Shaping gas exchange under drought: Seasonal and interspecific variation in photosynthetic traits of Pinus halepensis and Pinus brutia in Mediterranean forests

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As Mediterranean pine forests are increasingly exposed to intense drought events that may threaten their survival, it is important to improve our understanding of their ecophysiological responses to climatic drivers under different temporal and spatial scales. We studied key photosynthetic traits in different ecosystems of Pinus halepensis and the less studied Pinus brutia by applying an integrated, multi-site and multi-season comparative approach. For this purpose, we performed seasonal gas exchange measurements across three sites (Lesvos, Sani, and Xanthi) and ten plots in Greece to assess how species-specific stomatal strategies, microclimatic drivers, and stand structure shape carbon assimilation. We assessed CO 2 - and light-saturated photosynthetic capacity (A max ), stomatal conductance (g s ), and intrinsic water use efficiency (iWUE ) , and their responses to vapor pressure deficit (VPD) and the land surface water index (LSWI). No effect of stand structure, but significant variation among sites and seasons in A max , g s , and iWUE was detected. P. halepensis in Sani and P. brutia in Xanthi exhibited a bimodal photosynthetic activity, showing the highest A max in spring and its recovery in autumn or winter under favorable microclimate, reflecting an adaptive strategy to exploit favorable microclimatic windows. P. brutia in Lesvos, characterized by the highest VPD, exhibited a more conservative seasonal pattern with a peak of A max only in spring, reflecting prolonged atmospheric constraints on stomatal opening. Overall, atmospheric drought vs. land surface water availability had a stronger control on the A max of both species, reflecting a prioritization of hydraulic safety, in which both species employ isohydric stomatal regulation to increase their intrinsic water use efficiency at the cost of carbon uptake. However, P. brutia showed a less steep decline in A max under high atmospheric water demand, indicating a less isohydric response than P. halepensis that allows the maintenance of photosynthetic activity over a wider range of VPD and offers an advantage under increasingly intensified drought regimes. • Microclimate rather than stand structure drives gas exchange at the studied pines. • A bimodal photosynthesis pattern occurs if drought is alleviated in winter or autumn. • Atmospheric drought controlled gas exchange more than land moisture across sites. • P. brutia showed less isohydric responses than P. halepensis . • P. brutia exhibited a slower photosynthetic decline over wider atmospheric drought.

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