Heterogeneous Forest Landscapes Research Articles

Spatial heterogeneity of lodgepole pine sapling densities following the 1988 fires in Yellowstone National Park, Wyoming, USA

Large disturbances create spatial heterogeneity in vegetation re-establishment, and documenting such variability is critical for understanding and predicting succession. We quantified the spatial heterogeneity of lodgepole pine sapling densities 10 years after the 1988 fires in Yellowstone National Park using color infrared orthophotographs. Densities were classified across the landscape at an accuracy of 70.9%, and landscape metrics were used to characterize their spatial variability. Densities ranged from 0 to > 500 000 saplings/ha, but >60% of the burned area was represented by densities <5000 stems/ha. The burned area consisted of small patches averaging 1.5 ha in area at a mean patch density of 68 patches/100 ha. Densities occurred in nearly equal proportions across the landscape (Shannon's evenness = 0.85) but were well dispersed (contagion index ≈ 20%), suggesting that densities varied in a complex, fine-grained mosaic across the landscape, with high-density patches occurring within a matrix of larger, lower density patches. High-density patches were similar in area to severe surface fires, suggesting that burn severity is an important explanatory variable for spatial variation in sapling density. Large, stand-replacing fires may result in heterogeneous forest landscapes rather than homogenous forests of uniform structure, which may have important consequences for postfire ecological processes.

Species-environment relationships and vegetation patterns: effects of spatial scale and tree life-stage

Do relationships between species and environmental gradients strengthen or weaken with tree life-stage (i.e., small seedlings, large seedlings, saplings, and mature trees)? Strengthened relationships may lead to distinct forest type boundaries, or weakening connections could lead to gradual ecotones and heterogeneous forest landscapes. We quantified the changes in forest dominance (basal area of tree species by life-stage) and environmental factors (elevation, slope, aspect, intercepted photosynthetically active radiation (PAR), summer soil moisture, and soil depth and texture) across 14 forest ecotones (n = 584, 10 m #x00D7; 10 m plots) in Rocky Mountain National Park, Colorado, U.S.A. Local, ecotone-specific species-environment relationships, based on multiple regression techniques, generally strengthened from the small seedling stage (multiple R2 ranged from 0.00 to 0.26) to the tree stage (multiple R2 ranged from 0.20 to 0.61). At the landscape scale, combined canonical correspondence analysis (CCA) among species and for all tree life-stages suggested that the seedlings of most species became established in lower-elevation, drier sites than where mature trees of the same species dominated. However, conflicting evidence showed that species-environment relationships may weaken with tree life-stage. Seedlings were only found in a subset of plots (habitats) occupied by mature trees of the same species. At the landscape scale, CCA results showed that species-environment relationships weakened somewhat from the small seedling stage (86.4% of the variance explained by the first two axes) to the tree stage (76.6% of variance explained). The basal area of tree species co-occurring with Pinus contorta Doug. ex. Loud declined more gradually than P. contorta basal area declined across ecotones, resulting in less-distinct forest type boundaries. We conclude that broad, gradual ecotones and heterogeneous forest landscapes are created and maintained by: (1) sporadic establishment of seedlings in sub-optimal habitats; (2) survivorship of saplings and mature trees in a wider range of environmental conditions than seedlings presently endure; and (3) the longevity of trees and persistence of tree species in a broad range of soils, climates, and disturbance regimes.