Fine root biomass, morphology, and chemistry depth distributions vary with precipitation and soil fertility in four lowland tropical forests

Background/Question/Methods

Tropical forests have the largest soil carbon pools, plant biomass stocks, and plant species‐richness on Earth. Fine-roots play a fundamental role in the cycling of water, nutrients and carbon. Fine-root traits, such as morphology, can indicate resource acquisition strategies among forests and with depth. For example, long, thin fine-roots have been associated with greater resource acquisition in dry or infertile conditions. Variation in resource acquisition strategies is likely to vary among tropical forests, depending on moisture and soil nutrient availability. We hypothesize that spatial variation in fine-root traits maximizes absorptive surface area relative to mass in conditions of resource scarcity, such as in deeper soils or in forests with infertile soils. To address this, we compared fine-root biomass, diameter, carbon and nitrogen content, specific root length (SRL), specific root area (SRA), root tissue density (RTD) and AMF colonization among four distinct lowland tropical forests in Panama varying in rainfall and soil fertility. We compared fine-roots to 1.2-m depth, separated into depth increments, for 8 plots per forest (32 plots). We tested the effect of depth, site and their interaction on each trait using ANCOVA and using post-hoc regressions with depth or pairwise comparison among sites where main effects were significant.

Results/Conclusions

Site, depth, and their interaction were significant predictors of fine-root biomass, diameter and N content. Site and depth were also significant predictors of SRL, SRA, RTD, and C/N ratio, with no interaction. Site was also a significant predictor of fine-root C content, with no effect of depth. However, community-scale AMF colonization rates in surface soils did not vary among the four forests. Post-hoc tests revealed that fine-root biomass was largest at the wettest and most infertile site (3300 MAP, infertile), which also had the lowest SRL and SRA, indicating a large quantity of shorter roots. In contrast, RTD was greatest at the driest site (2350 MAP). Post-hoc tests also revealed that fine-root diameter, SRL, SRA and C/N ratios increased with depth, while RTD and N content decreased, such that shallow fine-roots were shorter and denser, while deeper fine-roots were longer and less dense. These patterns in fine-root morphology and tissue chemistry appear to indicate differences in resource acquisition strategy among the four tropical forests and with depth. The functional implications of these shifts in fine root-traits merit further investigation, and could help us understand how different tropical forests might respond to drying and changes in nutrient availability.