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In semi-arid ecoregions, trees are restricted to river valley floodplains where river water supplements the limited precipitation. To characterize the associated diurnal and seasonal dynamics in hydrology and water relations, we studied narrowleaf cottonwoods (Populus angustifolia) along a prairie river in Canada. From June through August, the shallow soil moisture was depleted but moisture remained higher above the alluvial groundwater table, which dropped to 2.3 m along with river recession. Throughout the summer, with the daily rise in temperature and insolation, foliar stomatal conductance (g s) and transpiration (E) increased to midday and then fell, thus maintaining the midday leaf water potential (Ψmd) above ~−1.7 MPa. This Ψmd approximated the water potential associated with 12% loss of xylem conductivity due to cavitation for branches (P 12); the Ψmd and P 12 varied independently across eight trees, providing differences in relative hydraulic risk. Sap flux density (F d) was measured with thermal dissipation probes, and revealed diurnal patterns similar to foliar E. In contrast to our expectation, the daily F d maxima were consistent through the summer despite the seasonal recession in water supply. Canopy conductances (G S), derived from F d, sapwood area and canopy area, declined with vapor pressure deficit (D) and fell slightly in late summer along with stomatal sensitivity, which reflects the magnitude of decrease in G S with increasing D. For spatial up-scaling, satellite-derived near-infrared reflectance of vegetation revealed the woodland phenology, with leaf expansion from May through June and gradual decline in photosynthetic productivity through the summer. Thus, the phreatophytic cottonwoods: (i) sustained substantial water use and productivity through the warm and dry summer, by (ii) accessing shallow soil moisture and then deeper alluvial groundwater, and (iii) providing diurnal stomatal regulation, to (iv) avoid xylem cavitation and (v) maintain fairly constant hydraulic conductance. [ABSTRACT FROM AUTHOR]

In dry ecoregions, trees are restricted to river valley floodplains where river water supplements the limited local precipitation. Around the Northern Hemisphere, cottonwoods, riparian poplars, are often predominant trees in floodplain forests and these ecological specialists require floods that create and saturate sand and gravel bars, enabling seedling recruitment. By pairing the interpretation of aerial photographs at approximately decade intervals with dendrochronology, we explored the coordination between river floods, geomorphic disturbance and colonization of plains cottonwoods (Populus deltoides) over eight meanders along the Red Deer River in the semi‐arid prairie of western Canada. This river has a relatively natural flow regime and minimal human alteration through the World Heritage Site of Dinosaur Provincial Park. We found that the 50‐year flood of 1954 increased channel migration and produced extensive accretion with downstream expansion of meander lobes and some channel infilling, which was followed by prolific cottonwood colonization. Those processes accompanied the major flood, while bank erosion and cottonwood losses were more gradual and continuous over the past half‐century. Results indicated even greater floodplain and woodland development after an earlier 100‐year flood in 1915. Each flood produced an arcuate band of mature cottonwoods and there were five to seven progressively older woodland bands across the floodplain, with each cottonwood age grouping increasing by about a half‐century. The 700 m wide floodplain was progressively reworked by the river through pulses of channel movement and floodplain and woodland development over approximately 250 years and correspondingly, the oldest cottonwoods were about 250 years old. [ABSTRACT FROM AUTHOR]

The Little Bow River (LBR) in western Canada naturally displayed an intermittent flow regime; the channel dried up most summers, excluding the development of riparian woodlands within this semi‐arid ecoregion. Around 1900, the Little Bow Canal was excavated to divert water from the adjacent Highwood River and with flow augmentation the LBR flow became continuous through the growth season. We hypothesized that the continuous regime enabled riparian woodland establishment and assessed conditions with sequential aerial photographs and field observations. Supporting the hypothesis, a few woodland groves established near the Highwood River where balsam poplars (Populus balsamifera) provided an abundant seed source. To investigate the basis for woodland development, we analysed historical hydrology of the LBR and assessed the four larger woodland groves, which included mature poplars, trembling aspen (P. tremuloides) and willow shrubs (Salix bebbiana and S. exigua). Each location had some bank excavation with channelization or gravel mining, and tree ageing through ring counts indicated gradual colonization and pulses of establishment after floods in 1920 and 1942. Thus, the conversion from an intermittent to continuous flow regime enabled woodland development which also benefited from excavations that created barren colonization sites. The study revealed four requirements for riparian woodland colonization in a dry region: (a) seeds, (b) barren sites, (c) bank saturation with higher river flow, and (d) sufficient river flows for tree and shrub survival and growth. While water withdrawal commonly degrades riverine ecosystems, flow augmentation can provide the opposite outcome, enhancing the river and riparian environments. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Prior appropriation (allocation), ‘first-in-time is first-in-right’, provides a dominant water policy in western North America and some other regions world-wide. Concerns with this chronological prioritization arose during the Canadian Oldman River Dam project, since anticipated water for environmental flows and an indigenous group would have been unreliable with junior licenses. It was considered that in Alberta prior allocation applies only to the natural flow and water captured in a reservoir during surplus is not later bound by license seniority. Consequently, the Oldman Dam operation ensures minimal environmental flows and thereafter, ‘sharing the shortage’ is applied, with licensees receiving partial allotments during shortages. A related test-case for adjacent rivers with publicly-funded reservoirs accompanied a water shortage in 2001. It was considered unacceptable to cut-off water to a food-processing plant, a town, and water co-operatives with recent licenses, and consequently irrigation districts and others agreed to share the shortage, with all receiving 60% allotments. Following success, water sharing was also approved for 2002 and 2007. The discretionary use of stored water and sharing the shortage represent ‘equitable allocation’, an alternative to prior appropriation, which may benefit other jurisdictions facing water shortages. [ABSTRACT FROM AUTHOR]

Poplars are trees of the angiosperm genus Populus, a group of generally diploid, dioecious, and deciduous pioneers that are widespread in forests around the Northern Hemisphere. Canada is a centre of poplar abundance and diversity, with 7 of the 29 global Populus species occurring naturally, and other species being introduced. Numerous interspecific hybrids also occur, including native hybrids that create centres of biodiversity, and artificial hybrid poplars that are planted for wood and fiber production, as well as for environmental and landscape applications. This paper introduces poplars and their distribution across Canada and explores Canadian and international contributions to poplar research. It also discusses papers of the special issue Poplar Research in Canada in the November 2007 and December 2007 issues of the Canadian Journal of Botany, which contains a collection of 19 contributions from Canadian institutions that report on a diverse range of topics including genomics and molecular biology, physiology, ecology, population genetics, and systematics. Many of these contributions address issues pertinent to silviculture and other applications, and one article chronicles poplar research and poplar utilization in Canada. These studies that appear in this special issue reflect the growing international emphasis on poplar, not only as an important and widespread group of trees, but also as a model that provides broader insights relevant to fundamental and applied sciences involving trees and forest systems. [ABSTRACT FROM AUTHOR]

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. 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. 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. 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. 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. [ABSTRACT FROM AUTHOR]

Major river floods provide powerful physical disturbances that create and expand gravel bars and islands. Their barren surfaces are suitable for seedling colonization by plants including poplars (cottonwoods) and willows that grow to contribute rich wildlife habitats and other ecosystem services. Conversely, in locations such as through bridges, riparian woodlands would impede future flows, elevating flood stages and increasing damage. It is consequently useful to understand where opportunity versus hazard locations occur and this encourages monitoring and predictive modeling of vegetation colonization. Colonization is related to the river inundation patterns but hydrodynamic modeling is difficult for complex channels and irregular features. Here we present an alternative empirical approach based on mapping of actual inundation patterns from a sequence of aerial photographs at different river flows. Based on field inventory of seedling distributions of different plants, we estimated inundation thresholds for six progressively lower riparian cover types: woodland, shrubland, perennial herbaceous, ruderal annuals, transition and barren. These were coordinated with the inundation patterns, enabling predictive mapping of the different vegetation classes to produce 'camo-maps'. These resemble camouflage with the irregular shapes and classification colors matching the vegetation types. The derived camo-maps were then compared with a lidar-based mapping of topography and vegetation structure. As an illustrative case study, we present the method for a large and complex gravel bar upstream from a bridge sequence and an expanded island below another bridge in Calgary Canada, where a hundred-year flood in June 2013 caused severe damage. The camo-maps reasonably represented the actual colonization patterns, although low flow years enabled some vegetation survival at lower positions than predicted. We present this as an effective and efficient empirical method to provide an alternative or complement to riparian vegetation projections based on hydrodynamic modeling. • River floods create gravel bars and islands that are colonized by trees and other plants • New riparian woodlands provide rich environments but can impede future flood flows • We introduce a method to analyze and project the locations of new riparian woodlands • This coordinates seedling recruitment with inundation patterns from aerial photographs • This enables predictive mapping of new riparian woodlands and other vegetation types [ABSTRACT FROM AUTHOR]

A growing need for sampling environmental spaces in high detail is driving the rapid development of non-destructive three-dimensional (3D) sensing technologies. LiDAR sensors, capable of precise 3D measurement at various scales from indoor to landscape, still lack affordable and portable products for broad-scale and multi-temporal monitoring. This study aims to configure a compact and low-cost 3D fusion scanning system (FSS) with a multi-segment Leddar (light emitting diode detection and ranging, LeddarTech), a monocular camera, and rotational robotics to recover hemispherical, colored point clouds. This includes an entire framework of calibration and fusion algorithms utilizing Leddar depth measurements and image parallax information. The FSS was applied to scan a cottonwood ( Populus spp.) stand repeatedly during autumnal leaf drop. Results show that the calibration error based on bundle adjustment is between 1 and 3 pixels. The FSS scans exhibit a similar canopy volume profile to the benchmarking terrestrial laser scans, with an r 2 between 0.5 and 0.7 in varying stages of leaf cover. The 3D point distribution information from FSS also provides a valuable correction factor for the leaf area index (LAI) estimation. The consistency of corrected LAI measurement demonstrates the practical value of deploying FSS for canopy foliage monitoring.

Several native poplar species meet at the margins of their natural distributions in southern Alberta, Canada. In this semi-arid area, poplars are obligate riparian species but they occupy several intergrading ecoregions. Populus deltoides Bartr. ex Marsh predominates in the warmest and driest eastern prairie ecoregions; P. balsamifera L. occupies the cooler and wetter western parkland and montane ecoregions; and P. angustifolia James and hybrids between the species occur in the intermediate grassland ecoregions. We investigated stomatal characteristics of these poplars in 51 genotypes collected across the range of ecoregions and grown in a semi-arid common garden. Stomatal length differed among genotypes within species but did not differ among species, ranging from 19 to 22 microm. Total stomatal densities (adaxial plus abaxial) differed among genotypes within species but were similar among species (290-420 stomata mm(-2)). Single-surface stomatal densities differed among species and consequently, the ratio of adaxial:abaxial stomatal density also differed, ranging from 0.94 for P. deltoides to 0.27 for P. balsamifera, with intermediate stomatal density ratios in P. angustifolia and hybrids. In a subsequent study of a subset of the same genotypes, stomatal density was correlated with stomatal conductance (r2 = 0.75) and the conductance ratios differed among species in the same manner as the stomatal density ratios. We conclude that: (1) diverse poplar genotypes respond similarly to a semi-arid environment by producing comparatively small and dense stomata; (2) differences in stomatal density underlie differences in stomatal conductance and differences among species in stomatal density ratio or conductance ratio may reflect adaptation to climatic differences among ecoregions; and (3) there is substantial variation in stomatal characteristics within and among species and hybrids in this area that could be useful for the selection and breeding of poplars adapted to different climatic conditions.