Catastrophic Disturbance Research Articles

PLANNING FOR PERSISTENCE IN MARINE RESERVES: A QUESTION OF CATASTROPHIC IMPORTANCE

Large-scale catastrophic events, although rare, lie generally beyond the control of local management and can prevent marine reserves from achieving biodiversity outcomes. We formulate a new conservation planning problem that aims to minimize the probability of missing conservation targets as a result of catastrophic events. To illustrate this approach we formulate and solve the problem of minimizing the impact of large-scale coral bleaching events on a reserve system for the Great Barrier Reef, Australia. We show that by considering the threat of catastrophic events as part of the reserve design problem it is possible to substantially improve the likely persistence of conservation features within reserve networks for a negligible increase in cost. In the case of the Great Barrier Reef, a 2% increase in overall reserve cost was enough to improve the long-run performance of our reserve network by >60%. Our results also demonstrate that simply aiming to protect the reefs at lowest risk of catastrophic bleaching does not necessarily lead to the best conservation outcomes, and enormous gains in overall persistence can be made by removing the requirement to represent all bioregions in the reserve network. We provide an explicit and well-defined method that allows the probability of catastrophic disturbances to be included in the site selection problem without creating additional conservation targets or imposing arbitrary presence/absence thresholds on existing data. This research has implications for reserve design in a changing climate.

Habitat disturbances differentially affect individual growth rates in a long‐lived turtle

AbstractDisturbances often help structure ecological communities, and their impacts may have consequences on population dynamics and long‐term species persistence. Should disturbances affect resources, a trade‐off may result between reproduction and individual growth, which in turn could affect the timing of sexual maturity in animals dependent on reaching a requisite size for the onset of maturation. We used a 14‐year mark–recapture dataset to determine the effects of catastrophic storms and the removal of nonindigenous vegetation on individual growth rates of a long‐lived turtle Terrapene carolina bauri. Adult male growth rates increased 19% after the disturbances, whereas female growth rates decreased by a similar percentage. Juvenile growth rates briefly increased, but as these animals became subadults, their growth rates slowed after disturbance, a change more pronounced in females than males. After the disturbances, the onset of male sexual maturity decreased by c. 1 years (from 10.8 to 9.5 years), female maturity was delayed by 2.5 years (from 8.5 to 11.0 years), and the subadult life stage was extended from 2 to 3.5 years. With resources and habitats similarly available to adults, we hypothesize that adult females diverted resources from growth to reproduction, whereas males allocated available resources to growth. Subadult growth rates decreased regardless of sex, implying that fewer or lower quality prey were available to smaller sized turtles in the years following disturbance. The results of this study suggest that habitat disturbances affected individual growth rates differently according to sex and life stage, which in turn affected the timing of maturation. In long‐lived vertebrates, such perturbations on life‐history traits such as growth rates likely affect population recovery, and may help to explain why turtle populations recover slowly following catastrophic disturbance, even when adult survivability is high.

Post-hurricane vegetation recovery in an urban forest

Urban forests are increasingly vulnerable to catastrophic disturbance, and their isolation and human use may challenge the ability of vegetation to recover spontaneously. We examined vegetation responses to recent hurricane disturbance in a temperate mixedwood urban forest: Point Pleasant Park in Halifax, Nova Scotia, which suffered over 70% canopy loss during Hurricane Juan in fall 2003. In 2005 we surveyed 30 paired plots with disturbed and intact tree canopies to assess early regeneration patterns and seed banks. Native early successional tree species dominated seed bank and seedling layers. Soil properties were similar between intact and disturbed urban plots and local reference forests, thus long-term woody debris removal, hurricane disturbance and subsequent clean-up activities have not caused substantial soil degradation. Non-native species were not abundant throughout the park but were concentrated at the park boundary adjacent to residential neighbourhoods. The results of this study suggest that urban forests can show natural successional trajectories after catastrophic disturbance, and management is probably not necessary for forest recovery in Point Pleasant Park. Conversely, intervention to speed up regeneration of shade-tolerant canopy species may be desired by local citizens, so managers will have to balance conflicting values in developing a restoration plan for the park.

Riparian areas in the Canadian boreal forest and linkages with water quality in streams

Riparian areas in the Canadian boreal forest represent the transition zone between terrestrial and aquatic ecosystems. We review factors that influence riparian vegetation communities and the associated interactions with boreal streams. Regional and local drivers (e.g., climate, edaphic factors, and natural disturbances) that affect upland boreal vegetation also operate in riparian areas. However, the proximity of riparian areas to the stream channel not only modifies some of these drivers, but it means that the stream itself becomes a driver of riparian vegetation dynamics. For example, hydrological disturbances like flooding and ice scour affect soil texture and alter successional pathways, sometimes completely denuding streambanks of vegetation. Even in riparian areas unaffected by such catastrophic disturbances, saturated soil conditions can influence riparian forest composition and nutrient cycling. Saturated soils support lower mineralization rates, therefore organic layers store relatively more carbon and nitrogen than adjoining upland soils, and primary productivity is generally lower. Saturated soils also have implications for the ability of the riparian area to “buffer” streams from inputs of water and nutrients. For example, reducing conditions in riparian soils could be the sites for nitrate removal from groundwater by denitrification. Spatial variation in weather, soils, vegetation cover, slope, accumulation of organic matter, geographic location, and relief undoubtedly add to the complexity of understanding the role of riparian systems in Canada’s vast boreal forest. However, the opportunity to identify patterns relating to riparian areas will assist in our understanding and management of these multifaceted systems.

Resilience of coral communities on an isolated system of reefs following catastrophic mass-bleaching

As a result of climate change, sea-water temperatures around the world are expected to increase, potentially causing more frequent and severe episodes of coral bleaching. In this study, the impact of elevated water temperatures at an isolated system of reefs was assessed by quantifying the changes in benthic communities over almost 10 years. Mass-coral bleaching in 1998 dramatically altered the community structure of the reefs, including a >80% relative decrease in the cover of hard and soft corals and a twofold increase in the cover of algae, but which did not include macroalgae. The magnitude of the impact varied among the different sites according to their initial cover and community structure, largely due to the differing susceptibilities of the dominant groups of hard corals. Subsequent increase in the cover of these groups varied according to their life history traits, such as modes of reproduction and rates of growth. Additionally, the increase in cover was strongly correlated with the magnitude of the impact at the different sites, suggesting that recovery was driven by processes acting over local scales. Six years after the bleaching, the hard corals had returned to approximately 40% of their pre-bleaching cover, but there was little change in the cover of soft corals, and the structure of most hard coral communities remained very different to that prior to the bleaching. These data provides insights into the degree to which coral communities are resilient to catastrophic disturbances, when they are isolated from other reef systems but not exposed to some of the chronic stressors affecting many reefs around the world.

The response of plant diversity to ecosystem retrogression: evidence from contrasting long‐term chronosequences

Following catastrophic disturbances, succession and vegetation development occur, but in the prolonged absence of these disturbances a decline (retrogressive) phase follows in which nutrient availability and tree biomass declines considerably. We measured plant diversity across six long‐term chronosequences that each included retrogressive stages in Australia, New Zealand, Alaska, Hawaii and Sweden. In contrast to theories predicting negative or hump‐shaped responses of tree diversity to biomass or soil fertility, tree species richness often peaked coincidentally with tree basal area (a surrogate of tree biomass), and declined during retrogression. Similar patterns were found regardless of whether or not species richness estimates were rarefraction‐adjusted to correct for variation in stem densities across plots. The Shannon‐Weiner diversity index sometimes showed the same pattern, but in two chronosequences was least when tree basal area peaked; this was driven by the domination of total basal area by single tree species in both cases. The decline in tree diversity during retrogression was often associated with reduced relative amounts of total phosphorus in soil. In contrast, total vascular plant species richness often increased during retrogression. These results demonstrate that forests with high tree diversity and biomass do not persist indefinitely in the long‐term absence of catastrophic disturbance, and that similar patterns occur across the boreal, temperate and subtropical zones.

FOREST STRUCTURE AND LIGHT REGIMES FOLLOWING MODERATE WIND STORMS: IMPLICATIONS FOR MULTI-COHORT MANAGEMENT

Moderate-severity disturbances appear to be common throughout much of North America, but they have received relatively little detailed study compared to catastrophic disturbances and small gap dynamics. In this study, we examined the immediate impact of moderate-intensity wind storms on stand structure, opening sizes, and light regimes in three hemlock-hardwood forests of northeastern Wisconsin. These were compared to three stands managed by single-tree and group selection, the predominant forest management system for northern hardwoods in the region. Wind storms removed an average of 41% of the stand basal area, compared to 27% removed by uneven-aged harvests, but both disturbances removed trees from a wide range of size classes. The removal of nearly half of the large trees by wind in two old-growth stands caused partial retrogression to mature forest structure, which has been hypothesized to be a major disturbance pathway in the region. Wind storms resulted in residual stand conditions that were much more heterogeneous than in managed stands. Gap sizes ranged from less than 10 m2 up to 5000 m2 in wind-disturbed stands, whereas the largest opening observed in managed stands was only 200 m2. Wind-disturbed stands had, on average, double the available solar radiation at the forest floor compared to managed stands. Solar radiation levels were also more heterogeneous in wind-disturbed stands, with six times more variability at small scales (0.1225 ha) and 15 times more variability at the whole-stand level. Modification of uneven-aged management regimes to include occasional harvests of variable intensity and spatial pattern may help avoid the decline in species diversity that tends to occur after many decades of conventional uneven-aged management. At the same time, a multi-cohort system with these properties would retain a high degree of average crown cover, promote structural heterogeneity typical of old-growth forests, and maintain dominance by late-successional species.

Framework for Assessing Viability of Threatened and Endangered Chinook Salmon and Steelhead in the Sacramento-San Joaquin Basin

Protected evolutionarily significant units (ESUs) of salmonids require objective and measurable criteria for guiding their recovery. In this report, we develop a method for assessing population viability and two ways to integrate these population-level assessments into an assessment of ESU viability. Population viability is assessed with quantitative extinction models or criteria relating to population size, population growth rate, the occurrence of catastrophic declines, and the degree of hatchery influence. ESU viability is assessed by examining the number and distribution of viable populations across the landscape and their proximity to sources of catastrophic disturbance.Central Valley spring-run and winter-run Chinook salmon ESUs are not currently viable, according to the criteria-based assessment. In both ESUs, extant populations may be at low risk of extinction, but these populations represent a small portion of the historical ESUs, and are vulnerable to catastrophic disturbance. The winter-run Chinook salmon ESU, in the extreme case, is represented by a single population that spawns outside of its historical spawning range. We are unable to assess the status of the Central Valley steelhead ESU with our framework because almost all of its roughly 80 populations are classified as data deficient. The few exceptions are those populations with a closely associated hatchery, and the naturally-spawning fish in these streams are at high risk of extinction. Population monitoring in this ESU is urgently needed.Global and regional climate change poses an additional risk to the survival of salmonids in the Central Valley. A literature review suggests that by 2100, mean summer temperatures in the Central Valley region may increase by 2-8°C, precipitation will likely shift to more rain and less snow, with significant declines in total precipitation possible, and hydrographs will likely change, especially the the southern Sierra Nevada mountains. Warming at the lower end of the predicted range may allow spring-run Chinook salmon to persist in some streams, while making some currently utilized habitat inhospitable. At the upper end of the range of predicted warming, very little spring-run Chinook salmon habitat is expected to remain suitable.In spite of the precarious position of Central Valley salmonid ESUs, there are prospects for greatly improving their viability. Recovering Central Valley ESUs may require re-establishing populations where historical populations have been extirpated (e.g., upstream of major dams). Such major efforts should be focused on those watersheds that offer the best possibility of providing suitable habitat in a warmer future.

Temporal and spatial variation in an Australian tropical rainforest

Abstract This study describes the floristics and structure of a 0.95‐ha lowland tropical rainforest plot at the Australian Canopy Crane Research Facility at Cape Tribulation, Queensland. Five years of post‐cyclonic change in forest floristics and structure following the passage of Tropical Cyclone ‘Rona’ in February 1999 are examined. Local and regional variation in tropical rainforest is examined in comparison with other lowland plots established nearby and mid‐elevation plots located elsewhere in north Queensland at Eungella, Paluma and the Atherton Tablelands. These plots are placed in a broader Australasian context along with lowland rainforest plots at Baitabag and Oomsis, Papua New Guinea. The 2005 survey found 680 stems of 82 species ≥10 cm d.b.h. on the crane plot, an increase of 30.3% in stems and 16.4% of species in the 5 years since the previous survey. The most abundant families were Meliaceae, Euphorbiaceae, Lauraceae, Myrtaceae and Apocynaceae and the most abundant species were Cleistanthus myrianthus, Alstonia scholaris, Myristica insipida, Normanbya normanbyi and Rockinghamia angustifolia. Temporal floristic and structural variation suggests that the crane site remains in an active stage of post‐cyclonic recovery. Local spatial variability in floristics and structure at Cape Tribulation exceeded the variation exhibited by a single plot over a period of 5 years, despite the impact of Cyclone Rona. This finding suggests a high degree of temporal stability within this stand of rainforest despite frequent catastrophic disturbances. The rainforests of Cape Tribulation constitute a relatively unique floristic community when observed in an Australasian context. Variation in rainforest community composition across the region shows the importance of biogeographical connections, the impacts of local topography, environmental conditions and disturbance history.

Impacts of a volcanic eruption on the forest bird community of Montserrat, Lesser Antilles

Volcanic eruptions are an important and natural source of catastrophic disturbance to ecological communities. However, opportunities to study them are relatively rare. Here we report on the effects of the eruption of the Soufrière Hills volcano on the forest bird community of the Lesser Antillean island of Montserrat. The island's species‐poor avifauna includes 11 restricted‐range species, including the Critically Endangered endemic Montserrat Oriole Icterus oberi. Analysis of monitoring data from 1997 to 2005 indicates that counts of most species were substantially lower following major ashfalls. However, this effect was short‐lived, with rapid population recovery in subsequent years. Furthermore, levels of seasonal rainfall appear to have been at least as important in determining population trends as ashfall. Overall, most species were at least as abundant at the end of the study as at the start, and no forest bird species have been extirpated from Montserrat. We discuss potential ecological drivers of ashfall impacts on populations: there is some evidence that terrestrial foragers were most severely affected.

Tsunami Impacts on Biodiversity of Seagrass Communities in the Andaman Sea, Thailand: (1) Seagrass Abundance and Diversity

Evaluation of the impact of a catastrophic disturbance on biodiversity is often difficult due to a lack of sufficient quantitative data on biological communities prior to the disturbance. Since 2001, we have been monitoring the abundance and biomass of seagrass and its associated animal community along the Andaman Sea coast of Thailand, an area that was hit by a tsunami originating in the Indian Ocean on Dec. 26, 2004. To quantitatively evaluate the effects of the disturbance on the biodiversity of the seagrass community we carried out a comparative analysis of seagrass species diversity and abundance based on pre-/post-disturbance data at several seagrass beds at Kuraburi, northern Phang-nga (most-affected region) and Trang (less-affected region). Species diversity, coverage and biomass of seagrass declined greatly after the tsunami at one seagrass bed (Thung Nang Dam) in Kuraburi. The effect of the tsunami was less obvious at two other beds a few kilometers away from Thung Nang Dam, and at the seagrass beds in Haad Chao Mai National Park in Trang (located ca. 200km south of Kuraburi), where temporal change before and after the tsunami was the least obvious. A steady decline in biomass was observed in the three seagrass beds that had not been severely affected by the tsunami, possibly associated with other types of disturbance such as river discharge. Analyses revealed that the effect of the tsunami on seagrass ecosystems was highly variable even on small spatial scales, and that other factors causing disturbance to the seagrass beds are important factors of the observed temporal variation.

Effects of microenvironmental heterogeneity on the seed‐to‐seedling process and tree coexistence in a riparian forest

AbstractIn a temperate riparian forest, the effects of substrate types, canopy gaps and conspecific seedfall density were investigated on the seed‐to‐seedling process for the five dominant species (Aesculus turbinata, Fagus crenata, Acer mono, Pterocarya rhoifolia and Cercidiphyllum japonicum). Densities of seedfall and subsequent seedling recruits were measured in the stand over a period of 6 years. A model assuming that local density of seedling recruits is proportional to seedfall density in the preceding year significantly explained a spatial variation in seedling recruits for all species. Several environmental factors were then added. Substrate composition had a positive effect on P. rhoifolia and C. japonicum. P. rhoifolia was favored by gravel substrate, which also explained the adult distribution of this species in this forest. C. japonicum appeared to be facilitated by a mineral‐soil substrate. However, the distributions of this substrate and adults of C. japonicum did not follow each other closely. A. mono was negatively affected by gaps, and F. crenata was negatively affected by conspecific seedfall density. In contrast, A. turbinata was not significantly affected by any of the environmental factors tested. The microenvironmental heterogeneity in this forest explained species coexistence to a limited extent in the context of seed‐to‐seedling processes. Performances at later stages of the life‐cycle and/or catastrophic disturbances (e.g. landslides) might have a stronger influence on species coexistence in this forest.

Regional processes in mangrove ecosystems: spatial scaling relationships, biomass, and turnover rates following catastrophic disturbance

Physiological processes and local-scale structural dynamics of mangroves are relatively well studied. Regional-scale processes, however, are not as well understood. Here we provide long-term data on trends in structure and forest turnover at a large scale, following hurricane damage in mangrove ecosystems of South Florida, U.S.A. Twelve mangrove vegetation plots were monitored at periodic intervals, between October 1992 and March 2005. Mangrove forests of this region are defined by a −1.5 scaling relationship between mean stem diameter and stem density, mirroring self-thinning theory for mono-specific stands. This relationship is reflected in tree size frequency scaling exponents which, through time, have exhibited trends toward a community average that is indicative of full spatial resource utilization. These trends, together with an asymptotic standing biomass accumulation, indicate that coastal mangrove ecosystems do adhere to size-structured organizing principles as described for upland tree communities. Regenerative dynamics are different between areas inside and outside of the primary wind-path of Hurricane Andrew which occurred in 1992. Forest dynamic turnover rates, however, are steady through time. This suggests that ecological, more-so than structural factors, control forest productivity. In agreement, the relative mean rate of biomass growth exhibits an inverse relationship with the seasonal range of porewater salinities. The ecosystem average in forest scaling relationships may provide a useful investigative tool of mangrove community biomass relationships, as well as offer a robust indicator of general ecosystem health for use in mangrove forest ecosystem management and restoration.

Calling Death's Bluff

However, doctors do have an earlier chance to intervene. By implanting a specialized device into a person's chest, they can equip the heart to recover instantly when death comes knocking. When the electrical leads of these so-called implantable cardioverter-defibrillators, or ICDs, detect an abnormal heart the attached pager-size generator delivers a shock that restores the normal cadence. Use of these devices began in the 1980s and has increased exponentially in recent years. This year, U.S. doctors will surgically implant some 200,000 defibrillators at a cost of about $30,000 to $50,000 per operation. Yet most of the devices will never see action. About four-fifths of defibrillators sit quietly in people's chests until their batteries conk out. Replacing a battery, which is necessary every 5 years or so, requires another round of surgery. Defibrillators also can be cantankerous. They sometimes mistake mild arrhythmia, or rhythm disruption, for a deadly spasm and shock the heart unnecessarily, causing pain but no permanent harm. And while some people die because a device fails to fire when it should-a problem that spawned massive recalls of certain models last year-many more perish because no one recognized that a defibrillator could have saved their lives. would like to improve their ability to distinguish people who truly need defibrillators from those who don't. current method uses a crude measure: the heart's mechanical efficiency at pumping blood. That value, called ejection fraction, has no direct relationship with the heart's electrical circuitry. Recent studies suggest that measures of the heart's electrical function and its responsiveness to the nervous system may reveal additional information about the heart's vulnerability to arrhythmia. Medical researchers recently concluded the first systematic test in which they weighed one such promising factor-an electrical characteristic called T-wave alternans-for deciding who should a defibrillator. They plan to announce their results at a medical meeting this November. If such trials prove that T-wave altemans and other new measures can supplement ejection fraction in clinical evaluation of patients, doctors may save more lives even though they implant fewer devices than they do now. HIGH PRICE AT THE PUMP With each contraction of the the organ's left ventricle squeezes blood into the body's arteries. A healthy chamber pumps out half to three-quarters of its contents, while a defective heart might expel 30 percent or less. This ejection fraction, expressed on a 100-point scale, can be gauged painlessly by performing a sonogram of the chest. An insult such as a myocardial infarction-commonly called a heart attack-causes muscle damage that can instantly trigger fatal arrhythmia, or it can permanently reduce ejection fraction. same muscle damage can also interfere with the conduction of electrical impulses through the thereby increasing the chance that a subsequent life-threatening arrhythmia will occur. Ejection fraction has been studied extensively and is clearly associated closely with sudden death, says cardiac electrophysiologist David S. Rosenbaum of Case Western Reserve University's MetroHealth Campus in Cleveland. form of arrhythmia that usually triggers cardiac arrest is called ventricular fibrillation. In that event, uncoordinated muscle contractions cause the ventricles to flutter ineffectuallv. malfunction develops when electrical impulses that control the pace and rhythm of the ventricles' contractions become irregular. The lethal event is electrical-a catastrophic disturbance in rhythm, says Richard Verrier, a cardiovascular electrophysiologist at Beth Israel Deaconess Medical Center in Boston. That's precisely the problem that defibrillators are designed to solve. People with an ejection fraction lower than 30 have been shown to benefit from defibrillators, so physicians currently use thatvalue as athrueshold when deciding who should a device. With that cut-off and other criteria, such as previous ventricular fibrillation or advanced heart failure, about 1.6 million people in the United States are eligible, according to Minneapolis-based device maker Medtronic. Yet ejection fraction reveals limited information about the heart's electrical integrity. You can have a lethal disturbance without having any [preexisting physical] damage to the heart, Verrier notes. Or, the organ can have damage that never leads to an arrhythtia. You're using a measure of mechanical abnormality to predict an electrical event, so you can't expect it to be overwhelmingly accurate; says Rosenbaum. Doctors have to implant 17 defibrillators to save one person. benefit is real, but it's modest and has to be weighed against the risk.' Hazards of the surgery include infections, clots, and internal bleeding. And when devices fire unnecessarily, patients get a flurry of shocks, says cardiologistJ. Thomas Bigger Jr. of Columbia University's New York-Presbyterian Hospital. The lethal event is electrical-a catastrophic disturbance in rhythm.

Species resistance and community response to wind disturbance regimes in northern temperate forests

Summary Severe winds are the predominant cause of natural disturbance in temperate forests of north‐eastern and north‐central North America. Conceptual models of the effects of wind disturbance have traditionally focused on the impacts of catastrophic disturbances and have painted a simple picture of how disturbance acts to maintain tree species diversity. These models ignore variation among species characteristics that could have important consequences for both resistance to and recovery from disturbance. We integrated an empirically parameterized, mechanistic model of windstorm mortality (WINDSTORM) and a seed‐mass‐based dispersal and recruitment model into a spatially explicit, individual tree model of forest dynamics (SORTIE) in order to create simulated long‐term ‘experiments’ designed to explore the sensitivity of forest composition and structure to species‐specific resistance to and recovery from disturbance. We found that species‐specific variation in resistance to wind mortality interacted strongly with: (i) shade tolerance characteristics, (ii) the medium‐term history of disturbances, (iii) the long‐term average severity of the disturbance regime and (iv) seedbed substrate dynamics to influence tree population dynamics and successional trajectories. We also examined how local and long‐distance dispersal affect response to wind disturbance. Ignoring differences among local dispersal characteristics overestimates the importance of dispersal‐limited species. Our results show that long‐distance immigration maintains species coexistence only if the immigration rate is very high relative to local dispersal. Despite this, stand‐scale models that ignore long‐distance dispersal can underestimate population dynamics of dispersal‐limited species. Our results indicate that landscape‐scale heterogeneity in structure and species composition in these forests is facilitated by synergisms between the stochastic nature of wind disturbance and complex interactions between species traits that govern: (i) resistance to disturbance, (ii) local dispersal and seedling establishment and (iii) competitive (i.e. growth/survival) traits that do not adhere to strict tradeoffs. We conclude that wind disturbance has much more complex and variable effects on long‐term dynamics of forest structure and composition than suggested by previous models. In particular, non‐catastrophic disturbances can generate important variation in forest dynamics that can either lead to dominance by a single or small number of species, or facilitate species coexistence.

After the Catastrophe

ECOLOGY/EVOLUTION The study of recolonization and succession after catastrophic disturbance can offer insights into the rules governing the assembly of ecological communities and how species interact during colonization and invasion, as well as the speed and trajectory of recovery. Catastrophes—and responses thereto—come in many forms. Planes et al. followed the recovery of coral reef fish assemblages after a thoroughly unnatural catastrophe: the underground nuclear tests carried out at Mururoa atoll in the Pacific between 1976 and 1995. Typically, the pressure wave from each test caused the instant death of all fish within 2000 m of the test site, while leaving the reef structure unchanged. Even so, the fish diversity and abundance that are characteristic of undamaged reef were restored within 1 to 5 years by immigration and recruitment from neighboring areas, suggesting that reef structure is a vital factor in community assembly. In contrast, Pitman et al. document a very slow recovery after a catastrophic flood that probably took place in an Ecuadorian tropical rain forest five centuries ago; tree species number has yet to recover to half that of neighboring unaffected areas, and there is a greater abundance of light-demanding early-successional species. — AMS Ecology 86 , 2578 (2005); J. Trop. Ecol. 21 , 559 (2005).

The Biogeochemistry of Carbon at Hubbard Brook

The Biogeochemistry of Carbon at Hubbard Brook

DEVELOPMENT OF TREE SIZE DISTRIBUTIONS IN DOUGLAS‐FIR FORESTS UNDER DIFFERING DISTURBANCE REGIMES

Two conceptual models describe the successional and structural development of mature and transitional stands toward old‐growth Pseudotsuga menziesii (Mirbel) Franco forests in the Pacific Northwest. The “catastrophic/maturational” model is based on infrequent catastrophic disturbances that initiate the successional cycle, while the “chronic/ partial fire” model is based on low‐ to moderate‐severity fires which, over long periods of time, can create old‐growth conditions through multiple disturbances. Although both models can produce similar structural characteristics, quantitative differences in speed and pathways of structural development with respect to these models are not well understood. To elucidate these quantitative differences, diameter distributions of live trees were investigated in a chronosequence of mature, transitional, and old‐growth stands subjected to these two disturbance models. Stands characterized by the “catastrophic/maturational” model exhibited varying degrees of rotated sigmoid distributions up to 250–300 years of age because of long establishment periods and differential growth and mortality rates of the component species. They also showed a modest trend toward reverse‐J distributions with increasing stand age. Stands characterized by the “chronic/partial fire” model had a stratum of distinctly older emergent trees with younger cohorts of Pseudotsuga menziesii, Tsuga heterophylla (Raf.) Sarg., and Thuja plicata Donn. and exhibited reverse‐J distributions within about 150–200 years after disturbance. These results suggest that reconsideration of current definitions of what constitutes old‐growth on Tsuga sites may be necessary. A conceptual model is presented, which links pathways of structural development to disturbance regimes. This model indicates that active management can hasten the development of old‐growth structure, even in some older forests.

DISTURBANCE HISTORY AND HISTORICAL STAND DYNAMICS OF A SEASONAL TROPICAL FOREST IN WESTERN THAILAND

Disturbances influence forest dynamics across a range of spatial and temporal scales. In tropical forests most studies have focused on disturbances occurring at small spatial and temporal scales (i.e., gap dynamics). This is primarily due to the difficulty of reconstructing long‐term disturbance histories of forests in which most tree species lack annual growth rings. Consequently, the role of past disturbances in tropical forests is poorly understood. We used a combination of direct and indirect methods to reconstruct the historical disturbance regime and stand development patterns in mature and regenerating seasonal dry evergreen forest (SDEF) in the Huai Kha Khaeng Wildlife Sanctuary in western Thailand. Direct estimates of long‐term establishment and growth patterns were obtained from 12 tree species that form annual growth rings as a consequence of the region's strong intra‐annual rainfall seasonality. Indirect estimates of establishment patterns were obtained from analyses of stand structure and individual tree architecture and application of age‐estimation models to 10 dominant canopy‐tree species using demographic data from a large‐scale, permanent forest‐dynamics plot.The combination of direct and indirect methodologies revealed a complex disturbance history in the seasonal evergreen forest over the past 250 years. In the mid‐1800s, 200–300 ha of forest were destroyed by a catastrophic disturbance, which led to the synchronous establishment of many of the trees that presently dominate the forest canopy. Since then widespread disturbances of variable intensity have occurred at least three times (1910s, 1940s, and 1960s). These disturbances created discrete temporal pulses of establishment in small to large gaps in the forest matrix across several square kilometers. Background mortality and gap formation were evident in every decade since 1790, but these varied in intensity and frequency.The SDEF retains a distinct structural and floristic legacy from the catastrophic disturbance of the mid‐1800s. The single‐age cohort that established after the disturbance has developed a complex three‐dimensional structure as a consequence of differences in interspecific growth patterns of the canopy‐tree species and subsequent disturbances of moderate and low intensity. While no single methodological approach provided a complete picture of the disturbance history and stand development patterns of the seasonal evergreen forest, taken together they offered new insights into the long‐term dynamics of a primary tropical forest. In particular, the study highlighted the role of disturbance at multiple spatial and temporal scales and varying intensities in determining the structure and composition of a complex, species‐rich tropical forest and raises important questions about the role of rare, catastrophic events on tropical forest dynamics.

Changes in soil microbial and nematode communities during ecosystem decline across a long-term chronosequence

Following the creation of new land surfaces, there is an initial build-up phase, but in the prolonged absence of catastrophic disturbance an ecosystem decline phase has often been observed. While a number of studies have investigated the changes in soil biota that occur during the build-up phase, few studies have investigated how the soil food web changes during the ecosystem decline phase, even though such studies may assist our understanding of biotic factors that contribute to long-term ecosystem changes. We investigated the response of soil microbial and nematode communities to ecosystem decline by studying each of four stages of a long-term (280,000 year) forested chronosequence caused by uplift of marine terraces in the Waitutu region of Fiordland National Park, New Zealand. With increasing chronosequence age there were large increases in ratios of C to N, C to P, and N to P in both the organic layer and mineral soil layer, indicative of greater nutrient (notably P) limitation over time. Variables related to soil microbial biomass and activity were lower on the older terraces when expressed on a per unit soil C basis, reflecting that the quality of the soil organic matter, which is the resource that supports microbial metabolism, declined over time. This in turn had important consequences for population densities of soil nematodes and enchytraeids, including both microbe-feeding and predatory groups. There were significant increases in the fungal:bacterial biomass ratio and in the fungal-feeding:bacterial-feeding nematode densities. Taken collectively, our results suggest a decline in soil microbial activity and soil fauna, and an increase in the relative importance of the fungal-based (vs. bacterial-based) energy channel during long-term ecosystem development on terraces of marine origin. This corroborates the hypothesis that the studied sites represent a retrogressive shift in soil organic matter quality over a long-term chronosequence.