Effects of climate on wood formation of Quercus robur L. and Quercus pyrenaica Willd. along a mediterraneity gradient in Galicia: an Integrated analysis using phenology, anatomy, and dendroecology

In woody plants, xylem tissue is involved in multiple key functions, such as long distance water and nutrient transport, mechanical support, and storage. Tree phenology and carbohydrate availability are assumed to influence earlywood formation in ring porous species, affecting hydraulic performance and wood production. However, the functional relationships between tree phenology, seasonal dynamics of growth, wood anatomy, and carbohydrate content have not been previously addressed for these species. Quercus robur is widespread across Europe, being abundant in areas under oceanic climatic influence. By contrast, Q. pyrenaica is frequent in mountain areas along the temperate Mediterranean transition, mainly in the Iberian Peninsula. Q. pyrenaica is therefore more tolerant to both winter frost and summer drought than Q. robur. This thesis combines the analysis of leaf phenology, xylogenesis, non structural carbohydrate (NSC) content in sapwood, and tree ring series in order to provide a mechanistic knowledge on the climatic regulation of leaf and cambial phenology in ring porous oaks, as well as to understand their consequences on wood anatomy and carbon allocation to growth. In addition, this investigation seeks to assess whether divergent stress tolerance strategies held by the coexisting Q. robur and Q. pyrenaica imply a contrasting adjustment of phenology, wood anatomy, and NSC content to shifting environmental conditions in the temperate Mediterranean transition. I selected three sites in Galicia (Spain) along a water availability gradient where both study species coexist. Leaf and cambial phenology were weekly monitored for ten individuals per site in 2012 and 2013. Microscopic xylem sections were obtained in order to register different anatomical parameters. Soluble sugars and starch concentrations were quantified in sapwood cores of 40 trees per species and site by using the anthrone method. Additionally, seasonal dynamics of NSC content were monitored bimonthly in 2012. To analyze tree ring series, two cores per tree were collected out of a total of 20 trees per site and species. Yearly series of tree ring widths and earlywood vessel areas enabled identifying those climatic drivers modulating xylem growth and anatomy. Warm conditions in late winter advanced phenology in both species, although cambial activity resumed earlier in Q. robur than in Q. pyrenaica. Whereas a longer period of earlywood formation resulted in larger vessels for Q. robur, earlywood vessel diameter in Q. pyrenaica probably relied on the rate rather than on duration of earlywood enlargement. The number of cambial cells at dormancy fostered growth capacity in both species, allowing for a longer active period and a wider xylem increment. Earlywood formation relied on stored carbohydrates, whilst current assimilates supported latewood growth concurrently with storage refilling. Feedbacks between soluble sugar content and earlywood vessels were stronger for Q. pyrenaica, which appeared to have a carbon saving strategy to face climatic extremes commonly occurring in the sub Mediterranean area. In study oaks, temperature during the dormant and quiescent periods played a key role in earlywood formation, probably through its effects on the timing of growth resumption and soluble sugar content. Water stress in late spring and summer was the main constrain for xylem production, although cold autumn conditions limited latewood formation at the most humid location. Cambial activity was highly resilient, as demonstrated by the post drought cambial resumption observed at the driest location in autumn. In addition, hydraulic efficiency appeared to be prioritized over safety under increasing xeric conditions, being concurrent with an increment in stored carbohydrates. These results suggest that the persistence of ring porous oaks in the Iberian Peninsula may be compromised under future climate change scenarios insofar extreme episodes hinder earlywood and foliage development in spring, even for Q. pyrenaica, despite its multiple adaptations to cope with environmental stress.