Physiological and Morphological Responses of Tamarix ramosissima and Populus euphratica to Altered Groundwater Availability

Riparian zones have a highly variable physical environment, including frequent groundwater fluctuations that range from inundation in the flooding season to drought in the dry season. Because natural flow variability increases with increased aridity, riparian plant communities in arid regions should experience a greater range and higher frequency in groundwater variation. One particular example is the riparian forest along the Tarim River, a 1321-km-long inland river located in the Tarim Basin, which is the most arid basin in China.

The Tarim riparian forest is dominated by Populus euphratica Oliv. Tamarix ramosissima Ledeb., the subdominant species in this region, is the P. euphratica’s only possible competitor. Although many studies have focused on the success of T. ramosissima in its invaded ranges, its major competitor in its home range, P. euphratica Oliv., historically has dominated riparian forests where both species occur naturally. Thus, identifying ecophysiological differences between T. ramosissima and its co-evolved competitor under varying hydrological conditions is important to understand how flow regimes affect dominance in its home range and promote invasion in new ranges.

Therefore, LI Jun et al. evaluated the ecophysiological and growth traits of P. euphratica and T. ramosissima seedlings in response to typical hydrological scenarios that frequently occur along the Tarim riparian forest. Seedlings of both species were grown in lysimeters, first under well-watered conditions and then exposed to different groundwater treatments: inundation, drought, and relatively shallow, moderate and deep groundwater.

The result indicated that T. ramosissima showed little growth whereas P. euphratica died after 45 days under inundation. Droughted seedlings of both species suffered from considerable water stress evidenced by slow growth, decreased total leaf area and specific leaf area, and decreased xylem water potential (ψ), maximum photosynthetic rate and carboxylation efficiency. Both species had better ecophysiological performances under shallow and moderate groundwater conditions. When groundwater declined below rooting depth, seedlings of both species initially experienced decreased ψ, but ψ of T. ramosissima recovered late in the experiment whereas P. euphratica maintained decreased ψ.

This ability of T. ramosissima to recover from water deficit might result from its rapid root elongation and subsequent ability to acquire groundwater, which in turn likely provides ecophysiological advantages over P. euphratica. The results suggested that recent groundwater declines along the Tarim River could facilitate T. ramosissima more due to its rapid response to changed groundwater availability. The result was published in Tree Physiology in 2013, 33(1): 57-68.