Assessing the drought effect on tree rings' width and xylem traits plasticity in Symplocos coccinea and S. speciosa: two rare and endemic Neotropical montane oak forest trees from eastern Mexico

Key message: Mexican Neotropical Symplocos species showed different wood anatomical climate adaptations and can help identify the effect of drought on relict-rare and endemic tree species. Abstract: Neotropical montane oak forests are exceptionally biodiverse with rich assemblages of relict-endemic species promoting endemism hotbeds. These ecosystems are threatened by climate change, such as drought events and a decrease in fog and drizzle periods. We used dendroecological and wood anatomical tools to assess the effect of historical drought events on annual growth rings and vessel anatomical traits of Symplocos coccinea and S. speciosa, two rare relict-endemic tree species occurring in the Neotropical montane oak forest understory layer. Xylem vessel anatomical traits are essential for assessing and predicting tree–climate response to hydric availability. We analyzed vessel lumen area, diameter, and frequency in the growth rings developed in drought and non-drought years in both Symplocos species. We found that summer temperatures between May and August affect the annual growth. Finally, our analyses demonstrated that high xylem plasticity allowed rapid hydraulic architecture adjustments to drought events, suggesting an essential role of diffuse-porous wood in Neotropical trees to adapt to actual climatic variations. The studied chronologies allow reconstruction of climate variations in the Neotropical montane oak forest from eastern Mexico.