Above-and Below-ground Plant-plant Interactions And Their Effects On Biomass-density Relationship In Plant

Plant-plant interactions (positive and negative) and population density regulating are central to ecology. Plant-plant interactions are the main mechanisms underlying density regulation, while density regulation is the presence of plant-plant interactions. Most previous studies on plant-plant interactions and population density regulation used only aboveground biomass to represent the whole individual, and ignored the distinction and interplay between different plant components (e.g., above-and below-ground parts). In this study, using combined approaches of model simulations, Chinese forest database and field investigation; we explored how above-and below-ground plant-plant interactions and their effects on biomass-density relationship in plant populations shift along the environmental gradients. Using two-layer "zone-of-influence" models, we first explored the interplay between above-and below-ground plant-plant interactions along an environmental gradient. Furthermore, we investigated the biomass-density relationship of different plant components across forest and shrub communities in China.1. Developping new two-layer zone-of-influence models and applying them to simulate above-and below-ground plant-plant interactionsPrevious one-layer zone-of influence model did not simulate shoot and root growth separately. Because competitive resources and mechanisms for shoots and roots are fundamentally different, we further extended the one-layer model to two layers. Here we used two-layer "zone-of-influence" models to simulate the effects of facilitation, size-asymmetry of competition, abiotic stress, resource availability and the balance of root-shoot growth on shoot and root interactions and their interplay along an environmental gradient. In the absence of facilitation, shoot and total competition became weaker, while root competition and the interplay between shoot and root competition were unchanged under increasing stress when root competition was completely symmetric. In contrast, shoot, root, total interactions and the interplay between shoot and root interactions were all negative, and they increased with increasing stress when root competition was size-symmetric. When facilitation was included in the models, net effects of shoot, root, total interactions and the interplay of root-shoot interactions were very different from those without facilitation, and many were positive under highly stressful conditions. The type of stress (non-resource or resource) did not significantly influence the simulation results. Our study provides an alternative interpretation of the interplay between above-and below-ground plant-plant interactions across an environmental gradient.3. Different above-and below-ground competition and optimal partitioning theory induce the different biomass-density relationship for different plant components in forest and shrub communitiesWe investigated the self-thinning lines for different plant components across forest and shrub communities in China. Neither the classical self-thinning rule (-3/2exponent), nor the universal scaling rule (-4/3exponent) can apply to all the self-thinning relationships for above-ground, below-ground and total biomass across these forest communities. There was no constant predicted value (e.g.,-1/2or-1/3for Ms;-1/2,-1/3or0for MB and ML) that can describe all the the scaling exponents for stem, branch and leaf biomass. Furthermore, in our study, the exponents of above-and below-ground biomass-density relationships in arid areas were greater than those in humid areas (smaller absolute value), these changes can be described by a power-function. The self-thinning lines for below-ground biomass were flatter than those for above-ground biomass across most of these forest communities. Stem biomass-density lines were steeper than corresponding branch and leaf ones across most of these forest communities. Below-ground biomass-density lines were steeper than those for above-ground biomass across most of shrub communities. This discrepancy can be explained in terms of the optimal partitioning theory. Our study provides a new mechanism underlying the variant biomass-density relationships along an environmental gradient.In summary, our results indicated that above-and below-ground plant-plant interactions and their effects on biomass-density relationship in plant communities shift along the environmental gradients. The mechanisms underlying these shifts are that:the interplay between above-and below-ground plant-plant interactions shift along an environmental gradientâ†'the difference between above-and below-ground self-thinning linesâ†'the difference among stem, branch and leaf biomass-density relationships.