Tropical forest root biomass, depth distributions, productivity and morphology vary across forests with rainfall, seasonality, and soil fertility

Amanda Longhi Cordeiro, Lee H. Dietterich and Daniela Cusack, Ecosystem Science & Sustainability, Colorado State University, Fort Collins, CO, S. Joseph Wright, Smithsonian Tropical Research Institute, Panama

Background/Question/Methods: Many tropical forests are predicted to become drier with climate change. Fine root dynamics play a fundamental role in the cycling of water, nutrients, and carbon in tropical forests, yet there is little data on how root dynamics will respond to long-term changes in moisture. For example, producing thinner roots (i.e., greater specific root length [SRL]) is considered a resource acquisition trait, yet how this trait responds to moisture is poorly understood for tropical forests. We hypothesized that seasonal and landscape-scale drying shift root production to deeper soils and toward greater specific SRL for improved water acquisition. We measured fine root productivity, standing stocks, and morphology using standing stocks and minirhizotron images to 1.2 m depth in 32 plots across four different lowland tropical forests that vary in soil fertility and mean annual precipitation.
Results/Conclusions: We found that fine root biomass decreased with depth across sites. At the soil surface, the wettest site had the largest fine root biomass stocks with the lowest SRL, indicating that large root biomass was used for resource acquisition, rather than long, thin roots. In contrast, the most fertile had the smallest fine root biomass and largest SRL, suggesting that this forest used long, thin, short-lived roots for resource acquisition, which might be related to water uptake since nutrients are plentiful. SRL in the wettest site increased with depth indicating root foraging for resources deeper. Fine root productivity from minirhizotron images decreased with depth across sites, and surface root production was greatest during the wet season and least during the dry season. For two sites (wettest and most fertile) deep root production (90-120 cm) increased during the dry season. Together, these data indicate that fine root biomass, production, and morphology vary with moisture availability, depth, soil fertility, and background rainfall level. These trends will be useful for predicting how climatic drying will affect root dynamics across heterogeneous tropical landscapes.