Wood anatomical and physiological differences between semiarid pine and oak, stand up for their tree ring sensibility to precipitation variability

Abstract

Reconstruction of past precipitation cycles using tree ring growth is a common practice to study climatic conditions from the past. However, several studies have attested that not all tree species provide with the same accuracy to reconstruct past climate and in particular precipitation. In this study, we present evidence that anatomical and morphological differences in trunk and root traits of two forest species provide different capacities to access restricted water sources, these explains the different sensitivities of tree-ring growth to annual precipitation by the species. To test this suggestion, we examined trunk anatomy, root distribution, tree-ring growth, natural isotopic label of tree rings (δ13C) as well as the water use efficiency (WUE) and leaf water potential (Ψleaf) for two tree species (oak - Quercus potosina and pine - Pinus cembroides) from a semiarid forest in central México. Results showed large differences in stem and root anatomy between species, with oak fine roots equipped with a triple layer of epidermal tissue containing calcium oxalate crystals (druses). These root characteristics permitted oak to explore deeper rock fractures and crevices reaching additional water sources, whereas pine fine roots were mostly distributed in the first 10 cm of the topsoil. Oak also possess trunk structural anatomy of specialized tissue formed by fiber tracheid's connecting vessel tissue to water storing, which is used during particular drought periods. Oak tree-ring growth and their δ13C values that corresponded with wet conditions were observed in years below average precipitation. In comparison, the same variables in pine changed accordingly to precipitation variability, since pine did not have traits helping to access additional water sources during drought.