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.     

Growth of riparian cottonwoods: a developmental pattern and the influence of geomorphic context

Cottonwoods are poplars (Populus sp.) adapted to riparian (streamside) zones and an understanding of their growth within these zones will assist with river management for cottonwood conservation and in the recognition of superior parental genotypes for hybrid poplar breeding programs. In this study we analyzed cottonwood growth in native riparian zones and compared growth along three study reaches of the Oldman River in Alberta, Canada that differed in geomorphic context, particularly the extent that the river channel was constrained by steep banks and bedrock. We used dendrochronology to analyze trunk growth patterns, and measured annual radial increments (RI) and basal area increments (BAI) of 278 narrowleaf cottonwoods (P. angustifolia), black cottonwoods (P. trichocarpa), their intrasectional hybrids, and natural intersectional hybrids with prairie cottonwoods (P. deltoides). The trees displayed common growth patterns with four phases: (I) a 3–7-year establishment phase with RI of about 1–2 mm/year, (II) a growth acceleration phase of about 15 years with RI increasing to the (III) RI growth peak of about 3 mm/year, and then (IV) the mature growth phase with relatively constant BAI and progressively declining RI. This general pattern was consistent across study reaches but the durations and growth rates of the phases differed along with forest stand structure. Along the unconstrained alluvial reach with a broad floodplain and dynamic channel, extensive and dense forest groves occurred. This increased tree competition, as evidenced by reduced RI and BAI in the mature phase. In contrast, along a constrained reach trees were restricted to sparse, narrow bands and their increased growth rates in the mature phase probably reflected reduced competition. Cottonwoods along the intermediate reach demonstrated an intermediate combination of forest and growth characteristics. Genotypic effects were slight although P. angustifolia had reduced RI during the establishment phase. These results demonstrate that within native riparian zones cottonwoods display an inherent growth pattern that reflects the trees' life history, and that growth rates and transitions are influenced by the geomorphic context that influences forest structure.     

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.     

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.     

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     

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.     

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.     

Drought stress and recovery of riparian cottonwoods due to water table alteration along Willow Creek,Alberta

A 5-m-deep gravel pit was excavated from 1996 to 1998 in the floodplain between Willow Creek, Alberta, and a grove of balsam poplars ('cottonwoods', Populus balsamifera L.) and water level at the pit was lowered 2.5 m through pumping. This interrupted the infiltration of stream water into the riparian groundwater and imposed drought stress on the cottonwoods. Trees in the drought-affected grove displayed extensive leaf senescence and abscission in late August 1998, while trees in nearby control groves remained green until autumnal senescence in late September. The precocious senescence was accompanied by a two-thirds reduction in leaf stomatal conductance (g _s) but mid-day leaf xylem water potentials (ψ_l) were only slightly reduced (?1.55 vs 1.42 MPa). Pumping ceased in 1999, the pit was partially refilled, and the hydraulic linkage between the stream and the riparian zone recovered. Subsequently in August 1999, g _s and ψ_l were similar for trees in the affected and control groves and senescence phenologies were similar in 1999 and 2000. Annual branch growth increments varied 3-fold across years between 1994 and 1999, but there was no reduction in these growth increments in the drought-affected trees in 1998 or 1999. This study supports the hydraulic linkage between a stream and the adjacent riparian zone in a semi-arid region and demonstrates the vulnerability of riparian cottonwoods to drought due to water table depletion. It also indicates rapid physiological recovery of cottonwoods following restoration of water availability.     

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.     

Direct seeding for riparian tree re-vegetation: Small-scale field study of seeding methods and irrigation techniques

Restoration of wetland and associated ecosystems is a major goal of land management agencies throughout the world. On the lower Colorado River, creation of riparian forests is planned to mitigate riparian habitat degradation by historic land-use conversions and river management. Current restoration practices use propagated plant stock. If direct seeding can be implemented, genetic and structural diversity could be enhanced at restoration sites even while reducing costs compared to vegetative propagation methods. A small-scale field study was implemented in Cibola, Arizona, to determine the effectiveness of direct seeding of Fremont cottonwood (Populus fremontii), Goodding's willow (Salix gooddingii), and coyote willow (S. exigua). For the first growing season, establishment of Fremont cottonwood averaged 7% of pure live seed rates for all treatments combined, whereas establishment of willows was less than 1%. Volunteer species were abundant, with grasses dominating cover and biomass after one growing season. Saltcedar (Tamarix ramosissima) established in abundance, but showed lower growth rates than Fremont cottonwood during the first growing season. Monitoring for three growing seasons indicated higher growth rates and survival of Fremont cottonwood compared to all volunteer species. Study results indicated that direct seeding of Fremont cottonwood is likely to be an efficient method for tree re-vegetation. Additional studies are required for willow species to determine if establishment from seed can be increased through enhanced weed control and elimination of Fremont cottonwood from the seed mix.     

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.     

Water allocation and climate-impacted riparian forest dynamics in the Ejina Oasis,Northwest China

Riparian forests in natural desert oases are extremely vulnerable to water shortages; of late these shortages have been associated with climate change and with increased human-led water allocation. This study covers a hundred-year history (1876–2017) of riparian forest growth at the Ejina Oasis, which is located in the lower reaches of the Heihe River basin of northwestern China. We collected tree cores from Populus euphratica, which is the major tree species found in the Ejina riparian forests. These samples allowed us to chart variations in riparian forest growth and to examine correlations between tree growth and local precipitation, temperature, drought indices, groundwater depth, and runoff volume from the middle reaches of the river. We found that groundwater depth (groundwater being mainly recharged by runoff) is the major factor limiting tree-stem radial growth. We compared runoff reconstruction series from upper reaches and P. euphratica radial growth in the lower reaches. We found a period of greatly decreased growth (1942–1951); which seems to have been due to human water diversion. We note that mountain runoff increased after 2000, but that riparian forest growth didn’t increase in tandem; the water that would otherwise have supported the forests had been diverted. Our study provides a warning for future water resource planning and suggests the desirability of policies that will balance the needs of natural ecosystems (riparian forests) with the requirements of artificial ecosystems (croplands).     

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.     

Seedling competition between native cottonwood and exotic saltcedar: implications for restoration

Altered hydrology of southwestern United States rivers has led to a decline in native cottonwood (Populus deltoides). Areas historically dominated by cottonwood have been replaced by invasive saltcedar (Tamarix chinensis). Restoration of historic hydrology through periodic flooding of riparian areas has been a means of restoring native species. However, due to similarity in germination requirements of cottonwoods and saltcedars, flooding may create an unwanted increase in the number of saltcedar seedlings. Therefore, we evaluated competitive aspects of these co-occurring species in an extant riparian habitat in the arid southwestern US. We measured effects of competition between cottonwood and saltcedar seedlings and among cottonwood seedlings during the first growing season following seedling establishment in 360, 0.5 × 0.5-m plots at the Bosque del Apache National Wildlife Refuge, New Mexico. We used five interspecific density treatments and five intraspecific density treatments. Cottonwood seedling biomass and height were twice that of saltcedar seedlings across all density treatments. As density of cottonwood increased, intraspecific competition increased in severity and biomass of cottonwood seedlings decreased. At 4 plants/0.25 m², cottonwood seedlings had the greatest biomass; whereas, survival was highest at 10 plants/0.25 m². Our results support greenhouse studies and suggest that if favorable germination conditions are established for cottonwood in floodplains, saltcedar seedlings that cogerminate could be outcompeted by native cottonwood seedlings.     

Abundance and Production of Riparian Trees in the Lowland Floodplain of the Queets River, Washington

Riparian zones associated with alluvial rivers are spatially dynamic, forming distinct vegetative mosaics that exhibit sharp contrasts in structure and processes related to the underlying biophysical template. The productivity of riparian plants, especially trees, influences streamside community characteristics as well as the forms and fluxes of organic matter to adjacent streams – thereby strongly impacting patterns of channel morphology, water flow, sedimentation, and habitat in rivers. As part of a comprehensive investigation of riparian dynamics in coastal rain forest rivers of the Pacific Northwest (USA), we examined riparian tree abundance (density, basal area, and biomass) and rates of production (basal area growth [BAI] and bole wood biomass increase [P]) of seven common species – red alder (Alnus rubra), Sitka spruce (Picea sitchensis), bigleaf maple (Acer macrophyllum), western hemlock (Tsuga heterophylla), black cottonwood (Populus trichocarpa), vine maple (Acer circinatum) and willow (Salix spp.) – in the lowland floodplain of the Queets River (Olympic National Park), Washington. Measurements were made annually for three years (1999 – 2001) in 16 permanent plots on three biophysical templates that formed a toposequence – active floodplain, young terrace and mature terrace. Stem density was highest in the active floodplain (∼27,000 stems/ ha), decreasing in the young terrace (∼2,700 stems /ha) and the mature terrace (∼500 stems/ha). Basal area and total stem biomass were lowest in the active floodplain (∼16 m2/ha and ∼18 Mg dry weight/ha, respectively) and higher on the young terrace (∼32 m2/ha and ∼134 Mg dry weight/ha) and on the mature terrace (∼69 m2/ha and ∼540 Mg dry weight /ha). Total plot-scale BAI was not significantly different among the physical templates with mean values ranging from approximately 1.4 (low terrace) to approximately 2.8 m2/ha/y (active floodplain). In contrast, P was significantly higher on the mature terrace (10.3 Mg/ha) than the active floodplain (3.2 Mg/ha) but there was no significant difference between young terrace (6.5 Mg/ha) and mature terrace. For the entire Queets River floodplain (57 km² over 77 km of river length), the mature terrace contributed 81% of the total annual production (28,764 Mg) whereas the active floodplain and young terrace accounted only for 5 and 14%, respectively. Overall, we show that riparian trees grow quickly in this coastal Pacific Northwest system and that the older riparian forests on mature terraces are the main contributors to stem production at the plot and floodplain scales for at least 350 years after stand initiation. This suggests that, in combination with the rapid lateral migrations of many alluvial rivers, the older riparian forests on those terraces are important and sustained sources of organic matter (especially large woody debris, LWD) that, over decades to centuries, shape the character of coastal rivers in the Pacific Northwest.     

How do riparian woody seedlings survive seasonal drought?

In semi-arid regions, a major population limitation for riparian trees is seedling desiccation during the dry season that follows annual spring floods. We investigated the stress response of first-year pioneer riparian seedlings to experimental water table declines (0, 1 and 3 cm day⁻¹), focusing on the three dominant cottonwood and willows (family Salicaceae) in California’s San Joaquin Basin. We analyzed growth and belowground allocation response to water stress, and used logistic regression to determine if these traits had an influence on individual survival. The models indicate that high root growth (>3 mm day⁻¹) and low shoot:root ratios (<1.5 g g⁻¹) strongly predicted survival, but there was no evidence that plants increased belowground allocation in response to drawdown. Leaf δ¹³C values shifted most for the best-surviving species (net change of +3.5 per mil from −30.0 ± 0.3 control values for Goodding’s willow, Salix gooddingii), implying an important role of increased water-use efficiency for surviving water stress. Both S. gooddingii and sandbar willow (S. exigua) reduced leaf size from controls, whereas Fremont cottonwood (Populus fremontii) sustained a 29% reduction in specific leaf area (from 13.4 to 9.6 m² kg⁻¹). The functional responses exhibited by Goodding’s willow, the more drought-tolerant species, may play a role in its greater relative abundance in dry regions such as the San Joaquin Basin. This study highlights the potential for a shift in riparian forest composition. Under a future drier climate regime or under reduced regulated river flows, our results suggest that willow establishment will be favored over cottonwood.     

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.     

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.