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Do isolated gallery-forest trees facilitate recruitment of forest seedlings and saplings in savannna?

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... These expansions depend on successful establishment of forest tree species because many constraints limit the establishment of forest tree species in savannah-woodland. For this reason, more emphasis has been placed on the importance of plant-plant interactions (i.e., facilitation) during the recruitment of forest tree species in savannah-woodland 8,9,10,11,12,13 . Facilitation has been shown to exert both a direct and indirect effect on establishment through the modi cation of abiotic and biotic conditions by particular plants, socalled nurse plants 14,15 . ...
... Previous studies suggest that seedling abundance and the survival of forest tree species under nurse plants is higher than in treeless areas of savannah-woodland 8,9,10 . These studies further suggest that the facilitative effects, such as amelioration of water stress, act by suppressing the occurrence of re and improving soil properties. ...
... afromontanum seedlings on the ground below. Fire suppression is very important because forest tree species are generally very vulnerable to re 8,10,11 . Previous empirical and theoretical studies on forest-savannah dynamics further suggest the importance of re suppression on forest expansion into adjacent savannah-woodland 3,42 . ...
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This study examined the mechanisms of facilitation and importance of seed dispersal during establishment of forest tree species in a tropical woodland. Seedling survival of Syzygium guineense ssp. afromontanum seedling (forest tree species) was monitored for 2.5 years in four different microsites in savannah-woodland in Malawi under Ficus natalensis (a potential nurse plant), Brachystegia floribunda (a woodland tree), and Uapaca kirkiana (a woodland tree), and in a treeless site. S. guineense ssp. afromontanum seed deposition was also monitored in the four sites, and the natural establishment of forest tree species was observed to confirm the importance of seed dispersal. Protection from fire was found to be the most important facilitation mechanism in this area, with a total fire-induced mortality rate of 43%. However, the rate was comparatively low under F. natalensis , where fire is thought to be inhibited due to the lack of light-demanding C4 grasses. B. floribunda also offered protection from fire, and seedling survival did not differ between these two microsites. However, only a few individual forest tree species naturally established under B. floribunda , indicating that the facilitative mechanism of fire suppression is not the only factor affecting establishment in this tropical woodland. A higher rate of seed dispersal was also observed under F. natalensis compared with the other three microsites, suggesting that dispersal processes are also critical for the establishment of forest tree species in woodland in this region.
... Many individual studies have quantified the ecological importance of scattered trees by comparing the levels of species richness of various groups of biota and the abundance of individual species or groups between areas supporting scattered trees and nearby open areas (Azihou, Kakaï, & Sinsin, 2014;Brooker, Osler, & Gollisch, 2008;Hooper, Legendre, & Condit, 2005). Such investigations have shown that the presence of scattered trees can positively affect the abundance and/or richness of several groups, including plants (Poltz & Zotz, 2011;Schlawin & Zahawi, 2008), birds (Barth et al., 2015;Pizo & Santos, 2011), bats Lumsden & Bennett, 2005), ants (Frizzo & Vasconcelos, 2013;Oliver, Pearce, Greenslade, & Britton, 2006), beetles (Oshawa, 2007), collembolans (Rossetti et al., 2015) and mites (Brooker et al., 2008). ...
... Considering that scattered trees were regarded as "biodiversity foci" in previous studies (e.g. Azihou et al., 2014;Dunn, 2000;Oshawa, 2007), we predicted there would be higher biodiversity levels near scattered trees. ...
... taxonomic families or guilds; for example, Ozanne et al., 2000) or (2) reported the abundance for the most abundant (prevalent) species only (e.g. Azihou et al., 2014). Using these criteria, we identified nine comparisons of community composition between scattered trees and open areas from eight studies, and 10 comparisons of community composition between scattered trees and habitat patches from nine studies. ...
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Scattered trees are thought to be keystone structures for biodiversity in landscapes world-wide. However, such trees have been largely neglected by researchers and their importance for biodiversity remains unclear. We completed a global meta-analysis to quantify relationships between scattered trees and the species richness, abundance and composition of vertebrates, arthropods and plants. First, we tested whether areas near scattered trees support higher levels of species richness and abundance than nearby open areas. Second, we compared levels of species richness and abundance in matrix areas with scattered trees and areas embedded within nearby habitat patches. We also compared the composition of biological communities inhabiting habitat patches, open areas and areas with scattered trees. A total of 62 studies contained suitable data for our quantitative analyses. The local abundance of arthropods, vertebrates and woody plants was 60%–430% greater and overall species richness was 50%–100% higher in areas with scattered trees than in open areas. Conversely, for herbaceous plants, there was no consistent relationship between species abundance and the occurrence of scattered trees, although species richness was, on average, 43% lower. The abundance and richness of all taxonomic groups was similar in matrix areas supporting scattered trees and habitat patches, although the species richness of epiphytes was, on average, 50% higher in habitat patches. Communities inhabiting habitat patches were more similar in composition to the communities inhabiting areas with scattered trees, and less similar to the communities of open areas. Synthesis and applications. Areas with scattered trees support greater levels of biodiversity than open areas, as well as communities that are more similar to those inhabiting habitat patches. Scattered trees can be regarded as keystone structures for vertebrates, arthropods and terrestrial plants in landscapes world-wide. The maintenance of scattered trees may be compatible with livestock grazing in some agricultural landscapes. Greater management effort and targeted, long-term policies are needed to retain or re-establish scattered trees in many farming landscapes in both forest and non-forest biomes around the world.
... We studied the effects of recurrent defoliation by the Fulani on the dynamics of African mahogany Khaya senegalensis (Meliaceae) seedlings and saplings in Benin (West Africa). K. senegalensis is a key component of woodlands and gallery forests and serves as a nursery tree for woody species in the savanna (Azihou et al. 2013b). Gallery forests are narrow and therefore have high light penetration and support significant grass cover, resulting in similar dynamics, including frequent fires, in both the savanna and the narrow belt of gallery forest. ...
... by safe sites and by disturbances such as fire, rather than by seed availability (Grubb 1977, Turnbull et al. 2000, Uriarte et al. 2010) and that harvest impacts seedling dynamics through changes in seedling survival, rather than changes in recruitment. Furthermore, we did not find evidence of conspecific density-dependent mortality (Janzen 1970, Connell 1971, but instead found a positive association between conspecific seedling density and seedling survival (Azihou et al. 2013b). This implies that seedling survival is driven by abiotic conditions and abundance of conspecific seedlings is primarily an indication of good conditions. ...
... Since burning negatively affects seedling survival, while canopy cover promotes seedling survival, the net effect is lower seedling densities in harvested populations and lower seedling survival with harvest in the drier region (Fig. 7). Another study in our study area found seedlings, but not saplings, of K. senegalensis under isolated trees in a savanna matrix, further demonstrating the importance of forest conditions on the transition from seedlings to saplings (Azihou et al. 2013b). ...
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Recurrent tree defoliation by pastoralists, akin to herbivory, can negatively affect plant reproduction and population dynamics. However, our understanding of the indirect role of defoliation in seedling recruitment and tree–grass dynamics in tropical savanna is limited. In West African savanna, Fulani pastoralists frequently defoliate several fodder tree species to feed livestock in the dry season. We investigated the direct and indirect effects of recurrent defoliation of African mahogany (Khaya senegalensis) by Fulani people on seedling (<2 cm basal diameter) and sapling dynamics in West Africa using four years of demographic data on seedling and sapling density, growth, and survival, coupled with fruit production and microhabitat data over the same time period. Tree canopy cover facilitated seedlings but had negative effects on sapling growth possibly via intraspecific competition with adult plants. Interspecific competition with grasses strongly reduced seedling survival but had a weak effect on sapling growth. Fire reduced seedling survival and weakly reduced growth of seedlings and saplings, but did not affect sapling survival. These results indicate that the effect of fire on seedlings and saplings is distinct, a mechanism suitable for an episodic recruitment of seedlings into the sapling stage and consistent with predictions from the demographic bottleneck model. Defoliation affected seedling density and sapling growth through changes in canopy cover, but had no effect on seedling growth and sapling survival. In the moist region, sapling density was higher in sites with low-intensity defoliation, indicating that defoliation may strengthen the tree recruitment bottleneck. Our study suggests that large-scale defoliation can alter the facilitative role of nurse trees on seedling dynamics and tree–sapling competition. Given that tree defoliation by local people is a widespread activity throughout savanna–forest systems in West Africa, it has the potential to affect tree–grass coexistence. Incorporating the influence of large tree defoliation into existing models of savanna dynamics can further our understanding of tree–grass coexistence and improve management. A rotating harvest system, which allows seedlings to recruit episodically, or a patchwork harvest, which maintains some nursery trees in the mosaic, could help sustain seedling recruitment and minimize the indirect effects of harvest.
... Despite their harsh conditions, gallery-forest trees isolated in savanna provide a wonderful opportunity to study colonisation. Azihou et al. (2013b) found that the microenvironment beneath isolated trees facilitates the early establishment and persistence of gallery forest seedlings resulting from LDD. The microenvironment beneath isolated trees is also a preferential survival site for forest seedlings, which is consistent with directed dispersal. ...
... Rocky outcrops, ferruginous and silty soils support savannas whilst gallery forest soil is clayey. Eight gallery forest tree species have some individuals isolated in savanna (Azihou et al. 2013b): Daniellia oliveri (Leguminosae-Caesalpinioideae), Khaya senegalensis (Meliaceae), Anogeissus leiocarpa (Combretaceae), Pterocarpus erinaceus (Leguminosae-Papilionoideae), Vitex doniana (Verbenaceae), Diospyros mespiliformis (Ebenaceae), Parkia biglobosa (Leguminosae-Mimosoideae) and Tamarindus indica (Leguminosae-Caesalpinioideae). ...
... The ability of forest species to colonise the adjacent savanna was inferred from the abundance of seedlings and their persistence in the new environment from the abundance of saplings. Because isolated trees facilitate the survival of forest seedlings relative to the less conducive environment in savanna (Azihou et al. 2013b), data recorded beneath isolated trees will be informative on directed dispersal whilst savanna plots will explain random dispersal. ...
Article
Long-distance dispersal (LDD) of plants is difficult to measure but disproportionately important for various ecological and evolutionary processes. Dispersal of seeds of gallery-forest trees in savanna provides an opportunity for the study of colonisation processes and species coexistence driven by LDD. Investigations were carried out on 91 isolated trees along four gallery forests sampled in the Biosphere Reserve of Pendjari, Benin. The abundance of adult trees within nearest gallery forest was combined with functional traits (species maximum height, seed weight, morphological adaptation for dispersal by wind, water, birds and mammals) to explain the floristic composition of forest seedlings and saplings under isolated trees and in savanna. Stepwise negative binomial regression was used to identify the most significant variables explaining abundance of seedlings and saplings beneath isolated trees and in savanna and then derive colonisation from seedlings and persistence from saplings. The maximum height of species and seed weight explained the highest proportion of variance in species colonisation. Morphological dispersal syndromes by wind and birds had poor explanatory importance. Species rare in gallery forest had higher potential to colonise new environments through LDD whilst abundant species had higher persistence abilities. Contrary to the predictions of the seedling-size effect, small-seeded species dominated the sapling stage. The findings revealed the strong dependence of LDD and subsequent colonisation and persistence processes on species traits specialised for a variety of dispersal vectors. They also suggest that LDD towards isolated trees established far away from gallery forest can be difficult.
... Life history of colonizing plants is also relevant: facilitation is often stronger for shade-tolerant and later-successional plants than early-successional and weedy species (Gómez-Aparicio et al. 2004). Species-specific facilitation has not been observed in every habitat (Olofsson et al. 1999) or combination of species (Azihou et al. 2013; Al-Namazi and Bonser 2020), which demonstrates the importance of both site and species combinations to plant-plant interactions. ...
... Drivers of woody plant abundance are species-specific, consistent with the hypothesis of species-specific facilitation (Callaway 1998;Reinhart et al. 2006;Cavieres et al. 2008;Azihou et al. 2013;Fagundes et al. 2018). Four species, Ailanthus altissima, Lonicera morrowii, Rhamnus cathartica, and Ulmus parvifolia were positively associated with A. altissima nurse trees. ...
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Facilitation by nurse plants shapes community development during primary succession. Human activity—especially in urban settings—has created environments like abandoned lots or post-industrial waste sites in which primary succession occurs. The importance of facilitation in these stressful urban habitats is likely pronounced. There is evidence that facilitative interactions are species-specific and reflect the interactions of different combinations of plant species. We test the hypothesis that colonizing species abundance will differ between focal host tree species due to differing effects on microhabitat character. Alternatively, colonizing species’ distributions may instead reflect environmental heterogeneity independent of host species. We sampled vegetation under 13 trees on a slagheap in Pittsburgh, PA, USA. Focal trees included Ailanthus altissima (n = 4), Rhamnus cathartica (n = 4), and Ulmus parvifolia (n = 5). Vegetation was surveyed and environmental variables were measured at the base of each tree. Species richness was compared between tree species at a larger cluster and smaller 1m² plot scales to assess scale-dependent patterns. Eight woody species were tested for differences in abundance between focal tree species. Shrubs and vines responded to different variables individually, including environmental factors and focal tree species. Non-native richness differed significantly with soil temperature but not between tree species. Though all focal trees in this study are non-native, we conclude that they facilitate establishment of both native and non-native species. Retention of nurse trees in post-industrial environments, even non-native individuals, may therefore contribute to revegetation of native species.
... This theory has been tested for many tropical trees (Cintra et al. 1997;Marhaver et al. 2013;Shea et al. 2004) and especially in the Amazonian forests. However, although the regeneration is poor for many trees species, there is a huge gap of information on the strength of this effect for tropical species in Sudanian region (Azihou et al. 2013). This may jeopardize the long-term conservation of tree species (Gignoux et al. 2009) and even affect the numerous services they provided to mankind (Heubach et al. 2011). ...
... To our knowledge, few studies have focused on the evaluation on the Janzen-Connell effect in Sudanian region, and few species have supported this effect even though the natural regeneration of many species is very challenging. For example, Azihou et al. (2013) found only two out of eight tree tropical savanna tree species such as Daniellia oliveri and Khaya senegalensis that supported this effect. In contrast, this effect has been widely tested for tree species in the Amazonian tropical rain forest (Clark and Clark 1984;Marhaver et al. 2013;Bagchi et al. 2010) and the Hawaiian Forest (Inman-Narahari et al. 2016). ...
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Afzelia africana is a tropical multi-use tree species distributed in West, East, and Central Africa. Understanding factors affecting seedlings is crucial to design an effective conservation and restoration plan for the species. Here, we examined the factors constraining seedling density and survival. We randomly established 203 circular plots of 5-m radius around adult trees in two different management systems (Agro-silvo-pastoral system and Protected Area). Then, we focused on the measurement of seedling density, survival/mortality in 2014 and 2015, and estimated disturbance and micro-environment features. We found that seedling mortality rate was higher in Agro-silvo-pastoral system than in Protected area, demonstrating the prominent role of the protected area for the species conservation. The more intense adult trees are pruned, the lower the seedling density beneath, showing a negative effect of pruning on seedling. Seedling density is relatively higher under large trees, revealing the seedlings recruitment bottleneck due to repeated fire and browsing. Furthermore, our data supported the Janzen–Connell effect, suggesting that seedlings are attacked by predators of their parents. Finally, we conclude that management system, pruning intensity, seedling predation determine the regeneration status of A. africana in Sudanian zone. Artificial regeneration program is necessary, as well as shelters and thorny protectors should be established around seedlings under reproductive trees. Firebreaks are also recommended to protect the seedlings and Afzelia stands.
... Livestock may thus be critical for the successional process at the early stages of espinal development after agricultural abandonment, fire, or severe woodcutting. Large herbivores can interact in complex ways with other disturbance factors such as fire to affect the overall balance between tree-tree and tree-grass competition and facilitation mechanisms (e.g., Holmgren et al. 2000, Van Langevelde et al. 2003, Armesto et al. 2009, Staver et al. 2009, Soliveres et al. 2012, Azihou et al. 2013b, Doughty et al. 2015. A key take-away message, in any case, is that most large herbivores had no effect on sclerophyllous tree presence under A. caven. ...
... Our results partially support the findings of related work on plant-plant facilitation in semiarid habitats. As in similar habitats, we found evidence of important effects of open woodland or savanna tree canopies on other plant species (Belsky et al. 1989, Hoffmann 2000, Azihou et al. 2013b, Bufford and Gaoue 2015, Stahlheber et al. 2015. However, our results do not reflect the fleshy fruit/Tertiary and dry fruit/Quaternary syndromes reported for nurse and beneficiary pairs in other mediterranean-climate habitats (Verd u et al. 2003, Soliveres et al. 2012. ...
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The successional pathways linking the Acacia caven-dominated savanna habitat “espinal” and the closed sclerophyllous forest of central Chile have long been debated. Previously, espinal was considered an invasive degradation of closed forest that tended toward desertification, could not be restored to forest, and had little ecological value. Recent GIS (Geographical Information System) research on land-use change has, however, detected apparent regeneration of sclerophyllous forest from espinal. This suggests that there is a successional path linking espinal and sclerophyllous forest. Here, we used surveys of transects in espinals and espinal–sclerophyllous forest transitions to ask whether (1) A. caven is an invasive species or a pioneer species, (2) forest regenerates by sclerophyllous trees “filling in” spaces between A. caven individuals, and then shading them out (plant–plant competition), or (3) forest regenerates by plant–plant facilitation between A. caven and other species, and (4) how current and historical management and condition affect these potential successional mechanisms. We find that A. caven establishes primarily in full sunlight and is unlikely to degrade forests via invasion. We also find, for the first time, evidence that A. caven is a nurse tree to several sclerophyllous forest tree-beneficiary species. Measurements of the under-canopy microhabitat of A. caven, compared to Lithraea caustica, another possible nurse species, suggest that it provides a balance between shade and soil moisture retention, making it a regeneration site not only for directed bird-dispersed seeds but also for undirected wind-dispersed ones. Conservation and restoration of espinals, especially in drier areas, could provide capacity for future dynamic successional pathways in central Chile.
... The pronounced differences in canopy effects across trees indicate that individual canopy traits and/or small-scale differences in abiotic site conditions override species-specific effects of canopy architecture. From previous studies we can infer that, besides canopy density, its size and height may also play an important role for the strength of facilitative effects (Weltzin & Coughenour 1990;Azihou et al. 2013). Canopy height may modulate shading effects over the course of the day, with lower canopies having stronger effects on microclimatic conditions (Belsky et al. 1993). ...
... We deduce that it is highly desirable for future studies to measure intraspecific variation in canopy traits, and to record abiotic site conditions on a microhabitat level. The only study we are aware of which simultaneously evaluated the importance of species identity and canopy size comes from a mesic West African savanna, and focused on tree recruitment in sub-canopy habitats (Azihou et al. 2013). This study found that canopy effects can be positive (facilitative), neutral or negative (suppressive), depending on species identity and canopy area. ...
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Do East African legume trees differ in their canopy effects? Have species with intermediate canopy density more pronounced facilitative effects on understorey primary production, community composition and diversity? How are canopy effects related to species' encroachment status, and modulated by a local aridity gradient? Upland and lowland environments of the Borana rangelands, an arid thornbush savanna in southern Ethiopia. We harnessed pastoralists' local ecological knowledge to rank the encroachment status of six legume tree species previously known as Acacia, and correlated this rank to a dendrometric proxy of canopy density. Vegetation relevés (1 m²) were placed in sub-canopy and adjacent inter-canopy habitats of four legume tree species that differed in canopy density and encroachment status (Vachellia bussei, V. drepanolobium, V. seyal, V. tortilis). Using mixed-effects ANOVA, we evaluated effects of tree species and habitat (sub-canopy or inter-canopy) on total, forb and grass biomass, and on species diversity, comparing results for lowland and upland sites. Effects on floristic composition were assessed via PERMANOVA and NMDS. Species' encroachment status and canopy density were negatively correlated. Most pronounced facilitative effects (more frequent and larger differences between sub-canopy and inter-canopy habitats) were found for a species with intermediate canopy density (V. bussei). Diversity in its understorey vegetation increased up to 32% (total biomass: 29%, forb biomass: 40%). In contrast, the species with the most open canopy (V. drepanolobium; also the most encroaching species) never exerted significant facilitative effects. In contrast to expectations, canopy effects were more frequent in (climatically less arid) upland environments, probably due to a redistribution of water within the landscape. Large differences in facilitative effects across tree individuals indicate that a high intraspecific variability of canopy traits and/or small-scale differences in abiotic site conditions have partly overridden species-specific differences in canopy architecture. Our study provides new insights on why encroaching legume tree species may decrease herbaceous production: they tend to have more open canopies, with small facilitative effects on sub-canopy herbaceous production. We thus recommend promoting an open savanna with the tree layer dominated by species with a high facilitative potential to maintain forage provision and species diversity.
... afromontanum seedlings. Fire suppression is very important because forest tree species are generally very vulnerable to fire 8,10,11 . Previous empirical and theoretical studies on forest-savannah dynamics further suggest the importance of fire suppression on forest expansion into adjacent savannah-woodland 3,46 . ...
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This study examined the mechanisms of facilitation and importance of seed dispersal during establishment of forest tree species in an Afrotropical woodland. Seedling survival of Syzygium guineense ssp. afromontanum was monitored for 2.5 years at four different microsites in savannah woodland in Malawi (southeastern Africa) under Ficus natalensis (a potential nurse plant), Brachystegia floribunda (a woodland tree), Uapaca kirkiana (a woodland tree), and at a treeless site. The number of naturally established forest tree seedlings in the woodland was also counted. Additionally, S. guineense ssp. afromontanum seed deposition was monitored at the four microsites. Insect damage (9% of the total cause of mortality) and trampling by ungulates (1%) had limited impact on seedling survival in this area. Fire (43%) was found to be the most important cause of seedling mortality and fire induced mortality was especially high under U. kirkiana (74%) and at treeless site (51%). The rate was comparatively low under F. natalensis (4%) and B. floribunda (23%), where fire is thought to be inhibited due to the lack of light-demanding C4 grasses. Consequently, seedling survival under F. natalensis and B. floribunda was higher compared with the other two microsites. The seedling survival rate was similar under F. natalensis (57%) and B. floribunda (59%). However, only a few S. guineense ssp. afromontanum seedlings naturally established under B. floribunda (25/285) whereas many seedlings established under F. natalensis (146/285). These findings indicate that the facilitative mechanism of fire suppression is not the only factor affecting establishment. The seed deposition investigation revealed that most of the seeds (85%) were deposited under F. natalensis. As such, these findings suggest that in addition to fire suppression, dispersal limitations also play a role in forest-savannah dynamics in this region, especially at the community level.
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Recurrent tree defoliation by pastoralists, akin to herbivory, can negatively affect plant reproduction and population dynamics. However, our understanding of the indirect role of defoliation in seedling recruitment and tree-grass dynamics in tropical savanna is limited. In West African savanna, Fulani pastoralists frequently defoliate several fodder tree species to feed livestock in the dry season. We investigated the direct and indirect effects of recurrent defoliation of African mahogany (Khaya senegalensis) by Fulani people on seedling (<2 cm basal diameter) and sapling dynamics in West Africa using four years of demographic data on seedling and sapling density, growth, and survival, coupled with fruit production and microhabitat data over the same time period. Tree canopy cover facilitated seedlings but had negative effects on sapling growth possibly via intraspecific competition with adult plants. Interspecific competition with grasses strongly reduced seedling survival but had a weak effect on sapling growth. Fire reduced seedling survival and weakly reduced growth of seedlings and saplings, but did not affect sapling survival. These results indicate that the effect of fire on seedlings and saplings is distinct, a mechanism suitable for an episodic recruitment of seedlings into the sapling stage and consistent with predictions from the demographic bottleneck model. Defoliation affected seedling density and sapling growth through changes in canopy cover, but had no effect on seedling growth and sapling survival. In the moist region, sapling density was higher in sites with low-intensity defoliation, indicating that defoliation may strengthen the tree recruitment bottleneck. Our study suggests that large-scale defoliation can alter the facilitative role of nurse trees on seedling dynamics and tree-sapling competition. Given that tree defoliation by local people is a widespread activity throughout savanna-forest systems in West Africa, it has the potential to affect tree-grass coexistence. Incorporating the influence of large tree defoliation into existing models of savanna dynamics can further our understanding of tree-grass coexistence and improve management. A rotating harvest system, which allows seedlings to recruit episodically, or a patchwork harvest, which maintains some nursery trees in the mosaic, could help sustain seedling recruitment and minimize the indirect effects of harvest.
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Understanding the role of termite mounds in biodiversity and ecosystem functioning is a priority for the management of tropical terrestrial protected areas dominated by savannahs. This study aimed to assess the effects of termite mounds on the diversity of plant functional types (PFTs) and herbaceous' net aboveground primary productivity (NAPP) in plant communities (PCs) of the Pendjari Biosphere Reserve. PCs were identified through canonical correspondence analysis performed on 96 phytosociological 'relevés' realized in plots of 900 m². PFTs' diversity was compared between savannahs and mounds' plots using generalized linear models. In each plot, 7 m² subplots were harvested and NAPP was determined. Linear mixed models were performed to assess change in herbaceous NAPP regarding species richness, graminoids' richness, specific leaf area and termite mounds. There is no specific plant community related to mounds. However, the occurrence of termite mounds induced an increase of woody and forbs diversity while the diversity of legumes and graminoids decreased. These diversity patterns led to decreasing of PCs' NAPP. This study confirms that termite-induced resource heterogeneity supports niche differentiation theory and increased savannah encroachment by woody species.
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This chapter illustrates using simulations how the assumed Janzen-Connell relationship between distance from parent and seed density can break down. Emphasis is given on spatial patterns of seed dispersal, the resultant spatial structure of seeds and the potential consequences for the population and the community. A special form of dispersal limitation is detailed, i.e. contagious seed dispersal, which is defined as the patchy deposition of seeds such that some sites receive many seeds and others receive few to none. By investigating contagious seed dispersal, the effects of spatial variability in seed dispersal curves on subsequent demographic processes are examined. The chapter explains where such dispersal is likely to take place and suggests potential outcomes for seed survival based on the resulting spatial deposition patterns. It illustrates how contagious seed dispersal relates to and modifies the original Janzen-Connell model and its community-level outcomes.
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Savannas intermingled with gallery forests are dynamic habitats typical in Africa. This study aims to determine if differences in species traits lead to non-overlapping distribution of gallery-forest and savanna species and abrupt transition between gallery forest and savanna. Tree species densities were measured in 375 plots of 1500 m2 covering a total sample area of 56.25 ha along forty 3-km transects located at right angles to a riverbed with gallery forest into surrounding savanna. Location, vegetation type, soil physical properties, erosion and fire occurrence were recorded as site factors. Data analysis included the quantification of co-occurrence patterns, threshold indicator taxa analysis and fuzzy set ordination. The gallery forest–savanna gradient predicted floristic composition of plots with a correlation of 0.595 but its accuracy was locally modified by the occurrence of fire and the physical properties of soil that coveredmore than 30% of the range of residuals. The distribution of gallery-forest and savanna tree species did not overlap. Along the gallery forest–savanna gradient, savanna species gradually increased in density while gallery-forest species showed a community threshold at 120 m from the river beyond the width of gallery forest. The forest species driving this trend should play an important role in the dynamics of gallery forest–savanna boundaries.
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1. The directed-dispersal (DrD) hypothesis constitutes one of the main explanations for the adaptive value of seed dispersal in spatially heterogeneous environments. Traditionally, the DrD hypothesis asserts non-random arrival to specific sites where establishment conditions are independently favourable. Yet, enhanced arrival might negatively affect the establishment in otherwise favourable sites through enhanced density-dependent mortality (DDM). Since both density effects and habitat suitability might differ among establishment phases, assessment of habitat suitability should encompass the entire establishment process.
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▪ Abstract Savannas occur where trees and grasses interact to create a biome that is neither grassland nor forest. Woody and gramineous plants interact by many mechanisms, some negative (competition) and some positive (facilitation). The strength and sign of the interaction varies in both time and space, allowing a rich array of possible outcomes but no universal predictive model. Simple models of coexistence of trees and grasses, based on separation in rooting depth, are theoretically and experimentally inadequate. Explanation of the widely observed increase in tree biomass following introduction of commercial ranching into savannas requires inclusion of interactions among browsers, grazers, and fires, and their effects on tree recruitment. Prediction of the consequences of manipulating tree biomass through clearing further requires an understanding of how trees modify light, water, and nutrient environments of grasses. Understanding the nature of coexistence between trees and grass, which under other ci...
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Question: Does the proximity of shrubs affect seasonal water stress of young Austrocedrus chilensis trees (a native conifer of the Austral Temperate Forest of South America) in xeric sites? Location: A. chilensis xeric forest in northwest Patagonia, Argentina. Methods: We examined the dependence of predawn twig water potential on tree development (seedling to adult) and proximity to nurse shrubs during spring and summer. We analysed spatial associations of seedlings, saplings and adult trees with nurse shrubs, and also evaluated if trees affected shrub canopy vitality. Results: Water stress in Austrocedrus trees was affected by shrub presence. Small trees (i.e.<0.5 m in height) growing in the open were most stressed, particularly in summer. Small trees growing within a shrub canopy had low water stress and little change between spring and summer. The opposite trend, however, was true for the medium-height category (i.e. 0.5-1.5 m in height); trees in this size category were more stressed when growing within the shrub canopy than in the open. Larger Austrocedrus trees (i.e.>2 m in height) were not affected by shrub presence. Austrocedrus trees were spatially associated with shrubs in all height classes; however, the percentage of living shrub canopy decreased with tree height. Conclusions: In xeric areas of northwest Patagonia, the strength and direction of interactions between A. chilensis and shrubs, in terms of tree water stress, are dynamic and modulated by tree size and environmental conditions. Overall, positive effects of shrubs on early developmental stages appear to be more important than subsequent negative interactions, since nursing effects could generate a spatial association of shrubs and Austrocedrus trees that persists through later successional stages. These findings shed light on mechanisms behind successional changes, and have important conservation and management implications.
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A high number of tree species, low density of adults of each species, and long distances between conspecific adults are characteristic of many low-land tropical forest habitats. I propose that these three traits, in large part, are the result of the action of predators on seeds and seedlings. A model is presented that allows detailed examination of the effect of different predators, dispersal agents, seed-crop sizes, etc. on these three traits. In short, any event that increases the efficiency of the predators at eating seeds and seedlings of a given tree species may lead to a reduction in population density of the adults of that species and/or to increased distance between new adults and their parents. Either event will lead to more space in the habitat for other species of trees, and therefore higher total number of tree species, provided seed sources are available over evolutionary time. As one moves from the wet lowland tropics to the dry tropics or temperate zones, the seed and seedling predators in a habitat are hypothesized to be progressively less efficient at keeping one or a few tree species from monopolizing the habitat through competitive superiority. This lowered efficiency of the predators is brought about by the increased severity and unpredictability of the physical environment, which in turn leads to regular or erratic escape of large seed or seedling cohorts from the predators.
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Scatter-hoarding animals spread out cached seeds to reduce density-dependent theft of their food reserves. This behaviour could lead to directed dispersal into areas with lower densities of conspecific trees, where seed and seedling survival are higher, and could profoundly affect the spatial structure of plant communities. We tested this hypothesis with Central American agoutis and Astrocaryum standleyanum palm seeds on Barro Colorado Island, Panama. We radio-tracked seeds as they were cached and re-cached by agoutis, calculated the density of adult Astrocaryum trees surrounding each cache, and tested whether the observed number of trees around seed caches declined more than expected under random dispersal. Seedling establishment success was negatively dependent on seed density, and agoutis carried seeds towards locations with lower conspecific tree densities, thus facilitating the escape of seeds from natural enemies. This behaviour may be a widespread mechanism leading to highly effective seed dispersal by scatter-hoarding animals.
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The mycorrhizal status of several representative shrub species (Lavandula spp. and Thymus satureioides) in Moroccan semiarid ecosystems, was evaluated as well as their contribution to the mycorrhizal potential of the soil. Furthermore, the rhizosphere soils collected under these target species were tested for their influence on the growth of Cupressus atlantica, a tree species whose natural stands has declined in this area. Soil samples were collected from the rhizosphere of L. stoechas, L. dentata and of C. atlantica existing in the experimental area. Control samples were randomly collected from bare soil sites, away from plant influence. All the target species formed AM symbiosis and the extent of AM fungal colonization was not significantly different between plant species. No significant difference was detected between the total number of AM fungal spores of the bare soil and those recorded in the root zones of target species and C. atlantica. Three genera of AM fungi (Scutellospora, Glomus and Acaulospora) were present in the rhizospheres of the plant species and in the bare soil. The number of mycorrhizal propagules in soil originating from around the four target plant species was significantly higher than the one in the bare soil (Figure 1). The most probable number (MPN) of mycorrhizal propagules per 100 g of dry soil ranged from 7.82 (bare soil) to 179.7 (L. dentata and C. atlantica) and 244.5 (L. stoechas and T. satureioides). As the total number of spores was not different for the soil of different origins, the increase of the mycorrhizal soil infectivity (MSI) mainly resulted from larger AM mycelial networks that constituted the main source of AM fungal inoculum. In addition, this MSI enhancement was linked with changes in the functioning of soil microbial communities. In a glasshouse experiment, the growth of C. atlantica seedlings was significantly higher in the C. atlantica and in the shrub species soils than in the bare soil. Although the AM inoculum potential is not sufficient to ensure the development of forest trees in Mediterranean ecosystems, the use of plant nurses such as T. satureioides or Lavandula spp. could be of great interest to restore a self-sustaining vegetation cover to act against desertification.
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Seed dispersal can be advantageous (1) in escape from density-or distance-dependent seed and seedling mortality, (2) by colonization of suitable sites unpredictable in space and time, and (3) by directed dispersal to particular sites with a relatively high probability of survival. Most previous research on the consequences of seed dispersal has focused on escape and colonization because adaptations ensuring directed dispersal are not expected under the paradigm of diffuse mutualism that characterizes the modern view of seed dispersal evolution. In this paper, I suggest that directed dispersal is more common than previously believed even in the absence of plant adaptations to promote it. Directed dispersal may be seen in particular among animal-dispersed plants and in arid ecosystems or successional areas, but has been overlooked due to the lack of detailed data on seed shadows generated by particular species, and the fact that the alternative advantages of dispersal are not mutually exclusive. Although directed dispersal is never thought to be the only advantage of dispersal, it may often be ecologically important if one dispersal vector has a disproportionate effect on plant recruitment. Furthermore, in human-disturbed and managed ecosystems, directed dispersal may be important in restoration. More studies detailing the consequences of different patterns of seed dispersal will be useful for conservation and management strategies.
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We present results of two experiments designed to identify the relative importance of dispersal distance, seedling density, and light conditions on pathogen-caused mortality of tropical tree seedlings. The field experiment on Barro Colorado Island, Panama, demonstrated that both an increase in dispersal distance and a decrease in seedling density reduce levels of damping-off disease among seedlings of Platypodium elegans, and that there is an interaction between the two factors. The results indicated significant variation among sites in pathogen activity and suggested that seedlings are more vulnerable to disease when establishing around their parent tree than around other conspecific trees.The second experiment in a screened enclosure used potted seedlings of 18 wind-dispersed tree species exposed to two levels of sunlight and seedling density. The results indicated that environmental conditions similar to those in light-gaps significantly reduce pathogen activity. They also confirmed that high seedling density increases disease levels, especially under shaded conditions.Seedlings of 16 of the 18 species experienced pathogencaused mortality, but in widely varying amounts. Seed weight was not a good predictor of a species' vulnerability to pathogens. Adult wood density, an indicator of growth rate and successional status, was inversely correlated with a species' vulnerability to pathogens. Fast-growing, colonizing species, whose seedlings require light-gaps, lacked strong resistance to seedling pathogens, relative to slow-growing species able to tolerate shade and escape seedling pathogens. We discuss these results in the context of seed dispersal as a means of escaping from seedling pathogens.
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Recruitment and survival of cacti in North American deserts are facilitated under the canopy of nurse plants. In the Sonoran desert, the giant cardon cactus (Pachycereus pringlei) is associated with ironwood (Olneya tesota) and mesquite (Prosopis glandulosa) trees. We hypothesized that P. pringlei seedlings would perform better under ironwood than mesquite, on the basis of the mature individuals patterns of association. In Bahia Kino, Sonora, we conducted a field experiment from 1992 to 2000 comparing the performance of P. pringlei seedlings under randomly selected O. tesota, and P. glandulosa trees. Results indicated that P. pringlei seedlings under P. glandulosa had significantly higher survival and were significantly taller than those under O. tesota after 8 years. Micro-environment and soil properties beneath both trees did not differed significantly, while tree physiognomies differed only in height at the base of the canopy and basal area. As a deciduous tree, P. glandulosa provided more litter to the ground than O. tesota. These results therefore did not support our initial hypothesis. We discuss how other biotic factors such as differential seed dispersal might explain why P. pringlei establishment is more strongly spatially associated with O. tesota.
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Investigations of the role of competition, predation and abiotic stress in shaping natural communities were a staple for previous generations of ecologists and are still popular themes. However, more recent experimental research has uncovered the largely unanticipated, yet striking influence of facilitation (i.e. positive species interactions) on the organization of terrestrial and aquatic communities. Modern ecological concepts and theories were well established a decade before the current renaissance of interest in facilitation began, and thus do not consider the importance of a wide variety of facilitative interactions. It is time to bring ecological theory up to date by including facilitation. This process will not be painless because it will fundamentally change many basic predictions and will challenge some of our most cherished paradigms. But, ultimately, revising ecological theory will lead to a more accurate and inclusive understanding of natural communities.
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In this review, we discuss the ecological and evolutionary consequences of plant- herbivore interactions in tropical forests. We note first that herbivory rates are higher in tropical forests than in temperate ones and that, in contrast to leaves in temperate forests, most of the damage to tropical leaves occurs when they are young and expanding. Leaves in dry tropical forests also suffer higher rates of damage than in wet forests, and damage is greater in the understory than in the canopy. Insect herbivores, which typically have a narrow host range in the tropics, cause most of the damage to leaves and have selected for a wide variety of chemical, developmental, and phenological defenses in plants. Pathogens are less studied but cause considerable damage and, along with insect herbivores, may contribute to the maintenance of tree diversity. Folivorous mammals do less damage than insects or pathogens but have evolved to cope with the high levels of plant defenses. Leaves in tropical forests are defended by having low nutritional quality, greater toughness, and a wide variety of secondary metabolites, many of which are more common in tropical than temperate forests. Tannins, tough- ness, and low nutritional quality lengthen insect developmental times, making them more vulnerable to predators and parasitoids. The widespread occurrence of these defenses suggests that natural enemies are key participants in plant de- fenses and may have influenced the evolution of these traits. To escape damage, leaves may expand rapidly, be flushed synchronously, or be produced during the dry season when herbivores are rare. One strategy virtually limited to tropical forests is for plants to flush leaves but delay "greening" them until the leaves are mature. Many of these defensive traits are correlated within species, due to physiological constraints and tradeoffs. In general, shade-tolerant species invest more in defenses than do gap-requiring ones, and species with long-lived leaves are better defended than those with short-lived leaves.
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Positive interactions often play an important role in structuring plant communities and increasing biological diversity. Using three scales of resolution, we examine the importance of a long-lived desert tree, ironwood (Olneya tesota) in structuring plant communities and promoting biological diversity in the Sonoran Desert. We examined the positive effects of Olneya canopies of different sizes on plant communities in mesic and xeric habitats along throughout the central Gulf Coast subregion of Sonora, Mexico. In xeric sites, Olneya canopies had strong positive effects on plant richness and abundance, and small positive effects on the size of plants, underscoring the role of facilitation in extreme environments. In mesic sites, Olneya canopies had very little effect on perennials and a negative effect on ephemeral richness, suggesting predominantly competitive effects in this less stressful environment. Overall, Olneya canopies increased biological diversity where abiotic stress was high, but did not increase diversity in more mesic areas. Thus. Olneya canopies caused consistent shifts in plant-community structure among xeric and mesic sites, but not when these were combined. Benefactor size also mediated positive interactions, with larger Olneya supporting larger perennials in both xeric and mesic sites. Thus, stress gradients and benefactor size both influenced the balance of facilitative and competitive effects under nurse-plant canopies, and the spatial scale at which facilitative shape community structure.
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The chapters of this book on seed dispersal are divided into four parts: (1) frugivores and frugivory (8 chapters); (2) seed and seedling shadows (7 chapters); (3) seed fate and establishment (eight chapters); and (4) management implications and conservation (six chapters). The book presents both recent advances and reviews of current knowledge.
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The initial spatial pattern of seed deposition influences plant population and community structure, particularly when that pattern persists through recruitment. In a vertebrate-dispersed rain forest tree, Virola calophylla, we found that spatially aggregated seed deposition strongly influenced the spatial structure of later stages. Seed dispersion was clumped, and seed densities were highest underneath V. calophylla females and the sleeping sites of spider monkeys (Ateles paniscus), the key dispersal agent. Although these site types had the lowest per capita seed-to-seedling survival, they had the highest seedling/sapling densities. Conversely, seed and seedling/sapling densities were lowest, and seed survival was highest, at sites of diurnal seed dispersal by spider monkeys. Negative density-dependent and positive distance-dependent seed survival thinned seed clumps. Nonetheless, the clumped dispersion at sleeping and parental sites persisted to the seedling/sapling stage because differences in seed deposition were large enough to offset differences in seed survival among these site types.
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In tropical West Africa, distribution patterns of forest islands in savannas are influenced by fires which occur regularly in the grass stratum. Along continuous forest-savanna transects in the Comoe National Park, the change in the amount and composition of non-woody phytomass was investigated from savanna to forest interior. This was correlated with the cover of vegetation strata above, soil depth, and the occurrence of seasonal surface fires. Phytomass mainly consisted of leaf litter in the forests (about 400 g m(-2) at the end of the rainy season, and about 600 g m(-2) at the end of the dry season) and of grasses in the savanna (about 900 g m(-2)). Low grass biomass appeared to be primarily the result of suppression by competing woody species and not of shallow soil. The occurrence of early dry-season fires seemed to be determined mainly by the amount of grass biomass as fuel because fires occurred in almost all savanna plots while forest sites remained unaffected. However, late dry-season fires will encounter higher amounts of leaf litter raising fire probability in forests. Due to the importance of the amount of combustible phytomass, fire probability and intensity might increase with annual precipitation in both savanna and forest.
Article
Total nonstructural carbohydrates (TNC) of roots were sampled in early autumn from saplings of four species that differ in light-dependent growth and survivorship ("shade tolerance"). The two deciduous species (Acer saccharum and Fraxinus americana) had higher TNC concentrations than the evergreens (Tsuga canadensis and Pinus strobus), presumably because of autumn build-up of reserves for spring refoliation. In separate comparisons of deciduous and coniferous pairs, A. saccharum and T. canadensis had higher low-light TNC concentrations and survivorship than F. americana and P. strobus, respectively. In high light, TNC levels were not significantly different between A. saccharum and F. americana and both species had > 98% survivorship. An analytical model of carbohydrate allocation demonstrates that variation in storage allocation can influence survivorship and growth and that the opportunity cost of storage is lower under low light. The model and empirical data are consistent with an observed corelation among species between growth determinancy and shade tolerance and a negative correlation between high-light growth rates and low-light survivorship. Allocation to storage may be an effective strategy of shade tolerance because it is relatively inexpensive under low light and provides a buffer against stresses.
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The higher flammability of tropical savanna, compared with forest, plays a critical role in mediating vegetation‐environment feedbacks, alternate stable states, and ultimately, the distribution of these two biomes. Multiple factors contribute to this difference in flammability, including microclimate, fuel amount and fuel type. To understand this transition in flammability, we studied fuel characteristics and microclimate across eight savanna–forest boundaries in south‐central Brazil. At each boundary, the environment was monitored for one week with automated measurements of near‐surface wind speed, air temperature, relative humidity and presence of dew. Manual measurements were performed to quantify fuel amounts and fuel moisture. These data were used to parameterize the fire behaviour model BehavePlus5 in order to simulate fire behaviour over the savanna–forest boundary. There were strong gradients across the boundary in all variables with the exception of total fuel load. During the day, savannas had higher wind speed and air temperature, and lower relative humidity and fuel moisture than forests. Although fuel loads were similar in savanna and forest, savanna was characterized by lower fuel bulk density, largely because of the presence of grasses. Based on these measurements, the fire behaviour model predicted savanna fires to be faster, more intense, and with greater flame lengths, relative to forest. A sensitivity analysis indicated that the primary cause of these differences was the low fuel bulk density characteristic of grassy fuels, with lesser contributions from wind speed, fuel moisture and total fuel load. These results indicate that the dominance of grassy fuels is the primary cause of the high flammability of savanna.
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In tropical West Africa, distribution patterns of forest islands in savannas are influenced by fires which occur regularly in the grass stratum. Along continuous forest2 at the end of the rainy season, and about 600 g m2). Low grass biomass appeared to be primarily the result of suppression by competing woody species and not of shallow soil. The occurrence of early dry-season fires seemed to be determined mainly by the amount of grass biomass as fuel because fires occurred in almost all savanna plots while forest sites remained unaffected. However, late dry-season fires will encounter higher amounts of leaf litter raising fire probability in forests. Due to the importance of the amount of combustible phytomass, fire probability and intensity might increase with annual precipitation in both savanna and forest.
Book
Change is constant in everyday life. Infants crawl and then walk, children learn to read and write, teenagers mature in myriad ways, and the elderly become frail and forgetful. Beyond these natural processes and events, external forces and interventions instigate and disrupt change: test scores may rise after a coaching course, drug abusers may remain abstinent after residential treatment. By charting changes over time and investigating whether and when events occur, researchers reveal the temporal rhythms of our lives. This book is concerned with behavioral, social, and biomedical sciences. It offers a presentation of two of today's most popular statistical methods: multilevel models for individual change and hazard/survival models for event occurrence (in both discrete- and continuous-time). Using data sets from published studies, the book takes you step by step through complete analyses, from simple exploratory displays that reveal underlying patterns through sophisticated specifications of complex statistical models.
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After a millenarian history of overexploitation, most forests in the Medi- terranean Basin have disappeared, leaving many degraded landscapes that have been re- colonized by early successional shrub-dominated communities. Common reforestation tech- niques treat these shrubs as competitors against newly planted tree seedlings; thus shrubs are cleared before tree plantation. However, empirical studies and theory governing plant- plant interactions suggest that, in stress-prone Mediterranean environments, shrubs can have a net positive effect on recruitment of other species. Between 1997 and 2001, we carried out experimental reforestations in the Sierra Nevada Protected Area (southeast Spain) with the aim of comparing the survival and growth of seedlings planted in open areas (the current reforestation technique) with seedlings planted under the canopy of preexisting shrub species. Over 18 000 seedlings of 11 woody species were planted under 16 different nurse shrubs throughout a broad geographical area. We sought to explore variation in the sign and magnitude of interactions along spatial gradients defined by altitude and aspect. In the present work, we report the results of a meta-analysis conducted with seedling survival and growth data for the first summer following planting, the most critical period for reforestation success in Mediterranean areas. The facilitative effect was consistent in all environmental situations explored (grand mean effect size d 1 5 0.89 for survival and 0.27 for growth). However, there were differences in the magnitude of the interaction, depending on the seedling species planted as well as the nurse shrub species involved. Additionally, nurse shrubs had a stronger facilitative effect on seedling survival and growth at low altitudes and sunny, drier slopes than at high altitudes or shady, wetter slopes. Facilitation in the dry years proved higher than in the one wet year. Our results show that pioneer shrubs facilitate the establishment of woody, late-successional Mediterranean spe- cies and thus can positively affect reforestation success in many different ecological settings.
Chapter
This book provides information on the historical and theoretical perspectives of biodiversity and ecology in tropical forests, plant and animal behaviour towards seed dispersal and plant-animal interactions within forest communities, consequences of seed dispersal, and conservation, biodiversity and management.
Article
Traditional ecological models have focused mainly on competition between plants, but recent research has shown that some plants benefit from closely associated neighbors, a phenomenon known as facilitation. There is increasing experimental evidence suggesting that facilitation has a place in mainstream ecological theory, but it also has a practical side when applied to the restoration of degraded environments, particularly drylands, alpine, or other limiting habitats. Where restoration fails because of harsh environmental conditions or intense herbivory, species that minimize these effects could be used to improve performance in nearby target species. Although there are few examples of the application of this "nursing" procedure worldwide, experimental data are promising, and show enhanced plant survival and growth in areas close to nurse plants. We discuss the potential for including nurse plants in restoration management procedures to improve the success rate of such projects.
Article
In a Neotropical pasture, I predicted that two characteristics of trees, type of fruit produced and amount of shade cast, would affect recruitment and growth of woody plants underneath them. I also predicted that woody plants that persisted in active pasture would affect the species assemblages under trees after pasture abandonment. To investigate these hypotheses, I examined the assemblages of recruits under several types of trees in active pasture and also under similar trees that were fenced off (enclosed) to simulate abandoned pasture. The trees were Ficus spp. (fleshy fruits, deep shade), Pentaclethra macroloba (dry fruits, deep shade), Cecropia spp. (fleshy fruits, sparse shade), and Cordia alliodora (dry fruits, sparse shade). Recruitment into "open" pasture plots (i.e., without trees) was also examined. In active pasture, the species assemblage of woody recruits depended on the tree under which they grew. The assemblage under Ficus was dense and diverse, under Cecropia and Cordia it was moderately dense and diverse, in open pasture it was sparse and species-poor, and under Pentaclethra it was dominated by its own seedlings. These patterns were found in the enclosed pasture as well, apparently because woody plants that had survived in the active pasture continued to grow after "abandonment." However, after enclosure, many new plants also became established, such that the enclosed pasture plots had almost twice as many woody plants and species as the active pasture plots. Growth of woody plants was most rapid under trees with the least shade (Cecropia, Cordia) and in open pasture. In contrast, growth of recruits was slower under the much shadier Ficus, and thus, in the initial stages of succession, Ficus appeared not to be as important a "recruitment focus" for woody plants. Growth of recruits under the equally shady Pentaclethra was also slow, but Pentaclethra seedlings readily established just outside the canopies of parent trees, where they grew quickly and created dense, monospecific stands. The results of this study suggest that patterns of early succession to forest after pasture abandonment will depend on the kinds of trees found in the pasture. Persistent woody recruits under trees in active pasture constitute sources of advanced regeneration that will substantially affect forest succession after pasture abandonment.
Article
Many studies have focused on positive-plant interactions such as nurse plants, which provide a sheltered subcanopy environment that benefits the nursed species. Most of these studies have focused on plant distribution and association patterns, while the microclimatic benefits are often assumed. This study quantifies 5 a.m. subcanopy temperatures as well as dew points beneath a common nurse tree of the Sonoran Desert, Cercidium microphyllum (palo verde, Fabaceae). Data are collected over 35 days in the winter (January and February) at six locations (at the base of the trunk, midway to the canopy edge, canopy edge, all to the north and to the south of the base of the tree) under each of two trees, as well as a control in the open.It is warmer beneath the tree than in the open, but also, it is warmer in the interior than at the canopy edge, and warmer to the south. Furthermore, differences in temperature between the subcanopy and the open site are greater on colder nights, and less pronounced on warmer winter nights, possibly due to the effect of cloud cover which often results in warmer overnight lows. In addition, variation in 5 a.m. temperature is greatest at the canopy edge and open, and temperature varies less in the interior where temperatures are also warmer. Subcanopy cover was quantified using fish-eye imagery. Results show that there is a significant relationship between 5 a.m. temperature and overlying cover.Dew point temperatures, surprisingly, were lower under the tree than at the canopy edge and in the open. That is, it is relatively dry under the nurse. This pattern can similarly be seen by distance and direction from the base of the tree (drier in the south). This may have to do with the nurse's roots and other vegetation growing beneath the nurse's canopy that compete for water in the ground, which leaves less available water to evaporate into the subcanopy air.
Article
Many previous works have been dedicated to the modeling of forest fires (or bush fires) using cellular automata (CA). Usually the transition rules used by the CA are either set or obtained by identification from experimental results. The main drawback of CA model for forest fires is the lack of sound knowledge on these transition rules. This work presents a physical model containing the main physical processes involved in bush fire propagation—convection, radiative transfer (nonlocal), and pyrolysis degradation—to define the neighborhood and the transition rule tables needed by the CA. These transitions can then be expressed as functions of the properties of vegetation and flames. On the basis on this model, a study is carried out to analyze the combined effects of humidity and occupation density of vegetation on the fire propagation. Several thresholds on these parameters have been defined to understand for which conditions the fire propagates indefinitely.
Article
Questions: Has fire suppression relaxed barriers to the exchange of species between savanna and forest? Do all species or a subset of species participate in this exchange? Would current vegetation structure persist if fire suppression were to cease? Location: A gallery forest edge in the Cerrado region of central Brazil that burned only once in the past 35 years. Methods: Density of tree seedlings, saplings and adults, leaf area index (LAI), tree basal area and diameter were surveyed in 12, 10 m × 70 m transects centred on and perpendicular to the forest–savanna boundary. Community composition was assessed using non-metric multi-dimensional scaling (NMDS). Results: Basal area and LAI declined substantially from forest to savanna, with an associated shift in species composition. Savanna tree species were nearly absent in the forest, but accounted for the majority of stems in the savanna. In contrast, forest species comprised 14% of adults and more than one-third of juveniles in the savanna. Despite the high diversity of trees (85 species) in the forest, five species play a particularly large role in this initial phase of forest expansion. Reintroduction of fire, however, would result in widespread topkill of juveniles and the majority of adult forest trees, thereby interrupting the succession towards forest. Conclusions: After 35 years during which the site burned only once, the savanna still remains dominated by savanna species. Nevertheless, the dominance of forest juveniles in border and savanna tree communities suggests that with a continued policy of fire suppression, the forest will continue to expand.
Article
1. It is often assumed that there is a trade-off between maternal provisioning and dispersal capacity, leading small-seeded species to disperse further than large-seeded species. However, this relationship between dispersal distance and seed mass has only been quantified for species from particular sites or with particular dispersal syndromes. 2. We provided the first large-scale, cross-species quantification of the correlations between dispersal distance and both seed mass and plant height. Seed mass was positively related to mean dispersal distance, with a 100-fold increase in seed mass being associated with a 4.5-fold increase in mean dispersal distance (R2 = 0.16; n = 210 species; P < 0.001). However, plant height had substantially stronger explanatory power than did seed mass, and we found a 5-fold increase in height was associated with a 4.6-fold increase in mean dispersal distance (R2 = 0.54; n = 211 species; P < 0.001). 3. Once plant height was accounted for, we found that small-seeded species dispersed further than did large-seeded species (R2 = 0.54; n = 181 species; slope = −0.130; P < 0.001); however, seed mass only added 2% to the R2 of the model. Within dispersal syndromes, tall species dispersed further than did short species, while seed mass had little influence on dispersal distance. 4. Synthesis. These findings enhance our understanding of plant life-history strategies and improve our ability to predict which species are best at colonizing new environments.
Article
Questions: What are the nurse effects of Rhodomyrtus tomentosa in degraded land of South China? Are canopy or soil factors responsible for the main nurse effect? Do facilitative effects increase with the shade tolerance of the target species?Location: Degraded shrubland in South China.Methods: Seedlings of three native climax woody species (Schima superba, Michelia macclurei, Castanopsis fissa) that differ in shade tolerance were subjected to four treatments by transplantation: (1) under the canopy of R. tomentosa shrubs; (2) in open interspaces without vegetation cover (control); (3) under the canopy of R. tomentosa from which canopies had been removed; and (4) in open interspaces without vegetation but covered by branches and leaves of R. tomentosa.Results: At low soil nutrient levels, increased canopy shade, soil porosity and soil moisture under the canopy of R. tomentosa increased seedling survival of the climax tree species S. superba, C. fissa and M. macclurei, and shoot height of S. superba. The nurse effect (a form of facilitation) of R. tomentosa depended more on canopy shade than on soil amelioration. The magnitude of facilitation or nurse effect was positively correlated with shade tolerance of the target species.Conclusions: Use of nurse plants in restoration differs from traditional reforestation (clearing and/or burning to reduce interspecific competition between target tree species and non-target species) because it focuses on positive interactions between nurse plants and target plants that increase establishment of target species and reduce time required for restoration. Because nurse effects of R. tomentosa shrubs tended to be larger on target species with greater shade tolerance, shade-tolerant plants are suggested as target species to accelerate restoration.
Article
Summary • The structure and composition of plant communities are influenced by positive and negative interactions between plants, the balance of which may change in intensity and sign through time and space, depending on availability of resources and on plant life history. • Over a 2-year period we analysed the balance of interactions between different life stages of a perennial grass, Stipa tenacissima, and a shrub, Cistus clusii, the dominant species in a semi-arid community in south-east Spain. • Cistus shrubs acted as nurses for juvenile Stipa plants, improving their water status, nutrient content, carbon assimilation rates and growth. The mechanisms underlying this facilitation effect were mainly the improvement of microclimatic conditions and soil physical and chemical properties under shrub canopies. By contrast, juvenile Stipa plants had an overall neutral effect on Cistus shrubs, although Cistus suffered some competitive effects during periods of water shortage. At this life stage, the short-term outcome of the interaction for both species was dependent on resource availability. • Close spatial association between adult plants had no negative effects for the interacting species, although positive effects most likely counterbalanced negative effects. • The long-term outcome of the interaction is reflected in the spatial distribution of both species, and determines population dynamics in this semi-arid plant community. • Our data show that the short-term balance of plant interactions may easily shift in response to environmental variability, which in turn may have important consequences for plant community structure. Journal of Ecology (2005) doi: 10.1111/j.1365-2745.2005.01033.x
Article
Summary 1 The objective of this study was to evaluate whether early successional shrubs facil- itate, tolerate or inhibit different stages of tree establishment in abandoned tropical pasture. 2 Seed rain, seed predation, seed germination and seedling survival of tropical forest trees in pasture grass, below small (< 25 m 2 ) shrub patches and below large (> 40 m 2 ) shrub patches, were compared in one abandoned pasture in Costa Rica over 2 years. 3 Seed rain of animal-dispersed trees was higher below both large and small shrub patches than below grass. Seed rain of wind-dispersed trees did not differ in the three patch types. 4 Predation of all animal-dispersed seeds combined and of three individual species was significantly higher below large and small shrub patches compared with below grass; predation of five species did not differ significantly in the three patch types. 5 Germination did not differ significantly in the three patch types for any of the species. 6 Seedling survival was highest below large shrub patches for three of four species. 7 Computer simulations of probabilities of seeds arriving in the pasture and surviving to the seedling stage suggest that early successional shrubs have a net facilitative effect on the early stages of forest tree seedling establishment compared with areas without shrubs in the pasture studied, although variance was high. Shrubs may facilitate, inhibit and tolerate different stages of tree seedling establishment.
Article
Summary 1. Physical defence traits of stems and leaves should enhance biomechanical strength and survival of seedlings. For eight neotropical tree species that differ widely in life- history strategies, we compared stem and leaf biomechanical traits of 1 and 7-month- old seedlings grown in the shaded forest understorey and in the laboratory. 2. Material traits (biomechanical traits per unit volume, mass or cross sectional area) were positively associated with seedling survival across species. Shade tolerant species that survive well in the forest understorey had stems and leaves with greater modulus of elasticity (stiffness), fracture toughness (resistance to tear), tissue density and fibre contents, compared to less shade tolerant species. 3. Seedling survival was most strongly correlated with stem tissue density at both 1 and 7 months (Spearman's correlation coefficient r s = 0·93 and 0·90), but was also strongly correlated with leaf density and stem toughness at 7 months ( r s = 0·93 and 0·89, respectively). 4. Multiple material traits were strongly and positively correlated with each other in both stems and leaves. However, these traits varied independently of seed and seedling size among species, indicating the unique importance of physical defence as functional traits. 5. Structural traits of stems that integrate size with material traits, including % critical buckling height, flexural stiffness, work-to-bend and stem flexibility, showed no significant interspecific correlation with seedling survival. 6. Modulus of elasticity and fracture toughness of stems generally increased as seedlings aged from 1 to 7 months, especially in species with high tissue density. In contrast, fracture toughness of leaf mid-vein and lamina showed inconsistent ontogenetic changes across species. 7. These results demonstrate that biomechanical traits including tissue density and fracture toughness should be considered as important functional correlates of seedling survival and overall life-history strategies of tree species.
Article
Summary • In severely disturbed habitats, the onset of resprouting as a persistence strategy might be problematic for tree species which do not accumulate sufficient reserves before the first disturbance event. This is due to the trade-off between the growth of reserves required to recover after disturbance and that of photosynthetic tissues. • In humid savannas, fire prevents trees from invading the whole landscape and nearby gallery forests have a completely different floristic composition. We test if the variations of survival during the first years of a young tree's life can explain the exclusion of forest species and the dominance pattern within savanna species. • Every six months for four years, we censused all seedlings and resprouts in 1 ha area of an annually burned savanna, to estimate their seasonal survival rates. We used capture–recapture statistical models to control for the probability of missing seedlings in the tall grass. • There were two main distinct patterns of survival among seedlings: ‘fire-responding’ species showed a 20–80% decrease in survival during the dry season, interpreted as mainly due to fire; ‘drought-responding’ species showed 20–80% variations in survival positively correlated to early-growing-season rainfall. • Yearly averaged survival probabilities of seedlings ranged between 0.10 and 0.63, reaching 0.850–0.996 for > 3-year-old resprouts of savanna species. Forest species showed no increase in survival with age. • A 4-year-survival-probability analysis showed that forest species were excluded from the savanna at the seedling stage. No parameter of the early survival curve related to the abundance of savanna species at the adult stage. • Synthesis. Savanna tree species follow two mutually exclusive main patterns of early survival probably related to fire and early-wet-season drought. The exclusion of forest species is consistent with a build up of reserves that is too slow due to the growth-resistance trade-off. We conclude from these findings that the use of resprouting as a persistence strategy is heavily constrained by disturbance frequency and imposes strong trade-offs on plant growth strategy.
Article
The strength of competitive and facilitative interactions in plant communities is expected to change along resource gradients. Contrasting theoretical models predict that with increasing abiotic stress, facilitative effects are higher, lower, or similar than those found under more productive conditions. While these predictions have been tested in stressful environments such as arid and alpine ecosystems, they have hardly been tested for more productive African woodlands. We experimentally assessed the strength of tree seedling facilitation by nurse trees in mesic and dry woodlands in Benin, West Africa. We planted seedlings of the drought-sensitive Afzelia africana and the drought-tolerant Khaya senegalensis under three microsite conditions (closed woodland, woodland gap, and open fields). Seedling survival was greater within woodlands compared with open fields in both the mesic and dry woodlands. The relative benefits in seedling survival were larger at the dry site, especially for the drought-sensitive species. Nevertheless, plant interactions became neutral or negative during the dry season in the drier woodland, indicating that the net positive effects may be lost under very stressful abiotic conditions. We conclude that facilitation also occurs in the relatively more productive conditions of African woodlands. Our results underscore the role of environmental variation in space and time, and the stress tolerance of species, in explaining competitive and facilitative interactions within plant communities. Abstract in French is available at http://www.blackwell-synergy.com/loi/btp.
Article
Abstract Araucaria Forest expansion over grassland takes place under wet climate conditions and low disturbance and it is hypothesized that isolated trees established on grassland facilitate the establishment of forest woody species beneath their canopies. Forest with Araucaria angustifolia is a particular type of Brazilian Atlantic Forest and the main forest type on the highland plateau in south Brazil, often forming mosaics with natural Campos grassland. The objectives of this paper were to evaluate the role of isolated shrubs and trees as colonization sites for seedlings of Araucaria Forest woody species on grassland, to determine which species function as preferential nurse plants in the process and the importance of vertebrate diaspore dispersal on the structure of seedling communities beneath nurse plants. The study was conducted in São Francisco de Paula, Rio Grande do Sul State, where we sampled isolated shrubs and trees in natural grassland near Araucaria Forest edges. Seedlings were counted and identified, and seedling diaspore dispersal syndromes, size and colour were registered. We detected 11 woody species with a potential role in nucleating grassland colonization by forest species. Beneath the canopies of nurse plants more forest species seedlings were found compared with open field grassland and the seedlings had diaspores mostly dispersed by vertebrates. Also, more seedlings were found under the canopy of A. angustifolia than beneath other nurse plant species. We conclude that A. angustifolia trees established on grassland act as nurse plants, by attracting disperser birds that promote colonization of the site by other forest species seedlings, and that under low level of grassland disturbance, conservation of frugivorous vertebrate assemblages may increase forest expansion over natural grassland and also facilitate the regeneration of degraded forest areas.
Article
Questions: In a natural grassland-forest mosaic: What is the influence of phylogeny and diaspore traits related to disperser attraction (DAT) on (1) seed size/number trade-off (SSNT) in woody species colonizing forest patches; (2) on the frequency of the species? 3. What is the influence of forest patch area on mean seed size and number. 4. Do phylogeny and DAT expressed at the species level affect this relationship? Location: Campos grassland and Araucaria forest in São Francisco de Paula, RS, Brazil, at ca. 29°28’ S; 50° 13’ W. Methods: Forest patches of different sizes in a grassland site recovering for ten years since human disturbances were surveyed by the relative abundance of vertebrate-dispersed woody saplings. We described colonizer species according to taxonomic phylogenetic relationships and diaspore type, size and color. We analyzed with a variation partitioning method their influence on SSNT and on species frequency in the patches. At the community level we regressed mean seed size and number on forest patch area and evaluated how these relationships were affected by phylogeny and DAT at the species level. Results: 1. Phylogeny and DAT mostly explained seed size and seed number per diaspore variation. 2. By controlling phylogeny and DAT influence the frequency of species in forest patches was positively associated with their seed number in the diaspores, and negatively associated with their seed size. 3. Mean seed size and seed number at the community level were positively associated with patch area. 4. When phylogeny and DAT influences on seed size were removed this relationship was stronger for seed size and weaker for seed number. Conclusions: 1. Energy allocation to dispersal in detriment of offspring survival increased the successful establishment of colonizer species in forest patches, despite phylogenetic relationships and DAT variation in their diaspores. 2. Although patch area exerted a selective pressure on seed size, habitat preferences of dispersers may also influence patch colonization.
Article
Summary1. Ecological edges (zones separating ecosystems or land cover types) can function as active boundaries, unique habitats and dynamic transition zones. Abiotic factors, species and species interactions exhibit strong responses to edges, and these responses – edge effects – can profoundly impact ecosystem structure and function.2. Edge effects may be altered by the presence or proximity of other nearby edges. This phenomenon – edge interaction – is poorly understood, though its importance is increasingly recognized. Edge interactions are likely in fragmented or patchy landscapes that contain many edges. In such landscapes, understanding how nearby edges interact may be critical for effective conservation and management.3. I examined edge interactions in an East African savanna. In this landscape, abandoned cattle corrals develop into treeless, nutrient-rich ‘glades’ that persist as preferentially grazed areas for decades to centuries. Glades represent important sources of structural and functional landscape heterogeneity and have major impacts on distributional patterns of plant and animals.4. I used existing variation in inter-glade distance to investigate the importance and strength of glade edge interactions for plants, Acacia ants and large herbivores. Specifically, I compared response patterns obtained from transects that extended outward from isolated glades (>250 m from another glade) and non-isolated glades (<150 m from another glade).5. Edge effect patterns between nearby glades differed significantly from patterns around isolated glades. When compared to areas outside isolated glades, areas between glades had almost twice the density of trees, half as much large herbivore use, reduced cover of glade-dominant grasses, and different Acacia ant communities. Many of the edge effects observed between non-isolated glades could not be inferred from effects around isolated glades.6.Synthesis. These findings suggest that edge interactions can alter plant and animal distributions in patchy landscapes. Edge effects near multiple edges can be stronger, weaker or qualitatively different from those near isolated edges. Such edge interactions can increase or decrease structural and functional continuity between nearby patches. Appropriate extrapolation of local edge effects in complex and fragmented landscapes will require greater understanding of edge interactions.
Article
Questions: 1. Do the species composition, richness and diversity of sapling communities vary significantly in differently sized patches? 2. Do forest patches of different sizes differ in woody plant colonization patterns? Location: São Francisco de Paula, Rio Grande do Sul, Brazil, 29°28'S,50°13'W. Methods: Three woody vegetation types, differing in structural development (patch size) and recovering for 10 years from cattle and burning disturbances, were sampled on grassland. We analysed the composition and complexity of the woody sapling communities, through relative abundance, richness and diversity patterns. We also evaluated recruitment status (residents vs. colonizers) of species in communities occurring in different forest patch size classes. Results: 1. There is a compositional gradient in sapling communities strongly associated with forest patch area. 2. Richness and diversity are positively correlated to patch area, but only in poorly structured patches; large patches present richness and diversity values similar to small patches. 3. Resident to colonizer abundance ratio increases from nurse plants to large patches. The species number proportion between residents and colonizers is similar in small and large patches and did not differ between these patch types. 4. Large patches presented a high number of exclusive species, while nurse plants and small patches did not. Conclusions: Woody plant communities in Araucaria forest patches are associated with patch structure development. Richness and diversity patterns are linked to patch colonization patterns. Generalist species colonize the understorey of nurse plants and small patches; resident species cannot recruit many new individuals. In large patches, sapling recruitment by resident adults precludes the immigration of new species into the patches, limiting richness and diversity.
Article
The initial spatial pattern of seed deposition influences plant population and community structure, particularly when that pattern persists through recruitment. In a vertebrate-dispersed rain forest tree, Virola calophylla, we found that spatially aggregated seed deposition strongly influenced the spatial structure of later stages. Seed dispersion was clumped, and seed densities were highest underneath V. calophylla females and the sleeping sites of spider monkeys (Ateles paniscus), the key dispersal agent. Although these site types had the lowest per capita seed-to-seedling survival, they had the highest seedling/sapling densities. Conversely, seed and seedling/sapling densities were lowest, and seed survival was highest, at sites of diurnal seed dispersal by spider monkeys. Negative density-dependent and positive distance-dependent seed survival thinned seed clumps. Nonetheless, the clumped dispersion at sleeping and parental sites persisted to the seedling/sapling stage because differences in seed deposition were large enough to offset differences in seed survival among these site types.
Article
Competition is a key process in plant populations and communities. We thus need, if we are to predict the responses of ecological systems to environmental change, a comprehensive and mechanistic understanding of plant competition. Considering competition, however, only at the population level is not sufficient because plant individuals usually are different, interact locally, and can adapt their behaviour to the current state of themselves and of their biotic and abiotic environment. Therefore, simulation models that are individual-based and spatially explicit are increasingly used for studying competition in plant systems. Many different individual-based modelling approaches exist to represent competition, but it is not clear how good they are in reflecting essential aspects of plant competition. We therefore first summarize current concepts and theories addressing plant competition. Then, we review individual-based approaches for modelling competition among plants. We distinguish between approaches that are used for more than 10 years and more recent ones. We identify three major gaps that need to be addressed more in the future: the effects of plants on their local environment, adaptive behaviour, and below-ground competition. To fill these gaps, the representation of plants and their interactions have to be more mechanistic than most existing approaches. Developing such new approaches is a challenge because they are likely to be more complex and to require more detailed knowledge and data on individual-level processes underlying competition. We thus need a more integrated research strategy for the future, where empirical and theoretical ecologists as well as computer scientists work together on formulating, implementing, parameterization, testing, comparing, and selecting the new approaches.
Article
Ecology Letters (2012) 15: 748–758 Forest and savanna biomes dominate the tropics, yet factors controlling their distribution remain poorly understood. Climate is clearly important, but extensive savannas in some high rainfall areas suggest a decoupling of climate and vegetation. In some situations edaphic factors are important, with forest often associated with high nutrient availability. Fire also plays a key role in limiting forest, with fire exclusion often causing a switch from savanna to forest. These observations can be captured by a broad conceptual model with two components: (1) forest and savanna are alternative stable states, maintained by tree cover-fire feedbacks, (2) the interaction between tree growth rates and fire frequency limits forest development; any factor that increases growth (e.g. elevated availability of water, nutrients, CO2), or decreases fire frequency, will favour canopy closure. This model is consistent with the range of environmental variables correlated with forest distribution, and with the current trend of forest expansion, likely driven by increasing CO2 concentrations. Resolving the drivers of forest and savanna distribution has moved beyond simple correlative studies that are unlikely to establish ultimate causation. Experiments using Dynamic Global Vegetation Models, parameterised with measurements from each continent, provide an important tool for understanding the controls of these systems.
Article
The ecological forces determining where within a landscape plants recruit and grow has been termed proximal habitat choice. Habitat choice is imposed first by a heterogeneous pattern of seed dispersal across the patches that make up the landscape and second by environmental variation that favors plant survival in some patches more than in others. Seed-seedling conflicts can occur during both of these phases of habitat choice if conditions or traits that are favorable for seeds are unfavorable for seedlings. During the dispersal phase, smaller seeds may have a greater probability of dispersal than larger seeds, and thus a greater probability of escape from predation, but they contain fewer reserves for support of the establishing seedling. After dispersal, environmental characteristics of a given patch type that lead to disproportionately high seed survival may lead to disproportionately low seedling survival. Considering three hypothetical landscapes, each composed of five patch types, I demonstrate that seed-seedling conflicts can have a major impact on both the overall quantity of recruitment at the landscape level and on the distribution of recruitment among patches. Available empirical evidence suggests these conflicts may be widespread in natural systems. Given their potential importance and extent, seed-seedling conflicts may play a previously unrecognized role in habitat choice.
Article
1 To survive in forest understoreys, seedlings must depend on carbohydrate reserves when they experience negative carbon balance imposed by occasional light reduction and tissue loss to herbivores and diseases. We present the first experimental evidence in support of this hypothesis, using seven woody neotropical species. 2 We transplanted seedlings that had recently expanded their first photosynthetic cotyledon or leaf to the forest understorey (1% of full sun) and quantified initial biomass and total non-structural carbohydrate (TNC) in stems, roots and storage cotyledons. We then randomly assigned seedlings to control and two stress treatments: light reduction (0.08% of full sun for 8 weeks) and complete defoliation. 3 First-year survival of control seedlings, a comparative measure of shade tolerance, differed widely among species. The two stress treatments reduced survival and relative growth rates (RGR) of all species. Shade-tolerant species were little impacted by the stress treatments, whereas the two least shade-tolerant species experienced 100% mortality. 4 In all treatments, 8-week and first-year survival was positively correlated with initial TNC pool size in stems and roots. By contrast, survival was generally not correlated with initial TNC concentration in any organ, TNC pools in cotyledons, seed mass or seedling biomass. 5 TNC in stems and roots, but not in cotyledons, decreased in response to light reduction and defoliation over 8 weeks. Leaf area recovery of defoliated seedlings was positively correlated with initial TNC pools in stems and roots. 6 First-year survival in each treatment was negatively correlated with 0–8 week RGR of control seedlings, suggesting higher stress tolerance of species with inherently slow growth rates in shade. RGR of control seedlings from 0 to 8 weeks was negatively correlated with initial TNC pools, but not concentrations, in stems and roots. After 8 weeks, RGR was positive for all species, without clear relationships with survival or TNC. 7 We conclude that carbohydrate storage in stems and roots enhances long-term survival in shade by enabling seedlings to cope with periods of biotic and abiotic stress. Carbohydrate storage is a key functional trait that can explain species differences in growth and survival that lead to species coexistence through niche assembly processes and life-history trade-offs.
Article
Large Acacia erioloba trees scattered through the sparse grassy vegetation of the arid oligotrophic are focal points for animal activity because they supply nests sites, shade and scarce food resources. Faeces, fallen nest material and carcass remains left below trees elevate levels of nutrients available to plants in the soil beneath large trees. Soil concentrations of N and K were two times greater and P concentrations 2.5times greater under canopies of A.erioloba. Plant species with fleshy fruits(Boscia, Grewia, Lycium, and Solanum spp.) occurred in 8% of treeless plots and beneath 17% of A.erioloba saplings, but there frequency increased to 90% beneath large trees. Dead A.erioloba trees were replaced by matrix vegetation(54%) and large shrubs with fleshy truits(28%) rather than by conspecifics(17%). The distribution of fleshy fruited plants in the Kalahari is thus rather dynamic and tied to the distribution of large trees. The shade beneath the canopies of large spreading trees was used by birds (particularly Kori Bustards Ardeotis kori)and mammals(mainly Springbok Antidorcas marsupialis, Gemsbok Oryx gazella, Blue Wildebeest Connochaetes taurinus & Bat-Eared Fox Otocyon megalotis) as a resting place during the heat of the day. Large raptors(>1.5kg) and vultures(>5.0kg) seldom parched on saplings or dead trees. nests of raptors, and other large communal nests of Sociable Weavers Philatarius socius were found mainly in large trees, and the nests of the Tree Rat Thallomys paedulcus were found in cavities in the stems of large trees. Acacia erioloba and the only other large tree, Acacia haematoxylon, apparently structure plant and animal communities and determine pattern and patch dynamics in this arid savanna. We suggest that their role in maintaining biodiveristy in the Kalahari cannot be served by the homogeneous thickets of stunted acacias that develop where the vegetation is overgrazed.
Article
The concept of a zone of influence, the area over which a plant alters the environment, forms the basis of many models of plant competition. Because of the logistical difficulties, we actually know little about the sizes and shapes of zones of influence belowground. Here we advocate obtaining data on plants' belowground zones of influence, including the length and distribution of lateral roots, in order to understand better how plants respond to their abiotic soil environment and to other plants. We provide several examples from recent work. First, we present an analysis of a large global data set which shows that maximum lateral root spread correlates with canopy size but that, for a given canopy size, maximum lateral root spread is greater in arid environments, and in coarse textured soils[does this relate to need to acquire water and the hydraulic conductivity of the soil?] Second, we use an experiment with the weedy Abutilon theophrasti to show how nutrient analogs as tracers yields information about lateral rot distribution within populations. In our experimental populations, teh belowground zone of influence extended well beyond neighbouring plants. Overlap in zones of influence in increased in nutrient patches. Third, we propose a new conceptual model of belowground zones of influence based on these and other data sets. The model assumes that teh probability of resource uptake or competing with a particular neighbour declines with distance from teh stem but that considerable uptake at great distances is still possible. It is also allows for plasticity in root distributions as might occur in spatially heterogeneous soils. Finally, we suggest how better information on the shapes and sizes of belowground zones of influence will help develop a more predictive framework for understanding plant competition.