Plant water use characteristics of five dominant shrub species of the Lower Rio Grande Valley, Texas, USA: implications for shrubland restoration and conservation

Abstract

The biogeographic distribution of plant species is inherently associated with the plasticity of physiological adaptations to environmental variation. For semi-arid shrublands with a legacy of saline soils, characterization of soil water-tolerant shrub species is necessary for habitat restoration given future projection of increased drought magnitude and persistence in these ecosystems. Five dominant native shrub species commonly found in the Lower Rio Grande Valley, TX, USA, were studied, namely Acacia farnesiana, Celtis ehrenbergiana, Forestiera angustifolia, Parkinsonia aculeata and Prosopis glandulosa. To simulate drought conditions, we suspended watering of healthy, greenhouse-grown plants for 4 weeks. Effects of soil salinity were also studied by dosing plants with 10% NaCl solution with suspended watering. For soil water deficit treatment, the soil water potential of P. glandulosa was the highest (-1.20 MPa), followed by A. farnesiana (-4.69 MPa), P. aculeata (-5.39 MPa), F. angustifolia (-6.20 MPa) and C. ehrenbergiana (-10.02 MPa). For the soil salinity treatment, P. glandulosa also had the highest soil water potential value (-1.60 MPa), followed by C. ehrenbergiana (-1.70 MPa), A. farnesiana (-1.84 MPa), P. aculeata (-2.04 MPa) and F. angustifolia (-6.99 MPa). Within the species, only C. ehrenbergiana and F. angustifolia for soil water deficit treatment and A. farnesiana for the salinity treatment had significantly lower soil water potential after 4 weeks of treatment (P < 0.05). We found that soil water potential, stomatal conductance and net photosynthesis of the species significantly reduced over time for both treatments (P < 0.05). We conclude that while all species exhibited capacities to withstand current water availability, some species demonstrated limited tolerance for extreme water stress that may be important for management of future shrub diversity in Lower Rio Grande Valley.

Keywords: Gas exchange; soil salinity; soil water deficit; soil water potential; thorn shrubs.

Figure 1.

Average soil water potential (SWP) of five native shrub species over 4 weeks for soil water deficit (SWD) and salinity treatments. ¶ Soil of C. ehrenbergiana and F. angustifolia had a significantly lower water potential value at week 4 for both treatments and A. farnesiana had a significantly lower water potential at week 4 of salinity treatment.

Figure 2.

Average values of stomatal conductance (g_s; a) and photosynthetic rate (P_n; b) for five native shrub species with 95% confidence intervals showing the result of control, SWD and salinity treatments, respectively. * Significantly lower g_s and P_n of each species compared with its control plants for SWD and salinity treatments. ¶ Significantly lower g_s value of F. angustifolia for salinity treatment only at week 4.