Salix Exigua Research Articles

Disturbance Regime Mediates Riparian Forest Dynamics and Physiological Performance, Arkansas River, CO

Tamarix ramosissima has caused dramatic morphological changes to riparian ecosystems and their bank structures over the last century throughout the southwestern United States. Growing as either small trees or dense stands of shoots, Tamarix species displace or actively outcompete native species of willow (Salix exigua) and cottonwood (Populus deltoides) on the Arkansas River in Colorado. Under normal conditions for colonization by native species, Tamarix seedlings are at a great competitive disadvantage due to slow above ground biomass accumulation. However, damming and the resulting altered disturbance regime may give Tamarix an advantage over native species. Damming on the Arkansas River, Colorado, dramatically reduces the intensity and recurrence interval of downstream flooding. Using two dams in eastern Colorado as proxies for flood control, we assessed physiological, demographic and abiotic factors in order to understand how flooding and damming might influence riparian community dynamics. Our study demonstrates that drought and salinity stress may influence native species recruitment and survival in areas with reduced flooding. Moreover, Tamarix water-use may be quite plastic in drought conditions, suggesting that it conserves water at below dam sites.

Erosional Consequence of Saltcedar Control

Removal of nonnative riparian trees is accelerating to conserve water and improve habitat for native species. Widespread control of dominant species, however, can lead to unintended erosion. Helicopter herbicide application in 2003 along a 12-km reach of the Rio Puerco, New Mexico, eliminated the target invasive species saltcedar (Tamarix spp.), which dominated the floodplain, as well as the native species sandbar willow (Salix exigua Nuttall), which occurred as a fringe along the channel. Herbicide application initiated a natural experiment testing the importance of riparian vegetation for bank stability along this data-rich river. A flood three years later eroded about 680,000 m(3) of sediment, increasing mean channel width of the sprayed reach by 84%. Erosion upstream and downstream from the sprayed reach during this flood was inconsequential. Sand eroded from channel banks was transported an average of 5 km downstream and deposited on the floodplain and channel bed. Although vegetation was killed across the floodplain in the sprayed reach, erosion was almost entirely confined to the channel banks. The absence of dense, flexible woody stems on the banks reduced drag on the flow, leading to high shear stress at the toe of the banks, fluvial erosion, bank undercutting, and mass failure. The potential for increased erosion must be included in consideration of phreatophyte control projects.

Responses of obligate versus facultative riparian shrubs following river damming

AbstractRiparian or streamside woodlands include obligate riparian trees and shrubs (obligates) that are restricted to streamside zones, and facultative riparian species that are abundant in, but not restricted to the riparian areas. Due to their distinctive life history requirements, it may be predicted that the ecological specialist obligates would be more vulnerable than the facultative generalists to impacts from river damming and flow regulation. We tested this along the Snake River through Hells Canyon, USA, where two native riparian shrubs dominate: the obligate sandbar willow (Salix exigua), and the facultative, netleaf hackberry (Celtis reticulata). We assessed riparian conditions over the past century by comparing ground‐level and aerial photographs taken after 1907 and in the 1950s in advance of three dams, versus recent conditions. These comparisons revealed three changes downstream from the dams: (1) the depletion of surface sands and sandbars and (2) reductions in sandbar willow versus (3) the proliferation of hackberry in dense bands above the typical high‐water line. The willow decline probably resulted from the depletion of sand following sediment trapping by the reservoirs, combined with changes in the seasonal water flow pattern. The increase in hackberry may have resulted from a beneficial ‘irrigation effect’ of daily water releases for power generation during the summer. The opposing responses reflect the plants' differing life histories and may partially resolve impacts of river regulation on alluvial sediments versus the instream flow pattern. We consider other riparian studies that suggest that obligates such as cottonwoods (Populus angustifolia, P. deltoides and P. fremontii) are highly vulnerable to river regulation, while facultative trees and shrubs such as trembling aspen (Populus tremuloides), wolf‐willow (Elaeagnus commutata) and velvet mesquite (Prosopis velutina) are more resilient. These results suggest that conservation of riparian woodlands should emphasize the ecological specialist obligates, while facultative species may be less vulnerable to river regulation. Copyright © 2009 John Wiley & Sons, Ltd.

Delayed tree mortality in the Atchafalaya Basin of southern Louisiana following Hurricane Andrew

Hurricanes can damage trees in forested wetlands, and the potential for mortality related to these storms exists due to the effects of tree damage over time. In August 1992, Hurricane Andrew passed through the forested wetlands of southern Louisiana with winds in excess of 225 kph. Although more than 78% of the basal area was destroyed in some areas, most trees greater than 2.5 cm dbh were alive and resprouting prolifically the following year (98.8%). Survival of most tree species was similarly high two years after the hurricane, but mortality rates of some species increased dramatically. For example, Populus heterophylla (swamp cottonwood) mortality increased from 7.8 to 59.2% (n = 76) and Salix interior (sandbar willow) mortality increased from 4.5 to 57.1% (n = 21). Stem sprouts on many up-rooted hardwood trees of other species were still alive in 1998, 6 years after the hurricane. Due to the understory tree species composition, regeneration, and high levels of resprouting, there was little change in species composition or perhaps a slight shift toward more shade and flood tolerant species six years following the hurricane event. Triadica sebifera (Chinese tallow) was found on some of the sites heavily disturbed by Hurricane Andrew, and may proliferate at the expense of native tree species.

Field Note: Phytoremediation of Chlorinated Ethenes in Seepline Sediments: Tree Selection

Phytoremediation of chlorinated ethene (CE)-contaminated water was investigated at the Savannah River Site in Aiken, SC, USA. Perchloroethylene (PCE) and trichloroethylene (TCE) are present where CE-contaminated groundwater currently outcrops in seepline soils. Results of constructed and planted test cells, filled with soil from a noncontaminated seepline area and supplied with CE-contaminated groundwater (48 ppb) in the field for one season are presented. These test cells were planted with loblolly pines, hybrid poplars, coyote willow, and sweet gum. Cis-dichloroethylene (cDCE), a byproduct from rhizosphere microbial activity, was detected in the soils as well as some tree tissues. All trees tested were found to uptake both PCE and TCE (5–50 pbb/gm dry wt).

Early-summer pheromone biology of Galerucella calmariensis and relationship to dispersal and colonization

Galerucella calmariensis (Coleoptera: Chrysomelidae) has become an effective biological control agent for purple loosestrife ( Lythrum salicaria). A male-produced aggregation pheromone was recently identified in this mostly univoltine beetle, and attractiveness to both sexes was demonstrated in the spring, when overwintered adults reproduce. Nothing was known about whether the pheromone functioned for the summer generation, when some of the beetles may undergo a brief reproductive period before entering diapause. Our results showed that both sexes of the beetles responded strongly to synthetic pheromone during summer, and males collected in the field during early summer and held under the prevailing photoperiod were able to emit pheromone. These males mated readily with field-collected females, and fertile eggs were laid, indicating a reproductive role for the pheromone during early summer. Males emitted pheromone well only when fed L. salicaria. Adult beetles were able to persist on marginal hosts such Salix interior and Rosa spp., but almost no pheromone was detected. Emission ceased and resumed as host access changed, even after a long interruption. Attractiveness of L. salicaria foliage was investigated. Six “green-leaf” volatiles were identified that were readily sensed by beetle antennae, but a preliminary synthetic blend of these was not attractive in the field. A model is presented for pheromone-mediated host colonization after dispersal, and relationships among pheromone biology, diapause, photoperiod, and host quality are discussed. Practical uses for the pheromone are noted, and new information is given about longevity of pheromone lures.

Ozone Sensitivity of 28 Plant Selections Exposed to Ozone Under Controlled Conditions

Ambient, ground-level ozone is the most important air pollutant affecting vegetation in the US. However, ozone-sensitive bioindicators need to be identified for use in field surveys to detect ozone-induced symptoms. To identify such bioindicators, 28 plant selections were exposed to ozone within greenhouse chambers during 2003 and 2004. Plants most sensitive to ozone included Asclepias incarnata (swamp milkweed), Asclepias syriaca (common milkweed), Cephalanthus occidentalis (buttonbush), Platanus occidentalis (American sycamore), Salix × cotteti (Bankers dwarf willow), Salix lucida (shining willow), Salix nigra (black willow), Salix sericea (silky willow), and Symphoricarpos albus (snowberry). Plants moderately sensitive included Aster novaeangliae (New England aster), Monarda didyma (bee-balm), Rhus aromatica (aromatic sumac), Salix discolor (pussy willow), Salix exigua (sandbar willow), Salix purpurea (basket willow), Sambucus ebulus (European dwarf elderberry), and Symphoricarpos spp. (mixture of “snowberries”). Plants more tolerant to ozone included Aster macrophyllus (bigleaf aster), Aster novibelgii (New York aster), Cercis canadensis (redbud), Populus maximowizii × trichocarpa (hybrid poplar), Rudbeckia laciniata (cutleaf, or tall green-headed coneflower), Salix amygdaloides (peach-leaved willow), Salix eriocephala (diamond willow), Sambucus canadensis (American elder), Sambucus nigra (European elder), Symphoricarpos orbiculatus (coralberry), and Viburnum trilobum (highbush-cranberry). The incidence and severity of ozone-induced symptoms varied with species, ozone concentration, and length of exposure. Pigmented adaxial leaf surface stipple, bifacial necrosis, and premature defoliation were common foliar symptoms induced by ozone; leaf reddening, bronzing, and chlorosis occurred less frequently. Species that produced classic ozone-induced stipple in this study may serve as useful bioindicators in field surveys to confirm that ozone injury occurs in many parts of the country. Such findings may have a positive impact on establishing more stringent air-quality standards to protect vegetation and the environment.

Do beavers promote the invasion of non-native Tamarix in the Grand Canyon riparian zone?

Beavers (Castor canadensis Kuhl) can influence the competitive dynamics of plant species through selective foraging, collection of materials for dam creation, and alteration of hydrologic conditions. In the Grand Canyon National Park, the native Salix gooddingii C.R.Ball (Goodding’s willow) and Salix exigua Nutt. (coyote willow) are a staple food of beavers. Because Salix competes with the invasive Tamarix ramosissima Ledeb., land mangers are concerned that beavers may cause an increase in Tamarix through selective foraging of Salix. A spatial analysis was conducted to assess whether the presence of beavers correlates with the relative abundance of Salix and Tamarix. These methods were designed to detect a system-wide effect of selective beaver foraging in this large study area (367 linear km of riparian habitat). Beavers, Salix, and Tamarix co-occurred at the broadest scales because they occupied similar riparian habitat, particularly geomorphic reaches of low and moderate resistivity. Once the affinity of Salix for particular reach types was accounted for, the presence of Salix was independent of beaver distribution. However, there was a weak positive association between beaver presence and Salix cover. Salix was limited to geomorphic settings with greater sinuosity and distinct terraces, while Tamarix occurred in sinuous and straighter sections of river channel (cliffs, channel margins) where it dominated the woody species composition. After accounting for covariates representing river geomorphology, the proportion of riparian surfaces covered by Tamarix was significantly greater for sites where beavers were present. This indicates that either Tamarix and beavers co-occur in similar habitats, beavers prefer habitats that have high Tamarix cover, or beavers contribute to Tamarix dominance through selective use of its native woody competitors. The hypothesis that beaver herbivory contributes to Tamarix dominance should be considered further through more mechanistic studies of beaver foraging processes and longterm plant community response.

Greenhouse and field assessment of phytoremediation for petroleum contaminants in a riparian zone

Greenhouse and field studies were conducted to evaluate the feasibility of phytoremediation for clean-up of highly contaminated sediments from Indiana Harbor. In the greenhouse study, plant species evaluated were willow ( Salix exigua), poplar ( Populus spp.), eastern gamagrass ( Tripsacum dactyloides), arrowhead ( Sagitaria latifolia), switchgrass ( Panicum virgatum), and sedge ( Carex stricta). Sediments with sedge, switchgrass, and gamagrass had significantly less residual total petroleum hydrocarbons (TPH) after one year of growth (approximately 70% reduction) than sediments containing willow, poplar, or no plants (approximately 20% reduction). Although not all polycyclic aromatic hydrocarbons (PAH) had concentration differences due to the presence of plants, residual pyrene concentrations in the unvegetated pots were significantly higher than in pots containing sedge, switchgrass, arrowhead, and gamagrass. As evaluated by TPH dissipation in the upper section of the pots, the sedge, switchgrass, and gamagrass treatments had higher TPH degradation than the unvegetated, willow and poplar treatments. These trends were similar for soil at the bottom of the pots, with the exception that in the switchgrass treatment, degradation was not significantly different than in the unvegetated soil. Two target contaminants, pyrene and benzo[ b]fluoranthene, showed differences in degradation between planted and unvegetated treatments. In the field study, phytoremediation plant species were eastern gamagrass ( T. dactyloides), switchgrass ( P. virgatum), and sedge ( C. stricta). In addition, rhizosphere characteristics of arrowhead ( S. latifolia) and sedge were assessed. Arrowhead- and sedge-impacted soils were found to contain significantly more PAH-degrading bacteria than unvegetated soils. However, over the 12-month field study, no significant differences in contamination were found between the planted and unplanted soils for TPH and PAH concentrations. TPH concentrations near the canal were greater than concentrations further from the canal, indicating that the canal may have served as a continuous source of contamination during the study.

Trace metals and sulfur in soils and forage of a chronic wasting disease locus

Environmental context. Chronic wasting disease (CWD) and other ‘prion’ diseases tend to cluster geographically, a characteristic that may indicate environmental risk factors, yet the biogeochemical features of regions with high prion disease incidence have rarely been investigated. This study, presenting soil, water and forage trace element analyses for a CWD cluster in Wisconsin, suggests that trace metal toxicity is unlikely to be a disease factor, but further indicates that willow browse could present a risk for copper deficiency in deer owing to high foliar sulfur concentrations. As copper deficiency leads to impaired immune function, it is hypothesised that the risk for CWD could be increased in environments where deer browse willow heavily. Abstract. In an effort to determine whether incidence of chronic wasting disease (CWD) in wild cervids is linked to biogeochemical risk factors of affected regions, soils, plants and surface waters in the Mt Horeb, Wisconsin CWD cluster were sampled and analysed for trace metals as well as major elements. The results revealed normal (relatively low) levels of trace metals in the soils, water and most plants, with no evidence of widespread heavy metal contamination in this region. It was concluded that incidence of CWD in Mt Horeb is unlikely to be linked to abnormally high concentrations of potentially zootoxic metals, including Pb, Zn, Cd or Mn, in soils waters or forages of the region. However, shrub willow, a common browse plant in the area, had high S levels in its leaves compared with other potential forage plants. Bioaccumulation of S was species-specific, with one willow species in particular (Salix exigua) having high foliar S in comparison with other species both in controlled greenhouse experiments as well as in the field near Mt Horeb. The willow S concentrations in the Mt Horeb CWD locus, averaging in excess of 4500 mg kg–1 in 2003 and remaining high in 2004, were high enough to have contributed to hypocuprosis in deer, and possibly to have caused polioencephalomalacia (PEM). It is hypothesised that conditions favourable to intensive deer browsing on shrub willow may exist periodically in Mt Horeb, leading to hypocuprosis and increased susceptibility to CWD.

Evaluation of woody plant restoration in the Middle Rio Grande: Ten years after

Native riparian cottonwood (Populus deltoides) dominated habitats are declining throughout the southwestern United States and being replaced by the exotic salt cedar (Tamarix ramosissima, T. chinensis) dominated systems. Restoration efforts have often involved removing salt cedar and simulating or restoring overbank flooding timed with seed rain of cottonwood and willow (Salix spp.). Success is then often assessed by examining seedling densities of the native species and their known competitive superiority over salt cedar at this stage. We measured plots in the Middle Rio Grande Valley, New Mexico, USA that had been cleared of salt cedar a decade earlier, compared the initial seedling community to the established community, and determined variables important in predicting establishment of cottonwood. During the decade since establishment, the proportion of cottonwood and seep willow (Baccharis glutinosa) increased, while salt cedar and coyote willow (S. exigua) decreased. Proportion of community foliar volume changed little for salt cedar (17.0 to 17.6%), while it increased for cottonwood (25.5 to 63.3%). Cottonwood dominated (> 70%) the overstory. Initially established woody stem density (all species) was the only predictor of the percent of cottonwood canopy cover a decade later as declining coyote willow allowed for greater cottonwood growth. No abiotic variables were important in predictions of subsequent cottonwood community establishment, probably because these variables were similar among sites in this active floodplain. High initial cottonwood and salt cedar seedling densities were related to lower cottonwood foliar volume and smaller individual tree diameters a decade later. Low initial densities of cottonwood allowed individuals to grow rapidly to a greater height, dominating the canopy. Restoration biologists should therefore consider areas that have low initial cottonwood seedling densities as potentially very successful if their goal is to produce a stand of large cottonwoods in a relatively short time.

Synchrony of Seed Dispersal, Hydrology and Local Climate in a Semi-arid River Reach in California

The temporal availability of propagules is a critical factor in sustaining pioneer riparian tree populations along snowmelt-driven rivers because seedling establishment is strongly linked to seasonal hydrology. River regulation in semi-arid regions threatens to decouple seed development and dispersal from the discharge regime to which they evolved. Using the lower Tuolumne River as a model system, we quantified and modeled propagule availability for Populus fremontii (POFR), Salix gooddingii (SAGO), and Salix exigua (SAEX), the tree and shrub species that dominate near-channel riparian stands in the San Joaquin Basin, CA. A degree-day model was fit to field data of seasonal seed density and local temperature from three sites in 2002–2004 to predict the onset of the peak dispersal period. To evaluate historical synchrony of seed dispersal and seasonal river hydrology, we compared peak spring runoff timing to modeled peak seed release periods for the last 75 years. The peak seed release period began on May 15 for POFR (range April 23–June 10), May 30 for SAGO (range May 19–June 11) and May 31 for SAEX (range May 8–June 30). Degree-day models for the onset of seed release reduced prediction error by 40–67% over day-of-year means; the models predicted best the interannual, versus site-to-site, variation in timing. The historical analysis suggests that POFR seed release coincided with peak runoff in almost all years, whereas SAGO and SAEX dispersal occurred during the spring flood recession. The degree-day modeling approach reduce uncertainty in dispersal timing and shows potential for guiding flow releases on regulated rivers to increase riparian tree recruitment at the lowest water cost.

Simulated recruitment of riparian trees and shrubs under natural and regulated flow regimes on the Wisconsin River, USA

Models that link ecological responses to hydrologic changes are important for assessing the effects of flow regulation on aquatic and riparian ecosystems. Based on the Recruitment Box Model, a graphical model used to prescribe environmental flows for cottonwood (Populus spp.) recruitment, we designed a simulation model to represent the influence of river flow dynamics on seedling recruitment of riparian pioneer woody plants. The model simulates the influence of temporal patterns of river stage on dispersal, germination, initial recruitment and over-winter survival of first-year seedlings of riparian pioneer shrubs and trees. We used the model to simulate seedling recruitment patterns for five species (Acer saccharinum, Betula nigra, Populus deltoides, Salix nigra and Salix exigua) on the Wisconsin River (Wisconsin, USA) under three flow scenarios: historic (1935–2002), simulated natural (1915–1975) and simulated regulated flows (1915–1975). Simulation results agreed well with field-observed relative differences among years (1997–2000) in seedling densities for the five focal species. Simulated successful recruitment years were highly synchronous among species, but species differed in their sensitivity to flows at different times during the growing season, consistent with among-species differences in seed dispersal timing. Comparison of simulated natural and regulated flows for 1915–1975 showed that flow regulation decreased monthly flow variability, increased late summer to winter baseflow and reduced the magnitude of spring peaks. Simulated recruitment and over-winter survival of tree seedlings of all species was enhanced under the regulated flow scenario, likely due to increased summer baseflow and reductions in peak flood magnitude. Our analyses show the utility of extending the Recruitment Box Model to include multiple species of riparian shrubs and trees, and the effects of post-colonization flows on their recruitment success. However, some key functional relationships between flow patterns and woody seedling demography (e.g. shear stress thresholds for seedling mortality) have not been adequately quantified and merit further study. Copyright © 2006 John Wiley & Sons, Ltd.

Willows, water regime, and recent cover change in the Peace–Athabasca Delta

:This study examines the relationships between water level changes and willow cover, one aspect of the controversy regarding ecosystem changes in the Peace–Athabasca Delta, northern Alberta, Canada. What water conditions are optimal for the delta’s willows? Does flooding cause willow dieback? Are there species-specific differences in susceptibility, and what attributes of flooding are related to dieback? How has willow cover changed in recent decades? Vegetation cover and its temporal response to flooding and drying were investigated for five common willow species in relation to water levels. Willow cover declined over the period 1993–2001, but cover changes differed among species. Willow cover decreased the most on the wetter transects, while drier transects increased in willow cover. Salix bebbiana and S. discolor appeared to be the most susceptible to flood-induced dieback, and S. petiolaris the least susceptible, while S. planifolia (the dominant willow in the delta) and Salix exigua were intermediate in flood susceptibility. Willow dieback was correlated with water depth, duration of flooding, and time since flooding. Large, old willows were more tolerant of flooding than small, young willows. A pulse of willow establishment occurred during the early 1980s that coincided with a drying period in the delta, increased regional wildfire activity, a decline in river discharge, and a decline in the level of Lake Athabasca. An increase in willow cover followed and reached a peak ca 1993. Flooding in the mid- to late 1990s resulted in a decline in willow cover. Given current climatic trends towards desiccation, willows may continue to be a dominant feature of the delta’s landscape.

OCCURRENCE, PERSISTENCE, AND EXPANSION OF SALTCEDAR (TAMARIX SPP.) POPULATIONS IN THE GREAT PLAINS OF MONTANA

Saltcedar (Tamarix spp.), a shrub native to Eurasia, is associated with major alterations to wetland and riparian systems in the southwestern United States. Since the 1960s saltcedar has been naturalized in northern states of the U.S. where its growth potential and impacts are not well known. Here, we describe the occurrence, age, size, and relative cover of saltcedar populations in several river basins in central eastern Montana, USA, to identify potential pat- terns of spread across the region and changes in individual populations as they age. Stands were aged according to the oldest saltcedar individuals and were sampled for dominant plant cover and soil properties. Multiple introductions appear to have occurred in Montana, with the oldest stands occurring on the Bighorn River in southern Montana. Saltcedar absolute and relative cover and stand area increased significantly with stand age, while native tree and shrub relative cover remained low across all stand ages. These results suggest that saltcedar stands establish where woody natives are not abundant and that they persist and expand over time. Although soil salinity remained constant, soil pH decreased with saltcedar stand age, indicating a possible effect of organic matter inputs. An analysis of annual wood increment of saltcedar and sandbar willow (a native with analogous growth form) stems along a latitudinal gradient showed that stem growth of both species did not differ significantly among regions. Stem growth decreased inversely with elevation for both species while growth responses to elevation did not differ between species. Our results show an increase in number of populations and continued viability of these populations. Mechanisms of saltcedar increases in this region are yet to be determined. Anthropogenic influences, such as saltcedar plantings, watershed alterations (e.g., river flow control), and habitat disturbances (e.g., cattle grazing or habitat clearing) may facilitate its spread in similar climates of the Great Plains.

TRANSVERSE AND LONGITUDINAL VARIATION IN WOODY RIPARIAN VEGETATION ALONG A MONTANE RIVER

This study explores how the relationship between flow and riparian vegetation varies along a montane river. We mapped occurrence of woody riparian plant communities along 58 km of the San Miguel River in southwestern Colorado. We determined the recurrence interval of inundation for each plant community by combining step-backwater hydraulic modeling at 4 representative reaches with Log-Pearson analysis of 4 stream gaging stations. Finally, we mapped bottomland surficial geology and used a Geographic Information System to overlay the coverages of geology and vegetation. Plant communities were distinctly arrayed along the hydrologic gradient. The Salix exigua Nuttall (sand-bar willow) community occurred mostly on surfaces with a recurrence interval of inundation shorter than 2.2 years; the Betula occidentalis Hooker (river birch) community peaked on sites with recurrence intervals of inundation between 2.2 and 4.6 years. The hydrologic position occupied by communities dominated by Populus angustifolia James (narrowleaf cottonwood) was strongly related to age of trees and species composition of understory shrubs. The fraction of riparian vegetation on surfaces historically inundated by the river decreased in the upstream direction from almost 100% near Uravan to <50% along the South Fork of the San Miguel River. In upstream reaches much of the physical disturbance necessary to maintain riparian vegetation is provided by valley-side processes including debris flows, floods from minor tributaries, landslides, and beaver activity. Where valley-side processes are important, prediction of riparian vegetation change based on alterations of river flow will be incomplete.

VEGETATION, SOILS, AND HYDROGEOMORPHOLOGY OF RIPARIAN PATCH TYPES OF A DRYLAND RIVER

From a landscape perspective, riparian corridors can be viewed as mosaics of vegetation patches. We delineated 10 patch types within the floodplain of the San Pedro River (Arizona) on the basis of physiognomy, dominant overstory species, and tree size class; and we assessed differences in hydrogeomorphology, vegetation structure, plant species richness, and soil chemistry and texture. Patches of tamarisk (Tamarix), an introduced species, fell within the continuum of variation shown by other patch types in the landscape mosaic. Among the tree-dominated types, cottonwood-willow (Populus-Salix) and tamarisk patches were inundated more frequently than mesquite (Prosopis) patches, while cottonwood-willow patches had shallower groundwater than tamarisk or mesquite patches. Due to the wetter conditions, cottonwood-willow patches had a high relative abundance of wetland and exotic species in the understory. Tamarisk patches and wet shrublands (Baccharis salicifolia–Salix exigua) had high woody stem densities while cottonwood-willow patches had dense canopy cover. In association with differences in canopy cover, cottonwood-willow patches had low herbaceous species richness but high woody species richness, while tamarisk patches had high herbaceous and low woody species richness. Soil electrical conductivity, silt content, organic matter content, and available phosphorus increased from young to old stands of both tamarisk and cottonwood-willow, often resulting in greater differences between patches of different size/age class than between patches with different dominant species. Surface soil salinity (electrical conductivity) was low in all patches, including those dominated by tamarisk. Nitrate was abundant in soils of tamarisk patches (perhaps reflecting their high clay content) and wet shrubland patches. Dry shrublands (Hymenoclea-Ericameria) and wet shrublands were similar to young forest patches in having coarse soils with little organic matter.

Dating floodplain sediments using tree‐ring response to burial

AbstractFloodplain sediments can be dated precisely based on the change in anatomy of tree rings upon burial. When a stem of tamarisk (Tamarix ramosissima) or sandbar willow (Salix exigua) is buried, subsequent annual rings in the buried section resemble the rings of roots: rings become narrower, vessels within the rings become larger, and transitions between rings become less distinct. We combined observations of these changes with tree‐ring counts to determine the year of deposition of sedimentary beds exposed in a 150‐m‐long trench across the floodplain of the Rio Puerco, a rapidly filling arroyo in New Mexico. This method reliably dated most beds thicker than about 30 cm to within a year of deposition. Floodplain aggradation rates varied dramatically through time and space. Sediment deposition was mostly limited to brief overbank flows occurring every few years. The most rapid deposition occurred on channel‐margin levees, which migrated laterally during channel narrowing. At the decadal timescale, the cross‐section‐average sediment deposition rate was steady, but there was a shift in the spatial pattern of deposition in the 1980s. From 1936 to 1986, sediment deposition occurred by channel narrowing, with little change in elevation of the thalweg. After 1986 sediment deposition occurred by vertical aggradation. From 1936 to 2000 about 27 per cent of the arroyo cross‐section filled with sediment. The rate of filling from 1962 to 2000 was 0·8 vertical m/decade or 85 m2/decade. Published in 2005 by John Wiley &amp; Sons, Ltd.

Field performance and biomass production of 12 willow and poplar clones in short-rotation coppice in southern Quebec (Canada)

Twelve clones of fast growing trees (willow and poplar) were planted in 1999 under short-rotation intensive culture (SRIC) on an abandoned farmland in southern Quebec. The plantation was established at a density of 18,000 trees per hectare from stem cuttings and no fertilizer and irrigation were applied. Trees performances were measured at regular interval during four growing seasons. The aims of the experiment were to compare the growth, insect and disease resistance of these clones in order to select those that have good potential for use as commercial biomass energy crops in northern regions of North America. The follow up of the growing performance has shown statistically significant differences between the clones. Poplar clones registered the highest aboveground biomass yield after 4 growing seasons (from 66.48 to 72.20 tDM ha-1). The best willow biomass productivity was obtained from clones SX64 (67.58 tDM ha-1) and clone SX61 (62.34 tDM ha-1). Only one willow clone S301 (Salix interior × S. eriocephala) was sensitive to leaf rust (Melampsora spp.) and clones SVQ (S. viminalis) and SV1 (S. dasyclados) were more prone to insect attacks. The results proved that some clones of S. miyabeana and S. sachalinensis were more productive and more resistant to insect and disease damage than S. viminalis which has been widely planted in SRIC in southern Quebec since many years.

SALIX EXIGUA CLONAL GROWTH AND POPULATION DYNAMICS IN RELATION TO DISTURBANCE REGIME VARIATION

Willows are important riparian colonizers. However, the predominant models of early riparian colonization, which emphasize seedling recruitment, are inadequate to explain the success of these species in light of the extremely low rates of seedling survival observed. We used molecular fingerprinting markers (AFLPs) to identify and characterize Salix exigua clones on six sites, ranging in size from 850 to 1150 m(2), located on two rivers. Clones as large as 325 m(2) were detected, and an average of six clones per site occupied 75% of the vegetated area. Building on Mahoney and Rood's recruitment box model, we propose a model whereby prolific clonal growth allows for long-term colonization of riparian zones, and the balance between the relative importance of seedling regeneration and clonal growth varies based upon disturbance regime. A reduction in disturbance regime resulted in greater clonal growth and reduced genotypic variation. It is probable that, with an extended reduction in disturbance, the Salix exigua component would be represented by fewer, larger clones and would eventually decline significantly when these clones are replaced by taller and more shade tolerant species.