Local Climate Forces Instability in Long-Term Productivity of a Mediterranean Oak Along Climatic Gradients.

Forests modify their productivity, composition, and distribution in response to global change. We studied the radial growth trends of the Western Mediterranean oak Quercus pyrenaica over the last two centuries to analyze whether trees exhibited instability in productivity in response to climatic changes. Trees were sampled to build annual growth chronologies following climatic gradients of increasing moisture availability and decreasing temperature with altitude and latitude. The species' response to climate showed high variability linked to local climatic conditions. The strength in the positive response of trees to moisture availability was inversely related to precipitation (that is, enhanced by higher water stress) whereas high temperature in the growing season was positive for tree-growth only at cold sites. The oldest ages of trees expanded back to the late 1500 s. These old-growth trees were located at the coldest sites and exhibited a long-term increase in productivity starting 150 years ago which could express a dominant positive effect of warming temperatures since the mid 1800 s at cold-humid sites. Conversely, trees at dry sites exhibited negative growth trends. Particularly low elevation stands located at latitudes below 40° displayed enhanced growth constraints with the increase in water stress around 1970, which suggests vulnerability of Quercus pyrenaica at the sampled altitudinal dry edge. The response of trees to future changes in climate should be monitored, particularly in threatened transitional zones. [ABSTRACT FROM AUTHOR]