Controls on ecosystem water-use and water-use efficiency: Insights from a comparison between grassland and riparian forest in the northern Great Plains

Highlights

• Evapotranspiration (ET) and net ecosystem CO₂ exchange were measured in three years.

• Growing season ET was 2.5 times higher in cottonwood forest than in grassland.

• Higher cottonwood water use was caused by higher LAI and access to groundwater.

• Growing season average WUE was higher in cottonwood forest than in grassland.

• Alluvial groundwater is required to support healthy riparian forest ecosystems.

Abstract

Within the grassland-dominated landscape of the North American Great Plains, riparian forest ecosystems exist along river floodplains. We compared cumulative evapotranspiration (ET) and ecosystem water-use efficiency (WUE) between a cottonwood forest and a nearby native grassland ecosystem in southern Alberta, using eddy covariance measurements during May-September (growing season) of three study years. Our objective was to test predictions about mechanistic controls on ecosystem water-use, and to provide insights into the amount of alluvial groundwater and stored soil water required to support a healthy riparian forest within the Great Plains biome. Grassland ET was dependent on precipitation inputs during the growing season. Cumulative growing season ET at the cottonwood site (375–451 mm) exceeded grassland ET (111–213 mm) by 2.1- to 3.4-fold depending on study year, despite slightly higher WUE in the cottonwood ecosystem. The difference in cumulative ET between ecosystems ranged from 238 to 264 mm in different years, in a region that normally receives 258 mm of cumulative precipitation during May-September. The large ET at the cottonwood site was caused by two-fold higher LAI, and associated greater canopy conductance than was apparent at the grassland site. The additional soil water required for the higher cottonwood ET was supplied by access to alluvial groundwater, which is recharged by river water, and was also supported by a larger soil volume to store water from precipitation and river flooding inputs. These factors resulted in a relatively long interval for cottonwood photosynthetic gas exchange that was consistent among years despite widely different environmental conditions, while the grassland had shorter growing season lengths that were constrained further as precipitation and soil moisture declined among years. Our analyses contribute to understanding the water requirements of these contrasting ecosystems and will help to improve management procedures for regulating river flow rates in order to sustain healthy riparian cottonwood ecosystems.

Introduction

The Great Plains is the second largest biome in North America and extends south from the aspen parkland and boreal forest areas in Alberta and Saskatchewan, Canada through the USA to New Mexico and Texas (Ostlie et al., 1997; Zhang et al., 2011). There is a substantial west to east gradient in precipitation that divides the native vegetation of the Great Plains into short-, mid- and tall grasslands or prairie ecosystems (Heisler-White et al., 2009; Wilcox et al., 2015). Productivity and water-use in prairie grassland ecosystems are strongly controlled by the amount of summer precipitation inputs, although the size of rain events and the interval between rain events can also significantly influence grassland ecosystem physiology (Knapp and Smith, 2001; Heisler-White et al., 2008, 2009; Knapp et al., 2008). Within this grassland-dominated landscape, riparian ecosystems also exist along the floodplains of rivers that distribute snow-melt water from the Rocky Mountains as they flow out through the prairies (Rood et al., 2003). Within the northern Great Plains of southern Alberta, river water also flows laterally out of the main river channels (“losing rivers”) creating an alluvial aquifer below the river floodplain (Hauer et al., 2016). A variety of important ecological processes connect the aquatic and terrestrial ecosystems through this alluvial aquifer and these ecological interactions promote regional biodiversity across the full spectrum from microbes to vertebrate animals (Hauer et al., 2016). Riparian cottonwood forests develop on these floodplains within the semiarid landscape that is otherwise treeless, in part supported by the alluvial groundwater which supplements any precipitation inputs to the soil (Snyder and Williams, 2000; Rood et al., 2003; Scott et al., 2003, 2000; Flanagan et al., 2017). The riparian forest ecosystems contribute many valued ecosystem services (Naiman et al., 2005; Hauer et al., 2016), but their health and survival has been threatened by reductions in river flows associated with dams that divert river water for agricultural irrigation, municipal and industrial consumption (Rood et al., 1995, 2003, 2005, 2008; Schindler and Donahue, 2006). This raises the fundamental question, how much river water is required to support healthy riparian forest ecosystems within the Great Plains biome? In order to help answer this question, and to provide some ecological perspective on the water-use requirements of riparian cottonwood forest ecosystems, our major objective in this study was to compare water-use and water-use efficiency between a native grassland ecosystem and a native, riparian cottonwood ecosystem in the northern Great Plains of southern Alberta. This comparison should provide insights into the amount of river water required to support a healthy riparian forest ecosystem in this region, as the local grasslands were expected to rely only on water sourced from summer precipitation inputs (Wever et al., 2002). More generally, this analysis would allow evaluation of some important factors influencing water-use and water-use efficiency in these two ecosystems.

In this paper we define water-use as cumulative ecosystem evapotranspiration (ET) during the growing season, and water-use efficiency (WUE) as the ratio of ecosystem photosynthesis to ecosystem ET. Several ecological and physiological factors can control ecosystem water-use and WUE, including: leaf area index (LAI), stomatal conductance, precipitation input, soil moisture content, access to groundwater, temperature, vapor pressure difference (VPD), and growing season length (Kelliher et al., 1995; Ponton et al., 2006; Beer et al., 2009). We predicted that cottonwood riparian forests and native grassland ecosystems should differ for several of these factors, as is described below. The contrasting dominant plant functional types associated with cottonwood forests and native prairie grassland will result in important differences in leaf and canopy photosynthetic gas exchange processes. Broad-leaf deciduous trees tend to have lower ratios of stomatal conductance to photosynthetic capacity than C₃ grasses (Smedley et al., 1991; Brooks et al., 1997), but the typically larger LAI of forests than grasslands results in canopy conductance being higher in the forest ecosystems (Kelliher et al., 1995; Wever et al., 2002; Flanagan et al., 2017). While precipitation input will be the same in nearby riparian forest and grassland ecosystems, groundwater access by deep-rooted, phreatophytic cottonwood trees should increase the supply of water to riparian forests above that available to grasslands (Scott et al., 2003, 2004). In addition, the depth of soil and permeable substrate beneath floodplain forests (2–3 m) exceeds that of native grasslands in the northern Great Plains (0–30 cm), and so the riparian forests should have greater soil water storage capacity. Local forest and grassland ecosystems are exposed to similar aerial environmental conditions (photosynthetically active radiation, temperature, VPD), but they differ in ecological strategies of the dominant plant functional types. In addition, the differing amounts of available soil water can result in contrasting growing season lengths for forest and grassland ecosystems. Grassland plants grow fast and use water quickly when it is readily available and then go dormant to survive times of water shortages (Flanagan and Adkinson, 2011). This ecological strategy of grassland plants can result in significantly shorter active growing seasons than is apparent for riparian forest ecosystems.

In this study, we compared ecosystem water-use and WUE between nearby riparian cottonwood forest and native grassland ecosystems in southern Alberta using eddy covariance measurements. Our goal was to test these predictions made above about mechanistic controls on ecosystem water-use, and to provide insights into the amount of alluvial groundwater and stored soil water that is required to support a healthy riparian forest ecosystem within the Great Plains biome. The study was conducted during May-September in three different years that had contrasting precipitation, air temperature and VPD, and so provided perspective on summertime environmental controls that influence water-use and WUE in riparian forest and grassland ecosystems in the northern Great Plains biome.