Mountain ecosystems are known to be particularly vulnerable to global environmental change. Faster warming rates have been observed at higher altitudes in mountains across the world, and alterations to fire regimes and nitrogen deposition rates are especially disruptive in high altitude environments. In recent years, multiple studies have documented significant changes in species distributions across altitudinal gradients. However, studies typically focus only on alpine tree-line environments excluding subalpine habitats, and rarely take into account local environmental variation as a factor that can modulate the responses of species to climate change. We used a combination of remote sensing, field population ecology, and dendrochronology methods, to explore how local environmental variation modulates distributional and growth responses of two species of endemic conifers from Central Mexico (Abies religiosa and Pinus hartwegii) to climate change. We collected data from 7 transects and 30 research plots between 3,600 m to 3,880 m and on different slope aspects on the western side of the Iztaccihuatl volcano, ~ 70 km southeast of Mexico City.
Results/Conclusions
We found substantial evidence of upslope expansion of A. religiosa populations into P. hartwegii habitat. There is an important mismatch between the present day adult ecotone of both species, roughly at 3,600 m, and the interspecific seedling to adult ratio equilibrium, found at 3,760 m. Colonization of A. religiosa is more intense and reaches higher elevations on north facing slopes, while P. hartwegii populations show very low recruitment at the lower end of their distribution, particularly on south facing slopes. Growth trends in P. hartwegii also show significant reductions on south facing slopes and at low elevations, while A. religiosa individuals exhibit more steady growth during the past decades. Moreover, a comparison between the early years of adult (>60 yro) and juvenile (<30 yro) pines, shows significant differences in growth rates, suggesting young individuals have had to cope with more adverse conditions relative to their parents. Lastly, a combination of remote sensing analysis with field data indicates that while intense fires have not occurred since 1986 in the area, fire has not been completely suppressed, potentially ruling out changes in the fire regime as mechanisms driving these patterns. Upcoming analyses using stable isotopes from tree rings plus tree survival data collected after this 2015-16 ENSO year will reveal more mechanistic processes at play. Our preliminary results reinforce the potential of using local scale field data to improve modeling accuracy and management effectiveness.