Rainfall seasonality shapes belowground root trait dynamics in an Amazonian tropical rainforest: A test of the stress-dominance hypothesis

The stress-dominance hypothesis (SDH) predicts that trait variation at the community level increases with the availability of limiting resources, driving spatial and temporal patterns in above-ground plant functional trait expression. Here, we test the assumption that the SDH also applies to fine roots responding to spatial and temporal fluctuations in soil resource availability. We monitored fine root mass and functional root traits associated with resource acquisition, that is specific root length (SRL), specific root tip abundance (SRTA) and branching index (BI), and traits related to stress tolerance, such as root diameter (RD) and tissue density (RTD) in a Central Amazonian tree community. To test for spatial differences in root traits, we separated the uppermost organic (O-A horizon, 0-5 cm) and mineral soil (B horizon, 5-15 cm) layers, and for temporal fluctuations, we investigated the relationship of precipitation on community-level root variation over a period of 27 months. In accordance with the SDH, we found that fine roots in the O-A horizon have on average 15% higher SRL, 23% higher BI, 32% higher SRTA and 15% lower RTD than those in the B horizon. Similarly, precipitation shifted the community over time to higher mean SRL, BI and SRTA (r = 0.92, 0.84 and 0.94, p < 0.0001 respectively), although trait shifts occurred in the trimester after the rainy season onset, revealing a time-lag between rainfall patterns and community response. We also detected a positive increase in trait range for SRL and SRTA with lagged precipitation (r = 0.90 and 0.79, p < 0.0001). On the other hand, traits related to stress showed a weaker negative relationship with instantaneous precipitation (r = -0.7 and -0.57, p = 0.046 and p = 0.1 for RD and RTD, respectively). Our results supported the SDH predictions that root systems will become more acquisitive in areas with more resources, and that the community will shift to more acquisitive but also broader trait dispersion as hydric stress decreases. We conclude that although higher resource availability may increase competition for acquisition, trait overdispersion seems to promote species coexistence. Our results highlight how dynamic root systems can be in response to environmental cues, cautioning the common practice of making conclusions about root traits adaptations to environmental gradients based on a single sampling observation.