Intraspecific variation in Janzen-Connell effect is mediated by stress and plant-soil feedbacks

Janzen-Connell (JC) effects, hypothesized to be mostly driven by negative plant-soil feedbacks (PSFs), are considered to be the key mechanism that regulates tropical forest plant diversity and coexistence. However, intraspecific variation in JC effects may weaken this mechanism, with the strength of PSFs being a potentially key variable process. We conducted a manipulated experiment with seedlings from two populations of Pometia pinnata (Sapindaceae), a tropical tree species in southwest China. We aimed to measure the intraspecific difference in PSF magnitude caused by inoculating the soil from different P. pinnata source populations and growing seedlings under differing light intensity and water availability treatments, and at varying plant densities. We found negative PSFs for both populations with the inoculum soil originating from the same sites, but PSFs differed significantly with the inoculum soil from different sites. PSF strength responded differently to biotic and abiotic drivers; PSF strength was weaker in low moisture and high light treatments than in high moisture and low light treatments. Our study documents intraspecific variation in JC effects: specifically, P. pinnata have less defences to their natively-sourced soil, but are more defensive to the soil feedbacks from soil sourced from other populations. Our results imply that drought and light intensity tended to weaken JC effects, which may result in loss of species diversity with climate change.

詹森-康奈尔效应（Janzen-Connell effects，简称JC效应）被认为主要由负植物-土壤反馈（plant-soil feedbacks，简称PSFs）驱动，是调控热带森林植物多样性与物种共存的核心机制。然而，JC效应的种内变异可能会削弱这一机制，而PSFs的强度正是其中潜在的关键调控过程。我们以中国西南部热带树种Pometia pinnata（无患子科Sapindaceae）两个种群的幼苗为实验材料，开展了控制实验。本研究旨在量化：接种来自不同P. pinnata源种群的土壤，并在不同光照强度、水分有效性处理以及不同植株密度下培育幼苗时，所引发的PSF强度的种内差异。研究发现，当接种土壤取自同一原站点时，两个种群均表现出负PSFs；但当接种土壤取自不同站点时，PSFs存在显著差异。PSF强度对生物与非生物驱动因子的响应存在分化：低水分、高光照处理下的PSF强度显著弱于高水分、低光照处理。本研究证实了JC效应存在种内变异：具体而言，P. pinnata对本土来源土壤的防御能力较弱，但对源自其他种群的土壤反馈则表现出更强的防御性。我们的研究结果表明，干旱与光照强度往往会削弱JC效应，这一现象可能会在气候变化背景下导致热带森林物种多样性丧失。