Seasonal Variation in Plant Hydraulic Traits of Tamarix ramosissima and Haloxylon ammodendron

In arid environments, water plays a dominant role in limiting and regulating the activities of life. The availability of water to individual plants in water-limited environments depends partially on local climatic and edaphic factors but also on the depth, lateral spread and degree of overlap of plant root systems. Effective hydraulic responses to varying soil moisture and evaporative demand are crucial to plant survival in arid ecosystems.

Tamarix ramosissima and Haloxylon ammodendron are two native dominant desert shrub species with different rooting patterns in the arid regions of central Asia. Dr. XU Guiqing, Prof. LI Yan and XU Hao, used the ecological and statistical methods to investigate the two species’ hydraulic responses to seasonal variations in water availability. They found that T. ramosissima had a deeper taproot (3.1 vs. 2.6 m) and a larger root surface area (3.02 vs. 1.28 m2) than H. ammodendron. For the deep-rooting T. ramosissima, the seasonal pattern of its predawn leaf water potential (ψ_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 ψ_pd was closely related to soil moisture in the upper layer at the depths of 0–0.2 and 0.6–0.8 m, which were 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 ψ_pd: in T. ramosissima, T_rmax gradually dropped with decreasing ψ_pd; in H. ammodendron, T_rmax showed no significant response to ψ_pd. For T. ramosissima, the major water resource was groundwater and vadose zone water, and ψ_lcontributed significantly to transpiration regulation. For H. ammodendron, the primary water resource was precipitation input, which was not sufficient to keep the ψ_l for this species outside the critical range of leaf shedding in summer. This study illustrates how species that co-occur and co-dominate in a given habitat may exhibit contrasting survival strategies based on distinct morphological and physiological characteristics.

This study was supported by the National Natural Science Foundation of China (Grant No. 40971042 and 40725002). The findings have been published in Ecological Research. doi: 10.1007/s11284-011-0858-8. The article can be linked as follows: http://www.springerlink.com/content/t613402052u05842.