Throughout interior Alaska (U.S.A.), a gradual warming trend in mean monthly temperatures occurred over the last few decades (approximatlely 2-4 degrees C). The accompanying increases in woody vegetation at many alpine treeline (hereafter treeline) locations provided an opportunity to examine how biotic and abiotic local site conditions interact to control tree establishment patterns during warming. We devised a landscape ecological approach to investigate these relationships at an undisturbed treeline in the Alaska Range. We identified treeline changes between 1953 (aerial photography) and 2005 (satellite imagery) in a geographic information system (GIS) and linked them with corresponding local site conditions derived from digital terrain data, ancillary climate data, and distance to 1953 trees. Logistic regressions enabled us to rank the importance of local site conditions in controlling tree establishment. We discovered a spatial transition in the importance of tree establishment controls. The biotic variable (proximity to 1953 trees) was the most important tree establishment predictor below the upper tree limit, providing evidence of response lags with the abiotic setting and suggesting that tree establishment is rarely in equilibrium with the physical environment or responding directly to warming. Elevation and winter sun exposure were important predictors of tree establishment at the upper tree limit, but proximity to trees persisted as an important tertiary predictor, indicating that tree establishment may achieve equilibrium with the physical environment. However, even here, influences from the biotic variable may obscure unequivocal correlations with the abiotic setting (including temperature). Future treeline expansion will likely be patchy and challenging to predict without considering the spatial variability of influences from biotic and abiotic local site conditions.

Landscape-scale modeling of reference period forest conditions and fire behavior on heavily logged lands. Ecosphere 5(3):32. http://dx.doi.org/10.1890/ES13-00294.1Abstract. Forest conditions prior to extensive land clearing are often used as a point-of-reference by ecologists and resource managers for characterizing the historical range of variability in forest conditions shaped by intact disturbance regimes. Quantitative data on forest reference conditions can be developed from forest surveys and reconstructions using dendroecology; however, these methods lack the spatial resolution needed for landscape management. In this paper, we combine predictive vegetation mapping methods with reference forest conditions inferred from early forest surveys, dendroecology, and fire simulation models to develop landscape-scale reference conditions for forest structure, forest fuels, fire frequency, and fire behavior using the Lake Tahoe Basin, California as an example.The dendroecological reconstruction method used for the Lake Tahoe Basin forests was not sensitive to variation in decomposition rates suggesting that our method provided robust estimates of reference period forest characteristics. The cluster analysis procedure identified five forest structure types (white fir, Jeffrey pine, red fir, lodgepole pine, and subalpine) and 15 subtypes. Each forest type had a characteristic composition, density, and basal area. Our random forests approach to classifying and mapping the spatial distribution of the five dominant reference forest structure types resulted in 51.5% classification accuracy using 14 physiographic and climatic variables. The random forests model to identify subtypes within each forest group had an average percent correct classification of 47.8%. The random forests model for fire intervals explained 67% of the variance in the point fire return interval estimates from fire-scarred trees. Estimates of reference period fuels modeled from stand structure suggested moderate fuel loads for reference forests. The predicted potential fire type for forest subtypes under extreme weather was surface fire except for red fir and the lodgepole pine subtypes with potential for crown fire. By characterizing the reference forest composition, structure, and disturbance frequency with a range of variability, managers can develop a forested landscape more resilient to changes in disturbance regime and climate. Although our approach was developed for the Lake Tahoe Basin, California, it could be applied to a wide range of forest landscapes to identify forest reference conditions.

Ice storms (major freezing rain events) periodically disturb forests in eastern NorthAmerica. The damage may vary spatially, especially in complex terrain. This study uses satellite imagery to investigate spatial heterogeneity of forest damage caused by ice storms that affected the Appalachian Mountains, Virginia during 1994. The results display a region-scale (southwest-to-northeast) gradient in damage that apparently corresponds to a gradient in the depth of ice that accumulated during the storms. Damage also varied topographically, particularly by aspect. Damage was most extensive on east-, southeast-and south-facing slopes; at middle elevations; and on slopes of moderate steepness.

Abstract. The effects of large-scale disturbances play a pivotal role in shaping ecosystem structure and function. Interactions between disturbances and a multitude of biophysical factors at different scales generate spatially heterogeneous patterns of damage on vegetated landscapes. However, research on largescale disturbances is often conducted at one spatial extent because of the challenges associated with quantifying patterns of damage in vegetation across multiple spatial extents. Consequently, the literature has identified many different and often conflicting biophysical controls of vegetation damage, which is likely a consequence of the historical and spatial contingency of particular geographic locations. We investigated the influence of biophysical factors and spatial extent of ice storm damage patterns across the entire Ouachita National Forest and in 65 individual watersheds. This allowed for an assessment of how spatial extent and contingency might influence statistical results and therefore any conclusions drawn from these analyses. Two pre-and post-storm Landsat 7 ETMþ scenes (October 6, 1999 and September 25, 2001) were acquired to map ice storm damage based on the normalized difference vegetation index (NDVI) change. Field sites verified the relationship between NDVI and actual forest damage from the storms. We derived topographic variables from a digital elevation model (DEM) and biological variables from GAPgenerated data. The relationship between ice storm damage patterns and biophysical factors across the entire national forest landscape was weak. However, the relationship between damage severity, topography, and forest type increased significantly at the watershed scale, though the specific variable displaying the strongest influence varied by individual watershed. This research demonstrates that complex topography and biological factors interact with the meteorological characteristics of ice storms to generate spatially heterogeneous damage patterns in forests at the watershed scale. It is likely that the damage patterns corresponding to a particular watershed will be repeated during future disturbances.