Absorbing roots areas and transpiring leaf areas at the tropical forest and savanna boundary in Brazil

Plants capture essential resources for growth via absorbing surfaces on both roots and leaves. As a result, the allocation of assimilates to these resource exchange surface areas are of fundamental importance to plant growth and survival. Previous work on tropical forests and savanna vegetation has mainly focused on broad root:shoot biomass ratios. Yet, uptake of CO₂ (leaves), water and nutrients (roots) is a surface area phenomenon. In this study we compared the root:leaf ratio of the active absorbing area at the ecosystem scale, within eight structurally diverse stands, that were chosen to characterise the transition between the Amazonian forest (closed canopy) and Brazilian Cerrado (savanna). We use an earth impedance method to quantify the absorbing root area index (RAI_absorbing) at each site, and compare these measurements to the more widely used fine root area index (RAI_fine). Surprisingly, we found that RAI_absorbing and RAI_fine were not correlated, leading us to conclude that the two measurements are not direct substitutes. Additionally, we compared both measures of RAI with the leaf area index (LAI) in these contrasting ecosystems. The resulting RAI_fine:LAI ratio (R²=0.85) was inversely proportional to basal area, with the highest values in the savanna vegetation. On the other hand, the RAI_absorbing:LAI ratio showed an opposite trend with basal area (R²=0.83), with highest values in the forest. We suggest this paradox may reflect different growth patterns by plants to access to adequate water and nutrient resources.