Climate–fire–vegetation interactions and the rise of novel landscape patterns in subalpine ecosystems, Colorado

Interactions at multiple scales can shape how forest ecosystems respond to both climate change and disturbance. At landscape scales, feedbacks cause vegetation and wildfire regimes to alter one another, while forest patterns at local scales can result from feedbacks between plants and their abiotic environment. Consequently, disturbances and abiotic changes may give rise to forest patterns that further affect processes like wildfire. Examples may exist in the history of subalpine ribbon forests, which are alternating areas of forest and meadow where snow drifting and linear bands of trees depend upon each other. Just as ongoing climate changes may change snow levels and the dynamics in these forests, past climate changes and wildfires may have generated the conditions suitable for ribbon forests. To examine feedbacks in subalpine forests and the potential that climate changes and fires gave rise to ribbon forests, we obtained six fossil pollen records from a subalpine landscape in Colorado. Forests there may have responded to climate change and widespread wildfires ˜1,000 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. The pollen records indicate that large vegetation changes coincided with the widespread wildfires at five of the six sites. Pollen assemblages consistent with ribbon forests first became important at this time as sagebrush (Artemisia) and other meadow taxa increased and conifers, especially spruce (Picea) declined. The ribbon forests emerged as novel fuel breaks at the time when temperatures rose ˜0.5°C but the number of sites burned per century declined. Cooling during the Little Ice Age then expanded the openings and extent of ribbon forests, probably helping to reduce the frequency of fires across our sites to its minimum. Synthesis. The rise of ribbon forests in northern Colorado illustrates how climate and fire can interact to rapidly transform landscapes and disturbance regimes. The interactions can produce forest patterns with unexpected consequences such as reduced wildfire despite regional warming.