Needle-Age-Dependent Photosynthesis in Evergreen Conifer Trees under Drought Stress

Conifer trees have developed morphological, anatomical, and physiological adaptations to maintain growth and photosynthesis at low water availability. Here, two crucial questions about the future arise: firstly, whether these adaptations are sufficient to maintain or increase growth and photosynthesis rates and secondly, whether boreal forests will remain effective carbon sinks to stock increasing concentrations of CO2 under global warming, which are associated with a higher frequency and severity of drought episodes. The photosynthetic machinery of evergreen conifer trees is protected against drought through the xeromorphic structure of their needles, that is, a thick epidermis, vascular layer, and stomata located deeply in the epidermis. The maintenance of many needle cohorts differing in age can also be regarded as an adaptation to drought; however, this diminishes the ability of conifers to recover faster the needles damaged by drought. Compared with young needles, old ones have reduced needle water, nitrogen content, net CO2 assimilation rates, stomatal conductance, and water and nitrogen use efficiency, but their leaf mass-to-area ratio increases non-linearly. Resources from older, shaded needles are translocated to younger and better-illuminated needles to increase overall photosynthetic capacity and improve the carbon balance, which is beneficial for survival and growth under drought. Stomatal closure is the first response to drought. Such a response may effectively protect the photosynthetic machinery at low or moderate water shortages and particularly, at high CO2 concentrations. Photochemical processes are less sensitive to drought than stomatal and mesophyll conductance and photosynthetic biochemistry.