Post-disturbance Periods Research Articles

Quantifying hurricane impacts on United States Virgin Islands reef fishes using a catchability invariant approach to compare uncalibrated survey indices

The United States Virgin Island's (USVI) coral reefs support many economically and ecologically important fish species. Located in the Caribbean Sea, they are subject to frequent severe weather disturbances, including two category 5 hurricanes, Irma and Maria, in 2017. The overarching goal of this study was to identify reef fish community impacts following these extreme events using discontinuous survey indices. Long-term (2001–2021), in situ, fishery-independent survey data using two methods were standardized to the extent possible in two regions of the USVI, St. Thomas/St. John (STT/STJ) and St. Croix (STX). Comparable data were assessed, 73 species collected on hard- bottom habitat, to identify the number of these species with significant changes in density and/or mean length between 2-yr survey intervals over the historic baseline (2001–2015), disturbance (2017–2019), and postdisturbance (2019–2021) periods. The results varied by region: STT/STJ had no disturbance impact and STX had a significant disturbance impact. In STX, 20 species had significant changes in density in the disturbance period compared to an average of 9.7 (SD 3.8) species for the baseline period. The proportion of species with significant density increases and decreases were similar suggesting that overall disturbance impacts are nuanced. Mean length observations were less informative, likely due to survey method and sample size changes. However, in combination with density they provided useful insights into the possible causes of population change. The successful use of discontinuous survey indices to obtain meaningful biological insights has broader applications to ecosystem and fishery datasets with similar limitations.

Carbon dynamics of Western North American boreal forests in response to stand-replacing disturbances

North American boreal forests are known to be an important carbon pool in boreal ecosystems, but have experienced extensive tree mortality and carbon loss due to multiple agents of stand-replacing disturbances in recent decades. However, the impacts of these stand-replacing disturbances on forest dynamics are still unknown. We used a recently developed remote-sensing based stand-replacing disturbance product, coupled with aboveground biomass (AGB), gross primary productivity (GPP) and leaf area index (LAI) datasets to estimate the impacts of stand-replacing disturbances (e.g., fires, logging and insect outbreaks) on the carbon balance of western North American boreal forests during 2000–2012. Our results showed that fire, logging and insect outbreaks resulted in AGB losses of 23.4, 16.6, and 4.7 Tg/yr, respectively. In the post-disturbance periods, AGB did not recover to its pre-disturbed levels in the 10th year, which is longer than the recovery time of GPP and LAI. Furthermore, the losses of AGB, GPP and LAI in fire events were the dominant factors for forest recovery after stand-replacing fire. Vapor Pressure Deficit (VPD), soil clay content, temperature and precipitation were the important factors for forest recovery after stand-replacing insect outbreaks and stand-replacing logging. When removing the impact of environmental factors, our results showed a smaller magnitude of AGB, GPP and LAI loss relative to the results including these factors, although similar recovery trajectories were observed among the two results. The results have important implications for understanding the effects of stand-replacing disturbances on the carbon dynamics of boreal forests, which is required to adopt effective forest management strategies after disturbance.

Impact of Channel Dredging and Straightening in an Atlantic White Cedar (Chamaecyparis thyoides L. (B.S.P.)) Freshwater Tidal Wetland

Stotts, S.; Callahan, J., and Gulledge, O., 2021. Impact of channel dredging and straightening in an Atlantic white cedar (Chamaecyparis thyoides L. (B.S.P.)) freshwater tidal wetland. Journal of Coastal Research, 37(5), 973–986. Coconut Creek (Florida), ISSN 0749-0208. Once prominent up the eastern coast of the United States, freshwater wetlands with Atlantic white cedar (AWC; Chamaecyparis thyoides L. (B.S.P.)) are now vanishing due to saltwater intrusion and anthropogenic landscape alterations. This study utilizes a historic, anthropogenic disturbance to provide insight into the AWC response to salinization. A major straightening and dredging project in the 1920s, including 16 oxbow cuts, allowed brackish water to move upstream in the St. Jones River in Dover, Delaware, leading to the die-off of an AWC swamp. The sampled AWC experienced climate change, storms, hydrologic alterations, and sea-level rise. Annual ring width and suppression event chronologies were developed from dead AWC and were compared to groupings of climate variables in the predisturbance (1895–1920) and postdisturbance (1928–1958) periods. Results of the dendrochronological analysis indicate that the AWC in this study died at an average age of 49.3 years, earlier than typically expected for such species. After the straightening and dredging, the AWC experienced changes in growth response to temperature while suppression events became positively correlated with elevated temperature (p = 0.024) and high precipitation (p = 0.019). The system also showed signs of becoming more sensitive to storm events in the postdisturbance period.

Carbon Balance and Streamflow at a Small Catchment Scale 10 Years after the Severe Natural Disturbance in the Tatra Mts, Slovakia

Natural disturbances (windthrow, bark beetle, and fire) have reduced forest cover in the Tatra National Park (Slovakia) by 50% since the year 2004. We analyzed carbon fluxes and streamflow ten years after the forest destruction in three small catchments which differ in size, land cover, disturbance type and post-disturbance management. Point-wise CO2 fluxes were estimated by chamber methods for vegetation-dominated land-use types and extrapolated over the catchments using the site-specific regressions with environmental variables. Streamflow characteristics in the pre- and post-disturbance periods (water years of 1965–2004 and 2005–2014, respectively) were compared to identify changes in hydrological cycle initiated by the disturbances. Mature Norway spruce forest which was carbon neutral, turned to carbon source (330 ± 98 gC m−2 y−1) just one year after the wind disturbance. After ten years most of the windthrow sites acted as carbon sinks (from −341 ± 92.1 up to −463 ± 178 gC m−2 y−1). In contrast, forest stands strongly infested by bark beetles regenerated much slowly and on average emitted 495 ± 176 gC m−2 year−1. Ten years after the forest destruction, annual carbon balance in studied catchments was almost neutral in the least disturbed catchment. Carbon uptake notably exceeded its release in the most severely disturbed catchment (by windthrow and fire), where net ecosystem exchange (NEE) was −206 ± 115 gC m−2. The amount of sequestered carbon in studied catchments was driven by the extent of fast-growing successional vegetation cover (represented by the leaf area index LAI) rather than by the disturbance or vegetation types. Different post-disturbance management has not influenced the carbon balance yet. Streamflow characteristics did not indicate significant changes in the hydrological cycle. However, greater cumulative decadal runoff, different median monthly flows and low flows and the greater number of flow reversals in the in the first years after the windthrow in two severely affected catchments could be partially related to the influence of the disturbances.

Using paired catchments to quantify the human influence on hydrological droughts

Abstract. Quantifying the influence of human activities, such as reservoir building, water abstraction, and land use change, on hydrology is crucial for sustainable future water management, especially during drought. Model-based methods are very time-consuming to set up and require a good understanding of human processes and time series of water abstraction, land use change, and water infrastructure and management, which often are not available. Therefore, observation-based methods are being developed that give an indication of the direction and magnitude of the human influence on hydrological drought based on limited data. We suggest adding to those methods a “paired-catchment” approach, based on the classic hydrology approach that was developed in the 1920s for assessing the impact of land cover treatment on water quantity and quality. When applying the paired-catchment approach to long-term pre-existing human influences trying to detect an influence on extreme events such as droughts, a good catchment selection is crucial. The disturbed catchment needs to be paired with a catchment that is similar in all aspects except for the human activity under study, in that way isolating the effect of that specific activity. In this paper, we present a framework for selecting suitable paired catchments for the study of the human influence on hydrological drought. Essential elements in this framework are the availability of qualitative information on the human activity under study (type, timing, and magnitude), and the similarity of climate, geology, and other human influences between the catchments. We show the application of the framework on two contrasting case studies, one impacted by groundwater abstraction and one with a water transfer from another region. Applying the paired-catchment approach showed how the groundwater abstraction aggravated streamflow drought by more than 200 % for some metrics (total drought duration and total drought deficit) and the water transfer alleviated droughts with 25 % to 80 %, dependent on the metric. Benefits of the paired-catchment approach are that climate variability between pre- and post-disturbance periods does not have to be considered as the same time periods are used for analysis, and that it avoids assumptions considered when partly or fully relying on simulation modelling. Limitations of the approach are that finding a suitable catchment pair can be very challenging, often no pre-disturbance records are available to establish the natural difference between the catchments, and long time series of hydrological data are needed to robustly detect the effect of the human activities on hydrological drought. We suggest that the approach can be used for a first estimate of the human influence on hydrological drought, to steer campaigns to collect more data, and to complement and improve other existing methods (e.g. model-based or large-sample approaches).

Understanding 48 years of changes in tree diversity, dynamics and species responses since logging disturbance in a subtropical rainforest

Identifying long-term impacts of logging disturbance on rainforests is crucial for biodiversity conservation. In this study, we evaluated changes in tree diversity and dynamics as well as species responses over 48years in a subtropical rainforest subject to intensive logging in north-eastern New South Wales (NSW) Australia. Some areas of this forest were intensively logged (≥70% of basal area removal) as part of silvicultural practices, in conjunction with other activities such as herbicide application and enrichment planting. Measurements between 1966 and 2014 of trees≥10cmdbh were undertaken in permanent plots in undisturbed and disturbed (logged) areas. In undisturbed areas, absence of logging and occurrence of minimal natural perturbations induced stable tree diversity and dynamics during the 48-year period. In disturbed areas, tree diversity, dynamics and species responses differed profoundly over time, and from undisturbed areas. Distinct trends were grouped in three post-disturbance periods. A first period in which tree removal and damage during logging operations induced constraining conditions that reduced richness, diversity, dominance, tree density, basal area and recruitment. A second period was characterised by a decline of tree density and basal area after post-logging mortality, but they both recovered in subsequent years. During this second period, richness and diversity increased and species responses such as colonisation, adaptation, facilitation, and inter- and intraspecific competition were exhibited. Finally, there was a third period in which species abundance (shade-tolerant and –intolerant species) increased, and richness and tree density surpassed levels in undisturbed areas. This diversity recovery was induced by species colonisation (from different functional groups), species re-appearance and interspecific competition. We found disturbance from intensive logging was the underlying mechanism for changes in tree diversity and the stimulus of numerous species interactions and responses at population and community levels. In disturbed areas, 48years of species responses promoted partial recovery of richness, species abundance and tree density. However, stand basal area was still lower than in undisturbed areas, and species dominance and floristic composition differ from 1966 conditions. In conclusion, intensive logging impacts still persist in disturbed areas, and more species responses are likely to occur during ongoing recovery.

Taxonomic- and trait-based recolonization dynamics of a riverine fish assemblage following a large-scale human-mediated disturbance: the red mud disaster in Hungary

We examined the recovery of a fish assemblage following the catastrophic spill of highly alkaline (pH ~13) red sludge into a lowland river (i) to characterize taxonomic- and trait-based colonization of fishes in the river in the first 3 years of the recovery and (ii) to determine which structural- and trait-based variables best predicted colonization. Species richness showed comparable values to pre-disturbance state <1 year after the spill. We found only moderate changes in the dominance of the most abundant species between pre- and post-disturbance periods, and consistent changes in the relative abundance of some rare species during the post-disturbance period. Frequency of occurrence (%) of the fishes in the watershed and their relative abundance (%) proved to be the most important predictor variables in colonization, whereas trait-based variables had a less important role. Our study about one of the largest scale and most serious documented fish kill shows that both taxonomic- and trait-based structure of fish assemblages can regenerate remarkably fast in a modified river and also shows that unfortunate chemical spills provide insights into the assembly of stream fish assemblages.

Progressive recovery of a tropical deforested stream community after a flash flood

AIM: In this study, we evaluated and compared community attributes from a tropical deforested stream, located in a pasture area, in a period before (PRED I) and three times after (POSD I, II, and III) a flash flood, in order to investigate the existence of temporal modifications in community structure that suggests return to conditions previous to the flash flood. METHODS: Biota samples included algae, macrophytes, macroinvertebrates, and fish assemblages. Changes in stream physical structure we also evaluated. Similarity of the aquatic biota between pre and post-disturbance periods was examined by exploratory ordination, known as Non-Metric Multidimensional Scaling associated with Cluster Analysis, using quantitative and presence/absence Bray-Curtis similarity coefficients. Presence and absence data were used for multivariate correlation analysis (Relate Analysis) in order to investigate taxonomic composition similarity of biota between pre and post-disturbance periods. RESULTS: Our results evidenced channel simplification and an expressive decrease in richness and abundance of all taxa right after the flood, followed by subsequent increases of these parameters in the next three samples, indicating trends towards stream community recovery. Bray-Curtis similarity coefficients evidenced a greater community structure disparity among the period right after the flood and the subsequent ones. Multivariate correlation analysis evidenced a greater correlation between macroinvertebrates and algae/macrophytes, demonstrating the narrow relation between their recolonization dynamics. CONCLUSIONS: Despite overall community structure tended to return to previous conditions, recolonization after the flood was much slower than that reported in literature. Finally, the remarkably high flood impact along with the slow recolonization could be a result of the historical presence of anthropic impacts in the region, such as siltation, riparian forest complete depletion, and habitat simplification, which magnified the effects of a natural disturbance.

Landscape-scale disturbance and protected areas: long-term dynamics of populations of the shrimp, Macrobrachium olfersi in lowland Neotropical streams, Costa Rica

Abstract:Freshwater migratory shrimps, an important component of tropical aquatic ecosystems, are vulnerable to land-use change during their upstream and downstream migrations. At La Selva Biological Station in the Sarapiquí region of Costa Rica, shrimp population data were collected between 1988 and 1989, before massive land-use change occurred downstream that could potentially affect shrimp recruitment upstream. Using generalized linear models and a Bayesian inference framework, the relative abundance of Macrobrachium olfersi between recent (2008–2011) and historical time periods (1988–1989) was compared in three stream reaches. Shrimp relative abundance in two stream reaches within the protected area of La Selva was relatively constant yearly and between recent post-disturbance (2008–2011) and historical pre-disturbance (1988–89) time periods. In contrast, a stream reach bordered by pasture accessible to fishermen, showed an 87% decrease in relative abundance between recent and historical time periods suggesting site-level disturbance, possibly from fishing. The lack of change between historical and contemporary sampling periods within interior-forest stream reaches suggests that shrimp populations in protected forested reaches are resistant or resilient to certain land-use changes occurring downstream.

An energetic perspective on the relationship between disturbance and ecosystem productivity

Previous ecological models for disturbance from energetic perspectives have focused on destructive pulses by which storages in a system are quickly drained during disturbance events and recovered thereafter. However, considering the wide range of disturbance intensities, frequencies, and durations in nature, disturbance effects on ecosystem energetics would be better understood by diversifying the disturbance effects on specific system configurations or energy pathways. Based on two hypotheses, we built simulation models of the variable disturbance–productivity relationships observed in a freshwater aquatic microcosm study. First, we hypothesized that disturbances will differentially alter the intrinsic rates of energy pathways in a system. Second, we hypothesized that there is a disturbance threshold where response of the intrinsic rates changes abruptly. Simulation results showed variable patterns of gross primary productivity (GPP) during the disturbance and post-disturbance periods under the diverse scenarios of disturbance effects on the intrinsic rates. Simulation results confirmed that the second hypothesis (i.e., disturbance threshold) was essential to achieve a U-shaped or peaked disturbance–productivity relationship. We evaluated the models by comparing them with the results of the microcosm tests, and suggested possible mechanisms of the variable disturbance–productivity relationships by varying parameters related to the disturbance effects on the intrinsic rates and the disturbance thresholds.

Density and habitat associations of Henslow's Sparrows wintering in saline soil barrens in southern Arkansas

Although the habitat requirements of breeding populations of Henslow's Sparrow (Ammodramus henslowii) have been examined, less is known about their habitat requirements and ecology during the nonbreeding season. We estimated population densities and quantified habitat associations of Henslow's Sparrows wintering in saline soil barrens in southern Arkansas. Densities of Henslow's Sparrows in the saline soil barrens were similar to those in the Longleaf Pine (Pinus palustris) Ecosystem of the southeastern United States, considered by many to be their primary wintering habitat. Henslow's Sparrows were closely associated with open areas with greater cover of Aristida spp. and globe beaksedge (Rhynchospora globularis), greater stem density at 11-20 cm above ground, more lichens, more herbaceous cover, more bare ground, greater occurrence of little bluestem (Schizacyrium scoparium )a s the tallest vegetation, less moss, and less shrub cover than randomly selected sites. In contrast to the results of studies conducted in the Longleaf Pine Ecosystem, the presence of Henslow's Sparrows in our study was not correlated with the height of the tallest vegetation. Our results indicate that saline soil barrens of southern Arkansas support a high density of wintering Henslow's Sparrows and do so for longer postdisturbance periods than longleaf pine savanna. We also found that stem density near the ground was similar to that reported from longleaf pine savanna, but only about half that observed on their breeding grounds. Areas used by Henslow's Sparrows had more lichen and less moss cover, suggesting that those areas were drier than random sites within the barrens. Further research is needed to determine if large populations of Henslow's Sparrows winter in other saline soil barrens and if fire influences habitat associations and densities in the barrens.

Vegetation response to disturbance in a coastal marsh in Texas

Disturbance is considered one of the main factors influencing plant species composition and diversity. We conducted a field study to address the plant community response in a coastal marsh to a major disturbance. In 1992, muskrats (Ondatra zibethicus L.) completely removed vegetation within a 450 ha area of intermediate coastal marsh in Texas, USA. We used vegetation data collected prior to the disturbance (1989-1991) as a baseline for comparison to that recorded annually for a decade (1992-2002) following the disturbance. We compared species diversity, richness, relative abundance, evenness, and species similarity between pre and postdisturbance periods to evaluate the temporal response of the disturbed plant community. Plant diversity in the study area returned to predisturbance levels after 10 years. Species diversity in the study area had two peaks following the main disturbance. These peaks are associated with fluctuations of the water levels in the area. Our results suggest that it is possible to control the sedge or grass dominance in a community by subjecting the area to a carefully timed willful disturbance (e.g., grazing or fluctuating water levels) to achieve management goals. However, vegetation composition in the area changed from a grass-dominated (predisturbance) to sedge-dominated (postdisturbance) community. At the conclusion of sampling in 2002, plant species abundance, evenness, dominance, and richness conditions reached levels similar to predisturbance. However, the species composition after a decade postdisturbance differed from that during the predisturbance period. Therefore, even though we are able to predict the return of species diversity, evenness, and richness of a community after a period following disturbance, the actual species composition of an intermediate marsh following recovery is difficult to determine accurately, as it is contingent on several biotic and abiotic conditions that prevail while the system recovers from disturbance.

ASSESSING RECOVERY IN A STREAM ECOSYSTEM: APPLYING MULTIPLE CHEMICAL AND BIOLOGICAL ENDPOINTS

Recovery dynamics in a previously disturbed stream undergoing remediation was assessed by measuring several chemical and biological endpoints. Three major trophic levels (periphyton, benthic macroinvertebrates, and fish communities) were analyzed along with various biological indicators of fish health including sub-organismal (biochemical and physiological) and individual-level responses. Ambient toxicity testing, water quality measurements, and fish bioaccumulation studies were also conducted to provide additional indicators of stream recovery. Information on chemical loading, biological exposure (bioaccumulation), and bioassays help identify the underlying or mechanistic basis of recovery, while endpoints measured on instream biota, and particularly those at higher levels of biological organization, are key responses for understanding the dynamics and ecological significance of recovery. When assessing recovery in aquatic ecosystems, it is important to conduct long-term field studies incorporating a variety of response variables that represent a wide range of sensitivities and response time scales. The value of different endpoints for assessing recovery depends on the inherent characteristics that are related to response variability and sensitivity, specificity and causal relationships to stressors, and ecological relevance. Long-term data sets incorporating a variety and range of these endpoints are needed to improve our understanding of natural variability in streams and provide a baseline against which effects of disturbance and subsequent recovery processes can be evaluated. The complexity of aquatic systems and their variable recovery dynamics suggest that no single measure is adequate for assessing aquatic ecosystem recovery and that a suite of chemical and biological endpoints is required for a more complete understanding of ecosystem dynamics and status during both the recovery and the post-disturbance periods. Such information should be valuable to environmental managers and regulators in helping to make more informed decisions regarding effective management and mitigation practices in disturbed systems.

Increased Particle Fluxes at the INDEX Site Attributable to Simulated Benthic Disturbance

Particle fluxes were measured 7 m above the sea bottom during the predisturbance, disturbance, and postdisturbance periods by using time series sediment traps attached to seven deep-sea moorings deployed in the INDEX experiment site in the Central Indian Basin. The predisturbance particle fluxes varied between 22.3 to 55.1 mg m−2 day−1. Increased and variable particle fluxes were recorded by the sediment traps during the disturbance period. The increase observed was 0.5 to 4 times more than the background predisturbance fluxes. The increases in particle fluxes (∼4 times) recorded by the sediment trap located in the southwestern direction (DMS-1) were the greatest, which could be the result of preferential movement of resuspended particles generated during the deep-sea benthic disturbance along the general current direction prevailing in this area during the experimental period. Also, the traps located closer to the disturbance area recorded greater fluxes than did the traps far away, across the Deep Sea Sediment Resuspension System path. This variability in recorded particle fluxes by the traps around the disturbance area clearly indicates that physical characteristics such as grain size and density of the resuspended particles produced during the disturbance had an important effect on particle movement. The postdisturbance measurements during ∼5 days showed a reduction in particle fluxes of ∼50%, indicating rapid particle settlement.