University of California, Berkeley; Lawrence Berkeley National Lab, United States
Abstract: Biomass allocation in plants is the foundation for understanding dynamics in ecosystem carbon balance, species competition, and plant-environment interactions. However, existing work on plant allometry has mainly focused on woody plants, with fewer studies having developed allometric equations for grass species. Grasses with different life histories (e.g. C3 vs C4 and annual vs perennial) can vary in their carbon investment by prioritizing growth of specific organs in order to optimize resource usage and outcompete co-occurring plants. Further, because grasses are important fuels for wildfire, the lack of grass allometry and allocation data adds uncertainty to process-based models that relate plant physiology to wildfire dynamics. To fill this gap, we conducted a greenhouse experiment with 11 common California grass species that vary in photosynthetic pathway and growth form. We harvested above- and below-ground biomass throughout the life cycle of each species and measured plant sizes, in order to quantify allometric relationships for leaf, stem, and fine root biomass, as well as for basal diameter, plant height and canopy area. Overall, plant size in terms of basal diameter is the best predictor for leaf and stem biomass, height, and canopy area. Including plant height as another predictor can improve model accuracy in predicting leaf and stem biomass and canopy area. Fine root biomass is a function of leaf biomass alone. Species vary in their allometric relationships with most of the variations occurring in plant height, canopy area, and stem biomass. We further explored potential trade-offs in carbon allocation across species regarding allocation between leaf and fine root, leaf and stem, and allocation to reproduction. We found that fast-growing plants allocate a greater fraction to reproduction. Additionally, the leaf-to-stem ratio decreases in response to the increase in plant height and specific leaf area. However, there is no difference in root-to-leaf ratio between either perennial and annual or C4 and C3 plants. Our study provides species-specific and functional-type-specific allometry equations for both aboveground and belowground organs of 11 grass species. These relationships enable biomass assessment in California grasslands with non-destructive measurements of plant basal diameter and height. The allometric relationships and trade-offs in carbon allocation across species can also help improve ecosystem models to better understand how species interact with coexisting plants and environment through change in morphology.
