Soil moisture does not affect stabilizing coexistence among tropical forest tree species

Valerie R. Milici, Ecology and Evolutionary Biology, University of Connecticut, Storrs, CT and Robert Bagchi, Ecology & Evolutionary Biology, University of Connecticut, Storrs, CT

Background/Question/Methods
There is considerable evidence supporting the hypothesis that fungal and oomycete fungal pathogens lead to stabilizing coexistence among tropical tree species, but whether and how this process is affected by abiotic conditions is less well understood. Within the tropics, tree species richness tends to increase as mean annual precipitation increases, which indicates that moisture may be an important abiotic variable altering the intensity of pathogen-mediated coexistence. Given the biology of both plants and soil pathogens, it is reasonable to predict that their relationship is sensitive to soil moisture availability.
We established a shadehouse experiment in Central Panama to assess whether soil moisture (100% vs. 50% water holding capacity) alters the ability of tree seedlings to invade heterospecific soil; invading from rarity being the key criterion for stabilizing coexistence. Seedlings from six shade-tolerant co-occurring species were grown in the experiment for a duration of six months, although some seedlings were harvested early due to COVID-19 shutdowns. Survival and performance in heterospecific soil were measured to quantify invasibility.
Results/Conclusions
Moisture did not affect the ability of seedlings to invade heterospecific soil, and we do not find evidence that soil moisture is an important abiotic variable that influences pathogen-mediated coexistence. Interestingly, evidence that species performed better in heterospecific soil varied among species and we did not find evidence to support their role in plant-mediated coexistence in general. Our previous work has shown that there is high variation among species in their susceptibility to pathogens near to conspecific adults, and it is possible that the species studied are coexisting via a combination of mechanisms and are not sensitive to soil pathogens. Given the empirical evidence supporting pathogen-mediated coexistence it is likely an important mechanism, however it may not be a universal phenomenon among tropical tree species.