Biomass Partitioning to Absorptive Roots in the Context of Multiple Resource Limitation

Plants often must cope with multiple resource limitations to optimize growth and reproduction. We examined how trees deploy their absorptive roots for the acquisition of water and nutrients by comparing tree biomass partitioning along hillslopes in the Susquehanna-Shale Hills Critical Zone. This was done by taking 250 soil cores to depth of refusal across the 8-ha catchment. Valley-floor trees were 30% taller with 20% higher leaf area and 20% higher leaf nitrogen concentration than trees on the ridge top. These results suggest nutrient and water limitations were much greater for trees on the ridgetop. Nonetheless, we did not find greater partitioning of biomass to absorptive root length on the ridgetop. Indeed, in the top 20 cm, root length density showed little spatial structure, with density differences from soil cores taken 1 m apart often as great as those of two cores taken at different hillslope positions. Collectively, our results suggest that theories of optimal biomass partitioning may have much stronger support in aboveground growth than in belowground growth for spatially heterogeneous, mixed-species forests.