Author: Knapp, A. K.; Fahnestock, J. T.; Hamburg, S. P.; Statland, L. B.; Seastedt, T. R.; Schimel, D. S.
Description: Landscape variation in soil water relations, leaf xylem pressure potential (°) and leaf-level net photosynthesis (A) in Andropogon gerardii, and net primary production (NPP) were evaluated during the 1989 and 1990 growing seasons in a northeast Kansas (USA) tallgrass prairie. Landscape patterns were assessed along transects that spanned upland and lowland topographic positions in an annually burned and a long-term unburned watershed. Landscape variability in volumetric soil water content (°) was significantly greater in the unburned watershed (coefficient of variation [CV] = 0.425 and 0.479 for 0-15 and 0-30 cm soil depths in unburned prairie vs. 0.285 and 0.330 for similar depths in the burned watershed). In both watersheds, significantly higher ° and total soil water content (0-30 cm) were measured in lowlands compared to uplands. Topographic anomalies, such as a lowland ridge, resulted in local, small-scale variation in soil moisture that equaled watershed variation. Variation across landscapes in predawn °, which was expected to reflect soil water content, was similar in both watersheds (CV = 0.312). Variation in midday @j was significantly greater across the burned than the unburned watershed in 1990 (maximum range in @j from uplands to lowlands was 0.708 MPa at predawn and 0.662 MPa at midday in the burned watershed). In both watersheds, variation in midday @j was much lower relative to °. Landscape patterns in leaf-level A in A. gerardii, the dominant species in this tallgrass prairie, were inconsistent when upland and lowland sites were compared. During an extended period of drought, A was significantly higher in plants in the unburned watershed. In both watersheds, NPP was strongly correlated with °. However, variability in NPP across topographic gradients in the unburned watershed was much less pronounced (CV = 0.224-0.245) than in the annually burned watershed (CV = 0.364-0.430). Moreover, the slope of the relationship between NPP and ° was significantly greater in the annually burned watershed. We propose that relatively uniform energy limitations across topographic gradients in unburned tallgass prairie, caused by detrital accumulation that absorbs/reflects sunlight, reduced topographic variability in NPP in unburned watersheds. This pattern occurred in unburned watersheds despite greater landscape variation in ° relative to burned watersheds. Analysis of long-term records of NPP from several watersheds supported the hypothesis that variability in NPP associated with topographic position is lower in unburned vs. burned watersheds. Variability in @j across watersheds and between years was muted by negative feedback of canopy leaf area (transpiring surface) on plant-soil water relations. We concluded that patterns in landscape variability in A and @j which may vary significantly over short time intervals, were not good predictors of seasonal carbon dioxide exchange or productivity in this tallgrass prairie. Nonetheless, interactions between A and @j, when combined with nitrogen and energy limitations to A. provide the mechanisms for integrated responses measured across these landscapes.
Subject headings: Andropogon gerardii; Plant leaf; Photosynthesis; Water potential; Grassland; Primary productivity; Soil plant relation; Water regime; Topography; Xylem; Kansas; Angiospermae; Environmental factor; Gramineae; Monocotyledones; Fodder crop; Spermatophyta; North America; America; United States
Publication year: 1993
Journal or book title: Ecology
Volume: 74
Issue: 2
Pages: 549-560
Find the full text: https://esajournals.onlinelibrary.wiley.com/doi/abs/10.2307/1939315
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Serial number: 4040