Leaf and stem physiological responses to summer and winter extremes of woody species across temperate ecosystems

Winter cold limits temperate plant performance, as does summer water stress in drought-prone ecosystems. The relative impact of seasonal extremes on plant performance has received considerable attention for individual systems. An integrated study compiling the existing literature was needed to identify overall trends. First, we conducted a meta-analysis of the impacts of summer and winter on ecophysiology for three woody plant functional types (winter deciduous angiosperms, evergreen angiosperms and conifers), including data for 210 records from 75 studies of ecosystems with and without summer drought across the temperate zone. Second, we tested predictions by conducting a case study in a drought-prone Mediterranean ecosystem subject to winter freezing. As indicators of physiological response of leaves and xylem to seasonal stress, we focused on stomatal conductance (g(s)), percent loss of stem xylem hydraulic conductivity (PLC) and photochemical efficiency of photosystem II (F-v/F-m). Our meta-analysis showed that in ecosystems without summer drought, g(s) was higher during summer than winter. By contrast, in drought-prone ecosystems many species maintained open stomata during winter, with potential strong consequences for plant carbon gain over the year. Further, PLC tended to increase and F-v/F-m to decrease from summer to winter for most functional types and ecosystems due to low temperatures. Overall, deciduous angiosperms were most sensitive to climatic stress. Leaf gas exchange and stem xylem hydraulics showed a coordinated seasonal response at ecosystems without summer drought. In our Mediterranean site subjected to winter freezing the species showed similar responses to those typically found for ecosystems without summer drought. We conclude that winter stress is most extreme for systems without summer drought and systems with summer drought and winter freezing, and less extreme for drought-prone systems without freezing. In all cases the evergreen species show less pronounced seasonal responses in both leaves and stems than deciduous species.