Infrequent Disturbances Research Articles

Tube-Based Discrete Controller Design for Vehicle Platoons Subject to Disturbances and Saturation Constraints

Cooperative adaptive cruise control (CACC) is a promising intelligent vehicle technology for improving traffic flow stability, throughput, and safety. One major control objective of CACC is to guarantee $\mathcal {L}_{p}$ string stability, i.e., $\mathcal {L}_{p}$ -norm measured disturbance is uniformly bounded along the vehicle string. Most existing methods for string stability are laborious for implementation without considering either heterogeneous disturbances (e.g., tracking errors and unmodeled dynamics) or saturation constraints (e.g., input saturation). The decentralized model predictive control (MPC) method, which is a widely used feedforward control for string stability, suffers the burdens of computation cost and intervehicular communication. To fill these gaps, we distinguish different types of disturbances and use different ways to handle them. We use feedforward control for large yet infrequent disturbances and feedback control for small yet frequent disturbances. Different from MPC, our feedforward control is event-triggered so that the intervehicle communication and planning costs can be significantly reduced. Different from pure robust feedback control, our combination of feedback and feedforward control could reduce the conservation of the controller. Theoretical analysis and simulations show that the proposed method guarantees $\mathcal {L}_{p}$ string stability of vehicle platoons considering heterogeneous disturbances and saturation constraints.

Testing conceptual models of early plant succession across a disturbance gradient

Abstract Studies of succession have a long history in ecology, but rigorous tests of general, unifying principles are rare. One barrier to these tests of theory is the paucity of longitudinal studies that span the broad gradients of disturbance severity that characterize large, infrequent disturbances. The cataclysmic eruption of Mount St. Helens (Washington, USA) in 1980 produced a heterogeneous landscape of disturbance conditions, including primary to secondary successional habitats, affording a unique opportunity to explore how rates and patterns of community change relate to disturbance severity, post‐eruption site conditions and time. In this novel synthesis, we combined data from three long‐term (c. 30‐year) studies to compare rates and patterns of community change across three ‘zones’ representing a gradient of disturbance severity: primary successional blast zone, secondary successional tree blowdown/standing snag zone and secondary successional intact forest canopy/tephra deposit zone. Consistent with theory, rates of change in most community metrics (species composition, species richness, species gain/loss and rank abundance) decreased with time across the disturbance gradient. Surprisingly, rates of change were often greatest at intermediate‐severity disturbance and similarly low at high‐ and low‐severity disturbance. There was little evidence of compositional convergence among or within zones, counter to theory. Within zones, rates of change did not differ among ‘site types’ defined by pre‐ or post‐eruption site characteristics (disturbance history, legacy effects or substrate characteristics). Synthesis. The hump‐shaped relationships with disturbance severity runs counter to the theory predicting that community change will be slower during primary than during secondary succession. The similarly low rates of change after high‐ and low‐severity disturbance reflect differing sets of controls: seed limitation and abiotic stress in the blast zone vs. vegetative re‐emergence and low light in the tephra zone. Sites subjected to intermediate‐severity disturbance were the most dynamic, supporting species with a greater diversity of regenerative traits and seral roles (ruderal, forest and non‐forest). Succession in this post‐eruption landscape reflects the complex, multifaceted nature of volcanic disturbance (including physical force, heating and burial) and the variety of ways in which biological systems can respond to these disturbance effects. Our results underscore the value of comparative studies of long‐term, ecological processes for testing the assumptions and predictions of successional theory.

The dynamics of rush circles in subalpine grassland

Geometric patterning in vegetation is often the result of cyclic succession, although circular forms sometimes result from the clonal growth of individual plants. We investigated whether cyclic succession or clonal growth created patterns in Poa subalpine grassland with rush (Empodisma minus) circles on the Central Plateau of Tasmania, Australia. We mapped change in the number, size and reflectance of the rush circles from 1953 to 2011 using aerial photographs, tested whether the circles occurred in distinct microenvironments, and whether the circles were likely to be individuals and determined whether the smallest circles were associated with disturbance. The number and area of circles increased with time. As circles became larger they became more reflective, evenly across their extent. The circles were associated with flatter patches in the plain. Surface soil characteristics did not differ between an exemplar circle and adjacent grassland. The Empodisma foliage in each circle tended to have colour consistently distinct from that of other circles in its vicinity, indicating that the circular form was a product of vegetative expansion of a single individual at a much more rapid velocity than the expansion of woody plant rings. The smaller circles were sometimes associated with frost-damaged Astelia alpina mounds, although far from confined to them. We did not find any instances of Empodisma circles <0.1 m in diameter. We deduced that the probability of establishment of new individuals of Empodisma in the grassland was extremely low, or depended on an infrequent disturbance. Although the increasing reflectance of circles with size and the lack of differentiation of soils between circle and grassland indicated the possibility of cyclic succession, the area covered by circles increased at the decadal time scale. The increases in circle coverage and number may have resulted from changes in climate and grazing regimes on the eastern Central Plateau since the 1940s.

A state-dependent model for assessing the population consequences of disturbance on income-breeding mammals

Human activities continue to expand in marine and terrestrial environments, leading to increased interactions with wildlife that can have negative impacts on population dynamics. Approaches for quantifying how these interactions translate to population-level effects are therefore crucial for effective management practices and balancing human-wildlife tradeoffs. We developed a method using state-dependent behavioral theory implemented via Stochastic Dynamic Programming (SDP) for predicting the population consequences of disturbance on the physiology and reproductive behavior of an income-breeding mammal, using California sea lions (Zalophus californianus) as a motivating species. Emergent properties of the model included reproductive characteristics associated with long-lived species, such as variation in the age at first reproduction, early termination of pregnancy, and skipped breeding. In undisturbed model simulations, reproductive rates and the average wean date were consistent with empirically-derived estimates from sea lions and other marine mammals, highlighting the utility of this model for quantifying fecundity estimates of data-deficient species and addressing fundamental ecological processes. In disturbed model simulations, exposure to prolonged, repetitive disturbances negatively impacted population growth; in addition, short, infrequent disturbances had the potential for adverse effects depending on the behavioral response of sea lions and the probability of being disturbed. The adverse effect of disturbance on population dynamics was due to a combination of reduced pup recruitment (survival to age one) resulting from a lower wean mass and increased abortion rates that led to skipped reproductive years, both of which have been documented for marine mammal populations experiencing natural fluctuations in prey availability. The derivation of state- and time-dependent reproductive decisions using an SDP model is an effective approach that links behavioral and energetic effects at the individual level to changes at the population level, and one that serves a dual purpose in the ability to quantify basic biological parameters and address ecological questions irrespective of disturbance.

Transitioning western U.S. dry forests to limited committed warming with bet‐hedging and natural disturbances

AbstractHistorical evidence suggests natural disturbances could allow more forest persistence, than expected from models, over 40 yr of transition to the net‐zero emissions needed to limit warming to <2.0°C (e.g., Paris Agreement). Forests must ultimately equilibrate with committed warming from accumulated emissions. Historical dry‐forest landscapes were heterogeneous from large, infrequent disturbances (LIDs) that reduced tree density and basal area, followed by slow, variable tree regeneration and recovery for 1–3 centuries. These together effectively provided bet‐hedging through stand‐ and landscape‐level heterogeneity that enhanced resistance and resilience to a diversity of unpredictable subsequent disturbances. Recent disturbances have not yet exceeded historical variability in rates and patterns, but could cause mortality of ∼26–51% of dry‐forest area in the transition. This also means 1/2 to 3/4 of dry‐forest area could escape most mortality and the mortality area could also have substantial forest persistence. Projections are unavailable for droughts or beetle outbreaks, but they recently caused about 3–4 times as much tree mortality as did moderate‐ to high‐severity fires. Mortality could reduce forest area if new trees do not regenerate, but 24 studies showed recent regeneration after high‐severity fires was slow, but indistinct from historical variability. Survival of smaller trees provided regeneration after beetle outbreaks and droughts. Regeneration in general was projected by 2060 to decline by ∼10% in one study and increase by 50% in another. If openings from disturbances increased, some grasslands and shrublands could be restored, increasing landscape heterogeneity and resistance to disturbance spread. Given these trends and our limited ability to prevent LIDs, I suggest (1) refocusing restoration to increase bet‐hedging resilience to droughts and beetle outbreaks by retaining small trees and diverse tree species, (2) expanding development of fire‐safe landscapes to protect people and infrastructure from unavoidable increased fire, (3) enabling more managed fire to restore and enhance stand‐ and landscape‐scale bet‐hedging, and (4) accepting that LIDs will revise resistance, resilience, and adaptation, which enhance forest persistence, particularly if post‐disturbance survivors are not logged and trees are not planted. Natural disturbance and slow recovery, if bet‐hedged to increase resistance and resilience, could enable substantial forest persistence.

Typhoon diverged forest succession from natural trajectory in the treeline ecotone of the Changbai Mountains, Northeast China

Abstract Windthrow is a typical gap phrase disturbance that changes forest composition but rarely alter succession trajectories of a forest stand. Catastrophic windthrow such as typhoons is a large but infrequent disturbance that can have long-lasting effects on forest ecosystems. Whether catastrophic windthrow resets successional pathways of forest ecosystems remains inconclusive. We compared forest structure and regeneration patterns between undisturbed and disturbed stands 30 years after a typhoon event in the treeline ecotone of the Changbai Mountains, Northeastern China. Measurements were made using WorldView-1 satellite imaging and fieldwork in two types of large plots, one located in areas affected by the typhoon event and the other in adjacent undisturbed areas. Results showed that community divergence at landscape scale occurred following the typhoon. In non-wind–disturbed stands, forest development followed the successional pathway of both broadleaved and coniferous trees. By contrast, the late-successional coniferous rather than the early successional broadleaved trees occupied the wind–disturbed stands. The alteration mechanism on the forest succession pathway was that herbs formed tall, dense, and persistent mono-dominant thickets after the typhoon, which inhibited the regeneration of broadleaved trees. In contrast to the common knowledge that forest would return to initial successional stage after catastrophic disturbance, we concluded that catastrophic windthrow, different from gap phase windthrow, altered forest successional trajectory.

低頻度大規模攪乱としての巨大津波が海岸林の樹木群集に与えた影響( 東日本大震災と砂浜海岸エコトーン植生 : 津波による攪乱とその後の回復)

低頻度大規模攪乱としての巨大津波が海岸林の樹木群集に与えた影響( 東日本大震災と砂浜海岸エコトーン植生 : 津波による攪乱とその後の回復)

Selection of a remote maternity roost by fringed myotis (Myotis thysanodes) in Carlsbad Cavern, New Mexico

Abstract In Carlsbad Cavern, New Mexico, a small maternity colony (∼100 individuals) of fringed myotis (Myotis thysanodes) regularly roosts >200 m below the surface of the ground and >1.5 km from the nearest opening to the cavern. With many passageways in the cavern that are closer to surface openings, we investigated why females selected a remote area to bear and raise young. Using radiotransmitters and light tags, we determined that bats traveled 1.8 or 2.1 km, depending on which of only two openings was used to exit the cavern. Air temperatures in the passageway containing the maternity colony and adjacent rooms were the warmest in the cavern, and air moisture also was greater in those areas. Warm and stable air temperatures, high humidity, low predation risk, and infrequent human disturbances were advantages of the roost site. For this colony of M. thysanodes, such advantages seemed to outweigh energetic costs and other disadvantages of commuting to a remote area of the cavern on a nightly basis.

Abiotic disturbances in Bulgarian mountain coniferous forests – An overview

Disturbances are among the most important processes that shape forest dynamics and landscapes. However the historic ranges of variability (HRV) of many coniferous forests in Europe and specifically in the mountains on the Balkan Peninsula are not well understood. We present the first overview of available data on disturbance events in coniferous forests in the high mountains in southwestern Bulgaria. Our study included data from historical publications, documents and archives, newer documentary data obtained through interviews with foresters and verified using satellite images, published studies and new tree-ring data. We documented at least 188 abiotic disturbance events in the Bulgarian mountains including fires (39%), windthrows (31%) and avalanches (20%) and fewer disturbances caused by snow and ice.Fires primarily affected Pine-dominated ecosystems, especially pure Pinus sylvestris and mixed Pinus sylvestris-Pinus nigra forests. Our tree-ring analysis also provided evidence for repeated fires in subalpine Pinus heldreichii and Pinus peuce forests during the last 500 years. Some of these fires affected different valleys within the same year, suggesting either large-scale events or the simultaneous occurrence of fires related to appropriate fuel and climate conditions. We also found data on several large fires in Picea abies-dominated ecosystems. Most of the fires were less than 100 ha, but some were larger, the most extensive of which was the Batalach fire (year 1890, approximately 9000 ha) in the Western Rhodopes. The highest frequency of fires was in 1880–1910, 1940–1950 and 2000–2010. We found evidence for at least 59 windthrows greater than 1ha, mainly in pure Picea abies and mixed Picea abies-Pinus sylvestris forests. There was high variability in the size of windthrows, ranging from small-scale, gap-sized blowdowns to large and infrequent disturbances affecting more than 300ha, the largest of which was the Beglika windthrow (1961, 3000ha). Although avalanches are considered important for subalpine forests on steep slopes they were generally poorly documented and data were limited to several avalanche cycles in 1955, 1963–1974 and in the last two decades. Snow-related damage was important mostly for young pine forests in which several events affected a high stock of wood in the 1930s, 1987, 1988 and 2015.Our data show that natural disturbances of various types and of a range of sizes are part of the natural dynamics of coniferous forests in Bulgaria. Further, our study contributes to the understanding of the historic range of variability of mountain forests in Europe.

Extreme rainfall impacts on soil CO2efflux in an urban forest ecosystem in Beijing, China

Extreme rainfall events are infrequent disturbances that affect urban environments and soil respiration (Rs). Using data measured in an urban forest ecosystem in Beijing, China, we examined the link between gross primary production (GPP) and soil respiration on a diurnal scale during an extreme rainfall event (i.e., the “21 July 2012 event”), and we examined diel and seasonal environmental controls on Rs. Over the seasonal cycle, Rs increased exponentially with soil temperature (Ts). In addition, Rs was hyperbolically related to soil volumetric water content (VWC), increasing with VWC below a threshold of 0.17 m3m−3, and then decreasing with further increases in VWC. Following the extreme rainfall event (177 mm), Rs showed an abrupt decrease and then maintained a low value of ∼0.3 μmol m−2s−1for about 8 h as soil VWC reached the field capacity (0.34 m3m−3). Rs became decoupled from Ts and increased very slowly, while GPP showed a greater increase. A bivariate Q10-hyperbolical model, which incorporates both Ts and VWC effects, better fits Rs than the Q10model in summer but not for whole year.

Phytoplankton size-diversity mediates an emergent trade-off in ecosystem functioning for rare versus frequent disturbances.

Biodiversity is known to be an important determinant of ecosystem-level functions and processes. Although theories have been proposed to explain the generally positive relationship between, for example, biodiversity and productivity, it remains unclear which mechanisms underlie the observed variations in Biodiversity-Ecosystem Function (BEF) relationships. Using a continuous trait-distribution model for a phytoplankton community of gleaners competing with opportunists, and subjecting it to differing frequencies of disturbance, we find that species selection tends to enhance temporal species complementarity, which is maximised at high disturbance frequency and intermediate functional diversity. This leads to the emergence of a trade-off whereby increasing diversity tends to enhance short-term adaptive capacity under frequent disturbance while diminishing long-term productivity under infrequent disturbance. BEF relationships therefore depend on both disturbance frequency and the timescale of observation.

Competing drivers lead to non-linear native–exotic relationships in endangered temperate grassy woodlands

Relationships between the diversity and abundance of native versus exotic species underpin management of disturbance regimes for conservation. Theory predicts negative, positive or neutral relationships depending on respective drivers, with greatest potential benefit when natives and exotics show opposing responses to management. We examined drivers of exotic plant cover and relationships with native plant richness using 12-year burning, mowing and grazing experiments in two representative temperate grassy eucalypt woodlands with contrasting histories of frequent versus infrequent disturbance. We hypothesized that disturbance and high resources favour exotics, and assessed whether natives and exotics covary positively due to common external drivers or negatively due to contrasting external drivers and/or competition. Positive relationships with rainfall and disturbance explained >80 % of the variation in exotic cover at both sites, supporting our first hypothesis. Native–exotic relationships were non-linear, with native richness first increasing rapidly with increasing exotic cover, then levelling and beginning to decrease. Common external drivers, particularly inter-annual rainfall, explained initial positive relationships, highlighting a prevalence of positive relationships at long temporal (as well as large spatial) scales. At the historically frequently-burnt site, a concomitant increase in native richness and exotic cover after fire contributed to the positive relationship, indicating a management trade-off. At the long-unburnt site, exotics increased but natives decreased with fire, suggesting dual benefits of low fire frequency. We conclude that relationships between exotic cover and native richness emerge from interactions among external drivers and competitive responses, with responses to external drivers dominating at low resources and negative interactions gaining importance as resources increase.

Litter microbial and soil faunal communities stimulated in the wake of a volcanic eruption in a semi‐arid woodland in Patagonia, Argentina

SummaryLarge‐scale disturbances can be important components of the temporal landscape of natural ecosystems, but generalities regarding ecosystem impacts are difficult due to their infrequent and unpredictable nature. Volcanic eruptions figure as one of the most prominent of these natural disturbances, but the effects on microbes and ground‐dwelling arthropods, which modulate carbon and nutrient turnover, are relatively unknown.We evaluated the effects of the 2011 Puyehue‐Cordón Caulle eruption in Patagonia, Argentina, on the litter and soil microbial and faunal communities in natural and afforested semi‐arid ecosystems located 70 km west of the epicentre of the eruption. We hypothesized that volcanic ash deposition would strongly reduce soil faunal and microbial communities due to insecticidal effects of ash on arthropods, with a concomitant reduction in ecosystem processes.Our objective was to quantify the impact of the volcanic eruption by comparing pre‐ and post‐eruption time points in the same study site, with nearly identical field methodology. We measured environmental variables of soil and litter moisture, pH, microbial biomass, and soil and litter microbial enzymatic activity. We evaluated ground‐dwelling arthropods and nematodes using pitfall traps and soil extraction, respectively. Additionally, a parallel, controlled‐condition experiment of simulated ash deposition was conducted to evaluate ash effects on litter decomposition and enzymatic activity.In the field, post‐eruption soils had lower soil water content, pH and soil organic matter. Additionally, nematode abundance and soil microbial enzyme activity were significantly reduced. In contrast, ground‐dwelling arthropods and litter enzymatic activity increased significantly. Finally, with simulated ash deposition, litter decomposition increased fourfold for native litter decomposition.Large‐scale disturbances may play a key role in biogeochemical cycling in affected natural ecosystems, but not necessarily due to their catastrophic effects. In contrast to our original predictions, we observed a marked stimulation of biotic activity and carbon turnover in the aftermath of the Puyehue volcanic eruption, which demonstrates that the biotic component of these ecosystems has a substantial capacity to respond to these disturbances in short time frames. These results can contribute to placing the role of these large‐scale infrequent disturbances in a more robust ecological context.ALay Summaryis available for this article.

Modeling Hurricane Exposure in a Caribbean Lower Montane Tropical Wet Forest: The Effects of Frequent, Intermediate Disturbances and Topography on Forest Structural Dynamics and Composition

Frequent intermediate disturbances can produce qualitatively different spatial heterogeneity and environmental variability than large infrequent disturbances, which facilitates the coexistence of disparate species types. We hypothesized that species coexistence will be maximized at sites exposed to recurring hurricanes with intermediate frequencies and effects. Consequently, we sought to determine if exposure vulnerability (EV) from three hurricanes with intermediate effects and frequencies (Ivan-2004, Dennis-2005 and Dean-2007), or the interaction between exposure and topography, could be used to explain forest structural dynamics and composition. We used data obtained in 2006 and 2012 from within 45, 25 × 25 m (2.8125 ha) permanent sample plots, established according to a randomized block design, and stratified according to elevation and aspect/sites (NE and SW facing ridges) in a tropical montane wet forest, John Crow Mountains, Jamaica. There was a significant reduction in basal area (BA) (14%), tree volume (10%), and density (26%) and there was a negative shift in the height profile of trees. Understory light (2008), stem density, and mortality increased with EV but decreased with aspect. The NE aspect/sites had higher EV after Dean and the other hurricanes. Consequently, BA, volume and density increased significantly for light-demanding species at the NE sites, but declined significantly overall with minimal changes at SW sites. Moreover, diversity was significantly higher at sites with higher EV for the three hurricanes. The frequent hurricanes with intermediate effects may have therefore maintained/increased spatial heterogeneity, which promoted the coexistence of species with disparate life histories at more exposed sites.

Linking spatiotemporal disturbance history with tree regeneration and diversity in an old-growth forest in northern Japan

Knowledge of long-term spatiotemporal effects of disturbances on forest structure, tree regeneration and species composition is key for understanding forest dynamics and predicting future forest responses to climate change. Here, we explore the spatiotemporal impact of disturbances of different severities on tree recruitment and diversity in species-rich oak-fir-maple forest in Hokkaido, a typhoon-prone area in northern Japan, over the past 230 years. The forest disturbance history was reconstructed by growth-release analysis from more than 45,500 tree rings of 385 trees belonging to 15 species. A mixed severity disturbance regime was prevalent over the study period. Altogether, 310 major and 293 moderate growth releases were identified. These were both temporally and spatially localized, with 80% of events detected in only four time periods: 1775–1784, 1815–1839, 1880–1909 and 1950–1979. Disturbances were followed by major recruitment pulses, each lasting around 30 years. Dendrochronological reconstructions alone indicate that severe (i.e. high proportion of releases), infrequent disturbances control tree regeneration and forest development (from oak-dominated forests to mixed-stand with higher proportion of shade-tolerant tree species). However, a combination of temporal and spatial pattern analysis revealed that less severe disturbances, creating small gaps, promote higher density and diversity of recruitment (altogether 19 tree species recorded) compared with severe disturbances. The latter create large forest gaps which became overgrown by dwarf bamboo and suppress tree regeneration. These results provide evidence that severe disturbances interacting with a strong biotic understory filter (as dwarf bamboo), can disrupt forest ecosystem dynamics by significantly reducing the extent and diversity of tree recruitment. Our findings are important as most climate models predict an elevated intensity of typhoons in Northeast Asia. We conclude that a combination of temporal and spatial analyses, as presented here, is necessary to disentangle the complex drivers of long-term forest dynamics.

Large, infrequent disturbance on a regulated river: response of floodplain forest birds to the 2011 Missouri River flood

Floodplain forests are dynamic habitats that support a high diversity and abundance of birds. Periodic flood disturbance is important in the establishment and maintenance of the heterogeneous mosaic of vegetation communities across the riverine landscape. Human suppression of disturbance regimes has been implicated in the decline of bird species in these systems. Because few large rivers are not subject to flood control by dams and levees, opportunities to study avian responses to flood disturbance are limited. A large magnitude, long‐duration flood event on the Missouri River, USA, during the summer of 2011 provided an opportunity to quantify post‐flood changes in forest bird densities and species richness relative to pre‐flood conditions on a riverine floodplain impacted by decades of flow regulation. We surveyed 75 forest sites on two segments of remnant floodplain forest along the Missouri National Recreational River (MNRR) in southeastern South Dakota and northeastern Nebraska and examined changes in density for 35 breeding landbird species from pre‐flood (2009–2010) to post‐flood (2012–2014) periods. We used a repeated measures ANOVA design to test the effects of year on average densities of birds and nesting guilds and confidence intervals to determine changes in densities of individual species and species richness. 19 of 35 focal species declined significantly one year after the flood (2012), but abundances of ten species recovered to pre‐flood densities or higher within two years. In 2012, density declines of six species and density increases of two species were significantly correlated with a decrease in woody vegetation density and percent shrub cover. Average bird densities and the density of shrub nesters rebounded in 2013 to pre‐flood levels and continued to increase through 2014. There were no significant changes in species richness at the level of forest habitat types between sampling years. Our results demonstrate short‐term resilience of floodplain bird species to a major disturbance despite declines in early successional habitat and minimal recovery of woody vegetation.

Dispersal Mutualism Incorporated into Large-Scale, Infrequent Disturbances.

Because of their influence on succession and other community interactions, large-scale, infrequent natural disturbances also should play a major role in mutualistic interactions. Using field data and experiments, I test whether mutualisms have been incorporated into large-scale wildfire by whether the outcomes of a mutualism depend on disturbance. In this study a seed dispersal mutualism is shown to depend on infrequent, large-scale disturbances. A dominant shrubland plant (Arctostaphylos species) produces seeds that make up a persistent soil seed bank and requires fire to germinate. In post-fire stands, I show that seedlings emerging from rodent caches dominate sites experiencing higher fire intensity. Field experiments show that rodents (Perimyscus californicus, P. boylii) do cache Arctostaphylos fruit and bury most seed caches to a sufficient depth to survive a killing heat pulse that a fire might drive into the soil. While the rodent dispersal and caching behavior itself has not changed compared to other habitats, the environmental transformation caused by wildfire converts the caching burial of seed from a dispersal process to a plant fire adaptive trait, and provides the context for stimulating subsequent life history evolution in the plant host.

Effects of varying organic matter content on the development of green roof vegetation: A six year experiment

Green roofs can potentially be used to tackle a variety of environmental problems, and can be used as development mitigation for the loss of ground-based habitats. Brown (biodiversity) roofs are a type of green roof designed to imitate brownfield habitat, but the best way of engineering these habitats requires more research. We tested the effects of altering organic matter content on the development of vegetation assemblages of experimental brown (biodiversity) roof mesocosms. Three mulch treatments were tested: (1) sandy loam, where 10mm of sandy loam mulch (about 3% organic matter by dry weight) was added to 100mm of recycled aggregate; (2) compost, where the mulch also contained some garden compost (about 6% organic matter by dry weight); and (3) no mulch, where no mulch was added. Mesocosms were seeded with a wildflower mix that included some Sedum acre, and vegetation development was investigated over a six-year period. Species richness, assemblage character, number of plants able to seed, and above-ground plant biomass were measured. Drought disturbance was an important control on plant assemblages in all mulch treatments, but there were significant treatment response interactions. The more productive compost treatment was associated with larger plant coverage and diversity before the occurrence of a sequence of drought disturbances, but was more strongly negatively affected by the disturbances than the two less productive treatments. We suggest that this was due to the over-production of plant biomass in the more productive treatment, which made the plants more vulnerable to the effects of drought disturbance, leading to a kind of ‘boom-bust’ assemblage dynamic. The ‘ideal’ amount of added organic matter for these green roof systems was very low, but other types of green roof that have a larger water holding capacity, and/or more drought resistant plant floras, will likely require more organic matter or fertiliser. Nonetheless, nutrient-supported productivity in green roof systems should be kept low in order to avoid boom-bust plant assemblage dynamics. Research into the best way of engineering green roof habitats should take place over a long enough multi-year time period to include the effects of temporally infrequent disturbances.

The longevity of broadleaf deciduous trees in Northern Hemisphere temperate forests: insights from tree-ring series

Understanding the factors controlling the expression of longevity in trees is still an outstanding challenge for tree biologists and forest ecologists. We gathered tree-ring data and literature for broadleaf deciduous (BD) temperate trees growing in closed-canopy old-growth forests in the Northern Hemisphere to explore the role of geographic patterns, climate variability, and growth rates on longevity. Our pan-continental analysis, covering 32 species from 12 genera, showed that 300-400 years can be considered a baseline threshold for maximum tree lifespan in many temperate deciduous forests. Maximum age varies greatly in relation to environmental features, even within the same species. Tree longevity is generally promoted by reduced growth rates across large genetic differences and environmental gradients. We argue that slower growth rates, and the associated smaller size, provide trees with an advantage against biotic and abiotic disturbance agents, supporting the idea that size, not age, is the main constraint to tree longevity. The oldest trees were living most of their life in subordinate canopy conditions and/or within primary forests in cool temperate environments and outside major storm tracks. Very old trees are thus characterized by slow growth and often live in forests with harsh site conditions and infrequent disturbance events that kill much of the trees. Temperature inversely controls the expression of longevity in mesophilous species (Fagus spp.), but its role in Quercus spp. is more complex and warrants further research in disturbance ecology. Biological, ecological and historical drivers must be considered to understand the constraints imposed to longevity within different forest landscapes.

Effects of a ‘natural’ flood event on the riparian ecosystem of a regulated large‐river system: the 2011 flood on the Missouri River, USA

AbstractFlow regulation has significantly altered hydrological, geomorphic, and ecological processes on the Missouri River. Cumulative effects are evident in declines in cottonwood (Populus deltoides) recruitment and in altered forest age structure and composition. Record runoff in 2011 exceeded reservoir capacity on the Upper and Middle Missouri, leading to a 500‐year recurrence interval flood in terms of volume, with the highest peak discharge in 59 years and flood durations of up to 3 months. We assessed the effects of this ‘large infrequent disturbance’ by comparing pre‐flood (2006–2009) to post‐flood (2012) changes in riparian forests along two unchannelized segments of the Missouri River. Live shrub and tree density declined sharply within young forest stands (<30 years). Higher proportions of non‐native (Elaeagnus angustifolia) and upland (Juniperus virginiana) trees showed evidence of recent mortality than did the native cottonwood. Sandbar area increased sharply from 2006 to 2012 and area of young forests declined, with particularly sharp declines in sapling stands that had established following the previous post‐dam record flow releases in 1997. Cottonwood recruitment was widespread in 2012, but nearly all seedling patches occurred on sandbars in the active channel rather than on overbank sites, with moderately high (61–77%) seedling mortality over 2012–2013. Physical and operational constraints within the regulated Missouri River limited the restorative effects of the 2011 flood and will likely limit future forest recovery. Process‐based riparian restoration would require restoring flow and sediment regimes that more closely mimic historical conditions, as well as overcoming the physical legacies of decades of flow regulation. Copyright © 2015 John Wiley & Sons, Ltd.