Altered Disturbance Regimes Research Articles

Shrub cover homogenizes small mammals\u2019 activity and perceived predation risk

Altered disturbance regimes, increasing atmospheric CO2, and other processes have increased woody cover and homogenized vegetation in savannas across the planet. African savannas with extensive versus minimal woody cover often have vastly different animal communities. However, we lack a clear mechanistic understanding of why animal communities are changing with vegetation structure. Our goal for this study was to understand how vegetation structure in an African savanna shaped the perceived predation risk of small mammals, hence affecting their activity. Using a reciprocal measure of standard giving-up-densities, amount of food eaten, we found sharp declines in rodents’ perceived predation risk and increased rodent activity underneath shrub cover. This response was consistent across species; however, species showed subtle differences in their responses to grassy vegetation. Our findings suggest that areas of minimal or extensive shrub cover (shrub encroachment) may be homogenizing rodents’ perceptions of predation risk and thus shaping their use of space.

Looking to the future: key points for sustainable management of northern Great Plains grasslands

The grasslands of the northern Great Plains (NGP) region of North America are considered endangered ecosystems and priority conservation areas yet have great ecological and economic importance. Grasslands in the NGP are no longer self‐regulating adaptive systems. The challenges to these grasslands are widespread and serious (e.g. climate change, invasive species, fragmentation, altered disturbance regimes, and anthropogenic chemical loads). Because the challenges facing the region are dynamic, complex, and persistent, a paradigm shift in how we approach restoration and management of the grasslands in the NGP is imperative. The goal of this article is to highlight four key points for land managers and restoration practitioners to consider when planning management or restoration actions. First, we discuss the appropriateness of using historical fidelity as a restoration or management target because of changing climate, widespread pervasiveness of invasive species, the high level of fragmentation, and altered disturbance regimes. Second, we highlight ecosystem resilience and long‐term population persistence as alternative targets. Third, because the NGP is so heavily impacted with anthropogenic chemical loading, we discuss the risks of ecological traps and extinction debt. Finally, we highlight the importance of using adaptive management and having patience during restoration and management. Consideration of these four points will help management and restoration of grasslands move toward a more successful and sustainable future. Although we specifically focus on the NGP of North America, these same issues and considerations apply to grasslands and many other ecosystems globally.

Changes in vegetation structure and composition of urban and rural forest patches in Baltimore from 1998 to 2015

Urban forests often occur as highly fragmented patches with many non-native plant species, altered disturbance regimes, environmental pollutants, and uncertain trajectories of plant community composition. In 1998, the Baltimore Ecosystem Study, a U.S. National Science Foundation-funded Long-Term Ecological Research (LTER) program, established eight forested plots to investigate long-term impacts of urbanization on natural ecosystems. All plots were located in the Baltimore metropolitan area with four in urban forest patches and four in a rural forest. In 1998, these forest patches had closed canopies with well-developed tree, shrub and vine layers, and extensive herbaceous cover. The 1998 alpha diversity (species richness) was higher in urban plots than in rural plots, whereas both plot types had similar forest structure and a relatively small number of non-native species. In 2015, we resampled these plots to investigate changes in plant structure, composition, and diversity based on the abundance, cover, and size of plant species. Trees and vines experienced minimal structural changes in all the plots. Sapling, shrub, and herbaceous abundances all declined over time in the rural plots. In the urban plots, however, only the sapling and herbaceous layers experienced declines. Despite having fewer structural changes, urban plots showed a greater shift in species composition than did rural plots. As in 1998, alpha diversity was lower in the rural plots. Beta diversity (community dissimilarity) decreased among rural plots but remained nearly unchanged in the urban plots, whereas beta turnover (species turnover) was much higher in the urban plots. These data suggest that the urban plots may have divergent compositional trajectories from the rural plots, which may help urban forests retain structural similarities through functional redundancy.

Eastern grasslands: Conservation challenges and opportunities on private lands

ABSTRACTGrasslands were once common in the eastern United States and provided habitat for numerous grassland‐dependent wildlife. However, native grasslands in this region have largely been eliminated as a result of conversion to other land uses (e.g., urban development, row‐crops), altered disturbance regimes (e.g., lack of fire and grazing), or conversion to introduced grasses for livestock production. Consequently, many wildlife species adapted to these environments have experienced protracted population declines, notably, northern bobwhite (Colinus virginianus), and other grassland‐dependent bird species. Restoration of native grasslands with appropriate native plant species composition, structure, and disturbance regimes will be key to restoring at‐risk wildlife. Indeed, the region still includes >20 million ha of grasslands, but these are dominated by introduced grass species and focused on cattle (Bos taurus) production. Any successful restoration strategy must engage the agricultural community, provide profitable forage production, and benefit habitat for wildlife. Furthermore, grassland ecosystems in humid regions (i.e., >95 cm annual rainfall) require frequent disturbance to prevent succession to forested conditions. Historically, these native grasslands were maintained by fire, herbivory, and their interaction. Consequently, wildlife communities in humid, native grasslands are adapted to those same disturbances and the conditions they create. Improving species composition by replacing introduced, sod‐forming grass species commonly used in production agriculture (e.g., tall fescue [Schedonorus arundinaceus] and bermudagrass [Cynodon dactylon]) with native grasses can be beneficial but must be accompanied by appropriate disturbance to achieve and maintain improved habitat. Therefore, developing a business case for producers to incorporate native grasses into regional agro‐grasslands that include disturbance through grazing may foster large‐scale improvements in habitat quality. Although Farm Bill conservation practices can make important contributions to conservation of eastern grassland‐dependent wildlife, such programs are dependent on uncertain and limited federal budgets and, to provide appropriate disturbance, require management intervention at a net cost to landowners and Farm Bill programs. Thus, there exists an opportunity to improve habitat at a scale that has the potential to sustain viable populations of grassland‐dependent wildlife. Therefore, we discuss the potential of a new conservation paradigm, one based on grazing systems that utilize native grasses, for filling that conservation gap. © 2019 The Wildlife Society.

Drivers of persistent post-fire recruitment in European beech forests

Climate change is expected to alter disturbance regimes including fires in European beech (Fagus sylvatica L.) forests. Regarding the resilience of beech forests to fire it is questionable whether seeds of this non-serotinous obligate masting seeder find advantageous conditions in a post-fire environment. The probability of recruitment success has been shown to increase when fire coincides with a mast year. However, the fire-induced recruitment window is poorly defined, and it is unclear how other interacting factors influence its duration. We used a space-for-time approach to model the relationships between post-fire beech recruitment, timing of seed mast events, and interacting environmental conditions using a zero-inflated model. Our results show that recruitment peaks 5–12 years after a fire, and continues throughout three decades post-fire. Beech recruitment in the post-fire period is driven by mast intensity interacting with (i) canopy opening as a consequence of progressive post-fire tree mortality and (ii) coverages of competing ground vegetation. Spring-summer moisture showed a weak positive effect on beech recruitment. We conclude that fires increase light availability, which in coincidence with a mast event results in pulses of beech recruitment. The delayed post-fire mortality of beech creates a recruitment window lasting for up to three decades, resulting in a higher-than-expected resilience of beech to individual fire disturbances.

Nitrogen alters effects of disturbance on annual grassland community diversity: Implications for restoration

Abstract In an era of anthropogenically altered disturbance regimes and increased nutrient loads, understanding how communities respond to these perturbations is essential for successful habitat restoration. Disturbance and resource supply can affect community diversity by altering community assembly processes, such as recruitment, mortality or competitive inequalities. The mechanisms behind community responses to these drivers will differentially affect multiple facets of diversity. Here we examine how factorial manipulations of disturbance (raking to remove above‐ground vegetation) and nitrogen supply affect taxonomic and phylogenetic diversity of predominantly annual California grassland communities spanning a 500‐km latitudinal and twofold rainfall gradient. The disturbance caused density‐independent biomass removal and increased access to resources such as space and light, thus mimicking demographic effects of disturbance as considered in ecological models and broadly applicable to empirical systems. We used paired metrics of richness, evenness and community composition to compare evidence from taxonomy and phylogeny. Disturbance increased species and phylogenetic diversity (richness and evenness metrics). However, nitrogen addition interacted with disturbance to reduce species richness and phylogenetic diversity. Undisturbed communities were more strongly clustered phylogenetically, but disturbance eroded this clustering such that communities became more random (i.e. indistinguishable from a null model of assembly). Species composition differed between disturbed and undisturbed communities, and many species were observed in only one community type. Disturbance interacted with nitrogen supply to alter phylogenetic composition of communities, and recently disturbed communities were more spatially variable in phylogenetic composition than undisturbed communities. Phylogenetic composition of communities also differed among nitrogen treatments. Synthesis. Our results suggest that disturbing these grassland communities by removing above‐ground vegetation increased community diversity by increasing recruitment. Seed addition following this type of disturbance is thus likely to be an effective restoration technique. However, we have shown that disturbance combined with nitrogen enrichment reduces community diversity. The mechanism for this enrichment effect does not appear to be linked to increased productivity leading to light limitation. This work suggests restoration efforts employing biomass removal must take nutrient availability into account to maximize local community diversity.

Scaling Ecological Resilience

Rapid climate change and altered disturbance regimes represent increasing stressors to the stability of existing ecosystems. Resilience is a widely used framework for post-disturbance response, but resilient responses are emergent properties resulting from component processes of persistence, recovery and reorganization, with different mechanisms at work in each mode. We present a model of scaled resilience, which allows resilience to be decomposed across scales of space, time, and levels of biological organization. Using case examples of post-fire resilience in dry conifer forests of interior western North America, we illustrate the increased clarity gained by separating scale-dependent mechanisms of persistence, recovery, and reorganization. We conclude by describing how the scaled resilience framework can be applied in land and fire management by distinguishing relevant management actions before, during, and after wildfire.

Fire Regime Alteration in Natural Areas Underscores the Need to Restore a Key Ecological Process

Research Natural Areas (RNAs) are federal lands designated to protect exemplary, relatively undisturbed ecosystems where ecological processes may proceed unencumbered with minimal human intervention. Ideally, RNAs serve as properly functioning reference sites for more heavily managed landscapes. However, many RNAs have been modified to some degree by past and ongoing human actions. In the western United States, these actions commonly result in altered disturbance regimes, most notably fire. Ecological disturbance regimes are important components of natural ecosystems, and major changes to such regimes challenge the usefulness of the RNA system as a reference network. To assess the extent of modern departure from their pre-Euroamerican settlement (i.e., pre-1850) fire regimes, we examined 64 RNAs on Forest Service lands in California. We found that 76% exhibited moderate to high fire regime departure. Of these, 87% are burning much less frequently than they would have under the presettlement fire regime and 13% are burning more frequently. Within RNAs, ecosystems historically characterized by frequent, mostly low-severity fire have missed multiple fire cycles and tend to burn at higher fire severities than expected under the presettlement fire regime. We present four case studies that demonstrate how recent wildfires have affected California RNAs. We also indicate where and how future fire management strategies could address altered fire regimes and more effectively sustain target ecosystems within RNAs. Our findings suggest that a re-examination of the hands-off approach to management in some protected natural areas like RNAs is needed.

Repeat disturbances have cumulative impacts on stream communities.

Climate change has altered disturbance regimes in many ecosystems, and predictions show that these trends are likely to continue. The frequency of disturbance events plays a particularly important role in communities by selecting for disturbance‐tolerant taxa.However, ecologists have yet to disentangle the influence of disturbance frequency per se and time since last disturbance, because more frequently disturbed systems have also usually been disturbed more recently. Our understanding of the effects of repeated disturbances is therefore confounded by differences in successional processes.We used in‐situ stream mesocosms to isolate and examine the effect of disturbance frequency on community composition. We applied substrate moving disturbances at five frequencies, with the last disturbance occurring on the same day across all treatments. Communities were then sampled after a recovery period of 9 days.Macroinvertebrate community composition reflected the gradient of disturbance frequency driven by differential vulnerability of taxa to disturbance. Diversity metrics, including family‐level richness, decreased, reflecting a likely loss of functional diversity with increasing disturbance frequency. In contrast, overall abundance was unaffected by disturbance frequency as rapid recovery of the dominant taxon compensated for strong negative responses of disturbance‐vulnerable taxa.We show that cumulative effects of repeated disturbances—not just the time communities have had to recover before sampling—alter communities, especially by disproportionately affecting rare taxa. Thus, the timing of past disturbances can have knock‐on effects that determine how a system will respond to further change.

Long‐term understory vegetation dynamics of mixed aspen forests in Rocky Mountain National Park, USA

AbstractAimsForests worldwide are subjected to increasing pressures from altered disturbance regimes, climate change, and their interactions. We resampled previously established vegetation plots to directly assess long‐term vegetation dynamics in the forest understory of mixed aspen forests, including species diversity, distribution, and composition.Study siteRocky Mountain National Park, Colorado, USA.MethodsWe resampled 89 vegetation plots that contained aspen (Populus tremuloides) in the original sampling in 1972/1973 in Rocky Mountain National Park to assess changes in understory diversity, non‐native species abundance, community composition, and elevation ranges. Analyses were performed at three spatial scales: landscape (all plots), ecotone (montane vs. subalpine), and seven forest “series” according to the classification presented in the original publication.ResultsUnderstory vegetation diversity did not significantly change at the landscape scale but increased in forests of the mesic montane series. Changes in diversity varied with elevation, with predominantly increases at lower elevations and decreases at higher elevations. Furthermore, species turnover and upward shifts were more pronounced at lower elevations. The proportion of plots containing non‐native species was similar between data sets, with 48% in 2012/2013 and 46% in 1972/1973. However, the number of non‐native species per plot increased considerably, especially in Pinus contorta forests. Significant shifts in understory community composition occurred in mesic montane forests and Pinus flexilis forests. Generally, higher floristic overlap was evident between forest types, indicating homogenization between 1972/1973 and 2012/2013 understory plant communities.ConclusionsAlthough our results indicate overall little change in understory communities between 1972/1973 and 2012/2013 in Rocky Mountain National Park, they also suggest that observed changes differ by elevation, possibly due to the interactions between elevation and changes in local climate. Our study underscores the importance of analyzing long‐term vegetation dynamics at different spatial scales and provides data from direct observations to improve the predictive power of vegetation models.

Spatial and Temporal Variability of the Impacts of Pinyon and Juniper Reduction on Hydrologic and Erosion Processes Across Climatic Gradients in the Western US: A Regional Synthesis

Pinyon (Pinus spp.) and juniper (Juniperus spp.) woodlands are an important vegetation type in the Great Basin, Colorado Plateau, and southwestern desert regions of the western US that is undergoing substantial changes associated with land management, altered disturbance regimes, and climate change. We synthesized literature on the ecohydrologic impacts of pinyon and juniper tree reductions across plot to watershed scales, short- and long-term periods, and regional climatic gradients. We found that the initial plot- to hillslope-scale ecohydrologic and erosion impacts of tree reduction on pinyon and juniper woodlands by fire, mechanical tree removal, or drought depend largely on: (1) the degree to which these perturbations alter vegetation and ground cover structure, (2) initial conditions, and (3) inherent site attributes. Fire commonly imparts an initial increased risk for hillslope runoff and erosion that degrades over time with vegetation and ground cover recovery whereas tree reductions by mechanical means pose fewer initial negative ecohydrologic impacts. Tree reduction by either approach can enhance understory vegetation and improve site-level ecohydrologic function over time, particularly on sites with an initially favorable cover of native herbaceous vegetation and a cool-season precipitation regime. Understory vegetation and ground cover enhancements appear to increase ecohydrologic resilience of some woodland communities to disturbances such as drought, fire, and insect infestations. In contrast, intensive land use, prolonged drought or repeated burning associated with invasions of fire-prone grasses can propagate long-term site degradation through persistent elevated runoff and erosion rates. Our synthesis suggests the annual precipitation requirement for increases in plot- to hillslope-scale soil water availability for herbaceous enhancement through tree removal likely ranges from 200–400 mm for sites in the Great Basin and northern Colorado Plateau (cool-season precipitation regimes), and, although suggested with great uncertainty, likely exceeds 400 mm for woodlands with rain-dominated precipitation regimes in the southwestern US. Overall, literature is inconclusive regarding tree reduction impacts on watershed-scale changes in groundwater and streamflow. To date, there is little evidence that drought-related changes to vegetation in pinyon and juniper woodlands substantially affect watershed-scale water availability and streamflow at the annual time scale. Our synthesis identifies key knowledge gaps to overcome in improving understanding of the ecohydrologic and erosion impacts of broadly occurring pinyon and juniper tree reductions in the western US.

Resilience and restoration of tropical and subtropical grasslands, savannas, and grassy woodlands

Despite growing recognition of the conservation values of grassy biomes, our understanding of how to maintain and restore biodiverse tropical grasslands (including savannas and open-canopy grassy woodlands) remains limited. To incorporate grasslands into large-scale restoration efforts, we synthesised existing ecological knowledge of tropical grassland resilience and approaches to plant community restoration. Tropical grassland plant communities are resilient to, and often dependent on, the endogenous disturbances with which they evolved-frequent fires and native megafaunal herbivory. In stark contrast, tropical grasslands are extremely vulnerable to human-caused exogenous disturbances, particularly those that alter soils and destroy belowground biomass (e.g. tillage agriculture, surface mining); tropical grassland restoration after severe soil disturbances is expensive and rarely achieves management targets. Where grasslands have been degraded by altered disturbance regimes (e.g. fire exclusion), exotic plant invasions, or afforestation, restoration efforts can recreate vegetation structure (i.e. historical tree density and herbaceous ground cover), but species-diverse plant communities, including endemic species, are slow to recover. Complicating plant-community restoration efforts, many tropical grassland species, particularly those that invest in underground storage organs, are difficult to propagate and re-establish. To guide restoration decisions, we draw on the old-growth grassland concept, the novel ecosystem concept, and theory regarding tree cover along resource gradients in savannas to propose a conceptual framework that classifies tropical grasslands into three broad ecosystem states. These states are: (1) old-growth grasslands (i.e. ancient, biodiverse grassy ecosystems), where management should focus on the maintenance of disturbance regimes; (2) hybrid grasslands, where restoration should emphasise a return towards the old-growth state; and (3) novel ecosystems, where the magnitude of environmental change (i.e. a shift to an alternative ecosystem state) or the socioecological context preclude a return to historical conditions.

Prioritizing conserved areas threatened by wildfire and fragmentation for monitoring and management.

In many parts of the world, the combined effects of habitat fragmentation and altered disturbance regimes pose a significant threat to biodiversity. This is particularly true in Mediterranean-type ecosystems (MTEs), which tend to be fire-prone, species rich, and heavily impacted by human land use. Given the spatial complexity of overlapping threats and species’ vulnerability along with limited conservation budgets, methods are needed for prioritizing areas for monitoring and management in these regions. We developed a multi-criteria Pareto ranking methodology for prioritizing spatial units for conservation and applied it to fire threat, habitat fragmentation threat, species richness, and genetic biodiversity criteria in San Diego County, California, USA. We summarized the criteria and Pareto ranking results (from west to east) within the maritime, coastal, transitional, inland climate zones within San Diego County. Fire threat increased from the maritime zone eastward to the transitional zone, then decreased in the mountainous inland climate zone. Number of fires and fire return interval departure were strongly negatively correlated. Fragmentation threats, particularly road density and development density, were highest in the maritime climate zone, declined towards the east, and were positively correlated. Species richness criteria showed distributions among climate zones similar to those of the fire threat variables. When using species richness and fire threat criteria, most lower-ranked (higher conservation priority) units occurred in the coastal and transitional zones. When considering genetic biodiversity, lower-ranked units occurred more often in the mountainous inland zone. With Pareto ranking, there is no need to select criteria weights as part of the decision-making process. However, negative correlations and larger numbers of criteria can result in more units assigned to the same rank. Pareto ranking is broadly applicable and can be used as a standalone decision analysis method or in conjunction with other methods.

Use of landscape simulation modeling to quantify resilience for ecological applications

AbstractGoals of fostering ecological resilience are increasingly used to guide U.S. public land management in the context of anthropogenic climate change and increasing landscape disturbances. There are, however, few operational means of assessing the resilience of a landscape or ecosystem. We present a method to evaluate resilience using simulation modeling. In this method, we use historical conditions (e.g., in North America, prior to European settlement), quantified using simulation modeling, to provide a comparative reference for contemporary conditions, where substantial departures indicate loss of resilience. Contemporary ecological conditions are compared statistically to the historical time series to create a resilience index, which can be used to prioritize landscapes for treatment and inform possible treatments. However, managing for resilience based on historical conditions is tenuous in the Anthropocene, which is characterized by rapid climate change, extensive human land use, altered disturbance regimes, and exotic species introductions. To account for the future variability of ecosystems resulting from climate and disturbance regime shifts, we augment historical simulations with simulations of ecosystem dynamics under projected climate and land use changes to assess the degree of departure from benchmark historical conditions. We use a mechanistic landscape model (FireBGCv2) applied to a large landscape in western Montana, USA, to illustrate the methods presented in this paper. Spatially explicit ecosystem modeling provides the vehicle to generate the historical and future time series needed to quantify potential resilience conditions associated with past and potential future conditions. Our methods show that given selection of a useful set of metrics, managers could use simulations like ours to evaluate potential future management directions.

Impacts of altered disturbance regimes on community structure and biodiversity mediated by fecundity–tolerance interactions

AbstractWe present an investigation of the effects of disturbance and fecundity–tolerance strategies on community composition. We develop a theoretical model and apply it to macrofaunal communities at deep‐sea hydrothermal vents. We characterize community outcomes and find that dominance, coexistence, and alternative stable equilibria can all result from the interplay of disturbance regimes and fecundity–tolerance interactions. We show that fecundity–tolerance trade‐offs can permit coexistence under disturbance, but a strict fecundity–tolerance trade‐off is not required for coexistence to arise. We further describe how coexistence depends on habitat availability and disturbance regimes. Generally, our model elaborates on fecundity–tolerance strategies as a new axis of trait variation in coexistence theory. Natural disturbance regimes vary considerably across regions, and anthropogenic disturbance to vent communities will escalate with the advent of deep‐sea mineral extraction. We demonstrate how anthropogenic changes to disturbance regimes may impact species diversity, pushing communities over thresholds leading to local species extinction.Recommendations for Resource Managers Conventional wisdom suggests that disturbance‐adapted communities will not be strongly affected by novel perturbations associated with resource extraction or other human activities. We show that differing fecundity–tolerance strategies can mediate coexistence in disturbance‐prone deep‐sea environments, but that small changes to the disturbance regime can alter community composition and result in species extirpation. Natural disturbance should not be used as a rationale for expected low impacts of anthropogenic disturbance, for example, mining activities in management of deep‐sea mineral extraction. Appropriate impact studies should be performed for all communities where disturbances, such as mining, are planned.

Seasonal shifts in export of DOC and nutrients from burned and unburned peatland-rich catchments, Northwest Territories, Canada

Abstract. Boreal peatlands are major catchment sources of dissolved organic carbon (DOC) and nutrients and thus strongly regulate the landscape carbon balance, aquatic food webs, and downstream water quality. Climate change is likely to influence catchment solute yield directly through climatic controls on run-off generation, but also indirectly through altered disturbance regimes. In this study we monitored water chemistry from early spring until fall at the outlets of a 321 km2 catchment that burned 3 years prior to the study and a 134 km2 undisturbed catchment. Both catchments were located in the discontinuous permafrost zone of boreal western Canada and had ∼ 60 % peatland cover. The two catchments had strong similarities in the timing of DOC and nutrient yields, but a few differences were consistent with anticipated effects of wildfire based on peatland porewater analysis. The 4-week spring period, particularly the rising limb of the spring freshet, was crucial for accurate characterization of the seasonal solute yield from both catchments. The spring period was responsible for ∼ 65 % of the seasonal DOC and nitrogen and for ∼ 85 % of the phosphorous yield. The rising limb of the spring freshet was associated with high phosphorous concentrations and DOC of distinctly high aromaticity and molecular weight. Shifts in stream DOC concentrations and aromaticity outside the early spring period were consistent with shifts in relative streamflow contribution from precipitation-like water in the spring to mineral soil groundwater in the summer, with consistent relative contributions from organic soil porewater. Radiocarbon content (14C) of DOC at the outlets was modern throughout May to September (fraction modern carbon, fM: 0.99–1.05) but likely reflected a mix of aged DOC, e.g. porewater DOC from permafrost (fM: 0.65–0.85) and non-permafrost peatlands (fM: 0.95–1.00), with modern bomb-influenced DOC, e.g. DOC leached from forest litter (fM: 1.05–1.10). The burned catchment had significantly increased total phosphorous (TP) yield and also had greater DOC yield during summer which was characterized by a greater contribution from aged DOC. Overall, however, our results suggest that DOC composition and yield from peatland-rich catchments in the discontinuous permafrost region likely is more sensitive to climate change through impacts on run-off generation rather than through altered fire regimes.

Wildfire and debris flows affect prey subsidies with implications for riparian and riverine predators

Ecosystems are connected through fluxes of nutrients, organic matter, and organisms. Disturbances that alter structure and function of one ecosystem may have consequences for other linked ecosystems. We investigated how wildfire and subsequent debris flows altered fluxes of invertebrates from tributaries in the Salmon River Basin, Idaho, USA. We compared fluxes of invertebrates downstream through drift and laterally via insect emergence from streams with varying disturbance histories (unburned, burned, and burned + debris flow) during two summers (3–4 years post fire and 2–3 years post debris flow). We observed that the combination of wildfire and debris flow increased the biomass export of invertebrates from tributaries to main-stem ecosystems 2–3 × compared to other streams. In contrast, aquatic insect emergence did not differ in magnitude among streams of different disturbance histories, but instead diverged in timing. Underwater surveys indicated trout in the main-stem river selected confluence habitats, with a tendency for stronger selection of confluences with burned streams. In a behavioral comparison between confluence and non-confluence habitats, rates of agonistic behavior were 4–6 × higher in confluence areas, indicating that confluences may be worth defending. Abundances of web-spinning spiders that depend on emerging insects did not vary with disturbance history in early-mid summer, but tended to be highest in riparian areas along burned streams by August. Because wildfire and debris flows are predicted to increase, our results elucidate potential pathways by which altered disturbance regimes may affect coupled aquatic-terrestrial ecosystems.

STEPWAT2: an individual‐based model for exploring the impact of climate and disturbance on dryland plant communities

AbstractThe combination of climate change and altered disturbance regimes is directly and indirectly affecting plant communities by mediating competitive interactions, resulting in shifts in species composition and abundance. Dryland plant communities, defined by low soil water availability and highly variable climatic regimes, are particularly vulnerable to climatic changes that exceed their historical range of variability. Individual‐based simulation models can be important tools to quantify the impacts of climate change, altered disturbance regimes, and their interaction on demographic and community‐level responses because they represent competitive interactions between individuals and individual responses to fluctuating environmental conditions. Here, we introduce STEPWAT2, an individual plant‐based simulation model for exploring the joint influence of climate change and disturbance regimes on dryland ecohydrology and plant community composition. STEPWAT2 utilizes a process‐based soil water model (SOILWAT2) to simulate available soil water in multiple soil layers, which plant individuals compete for based on the temporal matching of water and active root distributions with depth. This representation of resource utilization makes STEPWAT2 particularly useful for understanding how changes in soil moisture and altered disturbance regimes will concurrently impact demographic and community‐level responses in drylands. Our goals are threefold: (1) to describe the core modules and functions within STEPWAT2 (model description), (2) to validate STEPWAT2 model output using field data from big sagebrush plant communities (model validation), and (3) to highlight the usefulness of STEPWAT2 as a modeling framework for examining the impacts of climate change and disturbance regimes on dryland plant communities under future conditions (model application). To address goals 2 and 3, we focus on 15 sites that span the spatial extent of big sagebrush plant communities in the western United States. For goal 3, we quantify how climate change, fire, and grazing can interact to influence plant functional type biomass and composition. We use big sagebrush‐dominated plant communities to demonstrate the functionality of STEPWAT2, as these communities are among the most widespread dryland ecosystems in North America.

Disturbance, nutrients, and antecedent flow conditions affect macroinvertebrate community structure and productivity in an Arctic river

AbstractClimate change is expected to alter disturbance regimes and biogeochemical cycles that underlie the structure and function of ecosystems worldwide. In the Arctic, rapid warming is already affecting these processes via changes in precipitation and thawing permafrost. We assessed how anticipated changes in disturbance regimes and nutrient availability may affect an arctic river ecosystem (Kuparuk River, Alaska) by analyzing temporal patterns of biofilm chlorophyll mass and macroinvertebrate community structure and productivity. Our study incorporated an upstream reach (sampled 2001–2012) and a downstream reach (sampled 2011–2012) to which phosphorus (P) was added to simulate increases in nutrient supply that are anticipated as permafrost thaws. Greater hydrologic disturbance during the open‐water season correlated with reduced algal biomass and invertebrate secondary production (range ∼ 2–7 g DM m−2 yr−1) in the following spring and summer. Bed disturbing flows also altered macroinvertebrate community structure with distinct “high‐flow” and “base‐flow” assemblages documented. Recovery time was shorter for chlorophyll mass and macroinvertebrate production (∼ 1 yr) than community structure (∼ 3 yr). Experimental P‐addition increased algal biomass and invertebrate production, but also resulted in a third macroinvertebrate assemblage dominated by mobile grazers rather than filter‐feeders. Our results suggest that a decrease in the return interval for bed disturbing floods to < 4 yr will result in persistent changes in macroinvertebrate community structure and fundamental alterations to the food web. These results also demonstrate how arctic river communities may be affected by increases in the magnitude and variability of river discharge and nutrient supplies that are anticipated as the climate warms.

From the stand scale to the landscape scale: predicting the spatial patterns of forest regeneration after disturbance.

Shifting disturbance regimes can have cascading effects on many ecosystems processes. This is particularly true when the scale of the disturbance no longer matches the regeneration strategy of the dominant vegetation. In the yellow pine and mixed conifer forests of California, over a century of fire exclusion and the warming climate are increasing the incidence and extent of stand-replacing wildfire; such changes in severity patterns are altering regeneration dynamics by dramatically increasing the distance from live tree seed sources. This has raised concerns about limitations to natural reforestation and the potential for conversion to non-forested vegetation types, which in turn has implications for shifts in many ecological processes and ecosystem services. We used a California region-wide data set with 1,848 plots across 24 wildfires in yellow pine and mixed conifer forests to build a spatially explicit habitat suitability model for forecasting postfire forest regeneration. To model the effect of seed availability, the critical initial biological filter for regeneration, we used a novel approach to predicting spatial patterns of seed availability by estimating annual seed production from existing basal area and burn severity maps. The probability of observing any conifer seedling in a 60-m2 area (the field plot scale) was highly dependent on 30-yr average annual precipitation, burn severity, and seed availability. We then used this model to predict regeneration probabilities across the entire extent of a "new" fire (the 2014 King Fire), which highlights the spatial variability inherent in postfire regeneration patterns. Such forecasts of postfire regeneration patterns are of importance to land managers and conservationists interested in maintaining forest cover on the landscape. Our tool can also help anticipate shifts in ecosystem properties, supporting researchers interested in investigating questions surrounding alternative stable states, and the interaction of altered disturbance regimes and the changing climate.