Physiological Interactions Along Resource Gradients in a Tallgrass Prairie

Abstract

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.

Improving canopy processes in the Community Land Model version 4 (CLM4) using global flux fields empirically inferred from FLUXNET data

“…Canopy gradients of N a related to the M a profile are commonly observed in forests [Ellsworth and Reich, 1993;Hollinger, 1996;Carswell et al, 2000;Meir et al, 2002;Niinemets, 2007;Lloyd et al, 2010]. Canopy nitrogen gradients have been observed in grasslands [Schimel et al, 1991;Anten et al, 1998] and other herbaceous plant assemblages [Hirose and Werger, 1987;Hirose et al, 1988] and also in agricultural crops [Evans, 1993;Dreccer et al, 2000;Drouet and Bonhomme, 2004;Gastal and Lemaire, 2002]. An empirical exponential nitrogen profile allows for gradients in photosynthetic capacity in vegetation where gradients in leaf morphology may not be valid.…”

Environmental control of leaf area production: Implications for vegetation and land‐surface modeling

“…Alleviation of environmental or resource stress will result in higher rates of GPP and greater investment of carbon into leaf area production. A good proportion of natural ecosystems likely lie within this part of the LAI response curve, evidenced by the bulk of field studies that indicate a strong correlation between resource availability, GPP and maximum LAI [Schimel et al, 1991;Sala et al, 1994;Fassnacht and Gower, 1997;Vogel and Gower, 1998;Ares and Fownes, 1999;Balster and Marshall, 2000;Lane et al, 2000;Le Dantec et al, 2000;Bolstad et al, 2001].…”

Physiological drought tolerance and the structuring of tallgrass prairie assemblages

“…Eigenvalues represent the ratio of variation explained by a given axis to the amount of variation explained by chance. v www.esajournals.org shallow (often ,25 cm) (Schimel et al 1991) and upland species frequently experience low soil moisture (Nippert and Knapp 2007a). As such, either the predominance of species with low W crit is a legacy from infrequent, severe droughts or low-intensity soil moisture stress is frequent v www.esajournals.org lowland community in the past or in a subset of years.…”