Landscape-scale patterns in tree occupancy and abundance in subarctic Alaska

Roland, Carl; Schmidt, Joshua; Nicklen, Fleur

Recent studies suggest climate warming in interior Alaska may result in major shifts 2 from spruce-dominated forests to broadleaf-dominated forests or even grasslands. To quantify 3 patterns in tree distribution and abundance and investigate the potential for changes in forest 4 dynamics through time, we initiated a spatially-extensive vegetation monitoring program 5 covering 1.28 million hectares in Denali National Park and Preserve (DNPP). Using a 6 probabilistic sampling design, we collected field measurements throughout the study area to 7 develop spatially-explicit Bayesian hierarchical models of tree occupancy and abundance. These 8 models demonstrated a strong partitioning of the landscape among the six tree species in DNPP, 9 and allowed us to account for and examine residual spatial autocorrelation in our data. Tree 10 distributions were governed by two primary ecological gradients: 1) the gradient from low 11 elevation, poorly-drained, permafrost-influenced sites with shallow active layers and low soil pH 12 (dominated by Picea mariana) to deeply-thawed and more productive sites at mid-elevation with 13 higher soil pH on mineral substrate (dominated by Picea glauca); and 2) the gradient from older, 14 less recently disturbed sites dominated by conifers to those recently affected by disturbance in 15 the form of fire and flooding with increased occupancy and abundance of broadleaf species. We 16 found that the establishment of broadleaf species was largely dependent on disturbance, and 17 mixed forests and pure stands of broadleaf trees were relatively rare and occurred in localized 18 areas. Contrary to recent work in nearby areas of interior Alaska, our results suggest that P. 19 glauca distribution may actually increase in DNPP under warming conditions rather than 20 declining as previously predicted, as P. glauca expands into areas formerly underlain by 21 permafrost. We found no evidence of a shift to broadleaf forests in DNPP, particularly in the 22 poorly-drained basin landscape positions which may be resistant to such changes. Overall, our 23 results indicate that probabilistic sampling conducted at a landscape-scale can improve inference

Landscape-scale patterns in tree occupancy and abundance in subarctic Alaska

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