Trees (2003) 17:351–358 DOI 10.1007/s00468-003-0245-3
ORIGINAL ARTICLE
Nadine M. Amlin · Stewart B. Rood
Drought stress and recovery of riparian cottonwoods due to water table alteration along Willow Creek, Alberta Received: 26 August 2002 / Accepted: 30 December 2002 / Published online: 8 February 2003 Springer-Verlag 2003
Abstract A 5-m-deep gravel pit was excavated from 1996 to 1998 in the floodplain between Willow Creek, Alberta, and a grove of balsam poplars (‘cottonwoods’, Populus balsamifera L.) and water level at the pit was lowered 2.5 m through pumping. This interrupted the infiltration of stream water into the riparian groundwater and imposed drought stress on the cottonwoods. Trees in the drought-affected grove displayed extensive leaf senescence and abscission in late August 1998, while trees in nearby control groves remained green until autumnal senescence in late September. The precocious senescence was accompanied by a two-thirds reduction in leaf stomatal conductance (gs) but mid-day leaf xylem water potentials (yl) were only slightly reduced (1.55 vs 1.42 MPa). Pumping ceased in 1999, the pit was partially refilled, and the hydraulic linkage between the stream and the riparian zone recovered. Subsequently in August 1999, gs and yl were similar for trees in the affected and control groves and senescence phenologies were similar in 1999 and 2000. Annual branch growth increments varied 3-fold across years between 1994 and 1999, but there was no reduction in these growth increments in the drought-affected trees in 1998 or 1999. This study supports the hydraulic linkage between a stream and the adjacent riparian zone in a semi-arid region and demonstrates the vulnerability of riparian cottonwoods to drought due to water table depletion. It also indicates rapid physiological recovery of cottonwoods following restoration of water availability. Keywords Cottonwoods · Drought · Populus balsamifera · Stress · Water relations
N. M. Amlin · S. B. Rood ()) Department of Biological Sciences, University of Lethbridge, Alberta, T1K 3M4, Canada e-mail:
[email protected] Tel.: +1-403-3292327 Fax: +1-403-3292242
Introduction Riparian zones, river valley floodplains, provide dynamic interfaces between aquatic and terrestrial ecosystems (Naiman and Dcamps 1997). In many regions of the Northern Hemisphere these zones are dominated by cottonwood (Populus spp.) forests that provide especially rich wildlife habitats and are favored areas for human activities. Riparian cottonwoods along many rivers have collapsed over the past century and this has often been attributed to impacts from river damming and flow regulation (Rood and Mahoney 1990; Busch and Smith 1995). Each river development project provides a major intervention that can provide a study opportunity regarding riparian ecophysiology. Accompanying a new dam in southern Alberta, the Pine Coulee Project along Willow Creek, we recognized an opportunity to investigate the ecophysiological dependency of riparian cottonwoods on riparian groundwater. A large, 5-m-deep gravel pit was excavated to provide materials for the offstream dam and was situated between the creek and a grove of balsam poplars, Populus balsamifera L. To permit gravel mining, drainage ditches were excavated and water was pumped from the pit. Subsequently, in mid-August, 1998, we observed that trees in the affected-grove displayed extensive leaf senescence and abscission whereas trees in nearby control groves displayed normal, green leaves. A number of studies have indicated that in semi-arid regions, riparian cottonwoods are dependent on groundwater that infiltrates from the adjacent stream (Rood and Mahoney 1990; Busch et al. 1992; Rood et al. 1995; Stromberg and Patten 1996; Scott et al. 1999). We consequently hypothesized that the water pumping from the gravel pit had interrupted the hydraulic linkage between the stream and the riparian grove and this imposed drought stress on the cottonwoods that induced precocious senescence and abscission (Albertson and Weaver 1945; Scott et al. 1999; Rood et al. 2000a). We had missed the opportunity to study the initial physiological responses to the hydraulic interruption but
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learned that water pumping would cease and the gravel pit would be refilled. We thus anticipated physiological recovery that would be accompanied by similar patterns of senescence and abscission in the recovering grove and in nearby control groves. We also expected correlations between senescence patterns and underlying physiological patterns of xylem water potential and stomatal conductance (Kelliher et al. 1980; Braatne et al. 1992; Mahoney and Rood 1992; Blake et al. 1996; Rood et al. 2000b). These would be influenced by water demand due to temperature and humidity that determine vapor pressure deficit (Hogg and Hurdle 1997; Zhang et al. 1999). Due to the apparent drought stress, we also expected that annual branch elongation would be retarded during and possibly following the hydraulic interruption (Willms et al. 1998; Scott et al. 1999). The present study was thus conducted to compare ecophysiological conditions of the trees adjacent to the gravel pit with control trees in 1998 during the period of water pumping, and subsequently in 1999 after the hydraulic linkage between the stream and the riparian groundwater was restored.
Materials and methods Study site The Pine Coulee Project is located along the east slope of the Porcupine Hills, a feature of the Rocky Mountain foothills of southwestern Alberta, Canada (5007'N, 11345'W). The Project includes a 12 m dam on Willow Creek that creates a small reservoir from which a canal diverts water to fill an offstream reservoir in Pine Coulee, the valley of Pine Creek, a small tributary of Willow Creek. A zoned, earth-fill dam across Pine Coulee was built with clay, sand and gravel with the latter materials being excavated from the study site in the floodplain along Willow Creek. This gravel pit provided the hydraulic intervention of the present study (Fig. 1). Gravel pit excavation commenced in the spring of 1996. As the pit deepened, water was pumped out to draw-down the water table below the zone of gravel extraction. Shallow ditches were excavated across the gravel pit to enable water drainage to the pump inlets. Gravel excavation and water pumping continued through to 1998 when the gravel pit reached a depth of 5 m. In late fall 1998, pumping ceased and the gravel pit was partially refilled with excavation spoil for a wetland and riparian habitat enhancement project. Study transects for water table measurements To investigate elevational profiles and water table patterns, crosssectional transects were established perpendicular from Willow Creek across the floodplain and through the drought-affected and control groves (Fig. 1) and surveyed (elevations to €0.5 cm and distances to €1 m). Water table monitoring wells were installed along the transects in early June, 1999. For each well, a 20-cm diameter auger was used to drill a 5-m-deep hole and a 6-m-long, 5cm-diameter rigid plastic pipe was inserted down the auger shaft. The well pipes were slit at 1-m intervals to permit water passage. Surface elevations (‘stage’) of Willow Creek and water table elevations in the wells were measured weekly through the summer of 1999 to determine the water table slope that reflects the direction of groundwater flow. Creek surface elevations were surveyed (€0.5 cm) and well water table depths were determined by averaging triplicate readings with a Solinst (Georgetown, Ont., Canada) 101 water level meter with millimeter gradations.
Fig. 1 An aerial photograph of the study site at the Pine Coulee Project along Willow Creek, Alberta, 25 August 1999, showing the positions of the partially refilled gravel pit, drought-affected and control cottonwood groves, transects and water table monitoring wells, designated by x Drought-affected transect well 3 (3D) was at the Water Survey of Canada Pine Creek hydrometric gauging station (Station no. 05AB045) and other elevations were corrected to this benchmark. Observations of senescence and grove characteristics Observations of senescence and abscission involved overviews of the riparian groves from the adjacent prairie upland above the valley. The drought-affected and control groves were observed and groups of about 20 trees were rated from 1 to 5, indicating: fully green, partially senescent (yellow-green), fully senescent (yellow), partially abscised, or fully abscised, respectively. These overviews occurred periodically from August to October 1998, and July to October 1999 and 2000. A 500-m elevation airplane flight in late August, 1999 provided an additional aerial overview. Trunk densities were determined from counts of trunks in 50~20 m quadrats from each grove. Diameters from 10 trunks in each quadrat were measured at 1.4 m above the substrate surface. Water relations investigations On 31 August, 3, 4, 5, 8, and 15 September 1998, and 4, 6, 9 and 18 to 21 August 1999, stomatal conductances (gs), mid-day leaf xylem water potentials (yl), leaf senescence and environmental conditions and were analyzed from 24 cottonwood trees in the droughtaffected grove and 20 trees in the control grove. The cottonwoods were balsam poplars, Populus balsamifera L., except for two narrowleaf cottonwoods, P. angustifolia James, in the control grove. Trees were chosen that had substantial branches that could be reached with a 2-m step-ladder (most trees qualified) with selection through a 10–15 m walk after each tree in a random direction between 90 and 270 from the prior direction. Trees were numbered with metal tags and repeatedly sampled in 1998 and
353 1999. At each sampling, each tree was visually assessed with respect to the proportion of senescence and abscission (20% increments each). Water relations data were collected between 1000 and 1600 hours daily since poplars (cottonwoods) generally demonstrate maximal transpiration and minimal water potential during this period (Blake et al. 1994; Tyree et al. 1994), other riparian cottonwood studies have revealed sensitivity of mid-day yl (Smith et al. 1991; Busch and Smith 1995), and reduced mid-day yl could be particularly associated with xylem cavitation that may result in branch sacrifice or shoot mortality (Tyree et al. 1994; Rood et al. 2000a). Sampling alternated between the drought-affected and control groves, generally involving measurements of two or three trees and then changing grove. For each tree, the gs was calculated (McDermitt 1990) from measurements from each of ten leaves with a Li-Cor (Lincoln, Neb.) LI-1600 steady state diffusion porometer on the abaxial leaf surface, the dominant surface for foliar transpiration in poplars (Blake et al. 1984). Leaf and ambient temperatures, photosynthetically active radiation, relative humidity and time were also recorded for each leaf. Each leaf was assessed relative to color with a three-point scale (0.25 increments) from green (1) to yellow (2) and brown (3). For each of five of the leaves, after gs measurement the leaf petiole was cut and yl was measured using a Scholander-type pressure chamber (PMS Instrument, Corvallis, Ore.). Both yl and gs were measured from 338 and 296 leaves in 1998 and 1999, respectively, from 20 and 24 trees in the drought-affected and control, groves, respectively, with the same trees sampled in both years. The gs were determined from about twice as many leaves as yl for a total of 1,200 gs measurements. Branch growth increments Annual branch growth increments (Willms et al. 1998) from 1994 to 1999 were measured on five branches from each of ten trees within the drought-affected and control groves (€1 mm). For statistical analysis, the annual branch growth increments were represented as the percentage of the total branch growth over the 6 years. Leaf water relations and autumnal senescence To investigate the possible differences in the relationships between yl and gs in green versus senescing cottonwood leaves, a separate comparison was conducted along the Oldman River at Lethbridge, AB, in mid-September, 2000. In this study, yl and gs were determined from green versus yellow leaves of various trees of P. angustifolia, P. balsamifera and the prairie cottonwood, P. deltoides. Hydrology Discharges (Q) of Willow Creek were obtained from Hydat, Environment Canada’s historical streamflow database, for hydrometric station 05AB021, ’Willow Creek near Claresholm’, situated downstream of the study site. Data analyzed included daily mean Q for 1998 and 1999, and annual and monthly mean Q for 1944–1999. Statistical analyses with Statview 4.5 (Abacus Concepts, Berkeley, Calif.) included non-paired t-tests and non-parametric Mann-Whitney U-tests to investigate differences of the droughtaffected versus control groves. Comparisons across trees within the groves and comparisons of the branch growth increments involved analyses of variance (ANOVAs) and Fisher’s Protected Least Significant Differences (PLSD) post-hoc tests, and non-parametric Kruskal-Wallis tests. Correlation matrices were determined for the environmental variables and water relations measurements. Regression analyses were conducted with CA-Cricket Graph III (version 1.5.3, Computer Associates, Islandia, N.Y.).
Results Trunk densities and sizes did not significantly differ across the groves as densities ranged from about 40 to 80 trunks/1,000 m2 and diameters generally ranged from about 20 to 60 cm in both groves. Based on annual ring widths of exposed stumps, these trunks would have ranged from about 30 to 100 years in age. In late summer 1998, the drought-affected grove showed precocious leaf senescence and abscission as the majority (>60%) of the leaves had senesced or abscised by late August. At that time, the control grove had just commenced senescence and displayed little abscission (
