Tree-rings have become increasingly important to assess forest vulnerability to climate change. With their long-term records and annual resolution of carbon accumulation they represent an important tool for carbon cycle research, as an interface between forest inventories, eddy covariance measurements and satellite data. Previous large-scale assessments to derive climate sensitivities and future growth scenarios have often used chronologies stored in the International Tree-Ring Data Bank (ITRDB). Yet, most of the sites in the ITRDB were sampled at locations with marginal growing conditions - to maximize the climate signal – likely not representative of the average forest ecosystem.
We compare climate sensitivities of Douglas-fir (Pseudotsuga menziesii), ponderosa pine (Pinus ponderosa) and common pinyon (Pinus edulis) chronologies from the ITRDB with tree-ring series collected by the USFS Forest Inventory and Analysis (FIA) program throughout the Interior West of the United States. FIA increment cores were sampled from geographically unbiased plots established with the goal of making population-level assessments. For each sample at every ITRDB site we performed linear regression against gridded climate data and compared climate sensitivities to those of all FIA tree-ring time-series within a search radius of 50 km.
Results/Conclusions
In agreement with previous research we found for all three species an increase in climate sensitivities of the FIA trees (as well as the ITRDB samples) to winter precipitation (previous November through current year March) and annual mean maximum temperatures with decreasing latitude and elevation, and increasing longitude. Furthermore, for Douglas-fir, ITRDB trees were on average >2.8 times more sensitive to winter precipitation variability than surrounding FIA trees, independent of their location throughout the Interior West. Sensitivity of ITRDB Douglas-fir trees to mean maximum temperature variability was also systematically increased. On average, ITRDB trees displayed a 1.27±0.36 times higher inter-annual variability (expressed as standard deviation) than the FIA trees, indicative of higher sensitivity to varying climate conditions, specifically at continental locations and lower elevations. We also found similar tendencies of heightened sensitivity among ITRDB ponderosa pine and common pinyon.
Caution is advised on the sole use of ITRDB chronologies to infer climatic sensitivities for future forest vulnerability assessments at the population level. We recommend that future efforts incorporate systematic sampling campaigns along ecological or other gradients representative of: i) the full climate-space occupied by the species, and; ii) the demographic characteristics of the species or forest ecosystem of interest.