Seasonal variation in plant hydraulic traits of two co-occurring desert shrubs, Tamarix ramosissima and Haloxylon ammodendron, with different rooting patterns

Effective hydraulic responses to varying soil moisture and evaporative demand are crucial to plant survival in arid ecosystems. This study was carried out during two growing seasons (2004-2005) on two typical desert shrub species, Tamarix ramosissima and Haloxylon ammodendron, co-occurring in the Gurbantonggut Desert (Central Asia), to investigate their hydraulic responses to seasonal variations in water availability. The root distribution was studied by excavating the intact root systems. Leaf-specific apparent hydraulic conductance (K (l)) for the two species was calculated based on leaf water potential (psi (l)) and transpiration rate (T (r)), which were monitored during the growing seasons. T. ramosissima had a deeper taproot (3.1 vs. 2.6 m) and a larger root surface area (3.02 vs. 1.28 m(2)) than H. ammodendron. Combined with a higher psi (l), this meant that it maintained a better water status in general. For the deep-rooting T. ramosissima, the seasonal pattern of its predawn leaf water potential (psi (pd)) was in high accordance with the seasonal changes in soil moisture at a depth of 2.6-2.8 m, which was largely influenced by the upflow of groundwater through capillary ascent, and barely responded to rain events. For the shallow-rooting H. ammodendron, the seasonal pattern of psi (pd) was closely related to soil moisture in the upper layer at a depth of 0-0.2 and 0.6-0.8 m, which was recharged periodically by rain events and responded acutely to rainfall above 5 mm. The two species differed in their maximum transpiration rates (T (rmax)) and psi (pd): in T. ramosissima, T (rmax) gradually dropped with decreasing psi (pd); in H. ammodendron, T (rmax) showed no significant response to psi (pd). For T. ramosissima, the major water resource was groundwater and vadose zone water, and psi (l) contributed significantly to transpiration regulation. For H. ammodendron, the primary water resource was precipitation input, which was not sufficient to keep the psi (l) for this species outside the critical range of leaf shedding in summer. Thus, for these two representative species that share the same habitat, contrasting response strategies to water limitation were observed in relation to water acquisition and root distribution characteristics.