We studied stomatal responses to decreasing predawn water potential (Ψ_pd) and increasing leaf-to-air water vapor pressure difference (VPD) of co-occurring woody Mediterranean species with contrasting leaf habits and growth form. The species included two evergreen oaks (Quercus ilex subsp. ballota (Desf.) Samp. and Q. suber L.), two deciduous oaks (Q. faginea Lam. and Q. pyrenaica Willd.) and two deciduous shrubs (Pyrus bourgaeana Decne. and Crataegus monogyna Jacq.). Our main objective was to determine if stomatal sensitivity is related to differences in leaf life span and leaf habit. The deciduous shrubs had the least conservative water-use characteristics, with relatively high stomatal conductance and low stomatal sensitivity to soil and atmospheric drought. As a result, Ψ_pd decreased greatly in both species during the growing season, resulting in early leaf abscission in the summer. The deciduous oaks showed intermediate water-use characteristics, having maximum stomatal conductances and CO₂ assimilation rates similar to or even higher than those of the deciduous shrubs. However, they had greater stomatal sensitivity to soil drying and showed less negative Ψ_pd values than the deciduous shrubs. The evergreen oaks, and especially the species with the greatest leaf longevity, Q. ilex, exhibited the most conservative water-use behavior, having lower maximum stomatal conductances and greater sensitivity to VPD than the deciduous species. As a result, Ψ_pd decreased less during the growing season in the evergreens than in the deciduous species, which may contribute to greater leaf longevity by avoiding irreversible damage during the summer drought. However, the combination of low maximum CO₂ assimilation rates and high stomatal sensitivity to drought must have a negative impact on the final carbon budget of leaves with a long life span.

• The relationships between leaf longevity and the average photosynthetic capacity of the different leaf age-classes present in the crown were studied in several tree species to understand the ef...

1 It has been proposed that in longer‐living leaves the allocation of biomass to structural components is greater than in shorter‐living leaves, leading to a greater leaf mass per area (LMA) and to lower assimilation rates. However, direct evidence in support of this hypothesis is very scarce. 2 In the present work we investigated the relationships between leaf duration and LMA, leaf thickness and fibre concentrations (cellulose, hemicellulose and lignin) in five oak species, five pine species and three additional tree species, differing in leaf life spans. Correlations among leaf life span and the other leaf traits were obtained both across species (TIPs) and as phylogenetically independent contrasts (PICs). 3 Leaf thickness and LMA increased steadily with leaf longevity. No relationship was found between leaf longevity and the lignin concentration per unit leaf mass. Evergreen leaves were found to have higher mean concentrations of cellulose and hemicellulose than deciduous ones. However, no relationship was observed between leaf longevity and the concentration of structural carbohydrates across the set of evergreen species, although PIC correlations revealed increases in cellulose with leaf longevity within particular lineages. 4 Our findings reveal that leaf reinforcement by structural carbohydrates depends on leaf habit (deciduous vs. evergreen) and, within a given lineage, also on leaf longevity. However, among the evergreen species co‐occurring in a particular environment, leaf duration may apparently be increased, with no need for increases in the concentration of structural components per unit leaf mass.

The competitive equilibrium between deciduous and evergreen plant species to a large extent depends on the intensity of the reduction in carbon gain undergone by evergreen leaves, associated with the leaf traits that confer resistance to stressful conditions during the unfavourable part of the year. This study explores the effects of winter harshness on the resistance traits of evergreen leaves. Leaf mass per unit area (LMA), leaf thickness and the concentrations of fibre, nitrogen (N), phosphorus (P), soluble protein, chlorophyll and ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) were determined in three evergreen and two deciduous species along a winter temperature gradient. In the evergreen species, LMA, thickness, and P and structural carbohydrate concentrations increased with the decrease in winter temperatures. Nitrogen and lignin concentrations did not show definite patterns in this regard. Chlorophyll, soluble proteins and Rubisco decreased with the increase in winter harshness. Our results suggest that an increase in LMA and in the concentration of structural carbohydrates would be a requirement for the leaves to cope with low winter temperatures. The evergreen habit would be associated with higher costs at cooler sites, because the cold resistance traits imply additional maintenance costs and reduced N allocation to the photosynthetic machinery, associated with structural reinforcement at colder sites.

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