Evidence of belowground community structuring and niche divergence among coexisting species in a temperate forest

Background/Question/Methods: Recent studies in plant communities suggest that deterministic mechanisms, rather than stochastic, explain trait distributions along environmental gradients. Then, community assembly processes such as habitat filtering and niche segregation seem to be responsible for most of the observed variation in functional trait shifts across landscapes. These studies, however, have neglected belowground plant traits, even though fine roots (< 1mm in diameter) play a key role in plant resource acquisition. Here, our goals were to quantify root trait plasticity under natural conditions, and determine to what extent trait variation is explained by niche-based or stochastic mechanisms. We conducted our study in a temperate hardwood forest in northeast Ohio. We collected >120 soil cores (15 cm deep) and associated fine roots. Using molecular barcoding techniques we identified 750 root system modules belonging to 14 different canopy tree species, and measured eight different root traits including biomass, specific root length (SRL), specific root tip abundance (SRTA), root tissue density (RTD) and fractal dimension (FD). Additionally, we measured 17 different soil variables.

Results/Conclusions: Our findings indicated that 1) variations in root biomass were largely related to soil organic matter and pH conditions and to a lesser degree species root productivity. However, some species such as Acer rubrum produced substantially more roots than expected based on tree abundance observations, whereas others like Fagus grandifolia produced less. 2) Although interspecific differences were the most important factor explaining community trait variation, soil conditions partially explained changes in root traits at the community level. There was a significant trend for higher SRL and SRTA and lower RTD as soil resources increased. However, most species showed similar ranges on soil preferences, suggesting a weak filtering effect on root trait distribution. 3) In contrast, coexisting species showed a more evenly spread distribution of trait values than the null expectation, particularly for SRL and FD, suggesting that non-random trait sorting occur at relatively small scales. We conclude that root traits of coexisting species vary widely because morphological differentiation would lead species to exploit distinct niches in the same location, rather than reflecting morphological specialization for alternative soil conditions. We believe that this study highlights the importance of niche- based mechanisms defining the belowground structure of plant communities, opening new avenues to the understanding of plant community structuring in natural systems.