Variable climate response differentiates the growth of Sky Island Ponderosa Pines

Key message The seasonally cool and moist conditions of spring improved the growth of two co-occurring ponderosa pine species, which displayed different seasonal climatic responses and length of correlations to drought. We examined the climatic sensitivity of two partially sympatric pine species growing at their transition zone in the Santa Catalina Mountains, AZ, USA. Pinus arizonica is found at lower elevations compared to P. ponderosa var. brachyptera . Ring widths were measured in trees at two sites and correlated with precipitation, temperature, and Palmer Drought Severity Index to assess the influence of climate on growth. The two species were analyzed within and between sites, which have similar elevation, aspect, and species composition, although soils at the two sites have different water-holding capacities. Response function analyses of P. arizonica [sampled near its upper (and wetter) elevation limit], and P. ponderosa var. brachyptera [sampled near its lower (and drier) elevation limit] indicated that annual growth correlated positively and strongly with spring precipitation at both study locations. Local site conditions had a major impact on tree growth and variability in site conditions helped resolve the differences in species’ response to climate. For example, at the less dry site, growth of the lower-elevation pine ( P. arizonica ) responded to early-winter precipitation, while P. ponderosa var. brachyptera did not. Also, correlation analysis indicated that P. arizonica’s growth was more sensitive to drought for longer periods than P. ponderosa var. brachyptera . Finally, partial temperature-growth correlations of P. arizonica and P. ponderosa var. brachyptera indicated growth was limited by increased growing season and winter respiration, respectively. Rising night-time temperatures during spring significantly reduced growth of P. arizonica at Mt. Lemmon. These findings demonstrate subtle yet meaningful interspecies differences in sensitivity to seasonal moisture stress and use of carbon resources.