James E. Roelle

Latitudinal variation in cold hardiness in introduced Tamarix and native Populus

To investigate the evolution of clinal variation in an invasive plant, we compared cold hardiness in the introduced saltcedar (Tamarix ramosissima, Tamarix chinensis, and hybrids) and the native plains cottonwood (Populus deltoides subsp. monilifera). In a shadehouse in Colorado (41°N), we grew plants collected along a latitudinal gradient in the central United States (29–48°N). On 17 occasions between September 2005 and June 2006, we determined killing temperatures using freeze‐induced electrolyte leakage and direct observation. In midwinter, cottonwood survived cooling to −70°C, while saltcedar was killed at −33 to −47°C. Frost sensitivity, therefore, may limit northward expansion of saltcedar in North America. Both species demonstrated inherited latitudinal variation in cold hardiness. For example, from September through January killing temperatures for saltcedar from 29.18°N were 5–21°C higher than those for saltcedar from 47.60°N, and on September 26 and October 11, killing temperatures for cottonwood from 33.06°N were >43°C higher than those for cottonwood from 47.60°N. Analysis of nine microsatellite loci showed that southern saltcedars are more closely related to T. chinensis while northern plants are more closely related to T. ramosissima. Hybridization may have introduced the genetic variability necessary for rapid evolution of the cline in saltcedar cold hardiness.

Survival of plains cottonwood (Populus deltoides subsp. monilifera) and saltcedar (Tamarix ramosissima) seedlings in response to flooding.

We examined the response of first year saltcedar (Tamarix ramosissima) and plains cottonwood (Populus deltoides subsp.monilifera) seedlings to flooding in fall (25 days) and spring (28 days) using potgrown plants (12–18 individuals/26.5-liter pot). Seedlings were initially counted in all pots prior to fall treatment. Survival was calculated as the proportion of seedlings in cach pot still alive following spring treatment. Mean survival rates of seedlings flooded in fall (saltcedar =0.8%, cottonwood=20.8%, n=14 pots) were lower compared to the spring flooding treatment (saltcedar=91.1%, cottonwood=92.2%, n=13) and control (saltcedar=93.9%, cottonwood =98.7%, n=14). We used multiple response permutation procedures to detect omnibus distributional differences in survival data (total tests=9) because assumptions of normality and equal variance were not met. Survival distributions differed between saltcedar and cottonwood fall flooding groups (P<0.0001) and between fall flooding and control groups for both species (P<0.0001). No difference in survival distributions were detected between species or treatments for the control and spring treatment groups (P>0.07). Smaller size and consequent lack of energy reserves may account for lower survival of saltcedar compared to cottonwood in the fall treatment and for lower survival of both species in the fall treatment compared to the spring treatment. Fall flooding for controlling first year saltcedar seedlings is suggested as a potentially useful technique in riparian habitat restoration and management in the southwestern United States.

Early Vegetation Development on an Exposed Reservoir: Implications for Dam Removal

The 4-year drawdown of Horsetooth Reservoir, Colorado, for dam maintenance, provides a case study analog of vegetation response on sediment that might be exposed from removal of a tall dam. Early vegetation recovery on the exposed reservoir bottom was a combination of (1) vegetation colonization on bare, moist substrates typical of riparian zones and reservoir sediment of shallow dams and (2) a shift in moisture status from mesic to the xeric conditions associated with the pre-impoundment upland position of most of the drawdown zone. Plant communities changed rapidly during the first four years of exposure, but were still substantially different from the background upland plant community. Predictions from the recruitment box model about the locations of Populus deltoides subsp. monilifera (plains cottonwood) seedlings relative to the water surface were qualitatively confirmed with respect to optimum locations. However, the extreme vertical range of water surface elevations produced cottonwood seed regeneration well outside the predicted limits of drawdown rate and height above late summer stage. The establishment and survival of cottonwood at high elevations and the differences between the upland plant community and the community that had developed after four years of exposure suggest that vegetation recovery following tall dam removal will follow a trajectory very different from a simple reversal of the response to dam construction, involving not only long time scales of establishment and growth of upland vegetation, but also possibly decades of persistence of legacy vegetation established during the reservoir to upland transition.

Resource Development and Management in Jackson Hole, Wyoming

Abstract Modelers of natural resource systems are often frustrated because potential users of simulation models do not benefit from the increased understanding of system structure and behavior that accrues from the modeling process. This paper describes an attempt to extend the benefits of the modeling process by involving potential users in the initial construction and evaluation of a simple simulation model concerning resource development and management in Jackson Hole, Wyoming.