Leaf Mechanical Strength Corresponds to Tissue Water Relations in Twelve Species of California Ferns

The dominant vegetation types in southern California’s coastal foothills are chaparral and costal sage scrub. Chaparral shrubs have mechanically strong evergreen leaves whereas coastal sage scrubs bear mechanical weak, facultative deciduous leaves. What about the ferns that live in the understory of these vegetation types, especially considering their adaptations to a summer dry, Mediterranean-type climate? We tested the hypothesis that some fern leaves are stronger than others and mechanically strong leaves are associated with greater dehydration tolerance. Twelve fern species were examined. Tissue water relations were assessed via pressure volume curves using Scholander-Hammel pressure chambers. We estimated osmotic potential at saturation (Ψs,sat) and at the turgor loss point (Ψs,tlp). We examined pinna strength using an Instron Mechanical Testing Device to measure Young’s Modulus (YM) and tensile stress at break (TSB). We also measured vein density to determine if it was associated with mechanical strength. We found significant differences among our 12 fern species. Young’s Modulus was positively correlated with dehydration tolerance of leaf tissues, increasing with osmotic potentials at saturation (r2 = 0.514) and osmotic potentials at the turgor loss point (r2 = 0.536). Consistent with our initial hypothesis, we also found vein density to increase with mechanical strength and Young’s Modulus to increase with increasing tensile stress at break. We conclude that similar to species of chaparral shrubs and coastal sage scrubs in California the mechanical strength that increasing mechanical strength of leaves may be associated with increasing dehydration tolerance of their cellular tissues