Do plants need niches? Some recent developments in plant community ecology

Coupled plant traits adapted to wetting/drying cycles of substrates co-define niche multidimensionality

Theories attempting to explain species coexistence in plant communities have argued in favour of species' capacities to occupy a multidimensional niche with spatial, temporal and biotic axes. We used the concept of hydrological niche segregation to learn how ecological niches are structured both spatially and temporally and whether small scale humidity gradients between adjacent niches are the main factor explaining water partitioning among tree species in a highly water-limited semiarid forest ecosystem. By combining geophysical methods, isotopic ecology, plant ecophysiology and anatomical measurements, we show how coexisting pine and oak species share, use and temporally switch between diverse spatially distinct niches by employing a set of functionally coupled plant traits in response to changing environmental signals. We identified four geospatial niches that turned into nine, when considering the temporal dynamics of the wetting/drying cycles in the substrate and the particular plant species adaptations to garner, transfer, store and use water. Under water scarcity, pine and oak exhibited water use segregation from different niches, yet under maximum drought when oak trees crossed physiological thresholds, niche overlap occurred. The identification of niches and mechanistic understanding of when and how species use them will help unify theories of plant coexistence and competition.

Towards intercrop ideotypes: non-random trait assembly can promote overyielding and stability of species proportion in simulated legume-based mixtures

BACKGROUNDS AND AIMS

A major challenge when supporting the development of intercropping systems remains the design of efficient species mixtures. The ecological processes that sustain overyielding of legume-based mixtures compared to pure crops are well known, but their links to plant traits remain to be unravelled. A common assumption is that enhancing trait divergence among species for resource acquisition when assembling plant mixtures should increase species complementarity and improve community performance.

METHODS

The Virtual Grassland model was used to assess how divergence in trait values between species on four physiological functions (namely light and mineral N acquisition, temporal development, and C-N use efficiency) affected overyielding and mixture stability in legume-based binary mixtures. A first step allowed us to identify the model parameters that were most important to interspecies competition. A second step involved testing the impact of convergent and divergent parameter (or trait) values between species on virtual mixture performance.

RESULTS

Maximal overyielding was achieved in cases where trait values were divergent for the physiological functions controlling N acquisition and temporal development but convergent for light interception. It was also found that trait divergence should not affect competitive abilities of legume and non-legumes at random. Indeed, random trait combinations frequently led to reduced mixture yields when compared to a perfectly convergent neutral model. Combinations with the highest overyielding also tended to be associated with mixture instability and decreasing legume biomass proportion. Achieving both high overyielding and mixture stability was only found to be possible under low or moderate N levels, using combinations of traits adapted to each environment.

CONCLUSIONS

No simple assembly rule based on trait divergence could be confirmed. Plant models able to infer plant-plant interactions can be helpful for the identification of major interaction traits and the definition of ideotypes adapted to a targeted intercropping system.

Multi-scale analysis on species diversity within a 40-ha old-growth temperate forest

In order to better explore the maintenance mechanisms of biodiversity, data collected from a 40-ha undisturbed Pinus forest were applied to the Individual Species-Area Relationship model (ISAR) to determine distribution patterns for species richness. The ecological processes influencing species abundance distribution patterns were assessed by applying the same data set to five models: a Log-Normal Model (LNM), a Broken Stick Model (BSM), a Zipf Model (ZM), a Niche Preemption Model (NPM), and a Neutral Model (NM). Each of the five models was used at six different sampling scales (10 m × 10 m, 20 m × 20 m, 40 m × 40 m, 60 m × 60 m, 80 m × 80 m, and 100 m × 100 m). Model outputs showed that: (1) Accumulators and neutral species strongly influenced species diversity, but the relative importance of the two types of species varied across spatial scales. (2) Distribution patterns of species abundance were best explained by the NPM at small scales (10 m-20 m), whereas the NM was the best fit model at large spatial scales. (3) Species richness and abundance distribution patterns appeared to be driven by similar ecological processes. At small scales, the niche theory could be applied to describe species richness and abundance, while at larger scales the neutral theory was more applicable.

Ellenberg's water table experiment put to the test: species optima along a hydrological gradient

An important aspect of niche theory is the position of species' optima along ecological gradients. It is widely believed that a species' ecological optimum takes its shape only under competitive pressure. The ecological optimum, therefore, is thought to differ from the physiological optimum in the absence of interspecific competition. Ellenberg's Hohenheim water table experiment has been very influential in this context. However, the water table gradient in Ellenberg's experiment was produced by varying the soil thickness above the water table, which confounded the potentially disparate impacts of water table depth (WTD) and soil depth on species growth. Accordingly, here we have re-evaluated Ellenberg's work. Specifically, we tested the hypothesis that physiological and ecological optima are identical and unaffected by interspecific interaction. We used the same six grasses as in Ellenberg's experiments, but in our mesocosms, WTD was varied but soil depth kept constant. The design included both an additive component (with/without plant interaction) and a substitutive component (monocultures vs. species mixtures). The results show that the physiological optima along the hydrological gradient varied greatly between species, even in the absence of interspecific interaction. Within species, however, physiological and ecological optima appeared identical in most cases, irrespective of the competition treatment. We conclude that the 'physiological capacity' of species largely determines where they are able to persist and that any impact of interspecific interaction is only marginal. These findings are at variance with Ellenberg's rule, where competition is considered to shift the distribution of a species away from its physiological optimum.

Effects of dispersal on total biomass in a patchy, heterogeneous system: Analysis and experiment

An intriguing recent result from mathematics is that a population diffusing at an intermediate rate in an environment in which resources vary spatially will reach a higher total equilibrium biomass than the population in an environment in which the same total resources are distributed homogeneously. We extended the current mathematical theory to apply to logistic growth and also showed that the result applies to patchy systems with dispersal among patches, both for continuous and discrete time. This allowed us to make specific predictions, through simulations, concerning the biomass dynamics, which were verified by a laboratory experiment. The experiment was a study of biomass growth of duckweed (Lemna minor Linn.), where the resources (nutrients added to water) were distributed homogeneously among a discrete series of water-filled containers in one treatment, and distributed heterogeneously in another treatment. The experimental results showed that total biomass peaked at an intermediate, relatively low, diffusion rate, higher than the total carrying capacity of the system and agreeing with the simulation model. The implications of the experiment to dynamics of source, sink, and pseudo-sink dynamics are discussed.

A need for niches?

The idea that different species must have distinct ecologies if they are to coexist has been challenged recently by the claim that some models involving stochastic factors or clumped spatial distributions permit stable coexistence of species that are identical or differ only in competitive ability. However, these models have been misinterpreted; except in rather limited circumstances, they provide further support for the notion that species must be sufficiently ecologically distinct to coexist stably. The possible, limited, exceptions to this rule involve social factors by which individuals of a species discriminate between heterospecifics and conspecifics without there being any true ecological differences between species.

Assessing uncertainties in a second-generation dynamic vegetation model caused by ecological scale limitations

*Second-generation Dynamic Global Vegetation Models (DGVMs) have recently been developed that explicitly represent the ecological dynamics of disturbance, vertical competition for light, and succession. Here, we introduce a modified second-generation DGVM and examine how the representation of demographic processes operating at two-dimensional spatial scales not represented by these models can influence predicted community structure, and responses of ecosystems to climate change. *The key demographic processes we investigated were seed advection, seed mixing, sapling survival, competitive exclusion and plant mortality. We varied these parameters in the context of a simulated Amazon rainforest ecosystem containing seven plant functional types (PFTs) that varied along a trade-off surface between growth and the risk of starvation induced mortality. *Varying the five unconstrained parameters generated community structures ranging from monocultures to equal co-dominance of the seven PFTs. When exposed to a climate change scenario, the competing impacts of CO(2) fertilization and increasing plant mortality caused ecosystem biomass to diverge substantially between simulations, with mid-21st century biomass predictions ranging from 1.5 to 27.0 kg C m(-2). *Filtering the results using contemporary observation ranges of biomass, leaf area index (LAI), gross primary productivity (GPP) and net primary productivity (NPP) did not substantially constrain the potential outcomes. We conclude that demographic processes represent a large source of uncertainty in DGVM predictions.

Specialization for canopy position by hemiepiphytic Ficus species in a Bornean rain forest

The growth habits of five species of hemiepiphytic fig trees (Ficus subgenus Urostigma) were compared to see if any species occupied distinct canopy positions or showed unique host preferences in lowland rain forest in Gunung Palung National Park, Indonesian Borneo. Highly significant differences among species were found in the height of establishment on hosts, the level at which fig crowns were deployed, and the size and taxa of host trees. Ficus stupenda, F. dubia, and F. subtecta all had mean establishment heights between 29 and 31m above the ground, had their crowns in the upper canopy, and predominantly occupied large trees in the Dipterocarpaceae. F. consociata and F. xylophylla established at significantly lower mean heights of 24 and 18 m respectively, had their crowns predominantly in lower canopy layers, and disproportionately occupied smaller non-dipterocarp trees. These patterns were consistent both within lowland forest on alluvial terraces, which was most intensively sampled, and when data were combined from three other contiguous lowland forest habitats. Ficus species appear to be specialized to some extent for distinct microhabitats within the canopy.

Indicating appropriate groundwater tables for desert river-bank forest at the Tarim River, Xinjiang, China

Based on data collected over 2 years of monitoring the lower reaches of the Tarim River, the groundwater table depth was divided into six classes; 0 to 2 m, 2 to 4 m, 4 to 6 m, 6 to 8 m, 8 to 10 m, >10 m. We investigated the vegetation in this area to measure the influence of groundwater table depth on plant diversity and species ecological niche. The results indicated that plant diversity was highest at the 2 to 4 m groundwater table depth, followed by that at 4 to 6 m, and then that at 0 to 2 m. When the groundwater depth dropped to below 6 m, species diversity decreased dramatically, and the slope of Hill's index tended to level off. The ecological niche of the major species in this area initially expanded as the groundwater level dropped. The widest niche appeared at the 4 to 6 m groundwater table depth and gradually narrowed with deepening groundwater. Ecological niche analysis also revealed that the 4 to 6 m groundwater table depth was associated with the lowest degree of niche overlap and the richest variety of species. Our findings indicate that in the lower reaches of the Tarim River, the groundwater table depth must be a minimum of 6 m for vegetation restoration; it should be maintained at 2 to 4 m in the vicinity of the water path, and at 4 to 6 m for the rest of this arid area.

The Fluctuation Niche in Plants

Classical approaches to niche in coexisting plants have undervalued temporal fluctuations. We propose that fluctuation niche is an important dimension of the total niche and interacts with habitat and life-history niches to provide a better understanding of the multidimensional niche space where ecological interactions occur. To scale a fluctuation niche, it is necessary to relate environmental constrictions or species performance not only to the absolute values of the usual environmental and ecophysiological variables but also to their variances or other measures of variability. We use Mediterranean plant communities as examples, because they present characteristic large seasonal and interannual fluctuations in water and nutrient availabilities, along an episodic-constant gradient, and because the plant responses include a number of syndromes coupled to this gradient.

THE COMPARATIVE EVIDENCE RELATING TO FUNCTIONAL AND NEUTRAL INTERPRETATIONS OF BIOLOGICAL COMMUNITIES

Neutral and functional theories provide rival interpretations of community patterns involving distribution, abundance, and diversity. One group of patterns describes the overall properties of species or sites, and derives principally from the frequency distribution of abundance among species. According to neutral theory, these patterns are determined by the number of individuals of novel type appearing each generation in the community, whereas functional theory relates them to the distribution of the extent of niches. A second group of patterns describes the spatial attributes of communities, and derives principally from the decay of similarity in species composition with distance. Neutral theory interprets these patterns as consequences of local dispersal alone, whereas the functional interpretation is that more distant sites are likely to be ecologically different. Neutral theory often provides good predictions of community patterns, yet is at variance with a wide range of experimental results involving the manipulation of environments or communities. One explanation for this discrepancy is that spatially explicit models where selection is generally weak, or where selection acts strongly on only a few species at each site, have essentially the same output as neutral models with respect to the distribution of abundance and the decay of similarity. Detecting a non-neutral signal in survey data requires careful spatial or phylogenetic analysis; we emphasize the potential utility of incorporating phylogenetic information in order to detect functional processes that lead to ecological variation among clades.