Hydroclimatic drivers of the growth of riparian cottonwoods at the prairie margin: River flows,river regulation and the Pacific Decadal Oscillation

Cottonwoods, riparian poplars, are facultative phreatophytes and can obtain water from shallow soil moisture originating from rainfall, or from the deeper capillary fringe above the alluvial water table that is recharged by river water infiltration. The correspondence between cottonwood growth and river flows should reveal the dependency upon alluvial groundwater and subsequently, the vulnerability to reduced river flows. To explore this association, we analyzed historic growth patterns of plains cottonwoods (Populus deltoides) along the Red Deer River (RDR), which is at the northwestern limit of the North American Great Plains. We developed chronologies of yearly radial increments (RI) and basal area increments (BAI) and explored correspondences with the environmental records from the past century. In this semi-arid region, the RI or BAI were not correlated with local precipitation while negative correlation with growth season temperature (T) (r = −0.37, p < 0.01) could reflect reduced growth with hot summers. There was correlation between growth and annual river discharge (Q, and particularly log Q that approximates river stage) and this increased with two year averaging (r = 0.51, p < 0.01), reflecting carry-over in the watershed hydrology and in the ecophysiological response. There was correspondence with the Pacific Decadal Oscillation index (PDO, r = −0.45, p < 0.01), which provides multi-decade transitions that influence Rocky Mountain headwater precipitation and other weather characteristics, and river flows. The combination of Q, PDO and T provided the strongest multiple regression model, accounting for 44% of the historic growth variation (52% correspondence for 1953–2013). The RDR was dammed in 1983, enabling winter flow augmentation, but summer flows were sustained and cottonwood growth and the streamflow correspondence persisted. This indicates that it is the pattern of dam operation and not damming per se that determines the fate of established riparian cottonwoods downstream. This study revealed that these cottonwoods are phreatophytic and dependent upon alluvial groundwater that is recharged from the river. This provides a research strategy to determine whether riparian woodlands along other regulated rivers are similarly groundwater-dependent and could be vulnerable to river flow reductions from excessive water withdrawal for irrigation or other uses, or with climate change.     

Differing influences of natural and artificial disturbances on riparian cottonwoods from prairie to mountain ecoregions in Alberta, Canada

Aim Ecoregions represent biophysical zones where environmental factors enable the development of particular plant communities. Ecoregions are generally large but abrupt transitions occur in areas with rapid physical change. A particularly abrupt transitional sequence occurs in the Rocky Mountain region of south‐western Alberta where fescue prairie, aspen parkland and mountain ecoregions occur within 15 km. To investigate plant adaptation across ecoregions, our study investigated the influences of a natural disturbance (flooding) and an artificial disturbance (cattle grazing) on reproductive and population processes of black cottonwood (Populus balsamifera subsp. trichocarpa, Torr. & Gray), the dominant riparian tree. Location We studied cottonwoods throughout their elevational range along two free‐flowing, first‐order streams, Yarrow and Drywood creeks. Cottonwood was the only prominent tree in the prairie ecoregion, the dominant riparian tree in the parkland and extended upward through the montane ecoregion where it was a pioneer species for the mixed coniferous–deciduous woodland. Cottonwoods did not occur in the higher elevation sub‐alpine ecoregion. Methods Thirty‐six cross‐sectional sampling transects were located across the three ecoregions with cottonwoods, and in ungrazed and grazed areas of each ecoregion. Rectangular 100 m² tree and 2 m² seedling quadrats were positioned along the transects, and substrate and vegetation were assessed. Historic hydrological data were analysed relative to flood recurrences and seasonal flow patterns. Results Overall, the cottonwoods displayed a sawtooth shaped ‘punctuated progressive age structure’ with many young trees, progressively fewer older trees, and about four pulses of increased recruitment over the past century. This was considered to provide a healthy cottonwood population and recruitment pulses were apparently associated with flood events with appropriate peak timing and magnitude and a gradual post‐flood stage recession. However, analyses of tree, sapling and seedling data indicated that flood‐associated seedling recruitment was less important and clonal processes were more important for cottonwood recruitment in the montane ecoregion, the highest ecoregion with cottonwoods. The correlation between flood events and cottonwood recruitment was strongest in the mid‐elevation parkland ecoregion suggesting greater reliance on flood‐associated seedling recruitment. There was little correlation with flooding and limited recruitment in the fescue prairie ecoregion in recent decades and the disturbed age structure probably results from cattle impacts that have prevented recruitment and produced a decrepit cottonwood forest population. Main conclusions These analyses suggested that a healthy cottonwood population displayed a sawtooth shaped ‘punctuated progressive age structure’ and that cottonwood reproduction processes varied across ecoregions with increased clonality in the highest montane ecoregion. Cattle grazing impacts on reproduction were most severe in the lowest prairie ecoregion that is treeless except for the riparian zone. We conclude that appropriate strategies of instream flow regulation, land‐use policies and practices, and conservation and restoration efforts should be refined according to ecoregion to recognize the differences in cottonwood reproductive and population ecology.     

Root architecture of riparian trees: river cut-banks provide natural hydraulic excavation, revealing that cottonwoods are facultative phreatophytes

Plant root architecture reveals the sources of water and nutrients but tree root systems are large and difficult to analyze. With riparian (floodplain) trees, river cut-banks provide natural hydraulic excavation of root systems and this presents a unique study opportunity. Subsequently, we developed the ‘Cut-bank Root Method’, a simple, quantitative approach for analyzing the distribution of coarse roots, based on analyses of photographs of river cut-banks. These reveal the vertical extent of roots and median root depths (R_d). We applied this method along six rivers draining the Canadian Rocky Mountains and observed tenfold difference in R_d. The floodplain forests were dominated by cottonwoods and from mountain to prairie zones we observed progressively deeper roots of Populus trichocarpa (black cottonwood, R_d ~ 0.3 m), P. balsamifera (balsam poplar), P. angustifolia (narrowleaf cottonwood), and P. deltoides (prairie cottonwood, R_d ~ 0.9 m), which had R_d similar to P. fremontii (Fremont cottonwood) in Nevada, USA. Roots were shallower for co-occurring facultative riparian trees, with R_d ~ 0.1 m for P. tremuloides (trembling aspen) and Picea glauca (white spruce). Across the Canadian sites, R_d for cottonwoods were strongly associated with a growth season moisture index (May through September precipitation—potential evapotranspiration; R² = 0.97, P < 0.001). Thus, in wetter climates, riparian cottonwoods were shallow-rooted and would be more dependent upon rain than stream flow. Conversely, in the drier semi-arid regions the cottonwoods were phreatophytic, with deeper root systems in the capillary fringe above the alluvial ground-water table. These phreatophytic cottonwoods would be highly dependent upon stream flow and vulnerable to declining river flows due to river regulation or climate change.     

Localized temperature adaptation of cottonwoods from elevational ecoregions in the Rocky Mountains

To investigate temperature adaptation of cool-climate trees, we studied 14 cottonwood genotypes from riparian (streamside) zones in three ecoregions differing in elevation in the Rocky Mountains of Alberta, Canada. Black cottonwoods (Populus trichocarpa) were collected from the higher, cooler montane and intermediate (aspen) parkland, and narrowleaf cottonwoods (P. angustifolia) were collected from the lower, warmer fescue prairie. The genotypes were grown in growth chambers under three temperature regimes reflecting the ecoregion variation. Sapling growth (dry weight) varied significantly across temperatures and for the genotypes from within and particularly across the ecoregions. Significant temperature × genotype interactions further indicated differentiation of the temperature response. Growth of the montane clones increased by 209% between 15/10 and 20/15°C and was 37% lower at 25/20°C. In contrast, genotypes from the lower ecoregions grew more slowly at the cool and intermediate temperatures (59 and 58% of montane) and then demonstrated constant (−3% parkland) or slightly increased (+16% prairie) growth at 25/20°C. This suggests the existence of P. trichocarpa ecotypes, localized populations with different temperature responses. This differentiation may explain our previous observation of comparable growth across these ecoregions despite substantial temperature variation, and the existence of ecotypes would produce a range of responses to climate warming that should produce an upward shift of the mountain ecoregions.     

Functional flows: an environmental flow regime benefits riparian cottonwoods along the Waterton River,Alberta

With drainage from the Waterton‐Glacier International Peace Park, the Waterton River was dammed in 1964 to trap spring flow and permit offstream diversion for irrigation. Field observations in the 1980s indicated some decrepit riparian woodlands suggesting drought stress of the black and narrowleaf cottonwoods (Populus trichocarpa, P. angustifolia) due to insufficient in‐stream flows. Subsequently, an environmental flow regime commenced in 1991 and provided “functional flows,” deliberately regulating in‐stream flow components intended to restore ecological processes and particularly (1) an increase of the minimum flow from 0.93 to 2.27 m³/s (mean discharge 21.9 m³/s) and (2) flow ramping, gradual recession after the spring peak. This study investigated the historic flow patterns and the growth, population age structure, and spatial distributions of riparian cottonwoods along the free‐flowing upstream and regulated downstream reaches over four dam operations intervals: the free‐flowing pre‐dam condition; the initial dammed interval to the mid‐1970s; a post‐dam and drought interval in the 1980s; and with the environmental flow regime. Analyses of sapling, shrub‐, and tree‐sized cottonwoods included tree ring analyses to determine ages and growth patterns, and distributions were assessed relative to streamside elevations and sediment textures. These indicated that there has been progressive cottonwood colonization after damming but the colonization band dropped in elevation with the reduced flow regime and the future woodlands could become narrower. The tree ring analyses indicated that the growth of established trees benefited from the functional flows and the increase in minimum flow was probably particularly beneficial to the riparian cottonwoods.     

Mechanisms of Riparian Cottonwood Decline Along Regulated Rivers

Decline of riparian forests has been attributed to hydrologic modifications to river flows. However, little is known about the physiological and structural adjustments of riparian forests subject to modified flow regimes, and the potential for forest restoration using historic flow regimes is poorly understood. In this paired river study, we compared hydrology, water relations, and forest structure in cottonwood-dominated floodplains of the regulated Green River to those of the unregulated Yampa River. We measured floodplain groundwater levels, soil water availability, cottonwood xylem pressure (Ψ_xp), and leaf-level stomatal conductance (g_s) to assess current impacts of river regulation on the water status of adult cottonwoods. We also simulated a flood on the former floodplain of the regulated river to evaluate its impact on cottonwood water relations. Canopy and root structure were quantified with estimates of cottonwood leaf area and percent live canopy and root density and biomass, respectively. Regulation of the Green River has lowered the annual peak flow yet raised minimum flows in most years, resulting in a 60% smaller stage change, and lowered soil water availability by as much as 70% compared to predam conditions. Despite differences in water availability, daily and seasonal trends in Ψ_xp and g_s were similar for cottonwoods on the regulated and unregulated rivers. In addition, soil water added with the experimental flood had little effect on cottonwood water relations, contrary to our expectations of alleviated water stress. Green River cottonwoods had 10%–30% lower stand leaf area, 40% lower root density, and 25% lower root biomass compared with those for Yampa River cottonwoods. Our results suggest that water relations at the leaf and stem level are currently similar for Yampa and Green River trees due to structural adjustments of cottonwood forests along the Green River, triggered by river regulation.     

Effects of flooding on leaf development,transpiration, and photosynthesis in narrowleaf cottonwood,a willow-like poplar

The narrowleaf cottonwood, Populus angustifolia, occurs in occasionally flooded, low elevation zones along river valleys near the North American Rocky Mountains. This small poplar has narrow leaves and fine branching and thus resembles willows, which are commonly flood-tolerant. We investigated the flood response of narrowleaf cottonwoods and a related native hybrid, jackii cottonwood (P. × jackii = P. balsamifera × P. deltoides), by studying saplings of 24 clones in a greenhouse, with some pots being inundated to provide the flood treatment. Flooding slightly reduced leaf numbers (−10%), and leaf sizes were reduced by about 21% in female P. angustifolia versus a 50% reduction in the female hybrids. Flooding-reduced stomatal conductance and net photosynthetic rate, and reduced transpiration particularly in P. × jackii. The effects on foliar gas exchange declined over a 5-week interval, suggesting compensation. The moderate impact of flooding supports the hypothesis that narrowleaf cottonwoods are flood-tolerant, and we anticipate that these trees could provide traits to increase the flood tolerance of fast-growing hybrid poplars. The results further indicate that female cottonwoods may be more flood-tolerant than males, and females could be more successful in lower, flood-prone sites.     

Consistent growth of black cottonwoods despite temperature variation across elevational ecoregions in the Rocky Mountains

Globally, water and temperature provide the dominant environmental determinants of tree distribution and growth. In riparian or streamside zones, groundwater is abundant, and we consequently predicted that temperature would limit the growth of riparian cottonwoods in a cool climate northern mountain region. To investigate this association, we analyzed tree rings of 167 black cottonwoods, Populus trichocarpa, along two adjacent Rocky Mountain creeks in Alberta. Cottonwoods were sampled from 1700 m, near their upper elevational limit, down 500 m through three progressively warmer ecoregions, the montane, aspen parkland, and fescue prairie. Across these zones, June through September mean temperatures rose from 12.4 to 16.2°C (lapse rate = 0.67°C/100 m), and there was subsequently a 42% increase in growing degree days (base 5°C, GDD₅) from 900 GDD₅ at the trees’ upper limit. Despite this variation, growth rate of most trees was fairly consistent across the ecoregions; trunk diameter versus age associations were relatively similar (r ² = 0.85) with an estimated 14% increase in trunk sizes of 50 year-old trees with decreasing elevation. In all ecoregions, developmental patterns were prominent as annual radial increments increased up to about 20 years, and then progressively declined to an apparent lethal threshold of about 0.4 mm/year at about 100 years. Basal area increments also increased through the juvenile phase, but remained fairly constant thereafter. The weak association between growth and temperature suggests that other environmental factors limited growth rates or there were differences in temperature adaptation across these elevational ecoregions. The results suggest that predicted regional climate warming may not substantially promote the growth rates of these Rocky Mountain trees.     

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.     

Branch sacrifice: cavitation-associated drought adaptation of riparian cottonwoods

In their native riparian zones (floodplains), Populus deltoides (prairie cottonwood) and P. fremontii (Fremont cottonwood) commonly experience substantial branch die-back. These trees occur in semi-arid areas of North America and unexpectedly given the dry regions, they are exceptionally vulnerable to xylem cavitation, drought-induced air embolism of xylem vessels. We propose that the vulnerability to cavitation and branch die-back are physiologically linked; drought-induced cavitation underlies branch die-back that reduces transpirational demand enabling the remaining shoot to maintain a favorable water balance. This proposal follows field observation along various western North American rivers as precocious branch senescence, the yellowing and death of leaves on particular branches during mid- to late summer, was common for P. deltoides and P. fremontii during hot and dry periods of low stream-flow. Branches displaying precocious senescence were subsequently dead the following year. The proposed association between cavitation, precocious senescence and branch die-back is also supported by experiments involving external pressurization of branches to about 2.5 MPa with a branch collar or through an adjacent cut-branch. The treatments induced xylem cavitation and increased leaf diffusive resistance (stomatal closure) that was followed by leaf senescence and branch death of P. deltoides. P. trichocarpa (black cottonwood) appeared to be less affected by the pressurization treatment and this species as well P. angustifolia (narrowleaf cottonwood) and P. balsamifera (balsam poplar) seldom display the patchy summer branch senescence typical of P. deltoides and P. fremontii. ’Branch sacrifice’ describes this cavitation-associated senescence and branch die-back that may provide a drought adaptation for the prairie and Fremont cottonwoods. Received: 13 May 1999 / Accepted: 4 November 1999     

Branch growth of riparian cottonwoods: a hydrologically sensitive dendrochronological tool

 The conservation of riparian (river valley flood plain) forests relies on the provision of instream flows that are sufficient to sustain tree growth. In the present study, annual branch growth increments were investigated as an indicator of environmental favorability for riparian cottonwoods. Trees of three species, Populus angustifolia, P. balsamifera, and P. deltoides, and their natural interspecific hybrids, were studied at five sites along the Oldman and South Saskatchewan rivers in Alberta, Canada. Annual branch growth increments for the interval from 1983 to 1992 were positively correlated with stream flows (r ² = 0.79 at Lethbridge) and slightly negatively correlated with weather variables that contribute to water demand: evaporation, temperature, wind, and/or sunshine. The combination of January to May stream flow (water supply) and June evaporation (water demand) almost entirely accounted for the branch growth variation across years (r ² = 0.91 at Lethbridge). Tree ring increments were also investigated but were less closely correlated than branch increments across trees or with stream flow. Branch growth increments thus provide an accurate but short duration (1 or 2 decades) record of environmental favorability for growth. The close correlation between branch growth and stream flow indicates that water is the principal limitation to growth of these riparian cottonwoods and that these trees obtained their water from a source linked to the stream, the riparian water table. Analyses of branch increments should provide a management tool for (i) determining instream flow needs for riparian cottonwoods and (ii) analyzing impacts of stream flow alterations due to river damming or water diversion. Received: 8 May 1997 / Accepted: 23 September 1997     

Resizing a River: A Downscaled,Seasonal Flow Regime Promotes Riparian Restoration

While riverine organisms are adapted to the natural flow regime, it is impractical to fully restore natural flows along most regulated rivers. We propose an alternative with the delivery of downscaled flow regimes that provide the seasonal patterns that are essential for aquatic and riparian ecosystems. The Bridge River in British Columbia provided a novel case study as a downscaled flow regime commenced in 2000 along a reach that had generally experienced no flow for the prior half‐century. The experimental flow delivered a mean discharge of about 3 m³/s, versus the pre‐dam mean of 100 m³/s, with a seasonal pattern that mimicked the natural snowmelt‐dominated pattern. To assess the environmental response, we investigated black cottonwoods, Populus trichocarpa, the dominant riparian trees, in the pre‐flow versus post‐flow intervals, using tree ring interpretation for growth analyses and age determination. Sparse mature trees established prior to the 1948 damming did not show significant growth changes in the pre‐ versus post‐flow intervals. In contrast, younger trees that established closer to the river in the decade prior to 2000 displayed significant growth increases by 2002, and juveniles established after 2000 demonstrated faster initial growth than juveniles established before 2000. Further, bands of cottonwood saplings resulted from seedling recruitment along the new river fringe, particularly in 2002, 2003, and 2004, years with gradual flow recession. These responses demonstrate that a downscaled, seasonal flow regime provided environmental benefit, thereby restoring some river function and resulting in a resized river flanked by narrow and reproducing cottonwood bands.     

Environmental tolerance of an invasive riparian tree and its potential for continued spread in the southwestern US

Questions: Exotic plant invasion may be aided by facilitation and broad tolerance of environmental conditions, yet these processes are poorly understood in species‐rich ecosystems such as riparian zones. In the southwestern United States (US) two plant species have invaded riparian zones: tamarisk (Tamarix ramosissima, T. chinensis, and their hybrids) and Russian olive (Elaeagnus angustifolia). We addressed the following questions: (1) is Russian olive able to tolerate drier and shadier conditions than cottonwood and tamarisk? (2) Can tamarisk and cottonwood facilitate Russian olive invasion? Location: Arid riparian zones, southwestern US. Methods: We analyzed riparian tree seedling requirements in a controlled experiment, performed empirical field studies, and analyzed stable oxygen isotopes to determine the water sources used by Russian olive. Results: Russian olive survival was significantly higher in dense shade and low moisture conditions than tamarisk and cottonwood. Field observations indicated Russian olive established where flooding cannot occur, and under dense canopies of tamarisk, cottonwood, and Russian olive. Tamarisk and native riparian plant species seedlings cannot establish in these dry, shaded habitats. Russian olive can rely on upper soil water until 15 years of age, before utilizing groundwater. Conclusions: We demonstrate that even though there is little evidence of facilitation by cottonwood and tamarisk, Russian olive is able to tolerate dense shade and low moisture conditions better than tamarisk and cottonwood. There is great potential for continued spread of Russian olive throughout the southwestern US because large areas of suitable habitat exist that are not yet inhabited by this species.     

C : N : P stoichiometry of dominant riparian trees and arthropods along the Middle Rio Grande

1. We examined the role of flooding on the leaf nutrient content of riparian trees by comparing the carbon : nitrogen : phosphorus (C : N : P) ratio of leaves and litter of Rio Grande cottonwood (Populus deltoides ssp. wislizenii) in flood and non‐flood sites along the Middle Rio Grande, NM, U.S.A. The leaf C : N : P ratio was also examined for two non‐native trees, saltcedar (Tamarix chinensis) and Russian olive (Elaeagnus angustifolia), and six species of dominant riparian arthropods. 2. Living leaves and leaf litter of cottonwoods at flood sites had a significantly lower leaf N : P ratio and higher %P compared with leaves and litter at non‐flood sites. A non‐flood site downstream from wastewater effluent had a significantly lower litter C : N ratio than all other sites, suggesting N fertilisation through ground water. The non‐native trees, saltcedar and Russian olive, had higher mean leaf N content, N : P ratio, and lower C : N ratio compared with cottonwoods across study sites. 3. Riparian arthropods ranged from 5.2 to 7.1 for C : N ratio, 56–216 for C : P ratio, and 8.9–34 for N : P ratio. C content ranged from 25 to 52% of dry mass, N content from 4.7 to 10.8%, and P content from 0.59 to 1.2%. Differences in stoichiometry between high C : nutrient leaf litter and low C : nutrient invertebrates suggests possible food‐quality constraints for detritivores. 4. These results suggest that spatial and temporal variation in the C : N : P ratio of cottonwood leaves and leaf litter is influenced by surface and subsurface hydrologic connection within the floodplain. Reach‐scale variation in the elemental composition of riparian organic matter inputs may have important implications for decomposition, nutrient cycling, and food webs in river floodplain systems.     

The Role of Abandoned Channels as Refugia for Sustaining Pioneer Riparian Forest Ecosystems

In disturbance-prone ecosystems, organisms often persist in spatial refugia during stressful periods. A clear example is the colonization of abandoned river channels by pioneer riparian trees. Here, we examine the prominence of this establishment pathway for a foundation tree species (Fremont cottonwood, Populus fremontii) within the riparian corridor of a large river, the Sacramento River in central California. We quantified the total proportion of forest that initiated as a result of channel abandonment for a 160-km reach, analyzed concurrent patterns of tree establishment with floodplain accretion and sedimentation history, and developed a conceptual model of biogeomorphic evolution of abandoned channels. Historical air photo analysis indicated that stands associated with abandoned channels comprised more than 50% of the total extant cottonwood forest area. Tree-ring evidence showed that cottonwood stands commonly developed immediately following abandonment, and the recruitment window ranged from 4 to 40 years, but was less than 10 years at most sites. Rates of floodplain rise and fine sediment accumulation were high in young sites and decreased logarithmically over time. Together, these results suggest that abandoned channels are an important refuge for cottonwood recruitment, that the greatest opportunity for colonization occurs within a short period after the cutoff event, and that sedimentation processes influence the duration of the colonization window. On rivers where tree recruitment along the active channel is severely limited by hydrologic regulation and/or land management, abandoned channel refugia may play an even more important role in sustaining an ecologically functional riparian corridor. Preserving bank erosion, active meander corridors and forest regeneration zones created by cutoff events are therefore key conservation measures on shifting rivers.     

Genetic analysis of admixture and patterns of introgression in foundation cottonwood trees (Salicaceae) in southwestern Colorado,USA

Cottonwoods are well known as foundation riparian trees that support diverse communities and drive ecosystem processes. Although hybridization naturally occurs when the distributions of two or more cottonwood species overlap, few cottonwood hybrid zones have been genetically characterized. We use genetic and genomic analyses to characterize patterns of admixture and introgression for a newly described hybrid zone at the intersection of three species (Populus L. Salicaceae—Populus deltoides, Populus fremontii, and Populus angustifolia) in southwestern Colorado, USA. Analysis of nuclear and chloroplast microsatellite marker data detected substantial genetic variation among individuals, revealing that (1) hybridization is occurring between two, not three, species (P. deltoides and P. angustifolia); (2) gene flow is bidirectional; (3) hybrids are not abundant (admixture detected in only 34 of 270 trees), with most being early-generation F₁ hybrids; (4) cytonuclear disequilibria exists and F₁ hybrids tend to retain P. deltoides—like chloroplasts; and (5) roughly 30 % of the nuclear markers deviated from a neutral pattern of introgression, suggesting that selection may play a role in shaping the genetic structure of the hybrid zone in this region. Overall, our results show that despite strong selection maintaining species divergence, transfer of allelic variants across species boundaries can occur. Our study assesses the fine-scale genetic structure of hybridization between P. angustifolia and P. deltoides and lays the foundation for examining how geographic differences in hybrid zone dynamics arise and may influence subsequent ecological and evolutionary processes.     

Hydrologic linkages between a climate oscillation,river flows,growth, and wood Δ13C of male and female cottonwood trees

To investigate climatic influence on floodplain trees, we analysed interannual correspondences between the Pacific Decadal Oscillation (PDO), river and groundwater hydrology, and growth and wood ¹³C discrimination (Δ¹³C) of narrowleaf cottonwoods (Populus angustifolia) in a semi‐arid prairie region. From the Rocky Mountain headwaters, river discharge (Q) was coordinated with the PDO (1910–2008: r² = 0.46); this pattern extended to the prairie and was amplified by water withdrawal for irrigation. Floodplain groundwater depth was correlated with river stage (r² = 0.96), and the cottonwood trunk basal area growth was coordinated with current‐ and prior‐year Q (1992–2008: r² = 0.51), increasing in the mid‐1990s, and decreasing in 2000 and 2001. Annual Δ¹³C decreased during low‐flow years, especially in trees that were higher or further from the river, suggesting drought stress and stomatal closure, and male trees were more responsive than females (?0.86 versus ?0.43‰). With subsequently increased flows, Δ¹³C increased and growth recovered. This demonstrated the linkages between hydroclimatic variation and cottonwood ecophysiology, and we conclude that cottonwoods will be vulnerable to drought from declining river flows due to water withdrawal and climate change. Trees further from the river could be especially affected, leading to narrowing of floodplain forests along some rivers.     

Assessing the extent and diversity of riparian ecosystems in Sonora, Mexico

Conservation of forested riparian ecosystems is of international concern. Relatively little is known of the structure, composition, diversity, and extent of riparian ecosystems in Mexico. We used high- and low-resolution satellite imagery from 2000 to 2006, and ground-based sampling in 2006, to assess the spatial pattern, extent, and woody plant composition of riparian forests across a range of spatial scales for the state of Sonora, Mexico. For all 3rd and higher order streams, river bottomlands with riparian forests occupied a total area of 2,301 km². Where forested bottomlands remained, on average, 34% of the area had been converted to agriculture while 39% remained forested. We estimated that the total area of riparian forest along the principal streams was 897 km². Including fencerow trees, the total forested riparian area was 944 km², or 0.5% of the total land area of Sonora. Ground-based sampling of woody riparian vegetation consisted of 92, 50 m radius circular plots. About 79 woody plant species were noted. The most important tree species, based on cover and frequency, were willow species Salix spp. (primarily S. goodingii and S. bonplandiana), mesquite species Prosopis spp. (primarily P. velutina), and Fremont cottonwood Populus fremontii. Woody riparian taxa at the reach scale showed a trend of increasing diversity from north to south within Sonora. Species richness was greatest in the willow-bald cypress Taxodium distichum var. mexicanum—Mexican cottonwood P. mexicana subsp. dimorphia ecosystem. The non-native tamarisk Tamarix spp. was rare, occurring at just three study reaches. Relatively natural stream flow patterns and fluvial disturbance regimes likely limit its establishment and spread.     

Breeding Birds in Riparian and Upland Dry Forests of the Cascade Range

ABSTRACT We quantified breeding bird abundance, diversity, and indicator species in riparian and upland dry forests along 6 third- to fourth-order streams on the east slope of the Cascade Range, Washington, USA. Upland dry forest on southerly aspects was dominated by open ponderosa pine (Pinus ponderosa) and dry Douglas-fir (Pseudotsuga menziesii) plant associations. Upland mesic forest on northerly aspects was dominated by closed-canopy Douglas-fir or dry grand fir (Abies grandis) plant associations. Riparian overstory vegetation was dominated by black cottonwood (Populus trichocarpa) plant associations with a prominent hardwood tree and shrub component. We quantified bird assemblages, diversity, and abundance from parallel point transects on riparian and adjacent dry and mesic upslope forests. We detected 80 bird species from >12,000 point-transect observations during 1998–1999. Eighteen species accounted for 75% of all detections. Species richness and evenness were similar in all 3 forest types, with approximately 35 species and high evenness (0.85) in each forest type. Bird species assemblages differed among dry, mesic, and riparian forest types, with the greatest differences between riparian and both dry and mesic upland forests. Riparian forest had the greatest number (9) of strong characteristic, or indictor, species among the 3 forest types. Upland mesic forest was characterized by 7 indicator species. Upland dry forest had 4 indicator species. Our results indicate that current standards and guidelines for riparian buffers zones would allow for avian refuge and corridor functions along these streams. Forest managers could use our indicator species to predict and monitor shifts in upland forest species composition from thinning and prescribed burning practices that are used to reduce fuels in uplands and to reduce continuity of fire effects between riparian and upland zones.     

Selection for genetics‐based architecture traits in a native cottonwood negatively affects invasive tamarisk in a restoration field trial

Climate change and competition from invasive species remain two important challenges in restoration. We examined the hypothesis that non‐native tamarisk (Tamarix spp.) reestablishment after aboveground removal is affected by genetics‐based architecture of native Fremont cottonwood (Populus fremontii) used in restoration. As cottonwood architecture (height, canopy width, number of stems, and trunk diameter) is, in part, determined by genetics, we predicted that trees from different provenances would exhibit different architecture, and mean annual maximum temperature transfer distance from the provenances would interact with the architecture to affect tamarisk. In a common garden in Chevelon, AZ, U.S.A. (elevation 1,496 m), with cottonwoods from provenances spanning its elevation distribution, we measured the performance of both cottonwoods and tamarisk. Several key findings emerged. On average, cottonwoods from higher elevations were (1) two times taller and wider, covered approximately 3.5 times more basal area, and were less shrubby in appearance, by exhibiting four times fewer number of stems than cottonwoods from lower elevations; (2) had 50% fewer tamarisk growing underneath, which were two times shorter and covered 6.5 times less basal area than tamarisk growing underneath cottonwoods of smaller stature; and (3) the number of cottonwood stems did not affect tamarisk growth, possibly because the negative relationship between cottonwood stems and basal area. In combination, these findings argue that cottonwood architecture is affected by local conditions that interact with genetics‐based architecture. These interactions can negatively affect the growth of reinvading tamarisk and enhance restoration success. Our study emphasizes the importance of incorporating genetic and environmental interactions of plants used in restoration.