Warming effects on tree-ring variables in P. hartwegii Lindl. at the extremes of its natural elevational distribution in central Mexico

Abstract

In alpine forests, the impact of warming differs at the upper and lower extremes of the elevation gradient causing trees to compensate through different adaptive mechanisms. The objective was to retrospectively evaluate the radial growth response to recent interannual climate variation in Pinus hartwegii trees growing at the extremes of their elevational gradient on two volcanic mountains in central Mexico (La Malinche and Nevado de Toluca). Increment cores collected from 174 P. hartwegii trees were used to estimate time-series of growth ring including ring width and density variables, as well as basal area increment (BAI) and ring biomass (BIO) for 1964–2016. Differences in average radial growth and time-trends of ring variables in response to climate variables were estimated based on correlations of ring variables, BAI, and BIO with inter-annual variation in temperature, rainfall, and length of the frost-free period (FFP). Results showed higher average values of ring width and density variables, as well as BAI and BIO (i.e., higher productivity) at low elevation on both mountains. Time-trends for BAI and BIO were positive at both ends of the elevation gradient, but ring width and density variables followed opposite trends. Inter-annual variation of ring variables was explained more by temperature than by rainfall or FFP; however, temperature was negatively correlated with RW and positively with RD, even though BAI and BIO were positively correlated with temperature (All them with P < 0.001) at both elevational end on both mountains. For most of the ring variables, the total variation explained by the ANOVA model was more related to the linear warming trend (greater than 67%). Warming is affecting radial growth of P. hartwegii trees differently at high versus low elevation on both mountains, showing warning signs from the perspective of functional traits, but not in terms of productivity.