Mature Cottonwood Research Articles

Riparian recruitment persists after damming: Environmental flows and coupled colonization of cottonwoods and willows following floods along a dryland river

AbstractFor riparian woodlands, occasional floods provide geomorphic disturbance that creates barren colonization sites, and river stage patterns that enable seedling establishment. To investigate impacts from river damming and instream flow regulation on these processes, we studied the lower Red Deer River in the semi‐arid region of Alberta, Canada. Dickson Dam was implemented in 1983 and although major flood peaks in June 2005 and June 2013 were attenuated by about one‐quarter, substantial seedling establishment of plains cottonwoods (Populus deltoides) was observed. Cross‐sectional transects in 2014 and 2015 revealed the reproductively mature cottonwood band from 2005, while the 2013 colonization was limited to low elevations of 0.3–1.3 m above the base river stage. Seedling numbers and the elevational ranges were probably reduced by abrupt (>4 cm/day) river stage reductions in July 2005 and in July 2013. In addition to direct seedling establishment on barren sandbars, we observed “coupled colonization” with cottonwood recruitment within sparse patches of sandbar willows (Salix exigua). The flood‐tolerant willows stabilize the bars and increase aggradation through sediment trapping. Thus, cottonwood colonization persisted after damming, indicating that the pattern of downstream flow regulation was important, rather than damming per se. To sustain riparian recruitment along regulated rivers in dry regions, we recommend: (1) that floods be allowed as feasible, (2) higher river stages during seed dispersal, (3) flow ramping (gradual summer recession) for seedling survival, (4) sufficient growing season flows to avoid drought‐induced mortality, and (5) that willows be encouraged as well as cottonwoods.

Putah Creek hydrology affecting riparian cottonwood and willow tree survival.

Creating or recruiting new riparian forests to improve Lower Putah Creek (LPC) ecosystem functions is challenging under the modified stream flow regime developed after historic gravel mining and installation of Monticello Dam upstream. Hydrologic connectivity between riparian trees, shallow groundwater, and the low flow channel is essential towards maintaining these forests and related habitats through the annual summer and multi-year drought periods typical in this Sacramento Valley region of California. Despite increased average summer flows, significant mature cottonwood and willow tree mortality along the LPC riparian areas below the Putah Creek diversion dam in 2014 raised concerns over the soil and hydrologic factors affecting riparian vegetation survival. A forensic analysis was conducted combining annual canopy coverage fractions and tree ring studies with daily soil-water balances, low flow records, and available groundwater level Information from the past few decades to determine the key hydrologic factors affecting riparian tree survival along the LPC. The 2011-2016 drought was linked with greater than prior average soil-water deficits in 2012-2015 and lower initial soil-water storage on March 1 of 2012 and 2014 that would be expected to stress the trees. However, such stress was not apparent in decreased tree ring spacing during this period from mature (40-50years old), deceased, and living trees. Tree canopy coverage declined dramatically (by as much as 50% as compared to the previous decade average) only in the summer of the 2014 despite a ~ 35% increase in average summer flows from 2011 to 2014. However, the regional water table aquifer levels declined at an average rate of ~ 35mm/day in 2014 (as compared to ~ 17mm/day in previous decade) and by several meters overall between 2011 and 2016 suggesting that deceased trees lacked access to the water table aquifer or lateral stream seepage. The increased rates of water table decline and overall depth may be associated with a large increase in adjacent irrigated almond orchard areas in 2014-2016. Knowledge of the dynamic hydrologic factors controlling sustainability of riparian trees should better inform and guide future tree restoration efforts along the LPC.

Regeneration and invasion of cottonwood riparian forest following wildfire

Populus deltoides is considered to be a weak resprouter and highly susceptible to wildfire, but few post‐wildfire studies have tracked P. deltoides response and resprouting within the Great Plains of North America. Following a wildfire in southwestern Kansas, U.S.A., we surveyed burned and unburned areas of a cottonwood riparian forest along the Cimarron River that included a major understory invader, tamarisk (Tamarix ramosissima Ledeb.). We tested the following hypotheses, which are consistent with the current understanding of P. deltoides response to wildfire in the Great Plains: (1) regeneration of P. deltoides will be low in areas burned by the wildfire; (2) the number of dead P. deltoides individuals will be greater in the wildfire than unburned areas; and (3) tamarisk regeneration will be higher than P. deltoides regeneration in the wildfire areas because tamarisk is considered a stronger resprouter. We found evidence contrary to two of our hypotheses 3 years following the wildfire. (1) P. deltoides regeneration was high following the wildfire, averaging 692 individuals/ha. (2) The number of dead mature cottonwood trees was greater in wildfire plots than in unburned plots. (3) There was more P. deltoides regeneration than tamarisk regeneration following wildfire. These findings, which diverge from the majority of studies examining P. deltoides regeneration in the Great Plains, suggest that differing local environmental and forest stand conditions, coupled with the timing and intensity of the fire, could be important determinants of riparian forest species' responses to wildfire.

Platte Prairies Ants (Hymenoptera: Formicidae) in Restored and Remnant Prairie Tracts in Central Nebraska

ABSTRACT In July 2012, we sampled ant species in remnant and restored prairie tracts in the Platte River Prairies, managed by The Nature Conservancy of Nebraska. The sampling period coincided with TNC's July 2012 “Week of Insects”, an event which involved both general public and invited expert participation, and was an opportunity for both education and citizen science. In four days of sampling, even under severe drought conditions, we encountered 22 species of ants. Fifteen species were found in never-plowed, remnant prairies, and four species were found only in them. Thirteen species were found on restored (planted) prairies, with three of these found only in the planted prairies. Four species were exclusively associated with mature cottonwood trees near the site headquarters.

Temporal and Spatial Variation in Riparian Vegetation and Floodplain Aquifers on the Regulated Dolores River, Southwest Colorado, USA

The importance of flow variability and floodplain water table recharge for the establishment and long-term survival of riparian vegetation has been well-documented. However, temporal and spatial variation in floodplain aquifers has received less attention, although native species can have narrow tolerances for groundwater decline. Our observations of decreased cottonwood cover on floodplains and increased willow cover on river banks since dam completion on the Dolores River led to comparisons between three long-term study sites above and below McPhee Dam. We summarize 5 years (2010–2014) of shallow groundwater well data from transects of three wells per site. Vegetation cover data were collected from quadrats and line-intercept transects. In the willow zone, groundwater well levels mirror in-channel flows and rarely drop below 0.6 m from ground surface. Willow cover and stem counts on point bars are higher at dammed sites. Wells in the cottonwood zone indicate that alluvial recharge happens only during prolonged peak discharge during spring snowmelt or dam release. Years with no dam spill reduced connectivity between surface flows and groundwater, and groundwater depth dropped to between 2 and >2.5 m. Long-term data below the dam indicate that canopy cover of the dominant cottonwoods has declined over time (48% in 1995, 19% in 2003), especially in the wake of severe drought. Mature cottonwood cover is significantly higher at the undammed site (p = 0.025). Our results indicate that floodplain habitats below dams exist under artificially extreme drought and inform how biologically diverse riparian systems will be impacted by a drying climate. Copyright © 2016 John Wiley & Sons, Ltd.

A STELLA Model to Estimate Water and Nitrogen Dynamics in a Short-Rotation Woody Crop Plantation.

Although short-rotation woody crop biomass production technology has demonstrated a promising potential to supply feedstocks for bioenergy production, the water and nutrient processes in the woody crop planation ecosystem are poorly understood. In this study, a computer model was developed to estimate the dynamics of water and nitrogen (N) species (e.g., NH-N, NO-N, particulate organic N, and soluble organic N [SON]) in a woody crop plantation using STELLA (tructural hinking and xperiential earning aboratory with nimation) software. A scenario was performed to estimate diurnal and monthly water and N variations of a 1-ha mature cottonwood plantation over a 1-yr simulation period. A typical monthly variation pattern was found for soil water evaporation, leaf water transpiration, and root water uptake, with an increase from winter to summer and a decrease from summer to the following winter. Simulations further revealed that the rate of soil water evaporation was one order of magnitude lower than that of leaf water transpiration. In most cases, the relative monthly water loss rates could be expressed as evapotranspiration > root uptake > percolation > runoff. Leaching of NO-N and SON depended not only on soil N content but also on rainfall rate and duration. Leaching of NO-N from the cottonwood plantation was about two times higher than that of SON. The relative monthly rate of N leaching was NO-N > SON > NH-N. This study suggests that the STELLA model developed is a useful tool for estimating water and N dynamics from a woody crop plantation.

A STELLA Model to Estimate Soil CO2 Emissions from a Short-Rotation Woody Crop

The potential for climatic factors as well as soil–plant–climate interactions to change as a result of rising levels of atmospheric CO2 concentration is an issue of increasing international environmental concern. Agricultural and forest practices and managements may be important contributors to mitigating elevated atmospheric CO2 concentrations. A computer model was developed using the Structural Thinking and Experiential Learning Laboratory with Animation (STELLA) software for soil CO2 emissions from a short-rotation woody crop as affected by soil water and temperature regimes, root and microbial respiration, and surficial processes such as rainfall, irrigation, and evapotranspiration. The resulting model was validated with good agreement between the model predictions and the experimental measurements prior to its applications. Two scenarios were then chosen to estimate both diurnal and annual soil CO2 emissions from a 1-ha mature cottonwood plantation as affected by soil temperature, soil (i.e., root and microbial) respiration, and irrigation. The simulation resulted in typical diurnal soil respiration and CO2 emission patterns, with increases from morning to early afternoon and decreases from early afternoon to midnight. This pattern was driven by diurnal soil temperature variations, indicating that soil temperature was the main influence on soil respiration and CO2 efflux into the atmosphere. Our simulations further revealed that the average seasonal soil respiration rate in summer was 1.6 times larger than in winter, whereas the average seasonal CO2 emission rate in summer was 1.77 times larger than in winter. Characteristic annual variation patterns for soil respiration and CO2 emission also were modeled, with both increasing from January 1 through June 30 followed by steady declines from September 1 through December 31. These results suggest that the STELLA model developed is a useful tool for estimating soil CO2 emission from a short-rotation woody crop plantation.

Transpirational Water Loss in Invaded and Restored Semiarid Riparian Forests

The invasive tree, Tamarix sp., was introduced to the United States in the 1800s to stabilize stream banks. The riparian ecosystem adjacent to the middle Rio Grande River in central New Mexico consists of mature cottonwood (Populus fremontii) gallery forests with a dense Tamarix understory. We hypothesized that Populus would compensate for reduced competition by increasing its water consumption in restored riparian plots following selective Tamarix removal, resulting in similar transpiration (T) among stands. The northern study site included a Populus stand invaded by Tamarix (INVN) and a restored Populus‐only stand (RESN), as did a southern site (INVS and RESS) approximately 80 miles south. At each site, 20 × 20–m plots were established where up to 16 stems were monitored throughout the 2004 growing season using thermal dissipation sapflow sensors. Populus sapflux rates were greater in restored stands, suggesting those trees compensated for understory removal by using more water. Sapflow was scaled to estimate stand‐level T based on a quantitative assessment of sapwood basal area (Asw) by species. Although exotic species represented 85 and 91% of the total stems in the invaded stands, it amounted to only 3% (INVS) and 4% (INVN) of the total Asw, contributing proportionately less to T compared to Populus. Our results indicate that removing Tamarix from the Populus understory in this riparian forest had a minimal impact on stand water balance. Riparian restoration of the type discussed herein should focus primarily on enhancing riparian health rather than generating water.

A conditional trophic cascade: Birds benefit faster growing trees with strong links between predators and plants

Terrestrial systems are thought to be organized predominantly from the bottom-up, but there is a growing literature documenting top-down trophic cascades under certain ecological conditions. We conducted an experiment to examine how arthropod community structure on a foundation riparian tree mediates the ability of insectivorous birds to influence tree growth. We built whole-tree bird exclosures around 35 mature cottonwood (Populus spp.) trees at two sites in northern Utah, USA, to measure the effect of bird predation on arthropod herbivore and predator species richness, abundance, and biomass, and on tree performance. We maintained bird exclosures over two growing seasons and conducted nondestructive arthropod surveys that recorded 63652 arthropods of 689 morphospecies representing 19 orders. Five major patterns emerged: (1) We found a significant trophic cascade (18% reduction in trunk growth when birds were excluded) only at one site in one year. (2) The significant trophic cascade was associated with higher precipitation, tree growth, and arthropod abundance, richness, and biomass than other site-year combinations. (3) The trophic cascade was weak or not evident when tree growth and insect populations were low apparently due to drought. (4) Concurrent with the stronger trophic cascade, bird predation significantly reduced total arthropod abundance, richness, and biomass. Arthropod biomass was 67% greater on trees without bird predation. This pattern was driven largely by two herbivore groups (folivores and non-aphid sap-feeders) suggesting that birds targeted these groups. (5) Three species of folivores (Orthoptera: Melanoplus spp.) were strong links between birds and trees and were only present in the site and the year in which the stronger trophic cascade occurred. Our results suggest that this trophic system is predominately bottom-up driven, but under certain conditions the influence of top predators can stimulate whole tree growth. When the most limiting factor for tree growth switched from water availability to herbivory, the avian predators gained the potential to reduce herbivory. This potential could be realized when strong links between the birds and plant, i.e., species that were both abundant herbivores and preferred prey, were present.

Preemergence Control of Black Cottonwood in Nursery Containers

Abstract Two experiments were conducted to evaluate preemergence herbicides for control of black cottonwood (Populus trichocarpa) in nursery containers. In 2006, granular preemergence herbicides were applied to recently filled, weed-free containers in May just prior to seed release from mature cottonwood trees. Flumioxazin provided the most effective cottonwood control, although control with isoxaben + trifluralin, oxyfluorfen + oryzalin, and pendimethalin was also effective. In 2007, containers were filled February 15, and herbicides were applied to separate groups of containers on February 22, March 14, April 16, and May 15. Containers were over-seeded with cottonwood for a 2-week period starting on May 16. Control with most herbicides improved as the date of application neared the date of seeding. Flumioxazin provided the most effective control throughout the experiment, but control from it also improved as the date of application neared the time of seeding.

HYDROLOGIC REGIMES AND RIPARIAN FORESTS: A STRUCTURED POPULATION MODEL FOR COTTONWOOD

Riparian cottonwood (Populus deltoides) forests form the one of the most extensive deciduous forest ecosystems in arid regions of the western United States. However, cottonwood populations are threatened by flow alteration and channel degradation caused by dams, water diversions, and groundwater pumping. We developed a stochastic, density- dependent, population model to (1) consolidate information concerning cottonwood pop- ulation dynamics in a conceptual and analytical framework, (2) determine whether complex forest stand dynamics can be predicted from basic cottonwood vital rates and river hy- drology, and (3) aid in planning prescribed floods by projecting how altered flow regimes might affect populations. The model describes how annual variation in the hydrograph affects cottonwood mortality (via floods and droughts) and recruitment (via scouring of new habitat and seedling establishment). Using parameter values for the undammed Yampa River in Colorado, we found that abundances of seedlings and younger trees followed a boom-bust cycle driven by high flood mortalities while reproductive adult abundance fol- lowed a less erratic 5-15-yr periodicity driven by multiyear sequences of flows favorable to stand recruitment. Conversely, chance occurrences of multiple drought years eliminated cottonwood from up to 50% of available habitat, providing opportunities for competing plant species to establish. By simulating flow alterations on the Yampa ranging from chan- nelization (many floods/droughts) to damming (few floods/droughts), the model suggested that mature cottonwood forest should be most abundant near the observed natural flow regime. Model analysis also suggested that flow regimes with high flood frequencies result in stable (albeit small) population sizes, while stable flows result in highly variable pop- ulation sizes prone to local extinction.

FACTORS INFLUENCING EXPANSION OF THE BREEDING DISTRIBUTION OF BEWICK'S WREN INTO RIPARIAN FORESTS OF THE RIO GRANDE IN CENTRAL NEW MEXICO

Abstract In central New Mexico, breeding populations of Bewick's wren (Thryomanes bewickii) recently colonized riparian forests of the Middle Rio Grande and have increased rapidly to become the sixth most abundant breeding bird species in bosque habitats. In 1997–1998, I studied the potential roles that cottonwood forest succession and alien plant species invasion might have had in facilitating this change in the distribution of the wren. Habitat preference and nest cavity limitation were examined at 12 sites near Albuquerque, New Mexico. Bewick's wrens nested only in native tree species, especially large cottonwoods (Populus deltoides). Territories defended by males had greater coverage of mature cottonwood and lower amounts of open, early successional habitats than areas not occupied by wrens. Analysis of wren abundance data from 70 sites throughout the Middle Rio Grande found wren abundance to be highest at sites dominated by cottonwoods, especially at sites having alien salt cedar (Tamarix chinensis) ...

Evapotranspiration at the land/water interface in a semi‐arid drainage basin

1. Evapotranspiration (ET) is a major source of water depletion from riverine systems in arid and semiarid climates. Water budgets have produced estimates of total depletions from riparian vegetation ET for a 320‐km reach of the Middle Rio Grande, New Mexico, U.S.A., that have ranged from 20 to 50% of total depletions from the river.2. Tower‐based micrometeorological measurements of riparian zone ET throughout the growing season using three‐dimensional eddy covariance provided high quality estimates of ET at the stand scale.3. A dense stand of salt cedar (111–122 cm year–1) and a mature cottonwood (Populus deltoides ssp. wislizenia Eckenwelder) stand with an extensive understory of salt cedar (Tamaria ramosissima Ledeb) and Russian olive (Eleagnus angustifolia L.) (123 cm year–1) had the highest rates of annual ET. A mature cottonwood stand with a closed canopy had intermediate rates of ET (98 cm year–1). A less dense salt cedar stand had the lowest rates of ET (74–76 cm year–1).4. Summer leaf area index (LAI) measurements within the four stands were positively correlated with daily ET rates. LAI measurements throughout the growing season coupled to riparian vegetation classification is a promising method for improving riverine corridor estimates of total annual riparian zone ET along a reach of river.5. Combining recent estimates of the extent of riparian vegetation along the 320 km length of the Middle Rio Grande, from Landsat 7 imagery with annual growing season measurements of ET at the four riparian stands yields a first‐order riverine corridor estimate of total riparian zone ET of 150–250 × 106 m3 year–1. This is approximately 20–33% of total estimated depletions along this reach of river.

The discrimination of cottonwood clones in a mature grove along the Oldman River in southern Alberta

In southwestern Alberta, the prairie cottonwood (Populus deltoides Bartr.), balsam poplar (Populus balsamifera L.), narrowleaf cottonwood (Populus angustifolia James), and interspecific hybrids provide the foundation of the biologically rich riparian forests. In addition to seedling-based reproduction, these cottonwoods are capable of clonal propagation, the extent of which is poorly understood. To investigate clonality in mature cottonwoods, a method for clone recognition was investigated. Between 1995 and 1997, the morphology and phenology of each tree in a mature cottonwood grove along the Oldman River were characterized. In decreasing order of utility, the characteristics most effective in revealing clones were sex, leaf shape, floral (inflorescence) phenology, and leaf phenology: flushing, senescence, and abscission. Independent traits of poplar bud gall mite (Aceria parapopuli Keifer) susceptibility and trunk architecture were less useful in clone delineation but validated the clonal determinations. Based on the analysis, the grove's 391 trunks (>10 cm diameter) included 115 genotypes, 48 single-trunked individuals, and 67 multiple-trunked clones. The clones (genets) contained from 2 to 53 trunks (ramets). It was found that 88% of trunks belonged to clonal groups, a proportion that was higher than anticipated. The extensive capacity for clonal recruitment should thus be considered in analyses of cottonwood reproductive ecology and cottonwood conservation and restoration programs.Key words: asexual reproduction, clone delineation, cottonwoods, morphology, phenology, Populus.

Russian olive invasion into cottonwood forests along a regulated river in north-central Montana

Russian olive (Elaeagnus angustifolia L.) is an exotic tree that is invasive in western North America where it may replace native riparian cottonwood forests. We mapped the occurrence of Russian olive and measured the size, density, and age of it and cottonwood in sandbar, low-terrace, and high-terrace habitats along the lower Marias River below Tiber Dam in Montana. Russian olive occurs along the entire study reach but is more abundant near domesticated plantings. It establishes in moist lower terrace habitat as well as under mature cottonwood on high terraces. Seventy-seven percent of cottonwood trees in all size classes were damaged by beavers in low terrace sites, while only 22% of Russian olives showed damage. Since construction of Tiber Dam, cottonwood establishment has been restricted to lower terrace sites. Beaver prevent cottonwood from developing a mature canopy close to the river while having little effect on the continued invasion of Russian olive. Riparian cottonwood forests will eventually be replaced by Russian olive as old cottonwoods die on upper terraces and young plants on low terraces are removed by beaver or shaded by the less palatable species.Key words: Russian olive, cottonwood, riparian, beaver, exotic, dam.

Patterns of clonal occurrence in a mature cottonwood grove along the Oldman River, Alberta

The conservation of the remaining riparian woodlands of western North America will benefit from a better understanding of cottonwood (poplar) regeneration. Following our discrimination of the 67 cottonwood clones present in an island population of 391 trunks, the present investigation characterized the grove's clonal architecture in terms of species, sex, spatial patterns, and beaver impacts. Large clones containing up to 53 trunks occurred near the river channel in areas prone to flooding, scouring, deposition, and beaver damage. Populus section Tacamahaca Spach. species Populus angustifolia James and Populus balsamifera L. produced larger (mean diameter = 9.2 m) clones with more ramets (mean = 6.7) than the section Aigeiros Duby species Populus deltoides Bartr. (7.3 m diameter, 2.7 ramets). The increased clonal nature of the Tacamahaca species is probably adaptive along steeper gradient streams in foothill regions where increased physical disturbance encourages clonal sprouting. Conversely, P. deltoides occurs along more gradually sloped reaches with finer substrates and smoother stage patterns that would favor seedling recruitment. The grove's clonal structure accounted for the female-biased sex ratio among trunks (204 females : 116 males) since the sex ratio was even among genets (49 females : 51 males). Clonality has contributed substantially to regeneration and grove composition, favoring certain genotypes and influencing woodland structural diversity and spatial distributions. Clonal characteristics differ across Alberta's cottonwood species and would influence the distribution of these native trees.Key words: asexual reproduction, beavers, clonality, cottonwoods, Populus, sex.

Patterns of clonal occurrence in a mature cottonwood grove along the Oldman River, Alberta

Patterns of clonal occurrence in a mature cottonwood grove along the Oldman River, Alberta

Simulating Succession of Riparian Spruce Forests and White-Tailed Deer Carrying Capacity in Northwestern Montana

Abstract Rates of succession were determined during the period 1945-80 in six floodplain plant communities along the North Fork of the Flathead River, Montana. A succession model was developed to forecast the long-range effects of land-use change on the carrying capacity of white-tailed deer winter range. The areas occupied by mature spruce (Picea englemannii x glauca hybrid) and spruce-black cottonwood (Populus trichocarpa) forests on the floodplain were relatively constant between 1945-1980. Early seral types were more variable due to short-term variations in river flow. Successional modelling demonstrated the effects of two timber harvesting strategies on white-tailed deer populations. Simulated populations declined following two timber harvesting schedules, but recovered after cessation of harvest. An additional application of the model showed the potential effects of increased erosion rates on deer populations and demonstrated the versatility of land succession models for resource monitoring. West. J. Appl. For. 2(3):80-83, July 1987

Rooting Dormant Cuttings of Mature Cottonwood

Rooting Dormant Cuttings of Mature Cottonwood