Riparian Forest Structure Research Articles

The role of functional and phylogenetic diversity in riparian tree vegetation on leaf litter breakdown in rivers

While how taxonomic diversity mediates changes in ecosystem function is well‐studied, how deeper dimensions of biodiversity drive important processes is understudied. The overarching goal of this work was to determine the role of these dimensions of biodiversity independently and/or interactively in explaining carbon processing in rivers. Here, we explicitly link community structure and subsequent traits of riparian forests to adjacent ecosystem processing of carbon (e.g. leaf litter). This was accomplished by examining how forests are actually structured in addition to experimental manipulations of phylogenetic and functional diversities of riparian forest community inputs of leaf litter to streams. Experimental field manipulations were carried out in three Piedmont headwater streams to answer the following questions: 1) does existing variation in taxonomic, functional and phylogenetic diversity of riparian communities differentially drive decomposition in rivers? And 2) independent of taxonomic diversity, how does functional and phylogenetic diversity of leaf litter assemblages influence rates of decomposition in rivers? We observed significant interspecific variation in breakdown among 30 riparian tree species, in addition to significant relationships between breakdown rate and important foliar tissue chemistries. Breakdown of mixtures that reflected the composition of the riparian species composition did not vary with functional nor phylogenetic diversity, but breakdown of litter mixtures was higher than that of single species. In a separate study, when manipulated independently, functional and phylogenetic diversity were positively related to breakdown, and explained similar degrees of variation. These results are important to understand in light of deepening knowledge of the role different dimensions of biodiversity take in explaining ecosystem function.

Floristic composition, structure and diversity of riparian forests in southwestern Nigeria: conservation is inevitable

The Nigerian riparian forest ecosystems had declined in extent and distribution and this had been attributed mainly to land use change. This study intended to provide understanding links between plant diversity, composition, structures and disturbances both anthropogenic and natural processes inducing the vegetation dynamics. Nine study sites were used for this study, within each site, five (5) plots (0.25 ha in size) were marked out, placed systematically at an interval of 10 m along transect. Complete enumeration of plants species was carried out and identified to species level. Diversity indices and structural parameters were determined and anthropogenic activities were ranked. A total number of 233 plant species were identified, belonging to 80 families; out of which, Euphorbiaceae and Apocynaceae were dominant families. The density and basal area ranged from 2200 to 6000 ha−1 and 2.59 to 17.58 m2 ha−1 respectively across the study sites. Pterocarpus santalinoides, Alchornea cordiflora, Chassalia kolly, Tetracera spp., Fimbristylis, Bambusa vulgaris and Cyrtosperma senegalense were the dominant species. The Shannon diversity index ranged from (1.38–3.49), Simpson (0.66–0.97) and Evenness diversity was (0.43–0.84). Fisher alpha (10.03–30.21) and Whittaker beta diversity (0.36–0.89) values were highest in Ipetumodu (site VIII) and lowest in Ilesha (site II). Seventy‐three (73%) of the species in this study had low important value index (IVI). The dominance of some lianas and herbaceous species in the riparian forest sites showed disturbances, stages of ecological succession and regeneration of the vegetation. Conservation is inevitable towards maintaining and protection of species diversity, ecosystem roles and services of these forests in Nigeria.

SOIL CHEMICAL PROPERTIES AND PHYTODIVERSITY OF RIPARIAN FOREST LAND NEAR NANWAN LAKE

The riparian zone of reservoir is a spatially fluctuating ecotone (between terrestrial and aquatic ecosystems) and is an important area for nutrients’ circulation and materials’ flow. Riparian forest plays an important role in the stability of riparian habitat. As yet, the relationship between soil chemical properties and biodiversity of riparian forest near reservoir has not been thoroughly elaborated. In this study, we analyzed the soil chemical properties (total nitrogen and carbon, available phosphorous and kalium) and community structure characteristics of trees (diameter at breast height, tree height, canopy width), shrubs and herbaceous (Shannon diversity index, Simpson dominance index, species richness index, Pielou uniformity index) in the riparian forest land at the tail section that is a part of Nanwan lake reservoir in China. Results showed that the structure of riparian forest near reservoir of Nanwan lake represents a stable community. There was no significant difference in soil chemical properties and vegetation biodiversity between the type of centre’s island and the type of peninsula. The range of the nutrients’ content in forest land (nеar Nanwan lake reservoir), respectively, is: total carbon (TC) – 7,8–19,5 g/kg, total nitrogen (TN) – 0,72–1,49 g/kg, available phosphorous (AP) – 1,89–3,83 mg/kg, available kalium (AK) – 48,0–100,5 mg/kg. The soil pH value of the riparian forest land near Nanwan Lake reservoir is low due to strong acid reaction, so the toxic effects of aluminum should be considered. In the RDA analysis, the first axis is explained 73,16 % of the biodiversity factors of trees, shrubs and herbaceous, and the second axis – 6,48 %. The reflection of the presence of shrub-herbaceous layer in the values of the chemical properties of soil is significant: this layer is an important source of soil organic matter in the coastal zone and has a positive effect on soil quality. Therefore, attention should be paid to maintaining the stability of community structure in understory shrub-herbaceous layer.

Propriedades físicas da camada superficial do solo em florestas ripária no Brasil central: resistência à infiltração e penetração

ABSTRACT Plant composition, diversity and structure of riparian forests of Central Brazil are well known. However, little is known about soil physical properties under these forests. This knowledge is important as a baseline for biodiversity restoration and ecosystem services that occur in riparian zones. In order to bridge this gap, here we assessed the infiltration capacity and soil penetration resistance in a plinthic soil under gallery forest in Planaltina, Distrito Federal, Brazil. We measured infiltration capacity (Mini-Disk infiltrometer) and soil penetration resistance (Stolf penetrometer) following linear transects. The plinthic soil had high infiltration capacity and low penetration resistance. Our infiltration estimate is in the middle range when compared to other permeability studies in tropical forests. Like their counterparts, high biological activity along with the lack of disturbance are the likely explanations for such high topsoil permeability to water.

A dynamic riparian forest structure model for predicting large wood inputs to meandering rivers

AbstractFluvial processes strongly influence riparian forests through rapid and predictable shifts in dominant species, tree density and size that occur in the decades following large floods. Modelling riparian forest characteristics based on the age and evolution of floodplains is useful in predicting ecosystem functions that depend on the size and density of trees, including large wood delivered to river channels, forest biomass and habitat quality. We developed a dynamic model of riparian forest structure that predicts changes in tree size and density using floodplain age derived from air photos and historical maps. Using field data and a riparian forest chronosequence for the 160‐km middle reach of the Sacramento River (California, USA), we fit Weibull diameter distributions with time‐varying parameters to the empirical data. Species were stratified into early and late successional groups, each with time‐varying functions of tree density and diameter distributions. From these, we modelled how the number and size of trees in a stand changed throughout forest succession, and evaluated the goodness‐of‐fit of model predictions.Model outputs for the early successional group, composed primarily of cottonwoods and willows, accounted for most of the stand basal area and large trees >10 cm DBH for the first 50 years. Post‐pioneer species with slower growth had initially low densities that increased slowly from the time of floodplain creation. Within the first 100 years, early successional trees contributed the most large wood that could influence fluvial processes, carbon storage, and instream habitat. We applied the model to evaluate the potential large wood inputs to the middle Sacramento River under a range of historical bank migration rates. Going forward, this modelling approach can be used to predict how riparian forest structure and other ecosystem benefits such as carbon sequestration and habitat quality respond to different river management and restoration actions.

AN ECOLOGICAL ASSESSMENT OF THE STATE OF SMALL RIVERS IN THE MIDDLE TAIGA SUBZONE OF KARELIA

A multidisciplinary survey of three small rivers emptying to Lake Onego was carried out; information was collected regarding the aquatic biota, the species and age structure of riparian forests, their floristic diversity, and soils. The integral indicator chosen to assess the ecological condition of the watercourses was the characteristics of the rivers’ fish fauna, in particular fish of the Salmonidae family. Surveys of the rivers and riparian forest communities yielded records of 181 vascular plant species (11 of them macrophytes), 11 moss species, 2 lichen species; the fish fauna was made up of 9 species. Overall, the riparian forests featured a relatively wide range of typological diversity. The tree stands varied from the poor Ledum types (site class V) to the most productive Oxalis types (site classes III–I). Analysis of the tree species and age structure of forests in the riparian buffers revealed a prevalence of stable coniferous tree stands in good condition. Statistically reliable correlations were detected between the fish fauna and the tree species composition and inventory characteristics of the riparian forest stands, soil morphological characteristics, and the species diversity of the living ground cover and macrophytes. The species diversity of the riparian and the aquatic biotas were found to correlate positively with each other. Fish numbers were seen to negatively correlate with some characteristics of riparian forest soils (forest floor thickness, podzolic horizon), while the correlation with the humus horizon thickness was positive. Significant positive correlations were identified between aquatic organisms (fish, zoobenthos) and a majority of the stream’s hydrological parameters. The resultant up-to-date information about water-forest system components is important for understanding the role of small streams, which, alongside the scant officially recognized “spawning” rivers, are an ecological niche in the life cycle of salmonid fishes in Northwest Russia.

Soil Bacterial Community Responds to Land-Use Change in Riparian Ecosystems

Riparian forests were frequently cleared and converted to agricultural pastures, but in recent times these pastures are often revegetated in an effort to return riparian forest structure and function. We tested if there is a change in the soil bacterial taxonomy and function in areas of riparian forest cleared for agricultural pasture then revegetated, and if soil bacterial taxonomy and function is related to vegetation and soil physicochemical properties. The study was conducted in six riparian areas in south-eastern Australia, each comprising of three land-use types: remnant riparian forest, cleared forest converted to pasture, and revegetated pastures. We surveyed three strata of vegetation and sampled surface soil and subsoil to characterize physicochemical properties. Taxonomic and functional composition of soil bacterial communities were assessed using 16S rRNA gene sequences and community level physiological profiles, respectively. Few soil physiochemical properties differed with land use despite distinct vegetation in pasture relative to remnant and revegetated areas. Overall bacterial taxonomic and functional composition of remnant forest and revegetated soils were distinct from pasture soil. Land-use differences were not consistent for all bacterial phyla, as Acidobacteria were more abundant in remnant soils; conversely, Actinobacteria were more abundant in pasture soils. Overall, bacterial metabolic activity and soil carbon and nitrogen content decreased with soil depth, while bacterial metabolic diversity and evenness increased with soil depth. Soil bacterial taxonomic composition was related to soil texture and soil fertility, but functional composition was only related to soil texture. Our results suggest that the conversion of riparian forests to pasture is associated with significant changes in the soil bacterial community, and that revegetation contributes to reversing such changes. Nevertheless, the observed changes in bacterial community composition (taxonomic and functional) were not directly related to changes in vegetation but were more closely related to soil attributes.

Agricultural land-use change alters the structure and diversity of Amazon riparian forests

Riparian forests play key roles in protecting biodiversity and water resources, making them priorities for conservation in human-dominated landscapes, but fragmentation associated with expanding tropical croplands threatens their ecological integrity. We compared the structure of tropical riparian forests within intact and cropland catchments in a region of intensive soybean production in the southeastern Brazilian Amazon. We studied forest plots (varying from 120 to 210 m long) that bisected riparian zone forests and headwater streams in ten catchments. Four plots were within large areas of intact primary forest and six were in bands of protected riparian forest along streams within croplands as required by the Brazilian Forest Code. We found that riparian forests in croplands harbored fewer species of trees and seedlings/saplings, and had higher proportions of opportunistic, pioneer tree species. We also found greater variation in tree species composition, and higher internal dissimilarity in croplands compared with forests. The observed patterns in tree species composition were driven mainly by differences between riparian forest-cropland edges and those bordering intact upland forests. Forests nearest to streams in cropland and forested catchments were more similar to one another. Results suggest that wider buffers are needed at the edges of croplands to maintain riparian forest structure. The minimum 30-m riparian buffers now required by the Brazilian Forest Code may thus be insufficient to prevent long-term shifts in riparian forest species composition and structure.

Using Vegetation Guilds to Predict Bird Habitat Characteristics in Riparian Areas

Within arid regions riparian forests support high bird diversity compared to surrounding uplands. In these same regions, water demands for agriculture, urbanization, and recreation have altered the structure and composition of riparian forests and degraded bird habitat. Along rivers, plants with similar responses to flood disturbance and water availability can be grouped into functional guilds using traits. The use of plant guilds can mechanistically link bird distributions to traits such as canopy height, specific leaf area, and growth form. We demonstrate that bird species richness, abundance, and diversity are related to the heterogeneity of vegetation structure and plant guilds along a perennial river in the southwestern U.S. High-quality habitat can be explained by canopy cover, foliage height diversity, and foliage cover in the understory. The tall tree guild (dominated by Salix gooddingii) was the strongest predictor of bird habitat followed by drought tolerant shrubs (dominated by Prosopis velutina) demonstrating that riparian shrublands should be considered for conservation alongside gallery forests. Projected changes in flow regimes may result in homogenization of riparian vegetation and reduce habitat quality for migratory and breeding land birds. Practitioners can use developed methods to group vegetation by guilds to focus bird conservation efforts in arid ecosystems.

Lateral and longitudinal distribution of riparian vegetation along an ephemeral river in Namibia using remote sensing techniques

Ephemeral rivers are key features of desert ecosystems, providing shallow groundwater and occasional surface flow, but compared to perennial systems, the structure of riparian forests in ephemeral rivers and their interactions with the hydrologic regime are poorly understood. This study examines the distribution of riparian woody species in the ephemeral Kuiseb River in central Namibia. Unmanned aerial vehicle (UAV) and satellite imagery were used to examine the structure of the riparian forest across lateral and longitudinal gradients. Woody plant abundance peaked in the terminal alluvial zone, the reach of the river where floods typically run dry. The five dominant species exhibited similar spatial patterns at latitudinal and longitudinal scales. Faidherbia albida and Salvadora persica were found more proximal to the active channel and concentrated in the terminal alluvial zone. Acacia (Vachellia) erioloba, Euclea pseudebenus, and Tamarix usneoides were more evenly dispersed laterally across the riparian zone and along the river's longitudinal profile. These spatial patterns correspond to plant physiological properties of hydrological dependence and gradients of water availability and soil composition. The tight coupling of plant distributions with water availability highlights the need for long-term monitoring of ephemeral riparian vegetation in light of potential future changes to the global hydrologic regime.

Soil and Nutrient Cycling Responses in Riparian Forests to the Loss of Ash (Fraxinus spp. L) from Emerald Ash Borer (Agrilus planipennis, Fairmaire)

Emerald ash borer (EAB) is an alien invasive species that is spreading across Canada and the United States killing ash trees. In riparian forests where ash may be abundant; loss of ash can induce significant structural changes; including the creation of canopy gaps; changes in light penetration; expansion of ground vegetation; and alteration of soil nitrogen and carbon cycling. In 2014 and 2015, we examined the effects of EAB-caused gaps in riparian forests on soil nutrient dynamics. Two sites with different infestation timelines, a “new” site (mortality in past 2–3 years) and an “old” site (infested 10 years previous) were selected to determine temporal differences in effects of canopy gaps created by ash loss on litterfall, herbaceous ground vegetation, and soil nutrient cycling. Within both sites, plots with clustered dead ash (canopy gap plots—CG) were paired with nearby plots of full canopy and no ash (canopy closed plots—CC), and differences between paired plots determined. Total litterfall was observed at all sites but was only significant at the new infestation site. Reductions in leaf litter deposition in CG plots resulted in reduced N and C flux to the forest floor but soil C and N concentrations, and nitrogen mineralization rates, were not significantly different between CG and CC plots. Nitrate concentration in soil solution was significantly greater in CG plots compared to CC plots at the new infestation sites but showed the opposite trend at the old infestation sites. Herbaceous ground vegetation biomass was significantly greater (up to 10×) in CG plots than in CC plots. Overall, despite changes to riparian forest canopy structure and litterfall, there was no significant difference in soil nutrient cycling between EAB-induced canopy gaps and closed canopy plots after 10 years, suggesting a high resilience of riparian forest soils to EAB infestation

Divergence of riparian forest composition and functional traits from natural succession along a degraded river with multiple stressor legacies

Prolonged exposure to human induced-stressors can profoundly modify the natural trajectory of ecosystems. Predicting how ecosystems respond under stress requires understanding how physical and biological properties of degraded systems parallel or deviate over time from those of near-natural systems. Utilizing comprehensive forest inventory datasets, we used a paired chronosequence modelling approach to test the effects of long-term channelization and flow regulation of a large river on changes in abiotic conditions and related riparian forest attributes across a range of successional phases. By comparing ecological trajectories between the highly degraded Rhône and the relatively unmodified Drôme rivers, we demonstrated a rapid, strong and likely irreversible divergence in forest succession between the two rivers. The vast majority of metrics measuring life history traits, stand structure, and community composition varied with stand age but diverged significantly between rivers, concurrent with large differences in hydrologic and geomorphic trajectories. Channelization and flow regulation induced a more rapid terrestrialization of the river channel margins along the Rhône River and accelerated change in stand attributes, from pioneer-dominated stands to a mature successional phase dominated by non-native species. Relative to the Drôme, dispersion of trait values was higher in young forest stands along the Rhône, indicating a rapid assembly of functionally different species and an accelerated transition to post-pioneer communities. This study demonstrated that human modifications to the hydro-geomorphic regime have induced acute and sustained changes in environmental conditions, therefore altering the structure and composition of riparian forests. The speed, strength and persistence of the changes suggest that the Rhône River floodplain forests have strongly diverged from natural systems under persistent multiple stressors during the past two centuries. These results reinforce the importance of considering historical changes in environmental conditions to determine ecological trajectories in riparian ecosystems, as has been shown for old fields and other successional contexts.

Drivers of plant species composition in alder-dominated forests with contrasting connectivity

The effects of local and regional environmental variables as well as spatial gradients on the plant species composition of two types of alder-dominated forests (riparian forests and alder carrs) with contrasting connectivity were studied across the Western Carpathians from Hungary through Slovakia to Poland. We used large vegetation (240 sampling plots) and environmental (24 variables) datasets, which were accompanied by spatial variables represented by principal coordinates of neighbour matrices. Canonical correspondence analysis (CCA) of the two datasets revealed 13 and 29 variables with significant effects on variation in species composition of alder carrs and riparian alder forests, which jointly explained 41.2% and 36.4% of the variability, respectively. Altitude was the most important factor explaining 7.7% of the variability in the species composition of alder carrs and 8.2% in riparian alder forests. Variation partitioning in CCA revealed that local variables were crucial drivers for species composition patterns in alder carrs, while spatial processes unrelated to the measured environmental variables shaped the vegetation structure of riparian forests.

Legacy effects of emerald ash borer on riparian forest vegetation and structure

High levels of ash (Fraxinus spp.) mortality have been reported in areas of North America following invasion of emerald ash borer (EAB) (Agrilus planipennis Fairmaire) (Coleoptera: Buprestidae), but little is known about the impacts of EAB in riparian forests, particularly along first order streams. We delineated canopy gaps resulting from EAB-caused ash mortality along four first order streams in each of three distinct watersheds, which represented a temporal gradient of the EAB invasion across southern Michigan. Time lapsed aerial imagery showed canopy gaps were apparent by 2005, 2010, and 2012 in southeast, central and southwest sites, respectively. We assessed overstory trees, regeneration, shrubs, herbaceous plants, coarse woody debris, and photosynthetically active radiation in each canopy gap and in forests surrounding the gap. Canopy gaps were originally dominated by green ash (F. pennsylvanica) and black ash (F. nigra) but more than 85% of ash basal area was killed by EAB across all sites. Live basal area was greater in forests than in gaps, but was similar among all three watersheds. A lower percentage of dead ash trees remained standing and volume of ash coarse woody debris was higher in southeastern sites, with the longest history of EAB invasion, than in central and southwestern sites. Species richness of overstory trees was higher in forests (34 species) than in gaps (27 species). Photosynthetically active radiation, measured 1.5 m above ground, was 55% higher in canopy gaps than in adjacent forests during summer, but was similar during winter. Species composition of pre-EAB forest overstories and current saplings were similar among canopy gaps; overall, 66.3% of saplings were ash. Sapling species in forested areas adjacent to gaps were often dominated by Acer rubrum, while Quercus rubra, Prunus serotina, Acer saccharum and A. rubrum were common in overstories. Seedlings were rare within gaps now dominated by dense mats of sedges (Carex spp.). Although ash has been functionally lost from riparian forest overstories in most of southern Michigan, consistently high sapling densities, combined with reduced EAB densities, could facilitate ash persistence in post-invasion areas. If EAB densities increase, however, recruitment of ash saplings into the overstory will be limited. Continued monitoring will be important to assess the future status of ash and overstory dynamics in of post-EAB riparian forests.

Modelling riparian forest distribution and composition to entire river networks

AbstractAim: Developing a methodology to map the distribution of riparian forests to entire river networks and determining the main environmental factors controlling their spatial patterns.Location: Cantabrian region, northern Spain.Methods: We mapped the riparian forests at a physiognomic and phytosociological level by delimiting riparian zones and generating vegetation distribution models based on remote sensing data (Landsat 8 OLI and LiDAR PNOA). We built virtual watersheds to define a spatial framework where the catchment environmental information can be specified for each river reach, in combination with the vegetation map. In order to determine the drivers that play a significant role in the observed spatial patterns in riparian forests, based on our data sets we modelled interactions between environmental information and riparian vegetation by using the Random Forest algorithm.Results: The modelling results obtained reliably reproduced the variation of riparian forest structure and composition across Cantabrian watersheds. The produced maps were highly accurate, with a more than 70% overall accuracy for forest occurrence. A clear differentiation between Eurosiberian (habitats 91E0 and 9160) and Mediterranean (92E0) riparian forests was shown on both sides of the mountain range. Topography and land use were the main drivers defining the distribution of riparian forest as a physiognomic unit. In turn, altitude, climate and percentage of pasture were the most relevant factors determining their composition (phytosociological approach).Conclusions: Our study confirms that anthropic control ultimately defines the distribution of vegetation in the riparian area at a regional to local scale. Human disturbances constrain the extension of forest patches across their potential distribution defined by topoclimatic boundaries, which establish a clear limit between Mediterranean and Eurosiberian biogeographical regions.

Riparian and adjacent forests differ both in the humid mountainous ecoregion and the semiarid lowland

Few studies have quantified changes in riparian and adjacent forest across landscape units. In this study, the composition and structure of riparian and adjacent forest were compared in a humid and a semiarid ecoregion in northwestern Argentina: the Yungas forest and the Western Chaco. We expected that differences between riparian and adjacent zones could be less marked in humid than in semiarid regions. Ten sites were surveyed with a block design. An Importance Value Index, Rank-Abundance curves, and Analysis of Similarity and multivariate analyzes (NMDS) were performed to evaluate differences between forests. Stream and floodplain widths, lateral, and longitudinal slopes of streamside were analyzed by a principal components analysis (PCA). NMDS and PCA axes were correlated to analyze the relations among physical and biological arrangements. Results revealed that riparian forest may be very different from the adjacent in both ecoregions. Marked differences in geomorphological and physical features of streamsides were found between ecoregions and they were strongly associated with assemblage distribution. In Yungas forest, dominant species were different at all sites, according to the altitudinal stratification of this region. Within Western Chaco the species Salix humboldtiana Willd. and Tessaria integrifolia Ruiz and Pav., were commonly dominant in riparian sectors. The dominance of these species in both sectors by the widest rivers could indicate that the dimensions of the riparian zone in those sites are greater than those by the smaller streams. Our study reinforced the concept of riparian zones as dynamic ecosystems and we propose considering a landscape perspective in managerial decision making.

Lack of forest tree seedling recruitment and enhanced tree and shrub growth characterizes post-Tsuga canadensis mortality forests in the southern Appalachians

The loss of Tsuga canadensis from invasion by hemlock woolly adelgid (Adelges tsugae, HWA) has altered ecosystem structure and function in forests across the eastern United States. In southern Appalachian forests, T. canadensis co-occurred with hardwood species and an evergreen Rhododendron maximum shrub layer in riparian and cove positions. In this region, HWA infestation was detected in 2003, with mortality reaching 97% by 2014. In this study we examined responses of light, soil moisture, tree seedling density, and overstory and understory vegetation growth from 2004 to 2014 following HWA infestation and mortality of T. canadensis. We hypothesized that seedling recruitment and vegetation growth would continue to increase over time as observed with initial trends reported through 2009, and that species that associate with ectomycorrhizal (ECM) fungi may benefit more than those that do not due to the pulse of organic material in these stands from dead T. canadensis trees. Light transmission measured at 1 m above the ground increased from 2006 to 2009, but gradually decreased from 2015 to 2017. Basal area of overstory non-Tsuga trees increased only marginally, and there was no recruitment of tree seedlings to the overstory size class, even though seedling density of deciduous species increased initially. Increased basal area and stem density of R. maximum may explain the light and seedling responses, as this species can inhibit tree seedling recruitment by limiting light and nutrients. Overstory species with the highest basal area increment (BAI) in the post-T. canadensis stands were Pinus rigida, Betula lenta and Quercus coccinea, which all associate with ECM fungi. However, not all ECM tree species grew significantly more following T. canadensis mortality compared to pre-mortality growth rates—only those ECM species that had high growth rates prior to mortality did. After a decade, growth of both overstory trees and R. maximum has not compensated for the loss of T. canadensis. Active management of R. maximum, which may involve the removal of the evergreen shrub and soil organic layer, may be required to allow for diverse tree seedling recruitment; and subsequently, restore riparian forest structure, diversity, and function.

What Matters Most: Are Future Stream Temperatures More Sensitive to Changing Air Temperatures, Discharge, or Riparian Vegetation?

AbstractSimulations of stream temperatures showed a wide range of future thermal regimes under a warming climate — from 2.9°C warmer to 7.6°C cooler than current conditions — depending primarily on shade from riparian vegetation. We used the stream temperature model, Heat Source, to analyze a 37‐km study segment of the upper Middle Fork John Day River, located in northeast Oregon, USA. We developed alternative future scenarios based on downscaled projections from climate change models and the composition and structure of native riparian forests. We examined 36 scenarios combining future changes in air temperature (ΔTair = 0°C, +2°C, and +4°C), stream discharge (ΔQ = −30%, 0%, and +30%), and riparian vegetation (post‐wildfire with 7% shade, current vegetation with 19% shade, a young‐open forest with 34% shade, and a mature riparian forest with 79% effective shade). Shade from riparian vegetation had the largest influence on stream temperatures, changing the seven‐day average daily maximum temperature (7DADM) from +1°C to −7°C. In comparison, the 7DADM increased by 1.4°C with a 4°C increase in air temperature and by 0.7°C with a 30% change in discharge. Many streams throughout the interior western United States have been altered in ways that have substantially reduced shade. The effect of restoring shade could result in future stream temperatures that are colder than today, even under a warmer climate with substantially lower late‐summer streamflow.

Effects of a large flood on woody vegetation along the regulated Missouri River, USA

AbstractIn 2011, a large, long‐duration flood occurred on the regulated Missouri River following six decades without flooding. This study evaluated the effects of the flood on riparian forest structure and composition. In 2012, 168 forest sites sampled in 2006–2009 were resampled on five floodplain segments between Montana and Missouri, with 80 sampled again in 2013–2014. Changes in riparian forest area over 2006–2012, by age class and segment, were assessed using aerial imagery and GIS. Repeated‐measures analysis of variance was used to examine (a) the initial effects of the flood (preflood to 2012) on tree and shrub stem densities, (b) postflood (2012–2014) changes in stem density, and (c) species‐level responses for cottonwood (Populus deltoides), eastern red cedar (Juniperus virginiana), and Russian olive (Elaeagnus angustifolia). Across the study area, forest area declined 6–38% among age classes, with the greatest declines in the youngest classes. Tree density declined 19–49% across segments from preflood to 2012 but did not change significantly from 2012 to 2014. Shrub density declined 52–89% across segments from preflood to 2012, with 73–78% declines in the two youngest age classes, but increased by 42% from 2012 to 2014. Cottonwood and Russian olive, but not red cedar, showed partial recovery in the shrub/sapling layer, increasing from 2012 to 2014. Although flooding is important for floodplain forest health, the 2011 flood had mixed effects, with significant mortality of native floodplain trees and shrubs and only limited cottonwood recruitment. The initial decline in invasive species (Russian olive and red cedar), however, suggests that flooding may be an effective management tool.

Limited resilience in hotspots of functional richness: the Mediterranean riparian shrublands

Functional diversity indices are increasingly being used to describe plant community assembly processes and ecosystem functioning. However, their relevance for predicting alterations in ecosystem functioning of riparian plant communities is still largely unknown. We investigated the functional patterns of riparian forests along environmental gradients, using biological and environmental data from 189 well-preserved riverine locations in mainland Portugal. We calculated functional diversity indices (e.g. Richness, Redundancy, Divergence) for four riparian forest types, namely the Alder woodlands, Ash woodlands, Tree-heath shrublands and Mediterranean shrublands, using their plant species composition and 25 plant functional traits. We used multiple linear regression to predict the effect of the environment in the functional structure of riparian forests and ultimately evaluate the resilience of the riparian forests to environmental fluctuations. We found that Mediterranean shrublands have a significantly higher Functional Richness and a lower Functional Redundancy in comparison to the other riparian forest types. Both regional and habitat variables were important for predicting the functional diversity of riparian forests, with varying patterns according to forest types. In particular, we found that the redundancy of Mediterranean shrublands is mostly affected by precipitation, suggesting their potential vulnerability to climate change in the study area. Our results suggest the usefulness of functional diversity measures for conservation and monitoring the ecological functioning of riparian forests.