Abstract
AbstractRecent shifts in global forest area highlight the importance of understanding the causes and consequences of forest change. To examine the influence of several potential drivers of forest cover change, we used supervised classifications of historical (1938–1940) and contemporary (2015) aerial imagery covering a 2932‐km2 study area in the northern Front Range (NFR) of Colorado and we linked observed changes in forest cover with abiotic factors, land use, and fire history. Forest cover in the NFR demonstrated broad‐scale changes 1938–2015 and overall cover increased 7.8%, but there was notable spatial variability and many sites also experienced Forest Loss. Recent (1978–2015) wildfire was the largest single driver of Forest Loss, with fires burning 14.3% of the total study area. Recently burned areas showed net losses of 36.9% forest cover. Reasons for Forest Gain were more complex, with elevation, past mining density, fire history, and topographic heat load index being the strongest predictors of increases in forest cover. Historical mining activity is one of the dominant anthropogenic impacts in ecosystems in the NFR and it had a complex, non‐linear relationship with 20th‐century changes in forest cover. Subalpine stands originating after stand‐replacing fires circa mid‐1800s to early 1900s showed some of the greatest gains in forest cover, indicative of slow and continuous post‐fire recovery through the 20th century. We also investigated factors such as land ownership, road density, forest management activities, and development intensity, which played detectable, but more minor roles in observed change. Twentieth‐century changes in forest cover throughout the NFR are a result of ecological disturbances and anthropogenic influences operating at varying timescales and overlaid upon variability in the abiotic environment.
Highlights
Studies using forest inventories and remotely sensed data have identified declines in global forest area c. 1990s–2010s (Hansen 2013, Keenan et al 2015), though longer-term trends may differ in direction and magnitude (Song et al 2018)
Changes in forest cover and contributing factors Between 1938 and 2015, forest cover increased from 56.5% to 64.3% throughout the northern Front Range (NFR), a net gain of 7.8% (Fig. 4; Appendix S5: Table S1)
Changes in forest cover were influenced by abiotic factors and human influences Consistent with previous studies in the NFR and in California, United States (Platt and Schoennagel 2009, Lydersen and Collins 2018), we noted that forest cover changes varied with elevation and that mesic, N-facing grid cells showed lower increases in cover through the 20th century than did grid cells with other aspects
Summary
Studies using forest inventories and remotely sensed data have identified declines in global forest area c. 1990s–2010s (Hansen 2013, Keenan et al 2015), though longer-term trends may differ in direction and magnitude (Song et al 2018). The effects of wildfire activity on forest cover change are complex because fire is expected to influence forest cover through both initial losses and subsequent recovery. From 1984 to 2015, warming and drying trends contributed to increases in the area burned in large wildfires across much of the western United States (Abatzoglou and Williams 2016). These increases in wildfire activity have occurred in tandem with poor post-fire recovery in many western conifer forests (Stevens-Rumann et al 2018). Recent wildfire activity, combined with poor postfire recovery, may be leading to forest cover declines through the western United States at the onset of the 21st century
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