Abstract: Global environmental change threatens to disrupt key processes that structure ecological communities. One such process in plant communities is the interaction between plants and soil microorganisms, which regulates multiple outcomes including ecosystem function and biodiversity maintenance. Microbially mediated plant species coexistence is determined by the relative strength of microbially mediated stabilization and fitness differences, but the strength and direction of microbial effects on plant growth can vary with abiotic context, complicating predictions of species coexistence under global change. Evaluating context-dependence in plant-microbe interactions is especially important in human-modified forests, which encompass considerable environmental heterogeneity over small spatial scales. For example, in closed-canopy forests, habitat fragmentation creates marked gradients in light and water availability from the edge to interior of fragments, directly affecting plants and soil microorganisms, and also their interactions.
To evaluate how habitat fragmentation affects microbially-mediated plant coexistence, we established a pairwise plant-soil feedback (PSF) experiment in the Western Ghats Biodiversity Hotspot in Karnataka, India. Specifically, the experiment evaluates how variation in light and soil moisture levels affects PSF between Symplocos racemosa and Litsia floribunda – species that are commonly found in both edge and interior microsites in fragmented forests. Following the canonical PSF experiment design, we transplanted individual seedlings into pots that were inoculated with soil from conspecific or heterospecific adults. These treatments yield the data to quantify a theory-derived metric for microbially-mediated stabilization. We also included control treatments (no inoculum and a forest-collected but not species-specific inoculum), which we use to quantify microbially-mediated fitness differences. Each soil-by-environment treatment is replicated across six blocks, yielding a total of 192 plants.
As of February 2023, after two months of plant growth, we see interactive effects of environment and soil type on plant performance. For example, the lack of a robust soil community (sterile control) results in substantially lower seedling growth rates in both focal species only in the low-light/low-water treatment, which suggests that soil microbes may help ameliorate drought stress. We plan to complete the experiment by June 2023 (six months of plant growth), before the onset of the monsoon (rainy) season, such that the experiment captures microbial effects on plant growth and survival during the seedlings’ first complete dry season – a critical stage in this system. This study is among the first to quantify edge effects on plant-microbe interactions in tropical forests, and the first study of plant-soil feedback in the Western Ghats.