Intra-annual growth dynamics of Mediterranean pines and junipers determines their climatic adaptability

Abstract

Mediterranean trees and shrubs adjust physiological processes to ensure their functioning under shifting dry conditions. However, little is known about spatial and between-species differences in cambial dynamics and how their temporal variability determines climatic adaptability. We used the Vaganov‒Shashkin process-based model to simulate five decades of daily cambial dynamics of coexisting junipers (shrubs/short-stature trees) and pines (tall trees) at two climatically contrasting continental sites in northeastern Spain. We hypothesized that different sites and growth forms may shift from unimodal to bimodal growth patterns, which in turn determines their ability to cope with climate stress. The model produced simulations coherent with ring-width chronologies, xylogenesis data and field monitoring of soil moisture. The model successfully captured differences in growth patterns between sites, but it was unable to highlight differences between coexisting species. The cambial dynamics of all the species were controlled by soil moisture in summer, resulting in bimodal or right-skewed unimodal growth patterns. Simulated cambial dynamics were strongly bimodal, with two distinct peaks in spring and autumn and frequent growth cessation during summer at the dry site with Juniperus thurifera and Pinus halepensis. The growth dynamics of both species at the cold site (Juniperus communis, Pinus uncinata) were rather right-skewed unimodal, suggesting reduced but continuous summer cambial activity. Species at the cold site responded to increasing moisture limitation over the last decades by earlier onset of cambial activity and increased spring cambial kinetics. The shifting of spring growth phenology and kinetics was less effective at the dry site, where both species reduced their growth during summer significantly. Our results suggest that growth adaptability on intra-annual scale determines the performance of woody plants under ongoing climate change in drought-prone regions. High responsiveness of cambial activity corroborates other physiological adaptations which jointly determine the climate-change resistance of junipers and pines.