Desert woody plant functional strategies are related to facilitation and precipitation variability

Background/Question/Methods .  Life-history strategies of desert perennial plants are only roughly defined, and generally lack a quantitative functional basis.  Both the traits associated with mature plant functional strategies as well as those related to species’ regeneration niches (whether they require facilitation or not) are not known, and may provide key insights into demographic structure, population dynamics and community assembly.  We assessed the relationships between mature plant leaf traits, water use efficiency and plant size (a strong correlate of longevity and reproductive frequency) among 54 species of woody plants from the Mojave, Sonoran and Chihuahuan Deserts to identify axes of functional differentiation.  For 20 of these species, we also assessed functional traits and short-term drought survival of seedlings grown under controlled conditions.  We then tested whether seedling and mature plant functional strategies were related to the regeneration niche across species.

Results/Conclusions .  Three axes of differentiation were identified for mature plants, representing leaf traits associated with photosynthetic capacity, large plants with dense leaves, and high leaf mass with water use efficiency.  Surrogates of leaf function – nitrogen mass, surface area and water mass per leaf – were positively allometrically related to leaf dry mass, in contrast to the negative allometric relationships found at the global scale.  Woody desert plants also had significantly lower values of each of these metrics, except surface area, than shrubs globally.  Taken together, these results suggest that the small leaves of desert perennials are proximately carbon limited, due to total plant water limitation.  Relationships between leaf traits, water use efficiency and growth form exhibited unique functional strategies related to the manner of resource assimilation and allocation in response to scarce and variable precipitation.  Seedlings exhibited two axes of functional differentiation, one positively associated with survival, root density, seed and seedling size, with the other related to relative growth rate, LMA and aboveground allocation.  Species that require facilitation exhibited conservative functional strategies, whereas colonizers of bare ground had high growth rates and low short-term survival, contrary to predictions.  High seed density and opportunistic responses to precipitation pulses permit colonization, whereas facilitation-dependence is largely a function of low herbivory tolerance.  Colonizer traits were conserved through ontogeny, whereas facilitation-dependent species maintained a generally conservative strategy but were more flexible in their mature strategies.  This is the first study to identify the seedling functional basis of facilitation, and the most comprehensive assessment of desert perennial functional strategies.