Natural Disturbance Regimes Research Articles

Day Roosts of Myotis (Mammalia: Chiroptera) in an Arid Riparian Corridor in Southwestern New Mexico

Riparian corridors in the western United States harbor diverse biological communities that are threatened by reductions in available freshwater, changes to natural disturbance regimes, and anthropogenic disturbances. Limited data are available about bat roosts in riparian habitats in the southwestern United States. We examined day roosts of 3 sympatric Myotis species, the southwestern myotis (Myotis auriculus), Arizona myotis (Myotis occultus), and Yuma myotis (Myotis yumanensis), along the Mimbres River in southwestern New Mexico. We tracked 3 M. auriculus, 3 M. occultus, and 1 M. yumanensis to 16 Fremont's cottonwoods (Populus fremontii), 2 velvet ash (Fraxinus velutina), and 1 Arizona walnut (Juglans major) within the floodplain. Roost trees generally had a greater diameter and greater likelihood of exhibiting fire damage than nearby trees. Moreover, several roosts were in tree species not previously known to be used by these bat species, and bats did not roost in human-made structures in this arid riparian corridor.

Metrics and Models for Quantifying Ecological Resilience at Landscape Scales

An explicit link between the abiotic environment, the biotic components of ecosystems, and resilience to disturbance across multiple scales is needed to operationalize the concept of ecological resilience. To accomplish this, managers must be able to measure the ecological resilience of current conditions and project resilience under future scenarios of landscape change. The goal of this paper is to present metrics and describe a process for using geospatial data, landscape pattern analysis and landscape dynamic simulation modeling to evaluate ecosystem resilience at management scales. The dynamic equilibria of species abundances, community structure, and landscape patterns that are produced under a given combination of abiotic conditions, such as topography, soils, and climate, can form a foundation to define desired conditions and measure resistance and resilience. The degree of forcing required to push the system from this dynamic range is a measure of resistance, and the rate of return to the dynamic range after the perturbation is a measure of the resilience and recovery of the system. Several tools from the field of landscape ecology are useful in defining the dynamic range of an ecosystem under natural regulation and to measure the forcing required to drive departure and the rate of recovery. Simulation models provide means to quantify the expected range of species abundance, community structure, and landscape patterns under a variety of scenarios, including the natural disturbance regime, current disturbance regime, and possible future regimes under alternative management and climate scenarios. Landscape pattern analysis and multivariate trajectory analysis provide a means to quantify conditions and change vectors relative to this desired range. Together this combination of tools provides a means to define the conditions of a desired state for an ecosystem, to quantify the degree of resistance and resilience of the system to perturbation, and to measure and monitor the departure from the range of natural variability in the system dynamics.

A 15,400-year long record of vegetation, fire-regime, and climate changes from the northern Patagonian Andes

Paleoecological studies from the northern Patagonian Andes (40–44°S) have identified past changes in vegetation, fire regimes and paleoclimate since the last glaciation, including variations in strength and position of the Southern Westerly Winds (SWW). The extent to which records west and east of the Andes provide a congruent paleoclimatic history, however, has not been explored in detail in the literature. Physical and biological contrasts are evident between these regions today and are to be expected in paleoclimate reconstructions. In this context, we present pollen and charcoal records from sediment cores collected in Lago Espejo, a small closed-basin lake located in the core sector of the northern Patagonian Andes that spans uninterrupted the last ∼15,400 years. Following glacier withdrawal, the vegetation surrounding Lago Espejo features scattered Nothofagus woodlands, including relatively thermophilous rainforest trees between ∼15,400 and 14,400 cal yr BP. The disappearance of these trees and an abrupt rise in Nothofagus at ∼14,400 cal yr BP mark the establishment of closed-canopy forests during the Antarctic Cold Reversal, followed by increases in the cold-tolerant hygrophilous conifer Podocarpus nubigena during the Younger Dryas (∼12,700–11,500 cal yr BP). The Holocene vegetation consists of Nothofagus-dominated forests with modest variation in composition and structure until the present, attesting to the resilience of these forest communities to climate change and natural disturbance regimes. Rapid deforestation, anthropogenic fires and the establishment of artificial meadows with exotic herbs introduced by Europeans at ∼150 cal yr BP, triggered a rapid, large-magnitude landscape transformation unprecedented in the last 14,000 years. The timing and structure of vegetation changes revealed by the Lago Espejo record suggest that changes in the SWW were the main driver of vegetation and fire regimes in the Andes of northern Patagonia over the last 15,400 years. Comparison between multiple reconstructions from northern Patagonia reveals overall coherent vegetation and fire regime changes in the western and Andean sectors, and a spatially variable and more divergent behaviour in sites located further east. This spatial patter is akin to the present-day correlation between precipitation and SWW in this region.

Atmospheric change as a driver of change in the Canadian boreal zone1

Global anthropogenic emissions of greenhouse gases and hazardous air pollutants have produced broad yet regionally disparate changes in climatic conditions and pollutant deposition in the Canadian boreal zone (the boreal). Adapting boreal resource management to atmospheric change requires a holistic understanding and awareness of the ongoing and future responses of terrestrial and freshwater ecosystems in this vast, heterogeneous landscape. To integrate existing knowledge of and generate new insights from the broad-scale impacts of atmospheric change, we first describe historical and present trends (∼1980–2015) in temperature, precipitation, deposition of hazardous air pollutants, and atmospheric-mediated natural disturbance regimes in this region. We then examine their associations with ecosystem condition and productivity, biological diversity, soil and water, and the carbon budget. These associations vary considerably among ecozones and likely undergo further changes under the emerging risks of atmospheric change. We highlight the urgent need to establish long-term, boreal-wide monitoring for many key components of freshwater ecosystems to better understand and project the influences of atmospheric change on boreal water resources. We also formulate three divergent future scenarios of boreal ecosystems in 2050. Our scenario analysis reveals multiple undesirable changes in boreal ecosystem structure and functioning with more variable atmospheric conditions and frequent land disturbances, while continuing business-as-usual management of natural resources. It is possible, though challenging, to reduce unwanted consequences to ecosystems through management regimes focussed on socio-ecological sustainability and developing resilient infrastructure and adaptive resource-management strategies. We emphasize the need for proactive actions and improved foresight for all sectors of society to collaborate, innovate, and invest in anticipation of impending global atmospheric change, without which the boreal zone will face a dim future.

Investigation of natural resilience capacity of soil features affected by low severity ground wildfire after three years in Mediterranean forest ecosystem

Wildfires are one of the succession dynamics in the ecosystem, however forest ecosystems have natural resilience capacity to combat with natural disturbance regimes depend on local ecologic properties. This study was conducted to evaluate natural resilience capacity status of the soil’s physical, chemical and hydrological features exposed to low severity ground wildfire after three years in the Bulutoglu village, Kahramanmaras. Particle size distribution, soil reaction (pH), electrical conductivity (EC), organic matter content (OM), dispersion ratio (DR), moisture content (MC), field capacity (FC), colloid/moisture equivalent (CM), particle density (PD), bulk density (BD) and porosity ratio (PR) analysis were performed on two groups of soils (burned and unburned counterpart). The environmental sensitivity index (ESI) include soil, vegetation, climate, and management quality of the study area was determined by MEDALUS methodology. According to the results, there is no environmental sensitivity in the study area. It was determined that the negative effects of soil properties improved significantly after three years from the wildfire except for the PR and BD values. There was no statistically significant difference between the analyzed two soil samples groups. It was concluded that the difference between the bulk density and porosity ratios is not only due to the effects of the fire but also with grazing pressure, especially on the unburned area. According to the results, the burned area suffered from low severity ground wildfire has substantially been naturally rehabilitated itself within three years. Environmentally sensitivity (ESI) of the study area was a play an important role in the recovery of soil features. Additionally recommended avoiding some activities that will compress the soil for increasing natural resilience capacity after a wildfire.

Early arboreal colonization, postglacial resilience of deciduous Nothofagus forests, and the Southern Westerly Wind influence in central-east Andean Patagonia

The history and dynamics of deciduous Nothofagus forests along the eastern slopes of the central Patagonian Andes (44°-49°S) remain insufficiently studied and understood, particularly at timescales ranging from centuries to millennia. Available fossil pollen records point to time-transgressive responses of the arboreal vegetation to climatic changes during the Last Glacial Termination (T1) and early Holocene, and spatial heterogeneity since then along north-south, east-west, and elevation transects. The degree to which these results represent biogeographic and climatic trends, varying environmental gradients, or site-specific phenomena has not been assessed in detail. Here we present a fossil pollen and macroscopic charcoal record from Lago Churrasco (45°41′S, 71°49′W), a small closed-basin lake located in the deciduous Nothofagus forest zone of the central-east Andes of Chilean Patagonia. Our results suggest that Nothofagus trees colonized newly deglaciated terrains at ∼16,000 cal yr BP and formed scrublands/woodlands several millennia earlier than reported by previous studies east of the Andes. This suggests expansion and local densification of tree populations sourced from the eastern margin of the Patagonian Ice Sheet during the Last Glacial Maximum, with the additional implication that temperature and precipitation conditions favorable for tree survival and reproduction developed early during T1. We posit that the amount of moisture delivered by the Southern Westerly Winds was not a limiting factor for arboreal expansion during T1 in this sector of the central Patagonian Andes. Closed-canopy Nothofagus forests established at ∼10,000 cal yr BP and have remained essentially invariant despite climate change and natural disturbance regimes. This resilience was challenged and exceeded by human disturbance during the 20th century through the use of fire, leading to deforestation and spread of invasive exotic species in an extraordinarily rapid event. Our record suggests a permanent influence of the Southern Westerly Winds over the last 10,000 years, with relatively modest variations at centennial and millennial timescales.

Ecological effects of clearcutting practices in a boreal forest (Arkhangelsk Region, Russian Federation) both with and without FSC certification

Abstract The Forest Stewardship Council (FSC) is widely recognized as certification standard for responsible forest management across all major forest biomes. Still, only few field studies assessed if FSC certification generates measurable positive impacts in a managed forest ecosystem. Consequently, unequivocal evidence on ecological effectiveness of FSC is scarce. Especially the Russian boreal forests, where FSC has shown a rapid growth of certified areas, have hardly been studied regarding ecologically positive achievements delivered by certification compared to conventional practices. In this case study, systematic research was carried out to assess the ecological effectiveness of FSC in the boreal forests of the Arkhangelsk Region, Russian Federation. At harvested sites belonging to two companies, managed with and without FSC-certification respectively, and at a primary reference forest, 95 variables related to five indicators (living tree biomass, deadwood, rejuvenation, vegetation, and microclimate regulation) were empirically assessed. With regard to the most ecologically relevant parameters chosen for this study, there were very few substantial differences of FSC-certified forestry operations to conventional ones. This can be explained with the equally high amounts of harvested timber (ca. 97% extracted) as well as the practice of large-scale clearcutting (up to >50 ha), generating significant structural and functional ecological changes in the forest ecosystem. The findings suggest that there is no real change in forest operations towards an ecologically responsible harvesting. Rather, FSC-certified forestry contributes to the ongoing loss of primary forests in the remaining Intact Forest Landscapes, and FSC does not seem to slow down the loss of ecosystem functionality. Actually, the large-scale clearcuts do not resemble any comparable natural disturbance regime and produce risky ecological changes related to micro- and mesoclimate. Our data show substantial heating of the large clearcuts and the loss of ecosystemic buffering capacity, being highly relevant in the context of vulnerability to climate change.

Silvicultural implications from analyzing light induced height growth development of eight North American juvenile tree species in mixed-conifer forests

Abstract Modern silviculture is based on simulating natural disturbance regimes on the one side and reducing their impact on the other side. Disturbances modify light regimes of forests along complex spatial patterns, and species participating in the ecosystem tend to have unique adaptations to the multitude of light conditions found in forests. We studied the height growth reaction of saplings from eight native North American tree species to different light availabilities in a Sierra Nevada mixed conifer forest. Large height growth differences occurred between species. Height growth increased for most species with increasing light availability. However, black oak but also incense cedar and tanoak reached an optimal value or saturation point of light availability at intermediate light levels, above which their growth stayed the same or slightly declined on average. A variety of stand conditions, representing the range from early to late successional stages, are beneficial to maintain tree species diversity across stands. Such a gradient of conditions can be achieved by diversifying silvicultural regimes and using regeneration approaches that mimic a range of disturbance intensities. The results add to knowledge on the species’ ecology by specifying their height growth in dependence of a quantified light environment. The quantification of the light environment can directly be linked to % canopy cover and a range of regeneration methods, respectively. With this, the results may help achieve a more precise and target-oriented management of Sierra Nevada mixed conifer forests.

Disturbance Ecology in the Anthropocene

With the accumulating evidence of changing disturbance regimes becoming increasingly obvious, there is potential for disturbance ecology to become the most valuable lens through which climate-related disturbance events are interpreted. In this paper, I revisit some of the central themes of disturbance ecology and argue that the knowledge established in the field of disturbance ecology continues to be relevant to ecosystem management, even with rapid changes to disturbance regimes and changing disturbance types in local ecosystems. Disturbance ecology has been tremendously successful over the past several decades at elucidating the interactions between disturbances, biodiversity, and ecosystems, and this knowledge can be leveraged in different contexts. Primarily, management in changing and uncertain conditions should be focused primarily on the long-term persistence of that native biodiversity which has evolved within the local disturbance regime, and is likely to go extinct with rapid changes to disturbance intensity, frequency, and type. Where possible, conserving aspects of natural disturbance regimes will be vital to preserving functioning ecosystems and to that native biodiversity that requires disturbance for its continued existence, though these situations may become more limited over time. Finally, scientists must actively propose management policies that incorporates knowledge of disturbance ecology. Successful policies regarding changing disturbance regimes for biodiversity will not merely be reactive, and will recognize that for natural ecosystems as for human society, not all desired outcomes are simultaneously possible.

Soil erosion as a resilience drain in disturbed tropical forests

Tropical forests are threatened by intensifying natural and anthropogenic disturbance regimes. Disturbances reduce tree cover and leave the organic topsoil vulnerable to erosion processes, but when resources are still abundant forests usually recover. Across the tropics, variation in rainfall erosivity – a measure of potential soil exposure to water erosion – indicates that soils in the wetter regions would experience high erosion rates if they were not protected by tree cover. However, twenty-first-century global land cover data reveal that in wet South America tropical tree cover is decreasing and bare soil area is increasing. Here we address the role of soil erosion in a positive feedback mechanism that may persistently alter the functioning of disturbed tropical forests. Based on an extensive literature review, we propose a conceptual model in which soil erosion reinforces disturbance effects on tropical forests, reducing their resilience with time and increasing their likelihood of being trapped in an alternative vegetation state that is persistently vulnerable to erosion. We present supporting field evidence from two distinct forests in central Amazonia that have been repeatedly disturbed. Overall, the strength of the erosion feedback depends on disturbance types and regimes, as well as on local environmental conditions, such as topography, flooding, and soil fertility. As disturbances intensify in tropical landscapes, we argue that the erosion feedback may help to explain why certain forests persist in a degraded state and often undergo critical functional shifts.

Conserving bees in destroyed landscapes: The potentials of reclaimed sand mines

Sand mines represent anthropogenically impacted habitats found worldwide, which bear potential for bee conservation. Although floral resources can be limited at these habitats, vegetation free patches of open sandy soils and embankments may offer good nesting possibilities for sand restricted and other bees. We compared bee communities as found in three reclaimed sand mines and at adjacent roadside meadows in Maryland, USA, over two years. Both sand mines and roadsides hosted diverse bee communities with 111 and 88 bee species, respectively. Bee abundances as well as richness and Shannon diversity of bee species were higher in sand mines than at roadsides and negatively correlated with the percentage of vegetational ground cover. Species composition also differed significantly between habitats. Sand mines hosted a higher proportion of ground nesters, more uncommon and more ‘sand loving’ bees similar to natural sandy areas of Maryland. Despite the destruction of the original pre-mining habitat, sand mines thus appear to represent a unique habitat for wild bees, particularly when natural vegetation and open sand spots are encouraged. Considering habitat loss, the lack of natural disturbance regimes, and ongoing declines of wild bees, sand mines could add promising opportunities for bee conservation which has hitherto mainly focused on agricultural and urban habitats.

Fire and life history affect the distribution of plant species in a biodiversity hotspot

AbstractAimSpecies distribution models (SDMs) provide valuable insights into species–environment relationships and potential climate change impacts on diversity. Most SDMs do not account for the role of natural disturbance regimes such as fire in determining current and future species distributions, or how species traits mediate their response to these stressors. Here, we investigate the importance of fire in determining the distributions of species in fire‐prone fynbos vegetation, and how this varies in relation to different life history traits (growth form and fire‐response strategy).LocationCape Floristic Region, South Africa.MethodsWe modelled the distribution of 104 plant species with different life history traits, using Maxent. The model included five climatic variables, one edaphic and one fire variable. Post hoc analyses of model output and permutation procedures were conducted to assess variable importance across different life history traits. We accounted for phylogenetic autocorrelation using sister species comparisons.ResultsPermutation importance scores identified fire return interval as a major determinant of fynbos species’ distributions. Linear mixed effect analyses revealed that seeder species were significantly more sensitive to fire than resprouters. Coefficients from the (linear) response curves of the different predictors indicated that the occurrence of species across all life histories was negatively associated with longer fire return intervals.Main conclusionsFire and life history traits governing species’ response to fire are key factors determining species distributions in our study system. SDMs that ignore the role of fire in driving species distributions, and how this varies across different life history types, compromise our ability to understand species–environment relationships in fire‐prone ecosystems. There is great need for better spatial data describing historical, current and future fire regimes and for models that can incorporate different responses based on species life histories, to improve vulnerability assessments for fire‐prone ecosystems.

Impact of Disturbances on the Carbon Cycle of Forest Ecosystems in Ukrainian Polissya

Climate change continues to threaten forests and their ecosystem services while substantially altering natural disturbance regimes. Land cover changes and consequent management entail discrepancies in carbon sequestration provided by forest ecosystems and its accounting. Currently there is a lack of sufficient and harmonized data for Ukraine that can be used for the robust and spatially explicit assessment of forest provisioning and regulation of ecosystem services. In the frame of this research, we established an experimental polygon (area 45 km2) in Northern Ukraine aiming at estimating main forest carbon stocks and fluxes and determining the impact caused by natural disturbances and harvest for the study period of 2010–2015. Coupled field inventory and remote sensing data (RapidEye image for 2010 and SPOT 6 image for 2015) were used. Land cover classification and estimation of biomass and carbon pools were carried out using Random Forest and k-Nearest Neighbors (k-NN) method, respectively. Remote sensing data indicates a ca. 16% increase of carbon stock, while ground-based computations have shown only a ca. 1% increase. Net carbon fluxes for the study period are relatively even: 5.4 Gg C·year−1 and 5.6 Gg C C·year−1 for field and remote sensing data, respectively. Stand-replacing wildfires, as well as insect outbreaks and wind damage followed by salvage logging, and timber harvest have caused 21% of carbon emissions among all C sources within the experimental polygon during the study period. Hence, remote sensing data and non-parametric methods coupled with field data can serve as reliable tools for the precise estimation of forest carbon cycles on a regional spatial scale. However, featured land cover changes lead to unexpected biases in consistent assessment of forest biophysical parameters, while current management practices neglect natural forest dynamics and amplify negative impact of disturbances on ecosystem services.

Long-term tree-ring derived carbon dynamics of an experimental plantation in relation to species and density in Northwestern Ontario, Canada

Afforestation can result in significant accumulations of carbon (C) in both soils and vegetation, which can make an important contribution to GHG emission mitigation. However, there remains a critical knowledge gap about ecosystem C stocks and fluxes at afforested sites, which is required in order to understand the effect of management choices like species selection and initial planting density. A hybrid biometric modelling approach was used to reconstruct ecosystem C stocks and fluxes at an experimental spacing trial consisting of three species (red pine, white spruce, and black spruce) planted at three densities, 63 years after it was established on former agricultural land in Northwestern Ontario, Canada. Wood volume and increment were larger for closer spacing and for red pine relative to black or white spruce, and translated to analogous differences in estimates of ecosystem C dynamics. At young ages, net ecosystem production was largest at close spacing, but this difference attenuated over time and was similar in recent decades regardless of initial spacing. Estimated carbon stock and flux values were generally congruent with available estimates from independent studies at analogous sites, with the exception of litterfall which showed greater differences. Plots at the site were estimated to be C sinks on an annual basis immediately after establishment under the assumption that the site was previously under long-term agricultural land use, but anywhere from 10 to 20 years following establishment if the site was modelled as if it was a stand in the managed forest historically subject to natural disturbance regimes.

Predictors of habitat use and nesting success for two sympatric species of boreal woodpeckers in an unburned, managed forest landscape

Forest harvesting reduces mean stand age and dead wood amount. Among species dependent upon older forests and dead wood, primary excavators such as woodpeckers play an important ecological role by providing cavities for many other species. Conservation of these species thus requires striking a balance between timber harvesting and dead wood supply, especially under moderate natural disturbance regimes. In this study, we quantified habitat use in two closely-related, sympatric species: Black-backed (Picoides arcticus) and American Three-toed (P. dorsalis) Woodpecker, in the highlands of northern New Brunswick, Canada. Natural fires are rare in this ecoregion and forest harvesting is the main agent of landscape change, along with infrequent outbreaks of spruce budworm (Choristoneura fumiferana). Based on its habitat use patterns in other ecoregions, we hypothesized that Black-backed Woodpecker would select nest sites in mature coniferous forest stands and we expected that it would achieve a higher nest survival rate in forest stands than within, or at the edge of, cutblocks. Owing to sample size limitations for American Three-toed Woodpecker, statistical analyses were restricted to its occurrence patterns. Both species co-occurred, and there was no difference in detectability following conspecific and heterospecific playbacks. Contrary to our prediction, Black-backed Woodpecker selected nest sites with higher proportions of recently cut areas within 50 m than at randomly selected sites. Black-backed Woodpecker selected cutblocks for nesting when suitable snags were available and mature forest stands were present nearby as foraging habitat. For the species to persist in this landscape, the supply of snags must be continuous and forest stands must be sufficiently mature to host saproxylic insects.

Herbaceous Vegetation Responses to Gap Size within Natural Disturbance-Based Silvicultural Systems in Northeastern Minnesota, USA

The use of silvicultural systems that emulate aspects of natural disturbance regimes, including natural disturbance severities and scales, has been advocated as a strategy for restoring and conserving forest biodiversity in forests managed for wood products. Nonetheless, key information gaps remain regarding the impacts of these approaches on a wide range of taxa, including understory plant species. We investigated the 6- or 7-year response of herbaceous vegetation to natural disturbance-based silvicultural harvest gaps in a northern hardwood forest in Northeastern Minnesota. These results indicate that harvest gaps are effective in conserving understory plant diversity by promoting conditions necessary for disturbance-dependent understory plant species. However, harvest gaps also contained non-native invasive plant species.

A spatio-temporal, landscape perspective on Acacia dealbata invasions and broader land use and cover changes in the northern Eastern Cape, South Africa.

Biological invasions and human land use both have the potential to drastically alter the patterns and processes of landscapes, driving habitat fragmentation and altering natural disturbance regimes. The proliferation of an invasive species depends on composition and configuration of the landscape, as well as the invasiveness of the species. To effectively manage a highly invasive species, such as Acacia dealbata, it is crucial to understand the historical progression of the invasion within the landscape. This study sought to examine the landscape dynamics of biological invasions by tracking the historical spread of A. dealbata and broader land use/land cover (LULC) changes at different spatio-temporal scales in the northern Eastern Cape. A time-series of aerial photographs were systematically classified according to designated A. dealbata and LULC categories in ArcGIS to track the changes in the extent and rate of spread of A. dealbata. Markedly dynamic, multi-directional, and spatio-temporally variable LULC transitions were observed across the northern Eastern Cape over the last six decades. A. dealbata frequently retained a high proportion of cover over time, and despite the loss of cover to other LULC classes, a net increase in A. dealbata cover occurred as it spread at an overall annual rate of 0.11-0.21%, occupying approximately 8-18% of land cover across all sampled sites by 2013. Any management interventions to limit or control A. dealbata should therefore consider the spatio-temporal and LULC nuances of landscapes.

Distinguishing disturbance from perturbations in fire-prone ecosystems

Fire is a necessary ecosystem process in many biomes and is best viewed as a natural disturbance that is beneficial to ecosystem functioning. However, increasingly, we are seeing human interference in fire regimes that alters the historical range of variability for most fire parameters and results in vegetation shifts. Such perturbations can affect all fire regime parameters. Here, we provide a brief overview of examples where anthropogenically driven changes in fire frequency, fire pattern, fuels consumed and fire intensity constitute perturbations that greatly disrupt natural disturbance cycles and put ecosystems on a different trajectory resulting in type conversion. These changes are not due to fire per se but rather anthropogenic perturbations in the natural disturbance regime.

Barriers to natural regeneration in temperate forests across the USA

For millennia, natural disturbance regimes, including anthropogenic fire and hunting practices, have led to forest regeneration patterns that created a diversity of forest lands across the USA. But dramatic changes in climates, invasive species, and human population, and land use have created novel disturbance regimes that are causing challenges to securing desired natural regeneration. Climate is an ever-present background disturbance and determinant of species distribution. Changes in certain other factors such as large herbivore populations, wildfire, and pests modify forest composition and structure, and are common barriers to natural regeneration of desired species. Changes in long-standing disturbance regimes have led to the homogenization of forest landscape composition and structure. Today, forests have low regeneration potential and are low in resilience. They have reduced productivity and are prone to widespread health issues including severe forest mortality. In addition to epidemics of native invasive species due to climate change and availability of habitat at landscape scales, the continued introduction and spread of non-native pests and diseases are causing large-scale forest mortality. These ecological changes have cascading ecological consequences such as increases in severe wildfire, which pose new barriers to natural regeneration. Equally challenging are the barriers to natural regeneration that arise from social, political and economic factors. To address many of these issues requires active management that links all critical stages in the regeneration niche necessary for achieving desired regeneration to sustain forest development and production in a socially acceptable manner and economically viable market system.

How maintenance and restoration measures mediate the response of riparian plant functional composition to environmental gradients on channel margins: Insights from a highly degraded large river

Riparian habitats are transitional zones where strong environmental gradients shape community. To prevent flood risks and channel migration on managed rivers, civil engineering techniques have been widely used. Recently, ecological restoration of rivers has become a major issue. However, given the alteration of natural disturbance regimes induced by human infrastructures, the real added-value of these restoration actions is questionable. Thus, a major challenge is to better understand whether changes in abiotic conditions induced by human activity influence the response of plant communities to environmental gradients. Studying a highly degraded large river, we evaluated the effect of the elevation and soil texture gradients on plant functional composition and assessed whether human-mediated environment gradients, achieved through maintenance and restoration measures, shape community structure. In the summer of 2017, we sampled 17 geomorphic surfaces, mostly gravel bars, along the Rhône River and its tributaries that were either repeatedly cleared (brush clearing vs plowing), newly reprofiled or naturally rejuvenated by high flows. The results show shifts in trait values with elevation and convergence in plant traits with increasing proportion of fine sediments. The co-occurrence of species with contrasting traits was higher in highly disturbed environments, revealing the importance of rejuvenation processes. However, the influence of both environmental gradients was mediated by human activity. For maintenance measures, plowing was better able to promote species diversity and limited biotic homogenization along environmental gradients. Among the three geomorphic surfaces, naturally rejuvenated bars were the most stressful environments, hosting distinct functional assemblages, while communities on newly reprofiled banks were in the same ecological trajectories as repeatedly cleared bars. To promote an effective ecological restoration of riparian zones, (i) a greater variability of the minimum flow is needed, (ii) bedload transport restoration should be a priority and (iii) reprofiled banks should better mimic the landforms of natural river margins.