The water relations of two evergreen tree species in a karst savanna

Abstract

The ecohydrology of karst has not received much attention, despite the disproportionally large contribution of karst aquifers to freshwater supplies. Karst savannas, like many savannas elsewhere, are encroached by woody plants, with possibly negative consequences on aquifer recharge. However, the role of savanna tree species in hydrological processes remains unclear, not least because the location and water absorption zones of tree roots in the spatially complex subsurface strata are unknown. This study examined the water sources and water relations of two savanna trees, Quercus fusiformis (Small) and Juniperus ashei (Buchholz) in the karst region of the eastern Edwards Plateau, Texas (USA). Stable isotope analysis of stem water revealed that both species took up evaporatively enriched water during the warm season, suggesting a relatively shallow water source in the epikarst, the transition zone between soil and bedrock. Q. fusiformis had consistently higher predawn water potentials than J. ashei during drought, and thus was probably deeper-rooted and less capable of maintaining gas exchange at low water potentials. Although the water potential of both species recovered after drought-breaking spring and summer rain events, associated shifts in stem water isotope ratios did not indicate significant uptake of rainwater from the shallow soil. A hypothesis is developed to explain this phenomenon invoking a piston-flow mechanism that pushes water stored in macropores into the active root zones of the trees. Epikarst structure varied greatly with parent material and topography, and had strong effects on seasonal fluctuations in plant water status. The study suggests that tree species of the Edwards Plateau do not commonly reduce aquifer recharge by tapping directly into perched water tables, but more likely by reducing water storage in the epikarst. A more general conclusion is that models of savanna water relations based on Walter's two-layer model may not apply unequivocally to karst savannas.

Contrasting watershed-scale trends in runoff and sediment yield complicate rangeland water resources planning

“…Changes in land use and land cover -and even the impoundment of small reservoirs -have had negligible impacts on streamflow. These results confirm and add new insight to other research showing that woody plants in this region are shallow rooted and do not rely on deeper, perennial water sources (Heilman, 2009;Schwartz et al, 2013;Schwinning, 2008). It is still not understood why base flow showed a proportional increase .…”

Reviews and syntheses: on the roles trees play in building and plumbing the critical zone

“…This idea has led to the view that plants may utilize different niches (Silvertown et al, 2015) by partitioning their roots according to the hydrological conditions of different layers (e.g., Walter's two-layer hypothesis). Specifically, Walter's hypothesis states, in part, that shallow and deeply rooted plants do not compete for the same water resources (Walter, 1939;Weltzin and McPherson, 1997;Schenk and Jackson, 2002a;Schwinning, 2008;Holdo, 2013;Ward et al, 2013).…”

Characteristics of Point Recharge in Karst Aquifers

“…For example, 25% of drinking water supplies are sourced from a geological setting covering approximately 10% of the earth's land surface [9][10][11], and comprise some 40% of the groundwater of the United States [12]. In Europe, more than 30% of the land surface is made up of karst exposures with more than half the drinking water supply derived from karst aquifers in some countries [13].…”