The ultimate importance of paternal contributions to fitness and of post-pollination selection in flowering plants have remained elusive, largely because of the technical difficulty of assigning paternity. I review empirical studies that use heritable markers to investigate per-fruit seed paternity in natural populations and after experimental multiple-donor pollination. Thirty-one studies covering 23 species from 16 plant families document that in natural populations seeds from a single fruit are often fathered by multiple pollen donors (5 species from 5 families), that donors can differ significantly in seed-siring success (8 species from 6 families), that variation in pollen tube growth rates can be heritable (n = 1 out of 4 studies), that donor and recipient genotypes can simultaneously affect paternity (n = 2), and that temporal order of pollen deposition (n = 1) and environmental effects(n = 2) affect the outcome of pollen competition. These studies also investigate the role of post-pollination selection in the avoidance of inbreeding and for species boundaries. Most studies of male reproductive success in plants to date base on isozyme electrophoresis. The availability in the last decade of highly polymorphic molecular markers such as microsatellite DNA has been expected to open new possibilities to investigate competition and selection during the gametophytic phase. Yet, to date, there is still need for greater data wealth on seed paternity to test theories of sex allocation and to gain deeper understanding of floral trait evolution and of the evolutionary consequences of post-pollination selection in flowering plants.
In December 2004, a huge tsunami in SE Asia claimed many lives and caused catastrophic damage. This event has stimulated a debate about the role played by coastal ecosystems such as mangrove forests and coral reefs in protecting low-lying coastal areas. While some observers claim that these ecosystems play an important role, others are more sceptical and fear that financial resources may be diverted away from tsunami preparedness programmes to ecosystem rehabilitation schemes. Here, we review the role of coastal ecosystems in mitigating sea wave hazards. In particular, we examine the influence of coastal vegetation in severely affected parts of Aceh and Southern Thailand during the 2004 tsunami, and include direct observations made during two field visits in 2006.
Numerous scientific studies illustrate that coastal ecosystems are important in protecting the coast against normal and extreme wind-driven waves, including cyclonic storm surges. However, much less information is available concerning the role of coastal ecosystems in protecting against tsunamis, and most studies have been too limited in scope to be able to generalise over large geographic areas and diverse conditions. In particular, few studies have taken account of the high variability in the energy and speed of tsunami waves along different coastline stretches, making it difficult to compare the role of ecosystems in different regions. Other aspects that have been little studied include the spatial patterns of impact, e.g. flow diversion and channelling as a result of hydraulic resistance, and the effects of tree breaking and flow debris. However, despite these limitations, some tentative conclusions can be drawn about the protection provided by ecosystems against tsunami waves:
(1) The most important predictor variables of the tsunami hazard (i.e. set-up and inundation) at the regional and landscape levels are distance to the tsunami source and coastal topography, in particular near-shore bathymetry.
(2) The influence of coral reefs on tsunami waves is complex. While closed, intact reefs can provide some protection, water may be accelerated through channels in fragmented reefs, causing even greater destruction on land.
(3) In contrast, seagrass beds appear to provide a more consistent buffering against tsunamis, though more data are needed to quantify this effect.
(4) In several locations (particularly farther away from the tsunami source), mangroves and other vegetation probably provided some protection against the 2004 tsunami; in numerous other locations, however, vegetation provided no protection or may in some cases even have increased the hazard (e.g. by contributing to flow debris, or by channelling water flows). It seems that whether or not vegetation does provide protection depends on many factors, including stand size, density, species composition, structure and homogeneity. Overall, the value of vegetation as a potential tsunami buffer is probably fairly minor, though it cannot be completely dismissed. Many more detailed spatial and hydro-dynamic analyses are needed before the hazard-vegetation interactions can be realistically modelled and visualised, e.g. on risk maps.
(5) Tsunami greenbelts should not be treated as alternatives to early warning systems. Greenbelts may only be considered as an economical (and multi-functional) means to provide relative hazard protection for material assets (e.g. infrastructure, agriculture).
We recommend a more holistic view, with tsunami hazard mitigation being seen as one of several services provided by coastal ecosystems. Equally, the value of other ecosystem services may be seen within the frame of risk management as they foster social stability and, in case of disaster, robustness. To assess the overall value of these services and to develop more accurate indicators of risk will require much more transdisciplinary research.
Allocation is one of the central concepts in modern ecology, providing the basis for different strategies. Allocation in plants has been conceptualized as a proportional or ratio-driven process (‘partitioning’). In this view, a plant has a given amount of resources at any point in time and it allocates these resources to different structures. But many plant ecological processes are better understood in terms of growth and size than in terms of time. In an allometric perspective, allocation is seen as a size-dependent process: allometry is the quantitative relationship between growth and allocation. Therefore most questions of allocation should be posed allometrically, not as ratios or proportions. Plants evolve allometric patterns in response to numerous selection pressures and constraints, and these patterns explain many behaviours of plant populations.In the allometric view, plasticity in allocation can be understood as a change in a plant's allometric trajectory in response to the environment. Some allocation patterns show relatively fixed allometric trajectories, varying in different environments primarily in the speed at which the trajectory is travelled, whereas other allocation patterns show great flexibility in their behaviour at a given size. Because plant growth is often indeterminate and its rate highly influenced by environmental conditions, ‘plasticity in size’ is not a meaningful concept. We need a new way to classify, describe and analyze plant allocation and plasticity because the concepts ‘trait’ and ‘plasticity’ are too broad. Three degrees of plasticity can be distinguished: (1) allometric growth (‘apparent plasticity’), (2) modular proliferation and local physiological adaptation, and (3) integrated plastic responses. Plasticity, which has evolved because it increases individual fitness, can be a disadvantage in plant production systems, where we want to optimize population, not individual, performance.
We investigated allocation to roots, stems and leaves of 27 species of herbaceous clonal plants grown at two nutrient levels. Allocation was analyzed as biomass ratios and also allometrically. As in other studies, the fraction of biomass in stems and, to a lesser extent, in leaves, was usually higher in the high-nutrient treatment than in the low-nutrient treatment, and the fraction of biomass in roots was usually higher under low-nutrient conditions. The relationship between the biomass of plant structures fits the general allometric equation, with an exponent 1 in most of the species. The different biomass ratios under the two nutrient conditions represented points on simple allometric trajectories, indicating that natural selection has resulted in allometric strategies rather than plastic responses to nutrient level. In other words, in most of the species that changed allocation in response to the nutrient treatment, these changes were largely a consequence of plant size. Our data suggest that some allocation patterns that have been interpreted as plastic responses to different resource availabilities may be more parsimoniously explained as allometric strategies.
The influence of plant diversity on belowground ecosystem processes and feedbacks of the latter on plant growth are still largely unexplored. A previous field experiment showed positive effects of plant diversity on diversity and activity of culturable soil bacteria. We examined soil carry-over effects of these experimental communities on subsequent plant growth and the development of soil microbial communities in pots. We inoculated sterile substrate with soil from the field experiment and then sowed three plant species - Dactylis glomerata, Plantago lanceolata and Trifolium pratense - as monocultures or 3-species mixtures into the pots. We measured the phytometers over a period of 14 months. Thereafter, we sampled the soil in 3-species mixtures to analyze microbial communities. The original plant species richness and number of functional groups had positive soil carry-over effects on phytometer germination. Furthermore, in mixtures, species richness also stimulated short-term biomass production of phytometers and catabolic diversity of culturable soil bacteria. The number of plant functional groups had a positive effect on the amount of prokaryotic phospholipid fatty acids (PLFAs) but negative effects on the amount of eukaryotic PLFAs and the activity of culturable soil bacteria. The carry-over effects of plant diversity on phytometer growth disappeared with time. However, specific effects of the presence of particular functional groups or species in the original plant communities remained significant and were sometimes paralleled by effects on soil microbial communities. These results suggest that in the short term, soil carry-over effects of plant diversity are mediated by a general stimulation of soil microbes whereas the longer-term effects of particular plant species are more likely due to compositional shifts in soil microbial communities. Although the mechanisms of soil carry-over effects in biodiversity experiments remain to be determined, our study demonstrates that they exist and probably involve soil microbial communities. This ‘memory’ of soils may be an important factor in ecosystem development.
The relationships between species’ functional traits and their demographic life histories are central to understanding the causes of life-history diversity. A previous study identified three independent demographic axes amongst 29 broadleaved tree species: a light-demand axis, a population turnover rate (r–K) axis, and one discriminating substrate requirements for recruitment. This study evaluates the basis of life-history variation amongst the 29 species by assessing the correlation between the three demographic axes above and 14 morphological, phenological, architectural and biomass-partitioning traits. A principal components analysis of the 14 traits revealed two main axes: (1) a ‘resource capture versus conservation’ axis separated species with large and thin leaf laminas, high leaf K, P and N concentrations, light wood and small seeds from species with opposite attributes, and (2) a ‘physiognomic’ axis separated species with compound leaves, high leaf/above-ground mass ratio, and tall first branch in saplings from species with opposite attributes. Subsequent paired correlations and a Procrustes superimposition analysis showed that species with attributes typical of high capture and low conservation of resources, small seeds and short stature had faster population turnover rates (short longevity, and high mortality and recruitment rates) than species with opposite attributes. Species with compound leaves, high leaf/above-ground mass ratio, and tall first branch as saplings (physiognomic axis) tended to be more light-demanding and to have fast diameter growth rates of stems ⩾10 cm diameter. The independence between attributes typical of resource capture or conservation and stem diameter growth rate is noteworthy; it differs from correlations often reported for saplings and could be attributed to ontogenetic changes in plant function. In addition, the clear link between attributes typical of resource capture or conservation and plant recruitment, survivorship and longevity is consistent with r–K theory and brings to light an important connection between a leading axis of plant evolutionary diversification (resource capture versus conservation) and rates of population turnover.
In the past insufficient attention has been paid to quantitative measurements of resource fluxes in ecosystems that undergo successional change. In this study, simultaneous changes in seven plant resources (photosynthetically active radiation (PAR), water, nitrogen, phosphorus, calcium, magnesium and potassium) are quantified by a chronosequence approach for a 300-yr-long secondary succession on poor soil from Calluna vulgaris heathland to Fagus sylvatica-Quercus petraea late-successional forest (heathland-to-forest succession).
Invasion ecology, the study of how organisms spread in habitats to which they are not native, asks both about the invasiveness of species and the invasibility of habitats: Which species are most likely to become invasive? Which habitats are most susceptible to invasion? To set the stage for considering these questions with regard to plants, we offer a two-way classification of nativeness and invasiveness that distinguishes natives, non-invasive non-natives and invasive non-natives. We then consider the current state of knowledge about invasiveness and invasibility. Despite much investigation, it has proven difficult to identify traits that consistently predict invasiveness. This may be largely because different traits favour invasiveness in different habitats. It has proven easier to identify types of habitats that are relatively invasible, such as islands and riverbanks. Factors thought to render habitats invasible include low intensities of competition, altered disturbance regimes and low levels of environmental stress, especially high resource availability. These factors probably often interact; the combination of altered disturbance with high resource availability may particularly promote invasibility. When biotic factors control invasibility, non-natives that are unlike native species may prove more invasive; the converse may also be true. We end with a simple conceptual model for cases in which high levels of environmental stress should and should not reduce invasibility. In some cases, it may be possible to manipulate stress to control biological invasions by plants.
We used a highly replicated study to examine vegetation characteristics between patches of intervened forest, abandoned agroforestry systems with coffee and actively managed agroforestry systems with coffee in a tropical landscape. In all habitats, plant structural characteristics, individual abundance, species richness and composition were recorded for the three plant size classes: adult trees, saplings and seedlings. Furthermore, bird species richness and composition, and seeds dispersed by birds were recorded. Tree abundance was higher in forest habitats while saplings and seedlings were more abundant in abandoned coffee sites. Although species richness of adult trees was similar in the three habitats, species richness of saplings and seedlings was much higher in forest and abandoned coffee than in managed coffee sites. However, in spite of their relatively low species richness, managed coffee sites are an important refuge for tree species common to the almost disappeared mature forest in the area. Floristic similarity for adult trees was relatively low between land use types, but clearly higher for seedlings, indicating homogenizing processes at the landscape level. More than half of the saplings and seedling were not represented by adults in the canopy layer, suggesting the importance of seed dispersal by birds between habitats. Our results show that each of the studied ecosystems plays a unique and complementary role as seed source and as habitat for tree recovery and tree diversity.
Complementary resource use is regarded as a mechanism that contributes to positive relationships between biodiversity and ecosystem functioning. Here, we used a biodiversity experiment composed of nine potentially dominant species (grasses: Alopecurus pratensis, Arrhenatherum elatius, Dactylis glomerata, Phleum pratense, Poa trivialis; legumes: Trifolium pratense, T. repens; non-legume herbs: Anthriscus sylvestris, Geranium pratense) to test for differences among monocultures and mixtures and for effects of species richness and the presence of particular species on the use of aboveground space. The number of rooting shoots determined in a line transect increased from monocultures to mixtures. Particularly, the presence of A. elatius in mixtures caused a higher shoot density at the community level. The number of pin contacts per sampling point (cumulative cover) at the community level, analysed with the point intercept method, was higher in mixtures than monocultures, and higher in mixtures with than without A. elatius. The effect was attributable to increased densities across the strata of the vertical stand profile as well as to an increase in community height. The impact of species richness on the use of aboveground space differed considerably between individual species. A. elatius achieved increased densities across all strata of the stand profile, while D. glomerata reached higher densities with a more pronounced use of space in the upper strata with increasing species richness of mixtures. Cumulative cover of P. pratense and A. pratensis was not affected by species richness, while the remaining species decreased space use mostly in the upper strata with increasing species richness or in mixtures with the competitively superior A. elatius. Our study shows that potentially dominant species are limited in their ability for adaptive responses to canopy shading. Nevertheless, the differential responses to species richness of individual species with regard to vertical niche occupation resulted in positive diversity effects on aboveground space use at the community level.
Scientists have long inferred the locations of past treelines from the distribution of deadwood above modern tree boundaries. Although it is recognized that deadwood above treeline may have decayed, the absence of such wood is routinely taken to imply the absence of trees for periods ranging from the past few millennia to the entire Holocene. Reconstructed treeline histories are then explained in terms of such variables as slope, drainage, temperature, solar insolation, and precipitation. While these variables certainly help determine where deadwood is to be found above treeline today, we suggest that they cannot always explain where it is not to be found. In the alpine environments of the western United States, archeological work has established a human presence during nearly the entire Holocene in portions of the Rocky Mountains and for over 5000 radiocarbon years in the Great Basin and Sierra Nevada. We suggest that prehistoric occupations may have stripped deadwood from the landscape in all of these areas. To the extent that this is true, reconstructions of past treelines from deadwood may reflect the human prehistory of an area as much as it reflects treeline history itself. We encourage evaluation of this hypothesis in areas of active dendrochronological and archeological research.
The physiologically active lateral rootlets of all main trees in temperate forests are colonised by ectomycorrhizal fungi, forming so-called ectomycorrhizas. These symbiotic organs are the sites of exchange of nutrients, mainly P and N, provided from the fungal partner, and C from the host. Emerging from the ectomycorrhizas, fungal hyphae exploit the soil for the mobilisation and absorption of water and nutrient elements. By doing so, they connect the tree roots intimately with the soil and provide anchorage. The deposition of acidifying pollutants into forest ecosystems is a potential threat to the health and vitality of forest trees because it leads to the acidification and eutrophication of forest soils. Pollutants are also a threat to the functioning of ectomycorrhizas. Increased N concentrations in the soil lead to enhanced fungal N uptake and storage, and to enhanced N transfer to the host plants, and therefore to higher plant biomass of above ground parts. In consequence, there is a decrease of C allocation to the plant roots. This in turn leads to reduced ectomycorrhization, and to reduced production of external mycelia and fruiting bodies. Soil acidification leads to enhanced availability of Al, heavy metals, and radionuclides in the soil, all of which can be toxic to plants and fungi. Reduced growth of roots and hyphae are amongst the first symptoms. In ectomycorrhizas, the hyphae of the fungal tissues contain vacuolar polyphosphates which have the ability to bind Al, heavy metals, radionuclides and N. These electronegative polymers of phosphates represent an effective storage and detoxifying mechanism which otherwise is lacking in roots. Therefore, ectomycorrhizas have the potential to increase the tolerance of trees to acidifying pollutants and to the increased availability in the soil of toxic elements.
Gentiana sect. Ciminalis consists of seven mostly ecologically or geographically vicariant and closely related species which are distributed throughout the South and Central European high mountains. The analysis of a RAPD data set and trn L-intron and ITS sequences resulted in slightly different phylogenetic hypotheses. In the preferred hypothesis the group consists of two completely resolved main lineages: 1) G. clusii and G. alpina. 2) G. dinarica, G. acaulis, G. ligustica, G. angustifolia and G. occidentalis. The most important conclusions we have drawn from this phylogenetic hypothesis and from the observed patterns of molecular variation are: 1) The calcifuge ecology of G. acaulis and G. alpina evolved independently from calcicole ancestors. 2) Among the calcicole taxa speciation proceeded from East to West in a simple vicariant pattern. 3) The application of a provisional molecular clock indicates that speciation events in sect. Ciminalis probably occurred in the Quaternary. 4) Differences in infraspecific genetic variation among the widespread species suggest that G. alpina probably experienced more recent dispersal or gene flow than G. clusii and G. acaulis. 5) The large number of mutations in the lineage leading to G. angustifolia, compared to the few mutations in the lineage leading to G. dinarica, may be a result of their different population histories. While the extant range of G. angustifolia was strongly affected by Quaternary climatic fluctuations, that of G. dinarica has had a more stable climatic history. 6) The low number of mutations and the basal position in one clade of the preferred cladogram leads to the conclusion that G. dinarica is the species most similar to the last common ancestor of sect. Ciminalis.
Phenotypic plasticity may play a key role in the adaptation of organisms to changing environmental conditions. A special case of plasticity is represented by heterophylly, the ability of semi-aquatic plants to produce different types of leaves below and above water. Submerged leaves are thin and lack both a cuticle and stomata, whereas aerial leaves are thicker, cutinized and bear stomata. The striking variability in the submerged, floating and aerial leaves of heterophyllous aquatics has historically been considered a paradigmatic example of adaptive phenotypic plasticity. An extensive body of developmental and physiological research reveals that heterophylly is quite often mediated by similar environmental cues across diverse taxa, which may imply a common underlying mechanism. Patterns of plasticity in response to environmental cues in the laboratory are consistent with the hypothesis of individual adaptation to heterogeneous environments, and the distribution of this trait among phylogenetically related aquatic angiosperms suggests either convergent or parallel evolution in their descent from terrestrial ancestors. Yet, critical evaluations of the ecological and evolutionary significance of this trait are scarce. In this essay, we discuss the patterns of plasticity revealed by experimental manipulative studies of heterophylly in the context of the general problem of adaptive phenotypic plasticity, and suggest avenues for future research that are needed in assessing the ecological and evolutionary significance of this trait.
The presence of diverse and species-rich plant lineages on oceanic islands is most often associated with adaptive radiation. Here we discuss the possible adaptive significance of some of the most prominent traits in island plants, including woodiness, monocarpy and sexual dimorphisms. Indirect evidence that such traits have been acquired through convergent evolution on islands comes from molecular phylogenies; however, direct evidence of their selective value rarely is obtained. The importance of hybridization in the evolution of island plants is also considered as part of a more general discussion of the mechanisms governing radiations on islands. Most examples are from the Hawaiian and Canarian floras, and in particular from studies on the morphological, ecological and molecular diversification of the genus Aeonium, the largest plant radiation of the Canarian Islands.
This paper introduces the integration of additive partitioning with species—area relationships to island biogeography in order to address the question “How are the pteridophyte and spermatophyte native and endemic flora of different oceanic archipelagos partitioned across islands?”.Species richness data of all endemic species and all native species of pteridophytes and spermatophytes were obtained for the Azores, Canaries and Cape Verde in the Atlantic Ocean and Galápagos, Hawaii and Marquesas in the Pacific Ocean. Additive partitioning of species diversity was used to quantify how much of the total diversity of an oceanic archipelago flora (γ-diversity) is due to (i) the mean species richness of the flora of each island (α-diversity), (ii) the variability in species richness of the floras across islands (βNestedness) and (iii) the complementarity in species composition of the floras of different islands (βReplacement). The analysis was separately performed for the native and endemic pteridophyte and spermatophyte floras.The diversity partitioning of the six archipelagos showed large differences in how the flora of each archipelago is partitioned among the α, βNestedness and βReplacement components, for pteridophytes and spermatophytes and for all endemic species and all native species. The α-diversity was more important for all native species than for endemic species and more important for pteridophytes than for spermatophytes, with the Azores showing outstanding high values of α-diversity. The βNestedness was higher for pteridophytes than for spermatophytes and higher for endemic species than for all native species in both pteridophytes and spermatophytes. The values of βReplacement suggested that: (i) the spermatophyte native flora is more differentiated across islands than the pteridophyte native flora and (ii) the pteridophyte endemic flora and, especially, the spermatophyte endemic flora are more differentiated across islands than the corresponding native flora. An outstanding value of βReplacement for endemic and all native spermatophytes was found in Hawaii, confirming the biogeographical island differentiation in this archipelago.
The production of artificial snow and the use of snow additives in ski resorts have increased considerably during the last 20 years. Their ecological consequences are the subject of environmental concerns. This review compiles studies about the ecological implications of ski pistes preparation in general and of artificial snow production. The main direct impacts of ski piste preparation on the vegetation are related to the compaction of the snow cover, namely the induction of soil frost, the formation of ice layers, mechanical damage and a delay in plant development. The vegetation reacts with changes in species composition and a decrease in biodiversity. Artificial snowing modifies some of these impacts: The soil frost is mitigated due to an increased insulation of the snowpack, whereas the formation of ice layers is not considerably changed. The mechanical impacts of snow-grooming vehicles are mitigated due to the deeper snow cover. The delay of the vegetation development is enhanced by a considerably postponed snowmelt. Furthermore, artificial snowing induces new impacts to the alpine environment. Snowing increases the input of water and ions to ski pistes, which can have a fertilising effect and hence change the plant species composition. Increasingly, snow additives, made of potentially phytopathogenic bacteria, are used for snow production. They enhance ice crystal formation due to their ice nucleation activity. Although sterilised, additives affected the growth of some alpine plant species in laboratory experiments. Salts are applied not only but preferably on snowed pistes to improve the snow quality for ski races. The environmental impacts of most salts have not yet been investigated, but a commonly used nitrate salt has intense fertilising properties. Although snowing mitigates some of the negative impacts of ski piste preparation in general, new impacts induced by snowing could be non-beneficial to the vegetation, which, however, has yet to be clarified.
Biogenic habitat creation refers to the ability of some organisms to create, maintain or destroy habitats. These habitat changes affect species diversity of natural communities, but it remains to be elucidated if this process also affects the link between ecosystem functions and species diversity. Based on the widely accepted positive relationships between ecosystem functions and species diversity, we hypothesize that these relationships should be different in biogenically created habitat patches as compared to unmodified habitat patches. We tested this hypothesis by assessing the effects of a high-Andean cushion plant, Azorella madreporica, which creates habitat patches with different environmental conditions than in the surrounding open areas with reduced vegetation cover. We used observational and experimental approaches to compare the plant biomass–species richness relationships between habitat patches created by A. madreporica cushions and the surrounding habitat without cushion plants. The observational assessment of these relationships was conducted by counting and collecting plant species within and outside cushion patches. In the experiment, species richness was manipulated within and outside cushion patches. The cushion plant itself was not included in these approaches because we were interested in measuring its effects. Results of both approaches indicated that, for a given level of species richness, plant biomass within cushions was higher than in the surrounding open areas. Furthermore, both approaches indicated that the shape of plant biomass–species richness curves differed between these habitat types. These findings suggest that habitat modifications performed by A. madreporica cushions would be positively affecting the relationships between ecosystem functions and species diversity.
Nucleotide sequence data from the chloroplast rbcL gene and the trnL-trnF intergenic spacer of 58 species in 38 genera were used to infer the phylogenetic affinities of Monimiaceae to other Laurales, and to assess whether the family in the traditional wide sense is monophyletic. Besides Monimiaceae, the Laurales comprise Calycanthaceae, Gomortegaceae, Hernandiaceae, and Lauraceae. Magnoliaceae and Myristicaceae were used as outgroups. Based on recent molecular data, Amborellaceae and Chloranthaceae, which have sometimes been included in the order, do not belong in the Laurales, and indeed trnL-trnF sequences of Amborella (Amborellaceae) and Hedyosmum (Chloranthaceae) were too different to be unambiguously aligned with the remaining sequences. Parsimony analyses of the trnL-trnF and trnL-trnF-rbcL data groups the genera into five major clades, Calycanthaceae, Atherospermataceae, Gomortega, Siparunaceae, and a weakly supported Monimiaceae s.str.-Lauraceae-Hernandiaceae clade. RbcL data alone provide no resolution at the family level. Many aspects of traditional intra-familiar classification of Monimiaceae are supported except that the sole perfect-flowered member of the family, the monotypic Sri Lankan Hortonia, is not basal (13 of 15–22 genera sampled). Instead, there are two major clades in Monimiaceae. One comprises the functionally dioecious monospecific Peumus from Chile plus the morphologically and functionally dioecious small genera Monimia from the Mascarenes and Palmeria from eastern Australia and New Guinea. The other consists of Hortonia and all remaining genera. The atherospermatoids are supported in their traditional circumscription (14 species, 7 genera, of which 10 and 6 were sampled). The neotropical genus Siparuna, different from recent classifications that have stressed its isolation, is genetically and morphologically very close to the West African species Glossocalyx longicuspis. Both taxa have unisexual flowers of the same general morphology, and both have unitegmic ovules. From the current data it seems that monoecy is basal in Siparuna, but more complete sampling of species with a faster evolving genetic marker is needed for a fuller understanding of the evolution of monoecy and dioecy in this genus.
Grazing, fire and selective tree cutting are major disturbances that shape species diversity in savanna ecosystems, yet their effects are highly variable. We carried out a factorial experiment with two levels to examine the effects of grazing, fire and selective tree cutting on herbaceous species richness, abundance and diversity on two sites in the Sudanian savanna-woodlands of Burkina Faso for 10 years (1994–2003). The results showed significant inter-annual variation in species richness, abundance and diversity at both sites (p<0.001), while main or combined effects of fire, grazing and selective cutting were very limited and varied between life forms and sites. Grazing tended to favour the diversity of perennial grasses; fire tended to influence the richness of annual grasses and abundance and diversity of perennial grasses while selective tree cutting had no effect on any of the vegetation attributes assessed. The combined effect of grazing, fire and selective cutting tended to increase the diversity of forbs. In many cases, the responses of herbaceous species to treatments were clearer on the site with deeper soils than the one with shallow soils. Depending on the site and treatments, the inter-annual variation in vegetation attributes was partly related to amount and/or frequency of rainfall and partly to inter-annual variation in grazing or fire intensity. It can be concluded that both disturbances and climatic condition influence the structure and diversity of herbaceous flora in the Sudanian savanna-woodland ecosystem. The responses were site-specific, which accentuates the importance of landscape-scale approaches to understand the impacts of disturbances on composition, structure and diversity of savanna ecosystems.
Ailanthus altissima (tree of heaven), Simaroubaceac, is an early successional tree, native to China and North Vietnam, which has become invasive in Europe and on all other continents except Antarctica. It is most abundant in urban habitats and along transportation corridors, but can also invade natural habitats. This paper reviews the literature on the morphology, distribution, ecology, habitat requirements, population biology, genetics, physiology, impacts, management and uses of this species. (c) 2007 Rubel Foundation, ETH Zilrich. Published by Elsevier GmbH. All rights reserved.
Invasive plant species are widely recognised to have severe ecological impacts in a wide range of ecosystems throughout the world, yet there are few experimental studies measuring community-level effects of invasive plant species. Thus most evidence is from correlative studies, and as such often cannot easily disentangle cause and effect. Through a combination of an addition and removal experiment and a correlative approach (multi-site comparisons), this study aimed to quantify the effects of a widespread invasive species, Mimulus guttatus, on species richness and soil properties of riparian plant communities.
The genus Oenothera includes a number of species alien to central Europe, which differ in their invasion success. The present study was designed to investigate how fecundity, growth rate of seedlings and competitive ability contribute to the invasion potential within this genus. The relative growth rate and response to interspecific competition from neighbouring vegetation were determined for 15 species. Relations between these characteristics and other species traits identified in previous studies (germination, seed production and seed mass) were examined and used to explain the invasion success of particular species in Europe. Ability to germinate in the light was the only significant predictor of invasion success in six European countries. Fecundity is another trait contributing to invasion success. The most successful invaders within the genus are poor at competing with native vegetation, and an ability to escape from competition seems crucial for invasion success. Oenothera biennis, the most successful invader, is not outstanding in any of the characteristics assessed but is well placed in most of them. We suggest that a ‘Jack-of-all-trades’ strategy may be beneficial for an invader. The parental chromosomes of the species studied had no effect on the distribution of traits; hence the conclusions drawn at the species level are not biased by the specific banding pattern within the genus (permanent translocation heterozygosity). The present study indicates that comparative studies of closely related species may indicate the determinants of invasion success if they include many characteristics of the complete life cycle.
Baldellia ranunculoides (L.) Parl. (Alismataceae) is a taxonomically problematic aquatic plant with an historically ill-defined distribution and global conservation status. This paper finds morphological, ecological and molecular evidence for two distinct taxa, probably best described as subspecies: (1) B. ranunculoides subsp. ranunculoides and (2) B. ranunculoides subsp. repens and provides detailed distribution data on their overlapping range, in different habitats, across the cool, high rainfall areas of western Europe and west Mediterranean. The two subspecies are amongst the relatively large number of threatened European and north Africa aquatic plants and this paper provides a systematic review of their relative conservation pressures and management needs, with particular emphasis on the status of both taxa in central Europe. Other observations indicate probable evolutionary relationships within B. ranunculoides s.l. and its associated taxa and the review points out where these and other research topics could potentially be pursued.
While allelopathy has been defined as plant-plant chemical interference, there has been much confusion about what the concept encompasses and how important it is in nature. We distinguish between (1) direct plant-plant interference mediated by allelochemicals, and (2) the effects of secondary compounds released by plants on abiotic and biotic soil processes that affect other plants.It very difficult to demonstrate direct effects of chemicals released by a plant on nearby plants. Although soil ecology-mediated effects of secondary plant compounds do not fit the classical concept of allelopathy, we find support in the literature for the hypothesis that the most important effects of compounds released into the soil environment by plants on other plants occur through such indirect effects. The emphasis on, and skepticism of, direct plant-plant allelopathic interference has led some researchers to demand unreasonably high standards of evidence for establishing even the existence of allelopathic interactions, standards that are not demanded for other plant-plant interactions such as resource competition. While the complete elucidation of the mechanisms by which allelochemicals function in the field is many years away, such elucidation is not necessary to establish the existence of allelopathic interactions.We propose that most of the phenomena broadly referred to as allelopathic interference are better conceptualized and investigated in terms of soil chemical ecology. Even when direct plant-plant allelochemical interference occur, the levels of allelochemicals in the environment and their effects on plants are heavily influenced by abiotic and biotic components of the soil ecosystem. Putting allelopathy in the context of soil ecology can further research and reduce some of the less fruitful controversy surrounding the phenomenon.
This study explores the basis for conservation action on Myracrodruon urundeuva Fr. Allemão (Anacardiaceae). This is a wide-ranging forest species occurring in Brazil and other South American countries, notably in the Cerrado region. This paper aims to provide a structured review of available knowledge of its biology, ecology, silviculture and management. Widely-scattered published reports have been critically considered and efforts made to highlight and resolve contradictions and inconsistencies. Information about this species is scanty and scattered, particularly in respect to its biology and ecology. Some effort has been applied to its domestication and improvement, but opinion on its silviculture and management is still controversial. The taxon is typical of open and deciduous forest in the Cerrado. Gaps in current knowledge relevant for conservation are identified and steps to fill them proposed. Where in situ conservation proved to be more appropriate, recommendations are made for the location of additional protected areas. Complementary ex situ and enrichment conservation actions are suggested for specific parts of the range where resource losses are already so extensive that in situ measures alone are inadvisable. Opportunities for refining the limited management and conservation knowledge are proposed by highlighting priorities for study of the taxon. Finally, future action is discussed in the context of the infrastructure of the national conservation sector.
We present a comprehensive analysis of factors affecting resource allocation and crown formation in a subarctic birch tree, Betula pubescens ssp. czerepanovii (Orlova) Hämet-Ahti. Using biomass measurements and digitized data on tree architecture, we investigated several hypotheses on various factors that may modify plant growth. We also analyzed the extent to which different mechanisms operate at different scales, ranging from individual shoots to the whole branches or trees. Different factors affected allocation at different levels of organization. Stem age had a minor effect, suggesting that similar control mechanisms operate at all stages of development. Fates of individual shoots were affected by their local growing conditions as indicated, for example, by the dependence of long shoot production on light. Buds formed in the current long shoots were likely to become new long shoots. In the innermost crown parts, radial growth had priority compared to long shoot production. Elongation of individual long shoots was controlled by two conflicting factors. Long distance from the roots suppressed growth, probably indicating costs associated with resource transportation, whereas a high level of light augmented growth. In contrast, growth of entire branches was not so clearly related to the availability of resources, but showed limitation due to allometric scaling. This set a relationship between the maximum long shoot number and the overall branch size, and may indicate allometric constraints to the way a tree is constructed. Strict allometric relationships existed also between other structural traits of mountain birch, most of them similar at all levels of branching hierarchy. However, despite the upper level restrictions set by allometry, source-sink interactions and localized responses of individual shoots operated as local processes that directed allocation towards the most favourable positions. This may be a mechanism for achieving efficient tree architecture in terms of resource intake and costs of transportation.
Allocation to reproduction is often examined at the whole plant level from an allometric perspective; however, investment in reproduction is a hierarchical process which represents the integration of adjustments at different hierarchical levels. This essay review presents some key issues in reproductive allocation by examining problems of allocation at different hierarchical levels within plant. At the whole plant level, the application of ‘Rensch's rule’ to plants is discussed as a mechanism that might explain size differences in relation to gender in dioecious species. At branch level, the existence of vegetative branches may be explained by hypotheses on branch specialization and costs of reproduction. However, vegetative branches might also constitute an ‘assurance’ for reproduction in variable environments. At a lower hierarchical level, fruit abortion is a process widely observed in plants that impose limits to our capacity to estimate and develop general laws about reproductive allocation in plants. The question of whether fruit abortion can be used by the parent to produce a more homogeneous progeny that limits asymmetric competition among sibs is discussed. Finally, it seems that in some cases the allocation within seeds is in accordance with some predictions of kin selection theory. Most of these topics contain open questions that deserve further research.
Since form and function are tightly integrated in plants, and since plant attributes often scale allometrically, it follows that plant allometry is inherently multivariate. Unfortunately, traditional statistical methods for studying allometric relationships are very restrictive and do not allow one to model multivariate allometric patterns that follow realistic biological hypotheses. In this paper I describe a new statistical test (‘d-sep test’) that allows one to test, and potentially falsify, alternative multivariate orderings of cause-and-effect in the context of allometry.
Seagrasses are a group of 12 genera of monocotyledonous plants in four families that have successfully colonised shallow coastal seas, probably since the Cretaceous. Variations in light availability and water movement are prime environmental factors for the growth of these marine angiosperms. An overall similarity in growth form and modular clonal architecture allows the generalisation that small species have short-lived shoots with rapidly elongating rhizome axes, whilst the larger species have longer-lived shoots that do not expand rapidly with rhizomes. Annual rhizome elongation rates range between 2 cm and 4 m among species. This range in expansion capacity is correlated with rhizome diameter in an allometric fashion (y=191x−1.5, r2=0.58, p<0.05). Rhizomes with a wider diameter also allow the storage of larger quantities of reserve carbohydrates to be mobilized during the adverse winter season at higher latitudes or for flowering. Repeated branching and the basal positioning of the meristems allow the formation and maintenance of seagrass meadows, and these are a prominent feature creating spatial heterogeneity on the sea floor down to a mean colonisation depth of 15.1±1.3 m (median 8 m, range 0.7–90 m, n=150). Spatial complexity is highest in multi-species seagrass beds, such as those of the Indo-Pacific region and Australia. Seagrass beds function as important coastal filters for nutrients and pollutants and display high carbon sequestration rates. Due to the recalcitrant nature of seagrass detritus, it forms a disproportionally high contribution (12%, but only 1% of productivity) to the carbon stored in ocean sediments. The services provided by these ecosystems to human society range from water quality improvement via nursery and feeding grounds for economically important fish to storm buffering and recreative amenity.
Extractions/applications of plant-resources were investigated in Mestizo communities in the Jenaro Herrera district, Peruvian Amazon. Knowledge differences between genders on forest resource uses were also examined. Two methods were applied: (1) a forest survey based on structured interviews of ten informants on potential uses of 334 pre-selected trees and lianas in permanent sample plots, and interviews of 20 male and female informants concerning potential uses of 85 trees and lianas; and (2) a household survey recording extracted materials in or near the dwellings of 73 families. The conclusions are: (i) most forest products are extracted from a limited number of species, whereas a large number of species is mentioned as important/potentially useful for identical purposes; (ii) coherence exists between extracted species and their abundance in easily accessible forest formations; (iii) knowledge of forest resources is gender correlated; and (iv) a combination of methods for collection of information on use and extraction of plant resources provides more detailed and profound results than using each method separately.
From the poles to the tropics flooding is a powerful discriminator in plant distribution. Although plants can be divided globally as to whether or not they are tolerant of high water tables, it does not follow that all flood-tolerant species achieve their ability to survive flooding by similar adaptations. Flooding implies a periodic but temporary rise of the water table, hence plants that live in such areas have an amphibious life style. Amphibious plants have to adjust, not only to inundation and the dangers of oxygen deprivation, but also to the eventual lowering of water tables and often sudden re-exposure to a fully aerated environment and the lack of the physical support that is provided by flooding. In this respect they are distinct from aquatic species that live constantly in water. It is often tacitly assumed that for amphibious species flooding is the stressed condition and non-flooding the norm. This pre-judgement is not appropriate, particularly as in many habitats the flooded condition predominates for a longer part of the year than the unflooded. For amphibians, re-adapting from the aquatic to the terrestrial habitat requires specialised adaptations, just as much as a change from unflooded to flooded. Many flood-tolerant species, including surface-rooting grasses and sedges, may not be tolerant of anoxia, and instead prevent the accumulation of an oxygen debt in submerged organs by aeration mechanisms, including oxygen diffusion through aerenchyma, thermally induced mass movement of air, and the elongation of submerged shoots. In other species, and particularly in perennial plants with buried perennating organs, flooding can impose prolonged periods of anaerobiosis (anoxia). Being able to survive such oxygen deprivation requires (1) energy reserves sufficient for cell maintenance, (2) the prevention of cytoplasmic acidosis under anoxia, and (3) the anaerobic mobilisation of starch reserves. Re-entry to the aerobic habitat is facilitated by (4) the dispersal and excretion of products that transfer hydrogen from anoxic or hypoxic tissues, either to the external environment, or to parts of the plant with access to oxygen, before the anaerobic tissues return to air, and (5) anti-oxidative activity to minimise post-anoxic injury.
Inundations of lakeshores are classical examples of how disturbance can influence community diversity and composition. As the occurrence and intensity of flooding are predicted to change dramatically as a result of climate change, predicting the consequences of such changes has become a major task for community ecology. Here we present abundance data of five species that comprise a species-poor community of high conservation value at lakeshores of Lake Constance over 17 years, during which one of the longest flood periods and the lowest water levels since 1890 occurred. We used simple regression models and increasingly sophisticated Markov chain models plus non-linear parameter estimation to put down abundance changes to direct effects of flooding on population-dynamic parameters and to indirect effects of flooding through modification of interspecific competition. We found a negative effect of flood duration on abundance changes for the non-specialist species Agrostis stolonifera and Phalaris arundinacea, but no effect on Carex acuta. The specialist species, Ranunculus reptans but not Littorella uniflora showed a positive effect of flooding. Data analysis revealed an unambiguous competitive hierarchy with the two graminoid species (C. acuta, P. arundinacea) being superior, and the habitat specialists being most sensitive to interspecific competition. We used estimated parameters to project the community dynamics under different flooding regimes. Long-term projection showed that the original community is threatened by two non-specialist species (C. acuta and P. arundinacea). Even if this forecast was influenced by various model limitations, it may indicate irreversible changes in soil fertility during the phase of high eutrophication between 1950 and 1980. Our study demonstrated that long-term abundance relevés combined with Markov modelling and predictive simulations are an important counterpart to detailed short-term studies. The combination of empirical and theoretical methods elucidates the interaction of biotic and abiotic factors in community change.
The ease with which arbitrarily amplified dominant (AAD) markers such as RAPD, AFLP and ISSR could be used to generate vast quantities of data led to their rapid application addressing a diverse range of biological questions. The most significant uses of AAD markers remain in genetic mapping, diagnostic fingerprinting and in the study of genetic structure within and between populations of individual species. AAD markers may also be utilised for phylogenetics and systematics, but have commonly been used inappropriately in such analyses due primarily to poor consideration of potential homoplasy. This review discusses the properties of AAD markers (including potential artefacts) that relate to their interpretation as characters for phylogenetic and systematic analysis. Although useful markers are those for which heritability and homology have been demonstrated, this is rarely done because it is potentially time consuming and expensive. A consequence of this is that homology, appropriate taxonomic level, and the resolving capacities of commonly employed hierarchical analyses are often not addressed. A pragmatic approach suggests that tangible monomorphism (e.g. 20%) across all taxa studied should be a minimum requirement before AAD markers could be considered for phylogenetic analyses. This would help to prevent comparison of taxa that are too divergent. Rather than being transferable across widely diverged taxa, AAD polymorphisms can provide useful discriminatory information for phylogenetic and systematic studies of closely related species and non-reticulating, subspecific lineages. Below these levels of taxon divergence, population genetic effects may swamp hierarchical signal in the data, while at greater levels of divergence, homoplasy is likely to be significant.
The frequency and dynamics of sexual and asexual reproduction were investigated in a dioecious epixylic hepatic, Anastrophyllum hellerianum, which has declined in recent decades in Finland as a consequence of forestry practices. In our investigation asexual reproduction by gemmae was the dominant mode of reproduction and specialised gemmiparous shoots were present in all colonies studied. The proportions of dead shoots were considerably higher among sex-expressing than among non-sexexpressing shoots. Our results suggest that lower reproductive investment is required for asexual than for sexual reproduction. For instance, no trade-off is detected between asexual reproduction and survival of the gemmiparous shoots in A. hellerianum. Sexual reproduction occurred only in 12% of the colonies and it was promoted by the following factors: medium shoot density, high proportion of sex-expressing shoots, an even sex ratio and very short distances between individuals representing opposite sexes. The ratio of dead males to dead females was significantly female-biased, which suggests higher mortality among female shoots. At the level of individual shoots, more spores than gemmae were produced. However, as a consequence of the low frequency of sporophyte-bearing shoots, gemma production highly exceeded spore production at the colony level. Furthermore, cultivation tests of the propagules showed that gemmae germinate faster than spores.
Positive interactions among native plant species are common in alpine habitats, particularly those where one species (nurse plant) generates microclimatic conditions that are more benign than the surrounding environment, facilitating the establishment of other species. Nonetheless, these microclimatic conditions could facilitate the establishment of non-native species as well. A conspicuous component of the alien alpine flora of the central Chilean Andes is the perennial herb Taraxacum officinale agg. (dandelion). In contrast to other alien species that are restricted to human-disturbed sites, T. officinale is frequently observed growing within native plant communities dominated by cushion plants. In this study we evaluated if T. officinale is positively associated with the cushion plant Azorella monantha. Via seedling survival experiments and gas-exchange measurements we also assessed the patterns of facilitation between cushions and dandelions, and explore the potential mechanisms of invasion by dandelions. T. officinale grows spatially positively associated with cushions of A. monantha. Survival of seedlings, as well as their net-photosynthetic rates and stomatal conductance, were higher within cushions than in open areas away from them, suggesting that the microclimatic modifications generated by this native cushion facilitates the establishment and performance of a non-native invasive species. Our results, as well as other recent studies, highlight the role of native communities in facilitating rather than constraining non-native plant invasions, particularly in stressful habitats such as alpine environments.
The dispersal of megathermal angiosperms between tectonic plates is reviewed on the basis of fossil evidence for the Cretaceous and Tertiary periods, since the radiation of the angiosperms, and the period of break-up of Gondwana. The combination of tectonic plate disassembly and redistribution, coupled with phases of global warming followed by pronounced cooling, has resulted in the formation of intermittent dispersal opportunities for frost-intolerant plants, and has been a major factor in determining the direction of angiosperm diversification. The Early Cretaceous radiation of angiosperms seems to show little relationship to the formation of Tethys. However, for the Late Cretaceous and Tertiary nine relevant dispersal routes can be differentiated that can be divided into two distinct categories: routes which formed following the break-up of Gondwana during the Late Cretaceous and Earlier Tertiary, when warm climates encouraged dispersal of megathermal elements globally, and routes which formed since the Middle Eocene, following phases of plate collision, as global climates were cooling down, inhibiting such dispersal. Most inter-plate dispersal of megathermal angiosperms took place in the Late Cretaceous and Early Tertiary at a time when global climates were markedly different from those of today, and the global area of megathermal vegetation several times greater than at present. Under such a scenario, it is likely than opportunities for speciation were much higher than for present-day megathermal plants.
Interactions with animals are potentially important in shaping the life histories of plants. However, we still largely lack the quantitative information that is necessary to compare plant-animal interactions and plant life histories. This is because most studies have focused on only a single interaction and a single component of fitness. This review suggests a protocol for how plant-animal interactions, as well as other, interactions can be quantitatively investigated.I review results from the forest herb Lathyrus vernus to examine how plant performance is influenced by mollusc damage of shoot buds, vertebrate grazing, leaf damage, pre-dispersal seed predation, flower herbivory and pollinator visitation. More specifically I ask: (1) What are the effects of these interactions on components of plant fitness? (2) How do such effects translate into total fitness? (3) Do trade-offs among components of plant fitness influence the net effect of animals? (4) Are antagonistic and mutualistic interactions influenced by the same plant traits?The results illustrate why it is often necessary to monitor plants over several seasons and over all life cycle phases to assess the total effects of environmental factors. In L. vernus many components of fitness were affected by multiple interactions with animals. A large proportion of these effects was carried over to subsequent seasons and could only be detected after one or two seasons. Assessing the effects on overall performance in perennial plants is particularly difficult because fitness components have very different influence on total fitness. The matrix model approach used to estimate the effect of herbivores, seed predators and pollinators on L. vernus provides one possible way to make such assessments for organisms with complex life cycles. The results show that the cryptic damage of emerging shoots has the largest negative effects on total fitness. Correlations between different components are an essential feature of many life histories and must be incorporated into demographic models if we want to assess the net effect of environmental factors. In L. vernus costs of reproduction were present and demographic modelling showed that the positive effects of an increased seed production after additional hand-pollination on total fitness was completely offset by poorer performance in the following year. Lastly, this study provides an example of how optimal trait values can be assessed when traits influence several fitness components and several interactions. In L. vernus a higher number of flowers was correlated to both an increased fruit set and an increased risk of grazing damage and these effects were, in turn, carried over to the next season. The results clearly illustrate the limitations of inferences about phenotypic selection pressures that are based on single fitness components or single interactions.Taken together the results for L. vernus show that the selective pressure constituted by a given amount of animal interference is highly dependent on the context, in terms of both plant life history and other interactions. A life cycle approach like the one outlined in this study may avoid several problems and provide the basis for assessments of the total effects of animals on plants. Such an approach must be based on in-depth studies of plants.
Man-made habitats in central Europe can be broadly divided into arable land with weed vegetation, and settlements and their surroundings, harbouring ruderal vegetation. The former is a predictable environment with frequent, regular and large-scale disturbances, while the latter is an unpredictable environment with irregular disturbances of varying spatial extent producing heterogeneous mosaics of different successional stages. We hypothesize that these differences in disturbance regimes select for different sets of biological and ecological plant traits in these two habitats. A data set of 2715 vegetation plots sampled in man-made habitats dominated by annual plants in the Czech Republic was combined with data on biological and ecological traits of vascular plants, mostly taken from the BiolFlor database. Differences due to temporal variation and location of plots in different climatic zones were partialled out using partial canonical correspondence analysis. Then the differences in traits of the plants growing on arable fields and in settlements were analysed using logistic and least-square regression models, both with and without phylogenetic correction. Plants growing on arable land were more often annuals, R-strategists, with overwintering green leaves, insect or self-pollinated, reproducing by seeds, with persistent seed banks and archaeophytes (i.e. those aliens that arrived prior to 1500). Plants growing in human settlements were more often biennials or perennials, C-strategists, wind-pollinated, flowering in mid summer, reproducing both by seeds and vegetatively, dispersed by wind or humans, neophytes (i.e. those aliens that arrived after 1500), species with high demands for light and nutrients and with more continental distribution ranges. Most associations between plant traits and habitats did not change after taking phylogenetic relationships into account. Traits strongly linked to phylogeny were especially modes of pollination and dispersal. By contrast, traits weakly linked to phylogeny included life strategy and alien status.
We consider the role that key structural traits, such as spinescence, pubescence, sclerophylly and raphides, play in protecting plants from herbivore attack. Despite the likelihood that many of these morphological characteristics may have evolved as responses to other environmental stimuli, we show that each provides an important defence against herbivore attack in both terrestrial and aquatic ecosystems. We conclude that leaf-mass–area is a robust index of sclerophylly as a surrogate for more rigorous mechanical properties used in herbivory studies. We also examine herbivore counter-adaptations to plant structural defence and illustrate how herbivore attack can induce the deployment of intensified defensive measures. Although there have been few studies detailing how plant defences vary with age, we show that allocation to structural defences is related to plant ontogeny. Age-related changes in the deployment of structural defences plus a paucity of appropriate studies are two reasons why relationships with other plant fitness characteristics may be obscured, although we describe studies where trade-offs between structural defence and plant growth, reproduction, and chemical defences have been demonstrated. We also show how resource availability influences the expression of structural defences and demonstrate how poorly our understanding of plant structural defence fits into contemporary plant defence theory. Finally, we suggest how a better understanding of plant structural defence, particularly within the context of plant defence syndromes, would not only improve our understanding of plant defence theory, but enable us to predict how plant morphological responses to climate change might influence interactions at the individual (plant growth trade-offs), species (competition), and ecosystem (pollination and herbivory) levels.
Seed dispersal is a fundamental life history trait in plants. Although the recent surge of interest in seed dispersal by ants (myrmecochory) has added greatly to knowledge on the ecology of seed dispersal and ant–plant mutualisms, myrmecochory also represents a unique opportunity to examine the links between seed dispersal and evolution in flowering plants. Here we review the taxonomic, phylogenetic and biogeographic distribution of myrmecochory in flowering plants. Myrmecochory is mediated by elaiosomes, i.e., lipid-rich seed appendages that attract ants and serve as rewards for dispersal. We surveyed the literature for evidence of elaiosomes in angiosperm plants to estimate the global prevalence of myrmecochory. We then searched the literature for phylogenetic reconstructions to identify myrmecochorous lineages and to estimate the minimum number of independent evolutionary origins of myrmecochory. We found that myrmecochory is present in at least 11 000 species or 4.5% of all species, in 334 genera or 2.5% of all genera and in 77 families or 17% of all families of angiosperm plants. We identified at least 101, but possibly up to 147, independent origins of myrmecochory. We estimated three or more origins in 13 families and found that at least half the genera are myrmecochorous in 10 families. Most myrmecochorous lineages were Australian, South African or northern temperate (Holarctic). A mapping of families containing myrmecochorous genera on a dated angiosperm supertree showed that myrmecochory has evolved in most of the major angiosperm lineages and that it is more frequent in younger families (crown group age <80 million years) than in older ones. We suggest that the relatively low physiological and energetic costs of producing an elaiosome and the consistent selective benefits of myrmecochory (dispersal, protection from seed predators and fire, safe and nutrient-rich microsites) explain the numerous evolutionary and developmental origins of myrmecochory in angiosperm plants, and we propose that elaiosomes thus provide one of the most dramatic examples of convergent evolution in biology.
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.
The extant embryo-bearing plants (mosses, ferns and seed plants) have an aquatic ancestor or ancestors. Today they dominate the terrestrial vegetation of the world. Embryo-bearing plants which now live in water have, for a second or even third time, re-invaded water. Aquatic species are found in 440 genera from 103 families of embryo-bearing plants. This survey attempts to find out how often the Event of evolving from land back to water has taken place among the living plants of today. Analysing the morphological and phylogenetic affinities of all aquatics at taxonomic levels from divisio to varieties of species indicate the Event has taken place, at least 222 times but it could have happened 271 or even more times (bryophytes 10–19 times, ferns seven times, seed plants 205–245 times). Aquatic plants have evolved from often very different genetic and ecological backgrounds. Also, they have evolved at different times; some old ones are aquatic at the level of order or family, while others, more recent, are isolated species in otherwise terrestrial genera or races within species. It is, nevertheless, wonderful that such a large variety of plants have evolved solutions to the single problem of remastering life in water.
Plants in the Arctic and subarctic face the problems posed by herbivory in addition to short growth seasons, low temperatures and low nutrient availability. Herbivores control plant performance by removing biomass, by altering resource availability, by altering the physical environment, and by changing the balance of competition. The main difference between effects of herbivores in the Arctic and at lower latitudes may be the relatively greater importance of changes in resource availability and the physical environment resulting from herbivore activity, and their consequences for plant competitive abilities.
The coexistence of woody and grassy plants in savannas has often been attributed to a rooting-niche separation (two-layer hypothesis). Water was assumed to be the limiting resource for both growth forms and grasses were assumed to extract water from the upper soil layer and trees and bushes from the lower layers. Woody plant encroachment (i.e. an increase in density of woody plants often unpalatable to domestic livestock) is a serious problem in many savannas and is believed to be the result of overgrazing in ‘two-layer systems’. Recent research has questioned the universality of both the two-layer hypothesis and the hypothesis that overgrazing is the cause of woody plant encroachment.We present an alternative hypothesis explaining both tree–grass coexistence and woody plant encroachment in arid savannas. We propose that woody plant encroachment is part of a cyclical succession between open savanna and woody dominance and is driven by two factors: rainfall that is highly variable in space and time, and inter-tree competition. In this case, savanna landscapes are composed of many patches (a few hectares in size) in different states of transition between grassy and woody dominance, i.e. we hypothesize that arid savannas are patch-dynamic systems. We summarize patterns of tree distribution observed in an arid savanna in Namibia and show that these patterns are in agreement with the patch-dynamic savanna hypothesis. We discuss the applicability of this hypothesis to fire-dominated savannas, in which rainfall variability is low and fire drives spatial heterogeneity.We conclude that field studies are more likely to contribute to a general understanding of tree–grass coexistence and woody plant encroachment if they consider both primary (rain and nutrients) and secondary (fire and grazing) determinants of patch properties across different savannas.
While sexual regeneration of plants after disturbance is relatively well understood, vegetative regeneration has attracted some attention only recently. Its role along environmental gradients and across biomes is poorly known and standard methods for assessment are not yet established. We review current knowledge about the role of bud banks in vegetative regeneration and the diversity of their modes of functioning. The similarities and differences between bud banks and seed banks are illustrated, focusing on dormancy, dispersability, seasonal dynamics, longevity and storage of carbohydrates. We try to formulate some principles that unify bud bank functioning across habitats and growth forms: (1) the bud banks consist of all buds which may be used for vegetative regeneration, including those formed adventitiously only after injury; (2) vertical distribution of buds reflects avoidance of disturbance; (3) seasonal changes in the bud bank make vegetative regeneration sensitive to timing of disturbance; and (4) ability to form adventitious buds provides a potential for vegetative regeneration from roots, stumps and leaves. Based on these principles, a simple classification of bud banks is presented similar to the classification of seed banks. Bud bank traits are considered in relation to severity, timing and frequency of disturbance. These include vertical distribution and seasonal fluctuations in the number of buds. Methods for quantitative assessment of bud numbers and resprouting capacity are reviewed, and a new approach based on indirect bud counts is proposed. The suggested concept of bud banks may be widely used in studies focusing on plant functional traits in relation to disturbance regimes at the levels of plant individuals, populations and communities. Its further development may incorporate adjustments for areas with non-seasonal climate and refinements for some growth forms, such as epiphytes.
Atriplex tatarica is a heterocarpic annual native to a wide area of Middle and western Central Asia, Asia Minor, North Africa, and Eastern Europe. It is an early successional species of disturbed habitats, in Central Europe mostly occupying road margins and waste places. This paper deals with taxonomic status, morphology, distribution, ecology, and population biology of this species with special emphasis on its population genetics.
Rates of tree growth in tropical forests reflect variation in life history strategies, contribute to the determination of species' distributional limits, set limits to timber harvesting and control the carbon balance of the stands. Here, we review the resources that limit tree growth at different temporal and spatial scales, and the different growth rates and responses of functional groups defined on the basis of regeneration strategy, maximum size, and species' associations with particular edaphic and climatic conditions.