Shifts in woodland distribution has occurred in many areas across the western United States over the past century due to changes in climate and historical land management practices, such as early settlement wood cutting, fire suppression, and livestock grazing. In areas where woodland expansion has occurred, tree reduction treatments are commonly used to restore open areas and improve forage for cattle and wildlife species. Land managers may focus on treating areas where tree expansion has occurred by reconstructing historical landscapes, or, if the management goal is to increase forage production, they may restore areas with the highest potential for improved forage. We used a combination of observational and experimental approaches to assess where historical changes in woodland distribution have occurred and where the greatest potential for improved forage production exists in juniper (Juniperus monosperma) dominated woodlands in southeastern Colorado. We reconstructed settlement-era woodland distributions using General Land Office records and used aerial imagery to compare historical and modern distributions across the study region of varying soil types. To determine which areas have the highest potential for improved forage, we used mechanical mastication treatments in three dominant soil types, which represent a broad gradient in soil depth.
Results/Conclusions
In our study area, approximately 61% of historically woodland areas are now treeless, whereas approximately 57% of historically open areas have experienced woodland expansion. Expansion generally occurred on shallow, rocky soil types, which support little herbaceous vegetation relative to the deeper soil types (p=0.003) and were consequently less likely to be affected by fire suppression or grazing. Masticated plots had lower perennial grass cover than control plots one year after treatment, likely as a result of recent disturbance and the presence of mulch piles (p=0.04). Additionally, perennial grass cover was greater on the deeper soils compared to the shallowest soil type one year after mastication (p=0.02). We expect that deeper soil types will experience a larger increase in perennial grass cover over time given these deeper soils support greater herbaceous cover. Consequently, the areas with the greatest potential for improved forage are not the areas experiencing the greatest increases in tree density. Further, it is unlikely that anthropogenic factors have led to increases in woodland expansion in these rocky, shallow soil types. As a result, if the management goal is to increase forage production, we recommend prioritizing treatments in the deeper soils that support greater amounts of herbaceous vegetation.