Physiological Interactions Along Resource Gradients in a Tallgrass Prairie

Spatial variability in availability of resources that limit photosynthesis (water and N) leads to variation in rates of astmosphere—biosphere exchange. N content and allocation are canopy properties that link ecosystem, physiological, and biophysical processes and that vary in space at scale relevant to atmosphere—biosphere interaction. We studied landscape—scale variation in these and related canopy properties in Kansas Tallgrass prairie (USA). The tallgrass ecosystem was studied to this investigation because primary production in the prairie is constrained by N availability. This work was designed to aid in interpretation and spatial extrapolation of gas exchange measurements made using aerodynamic techniques as part of FIFE (First ISLSCP Field Experiments), a NASA—supported study. We collected data on spatial distribution of biomass, leaf area index (LAI), canopy N mass, N concentration (N), and gas exchange along topographic and management gradients. We also measured height distribution of N, light interception, and gas exchange within canopies as a function of position in the landscape. Substantial variation in biomass, LAI, N accumulation, and N allocation occurred over time, with topography, and as a result of previous burning. The vertical gradient of N and photosynthetic capacity within canopies were correlated, in space and time, with biomass and canopy light interception. The gradients were steeper in high biomass sites than in low biomass sites. in addition, proportional N allocation to the upper layer increased with time (12% in June, 32% in August) as biomass increased. As nutrient uptake increased within the tallgrass landscape, biomass increased and light limitation in the lower canopy was induced. As this light limitation increased with increasing biomass, or with accumulation of dead vegetation, allocation of N to the upper canopy increased. Height distribution of photosynthetic capacity paralleled within—canopy N allocation and light interception. As resource ratios (light, water, and nitrogen) varied in the landscape, so did rates of gas exchange. This work suggests that interactions between light extinction, N allocation, and photosynthesis that have been proposed for monospecific stands apply to the multispecies, but structurally simple, canopy of the tallgrass prairie. Models of plant performance based on evolutionary arguments may provide a powerful basis for spatial extrapolation of atmosphere—ecosystem exchange rates from sites to landscapes and larger regions.