A landscape perspective on stability and change over 4000 years in northern Wisconsin

Background/Question/Methods

This study used paleoecological records from small lakes to address questions about long-term responses of different forest communities to climate change. Which community types respond most quickly and change most? Which communities 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.  Soil attributes, including a suite of 11 soil variables from the SSURGO database, were used to classify the landscape within 5 km of each of 13 small lakes where pollen was analyzed at a median interval of 70 years and grouped into 16 community types with hierarchical cluster analysis.  Transition matrices were calculated for shifts between each of the 16 community types over four time intervals and three subregions with different soil and landscape attributes. Charcoal particles >125 microns were counted to identify local fire events. Graminoid cuticle was tallied separately from other charcoal particle types.  Charcoal signature types were identified using a stratigraphically unconstrained incremental sum-of-squares cluster analysis of bCHAR, graminoid bCHAR, the square root of CHAR:bCHAR, and the mean number of fires/500 years (b= background, CHAR= charcoal accumulation rate). 

Results/Conclusions

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 decadal or longer 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 relative abundance of pollen types in jack pine communities fluctuated more rapidly at 70-year intervals than 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.