Kristjan Zobel

The species pool and its relation to species richness: Evidence from Estonian plant communities

Two types of species pool are distinguished. The regional species pool is defined as the set of species, occurring in a certain region (here: Estonia) which are capable of coexisting in a target community. The actual species pool is defined as the set of species present in a community. Field data from 14 different vegetation types in Estonia were used. The regional pool was compiled by including from the regional flora (1) all species for which the Ellenberg indicator values did not differ more than 1.5 relative units from the community mean and (2) all indifferent species. The actual pool was compiled by careful field observations. The aim of the paper is to test the validity of two null hypotheses about the species pool. HO 1 postulates that any size of the actual species pool is equally probable in the interval between zero and the size of the regional species pool. HO 2 postulates that any value of species richness per unit area (1 m 2 ) is equally probable in the interval between zero and the size of the actual species pool. To test the strengths of the relationships Monte Carlo modelling was used. It was shown that the relation between variables was stronger than proposed by the null models (P = 0.041 for HO 1 and P = 0.002 for HO 2 ). Consequently, the size of the actual species pool is largely determined by the regional species pool, and the species richness per 1 m 2 is largely determined by the actual pool. The results are discussed in the framework of coexistence theory. The size of the regional pool is determined by evolutionary (speciation) and historical (large-scale migration) processes. The size of the actual pool depends on local-scale migration, which can be a function of isolation, successional stage, local management history, etc.

Ecological assembly rules in plant communities-approaches, patterns and prospects

Understanding how communities of living organisms assemble has been a central question in ecology since the early days of the discipline. Disentangling the different processes involved in community assembly is not only interesting in itself but also crucial for an understanding of how communities will behave under future environmental scenarios. The traditional concept of assembly rules reflects the notion that species do not co‐occur randomly but are restricted in their co‐occurrence by interspecific competition. This concept can be redefined in a more general framework where the co‐occurrence of species is a product of chance, historical patterns of speciation and migration, dispersal, abiotic environmental factors, and biotic interactions, with none of these processes being mutually exclusive.

Functional species pool framework to test for biotic effects on community assembly

Functional trait differences among species are increasingly used to infer the effects of biotic and abiotic processes on species coexistence. Commonly, the trait diversity observed within communities is compared to patterns simulated in randomly generated communities based on sampling within a region. The resulting patterns of trait convergence and divergence are assumed to reveal abiotic and biotic processes, respectively. However, biotic processes such as competition can produce both trait divergence and convergence, through either excluding similar species (niche differences, divergence) or excluding dissimilar species (weaker competitor exclusion, convergence). Hence, separating biotic and abiotic processes that can produce identical patterns of trait diversity, or even patterns that neutralize each other, is not feasible with previous methods. We propose an operational framework in which the functional trait dissimilarity within communities (FDcomm) is compared to the corresponding trait dissimilarity expected from the species pool (i.e., functional species pool diversity, FDpool). FDpool includes the set of potential species for a site delimited by the operating environmental and dispersal limitation filters. By applying these filters, the resulting pattern of trait diversity is consistent with biotic processes, i.e., trait divergence (FDcomm > FDpool) indicates niche differentiation, while trait convergence (FDcomm < FDpool) indicates weaker competitor exclusion. To illustrate this framework, with its potential application and constraints, we analyzed both simulated and field data. The functional species pool framework more consistently detected the simulated trait diversity patterns than previous approaches. In the field, using data from plant communities of typical Northern European habitats in Estonia, we found that both niche-based and weaker competitor exclusion influenced community assembly, depending on the traits and community considered. In both simulated and field data, we demonstrated that only by estimating the species pool of a site is it possible to differentiate the patterns of trait dissimilarity produced by operating biotic processes. The framework, which can be applied with both functional and phylogenetic diversity, enables a reinterpretation of community assembly processes. Solving the challenge of defining an appropriate reference species pool for a site can provide a better understanding of community assembly.

On the species-pool hypothesis and on the quasi-neutral concept of plant community diversity

5. Conclusions5.1. According to the results from studying a broad variety of Estonian herbaceous communities (4.11) the question in 1.14 should be answered as:selection from a regional species pool into an actual species pool and selection from actual species pool into a microsite are mostlyrandom and neutral processesand they are not directed significantly by interspecific competition.5.2. Yet, the formation of a diversity pattern should be called aquasi-neutral process,mainly because the exclusion of species from communities due to asymmetric light competition is common during succession (when taller species outcompete shorter ones).

Foraging for space and avoidance of physical obstructions by plant roots: a comparative study of grasses from contrasting habitats.

Physical obstructions that reduce space for root growth can profoundly affect plant performance. The aim of this study was to investigate the ability of roots to avoid obstructions and forage for usable space, and to reveal the mechanism involved. Eight grass species from four genera were examined. Each genus included species characteristic of habitats with high and low nutrient availability. The ability to limit root mass and to adjust morphology within substrate containing obstructions in the form of gravel was investigated. A treatment with activated carbon, which adsorbs organic compounds, was used to examine the possible involvement of root exudates in responses to obstructions. Only species characteristic of nutrient-poor habitats restricted placement of root mass in substrate containing obstructions, and this response disappeared in the presence of activated carbon. Root morphological responses to obstructions differed from those shown in response to nutrient-poor conditions or compacted soil. These results suggest that the ability to avoid obstructions is dependent on the sensitivity of roots to their own exudates accumulating in the vicinity of obstructions. This is similar to other behavioural responses in which cues or signals are used to adjust growth before stressful conditions are encountered.

Plants are least suppressed by their frequent neighbours: the relationship between competitive ability and spatial aggregation patterns

Summary 1. Previous studies have concluded that spatial aggregation of conspecifics should benefit weak competitors and put stronger competitors at a disadvantage, thus promoting plant species coexistence. However, if competitive ability is viewed as a behavioural trait, it becomes evident that traits determining spatial patterns and competitive ability could co-evolve, resulting in greater dispersal in stronger competitors and reduced competitive ability in spatially aggregated species. 2. To test this prediction, we combined spatial data from a field survey of seven temperate grassland communities with the results of a common-garden competition experiment involving 28 focal species. 3. We found that species exhibiting strong conspecific aggregation and infrequent heterospecific encounters in the field maintained greater growth in competition with conspecifics than with heterospecifics. In contrast, species that mostly encountered heterospecific neighbours in the field achieved greater growth when surrounded by heterospecific than conspecific neighbours, indicating greater competitive ability. The observed patterns of conspecific aggregation were related to variation in clonal dispersal characteristics and there was a direct positive relationship between clonal dispersal distance and competitive ability. 4. Synthesis. Our study demonstrates that viewing competitive ability as a behavioural trait that imposes different costs and benefits on an individual depending on the identity of its neighbours can provide new insights into the long-debated topic of mechanisms promoting plant species coexistence.

To compete or not to compete: an experimental study of interactions between plant species with contrasting root behaviour

Game-theoretic models predict that plants with root systems that avoid belowground competition will be displaced by plants that overproduce roots in substrate shared with competitors. Despite this, both types of root response to neighbours have been documented. We used two co-occurring clonal species (Glechoma hederacea and Fragaria vesca) with contrasting root responses to neighbours (avoidance of competition and contesting of resources, respectively) to examine whether functional variation in other traits affected the success of each rooting strategy, leading to a different outcome from that predicted on the basis of root behaviour alone. Vegetative propagation rates, morphology and biomass allocation patterns were examined when each species was challenged with competition from physically separate ramets with either the same rooting strategy (intraclonal competition) or the contrasting rooting strategy (interspecific competition). Contrary to the predictions of game-theoretic models, the species that exhibits avoidance of root competition (Glechoma) was not competitively inferior to the species that does not (Fragaria). Glechoma achieved greater total mass in the interspecific treatment than in the intraclonal treatment. However, Fragaria did not experience more intense competition from Glechoma than it did in the intraclonal treatment. Strong interference between the two species appeared to be avoided because Glechoma invested preferentially in rapid exploitation of unoccupied space, whereas Fragaria invested in increasing the competitive ability and local persistence of established ramets. Our results suggest that interspecific trade-offs between traits related to competitive ability and resource exploitation can allow coexistence of species with contrasting rooting behaviours. Full assessment of the adaptive value of different root responses to neighbours therefore requires concurrent consideration of the combined effects of a wide array of functional traits.

Effects of additional illumination and fertilization on seasonal changes in fine-scale grassland community structure

. Fine-scale structure of a species-rich grassland was examined for seasonal changes caused by manipulated changes in the availability of above and below-ground resources (additional illumination with the help of mirrors and fertilization) in a field experiment. If changes induced by fertilization — which are expected to lead to a reduction in small-scale diversity —are due to intensified light competition, they should be compensated for by additional light input. Permanent plots of 40 cm × 40 cm were sampled by the point quadrat method at three angles (60°, 90° and 120° from the horizontal North-South direction), using a laser beam to position the quadrats, in early July and early September. The applied treatments did not cause apparent changes in plant leaf orientation. The degree of spatial aggregation of biomass increased seasonally in fertilized, non-illuminated plots: greater productivity at a constant light supply led to a faster growth rate of potentially dominant species, as compared to the subordinate ones. Additional illumination mitigated this effect of fertilization, indicating that the observed changes in biomass aggregation were due to increased light competition. There was a considerable seasonal decrease of variance ratio (ratio of observed variance of richness at a point and variance expected at random) in fertilized only and in illuminated only plots. In fertilized plots this was due to the positive relationship between biomass aggregation and expected variance of richness. Biomass constancy occurs to be inversely related to deficit in variance of richness. In illuminated plots, in contrast, only the observed variance of richness decreased seasonally, indicating a more uniform use of space by different species. Evidently, a deficit in variance of richness can be caused by drastically different processes, showing that the variance ratio statistic may not have a significant explanatory value in fine-scale community studies.

The space-use strategy of plants with different growth forms, in a field experiment with manipulated nutrients and light

The biomass allocation pattern in plants is known to depend on the below and above-ground resource availabilities. In a herbaceous multi-species stand, it can be expected that the effects of nutrient and light availability on plants’ general space-use strategy are fundamentally different. We hypothesized that nutrient status alters the amount of biomass produced per unit canopy volume (biomass density), but not so much the biomass vertical distribution pattern. Changes in light availability, in contrast, should affect the vertical distribution pattern of biomass but not biomass density. We were also interested in whether the effect of resource manipulation on a plant’s space-use strategy depends on its basic morphological characteristics (growth form).The results from a four-year permanent plot experiment in a species-rich grassland, with fertilization and additional illumination from mirrors applied to 40 × 40 cm plots, showed that our main hypothesis was correct. Fertilization significantly affected biomass density above as well as below-ground, while additional illumination generally did not. Light addition altered the vertical distribution pattern of above-ground biomass, which remained unaffected by the fertilizer treatment.The effects of resource manipulations on plants’ space-use strategy were strongly dependent on the basic morphological traits of species. Plants with a leafy stem (grasses and upright forbs) could significantly increase their above-ground biomass density with fertilization, while the ones without a leafy stem (sedges and rosette-forming forbs) could not. Additional illumination, however, significantly increased the amount of biomass per unit canopy volume only in species with narrow leaves (graminoids). The space-use strategy and vertical allocation pattern of extra resources clearly depend on the balance between light and nutrient resources and on plants’ growth forms.

An experimental test of diversity maintenance mechanisms, by a species removal experiment in a species-rich wooded meadow

Most theories of plant species coexistence assume the presence of diversity maintenance mechanisms, i.e. mechanisms enhancing species richness in a community. We wished to determine whether such a mechanism was operating by establishing a field experiment in the species-rich wooded meadow of Laelatu, western Estonia. Ten to seventeen subordinate species were removed periodically (for 4 years) from 10 permanent plots of 1×1 m (each plot had its specific list of excluded species; 10 plots served as control). Since the removed species were all subordinate ones, very little biomass was removed, but at the same time richness was reduced by 25–33%. If some diversity maintenance mechanism was operating, we would expect that immigration of other subordinate species would restore the original species richness.It was not possible to reject the null hypothesis of an identical immigration rate of new species into manipulated and control plots. The rate of small-scale species turnover was not affected by the removal of subordinate species. Interrelations of five richness characteristics were studied, by comparing empirical correlations among them, with those expected from a null model of random migration of species. The immigration rate of new species appeared to be related to the number of constant species, and immigration/extinction balance related to initial richness, more strongly than predicted by the null model. In the manipulated series these relationships matched the expectation from the null model. While the results generally support the so-called species pool hypothesis (and the carousel model), it seems that species small-scale turnover depends on the richness pattern in the studied grassland. In the case of plots with artificially reduced richness no departures can be detected from the random migration hypothesis.