Functional Strategies of Chaparral Shrubs in Relation to Seasonal Water Deficit and Disturbance

The study of interspecific variation in plant ecological strategies has revealed suites of traits associated with leaf life span and with maximum levels of water deficit (measured as leaf water potentials). Here, the relationship between these sets of traits was examined in a study of 20 co-occurring chaparral shrubs that vary in leaf habit, rooting depth, and regeneration strategies. Leaf life span (LLS) and minimum seasonal water potentials (ψmin) were not significantly correlated, suggesting that they are associated with independent aspects of functional variation. Multiple regression analyses of a large suite of physiological, functional, and phenological attributes in relation to these two "anchor traits" supported this view. Short LLS was significantly associated with high specific leaf area, high carbon assimilation and leaf nitrogen (per mass), early onset of growth, and a multistemmed, short stature growth form. This suite of traits was also associated with opportunistic regeneration following physical disturbance. Area-based gas exchange was not tightly linked to LLS. Low ψmin (i.e., greater water deficit) was associated with high wood density, small vessel diameters, thin twigs, low leaf area : sapwood area ratios, and early onset of leaf abscission. Among the evergreen species, this suite of traits was most characteristic of post-fire seeders, which depend on high drought tolerance for post-fire regeneration of seedlings. Plant stature was the only trait associated with both the LLS axis and the ψmin axis of functional variation. A two-dimensional strategy space, approximately defined by LLS and ψmin, can be used to distinguish contrasting strategies of drought tolerance vs. avoidance, and alternative modes of regeneration following fire and other disturbance. This conceptual scheme illustrates the strength of a trait-based approach to defining plant strategies in relation to resource availability and disturbance.