Restoration of riparian forest productivity lost as a consequence of flow regulation is a common management goal in dryland riverine ecosystems. In the northern hemisphere, dryland river floodplain trees often include one or another species of Populus, which are fast-growing, nutrient-demanding trees. Because the trees are phreatophytic in drylands, and have water needs met in whole or in part by a shallow water table, their productivity may be limited by nitrogen (N) availability, which commonly limits primary productivity in mesic environments. We added 20 g N m−2 in a 2-m radius around the base of mature Populus fremontii along each of a regulated and free-flowing river in semiarid northwest Colorado, USA (total n = 42) in order to test whether growth is constrained by low soil N. Twelve years after fertilization, we collected increment cores from these and matched unfertilized trees and compared radial growth ratios (growth in the 3-year post-fertilization period/growth in the 3-year pre-fertilization period) in paired t tests. We expected a higher mean ratio in the fertilized trees. No effect from fertilization was detected, nor was a trend evident on either river. An alternative test using analysis of covariance (ANCOVA) produced a similar result. Our results underscore the need for additional assessment of which and to what extent factors other than water control dryland riverine productivity. Positive confirmation of adequate soil nutrients at these and other dryland riparian sites would bolster the argument that flow management is necessary and sufficient to maximize productivity and enhance resilience in affected desert riverine forests.
Elevated CO2 does not offset greater water stress predicted under climate change for native and exotic riparian plants
Perry L.G., P.B. Shafroth, D.M. Blumenthal, J.A. Morgan, D.R. LeCain
In semiarid western North American riparian ecosystems, increased drought and lower streamflows under climate change may reduce plant growth and recruitment, and favor drought-tolerant exotic species over mesic native species. We tested whether elevated atmospheric CO2 might ameliorate these effects by improving plant water-use efficiency.
We examined the effects of CO2 and water availability on seedlings of two native (Populus deltoids spp. monilifera, Salix exigua) and three exotic (Elaeagnus angustifolia, Tamarix spp., Ulmus pumila) western North American riparian species in a CO2-controlled glasshouse, using 1-m-deep pots with different water-table decline rates.
Low water availability reduced seedling biomass by 70–97%, and hindered the native species more than the exotics. Elevated CO2 increased biomass by 15%, with similar effects on natives and exotics. Elevated CO2 increased intrinsic water-use efficiency (Δ13Cleaf), but did not increase biomass more in drier treatments than wetter treatments.
The moderate positive effects of elevated CO2 on riparian seedlings are unlikely to counteract the large negative effects of increased aridity projected under climate change. Our results suggest that increased aridity will reduce riparian seedling growth despite elevated CO2, and will reduce growth more for native Salix and Populus than for drought-tolerant exotic species.
Projecting climate change effects on cottonwood and willow seed dispersal phenology, flood timing, and seedling recruitment in western riparian forests
Tamarix spp. are introduced shrubs that have become among the most abundant woody plants growing along western North American rivers. We sought to empirically test the long-held belief that Tamarix actively displaces native species through elevating soil salinity via salt exudation. We measured chemical and physical attributes of soils (e.g., salinity, major cations and anions, texture), litter cover and depth, and stand structure along chronosequences dominated by Tamarix and those dominated by native riparian species (Populus or Salix) along the upper and lower Colorado River in Colorado and Arizona/California, USA. We tested four hypotheses:
The rate of salt accumulation in soils is faster in Tamarix dominated stands than stands dominated by native species,
The concentration of salts in the soil is higher in mature stands dominated by Tamarix compared to native stands,
Soil salinity is a function of Tamarix abundance, and
Available nutrients are more concentrated in native-dominated stands compared to Tamarix-dominated stands.
We found that salt concentration increases at a faster rate in Tamarix-dominated stands along the relatively freeflowing upper Colorado but not along the heavily regulated lower Colorado. Concentrations of ions that are known to be preferentially exuded by Tamarix (e.g., B, Na, and Cl) were higher in Tamarix stands than in native stands. Soil salt concentrations in older Tamarix stands along the upper Colorado were sufficiently high to inhibit germination, establishment, or growth of some native species. On the lower Colorado, salinity was very high in all stands and is likely due to factors associated with floodplain development and the hydrologic effects of river regulation, such as reduced overbank flooding, evaporation of shallow ground water, higher salt concentrations in surface and ground water due to agricultural practices, and higher salt concentrations in fine-textured sediments derived from naturally saline parent material.
Tree-Ring Records of Variation in Flow and Channel Geometry
The Northwestern Forested Mountains are ecologically diverse and geographically widespread, encompassing the mountain ecosystems of central and northwestern North America (CEC 1997; Figure 2.2). The ecoregion description is adapted from CEC (1997). Geographically, they extend from the Rocky Mountains and the Sierra Nevada north through the Siskiyous, the east side of the Cascade Range, and then east of the Coast Ranges to interior Alaska. Climatically, the region is characterized by a transition from a moist, maritime climate in the northwest, to a continental and drier climate in the Rockies in the southeast. Orographically generated rainfall creates both rain shadows and wet belts, often in close proximity.
The vegetation of the ecoregion is extremely diverse, with distinct community zonation occurring along elevation gradients. Alpine communities at the highest elevations contain various forb, lichen, and shrub associations. Subalpine communities include lodgepole pine (Pinus contorta), subalpine fir (Abies lasiocarpa), Pacific silver fir (Abies amabilis), grand fir (Abies grandis), and Engelmann spruce (Picea engelmannii). Mid-elevation forests are characterized by ponderosa pine (Pinus ponderosa), Rocky Mountain Douglas-fir (Pseudotsuga menziesii var. glauca), lodgepole pine, and quaking aspen (Populus tremuloides) in the east, and by western hemlock (Tsuga heterophylla), western red cedar (Thuja plicata), Douglas-fir (Pseudotsuga menziesii), and western white pine (Pinus monticola) in the west and southwest. White and black spruce (Picea glauca and P. mariana) dominate the Alaskan portion of the ecoregion. Vegetation of the interior valleys in the southern portion of the region includes big sagebrush (Artemisia tridentate), rabbitbrush (Chrysothamnus spp.), and antelope bitterbrush (Purshia tridentate).
Genetic and environmental influences on cold hardiness of native and introduced riparian trees
Friedman, J.S., J.E. Roelle, and B.S. Cade
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National Proceedings: Forest and Conservation Nursery Associations – 2011
To explore latitudinal genetic variation in cold hardiness and leaf phenology, we planted a common garden of paired collections of native and introduced riparian trees sampled along a latitudinal gradient. The garden in Fort Collins, Colorado (latitude 40.6°N), included 681 native plains cottonwood (Populus deltoids subsp. monilifera) and introduced saltcedar (Tamarix ramosissima, T. chinensis, and hybrids) collected from 15 sites from 29.2 to 47.6°N in the central United States. In the common garden, both species showed latitudinal variation in fall, but not spring, leaf phenology. This suggests that latitudinal gradient field observations in fall phenology are a result, at least in part, of the inherited variation in the critical photoperiod. Conversely, the latitudinal gradient field observations in spring phenology are largely a plastic response to the temperature gradient. Populations from higher latitudes exhibited earlier bud set and leaf senescence. Cold hardiness varied latitudinally in both fall and spring for both species. Although cottonwood was hardier than saltcedar in midwinter, the reverse was true in late fall and early spring. The latitudinal variation in fall phenology and cold hardiness of saltcedar appears to have developed as a result of multiple introductions of genetically distinct populations, hybridization, and natural selection in the 150 years since introduction.
Environmental flows for riparian restoration and Tamarix management
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Tamarisk Research Conference: current status and future directions, Oct. 3-4, 2006, Fort Collins, CO