Growth, wood anatomy and stable isotopes show species-specific couplings in three Mexican conifers inhabiting drought-prone areas

An improved 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 several traits (tree-ring width, formation of intra-annual density fluctuations - IADFs, wood anatomy, Δ13C and δ18O records) to retrospectively quantify these responses in three conifers inhabiting drought-prone areas in northwestern Mexico. A fir species (Abies durangensis) was studied in a higher altitude and slightly rainier site and two pine species were sampled in a nearby, lower drier site (Pinus engelmannii, Pinus cembroides). Tree-ring-width indices (TRWi) of the studied species showed a very similar year-to-year variability likely indicating a common climatic signal. Wood anatomy analyses done 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). Instead, cell wall thickness was widest in P. engelmannii and this species 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 studied species, while cell-wall thickness was negatively affected by this season's precipitation in all species but P. cembroides. Stable isotope analysis showed significantly lower values of Δ13C for P. cembroides and no significant δ18O differences between the three species, although 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 but likely gives few advantages on future climate scenarios with longer and frequent drought spells.