|HOME SCHEDULE AUTHOR INDEX SUBJECT INDEX|
Adaptive variation in the vulnerability of woody plants to xylem cavitation.
Maherali, Hafiz*,1, Pockman, William2, Jackson, Robert1, 1 Duke University, Durham, NC2 University of New Mexico, Albuquerque, NM
ABSTRACT- Although there are clear links between vulnerability to xylem cavitation and desiccation tolerance, considerable interspecific variation exists within and between climates. To understand the adaptive significance of this variation, and examine the potential for trade-offs with other traits, we studied relationships among xylem vulnerability to cavitation, mean annual precipitation (MAP) and specific hydraulic conductivity (KS). We compiled a database of 173 species from 52 seed plant families. Relationships were evaluated using standard correlations (AC) and phylogenetically independent contrast correlations (CC) calculated over a range of alternate seed plant phylogenies. Vulnerability to cavitation, expressed as the xylem tension at which 50% of conductivity was lost (50), varied from -0.18 to -9.9 MPa for angiosperms and from -1.5 to -14.1 MPa for conifers. Species in dry environments were generally more resistant to cavitation than those in wet environments (AC = -0.58, P <0.01). However, significant contrast correlations between 50 and MAP were found only in evergreen angiosperms (CC = -0.54, P <0.01) and conifers (CC = -0.41, P <0.01). In contrast, deciduous angiosperms had higher KS in dry environments (CC = -0.66, P <0.01), whereas there was no association between KS and MAP for evergreen angiosperms (CC = -0.16) or conifers (CC = 0.15). These results suggest that the evolution of deciduousness in dry environments may be of greater ecological significance than the evolution of high resistance to xylem cavitation. These results also suggest that the evolution of high KS may be associated with short leaf life span. Despite evidence of a tradeoff between 50 and KS among all species (AC = -0.31, P <0.05), this relationship was not significant in the independent contrast analysis (CC = -0.18), suggesting that the association between these two traits was driven by hydraulic differences between angiosperms and conifers.
KEY WORDS: hydraulic architecture, correlated evolution, adaptation, xylem