Recent studies have shown that accounting for intraspecific trait variation (ITV) may better address major questions in community ecology. However, a general picture of the relative extent of ITV compared to interspecific trait variation in plant communities is still missing. Here, we conducted a meta-analysis of the relative extent of ITV within and among plant communities worldwide, using a data set encompassing 629 communities (plots) and 36 functional traits. Overall, ITV accounted for 25% of the total trait variation within communities and 32% of the total trait variation among communities on average. The relative extent of ITV tended to be greater for whole-plant (e.g. plant height) vs. organ-level traits and for leaf chemical (e.g. leaf N and P concentration) vs. leaf morphological (e.g. leaf area and thickness) traits. The relative amount of ITV decreased with increasing species richness and spatial extent, but did not vary with plant growth form or climate. These results highlight global patterns in the relative importance of ITV in plant communities, providing practical guidelines for when researchers should include ITV in trait-based community and ecosystem studies.

Following catastrophic disturbances, succession and vegetation development occur, but in the prolonged absence of these disturbances a decline (retrogressive) phase follows in which nutrient availability and tree biomass declines considerably. We measured plant diversity across six long-term chronosequences that each included retrogressive stages in Australia, New Zealand, Alaska, Hawaii and Sweden. In contrast to theories predicting negative or hump-shaped responses of tree diversity to biomass or soil fertility, tree species richness often peaked coincidentally with tree basal area (a surrogate of tree biomass), and declined during retrogression. Similar patterns were found regardless of whether or not species richness estimates were rarefraction-adjusted to correct for variation in stem densities across plots. The Shannon-Weiner diversity index sometimes showed the same pattern, but in two chronosequences was least when tree basal area peaked; this was driven by the domination of total basal area by single tree species in both cases. The decline in tree diversity during retrogression was often associated with reduced relative amounts of total phosphorus in soil. In contrast, total vascular plant species richness often increased during retrogression. These results demonstrate that forests with high tree diversity and biomass do not persist indefinitely in the long-term absence of catastrophic disturbance, and that similar patterns occur across the boreal, temperate and subtropical zones.

Summary1. Ecosystem retrogression occurs during the very long-term absence of major disturbances, and it is characterized by decreases in productivity, decomposition rates and nutrient availability. Ratios of total soil nitrogen (N) to phosphorus (P) also characteristically increase during retrogression, but the nature of N inputs to ecosystems undergoing retrogression has seldom been explored. 2. We studied a 5000-year-old chronosequence involving 30 islands that differed greatly in history of disturbance (wildfire through lightning strike), with increasing time since disturbance leading to ecosystem retrogression. For each island, we quantified N inputs through biological fixation by cyanobacteria hosted by each of two feather moss species that dominate the ground layer vegetation ( Pleurozium schreberi and Hylocomium splendens ), and compared these with N inputs through atmospheric deposition. 3. Both N 2 fixation per unit land area and fixation per unit moss mass increased significantly with increasing time since disturbance for both moss species. As retrogression progressed, the amount of total N input through biological fixation increased to levels comparable to that of input through atmospheric deposition. 4. Across the chronosequence, N has been accumulating in the humus layer at a rate of 1·8 kg ha -1 year -1 in the absence of fire during the past 5000 years. The added N input from biological fixation in this area of low atmospheric N deposition helps explain this relatively high rate of sequestration. 5. Our results show that, contrary to several claims in the literature, biological N 2 fixation is not only important in early-successional ecosystems but also in late-successional systems that have undergone retrogression. This fixation can contribute both to the elevated N : P ratios that occur during retrogression and to accumulation of N capital in the soil. However, much of this N may exist in forms that are relatively unavailable to co-existing plant species.

Summary 1.There has been much debate about how losses of species and functional groups may affect the invasibility of vegetation, but little is understood about how invasibility differs across ecosystems or is driven by environmental context. 2. We studied the invasibility of field plots in two ongoing removal experiments set up across thirty lake islands in northern Sweden. These islands differ in size, and therefore soil fertility and productivity. One experiment involves full factorial removal of three functional groups (dwarf shrubs, mosses and tree roots), and the other involves full factorial removal of three species of dwarf shrub ( Vaccinium myrtillus , V. vitis-idaea and Empetrum hermaphroditum ). 3. We investigated the effects of removal treatments in both experiments on the invasibility of each of three species ( Betula pubescens , Pinus sylvestris and Picea abies ). This included a seed sowing study, and a seedling planting study, for each of the three species. 4. For the functional group experiment, removal of shrubs promoted invasibility by all species, and removal of mosses also had positive effects. For the species removal experiment, the two Vaccinium species exerted the strongest effects against invasibility. The floristic components that had the greatest effects represented only a small proportion of total plant biomass. 5. The effects of the removal of shrubs (or of either Vaccinium species) on invasibility often varied across island size classes. In these cases, removals usually had the greatest positive effects on the largest and most productive islands. In contrast, the effects of moss removals on P. sylvestris seedling survival were greatest on small islands. 6. These results show clearly that the effects of loss of components of the resident flora (at either the functional group or species level) on invasibility at the plot scale are context dependent, and can vary greatly across ecosystems. 7. Synthesis . Our results contribute to the ongoing debate about how loss of species and functional groups influences community-level processes, by showing that the effects of loss of resident biota on invasion of new species depends on the attributes of the biota that are lost and the ecosystems that they are lost from.

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