Inter-specific differences in eco-physiological characters of coexisting shrub species in a semi-humid woodland ecosystem

Background/Question/Methods

We investigated eco-physiological characteristics discrimination of four coexisting thorn scrub species (Acacia fernesiana, Bumelia celastrina, Parkinsonia aculata and Prosopis glandulosa) of a semi-humid woodland ecosystem of South Texas Refuge Complex (STRC), USA, using water potential and gas exchange characteristics. The purpose of the study was to determine if the coexisting species are functionally similar in terms of water potential and gas exchange characteristics under range of soil water availability. Ten individuals of each species were randomly selected from an undisturbed site at STRC. For each species, five individuals were treated with water at midday of summer (June 17^th) 2010, the amount of water being contingent to the size of stem (approximately 8.7 L. water/cm basal diameter of stem) before observation. Predawn leaf water potential (PWP) and midday net photosynthetic rate and stomatal conductance measurements were taken in summer (June 18^th) 2010.

Results/Conclusions

Water additions significantly increased plant water potential (p= 0.027) and stomatal conductance (p = 0.036) of all species.  However, no change was detected in net photosynthetic rate (p = 0.30). The PWP values of the species were also significantly difference among the species for watered plants (p = 0.049) and unwatered plants (p = 0.005). The net photosynthetic rate was not different among the species watered and not watered. In contrast, stomatal conductance was significantly different among the species watered (p = 0.014), and those that were not watered (p = 0.014).  To assess variation in maximum gas exchange capacitance, a companion greenhouse study was conducted with ample of water and nutrition, which showed the net photosynthetic rate and stomatal conductance were not different among the species. The result suggested that the co-existing species respond differently in additional water availability. The inter-specific variations in leaf water potential and stomatal conductance suggested that the species showed capacities to withstand a wider range of soil water status while maintaining similar net photosynthetic rates. We conclude that anticipated soil water stress is less likely to affect CO₂ uptake capacitance of these species in changing climate.