Landscape patterns of sapling density, leaf area, and aboveground net primary production in postfire lodgepole pine forests, Yellowstone National Park (USA)

Causes and implications of spatial variability in postfire tree density and understory plant cover for patterns of aboveground net primary production (ANPP) and leaf area index (LAI) were examined in ninety 11-year-old lodgepole pine (Pinus contorta var. latifolia Engelm.) stands across the landscape of Yellowstone National Park (YNP), Wyoming, USA. Field studies and aerial photography were used to address three questions: (1) What is the range and spatial pattern of lodgepole pine sapling density across the burned Yellowstone landscape and what factors best explain this variability? (2) How do ANPP and LAI vary across the landscape and is their variation explained by abiotic factors, sapling density, or both? (3) What is the predicted spatial pattern of ANPP and LAI across the burned Yellowstone landscape? Stand density spanned six orders of magnitude, ranging from zero to 535,000 saplings ha(-1), and it decreased with increasing elevation and with increasing distance from unburned forest (r(2) = 0.37). Postfire densities mapped from 1:30,000 aerial photography revealed that 66% of the burned area had densities less than 5000 saplings ha(-1) and approximately 25% had densities greater than 10,000 saplings ha(-1); stand density varied spatially in a fine-grained mosaic. New allometric equations were developed to predict aboveground biomass, ANPP, and LAI of lodgepole pine saplings and the 25 most common herbaceous and shrub species in the burned forests. These allometrics were then used with field data on sapling size, sapling density, and percent cover of graminoid, forb, and shrub species to compute stand-level ANPP and LAI. Total ANPP averaged 2.8 Mg ha(-1) y(-3) but ranged from 0.04 to 15.12 Mg ha(-1) y(-1). Total LAI averaged 0.80 m(2) m(-2) and ranged from 0.01 to 6.87 m(2) m(-2). Variation in ANPP and LAI was explained by both sapling density and abiotic factors (elevation and soil class) (ANOVA, r(2) = 0.80); abiotic variables explained 51%-54% of this variation. The proportion of total ANPP contributed by herbaceous plants and shrubs declined sharply with increasing sapling density (r(2) = 0.72) and increased with elevation (r(2) = 0.36). However, total herbaceous productivity was always less than 2.7 Mg ha(-1) y(-1), and herbaceous productivity did not compensate for tree production when trees were sparse. When extrapolated to the landscape, 68% of the burned landscape was characterized by ANPP values less than 2.0 Mg ha(-1) y(-1), 22% by values ranging from 2 to 4 Mg ha(-1) y(-1), and the remaining 10% by values greater than 4 Mg ha(-1) y(-1). The spatial patterns of ANPP and LAI were less heterogeneous than patterns of sapling density but still showed fine-grained variation in rates. For some ecosystem processes, postfire spatial heterogeneity within a successional stage may be similar in magnitude to the temporal variation observed through succession.