Growth, isotope records and quantitative wood anatomy reveal species-specific couplings in three Mexican conifers inhabiting drought-prone areas

An improvement of our understanding of how tree species will respond to warmer conditions and longer droughts requires comparing their responses across different environmental settings and considering a multi-proxy approach. We used different xylem traits (tree-ring width, formation of intra-annual density fluctuations –IADFs, wood anatomy, D¹³C and d¹⁸O records) to retrospectively quantify these responses in three conifers inhabiting two different drought-prone areas in northwestern Mexico. A fir species (Abies durangensis) was studied in a higher altitude and more humid site and two pine species were sampled in a nearby, drier site (Pinus engelmannii, Pinus cembroides). Tree-ring-width indices (TRWi) of all the species showed very similar year-to-year variability, likely indicating a common climatic signal throughout the whole region. Wood anatomy analyses, covering over 3.5 million measured cells, showed that P. cembroides lumen area was much smaller than in the other two species and it remained constant along all the studied period (over 64 years). Alternately, cell wall was ticker in P. engelmannii which also presented the highest amount of intra-annual density fluctuations. Climate and wood anatomy correlations pointed out that lumen area was positively affected by winter precipitation for all the species, while cell-wall thickness was negatively affected by current season precipitation in all species but P. cembroides, suggesting this taxon may be better adapted to withstand drought than its coexisting conifer with thinner cell walls resulting from wet winters. Stable isotope analysis showed in P. cembroides some of the lowest cellulose-Δ¹³C mean values ever reported in the literature for a forest tree species, although there were no particular trend differences between the studied species. As well, no significant δ¹⁸O differences where found between the three species, but they shared a common decreasing trend. With very distinct wood anatomical traits (smaller cells, compact morphology), P. cembroides stood out as the better-adapted species in its current environment and could be less affected by future drier climate. P. engelmannii and A. durangensis showed high plasticity at wood anatomical level, allowing them to promptly respond to seasonal water availability, however this feature may provide few advantages on future climate scenarios with longer and more frequent drought spells. Further research, including xylogenesis analysis and monitoring of different populations of these tree species, would be still necessary to reach a clearer understanding of their future responses to weather patterns. Our multi-proxy approach could be used in other forests to characterize the in situ functioning of trees, e.g. growth, water use, and development of strategies for forest management under the current climate change scenarios.