Structure and development of expanding western juniper woodlands as influenced by two topographic variables

Juniper and pinon woodlands have been expanding throughout the Intermountain West, USA since the late 1800s. Although causal factors attributed to woodland expansion have been documented, data are lacking that describe the influence of topographic features on rates of development and structural attributes of expanding woodlands. Our primary objective was to determine the relationship between stand-level developmental and structural attributes of four expanding western juniper (Juniperus occidentalis) woodlands with two topographic features commonly important to forest vegetation patterns, site exposure (an index of insolation exposure based on slope and aspect) and elevation. To accomplish this we measured tree density and age across four western juniper woodlands in Oregon and Idaho. Site exposure and elevation were consistently correlated with spatial and temporal components of woodland development and structure. Holding slope constant, a shift from a north to south aspect resulted in a 4.9 trees/ha/year decrease in tree establishment at a similar elevation. A 200 m rise in elevation was associated with a 1.8 trees/ha/year increase in tree establishment with similar exposure. A 100 m rise in elevation was associated with a 5-year decrease in the time required for a stand to reach dominance and a 22% increase in dominant tree density among stands with similar site exposure. Although significant, site exposure and elevation only explained a portion of the process of woodland dynamics. However, our data suggest exposure and elevation are contributing variables influencing woodland expansion across landscapes, resulting in spatial and temporal heterogeneity in stand structure and development. Models of woodland expansion incorporating landscape topographic features may be practical for identifying windows of opportunity for less costly restoration.