Intra-annual growth, gas exchange and foliar phenology response to drought in Quercus pyrenaica and Pinus sylvestris
Climatic scenarios for the Mediterranean region forecast increasing frequency and intensity of drought events. With climate change, Eurosiberian conifer Pinus sylvestris is expected to reduce its distribution range in the region, being displaced at low altitudes by more drought tolerant taxa such as sub-Mediterranean Quercus spp. We analyzed how P. sylvestris and Quercus pyrenaica, a ring-porous species, modify the relationships among intra-annual growth dynamics, leaf phenology and gas exchange under enhanced drought conditions. A rainfall exclusion experiment was established in 2012 in a mountain forest stand located at the altitudinal distribution limit of these two species in Central Spain. Micro-cores were sampled from six trees per treatment and species every 7-15 days between April and November during the period 2012-2014 at both the rainfall exclusion and control plots. The number of tracheids, in P. sylvestris, and length of growth increments, in Q. pyrenaica, at each cell development phase (cambium, enlargement, wall-thickening and mature) was measured from stained microsections to analyze the effect of drought on cambial phenology and intra-annual growth. Foliar phenology was recorded on the same dates of micro-core sampling. Additionally, gas exchange and leaf water potential were measured in early-, mid- and late-summer between 2012 and 2014.
Growing season started earlier in Q. pyrenaica than in P. sylvestris. The beginning of wood formation seemed to be influenced by temperature and photoperiod, whereas the ending was significantly shorter in trees subjected to rainfall exclusion, indicating a strong influence of summer drought on the length of the growing season. In Q. pyrenaica, budburst and the lignification of the vessels in the first row occurred synchronously, indicating current-year vessels are functional during leaf formation. In P. sylvestris, budburst and blooming occurred when the ring was enlarging but tracheids had not yet lignified, which suggests that previous-year tracheids were used for water transport during early needle formation. These results also imply a strong reliance on stored carbohydrates at the beginning of the season in both species. Photosynthetic rates and stomatal conductance tended to be higher on control trees than on trees subjected to drought in P. sylvestris. Highest photosynthetic rates occurred during mid-summer (July-August), when cell enlargement rates were lowest. Recent decreasing growth trends in both species, particularly under the rainfall exclusion treatment, could indicate that both species will be vulnerable to increasing drought stress as a result of climate change, with P. sylvestris showing a higher risk of drought-induced mortality.