Ecological context tempers the response of plant communities to climate change
Clearly plant communities respond to climate change, but the ways in which ecological context can affect those responses have been difficult to assess. We used paleoecological records from small lakes to ask how long-term responses of forest communities to climate change differ with local landscape and ecological context. Which communities respond most quickly and change most? Which are most stable? Under what conditions do sudden state changes occur? We focused on the response of forest communities on sandy soils in northwestern Wisconsin to climate changes of the last 4000-8000 years. The 13 study sites differ in the abundance of potential fire breaks (lakes and wetlands) and soil quality within 5 km. Pollen from a sediment core collected from each lake was analyzed at ~70 year intervals and samples from all sites were grouped into 16 community types with hierarchical cluster analysis. Transition matrices were calculated for these 16 community types over four time intervals and three subregions with different landscape attributes. Charcoal particles >125 microns were counted to identify local fire events. Fire regimes were classified using a stratigraphically unconstrained cluster analysis of bCHAR, graminoid bCHAR, sqrt CHAR:bCHAR, and mean # fires/500 years (b= background, CHAR= charcoal accumulation rate).
Transition matrices calculated using >500 transitions show that jack pine (Pinus banksiana) communities with strong fire feedbacks remained stable during several periods of climate change, including multidecadal droughts and the increase in moisture at the beginning of the Little Ice Age ca. 700 cal yr BP. In contrast, oak (Quercus spp.) and white pine (Pinus strobus) communities were more likely to change species composition and disturbance regimes. The pollen assemblages in jack pine communities fluctuated more rapidly at 70-year intervals than in other community types, but pollen assemblages remained within the range of jack pine community types over thousands of years. We attribute this pattern of rapid fluctuations and long-term stability to the feedback between jack pine and stand-replacing fires. Communities with more oak and white pine had much larger and sometimes rapid state changes from oak- to mixed pine- to white pine-dominated forests. In some cases the state changes were coincident with relatively rapid climate changes at about 1500 and 700 years ago. Analysis of change in community types vs. smaller fluctuations in relative abundance at sites that differ in soil texture provides a framework for incorporation of long-term vegetation observations into models.