INTRA-ANNUAL CLIMATIC DRIVERS OF GROWTH AND WOOD ANATOMY AGREE WITH XYLOGENESIS PATTERNS IN TWO MEDITERRANEAN CONIFERS

In Mediterranean continental climates, trees face a double climatic stress, cold winters and dry summers. Improved understanding of the different climate sensitivity in coexisting tree species may help to better predict future forest dynamics in a global warming scenario. We measured wood anatomical features as related to weekly- to monthly-resolved weather records to determine the main climatic drivers of wood formation at intra-annual scales. Two coexisting Mediterranean conifers (Pinus halepensis and Juniperus thurifera) were considered to measure tree-ring width and anatomical variables (lumen transversal area and cell-wall thickness) using the software ROXAS. We related these variables with local climatic data (mean minimum and maximum temperatures, precipitation, soil moisture deficit) for the period 1970-2013. Both tree-ring width and lumen area showed similar year-to-year variability indicating common climatic signals acting on both variables. Tree-ring width increased as previous winter precipitation did. Lumen area responded positively to wet spring conditions, particularly in the case of the juniper. In juniper, cell-wall thickness increased when early summer conditions became drier. These observations are consistent with the presence of intra-annual density fluctuations (IADF) in the juniper latewood which increased in parallel with late-summer water balance. This confirms that latewood IADFs reflect changing water availability in the late growing season as larger tracheid lumens. The analysis of wood anatomical traits allowed inferring long-term growth and wood-anatomical responses to climate at intra-annual scales, which were consistent with previous observations of xylogenesis and the different sensitivity to climate of the two coexisting conifer species