COS 5-4 - The impact of experimental drought and rainfall seasonality on fine-root traits in Panamanian forests across rainfall and soil fertility variation

Abstract: Tropical forests have some of the largest soil carbon (C) pools, which are related, at least in part, to large stocks of root biomass. Despite the importance of root biomass for soil C storage, effects of climatic drying on root dynamics are poorly understood in tropical forests. In addition to total root biomass, root morphology can be important for both transfer of C into soils, and for plant function. We hypothesized that climatic drying promotes proliferation of fine root biomass in tropical forests, and that these roots would be longer and thinner (i.e., greater specific root length [SRL]), with greater AMF colonization for water acquisition, compared with control conditions. We predicted that these roots would also turn over more rapidly, and thus contribute to soil C stocks with drying. To address this, we conducted a partial throughfall reduction (Panama Rainforest Changes with Experimental Drying [PARCHED]) in four distinct seasonal, lowland forests. We collected soil and fine roots (< 2 mm diameter) using sequential coring and ingrowth cores during years 2-4 of the experiment at 0-10 and 10-20 cm depths. We tested the effects of depth, site, throughfall manipulation and season on fine roots using ANOVA, with post-hoc pairwise comparison.

Fine root biomass and productivity were significantly affected by throughfall exclusion, season, depth and site, AMF was affected by the throughfall exclusion, and other root traits mainly varied by site. Overall, there was greater fine root productivity when water was more available. For example, productivity was larger in late wet season, in the wettest site, and in control plots. Fine root standing stock was also larger in sites with larger MAP and lower fertility and in control plots. Root morphology on both ingrowth cores and standing stocks was not affected by throughfall exclusion, but varied by site. For example, roots had a smaller SRL and larger root diameter in the wettest infertile site, which had also greater root biomass and productivity. AMF colonization in all four sites was affected by throughfall exclusion and depth in the wet season. AMF colonization declined with soil depth and increased with throughfall exclusion. Therefore, plants might appear to be compensating for biomass reduction in throughfall exclusion plots with increased AMF interactions, rather than with changing morphology. Downstream effects on soil C stocks have yet to be determined, but could indicate a reduction in C inputs to soil C stocks with drying in tropical forests.