The facultative bimodal growth pattern in Quercus ilex – A simple model to predict sub-seasonal and inter-annual growth

Abstract

In Mediterranean climates, bimodal growth patterns, corresponding to two peaks in radial increment during favorable seasons, have been described in several tree species. However, we lack a better mechanistic understanding of bimodality and its potential responses to the predicted warming and aridification trends. Filling this research gap is important since growth duration affects the capacity of trees to form wood and uptake carbon. Here we used an 11-year (1994–2004) long record of dendrometer data of the Mediterranean Holm oak (Quercus ilex) and compared how climate related to radial increment in trees from the south- and the north-facing slopes. We also related climate variables to tree-ring width and the production of intra-annual density fluctuations (IADFs), which reflects bimodality. In this paper, we introduce a model called VS-Lite2 to simulate tree-growth dynamics, which is a modified version of the process-based Vaganov-Shashkin Lite model. The VS-Lite2 model adequately reproduced the bimodal intra‐annual pattern of radial growth, IADFs, and annual tree growth. Trees from the south-oriented slope grew more, produced more IADFs and showed a more marked bimodal pattern than trees from the north-facing slope. These differences agree with the observation that late-summer drought constrained growth. Therefore, radial-growth models should consider plastic bimodality and micro-environmental conditions in areas subjected to seasonal droughts.