Abstract
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 (gs), percent loss of stem xylem hydraulic conductivity (PLC) and photochemical efficiency of photosystem II (Fv/Fm). Our meta‐analysis showed that in ecosystems without summer drought, gs 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 Fv/Fm 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.
Highlights
Woody species in temperate forests show pronounced annual dynamics in growth and ecophysiology (Lo Gullo et al 2005, Gulías et al 2009, McCulloh et al 2011, Carnicer et al 2013, Granda et al 2013)
As indicators of physiological response of leaves and xylem to seasonal stress, we focused on stomatal conductance, percent loss of stem xylem hydraulic conductivity (PLC) and photochemical efficiency of photosystem II (Fv/Fm)
We focused on the photochemical efficiency of photosystem II, as a measure of photosynthetic function of leaves which is highly related to seasonal stress and may indicate acclimatization to extreme events (García-Plazaola et al 1999)
Summary
Woody species in temperate forests show pronounced annual dynamics in growth and ecophysiology (Lo Gullo et al 2005, Gulías et al 2009, McCulloh et al 2011, Carnicer et al 2013, Granda et al 2013). In this study we tested for contrasting physiological responses for woody functional types which experience distinct seasonal extremes during summer and winter across the temperate zone. For critical detail on the response of species experiencing winter freezing in a drought prone ecosystem, and because the meta-analysis approach can be misleading due to pooling data for average patterns across species growing under disparate conditions and measured with varying techniques, we conducted a case study of Mediterranean species of the three functional types in central Spain. Our data allowed a test of the hypothesis that stomatal dynamics are influenced by the status of the hydraulic system generally across seasons These hypotheses provided a baseline framework for understanding the impacts of seasonal extremes, and we provide a first test for different functional types across the temperate zone
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