Physiological Response to Groundwater Depth Varies Among Species and With River Flow Regulation

Abstract

We investigated the physiological response of two native riparian tree species (Populus fremontii and Salix gooddingii) and one exotic species (Tamarix chinensis) to groundwater availability along gradients of depth to groundwater at two rivers in Arizona. Depth to groundwater (DGW) at the dam-regulated Bill Williams River (BWR) was relatively constant and shallow (Ͻ4 m). Populus fremontii at BWR did not experience reduced water availability at deeper groundwater depths, as evidenced by high predawn water potential. However, leaf gas exchange of P. fremontii was sensitive to high vapor pressure deficit where surface flow was ephemeral at BWR. Lower predawn water potentials of S. gooddingii at BWR suggested reduced water availability at deeper groundwater depths, but these reductions did not adversely affect net photosynthetic rate. Along the range of depth to groundwater at BWR, all three species suffered little canopy dieback, and dieback was not related to depth to groundwater. Depth to groundwater at the freeflowing Hassayampa River (HRP) was much greater and declined more rapidly in the ephemeral reaches than at BWR. Both P. fremontii and S. gooddingii experienced reduced water availability at deeper groundwater depths at HRP, as evidenced by lower predawn water potential. Both species also experienced reduced leaf gas exchange at deeper groundwater depths. Canopy dieback of all species was higher at HRP than at BWR and increased with increasing DGW, especially when DGW fell below 3 m. There was evidence to support branch sacrifice in these three riparian tree species as a means of improving water status in the surviving shoot. However, branch sacrifice was insufficient to prevent mortality in some of the native trees where DGW fell below 3 m at HRP. In contrast to the native species, T. chinensis showed no change in water availability, leaf gas exchange, or canopy dieback with increasing DGW at either river. Leaf gas exchange was lower and dieback was greater for T. chinensis at HRP where depth to groundwater was greater than at BWR, but there was no mortality at either river. Our results show that deep groundwater is more detrimental to the physiological condition of P. fremontii and S. gooddingii than it is to T. chinensis. Also, the pronounced differences in DGW and tree physiological performance between BWR and HRP suggest that dam regulation can increase water availability to mature trees in some desert riparian ecosystems. Finally, our study also provides estimates of the range of DGW that can maintain healthy, mature P. fremontii and S. gooddingii trees.

Drought stress and recovery of riparian cottonwoods due to water table alteration along Willow Creek, Alberta

“…This conclusion is consistent with other studies of riparian cottonwoods growing in conditions of water table depletion (Scott et al 1999(Scott et al , 2000Horton et al 2001aHorton et al , 2001b. This conclusion is consistent with other studies of riparian cottonwoods growing in conditions of water table depletion (Scott et al 1999(Scott et al , 2000Horton et al 2001aHorton et al , 2001b.…”

“…As a foundation for the present study, it has been determined that phreatophytic riparian cottonwoods depend on the water table as their principal water source and depletions in riparian groundwater impose drought stress (Busch et al 1992;Busch and Smith 1995;Rood et al 1995;Stromberg et al 1997;Zhang et al 1999;Scott et al 2000;Shafroth et al 2000;Horton et al 2001a). Further, in semi-arid regions such as the Willow Creek site, the riparian water table is supported by water flowing from the adjacent stream into the floodplain alluvium creating a hydrologic situation referred to as an 'influent' condition and 'losing' stream (Gordon et al 1992).…”

Plant species richness in ephemeral and perennial reaches of a dryland river

“…Some species in this group are obligate phreatophytes that depend on shallow water tables (Horton et al 2001); others are shallow-rooted hydrophytes sustained by year-round or quasi-permanent surface Xows . Within the soil seed bank, however, the hydric species showed greater compositional similarity between hydrologic site types.…”

9.14 Reciprocal Relations between Riparian Vegetation, Fluvial Landforms, and Channel Processes

“…In the absence of such competitors, populations often shift to positions away from the stress and toward the habitat optima. There is an energetic trade-off between tolerance of inundation and tolerance of drought or water stress, with few riparian species being adept at both (Horton et al, 2001) ( Figure 1). (Kotowski et al, 2006).…”