The loss of biodiversity can greatly influence ecosystem functions. Hence understanding the mechanisms behind plant species coexistence and maintenance of plant species diversity has important implications for biodiversity conservation and ecosystem function improvement. Environmental heterogeneity has long been thought to promote plant species coexistence and plant species diversity through increasing niche availability. Theoretical and observational studies have supported this hypothesis. However, convincing evidence for the heterogeneity-diversity hypothesis delivered by appropriate experiments is very scarce. The aim of my thesis was to examine the effects of different types of soil heterogeneity, i.e. the spatial variation in soil nutrients, soil pH and plant-soil feedbacks, on plant competitive interactions, species coexistence and plant species diversity.
As a first step, I tested the potential of soil nutrient heterogeneity to promote plant species coexistence. In an experimental garden, two plant species with contrasting growth forms (i.e. phalanx vs. guerilla growth form) were planted in monocultures and in mixtures on homogenous and heterogeneous substrates consisting of low and high nutrient soil patches. In the plant mixtures, the two plants were either evenly distributed or planted in a clustered pattern. After two growing seasons, I found that soil nutrient heterogeneity increased the competitive ability of the competitive inferior species (guerilla growth form) and decreased that of the competitive superior species (phalanx growth form). The species with the guerilla strategy benefited more from the heterogeneous soil environment by selectively growing in high nutrient soil patches within heterogeneous soils. Apparently, soil nutrient heterogeneity does have the potential to promote plant species coexistence by slowing down the competitive exclusion process.
Further, I tested whether soil nutrient heterogeneity can promote plant species diversity in an experimental community and whether these effects depend on the spatial scale at which species diversity (focal scale) and soil heterogeneity (grain size or patch size) are measured. I found that horizontal soil heterogeneity in soil nutrients reduced plant species richness, regardless of the patch size of the heterogeneity treatments. This decline in species richness is likely because the dominant species benefited from the heterogeneous environment, and outcompeted other plant species. Moreover, vertical heterogeneity in soil nutrients also reduced plant species diversity when high nutrient soil was located in the bottom layer. In this case only deep-rooting plant species had access to high nutrient soil, and could outcompete other plant species that cannot utilize the resources in the deeper layers. Spatial heterogeneity in soil nutrients only influenced species diversity when determined at the 10 cm × 10 cm patch scale but not when determined at the 40 cm × 40 cm plot scale. Therefore, soil nutrient heterogeneity indeed influenced plant species richness but the effect was negative in these experimental communities. Such negative effects were more common when species diversity was quantified at the small scale, but diversity were not influenced by the spatial scale at which soil heterogeneity was measured (i.e. patch size), at least at the spatial scales used in this thesis.
In a parallel experiment, I repeated the horizontal heterogeneity experiment by manipulating soil pH. I found that soil heterogeneity in pH promoted plant species diversity when the soil pH patches were large, and that heterogeneous pH soil with large patches sustain a higher species diversity than heterogeneous pH soil with small patches, even though these effects were only significant at the final harvest. This is likely because heterogeneous pH soil with large patch sizes provided refuges for the subordinate and rare species. Such positive effects were only significant when species diversity was quantified at a small (10 cm × 10 cm patch) scale. These results thus support the classic heterogeneity-diversity hypothesis, and highlight the importance of spatial scales at which species diversity and soil heterogeneity are measured in heterogeneity-diversity relationships.
Subsequently, I focused on the effects of biotic factors, i.e. plant-soil feedbacks and spatial variation in plant-soil feedback on plant species coexistence. I examined how the abundance of a species in a plant community consisting of two species in long-term field plots (conditioning phase), via plant-soil feedbacks influences competition between these two species when they grow later in a greenhouse experiment (test phase) on soils collected from the field experiment. There was a negative relationship between the abundance of a species in the field plot and its relative competitiveness in the greenhouse experiment, probably due to allelopathic effects because this relationship was also true after elimination of soil biota. This negative density-dependent feedback effect varied between plant species, yet it has the potential to promote the coexistence of competing plant species through preventing the dominance of particular plant species.
Using soils collected from monoculture plots in the field experiment, in a greenhouse experiment, I created heterogeneous soil consisting of discrete patches of “own” soil (conditioned by the same plant species as the focal species) and “foreign” soil (conditioned by another plant species), and homogeneous soil where the “own” and “foreign” soils were evenly mixed. The difference in growth between the two competing plant species was smaller in heterogeneous soil than in homogenous soil, indicating that spatial heterogeneity in plant-soil feedback can reduce plant growth differences so that it may promote plant species coexistence. I also found that both competing plant species grew better in “foreign” soil patches than in “own” soil patches within the heterogeneous soil treatment. This can contribute to reduced growth differences in spatially heterogeneous plant-soil feedback conditions.
In conclusion, soil heterogeneity had an important influence on plant species coexistence and plant species diversity. However, the effects varied depending on the type of soil factors that were manipulated, as well on the spatial scales at which species diversity and soil heterogeneity were measured. Soil nutrient heterogeneity can promote plant species coexistence through equalizing the competitive ability between competing plant species, and reduce plant species diversity by promoting the dominance of particular plant species. Heterogeneity in soil pH may promote plant species diversity when the patch size of soil heterogeneity is sufficiently large by proving refuges for subordinate plant species. Plant-soil feedback is a key process in influencing plant species coexistence. These negative density-dependent feedbacks can promote plant species coexistence by preventing the predominance of one of the plant species. The spatial variation in plant-soil feedbacks also has the potential to promote coexistence by reducing the growth inequalities between the competing plant species. Future studies integrating spatial heterogeneity in different soil properties at various spatial scales are urgently needed to guide the restoration of plant species diversity.