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.     

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.     

Living fencerows of the Rio San Miguel,Sonora, Mexico: Traditional technology for floodplain management

In southwestern North America, agriculture is limited by both arable land and available water supplies. In the upper Rio San Miguel, as well as in other narrow river valleys of eastern Sonora, Mexico, floodplain farming is dependent upon living fencerows for its environmental stability. Propagated fencerows of willow and cottonwood maintain, extend, and enhance floodplain fields. These ecological filters also protect fields from cattle, harbor agents of biological control of pests, and provide renewable supplies of wood. Traditional Sonoran farmers do not perceive cottonwoods and willows as phreatophytic pests, as their Anglo-American neighbors do. The stability of the upper San Miguelagroecosystem con-trasts with severely eroded conditions within the region's other arid watersheds.     

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.     

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.     

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.     

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.     

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.     

Depressed hydraulic redistribution of roots more by stem refilling than by nocturnal transpiration for Populus euphratica Oliv. in situ measurement

During the night, plant water loss can occur either through the roots, as hydraulic redistribution (HR), or through the leaves via the stoma, as nocturnal transpiration (E_n), which was methodologically difficult to separate from stem refilling (R_e). While HR and E_n have been reported across a range of species, ecosystem, and climate zone, there is little understanding on the interactions between E_n and/or R_e and HR. As water movement at night occurs via gradients of water potential, it is expected that during periods of high atmospheric vapor pressure deficit (VPD), water loss via E_n will override water loss via HR. To test this hypothesis, sap flow in stems and roots of Populus euphratica Oliv. trees, growing in a riparian zone in a hyperarid climate, was measured once in a year. Nocturnal stem sap flow was separated into E_n and R_e using the “forecasted refilling” method. Substantial nocturnal sap flow (38% of 24‐hr flux on average) was observed and positively correlated with VPD; however, the strength of the correlation was lower (R² = .55) than diurnal sap flow (E_d) (R² = .72), suggesting that nocturnal stem sap flow was attributed to both water loss through the canopy and replenishment of water in stem tissues. Partitioning of nocturnal sap flow shows that R_e constituted approximately 80%, and E_n ~20%, of nocturnal sap flow. The amount of root sap flow attributed to redistribution was negatively related to E_d (R² = .69) and the amount of acropetally sap flow in stems, R_e (R² = .41) and E_n (R² = .14). It was suggested that the magnitude of HR is more strongly depressed by R_e that was recharge to the water loss via E_d than by E_n. It was consistent with whole‐tree water balance theory, that the nighttime upward sap flow to xylem, stem refilling and transpiration, may depress hydraulic redistribution of roots.     

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.     

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     

Factors affecting the regeneration and distribution of riparian woodlands along a northern prairie river: the Red Deer River,Alberta, Canada

Abstract. Flooding patterns and variations in the composition and successional trends of riparian vegetation in the upper and lower Red Deer River in southern Alberta, Canada, were studied in order to establish which flood regimes were most important in the regeneration and maintenance of riparian vegetation communities, with a particular focus on riparian poplars. The dominant riparian tree in the upper river is Populus balsamifera with some Picea glauca and in the lower river Populus deltoides (the plains cottonwood). Dendrochronological studies of the poplars along the river show that major periods of regeneration correspond with major flood events during the record period. Extensive cottonwood regeneration occurred in the period 1900–20, corresponding with a series of floods, some as high as the 1 in 100-year event. In addition, just prior to and during this period there was a significant reduction in use of the floodplain by bison, clearance of forests and a higher incidence of fires in upper reaches of the river and a series of high rainfall years. A series of floods greater than the 1 in 10-year flood occurred in the 1950s and stimulated the most extensive regeneration of poplars experienced since the 1920s. Parts of the lower Red Deer floodplain are now at elevations well above the 1 in 100-year flood event. It is suggested that fringe replenishment of riparian poplars is currently the dominant form of regeneration and that the large stands of mature poplars found on the floodplain, initiated during the end of the last century and first decades of this century, are unlikely to be replaced unless large floods (>1 in 50-year events) occur again. Construction of the Dickson Dam above the city of Red Deer in 1983 has led to attenuation of floods and a reduced likelihood that extensive flooding and poplar regeneration will occur again. A number of recommendations are made regarding flow management to both retain the fringe regeneration of poplars currently occurring and to stimulate more widespread regeneration of poplars on the floodplain.     

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.     

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.     

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.     

Determining factors of cottonwood planting survival in a desert river restoration project

Planting native riparian trees can help recover wildlife and fish habitat on a local scale, when full recovery of natural processes that sustain riparian ecosystems is infeasible. To help improve planting success, we determined which environmental factors and management practices most influenced survival of planted Populus fremontii (Fremont cottonwood) in a field experiment on the San Rafael River, Utah, U.S.A. We planted 474 approximately 2‐m‐tall trees and tracked survival for 1.25 years. We used logistic regression to evaluate whether tree height, elevation above the river channel, distance to existing cottonwood or Salix exigua (coyote willow), soil conductivity, soil texture, planting depth, planting method (mechanical auger vs. hand‐digging), and provision of natural and commercial supplements affected survival probability. Survival probability decreased with elevation above the river channel bottom and was greater in auger‐dug than hand‐dug holes. Survival probability was lower in soils with the highest salinity levels and was lower in sandy soils than soils with silt and clay. Survival may be improved by planting well above the channel to avoid flooding impacts but within 2 m above the channel in auger‐dug holes to ensure access to soil moisture. Testing soil salinity and texture in areas with suitable elevation could also help improve survival. Approximately 35% of trees survived to the end of the study period, indicating that planting can help recover riparian habitat locally, especially if survival is improved in future planting efforts. However, full recovery of desired riparian habitat throughout the floodplain will require natural flows.     

Knowledge-based classification of remote sensing data for the estimation of below- and above-ground organic carbon stocks in riparian forests

Floodplain forests play a crucial role in the storage of organic carbon (C_org). However, modeling of carbon stocks in these dynamic ecosystems remains inherently difficult. Here, we present the spatial estimation of C_org stocks in riparian woody vegetation and soils (to a depth of 1 m) in a Central European floodplain using very high spatial resolution remote sensing data and auxiliary geodata. The research area is the Danube Floodplain National Park in Austria, one of the last remaining wetlands with near-natural vegetation in Central Europe. Different vegetation types within the floodplain show distinct capacities to store C_org. We used remote sensing to distinguish the following vegetation types: meadow, reed bed and hardwood, softwood, and cottonwood forests. Spectral and knowledge-based classification was performed with object-based image analysis. Additional knowledge rules included distances to the river, object area, and slope information. Five different classification schemes based on spectral values and additional knowledge rules were compared and validated. Validation data for the classification accuracy were derived from forest inventories and topographical maps. Overall accuracy for vegetation types was higher for a combination of spectral- and knowledge-based classification than for spectral values alone. While water, reed beds and meadows were clearly detectable, it remained challenging to distinguish the different forest types. The total carbon storage of soils and vegetation was quantified using a Monte Carlo simulation for all classified vegetation types, and the spatial distribution was mapped. The average storage of the study site is 428.9 Mg C ha⁻¹. Despite certain difficulties in vegetation classification this method allows an indirect estimation of C_org stocks in Central European floodplains.     

Effects of sediment removal on nitrogen uptake by riparian plants in the higher floodplain of the Chikuma River, Japan

Riparian plants can use nitrogen (N) from soil and river water, but the use of river water N might be limited in higher floodplain environments of the Chikuma River. The purpose of this study is to reveal the relationship between N uptake by riparian plants and the floodplain topography (relative height and distance from a river channel). We examined the hypothesis that surface sediment removal from the higher floodplain increases river water N uptake by riparian plants by using a stable isotope analysis. The δ¹⁵N value of river water samples (ca. 8‰) were significantly higher than those of the soil extracts (ca. 3‰) in the study area. The δ¹⁵N value of riparian plants increased from +3.0‰ (standard deviation, SD ±2.1‰) before sediment removal to +9.6‰ (±2.1‰) after sediment removal, although there was no significant change in the δ¹⁵N value in N sources of soil and river water. The sediment removal enhanced frequency of flood disturbance, relative ground water level, and river water N uptake by riparian plants on the floodplain.     

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