Cottonwood Willow Research Articles

Development of a carbon calculator tool for riparian forest restoration

AbstractAim(s)Methods for estimating carbon sequestration in riparian forest biomass are needed to qualify riparian restoration and conservation projects for funding through carbon credits. As part of a proposed accounting methodology for California's cap‐and‐trade system, we described five riparian vegetation associations and created a tool to predict their rates of carbon accumulation under different restoration and land‐use scenarios.LocationCalifornia, USA.MethodsWe assembled a database of 654 forest inventory plots of known age (3–108 years) from replanted and naturally recruiting California riparian forests. We then used detrended correspondence analysis and agglomerative hierarchical clustering to detect vegetation structure in the plots and delineate distinct forest types. We used allometric equations to estimate biomass from individual trees’ diameters and calculate total biomass for each plot. Next, we fitted sigmoid growth functions to the plot biomass data to generate expected values for live‐tree biomass over a 100‐year period for each forest type. Finally, we created an algorithm to match user‐inputted data for an intended restoration project with a look‐up table that predicts carbon accumulation appropriate to the user's scenario.ResultsCalifornia riparian vegetation was divided into willow scrub, cottonwood–willow, mixed riparian, upland riparian and riparian woodland types. Total carbon sequestration in riparian vegetation and soil varied from 75.7 to 137.4 Mg C/ha at 30 years post‐restoration and from 95.1 to 175.8 Mg C/ha 100 years after stand initiation. Vegetation types dominated by taller, more shade‐tolerant trees developed more slowly than scrubby and pioneer stands, but came to a higher maximum biomass. Our models put riparian biomass accumulation in the range of the few extant literature values for mediterranean systems, but uncertainties are high, and soil carbon may be underestimated.ConclusionsOur workflow and methods should be transferable to the development of carbon accounting tools for any other woody vegetation type. However, our difficulty in finding appropriate published data for the analysis suggests a critical need for field surveys appropriate to biomass estimation in woodland and forest communities not exploited for timber. The most important measurements are the age of the stand and a census of tree species, height and diameter.

The long‐term legacy of geomorphic and riparian vegetation feedbacks on the dammed Bill Williams River, Arizona, USA

AbstractOn alluvial rivers, fluvial landforms and riparian vegetation communities codevelop as a result of feedbacks between plants and abiotic processes. The influence of vegetation on river channel and floodplain geomorphology can be particularly strong on dammed rivers with altered hydrology and reduced flood disturbance. We used a 56‐year series of aerial photos on the dammed Bill Williams River (Arizona, USA) to investigate how (a) different woody riparian vegetation types influence river channel planform and (b) how different fluvial landforms drive the composition of riparian plant communities over time. We mapped vegetation types and geomorphic surfaces and quantified how relations between fluvial and biotic processes covaried over time using linear mixed models. In the decades after the dam was built, woody plant cover within the river's bottomland nearly doubled, narrowing the active channel by 60% and transforming its planform from wide and braided to a single thread and more sinuous channel. Compared with native cottonwood–willow vegetation, nonnative tamarisk locally induced a twofold greater reduction in channel braiding. Vegetation expanded at different rates depending on the type of landform, with tamarisk cover on former high‐flow channels increasing 17% faster than cottonwood–willow. Former low‐flow channels with frequent inundation supported a greater increase in cottonwood–willow relative to tamarisk. These findings give insight into how feedbacks between abiotic and biotic processes in river channels accelerate and fortify changes triggered by dam construction, creating river systems increasingly distinct from predam ecological communities and landforms, and progressively more resistant to restoration of predam forms and processes.

Influence of monsoon‐related riparian phenology on yellow‐billed cuckoo habitat selection in Arizona

AbstractAimThe western yellow‐billed cuckoo (Coccyzus americanus occidentalis), a Neotropical migrant bird, is facing steep population declines in its western breeding grounds owing primarily to loss of native habitat. The favoured nesting habitat for the cuckoo in the south‐western United States is low‐elevation riparian forests and woodlands. Our aim was to explore relationships between vegetation phenology patterns captured by satellite phenometrics and the distribution of the yellow‐billed cuckoo, and to use this information to map cuckoo habitat.LocationArizona, USA.MethodsLand surface phenometrics were derived from satellite Advanced Very High‐Resolution Radiometer (AVHRR), bi‐weekly time‐composite, normalized difference vegetation index (NDVI) data for 1998 and 1999 at a resolution of 1 km. Fourier harmonics were used to analyse the waveform of the annual NDVI profile in each pixel. To create the models, we coupled 1998 satellite phenometrics with 1998 field survey data of cuckoo presence or absence and with point data that sampled riparian and cottonwood–willow vegetation types. Our models were verified and refined using field and satellite data collected in 1999.ResultsThe models reveal that cuckoos prefer areas that experience peak greenness 29 days later, are 36% more dynamic and slightly (< 1%) more productive than their average cottonwood–willow habitat. The results support a scenario in which cuckoos migrate northwards, following the greening of riparian corridors and surrounding landscapes in response to monsoon precipitation, but then select a nesting site based on optimizing the near‐term foraging potential of the neighbourhood.Main conclusionsThe identification of preferred phenotypes within recognized habitat can be used to refine future habitat models, inform habitat response to climate change, and suggest adaptation strategies. For example, models of phenotype preferences can guide management actions by identifying and prioritizing for conservation those landscapes that reliably exhibit highly preferred phenometrics on a consistent basis.

Growth Response of Coyote Willow (Salix exigua) Cuttings in Relation to Alluvial Soil Texture and Water Availability

A common approach to re-establishing cottonwood–willow habitat along regulated rivers is through installing dormant, rootless cuttings, yet there is little published information exploring floodplain characteristics that optimize growth of southwestern riparian willows planted in this manner. The goal of this project was to evaluate relationships between growth attributes of Salix exigua and soil texture and soil water availability. Monitoring plots were established in five willow swales planted with dormant S. exigua cuttings along the banks of the Middle Rio Grande in central New Mexico. Data analysis revealed significantly higher aerial cover, height, and stem density for S. exigua plants installed in plots with intermediate levels (15–25%) of fine textured soils distributed through the soil profile. Similar relationships were found in relation to soil water availability. Regression analysis of percent fines and available water at different depth increments provided limited explanation of variability in willow growth attributes at different plots. Findings indicate that S. exigua plants established from cuttings can achieve heights and aerial cover values similar to naturally established willow bars if the floodplain soil profile contains intermediate levels of fine textured soils and the maximum depth to groundwater is within 1.5 m of the ground surface. Where sites are dominated by coarse sand, S. exigua growth may be improved if maximum depth to groundwater is within 1 m of the ground surface.

An Integrative Assessment of the Effects of Tamarisk on Stopover Ecology of a Long-Distance Migrant Along the San Pedro River, Arizona

ABSTRACT. The riparian habitats of the arid southwestern United States provide critical stopover habitat for avian migrants, but they have been severely altered by invasive tamarisk (Tamarix spp.). The responses of birds to such habitat alterations are not well understood. We combined ecological and physiological measurements to provide an integrative assessment of how tamarisk invasion affects the stopover ecology of vernally migrating Wilson's Warblers (Wilsonia pusilla) along the San Pedro River in Arizona. Despite higher arthropod biomass in native cottonwood—willow habitat, the refueling rate of Wilson's Warblers, as measured by plasma metabolite profiling, was higher in tamarisk. Density and detections of Wilson's Warblers did not differ between habitats, but both measurements were significantly higher in cottonwood—willow habitat for the other members of the Wilson's Warbler foraging guild. Our measurements of food resources and migrant densities suggest that cottonwood—willow may provide high-qual...

Avian Density and Nest Survival on the San Pedro River: Importance of Vegetation Type and Hydrologic Regime

Abstract: Lowland riparian vegetation in the southwestern United States is critically important for maintaining a high richness and density of breeding birds. Further investigation is needed within riparian corridors, however, to evaluate the relative importance of vegetation type and hydrologic regime for avian density and nest survival as targets for regional conservation or restoration efforts. We estimated the densities of 40 bird species and for species grouped on the basis of nest height and dependence on surface water in gallery cottonwood–willow (Populus spp.–Salix spp.) forests, saltcedar (Tamarix spp.) shrub lands, and terrace vegetation types along a gradient in the hydrologic regime of the San Pedro River, Arizona, USA. We also assessed nest survival for shrub‐nesting insectivores and herbivores. Canopy‐nesting birds as a group and 14 individual bird species reached their greatest densities in cottonwood forests regardless of the hydrologic regime. Water‐dependent birds as a group reached their highest density in both intermittent‐ and perennial‐flow cottonwood stands, but certain species occurred almost exclusively in perennial‐flow sites. Two shrub‐nesting species and the brown‐headed cowbird (Molothrus ater) were most abundant in saltcedar shrub lands, and the brown‐headed cowbird was most abundant in saltcedar stands with intermittent flows. Mesquite (Prosopis spp.) and big sacaton (Sporobolus wrightii) grassland each maintained the highest densities of certain species within ≥1 hydrologic regime. Shrub‐nesting insectivores had the greatest nest survival in cottonwood, including Arizona Bell's vireo (Vireo bellii arizonae), and also had lower proportions of nests parasitized and preyed upon, although 95% confidence intervals among vegetation types overlapped. Nest survival for both shrub‐nesting insectivores and herbivores was lowest in intermittent‐flow saltcedar, although, again, confidence intervals overlapped. Nest survival was lower in parasitized than nonparasitized nests in mesquite and across vegetation types for Arizona Bell's vireo and in cottonwood for Abert's towhee (Pipilo aberti). Riparian management that maintains heterogeneous riparian vegetation types, including floodplain vegetation comprising cottonwood–willow gallery riparian forests with some stretches of perennial flow, are important for maintaining the high diversity and abundance of breeding birds on the San Pedro River and probably across the region. Cottonwood stands also appear to maintain highest nest survival for some shrub‐nesting birds.

Growth of Valley Oak (Quercus Lobata Nee) in Four Floodplain Environments in the Central Valley of California

Although there have been numerous studies of California upland oak regeneration and growth there has been no research investigating oak sapling growth in riparian environments. This study looks at the growth response of young valley oak Quercus lobata Nee, a dominant late successional riparian species in California, across environmental variation on a floodplain. Growth was measured over the course of three years at four different sites. The sites were chosen to represent the range of successional stages and surface age. Growth was significantly higher on younger, unforested sites. There was no difference in growth rate in the two forested sites early successional cottonwood willow forest and late successional mature oak forest. Herbivory was highest in the cottonwood willow forest, where density of young oaks was also highest. The impact of flooding was measured on the youngest floodplain surface, an open floodplain restoration site where acorns were planted the year our study began. There was a significant negative impact of flooding on sapling growth in all but the first year of growth. Taken together these results suggest that existing forest trees and flooding both inhibit the growth of valley oaks on the floodplain, and that restoration in open sites may be more successful than restoration under an existing canopy.

Dynamics of Fremont cottonwood (Populus fremontii) and saltcedar (Tamarix chinensis) populations along the San Pedro River, Arizona

Woodlands of the exotic saltcedar (Tamarix chinensis) have replaced forests of native Fremont cottonwood (Populus fremontii) and willow (Salixspp.) along many rivers of the American South-west. In the middle basin of the San Pedro River, saltcedar dominates only at the drier sites where the surface and ground-water conditions no longer support cottonwood–willow forests. At sites with perennial (or near-perennial) stream flow, saltcedar is co-dominant with Fremont cottonwood. However, saltcedar has been declining in importance at these sites, perhaps due to recent occurrence of conditions that favour cottonwood establishment (frequent winter flooding, high rates of stream flow during spring, exclusion of livestock). This shift provides evidence of capacity for self-repair in degraded Sonoran riparian ecosystems. In the upper basin, in contrast, saltcedar has increased in relative abundance at sites that show evidence of ground-water decline, signaling a need for vigilance in river management. Saltcedar is generally sparse in the upper basin, probably due to the combination of cool temperatures and persistence of perennial or near-perennial stream flows in most areas. Throughout the San Pedro River, saltcedar and cottonwood both have been influenced by changing flood patterns. Expansion of Fremont cottonwood populations and initial colonization by saltcedar both correlate with post-1960 increases in fall and winter flood frequency and decreases in summer flood size.