Vegetation response to climate change depends on the abundance, type, and distribution of firebreaks
The response of vegetation to future climate change is poorly understood at the spatial scales most relevant to human activities. Paleoecological methods can reconstruct vegetation responses to past climatic change at the landscape scale, but data are rarely collected at sufficient spatial and temporal scales to reveal how different parts of a landscape responded to climate changes. Previous work demonstrated that sites lacking abundant firebreaks responded differently to late-Holocene climate changes than those with abundant lakes and wetlands within 5 km. We hypothesized that during more severe, prolonged dry periods, wetlands would dry up and cease to offer protection from fire spread. Sites protected by wetlands in the late Holocene would have experienced increased fire frequency and developed fire-dependent jack pine forest similar to the vegetation around sites with little fire protection today. We developed a Holocene lake-level record to identify times of severe drought and compared vegetation changes at three sites, with different abundance and types of fire protection within 5km: unprotected (Metzger, 5.5% lakes, 1.5% wetlands), protected by wetland (Ferry, 10% lakes, 15% wetlands) and protected by lakes (Lonestar, 15% lakes, 10% wetlands).
Lake-levels at Cheney Lake, near the center of the sand plain, were ~1.5 m lower during the mid-Holocene (8000-4000 years ago), suggesting that the extent of wetlands would have been much reduced. During this period lakes would have provided the only effective fire protection. Our results from pollen analysis are consistent with the hypothesis. Pollen at Ferry Lake was dominated by jack/red pine during the mid-Holocene. Ferry contained an average of 53% jack/red pine and 13% oak pollen during the mid-Holocene with generally less red/jack pine and higher white pine and birch pollen in the late-Holocene. Metzger, with few firebreaks of any kind, maintained high jack/red pine pollen percentages throughout the last 8000 years (52% jack/red pine and 7% oak pollen). Lone Star, with more abundant lakes and few wetlands nearby, maintained both oak and pine forest during this time (27% jack/red pine and 31% oak pollen). We conclude that the abundance, type, and distribution of firebreaks on the landscape are important drivers of a changing vegetation mosaic in response to climate change. Our results demonstrate that paleoecological data with high spatial and temporal resolution provide a valuable tool to predict landscape responses to past and future climate change.