Signs of climatic variation in the width and density of P. hartwegii tree-rings at the ends of its elevational gradient in the mountains of central Mexico

Trees in alpine forests, being sessile and long-lived organisms, are exposed to intra- and inter-annual climatic variation, which is recorded in their growth rings. Specifically, trees growing at the ends of their natural distribution are the most affected due to in those sites the conditions are marginal and adverse for their development and growth. The objective was to evaluate how interannual climatic variation affects the width (RW) and density (RD) of tree-ring formation of P. hartwegii trees in two mountains of central Mexico (La Malinche and Nevado de Toluca). Increment cores collected from 174 P. hartwegii trees were used to estimate time-series of RW and RD for 1964–2016. Differences in average radial growth and time-trends of ring traits in response to climate variables were estimated based on correlations between RW and RD with inter-annual variation in temperature, rainfall, and length of the frost-free period (FFP). Time-trends of RW and RD were negative at both ends of the elevational gradient. Temperature was negatively correlated with RW and positively with RD. For both RW and RD, the total variation explained by the ANOVA model was more related to the linear warming trend. Warming is affecting radial growth of P. hartwegii trees differently at higher elevations than at lower elevations in both mountains, showing distinct warning signals through compensatory effects between RW and RD.