Journal of Ecology

Published by Wiley
Online ISSN: 1365-2745
Print ISSN: 0022-0477
The study area, located in the Laikipia County of central Kenya (a), shown in topographic detail (b). The 74 sampling sites are located across a strong rainfall gradient. The sampling sites included 27 ‘experimental’ sites, arranged in blocks and located across two different large-scale exclosure experiments (yellow circles), and 47 ‘landscape’ sites (see text for details). Landscape sites that were analysed using a paired design are shown in black, and additional landscape sites are in white. Sites are distributed normally across the rainfall gradient (c) and bimodally across an underlying soil gradient (d) including predominantly high-clay black cotton soils (black bars), and a continuum of transitional and red sand soils (red bars). Panel e shows the randomized block design (1 triplet block per experiment shown) used in the two experiments (KLEE and UHURU). Each experiment includes treatments that allow access to all animals (0), wildlife (W) or cattle (c; KLEE only) as well as other treatments (not used; shown in grey); distance between replicate blocks is not to scale.
Residual variation from the regression of mean vegetation height (a), total cover (b) and aerial cover (c) to rainfall, plotted against% dung cover (a proxy for total herbivore abundance). In experimental sites (red), the removal of herbivores leads to plant communities that are taller, with more cover, and more structural complexity, after correcting for rainfall effects (all relationships significant). In contrast, in landscape sites (blue), the loss of herbivores has little effect on vegetation structure (no significant relationships).
Effect sizes (loge response ratios) of wildlife removal on plant species richness across a gradient of rainfall (mean annual rainfall). The effects of wildlife loss on species richness are u-shaped in landscape sites (dashed line, unfilled circles; P = 0.02, R2 = 0.33), and linear in experimental sites (solid black line, filled circles; P = 0.07, R2 = 0.21). The grey shaded area represents the 95% confidence interval across all site pairs. Only the subset of sites that were spatially and temporally paired (n = 24) were used for these analyses. Model details in Table S2.
Results from discrimination analysis of plant community species composition (a) and growth form composition (b) show strong and consistent differences in composition between high wildlife (unfilled circles) and low wildlife sites (filled circles) in landscape sites (in blue) but not in experimental sites (in red). Important environmental drivers of plant community composition (shown with black arrows) are underlying soil parameters (% sand, silt and clay), annual rainfall and domestic livestock. The length of the arrow is proportional to the strength of the correlation. In panel b, the underlying patterns of growth form variation driving the differences are plotted in green arrows and green text.
1. Herbivores influence the structure and composition of terrestrial plant communities. However, responses of plant communities to herbivory are variable and depend on environmental conditions, herbivore identity and herbivore abundance. As anthropogenic impacts continue to drive large declines in wild herbivores, understanding the context dependence of herbivore impacts on plant communities becomes increasingly important.2. Exclosure experiments are frequently used to assess how ecosystems reorganize in the face of large wild herbivore defaunation. Yet in many landscapes, declines in large wildlife are often accompanied by other anthropogenic activities, especially land conversion to livestock production. In such cases, exclosure experiments may not reflect typical outcomes of human-driven extirpations of wild herbivores.3. Here, we examine how plant community responses to changes in the identity and abundance of large herbivores interact with abiotic factors (rainfall and soil properties). We also explore how effects of wild herbivores on plant communities differ between large-scale herbivore exclosures and landscape sites where anthropogenic activity has caused wildlife declines, often accompanied by livestock increases.4. Abiotic context modulated the responses of plant communities to herbivore declines with stronger effect sizes in lower-productivity environments. Also, shifts in plant community structure, composition and species richness following wildlife declines differed considerably between exclosure experiments and landscape sites in which wild herbivores had declined and were often replaced by livestock. Plant communities in low wildlife landscape sites were distinct in both composition and physical structure from both exclosure and control sites in experiments. The power of environmental (soil and rainfall) gradients in influencing plant response to herbivores was also greatly dampened or absent in the landscape sites. One likely explanation for these observed differences is the compensatory effect of livestock associated with the depression or extirpation of wildlife.5. Synthesis. Our results emphasize the importance of abiotic environmental heterogeneity in modulating the effects of mammalian herbivory on plant communities and the importance of such covariation in understanding effects of wild herbivore declines. They also suggest caution when extrapolating results from exclosure experiments to predict the consequences of defaunation as it proceeds in the Anthropocene.
R* for four resources (a) and the impacts of grazing (b) for exotic and native species. R* represents resource concentration in monocultures for DIN (dissolved inorganic nitrogen, nitrate + ammonium concentrations), phosphorus and soil moisture. R* for light (PAR – photosynthetically active radiation unconsumed) is the per cent of light above the canopy that reaches the soil surface. Grazing impacts are the log difference between biomass (or seed mass) in grazed plots and ungrazed plots. Means, standard error bars and P-values (in a) are from mixed effects models with exotic/native status as the fixed effect and species and block as random effects. The difference between grazed and ungrazed mass (on a log scale), standard error bars and P-values in (b) are from mixed effects models with exotic/native status, grazing, and their interaction as fixed effects and species and block random as effects.
Rank abundance in mixed plots of 11 native and exotic annuals as biomass and seed production in the absence (a) and presence (b) of grazing. Grey bars are native species, white bars are exotic species. Species abbreviations are listed in Table 1. Standard error bars are from five block-specific values of relative abundance (species mass/total mass). Insets show mean biomass and seed production (in g m−2) of exotic and native annual species (y axis on a log scale). Means, standard error bars and P-values in inset graphs are from mixed effects models with exotic/native status, grazing and their interaction as the explanatory variable for biomass or seed production, with species and block as random effects for status and grazing, respectively.
The relationship between competitive ability for nitrogen and light (R*– resource concentrations in monoculture) and relative abundance as biomass in ungrazed (a) and grazed mixtures (b). Lower values of R* indicate greater competitive ability for that resource. Each circle represents one of five native (grey) or six exotic (white) species. P-values and Kendall’s τ are based on two-tailed tests; Kendall’s τ is a measure of association between ranked variables. Results are similar for relationships between R* for nitrogen and light and relative abundance in seed mass (results not shown).
The dominance of invasive species is often assumed to reflect their competitive superiority over displaced native species. However, invasive species may be abundant because of their greater tolerance to anthropogenic impacts accompanying their introduction. Thus, invasive species can either be the drivers or passengers of change. We distinguish between these two possibilities in California grasslands currently dominated by Mediterranean annuals (exotics) and subjected to livestock grazing since European settlement. We focused on native annual grasses and forbs, an understudied species-rich component of the California flora, and Mediterranean annual grasses, currently dominant and among the first non-native plants introduced to the area. We established a field experiment with fenced and unfenced blocks in a cattle pasture. We measured concentrations of limiting resources (nitrogen, phosphorus, light and soil moisture) in monoculture plots as an index of competitive ability (i.e. R*). We then quantified grazing impacts on biomass and seed production in grazed vs. ungrazed monoculture plots. Finally, we measured biomass and seed production of each species competing in mixture plots, in the presence and absence of grazers. We found that native and exotic species did not differ in R* indices of competitive ability, i.e. concentrations of limiting resources in ungrazed native monoculture plots did not differ from concentrations in ungrazed exotic monoculture plots. By contrast, exotic annuals suffered less from grazing than native annuals, perhaps reflecting their longer evolutionary history with cattle grazing. Consistent with these results, native and exotic annuals were equally abundant in ungrazed mixtures, but exotic species overwhelmingly dominated grazed mixtures. Species able to draw down nitrogen and light to lower levels in monocultures (i.e. those with lower R* values) dominated biomass and seeds in mixed plots without grazers. However, R* did not predict the relative abundance of species in grazed plots. Moreover, the relative abundance of species in mixtures did not correlate with grazing impacts on their monocultures, implying that grazing alters inter-specific competitive dynamics. Synthesis. We demonstrate that the displacement of native annuals by Mediterranean annual grasses in California may largely have been driven by cattle grazing.
Grasses using the C4 photosynthetic pathway dominate today's savanna ecosystems and account for ∼20% of terrestrial carbon fixation. However, this dominant status was reached only recently, during a period of C4 grassland expansion in the Late Miocene and Early Pliocene (4–8 Myr ago). Declining atmospheric CO2 has long been considered the key driver of this event, but new geological evidence casts doubt on the idea, forcing a reconsideration of the environmental cues for C4 plant success. Here, I evaluate the current hypotheses and debate in this field, beginning with a discussion of the role of CO2 in the evolutionary origins, rather than expansion, of C4 grasses. Atmospheric CO2 starvation is a plausible selection agent for the C4 pathway, but a time gap of around 10 Myr remains between major decreases in CO2 during the Oligocene, and the earliest current evidence of C4 plants. An emerging ecological perspective explains the Miocene expansion of C4 grasslands via changes in climatic seasonality and the occurrence of fire. However, the climatic drivers of this event are debated and may vary among geographical regions. Uncertainty in these areas could be reduced significantly by new directions in ecological research, especially the discovery that grass species richness along rainfall gradients shows contrasting patterns in different C4 clades. By re-evaluating a published data set, I show that increasing seasonality of rainfall is linked to changes in the relative abundance of the major C4 grass clades Paniceae and Andropogoneae. I propose that the explicit inclusion of these ecological patterns would significantly strengthen climate change hypotheses of Miocene C4 grassland expansion. Critically, they allow a new series of testable predictions to be made about the fossil record. Synthesis. This paper offers a novel framework for integrating modern ecological patterns into theories about the geological history of C4 plants.
1.Adams and Zhang recently published one of the best studies so far of patterns of insect folivory along a latitudinal (climatic) gradient. They show clear negative trends in foliage loss in relation to temperature for certain groups of insect herbivores. 2.Although their suggestion that the plant–herbivore interaction may be more important in cooler climates could be valid, they did not bring up the complementary explanation that interactions between predators and herbivores could also vary with climate. There are indications that insect natural enemies may respond more positively than insect herbivores to an increase in temperature. We argue that higher predator pressure in warmer climates may partly explain the patterns observed by Adams and Zhang. 3.Synthesis.To further develop the important research concerning herbivory in a changing climate, both theoretically and empirically, plant ecologists and entomologists would mutually benefit from joining forces.
Invasive annual grasses introduced by European settlers have largely displaced native grassland vegetation in California and now form dense stands that constrain the establishment of native perennial bunchgrass seedlings. Bunchgrass seedlings face additional pressures from both livestock grazing and barley and cereal yellow dwarf viruses (B/CYDVs), which infect both young and established grasses throughout the state. Previous work suggested that B/CYDVs could mediate apparent competition between invasive exotic grasses and native bunchgrasses in California. To investigate the potential significance of virus-mediated mortality for early survivorship of bunchgrass seedlings, we compared the separate and combined effects of virus infection, competition and simulated grazing in a field experiment. We infected two species of young bunchgrasses that show different sensitivity to B/CYDV infection, subjected them to competition with three different densities of exotic annuals crossed with two clipping treatments, and monitored their growth and first-year survivorship. Although virus infection alone did not reduce first-year survivorship, it halved the survivorship of bunchgrasses competing with exotics. Within an environment in which competition strongly reduces seedling survivorship (as in natural grasslands), virus infection therefore has the power to cause additional seedling mortality and alter patterns of establishment. Surprisingly, clipping did not reduce bunchgrass survivorship further, but rather doubled it and disproportionately increased survivorship of infected bunchgrasses. Together with previous work, these findings show that B/CYDVs can be potentially powerful elements influencing species interactions in natural grasslands. More generally, our findings demonstrate the potential significance of multitrophic interactions in virus ecology. Although sometimes treated collectively as plant 'predators', viruses and herbivores may exert influences that are distinctly different, even counteracting.
Mean percentage (±1 SE) of leaf consumption for saplings of the 10 study species (Ardisia crenata, Camellia fraterna, Castanopsis eyrei, Cyclobalanopsis glauca, Eurya muricata, Lithocarpus glaber, Loropetalum chinense, Machilus thunbergii, Neolitsea aurata and Schima superba): (a) mean values per plot; (b) mean values per age class.
Relationship between herbivore damage of the single study species (arcsine-square-root-transformed) and species richness of woody plants across the diversity gradient of 27 study plots in subtropical China. Regression slopes (β; with their probabilities P) from single regressions show sign and magnitude of the relationships; regression lines indicate significant (black) and close to significant (grey) relationships.
Mean percentage of leaf damage per plot owing to insect herbivory in relation to species richness of trees and shrubs across a diversity gradient of 27 study plots in subtropical China (β = 0.001, P = 0.025).
1.Insect herbivory can strongly affect ecosystem processes, and its relationship with plant diversity is a central topic in biodiversity–functioning research. However, very little is known about this relationship from complex ecosystems dominated by long-lived individuals, such as forests, especially over gradients of high plant diversity. 2.We analysed insect herbivory on saplings of 10 tree and shrub species across 27 forest stands differing in age and tree species richness in an extraordinarily diverse subtropical forest ecosystem in China. We tested whether plant species richness significantly influences folivory in these highly diverse forests or whether other factors play a more important role at such high levels of phytodiversity. 3.Leaf damage was assessed on 58 297 leaves of 1284 saplings at the end of the rainy season in 2008, together with structural and abiotic stand characteristics. 4.Species-specific mean damage of leaf area ranged from 3% to 16%. Herbivory increased with plant species richness even after accounting for potentially confounding effects of stand characteristics, of which stand age-related aspects most clearly covaried with herbivory. Intraspecific density dependence or other abiotic factors did not significantly influence overall herbivory across forest stands. 5.Synthesis.The positive herbivory–plant diversity relationship indicates that effects related to hypotheses of resource concentration, according to which a reduction in damage by specialized herbivores might be expected as host plant concentration decreases with increasing plant diversity, do not seem to be major determinants for overall herbivory levels in our phytodiverse subtropical forest ecosystem. We discuss the potential role of host specificity of dominant herbivores, which are often expected to show a high degree of specialization in many (sub)tropical forests. In the forest system we studied, a much higher impact of polyphagous species than traditionally assumed might explain the observed patterns, as these species can profit from a broad dietary mix provided by high plant diversity. Further testing is needed to experimentally verify this assumption.
1. Senescence, the physiological decline that results in decreasing survival and/or reproduction with age, remains one of the most perplexing topics in biology. Most theories explaining the evolution of senescence (i.e. antagonistic pleiotropy, accumulation of mutations, disposable soma) were developed decades ago. Even though these theories have implicitly focused on unitary animals, they have also been used as the foundation from which the universality of senescence across the tree of life is assumed.2. Surprisingly, little is known about the general patterns, causes and consequences of whole-individual senescence in the plant kingdom. There are important differences between plants and most animals, including modular architecture, the absence of early determination of cell lines between the soma and gametes, and cellular division that does not always shorten telomere length. These characteristics violate the basic assumptions of the classical theories of senescence and therefore call the generality of senescence theories into question.3. This Special Feature contributes to the field of whole-individual plant senescence with five research articles addressing topics ranging from physiology to demographic modelling and comparative analyses. These articles critically examine the basic assumptions of senescence theories such as age-specific gene action, the evolution of senescence regardless of the organism's architecture and environmental filtering, and the role of abiotic agents on mortality trajectories.4. Synthesis. Understanding the conditions under which senescence has evolved is of general importance across biology, ecology, evolution, conservation biology, medicine, gerontology, law and social sciences. The question ‘why is senescence universal or why is it not?’ naturally calls for an evolutionary perspective. Senescence is a puzzling phenomenon, and new insights will be gained by uniting methods, theories and observations from formal demography, animal demography and plant population ecology. Plants are more amenable than animals to experiments investigating senescence, and there is a wealth of published plant demographic data that enable interpretation of experimental results in the context of their full life cycles. It is time to make plants count in the field of senescence.
1. Variation in disease resistance is a widespread phenomenon in wild plant–pathogen associations. Here, we review current literature on natural plant–pathogen associations to determine how diversity in disease resistance is distributed at different hierarchical levels – within host individuals, within host populations, amongst host populations at the metapopulation scale and at larger regional scales. 2. We find diversity in resistance across all spatial scales examined. Furthermore, variability seems to be the best counter-defence of plants against their rapidly evolving pathogens. We find that higher diversity of resistance phenotypes also results in higher levels of resistance at the population level. 3. Overall, we find that wild plant populations are more likely to be susceptible than resistant to their pathogens. However, the degree of resistance differs strikingly depending on the origin of the pathogen strains used in experimental inoculation studies. Plant populations are on average 16% more resistant to allopatric pathogen strains than they are to strains that occur within the same population (48% vs. 32% respectively). 4. Pathogen dispersal mode affects levels of resistance in natural plant populations with lowest levels detected for hosts of airborne pathogens and highest for waterborne pathogens. 5. Detailed analysis of two model systems, Linum marginale infected by Melampsora lini, and Plantago lanceolata infected by Podosphaera plantaginis, show that the amount of variation in disease resistance declines towards higher spatial scales as we move from individual hosts to metapopulations, but evaluation of multiple spatial scales is needed to fully capture the structure of disease resistance. 6.Synthesis. Variation in disease resistance is ubiquitous in wild plant–pathogen associations. Whilst the debate over whether the resistance structure of plant populations is determined by pathogen-imposed selection versus non-adaptive processes remains unresolved, we do report examples of pathogen-imposed selection on host resistance. Here we highlight the importance of measuring resistance across multiple spatial scales, and of using sympatric strains when looking for signs of coevolution in wild plant–pathogen interactions.
1. Senescence (an increase in the mortality rate or force of mortality, or a decrease in fertility, with increasing age) is a widespread phenomenon. Theories about the evolution of senescence have long focused on the age trajectories of the selection gradients on mortality and fertility. In purely age-classified models, these selection gradients are non-increasing with age, implying that traits expressed early in life have a greater impact on fitness than traits expressed later in life. This pattern leads inevitably to the evolution of senescence if there are trade-offs between early and late performance. 2. It has long been suspected that the stage- or size-dependent demography typical of plants might change these conclusions. In this paper, we develop a model that includes both stage- and age-dependence and derive the age-dependent, stage-dependent and age×stage-dependent selection gradients on mortality and fertility. 3. We applied this model to stage-classified population projection matrices for 36 species of plants, from a wide variety of growth forms (from mosses to trees) and habitats. 4. We found that the age-specific selection gradients within a life cycle stage can exhibit increases with age (we call these contra-senescent selection gradients). In later stages, often large size classes in plant demography, the duration of these contra-senescent gradients can exceed the life expectancy by several fold. 5. Synthesis. The interaction of age- and stage-dependence in plants leads to selection pressures on senescence fundamentally different from those found in previous, age-classified theories. This result may explain the observation that large plants seem less subject to senescence than most kinds of animals. The methods presented here can lead to improved analysis of both age-dependent and stage-dependent demographic properties of plant populations.
Relationships between species root and leaf traits: (a) Specific root length (SRL) and SLA, (b) root and leaf tissue density, (c) root and leaf N concentrations and (d) root and leaf P concentrations. Legend shows phylogenetic correlations (λ, phylogenetic least-squares model), and Pearson correlations (r, ordinary least-squares model), along with their respective significance (*P < 0.05, **P < 0.01, ***P < 0.001). Fitted lines taken from the model with the lowest Akaike Information Criterion (#). See Table S7 for full model details. Closed circles indicate woody angiosperms, open circles indicate conifers and open squares indicate tree ferns.
Species root traits against the vegetation CCA axis 1 scores. Note log scale on y-axes. The x-axis is positively correlated with site age (Pearson’s correlation r = 0.95, P < 0.001). SRL, specific root length; TD, tissue density and hair index is length of hairs per unit root length. Legend shows phylogenetic correlations [λ, phylogenetic least-squares (PGLS) model], and Pearson correlations (r, ordinary least-squares model), along with their respective significance (*P < 0.05, **P < 0.01, ***P < 0.001). Linear fitted lines taken from the model with the lowest Akaike Information Criterion (#). See Table S7 for full model details. For both SRL and root N:P, the dotted line is the significant linear PGLS model, while the solid line is a better fitting polynomial PGLS model [log(SRL) = 0.35*CCA12 + 0.29*CCA1 + 7.32, λ = 0.89, P < 0.002, AIC = 89.2; log(N:P) = 0.28*CCA12 + 0.13*CCA1 + 2.3, λ = 0.63, P < 0.001, AIC = 4.18]. Closed circles indicate woody angiosperms, open circles indicate conifers, closed squares indicate herbaceous angiosperms and open squares indicate ferns.
Community-level average trait values for (a) specific root length, (b) root diameter, (c) root tissue density, (d) branching intensity, (e) hair length per unit root length, (f) root N, (g) root P and (h) root N:P ratio, against log site age (Table S8). Values calculated using either even-weighting for all species present at each site (open circles), or abundance-weighted trait values (closed circles). In both cases, a single mean trait value for each species across all sites was used. Legend shows Pearson correlations (r) and significance (**P < 0.01, ***P < 0.001) of each relationship.
Relationships among species-level root traits [slope, significance and phylogenetic correlation (k)]
1. Below-ground plant functional traits regulate plant–soil interactions and may therefore strongly influence ecosystem responses to global change. Despite this, knowledge of how fine-root functional traits vary among plant species and along environmental gradients has lagged far behind our understanding of above-ground traits. 2. We measured species- and community-level root and leaf trait responses for 50 temperate rain forest species from 28 families of ferns, woody and herbaceous angiosperms and conifers, along a soil chronosequence in New Zealand that exhibits a strong gradient in soil nutrient availability. Relationships among species traits (both above- and below-ground) and their distribution along the chronosequence were tested using phylogenetic generalized least-squares regression to account for plant relatedness. 3. Distinctive root trait syndromes were observed; they were closely linked to species’ distribution along the chronosequence. Species growing in the strongly P-limited late stages of the chronosequence had relatively high specific root length (SRL), thin root diameter, high root tissue density, high levels of root branching and low root nutrient concentrations compared to intermediate stages. Species on the youngest site also had high SRL, but had low root tissue density, thick root diameter and high root nutrient concentrations. 4. Species root and leaf nutrient concentrations were positively correlated, reflecting the strong underlying gradient in soil fertility. In contrast, the relationship between SRL and SLA was more complex; there was a weak positive correlation between SRL and SLA, but this conflicted with stronger patterns of increasing SRL and declining SLA with increasing site age. 5. Community-averaged trait values calculated using presence/absence data showed similar trends to the species-level patterns. In contrast, community averages calculated using species abundance-weighted data showed weaker relationships with site age, particularly for morphological traits. This suggests that much of the variation in morphological traits between sites was driven by shifts in the presence of subordinate or ‘rare’ species rather than by changes in the dominant species. 6. Synthesis. Our study demonstrates co-ordinated species- and community-level changes in root traits along a soil chronosequence. These results highlight the influence of soil nutrition on plant functional traits and contribute to our understanding of the drivers of community assembly in a changing environment.
Map of Tsholotsho communal area in Zimbabwe indicating locations of the 10 study fields. C, clay soil; S, sand soil.
Distribution of the 10 most common ribotypes in five clay fields (a) and five sand fields (b), where the field number refers to the replicate field within each soil type. The ribotype abbreviations are listed in Appendix S1.
Means (+ 1 SE) of mycorrhizal colonization (a) and shoot dry weight (b) of sorghum inoculated with either Gi. rosea and Gi. gigantea (Gigaspora), Gl. etunicatum and Gl. mosseae (Glomus), or all four species (Gigaspora/Glomus), and grown in pots containing either clay, sand/clay or sand for 56 days. Different upper-case letters indicate differences within AM fungal treatment among soil types, and different lower-case letters indicate differences within soil type among AM fungal treatments using Tukey's HSD at α = 0.05.
the influences of environmental variables and spatial structure on the dispersion of arbuscular mycorrhizal fungal ribotypes by multivariate variance partitioning. Sig- nificant P
What may cause this disparity between the two AM fungal families cannot be deduced from our study, as clay and sand differ in many variables (Table 1) that do not act independently (as is evident by their close correlation in Fig. 2). Soil fertility can impact AM fungal community structure, as indicated by shifts in AM fungal communities along gradients of N deposition (Egerton-Warburton & Allen 2000), soil organic carbon (Johnson et al. 1991), or N and P additions (Johnson 1993). Indeed, organic carbon and total nitrogen appear to have been important factors partitioning niche space within the Glomeraceae in the current study (Fig. 2). However, the RDA indicated that the separation on the family level was driven by percentage clay, but why clay has a negative effect on Gigasporaceae remains unknown. Possibly, differential biomass allocation and growth patterns of the external hyphae between Gigasporaceae and Glomeraceae, as detected by Hart & Reader (2002) and Dodd et al. (2000), are affected by texture. Additionally, our glasshouse study indicated that sporulation may be impaired in clay (Table 3). This may have a greater negative effect on Gigasporaceae than on Glomeraceae, as spores are the main propagules for many Gigasporaceae AM fungi (Klironomos & Hart 2002). Further comparative studies with Scutellospora cerradensis (Sc. rt1), the only member of the Gigasporaceae that occurred predominately in clay soil in the field (Fig. 2), could be informative. Overall, to understand the underlying mechanisms for the adverse effects of clay on most Gigasporaceae, more basic ecological experiments on AM fungi clearly are needed.
Summary • A set of metapopulation models representing non-exclusive hypotheses for the persistence of a parasite metapopulation in time were used to examine spatial patterns and processes in a Triphragmium ulmariae–Filipendula ulmaria parasite–host interaction from the Skeppsvik Archipelago, northern Sweden, using ecological data from an 11-year study. • Models were fitted to the data set using maximum likelihood and subsequently ranked using parsimony criteria derived from information theory. • Over intermediate time-scales (10–100 years), the persistence of the parasite metapopulation is determined by core-satellite relationships, with a few high-quality patches with large host and parasite populations contributing to the overall stability of the parasite metapopulation. • Evidence was obtained for the existence of a complex hierarchical structuring of populations, with dispersal rates among populations on the same island being an order of magnitude greater than among islands in the same island chain. In turn, dispersal among populations on the same island chain were orders of magnitude higher than the baseline rate.
1 Horizontal and vertical heterogeneity of resource availability, coupled with the specialized use of resources by tree species, results in complex patterns of tree species distributions in tropical rain forests. We studied the horizontal and vertical distributions of 4014 individuals in 11 species of early successional Macaranga (Euphorbiaceae) in tropical rain forest in Sarawak, Malaysia.2 The horizontal distribution of individual trees was assessed with respect to crown light levels, establishment microsites, and broader scale variation in soil textural properties. Vertical distribution was assessed using an allometric approach to estimate maximum tree height (Hmax) and the slope of the sapling height–diameter relationship.3 Average light levels intercepted and the proportion of individuals in each of five crown illumination classes varied significantly among the 11 species. Species ranged from extremely high-light demanding, to quite shade tolerant. Average light levels intercepted by trees generally increased through ontogeny, but the ranking of species did not change significantly.4 Fewer individuals of the more shade-tolerant species established on disturbed microsites, irrespective of light levels. Among the more high-light demanding species, the proportion of trees on different types of disturbed sites varied.5 Trees of seven species were significantly more common on clay-rich soils, two preferred sand-rich soils, and two were not strongly affected by soil texture.6 Hmax ranged from 5.5 to 31.3 m and was negatively correlated with shade tolerance among species, although among the more high-light demanding species there was a wide range of tree sizes. Among species, Hmax was negatively correlated with both the slope and y-intercept of the sapling height–diameter relationship, indicating that small-statured species (also more shade tolerant) had more slender saplings than larger statured species.7 Heterogeneity of resource availability leads to differences in horizontal and vertical tree distribution, which are important for the coexistence of 11 Macaranga species.
Abstract • Seed limitation of plant species distribution has received much attention in recent years. The empirical evidence of seed limitation is, however, still weak, and mostly correlative. Unambiguous evidence can only be provided by long-term transplantation experiments. • We used an 11-year experiment, in which seeds of six perennial forest herbs were sown into 43 patches that were either occupied or unoccupied by the target species, to investigate the importance of seed limitation. In addition, we determined whether short-term (1–4 years) and long-term (11 years) experimental assessments provided similar results. Given that long-term experiments are generally not possible, we examined whether environmental factors and vegetation composition can correctly identify suitable habitats. • Our results show that the distribution of the forest herbs investigated were limited by the availability of seeds, and that this pattern was qualitatively consistent, over time. However, the number of initially unoccupied patches with successful recruitment decreased with time. • The fact that a similar trend in recruitment success was found in sowing experiments in initially occupied patches suggests that the decrease in recruitment success over time cannot solely be attributed to habitat suitability. Environmental factors, such as soil nutrients and vegetation composition, were in some cases correlated with recruitment success, but the strength of these relationships was often weak. • Our results support the notion that seed availability is an important limitation of plant species abundance and distribution. The results also point to several difficulties related to the empirical assessment of seed limitation. We therefore suggest a protocol that should enhance the quality of future experiments. Journal of Ecology (2006) 94, 1224–1232 doi: 10.1111/j.1365-2745.2006.01169.x
Summary • The Pleistocene is an important period for assessing the impact of climate change on biodiversity. During the Last Glacial Maximum (LGM; 21 000 years ago), large glaciers and permafrost reached far south in Europe. Trees are traditionally thought to have survived only in scattered Mediterranean refugia (southern refugia hypothesis), but a recent proposal suggests that trees may have been much more widely and northerly distributed (northern refugia hypothesis). • In this study, the southern vs. northern refugia hypotheses were investigated by estimating the potential LGM distributions of 7 boreal and 15 nemoral widespread European tree species using species distribution modelling. The models were calibrated using data for modern species distributions and climate and projected onto two LGM climate simulations for Europe. Five modelling variants were implemented. • Models with moderate to good predictive ability for current species range limits and species richness patterns were developed. • Broadly consistent results were obtained irrespective of the climate simulation and modelling variant used. Our results indicate that LGM climatic conditions suitable for boreal species existed across Central and Eastern Europe and into the Russian Plain. In contrast, suitable climatic conditions for nemoral tree species were largely restricted to the Mediterranean and Black Sea regions. Large proportions of these northern and southern regions would have been suitable for a number of boreal or boreal plus nemoral tree species, respectively. • These findings are consistent with recent palaeoecological and phylogeographic data regarding LGM distributions of trees and other boreal and nemoral taxa. • Synthesis. It is clear that the view of the LGM landscape in Europe as largely treeless, especially north of the Alps, needs to be revised. Trees were probably much more widespread during the LGM than hitherto thought, although patchily distributed at low densities due to low atmospheric CO2 concentrations and high wind-speeds. The findings presented here help explain the occurrence of mammal assemblages with mixtures of forest, tundra and steppe species at many localities in southern Central and Eastern Europe during the LGM, as well as the phylogeographic evidence for the extra-Mediterranean persistence of many boreal species.
1Small and isolated populations of species are susceptible to loss of genetic diversity, owing to random genetic drift and inbreeding. This loss of diversity may reduce the evolutionary potential to adapt to changing environments, and may cause immediate loss of fitness (cf. inbreeding depression). Together with other population size-dependent stochastic processes, this may lead to increased probabilities of population extinction.2This set of processes and theories forms the core of conservation genetics and has developed into the conservation genetics paradigm. Many empirical studies have concentrated on the relationship between population size and genetic diversity, and in many cases evidence was found that small populations of plants do indeed have lower levels of genetic diversity and increased homozygosity. Although less empirical attention has been given to the relationship between low genetic diversity, fitness and, in particular, evolutionary potential, the paradigm is now widely accepted.3Here we present five areas of the paradigm which could be refined, i.e. the ‘rough’ edges of the conservation genetics paradigm.4Treating population size and isolation not as interchangeable parameters but as separate parameters affecting population genetics in different ways could allow more accurate predictions of the effects of landscape fragmentation on the genetic diversity and viability of populations.5There is evidence that inbreeding depression may be a genotype-specific phenomenon, rather than a population parameter. This sheds new light on the link between population inbreeding depression and the expected increased probability of extinction.6Modern eco-genomics offers the opportunity to study the population genetics of functional genes, to the extent that the role of selection can be distinguished from the effects of drift, and allowing improved insights into the effects of loss of genetic diversity on evolutionary potential.7Incorporating multispecies considerations may result in the generally accepted notion that small populations are at peril being called into question. For instance, small populations may be less capable of sustaining parasites or herbivores.8Comparative studies of endangered, common and invasive species may be a valuable approach to developing conservation biology from a phenomenological case study discipline into one investigating the general principles of what sustains biodiversity.9The issues discussed set an agenda for further research within conservation genetics and may lead to a further refinement of our understanding and prediction of the genetic effects of habitat fragmentation. They also underline the need to integrate ecological and genetic approaches to the conservation of biodiversity, rather than regarding them as opposites.
Summary • Many studies in northern Europe, North and South America, describe regional trends of population densification at altitudinal and polar tree lines during the 20th century. The purpose of this study was (1) to ascertain if this regeneration enhancement is present across the alpine ecotones of the Pyrenees, (2) if synchronous recruitment trends are common among the studied populations and (3) to determine the tree limit stability during recent decades. • Twelve Pinus uncinata tree line populations were studied on the Iberian eastern range of the Pyrenees. Rectangular plots ranging from 940 to 7600 m2 were set along the forest-alpine grassland transition; more than 3600 P. uncinata individuals were mapped. Tree size and age were used to establish the demographic structure at each stand, and to characterize abrupt or smooth transition patterns along the tree line ecotone. A new procedure for estimating missing rings in off-centre cores was developed to ensure a correct interval for the age-classes distribution analysis. • Past and recent synchronous recruitment trends (mid 19th century, second half of the 20th century) were apparent at the tree line over the studied area of the Pyrenean range. The ecotone densification since the 1950s occurred in the context of climatic warming and substantial land use abandonment. Both gradual and step-like transition patterns in tree age and size along the ecotone were observed. • Regeneration enhancement in the last approximately 30 years appears as an abrupt change in population age structures, which could indicate the importance of feedback mechanisms for tree line recruitment dynamics. In 50% of the surveyed tree lines ecotone densification has been coupled to tree limit shifts in the recent past. This indicates both great tree limit sensitivity to short-term climatic changes and the presence of differential tree line dynamics at a regional scale. • Synthesis. The observed past and recent synchronous recruitment trends suggest the presence of regional climatic factors modulating tree line structure and dynamics. However, tree line dynamics in the Pyrenees have been widely affected by local anthropogenic activities. We suggest that the presence of step-like tree line transitions in tree age can be considered an evidence of recent human-induced disturbances when no other major natural disturbances affect the tree line dynamics.
Summary 1 The cover of plant species was recorded annually from 1988 to 2000 in nine spatially replicated plots in a species-rich, semi-natural meadow at Negrentino (southern Alps). This period showed large climatic variation and included the centennial maximum and minimum frequency of days with ≥ 10 mm of rain. 2 Changes in species composition were compared between three 4-year intervals characterized by increasingly dry weather (1988-91), a preceding extreme drought (1992-95), and increasingly wet weather (1997-2000). Redundancy analysis and  with repeated spatial replicates were used to find trends in vegetation data across time. 3 Recruitment capacity, the potential for fast clonal growth and seasonal expansion rate were determined for abundant taxa and tested in general linear models (GLM) as predictors for rates of change in relative cover of species across the climatically defined 4-year intervals. 4 Relative cover of the major growth forms present, graminoids and forbs, changed more in the period following extreme drought than at other times. Recruitment capacity was the only predictor of species' rates of change. 5 Following perturbation, re-colonization was the primary driver of vegetation dynamics. The dominant grasses, which lacked high recruitment from seed, therefore decreased in relative abundance. This effect persisted until the end of the study and may represent a lasting response to an extreme climatic event.
Summary • A literature comparison of 14 radiocarbon-dated macrofossil records of raised peat bog initiation indicates that there is a relationship between the prevailing climate and the character of the first ombrotrophic vegetation communities at peatland sites in Britain and Ireland. • All that is required for ombrotrophy is the separation of the mire growing surface from the influence of surface and subsurface waters. This could occur via vertical accumulation of the peat mass or a lowering of the water table. The establishment of bog species can be rapid once isolation occurs. • Peatlands may become ombrotrophic in a variety of water table conditions and climatic regimes. There are at least two distinctive routes to ombrotrophy, via a ‘dry-pioneer oligotrophic community’ or via a ‘wet-pioneer oligotrophic community’. • Tregaron South-east Bog does not fit the pattern suggested by the literature comparison. The Fen–Bog Transition (FBT) occurred in a period of increased effective precipitation but the first ombrotrophic community was indicative of relatively dry, ‘hummocky’ bog and a deep or unstable water table. • The transitional poor fen communities at Tregaron South-east Bog were short-lived. Sphagnum palustre mire lasted for 90 years compared to 300 years at Bolton Fell Moss in Cumbria, and the FBT was synchronous across much of the bog. • The Tregaron peatland complex has a long history of water table fluctuations as shown by the stratigraphy of the marginal peats. • Channel incision in the River Teifi could have contributed to the development of ‘dry hummock’ pioneer bog in the humid climatic conditions at 7300 cal. BP, by reducing the level of lagg streams.
1. Loss of plant species induced by adverse human influence and habitat fragmentation might be delayed due to a slow response to changed environmental conditions. This phenomenon is known as extinction debt, which can lead to the underestimation of actual threats to biodiversity. To adequately estimate the condition of recently fragmented plant communities and avoid future loss of species, good indicators of forthcoming extinctions are needed. 2. We studied the behaviour of genetic diversity of a relatively common habitat specialist grass Briza media in fragmented calcareous grasslands in which the extinction debt has been previously documented. Species richness in this system is shown to be determined by historical landscape patterns and human population densities prevailing centuries ago. We hypothesize that genetic diversity in this grassland system is related to current landscape patterns and contemporary human impact since genetic diversity might react more quickly to environmental changes than species diversity. 3. In contrast to species diversity, genetic diversity was indeed best described by current connectivity of grasslands. Additionally, genetic diversity was negatively related to current human population density, indicating an adverse effect of contemporary human settlements on studied species. The faster response of genetic diversity to changed environmental conditions compared to species richness was further supported by the absence of an expected correlation between species richness and genetic diversity. 4. Human population density a century ago had a positive effect on genetic diversity. A similar effect has been demonstrated for species richness in these grasslands, indicating that traditional land use in the past has supported the development of both genetic and species diversity. 5. Synthesis. Genetic diversity of Briza media in fragmented communities is reacting quickly to changes in landscape structure and anthropogenic pressure. Our results confirm that plant species can be prone to genetic deterioration due to habitat fragmentation and negative anthropogenic impact even if the decline in species richness has been delayed by extinction debt. Thus, a decrease of population genetic diversity in fragmented communities can be taken as the first indication of future species losses.
Summary 1 We characterized the dispersal potential and early growth traits of 14 tropical mangrove species in experiments where diaspores were immersed in various solutions of seawater and subsequently stranded onto surfaces with the same salinity. 2 Viviparous and non-viviparous species had similar buoyancy, seed weight and rates of root and shoot initiation, as well as early growth and salinity tolerance. This trait convergence may be related to selection against small, dormant diaspores in the unstable regeneration niche. 3 Differences in dispersal potential and early growth of 12 species were compared with known patterns of distribution (across the shore, along estuaries, regional occurrence and continental range size) to test if tidal sorting of diaspores could account for adult spatial patterns. 4 Diaspore buoyancy, orientation, lateral root initiation, shoot initiation and early shoot extension differed among species but none correlated with adult zonation across the shore or along estuaries. However, some back-shore species had diaspores that were buoyant and were slow to initiate lateral roots and shoots. Patterns of early growth were partially related to the distribution along estuaries but salinity responses contributed to this zonation in only three species. 5 Regional distributions were unrelated to dispersal potential. However, the tend - ency of infrequent species to show slow growth in full seawater may account for the under-saturation of species in estuaries with appropriate habitats. The range size of the tropical mangroves appears unrelated to their dispersal potential and early growth traits. 6 Early life history traits of 12 mangroves showed poor correlation with patterns of adult distribution across all spatial scales. Traits related to establishment were, however, stronger predictors of distribution than those associated with dispersal.
1. The long term impacts of historical land use on montane tropical forest tree species composition and structure is unknown. We used old plantation survey maps of 19th century coffee plantations in the Blue and Port Royal Mountains of Jamaica to investigate the effects of land use history on forest tree species composition and structure after 150–170 years of re-growth. 2. We sampled trees (≥ 10 cm d.b.h.) in montane forests along eight 200 m × 5 m transects that ran across old plantation boundaries. We assumed that agriculture had been abandoned for at least 150 years in the old plantations, based on the present forest cover which has now re-grown to resemble old growth forest, and that what was described as forest on old maps remained forested throughout the 19th and 20th centuries. Transects were located in montane forests between 1100 and 1500 m elevation, except for one transect which was in lower montane forest at 600 m elevation. For data analysis, transects were divided into three equal lengths, representing three forest types: old plantation, old edge and old growth. 3. There was no difference between old growth forest and old plantation forest or old edge forest in species density, endemic species richness or basal area. However we found a significantly higher percentage of individuals of endemic species in the old growth forest type. 4. The species composition of large and medium sized stems (20.1–95.0 cm) in the three forest types was different. The similarity between forest types of species with small stems (10–20 cm) suggests that succession follows an equilibrium model. 5.Synthesis. Old growth tropical montane forests are still different from forests on adjacent old plantations after 150–170 years of re-growth in having a higher percentage of individuals of endemic tree species, and in the tree species composition of large and medium-sized stems. While these old-secondary forests are not identical to old growth forest, they can have similar species density and basal area to old growth forests after two-centuries.
We assess the robustness of a 1500-year palaeoclimate data base and forest gap model in simulating northern Scandinavian boreal forest succession since ad 500. We also evaluate trends in species abundance in a nemoral woodland in southern Scandinavia by comparing modelling results to palaeoecological data. 2 Simulated biomass trends for Picea , Pinus and Betula growing at Penningholmen (northern Sweden) are strikingly similar to those observed in the local pollen record, particularly in the replication of the early Little Ice Age (LIA) decline in Betula and the late-LIA dieback of Picea and Pinus . 3 LIA decreases in Betula may therefore be partly due to the effects of climate on its competitive interactions with Pinus , as well as the previously proposed effects of insect herbivory. 4 Simulations of Draved Forest (western Denmark), suggest that Tilia is under-represented in modern-day Scandinavian nemoral woodlands, and consequently that the present dominance of Fagus probably reflects strong human-plant interactions from as early as the beginning of the 17th century. 5 This study highlights the importance, despite general limitations associated with vegetation models, of model-data comparisons for understanding mechanisms and processes underlying past forest succession, and emphasizes the usefulness of forest models for reconstructing climate influences on past vegetation.
Summary 1 Clonal populations face a trade-off between sexual recruitment and vegetative growth and, once established, may undergo continuous declines in genotypic diversity if their sexual recruits make poor competitors. The geological history of delta formation in the Lower Mississippi River Valley was used to age eight S. alterniflora marshes for use in a space-for-time substitution ranging over 1500 years, in order to determine the long-term effects of clonal growth on genotypic diversity in natural populations. 2 We also predicted that highly heterozygous clones are competitively superior, leading to an increase in the overall level of genetic diversity as a marsh ages and/or to an increasingly positive relationship between clone size and individual heterozygosity, and that the clumping of ramets within clones will occur over increasingly large distances as populations age, while the clumping of genetically related clones will become less pronounced as intraclonal competition begins to obscure the initial effects of localized seedling recruitment. 3 Using molecular markers to differentiate clones, we documented a decline in clonal richness at the rate of approximately 1% 100 years − 1 that was accompanied for the first 300-500 years by an increase in the distance over which clumping of ramets within genets occurred. Older populations, in the 500-1500-year range, showed evidence of clone fragmentation. 4 The spatial clustering of kin was observed for only two marshes, and exhibited no clear relationship with marsh age. 5 Whereas the overall level of genetic diversity was consistent among marshes and showed no clear relationship with marsh age, the relationship between heterozygosity and individual clone size became increasingly pronounced within older marshes. 6 Our results suggest that under natural conditions S. alterniflora marshes will rarely reach ages sufficient for the loss of all clonal diversity, or for the effects of inbreeding and drift to pose a significant threat to population viability.
Summary • Nutrient fluxes play a significant role in the interaction of myrmecophytic plants and their symbiotic ants. There is a clear flux from plants to ants via nectar or food bodies but nutrient fluxes from ants to plants are less obvious. • We report on a nitrogen flux from ants to plants in the association between Pheidole bicornis (Formicidae-Myrmicinae) and two myrmecophytic Piper species (P. fimbriulatum and P. obliquum, Piperaceae). Pulse experiments were performed by feeding ants with 15N-labelled glycine supplied in sucrose solution. Workers passed ingested label on to other ants and the brood by trophallaxis. • The distribution of label within the colony showed highest incorporation rates in larvae and in the working caste, while the reproductive caste received only a small amount of the 15N-labelled food. • Nutrient transfer from ants to plants occurred remarkably fast. Within 6 days, up to 25% of the nitrogen ingested by the ants was incorporated by the plants. • However, ant distribution within P. fimbriulatum plants did not correlate with the intra-plant uptake pattern of 15N, and ant-mediated nitrogen uptake by myrmecophytic P. fimbriulatum accounted for less than 1% of the plants’ above-ground nitrogen demand.
1. Sapling architecture may be an important determinant of performance traits, such as light interception and height growth, but few studies have examined the direct relationship between sapling architecture and growth properties. To study this relationship and the potential for strategic diversification, we analysed the growth properties in saplings of 16 Bornean tree species that differ in architecture. 2. Annual net production significantly differed amongst species and was positively correlated with total above-ground dry mass, total leaf area and crown area. In contrast, the net assimilation rate was weakly but negatively correlated with these architectural traits. The net assimilation rate was virtually independent of leaf size and specific leaf area. Relationships between sapling architecture and relative growth rate in mass were weak. 3. The relative growth rate in height did not significantly differ amongst species, although their total dry mass, a proxy for extension cost, varied fourfold across species for a given sapling height. This is because the proportional increase in net production with total dry mass, which is based on a larger total leaf area and larger crown area, cancelled out the higher extension cost. All architectural traits, including leaf size and specific leaf area, failed to predict height growth rate. 4. Synthesis. Relative growth rates in both mass and height were relatively independent of sapling architecture. Of the architectural traits, leaf size, specific leaf area and stem diameter were poor predictors of growth properties, even though they were considered functionally important. These results clearly reject the classic hypothesis that architectural variation leads to a trade-off between height growth and light interception, at least for the species that are under shaded conditions. However, functional variation ranging from species with high net production and low net assimilation rates (in saplings of equal height) to species with the opposite traits, which was accompanied by architectural variation in total dry mass and related size factors, may be important for the coexistence of these tree species. The possibility that small total dry mass may be advantageous in height growth under well-lit conditions should be examined in future studies.
Summary 1 Climate change in South Africa may threaten the sclerophyllous evergreen shrubs of this region. Available data suggest that they are not as tolerant of water stress as chaparral shrubs occurring in climatically similar California, USA. 2 Seventeen species from nine angiosperm families, including both fynbos and succu- lent karoo species, were studied at a field site in Western Cape Province, South Africa. Minimum seasonal pressure potential ( P min ), xylem specific conductivity ( K s ), stem strength against breakage (modulus of rupture, MOR), xylem density, theoretical vessel implosion resistance ( ) and several fibre and vessel anatomical traits were measured. 3 Species displayed great variability in P min , similar to the range reported for chaparral and karoo shrub species, but in contrast to previous reports for fynbos shrubs. 4 More negative P min was associated with having greater xylem density, MOR and . There was no relationship between P min and traits associated with increased water transport efficiency. 5 Xylem density integrates many xylem traits related to water stress tolerance, including P min , MOR and , as well as percentage fibre wall, parenchyma, vessel area and fibre lumen diameter. 6 Xylem density may be an integral trait for predicting the impact of climate change on evergreen shrubs.
1 Using 18 years of permanent-plot data from a 4-ha stand, we investigated whether stand dynamics is predictable, i.e. driven by competition (inferred from species shade tolerances), or unpredictable (driven by frequent exogenous disturbance or other factors). We also considered whether small disturbance might accelerate or retard succession. The study involved dynamics of stems ≥ 4.5 cm diameter at breast height (d.b.h.) for 10 important species in a southern mixed hardwood forest 80 years after selective removal of Pinus taeda.
Summary • We studied a cliff-face forest ecosystem dominated by a single long-lived tree species that has been previously shown to have slow, pulsed recruitment. We assessed the degree to which microsite and climatic variability over a long period of time control recruitment, morbidity and mortality of trees at a previously disturbed cliff site. • We sampled more than 2000 Thuja occidentalis (eastern white cedar) over an 18-year period using a series of dynamic cohorts. We also examined a smaller area more intensively for 9 years. • Microsite and climate both played a role in controlling emergence and survival. Seedlings emerged preferentially in horizontal microsites such as large ledges and shelves but survival there was poor, whereas crevices and smaller ledges had lower emergence but the best survival. Decaying logs, cliff edges, vertical cliff faces and the smallest ledges proved unsuitable for seedling recruitment. Very few seedlings survived for more than 5 years. • While spring and summer climate influenced emergence and early survival, climate effects decreased with increasing plant size and mortality at the later stages of recruitment was independent of climate. • Drought and pathogens were the most common causes of mortality in horizontal habitats, while drought and rockfall were important in vertical habitats. • There appear to be a finite number of safe sites on cliff faces, and recruitment to those sites limits the demographic changes in tree populations over time. • Long-term studies on long-lived species have the value of sorting real, but unimportant, short-term variation in plant response to climate and site conditions from the long-term trends that are principally responsible for moulding the structure of the ecosystem. Journal of Ecology (2005) doi: 10.1111/j.1365-2745.2005.01083.x
1Demographic trends of Pinus sylvestris L. (Scots pine) tree line populations are reported for a 32-year monitoring period (1973–2005). Functional and projective aspects of tree line performance were analysed by relating temporal variability and change of vital population parameters, such as natality/mortality, vigour, injuries, height growth and seed viability to contemporary variations in air and soil temperatures.2The size of the entire sampled population increased by 50% during the 32-year observation period and thereby pine has become a more prominent element on the landscape. This reverses a natural multicentennial or even millennial trend of tree line decline and recession.3Contrasting population trends were recorded for the subperiods 1973–87 and 1988–2005, viz. decline and increase, respectively. Mean summer temperatures (JJA) did not change perceivably over and between these intervals, although some exceptionally warm summers from 1997 onwards have contributed to population expansion by increased seed viability and seedling emergence. Winter temperatures (DJF) decreased significantly over the first subperiod and were consistently higher during the second, which has significantly lowered the mortality rates.4A functional link to winter temperature conditions was particularly stressed by the aetiology of individual plant vigour, injuries and final mortality. Classical symptoms of winter desiccation correlated significantly with low winter temperatures. This negative impact occurred with a high frequency during the decline phase and virtually ceased during the expansion phase from 1988 onwards, when winter air and root zone temperatures were raised to a consistently higher level.5Winter and summer temperatures in the air and soil, as well as positive feedback mechanisms and nonlinear responses, must be taken into account in the search for global or regional mechanical explanations for the tree line phenomenon. This insight helps to generate realistic tree line models for a high-CO2 world, when winter warming is usually predicted to be particularly large.
Summary 1 We estimated the dry, living, above-ground biomass (AGB) standing stock and its turnover in a 50-hectare forest plot located in moist tropical forest on Barro Colorado Island, Panama. The estimates were obtained using inventory data collected every 5 years from 1985 to 2000, including measurements of all trees ≥ 1 cm diameter. 2 Four different allometric regressions relating trunk diameter and height with AGB were compared. Based on the most consistent method, we estimated that the Barro Colorado forest holds 281 ± 20 Mg ha − 1 (1 Mg = 10 3 kg) of AGB, lianas included. A third of the AGB is stored in trees larger than 70 cm in diameter. 3 Stand-level AGB increment (growth plus recruitment) was highest in the period 1985-90 (7.05 ± 0.32 Mg ha − 1 year − 1 , mean ± 95% confidence limits based on samples of multiple hectares) and smallest in the period 1990-95 (5.25 ± 0.26 Mg ha − 1 year − 1 ), while AGB losses were similar during the three intervals (mean 5.43 ± 0.72 Mg ha − 1 year − 1 ). This resulted in significant differences in AGB change (defined as increment minus loss) among census intervals; including branchfalls, the AGB of Barro Colorado Island increased in 1985-90 (+0.82 ± 0.84 Mg ha − 1 year − 1 ), decreased in 1990-95 ( − 0.69 ± 0.82 Mg ha − 1 year − 1 ), and increased again in 1995-2000 (+0.45 ± 0.70 Mg ha − 1 year − 1 ). The 15-year average was +0.20 Mg ha − 1 year − 1 , but with a confidence interval that spanned zero ( − 0.68 to 0.63 Mg ha − 1 year − 1 ). 4 Branchfalls and partial breakage of stems had a significant influence on the AGB changes. They contributed an average of 0.46 Mg ha − 1 year − 1 to the AGB loss. About 5% of AGB increment was due to trees less than 10 cm in diameter. 5 To test whether the AGB of tropical forests is increasing due to climate change, we propose that in each forest type, at least 10 hectares of forest be inventoried, and that measurements of the small classes (< 10 cm diameter) as well as large size classes be included. Biomass loss due to crown damage should also be estimated.
Summary • It is suggested that arguments concerning the nature of primary plant strategies could have been resolved more rapidly by reference to older literature relating to the behaviour of solutes in the rhizosphere and by more active programmes of plant trait screening. • The critique of CSR theory in Craine (2005) is rejected largely on the basis that it misunderstands the role of fundamental and proximal controls on vegetation composition (sensu Welden & Slauson 1986). • The ‘way forward’ advocated in Craine (2005) is flawed in its exclusive reliance on competition experiments. Recent progress in community and ecosystem ecology is strongly related to an increasing recognition of the declining importance of competition in unproductive or heavily disturbed environments. Journal of Ecology (2007) 95, 227 –230 doi: 10.1111/j.1365-2745.2006.01163.x
Summary 1 Resource competition theory incorporates the mechanisms that underlie consumer- resource interactions and the trade-offs that constrain these mechanisms. Contrary to assertions by Craine, the concept of R* as the measure of resource reduction and the predictor of resource competition has not changed since it was proposed more than two decades ago. 2 Resource reduction, as summarized in R*, is readily observed. Soil concentrations of nitrate and water are decreased by plant uptake, and are lowered to different levels by different species. Tests have shown R* theory to correctly predict competitive outcomes for a variety of organisms and ecosystems. 3 Consumer-resource mechanisms are a building block for theories that incorporate other trade-offs faced by plants, such as those between competitive ability and dispersal. 4 Numerous plant traits interactively determine R* in a manner predictable from trait- based resource competition theory. The same traits shown by comparative research to be associated with plant dominance in low-nutrient habitats give lower R* values, greater predicted competitive ability and greater predicted abundances in nutrient- limited habitats. 5 Plant ecology needs closer links between analytical theory, observations and exper- iments. Simple verbal theories can generate novel ideas but the logical implications of such scenarios are best explored using the rigorous logic of mathematics. Predictions of theory can then be tested via experiments and comparative studies.
1. Fridley et al. (2007) introduced a technique of species habitat specialization assessment based on co-occurrence analysis of large species-plot matrixes, with a continuous metric (θ value) intended to reflect relative species niche width. 2. They used simulated data in order to demonstrate the functionality of the new method. I repeated their simulation and introduced three alternative scenarios with various patterns of species pool size along a simulated gradient. Results indicated that the co-occurrence based estimation of species niche width is dependent on the size of species pool at the position of species optima. This relationship was also revealed in an analysis of a real data set with Ellenberg indicator values as surrogates for environmental gradients. 3. I introduced a modification of the original algorithm, which corrects the effect of the species pool on the estimation of species niche width: the beta diversity measure based on additive partitioning was replaced with the multiplicative Whittaker's beta. Even after this, the method can satisfactorily recover the real pattern of species specialization only for unsaturated communities with a linear relationship between local and regional species richness. 4. Synthesis. This paper corrects the algorithm for co-occurrence based estimation of species specialization, introduced by Fridley et al. (2007), which was sensitive to the changes in species pool size along environmental gradients.
Summary • The distribution of biodiversity at multiple spatial scales has been traditionally investigated through the additive partitioning of γ-biodiversity (regional) into α-(within-site) and β-(among-site) components. • However, this decomposition is almost exclusively applied using species turnover among communities while two communities with no species in common can be very similar because they share some ‘biological’ similarity. • To overcome this limitation, Hardy & Senterre (2007) (J. Ecol., 95, 493–506) presented a new statistical framework partitioning the phylogenetic diversity into α- and β-components using the Rao's quadratic entropy. • We show that their decomposition is correct only when sites have the same total abundance, otherwise it may lead to negative β-diversity values. As an alternative, we provided a general decomposition of the quadratic entropy into α-, β- and γ-diversities. • Synthesis. We suggest that the ‘biological’ turnover quantified by the β-component of the regional quadratic entropy may help, at least, to disentangle dispersal vs. niche influences on biodiversity patterns.
True (simulation) α values, ‘scaling exponents’, subtracted from values estimated by a fitting method (Δα distributions) vs. sample size for all replicate species. Results of (a) maximum-likelihood estimation (MLE; Russo, Wiser & Coomes 2007) and (b) standardized major axis regression subject to a slope-of-one constraint (SMA slope-of-one; Coomes & Allen 2009) applied to replicate simulations of tree growth that incorporate moderate levels of biological variation in parameter values and measurement error (see text for details). We replicated each of 45 simulated species 100 times while fitting growth parameters individually to each replicate species. Levels of variation were chosen for this moderate case to generate distributions of simulated tree growth similar to the Costa Rican dry forest San Emilio data set (Enquist et al. 1999) while the level of diameter measurement error match estimates from another study of tropical tree growth (Metcalf, Clark & Clark 2009). Blue bars are confidence intervals around means (red dots) while grey ‘xs’ are Δα values for each simulated species. Unbiased parameter estimation results in a Δα mean of zero (horizontal line) while negative values are underestimates and positive values are overestimates.
Summary of simulation results for all biological variation and error scenarios
True (simulation) β values, ‘scaling normalizations’, subtracted from values estimated by a fitting method (Δβ distributions) vs. sample size for all replicate species. Results of (a) maximum-likelihood estimation (MLE; Russo, Wiser & Coomes 2007) and (b) standard major axis regression subject to a slope-of-one constraint (SMA slope-of-one; Coomes & Allen 2009) applied to replicate simulations of tree growth (see legend for a–b and text for details). Blue bars are confidence intervals around means (red dots) while grey ‘x’s are Δβ values for each simulated species. Unbiased parameter estimation results in a Δβ mean of zero (horizontal line) while negative values are underestimates and positive values are over estimates.
1. Coomes & Allen (2009) propose a new statistical method to test the Metabolic Scaling Theory prediction for tree growth rate size scaling (scaling constant α = 1/3) presented in Enquist et al. (1999). This method finds values of the scaling constant that yield standardized major axis (SMA) slopes of one in a comparison of allometrically transformed diameter census data. This SMA ‘slope-of-one’ method produces results that contrast with those generated by maximum-likelihood estimation (MLE; Russo, Wiser & Coomes 2007; Coomes & Allen 2009). 2. We hypothesize that the SMA slope-of-one method is inappropriate for this application because it assumes, unrealistically, that there is no biological or error variance in tree growth size scaling. To test our hypothesis, we simulate ‘allometric’ tree growth with biological and error variance in parameters and measurements. We find that the SMA slope-of-one method is sensitive to the amount of biological and error variance and consistently returns biassed parameter estimates, while the MLE method displays relatively little bias, particularly at larger sample sizes. 3. Synthesis. The conclusions of Coomes & Allen (2009) should be reconsidered in the light of our findings. Investigations of tree growth rate size scaling must consider the influence of biological and error variance in model-fitting procedures to ultimately unravel the effects of tree architecture and ecological factors on patterns of size-dependent growth.
Summary1. Freckleton, Watkinson & Rees (2009) criticize a recent paper by ourselves in this journal (Brooker & Kikvidze 2008) as well as our earlier work on competition importance (Brooker et al. 2005). In response, here we clarify our ideas with the aim of defining more clearly the key points of scientific debate, specifically (i) the definition of the importance of competition and (ii) its measurement.2. Freckleton, Watkinson & Rees (2009) interpret the classic paper by Welden & Slauson (1986) such that importance as a concept relates to long-term, population-level consequences of competition. However, we consider competition importance to be the proportional impact of competition relative to the overall impact of the environment, and our index Cimp expresses changes in competition importance – as defined by ourselves – along productivity gradients. We argue that our definition more accurately reflects the work of Welden & Slauson, as well as a more recent use of the concept (Grace 1991), which precedes the work of Freckleton & Watkinson (2001).3. We highlight that Cimp was never proposed as a general index of competition importance, but is readily applicable in certain circumstances. Notably, our index and the approaches to measuring competition importance as set out by Freckleton, Watkinson & Rees (2009) are not unrelated.4. We also discuss some recent additional responses to both our (2008) paper and that by Freckleton, Watkinson & Rees (2009), including applications of the concept of competition importance. Although the authors of these papers may not have used our index Cimp, they follow the same definitions for the overall concept of competition importance as ourselves.5. Synthesis. We conclude that the complex topic of biotic interactions, including the specific issue of the importance of competition, invites a range of approaches. Importantly, these approaches can be complementary and not conflicting. Here, we propose what we see as a sensible resolution to the current debate concerning the definition of competition importance, a resolution which is backed by the original source article, literature precedent and current usage.
Summary • Resurveys of regional floras allow assessment of long-term floristic change and to pinpoint driving forces behind these changes. Causes of floristic decline derived from such resurveys depend on the considered landscape, but are often associated with human activities especially in urbanized areas. Much could be learned from comparisons of contrasting landscapes. • Vascular plant abundance categories from a detailed, late 19th century flora were matched with distribution data from a late 20th century plant database in a rural area (Thiérache; 1673 km2) in northern France. Floristic change was characterized both in absolute numbers of extinct and extant species and by a measure for relative change in range size. Knowledge of land use changes and ecological correlates of floristic change permitted the causes for species change to be inferred and to identify which traits are associated with species vulnerability. Extinction rates were compared with similar studies from contrasting landscapes, taking into account the negative relationship between standardized extinction rates and log-transformed size of the study area. • Of the 959 species from the historical list, 186 (19.4%) may be considered regionally extinct. Most extinct species were already rare historically. Differences among habitats indicated strong declines for aquatic species and arable weeds and least change for forest species. Percentage species loss per year in Thiérache equalled 0.22, which was higher than two other similar sized predominantly rural landscapes. • Across the main habitat groups, relative species decline was always associated with relatively higher stress tolerance and lower competitiveness and biased towards therophytes. The main causes of species decline were management intensification, eutrophication, secondary succession in semi-natural open habitats and land use change. • Synthesis. Rural landscapes with no population density increase, no urbanization and a stable landscape configuration are believed to be less vulnerable; yet, on a relatively short time scale, without nature conservation measures, these landscapes may suffer strong species loss comparable to that in urbanized landscapes. Effective management of landscapes for biodiversity conservation whether they are urbanized or rural requires the development of a network of nature reserves.
1. Plants are simultaneously attacked by multiple herbivores and pathogens. While some plant defences act synergistically, others trade-off against each other. Such trade-offs among resistances to herbivores and pathogens are usually explained by the costs of resistance, i.e. resource limitations compromising a plant’s overall defence. 2. Here, we demonstrate that trade-offs can also result from direct negative interactions among defensive traits. We studied cyanogenesis (release of HCN) of lima bean (Fabaceae: Phaseolus lunatus) and effects of this efficient anti-herbivore defence on resistance to a fungal pathogen (Melanconiaceae: Colletotrichum gloeosporioides). 3. Leaf tissue destruction by fungal growth was significantly higher on high cyanogenic (HC) lima bean accessions than on low cyanogenic (LC) plants. The susceptibility of HC accessions to the fungal pathogen was strongly correlated to reduced activity of resistance-associated polyphenol oxidases (PPOs) in leaves of these plants. LC accessions, in contrast, showed high PPO activity, which was correlated with distinct resistance to C. gloeosporioides. 4. Experimentally applied, gaseous HCN reduced PPO activity and significantly increased the size of lesions caused by C. gloeosporioides in LC leaves. 5. Field observations of a wild lima bean population in Mexico revealed a higher infection rate of HC compared to LC plant individuals. The types of lesions observed on the different cyanogenic plants in nature were similar to those observed on HC and LC plants in the laboratory. 6. Synthesis. We suggest that cyanogenesis of lima bean directly trades off with plant defence against fungal pathogens and that the causal mechanism is the inhibition of PPOs by HCN. Our findings provide a functional explanation for the observed phenomenon of the low resistance of HC lima beans in nature.
Summary 1. This account presents information on all aspects of the biology of Orchis mascula (L.) L. (early purple orchid) relevant to understanding its ecological characteristics and behaviour. The main topics are presented within the framework of the Biological Flora of the British Isles: distribution, habitat, communities, responses to biotic factors, responses to environment, structure and physiology, phenology, floral and seed characteristics, herbivores and disease, history and conservation. 2. Orchis mascula is a native herb of the British flora. It is mostly found in woodland, copses, grasslands and open pastures, mostly on neutral or base-rich soils. It can also occur in hedgerows, scrub, on roadsides and railway banks, in grikes on limestone pavement and on moist cliff ledges. It is absent from very acid or very wet sites. It tolerates a sparse to moderately dense canopy, but it does not flower in deep shade. 3. Orchis mascula is a non-bulbous geophyte with little or no capacity for vegetative spread. The main perennating organ is a tuber (strictly a rootstem tuber). In most years the tuber generates a rosette of expanded leaves, and, at the end of every year of the plant's life, the tuber is replaced by at least one new tuber. Dormancy, the failure of above-ground parts to appear in a growing season, and the subsequent reappearance of full-sized photosynthetic plants in subsequent seasons, has been observed, but does not last longer than 1 year. The species is long-lived: it takes at least 4 years from first appearance above-ground to achieve flowering for the first time. The maximum recorded lifetime after first appearance is 13 years. 4. Orchis mascula is not autogamous and pollinators are necessary for successful pollination and fruit set. Flowers are nectarless, and pollinated by deceit, mainly by bumblebees and solitary bees. Natural levels of fruit set are usually < 20%. Hand-pollination can increase fruit set to approxi- mately 80%, indicating that seed production is strongly pollen-limited. The lowest four flowers are more likely to be pollinated than the upper flowers, probably because pollinators learn to avoid the species after discovering that it offers no reward. 5. As in most other European countries, Orchis mascula has declined in the British Isles, although it is not at threat of extinction at a national level. Most sites from which it has been lost are in central England and Scotland. Most losses have been caused by woodland clearance and coniferization, intensification of grassland management and ploughing. The cessation of traditional coppicing practises has also led to a decline in the abundance of O. mascula . Since the species is slow to colonize new forest stands or grasslands, management should focus mainly on conservation of ancient forest habitats and grasslands in which fertility is moderate to low and grazing is absent or low in intensity. Restoration of traditional coppicing practices could also lead to higher chances of the species flowering and surviving in the long-term.
1 We tested the hypothesis that competitive hierarchies are invariant with respect to changing nutrient supply. 2 The competitive performance of 26 shoreline plant species was determined experimentally as the relative ability to suppress the growth of a common indicator (phytometer) species, Penthorum sedoides. Each species was grown with the phytometer under each of two nutrient treatments created with different concentrations of a modified Hoagland’s solution (n = 5 replicates per species/treatment), for two growing seasons. 3 Although shifts in ranking of relative competitive performance were apparent between nutrient levels, competitive performance under high and low nutrient conditions was significantly correlated in both year 1 (r = 0.65) and year 2 (r = 0.76), when all species were considered. 4 At the broad community scale, the outcome of competitive interactions thus appears to be relatively predictable and independent of the environment, and therefore provides a useful tool for exploring and understanding community pattern. These results cannot address questions related to the outcome of competitive interactions between similar species or the effects of fine-scale pattern.
1. Understanding how vital rates and reproductive value change with age is fundamental to demography, life history evolution and population genetics. The universality of organism senescence has been questioned on both theoretical and empirical grounds, and the prevalence and strength of senescence remain a controversial issue. Plants are particularly interesting for studies of senescence since individuals of many species have been reported to reach very high ages. 2. In this study, we examined whether the herb Borderea pyrenaica, known to reach ages of more than 300 years, experiences senescence. We collected detailed demographic information from male and female individuals in two populations over 5 years. An unusual morphological feature in this species enabled us to obtain exact age estimates for each of the individuals at the end of the demographic study. 3. We used restricted cubic regression splines and generalized linear models to determine nonlinear effects of age and size on vital rates. We then incorporated the effects of age and size in integral projection models of demography for determining the relationship between age and reproductive value. As the species is dioecious, we performed analyses separately for males and females and examined also the hypothesis that a larger reproductive effort in females comes at a senescence cost. 4. We found no evidence for senescence. Recorded individuals reached 260 years, but growth and fecundity of female and male individuals did not decrease at high ages, and survival and reproductive value increased with age. The results were qualitatively similar also when accounting for size and among-individual vital rate heterogeneity, with the exception that male flowering probability decreased with age when accounting for size increases. 5.Synthesis. Overall, our results show that performance of both male and female plants of B. pyrenaica may increase rather than decrease at ages up to several centuries, and they support the notion that senescence may be negligible in long-lived modular organisms. This highlights the need to explore mechanisms that enable some species to maintain high reproductive values also at very high ages and to identify the evolutionary reasons why some organisms appear to experience no or negligible senescence.
1 Two palaeoecological data sets were used to study forest development in the boreo-nemoral zone of southern Sweden during the last 3000 years. Maps of forest types present in 1250 bc, ad 500 and today were compiled from regional pollen data and these were compared with 16 stand-scale pollen analyses. 2 The forest type maps showed a transition from mixed deciduous forest to coniferous forest consisting chiefly of Picea and Pinus. The stand-scale studies recorded the same general development despite site-specific trends. A detrended correspondence analysis displayed the successional trends of the stand-scale sites. All stands moved away from the rich deciduous forest represented by Alnus, Corylus, Quercus and Tilia via Betula and Carpinus to Picea and Pinus forest or, in two stands, to Fagus forest. 3 A rate of change analysis covering the last 3000 years showed that the changes recorded from the last 150 years were the most rapid, but represented the culmination of a transformation that was initiated 850 years earlier. These recent changes completely overshadowed the previous record. 4 The regional maps recorded relatively high proportions of Pinus in the eastern part of the study area throughout the period under investigation. The stand-scale studies indicated that this area had a high fire frequency, while the relatively recent increase in Pinus in south-west Sweden was better explained by anthropogenic influences. The stand-scale data suggested that the regional role of Pinus had been overestimated in southern Sweden 5 Corylus, Quercus and Tilia were the major species in the former forests, but began a slow decrease in importance around ad 700. The increasing cereal pollen record was related to the decline of the deciduous forest component, suggesting that anthropogenic activity has been the major driving force in its loss. 6 The combination of regional- and stand-scale studies has provided new insights into forest pattern and process. The local records capture the essential features of the regional record of vegetation history, record forest composition more faithfully than regional sites, and additionally yield insight into processes such as fires that have a regional significance.
Top-cited authors
John Philip Grime
  • The University of Sheffield
Ragan M Callaway
  • University of Montana
Johannes H C Cornelissen
  • Vrije Universiteit Amsterdam
Martin Hermy
  • KU Leuven
Stephen Hubbell
  • University of California, Los Angeles