Summary
Feedbacks at multiple scales can be important for shaping how forest ecosystems respond to both climate change and disturbance. At landscape scales, feedbacks likely exist between vegetation and wildfire regimes such that a change in one produces changes in the other. More locally, some forest patterns can result from feedbacks between plants and their abiotic environment. Alternating areas of forest and meadow (ribbon forests) in subalpine forest provide an example where both scales of feedbacks could be important with changes in climate-vegetation-fire interactions giving rise to local-scale feedbacks between snow drifting and forest extent that created the ribbon forests and further feedback to alter fuel continuity and fires regimes.
To examine the feedbacks in subalpine forests and the history of ribbon forests, we obtained six fossil pollen records from lakes across a subalpine landscape in Colorado. Forests there may have responded to climate change and widespread wildfires ca. 1000 years ago when >80% of sites on the landscape burned within a century. The fires coincided with regional warming, but the extent of burning declined before the climate cooled, possibly driven by changes in fuel structure and composition.
Results of cluster analyses of the pollen percentages indicate that large changes between successive sets of samples coincided with the widespread wildfires at five of the six sites. After the wildfires, sagebrush (Artemisia) and other meadow taxa increased as conifers, especially spruce (Picea), declined across the landscape, indicating that the forests opened.
Synthesis. The opening of the forests may have created fuel breaks across the mountain range that limited wildfire after temperatures rose ~0.5 °C. When the openings then became larger and the area covered by ribbon forest expanded during the Little Ice Age (LIA), the extent of fires further declined. Pollen assemblages associated with modern ribbon forests only became common across our study sites during the LIA when the frequency of fires across our sites reached its minimum. The rise of novel ribbon forests in northern Colorado thus illustrates how climate and fire can interact to rapidly transform landscapes and their disturbance regimes.