Abstract
Tree rings provide fruitful information on climate features driving annual forest growth through statistical correlations between annual tree growth and climate features. Indices built upon tree growth limitation by carbon sequestration (source hypothesis) or drought-driven cambial phenology (sink hypothesis) can be used to better identify underlying processes. We used both analytical frameworks on Quercus suber, a sparsely studied species due to tree ring methodological issues, and growing on a favorable sub-humid Mediterranean climate and deep soil conditions in Tunisia (North Africa). Statistical analysis revealed the major role of autumnal rainfall before the growing season on annual tree growth over the 1918–2008 time series. Using a water budget model, we were able to explain the critical role of the deep soil water refill during the wet season in affecting both the drought onset controlling growth phenology and the summer drought intensity affecting carbon assimilation. Analysis of recent climate changes in the region additionally illustrated an increase in temperatures enhancing the evaporative demand and advancing growth start, and a decline in rainfalls in autumn, two key variables driving stem growth. We concluded on the benefits of using process-based indices in dendrochronological analysis and identified the main vulnerability of this Mediterranean forest to autumnal rainfall decline, a peculiar aspect of climate change under summer-dry climates.
Highlights
Tree rings are valuable climate archives that bear witness to the yearly climate conditions that effectively drive trunk expansion over the span of decades to centuries [1]
The sink limitation hypothesis for Mediterranean climate states that annual tree growth is limited by phenological phases of cambial activity controlled by the dual role of cold temperatures in the winter and the onset/offset of drought [8], while the source limitation hypothesis states that stem growth is limited by carbon assimilation, highly affected by summer drought under Mediterranean climatic conditions
In our site, which is characterized by a favorable water balance on deep soil conditions, we identified that wood growth is mainly controlled by the water stored before the growing season and depends less on the spring and summer rainfall
Summary
Tree rings are valuable climate archives that bear witness to the yearly climate conditions that effectively drive trunk expansion over the span of decades to centuries [1]. They have a seasonal resolution, are absolutely dated, and are available in a wide range of climatic settings [2]. The sink limitation hypothesis for Mediterranean climate states that annual tree growth is limited by phenological phases of cambial activity controlled by the dual role of cold temperatures in the winter and the onset/offset of drought [8], while the source limitation hypothesis states that stem growth is limited by carbon assimilation, highly affected by summer drought under Mediterranean climatic conditions. Regional variations in soil field capacity within a homogeneous climate region that locally affect soil water balance and drought features [11] can lead to contrasted results in the statistical relationship between seasonal climate variable and tree rings
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