Have Forests Really Become Denser? an Object-oriented Case Study of a Key Premise in Wildfire Policy

In the wake of numerous catastrophic wildfires, forest management policies have been implemented in recent years in the United States, including the Healthy Forest Restoration Act of 2003. A key premise underlying these policies is that fire suppression has resulted in denser forests than were present historically in some forest types. We evaluate this premise for the montane zone of the northern Front Range, Colorado. Historical photographs from 1938 and 1940 were scanned, orthorectified, and overlaid on DOQQs from 1999. Using an object-oriented image classification technique, the photos were then finely segmented and classified into two classes: tree and nontree. Trees are heterogeneous in appearance in black and white aerial photography, so we employed separate membership functions to identify four visually distinct types: ‘interior forest’, ‘isolated trees’, ‘dark forest’, and ‘edge forest’. A particular challenge was making the classification robust to differences in illumination across topographic features. Based on the classification of fine objects, we then calculated the % tree cover within a larger set of objects for the two time periods. We estimate that average tree density across the study area increased by 4%, with considerable spatial variation within the landscape. The results of the analysis illustrate that, consistent with tree-ring evidence, the highest increase in tree density has taken place in areas characterized by low initial density, south-facing slopes, low elevations, and ponderosa-pine domination. In contrast, the highest elevation areas dominated by mixed-conifer and lodgepole pine forests demonstrated no significant change in tree cover.