West coast U.S. tree species show stronger evidence for shifts in density than range

Abstract

Climate change is associated with shifts in the geographic range of trees and other taxa globally. In North America and elsewhere tree species’ ranges are expected to shift at rates of meters to kilometers annually. However, empirically detecting a shift in the range of a tree species may be difficult due to the lack of quality long-term regional or national data. Evidence of range shifts is further confounded by apparently contradictory findings from separate studies that use different methods to evaluate species responses. In this study we examine U.S. Forest Service Forest Inventory Analysis plot revisit data in the states of California, Oregon, and Washington to examine shifts in the ranges and densities of tree species relative to values of temperature, precipitation, and maximum and minimum vapor pressure deficit (VPD) using a design-based estimation technique. We also examine changes in these four climate variables for years before and between plot visits. We found that the ranges for 49 tree species seldom shifted in the 10-year period of plot remeasurements, but depending on the climate variable considered, between seven and ten out of 30 species did exhibit density shifts, generally increasing in plots that were initially cooler, drier, and had lower VPD. Overall, plots increased in temperature and VPD values between time periods while they generally experienced declines in precipitation. These changes were frequently not constant across species plots. Temperature and precipitation values were often negatively associated with first visit values (cooler and wetter plots disproportionately warmed and dried). Maximum and minimum VPD generally increased most at plots with initially higher VPD values. These results indicate that the geographical range of U.S. west coast tree species may not change as quickly as their ecological niches geographically shift, although changes in species densities within their ranges may already be underway. The results also indicate that changes in environmental conditions may differ among and within species’ ranges.