Are plant adaptations to water stress and the chemical content of organic matter directly linked?

Background/Question/Methods

With future climate change, California is expected to experience drier conditions during times of the year when most plants are active, creating conditions of greater water stress. There is a great deal of uncertainty about the implications of plant responses to increased water stress for species distributions and other aspects of ecosystem function. A major physiological response of plants to water stress is cavitation, or introduction of air embolisms, into xylem. Embolisms block the flow of water and prevent water uptake, potentially leading to catastrophic failure of the vascular system. Plants with greater resistance to cavitation have been shown to have greater xylem density, suggesting that thicker, lignified fibers may provide the necessary vessel support to counter negative water potential. We tested the hypothesis that plant adaptations to water stress are correlated with plant chemical composition by choosing sites along a depth to watertable (DTW) gradient from 0.5-20 m in Owens Valley, California. Stems of dominant Owens Valley species were measured for lignin content and leaves were measured for isotopic composition. We further propose that increased resistance to vulnerability as a response to water stress is associated with increased lignification of xylem. To test this hypothesis, we measured lignin content of several shrub species in other semi-arid, Southern California ecosystems and compared these measurements with vulnerability to cavitation.

Results/Conclusions

Stem percent lignin did not vary with DTW in the grass species Distichlis spicata or the shrub species Atriplex torreyi; however, there was a significant positive correlation between stem percent lignin and DTW in the phreatophytic shrub species Ericameria nauseosa. Leaf carbon isotope composition (d¹³C) became more enriched with watertable depth, suggesting increased water stress with increased DTW. In Southern California ecosystems there was a significant negative correlation between the water potential at a 50% loss of hydraulic conductance and stem percent lignin in chaparral species, and a similar trend in coastal sage scrub species. We are currently evaluating this relationship in a broad range of species and ecosystems, and also investigating the implications for nutrient cycling. As lignin has been shown to be very resistant to decomposition, xylem composed of a greater proportion of lignin to other compounds may ultimately have consequences for plant litter decomposition and ecosystem nutrient cycling. Therefore, plant responses to water stress and the chemical composition of organic matter may be directly linked through the lignification of xylem.