Diversity-disturbance relationships: frequency and intensity interact

An influential ecological theory, the intermediate disturbance hypothesis (IDH), predicts that intermediate levels of disturbance will maximize species diversity. Empirical studies, however, have described a wide variety of diversity-disturbance relationships (DDRs). Using experimental populations of microbes, we show that the form of the DDR depends on an interaction between disturbance frequency and intensity. We find that diversity shows a monotonically increasing, unimodal or flat relationship with disturbance, depending on the values of the disturbance aspects considered. These results confirm recent theoretical predictions, and potentially reconcile the conflicting body of empirical evidence on DDRs.     

Diversity and Disturbances in the Antarctic Megabenthos: Feasible versus Theoretical Disturbance Ranges

The intermediate disturbance hypothesis (IDH) predicts a hump-shaped relationship between regional diversity and the disturbance rate. We tested the IDH for the megabenthos inhabiting the Antarctic sea floor, which is disturbed by iceberg scouring. We used models based on the empirical knowledge of succession to calculate the IDH curve for this system and to extrapolate the presently observable range of the IDH curve to higher and lower disturbance rates. Although the hump-shaped relationship has been found for a purely theoretical (extremely large) disturbance range, within the feasible disturbance range (assumed as realistic in the Antarctic region under climate change), the regional diversity of successional stages due to iceberg scouring strongly decreases with lower disturbance rates but levels off only slowly with higher disturbance rates. The reason is the unevenness in the lifetimes of the successional stages, in that early stages are short-lived whereas late stages are long-lived. With such unevenness, increasing disturbances support the early stages without jeopardizing the later ones. Additionally, we converted this regional diversity of stages to the regional diversity of taxa using a transformation formula based on empirical knowledge of the number and mean abundance of taxa in the particular stages. Our results suggest that a decrease in iceberg scouring due to climate change would be more detrimental to the diversity of the Antarctic megabenthos than an increase.     

Neutral communities may lead to decreasing diversity-disturbance relationships: insights from a generic simulation model

Many attempts have been made to confirm or reject the unimodal relationship between disturbance and diversity stated by the intermediate disturbance hypothesis (IDH). However, the reasons why the predictions of the IDH apply or fail in particular systems are not always obvious. Here, we use a spatially explicit, individual-based community model that simulates species coexistence in a landscape subjected to disturbances to compare diversity-disturbance curves of communities with different coexistence mechanisms: neutrality, trade-off mechanism and intraspecific density dependence. We show that the shape of diversity-disturbance curves differs considerably depending on the type of coexistence mechanism assumed: (1) Neutral communities generally show decreasing diversity-disturbance curves with maximum diversity at zero disturbance rates contradicting the IDH, whereas trade-off communities generally show unimodal relationships confirming the IDH and (2) density-dependent mechanisms do increase the diversity of both neutral and trade-off communities. Finally, we discuss how these mechanisms determine diversity in disturbed landscapes.     

The effect of colonization and competition processes on the relation between disturbance and diversity in plant communities

Many theoretical and field studies have emphasized the impact of disturbance in the dynamics and diversity of sessile organism communities. This view is best reflected by the Intermediate Disturbance Hypothesis (IDH), which states that a maximum of diversity is found in ecosystems or communities experiencing intermediate disturbance regimes or at an intermediate stage of development since the last major disturbance event. Although theoretical models based on competitive interactions tend to validate this hypothesis, a recent meta-analysis of field experiments revealed that the mono-modal relationship between disturbance and diversity might not be a general pattern. In this article, we investigate the relationship between disturbance and diversity through the study of patch models, combining two types of competitive interactions: with or without competitive hierarchy, with two mechanisms influencing colonization: negative frequency dependence in colonization rates and immigration. These combinations led to various disturbance-diversity patterns. In the model without competitive hierarchy (founder effect model), a decreasing relationship appeared to be the rule as mentioned in previous studies. In the model with competitive hierarchy, the IDH pattern was obtained for low frequency dependence and low immigration. Nevertheless, high negative frequency dependence in colonization rates led to a decreasing relationship between disturbance and diversity. In contrast, high immigration led to an increasing relationship. The coexistence window (the range of disturbance intensity allowing coexistence) was the widest for intermediate immigration rates. For random species assemblages, patterns with multiple peaks were also possible. These results highlight the fact that the mono-modal IDH pattern should not be considered a rule. Competition and colonization mechanisms have a profound impact on the relationship between disturbance and diversity.     

Functional traits and climate drive interspecific differences in disturbance-induced tree mortality

With climate change, natural disturbances such as storm or fire are reshuffled, inducing pervasive shifts in forest dynamics. To predict how it will impact forest structure and composition, it is crucial to understand how tree species differ in their sensitivity to disturbances. In this study, we investigated how functional traits and species mean climate affect their sensitivity to disturbances while controlling for tree size and stand structure. With data on 130,594 trees located on 7617 plots that were disturbed by storm, fire, snow, biotic or other disturbances from the French, Spanish, and Finnish National Forest Inventory, we modeled annual mortality probability for 40 European tree species as a function of tree size, dominance status, disturbance type, and intensity. We tested the correlation of our estimated species probability of disturbance mortality with their traits and their mean climate niches. We found that different trait combinations controlled species sensitivity to disturbances. Storm-sensitive species had a high height-dbh ratio, low wood density and high maximum growth, while fire-sensitive species had low bark thickness and high P50. Species from warmer and drier climates, where fires are more frequent, were more resistant to fire. The ranking in disturbance sensitivity between species was overall consistent across disturbance types. Productive conifer species were the most disturbance sensitive, while Mediterranean oaks were the least disturbance sensitive. Our study identified key relations between species functional traits and disturbance sensitivity, that allows more reliable predictions of how changing climate and disturbance regimes will impact future forest structure and species composition at large spatial scales.     

Temporal Changes in Tree Species and Trait Composition in a Cyclone-prone Pacific Dipterocarp Forest

Our understanding of the effects of tropical cyclones on species composition and dynamics of forest communities is mainly derived from studies that have considered single cyclonic events. Here we examined changes in the tree species and functional trait composition in an 8-ha Dipterocarp forest at Palanan in the northeastern Philippines that is subject to a high frequency of cyclonic disturbance (1–4 cyclones annually). The plot has been censused four times over a 16-year interval allowing us to consider the medium-term forest dynamics in response to repeated cyclones. We hypothesized that as the forest community in Palanan has been selected under frequent disturbance by cyclones, it should show little functional change across the census intervals. We analyzed changes in demography, species composition, and community-weighted functional traits (specific leaf area, leaf area, wood density, and specific growth rate) across the censuses and compared these against cyclone intensities during the census intervals. Demographic changes across census years suggest that the community responded to cyclonic disturbances through substantial turnover in the small- and medium-size individuals, and that there has been an increase in plot-level stem density and basal area across the measured period. Trait compositional changes from 1994 to 2010 were mostly small, but indicate a shift towards species with larger leaves and faster growth rates—traits that are associated with fast recovery after disturbance. These changes all coincide with a large intense cyclone between the second and third censuses, suggesting that cyclone strength, more than cyclone frequency, affects this forest.     

Drivers of productivity and its temporal stability in a tropical tree diversity experiment

There is increasing evidence that mixed‐species forests can provide multiple ecosystem services at a higher level than their monospecific counterparts. However, most studies concerning tree diversity and ecosystem functioning relationships use data from forest inventories (under noncontrolled conditions) or from very young plantation experiments. Here, we investigated temporal dynamics of diversity–productivity relationships and diversity–stability relationships in the oldest tropical tree diversity experiment. Sardinilla was established in Panama in 2001, with 22 plots that form a gradient in native tree species richness of one‐, two‐, three‐ and five‐species communities. Using annual data describing tree diameters and heights, we calculated basal area increment as the proxy of tree productivity. We combined tree neighbourhood‐ and community‐level analyses and tested the effects of both species diversity and structural diversity on productivity and its temporal stability. General patterns were consistent across both scales indicating that tree–tree interactions in neighbourhoods drive observed diversity effects. From 2006 to 2016, mean overyielding (higher productivity in mixtures than in monocultures) was 25%–30% in two‐ and three‐species mixtures and 50% in five‐species stands. Tree neighbourhood diversity enhanced community productivity but the effect of species diversity was stronger and increased over time, whereas the effect of structural diversity declined. Temporal stability of community productivity increased with species diversity via two principle mechanisms: asynchronous responses of species to environmental variability and overyielding. Overyielding in mixtures was highest during a strong El Niño‐related drought. Overall, positive diversity–productivity and diversity–stability relationships predominated, with the highest productivity and stability at the highest levels of diversity. These results provide new insights into mixing effects in diverse, tropical plantations and highlight the importance of analyses of temporal dynamics for our understanding of the complex relationships between diversity, productivity and stability. Under climate change, mixed‐species forests may provide both high levels and high stability of production.     

Forecasting intensifying disturbance effects on coral reefs

Anticipating future changes of an ecosystem's dynamics requires knowledge of how its key communities respond to current environmental regimes. The Great Barrier Reef (GBR) is under threat, with rapid changes of its reef‐building hard coral (HC) community structure already evident across broad spatial scales. While several underlying relationships between HC and multiple disturbances have been documented, responses of other benthic communities to disturbances are not well understood. Here we used statistical modelling to explore the effects of broad‐scale climate‐related disturbances on benthic communities to predict their structure under scenarios of increasing disturbance frequency. We parameterized a multivariate model using the composition of benthic communities estimated by 145,000 observations from the northern GBR between 2012 and 2017. During this time, surveyed reefs were variously impacted by two tropical cyclones and two heat stress events that resulted in extensive HC mortality. This unprecedented sequence of disturbances was used to estimate the effects of discrete versus interacting disturbances on the compositional structure of HC, soft corals (SC) and algae. Discrete disturbances increased the prevalence of algae relative to HC while the interaction between cyclones and heat stress was the main driver of the increase in SC relative to algae and HC. Predictions from disturbance scenarios included relative increases in algae versus SC that varied by the frequency and types of disturbance interactions. However, high uncertainty of compositional changes in the presence of several disturbances shows that responses of algae and SC to the decline in HC needs further research. Better understanding of the effects of multiple disturbances on benthic communities as a whole is essential for predicting the future status of coral reefs and managing them in the light of new environmental regimes. The approach we develop here opens new opportunities for reaching this goal.     

Hypertrophic phytoplankton and the Intermediate Disturbance Hypothesis

The provisions of Connell's Intermediate Disturbance Hypothesis (IDH) were investigated in relation to the behaviour of freshwater phytoplankton in a hypertrophic lagoon, paying special attention to the link between species-diversity and environmental disturbances. Phytoplankton diversity yielded different indices depending on the basic unit of calculations (cells, particles, phytoplankton units, biomass). Although their ranges were approximately equal, they did not covary so could not be considered mutually substitutable. For the purpose of IDH testing, biomass diversity was chosen.Equilibrium states were considered to obtain in those periods with a very high fraction of total phytoplankton biomass, shared by no more than three phytoplankton species. Disturbances were considered as counterparts of equilibrium states. Disturbance factors were mostly abiotic, environmental features of the lake operating on the phytoplankton community at different time scales (co-occurring and with 1- and 2-weeks' lags). These scales may relate to the time required to establish phytoplankton community structure.IDH could be suspected not to hold for the phytoplankton of this hypertrophic lake, which experienced seven near-equilibrium phases and six disturbance periods throughout the study. As a rule, equilibrium states lasted longer than disturbance periods. The expected relationships between both disturbance intensity or frequency were not shown. Furthermore, no relationship was demonstrated between diversity (and hence IDH) and the phytoplankton community change rate. Wind stress probably played a minor role in triggering disturbance events. Disturbances were shown partly to promote small-sized phytoplankton communities.Finally, a plea for studying hypertrophic phytoplankton in greater detail is stressed if its responses to disturbances are to be fully understood.     

What is the role of disturbance in catalyzing spatial shifts in forest composition and tree species biomass under climate change?

Mounting evidence suggests that climate change will cause shifts of tree species range and abundance (biomass). Abundance changes under climate change are likely to occur prior to a detectable range shift. Disturbances are expected to directly affect tree species abundance and composition, and could profoundly influence tree species spatial distribution within a geographical region. However, how multiple disturbance regimes will interact with changing climate to alter the spatial distribution of species abundance remains unclear. We simulated such forest demographic processes using a forest landscape succession and disturbance model (LANDIS-II) parameterized with forest inventory data in the northeastern United States. Our study incorporated climate change under a high-emission future and disturbance regimes varying with gradients of intensities and spatial extents. The results suggest that disturbances catalyze changes in tree species abundance and composition under a changing climate, but the effects of disturbances differ by intensity and extent. Moderate disturbances and large extent disturbances have limited effects, while high-intensity disturbances accelerate changes by removing cohorts of mid- and late-successional species, creating opportunities for early-successional species. High-intensity disturbances result in the northern movement of early-successional species and the southern movement of late-successional species abundances. Our study is among the first to systematically investigate how disturbance extent and intensity interact to determine the spatial distribution of changes in species abundance and forest composition.     

Forest response to increasing typhoon activity on the Korean peninsula: evidence from oak tree‐rings

The globally observed trend of changing intensity of tropical cyclones over the past few decades emphasizes the need for a better understanding of the effects of such disturbance events in natural and inhabited areas. On the Korean Peninsula, typhoon intensity has increased over the past 100 years as evidenced by instrumental data recorded from 1904 until present. We examined how the increase in three weather characteristics (maximum hourly and daily precipitation, and maximum wind speed) during the typhoon activity affected old‐growth oak forests. Quercus mongolica is a dominant species in the Korean mountains and the growth releases from 220 individuals from three sites along a latitudinal gradient (33–38°N) of decreasing typhoon activity were studied. Growth releases indicate tree‐stand disturbance and improved light conditions for surviving trees. The trends in release events corresponded to spatiotemporal gradients in maximum wind speed and precipitation. A high positive correlation was found between the maximum values of typhoon characteristics and the proportion of trees showing release. A higher proportion of disturbed trees was found in the middle and southern parts of the Korean peninsula where typhoons are most intense. This shows that the releases are associated with typhoons and also indicates the differential impact of typhoons on the forests. Finally, we present a record of the changing proportion of trees showing release based on tree‐rings for the period 1770–1979. The reconstruction revealed no trend during the period 1770–1879, while the rate of forest disturbances increased rapidly from 1880 to 1979. Our results suggest that if typhoon intensity rises, as is projected by some climatic models, the number of forest disturbance events will increase thus altering the disturbance regime and ecosystem processes.     

Current regeneration patterns at the tree line in the Pyrenees indicate similar recruitment processes irrespective of the past disturbance regime

Aim Impacts of global change, such as land‐use and climate changes, could produce significant alterations in the elevational patterns of alpine tree line ecotones and their adjacent vegetation zones. Because the responses of the tree line to environmental variations are directly related to successful tree regeneration, understanding recruitment dynamics is an indispensable step in tree line research. We aimed to compare potential ecological limitations on recent tree line regeneration in undisturbed and disturbed sites by analysing the demographic structure and spatiotemporal patterns of recruits and large trees. Location Alpine tree line ecotones comprising Pinus uncinata in the Catalan Pyrenees (north‐east Spain) and Andorra. Methods We assessed the demographic structure and spatial pattern of recent recruitment using techniques of point‐pattern and autocorrelation analyses. A total of 3639 P. uncinata individuals were mapped, measured and aged at 12 sites. To evaluate the effects of past disturbances on recent tree line response we compared tree lines that had either been recently affected by human‐induced disturbances or had remained undisturbed for many years. Results The age structure of the tree lines, together with the lack of an age gap between seedlings and saplings, did not indicate recent episodes of high seedling mortality and suggest that recruitment has been frequent under current climate conditions. Seedlings appeared highly aggregated at short distances (up to 3 m), irrespective of disturbance history, and were spatially segregated with respect to large trees. However, we found no evidence of patches of even‐aged seedlings, and our results suggest that dispersal events at intermediate distances (10–17 m) may be frequent. Autocorrelation analyses revealed different patterns of density and age of recruits between disturbed and undisturbed tree lines, but the strength and small‐scale clustering of seedlings and saplings were very similar between sites. Main conclusions We found no recruitment limitation on recent tree line dynamics in the Pyrenees. Furthermore, processes affecting tree recruitment seem to be similar among populations regardless of their past disturbance regime. Our results suggest that constraints on tree line dynamics causing differential responses between sites may operate on older life stages and not upon recruits, and that such constraints may be more contingent on local site conditions than on disturbance history.     

Multiple factors influence plant richness and diversity in the cold and dry boreal forest of southwest Yukon,Canada

The intermediate disturbance hypothesis (IDH) is regarded as an appropriate model for understanding plant diversity–disturbance relationships in the boreal region. However, concern has been raised over its applicability across such a broad region, in particular, in low productive areas of the region such as the cold, dry region of the southwest Yukon. We examined plant richness and diversity in relation to the IDH in the context of both disturbance and productivity gradients. We then tested whether environmental variation and resource quantity interacted with development stage to shape richness patterns in the region. We found that richness and diversity did not conform to the IDH but that environmental variation and resource quantity influenced richness and diversity patterns. Overall richness and diversity patterns were not influenced by development stage though plant functional group patterns were. Richness patterns varied between plant functional groups suggesting that divergent processes shape inter functional group richness within the same stands. Our findings highlight the influence of multiple factors governing patterns of plant richness and diversity in the cold dry boreal forests of the southwest Yukon.     

Species turnover differentiates diversity–disturbance relationships between aboveground vegetation and soil seedbank

An understanding of relationships between species diversity and disturbance gradient is important to comprehend the role of disturbances in the structure of plant communities. Although some studies have demonstrated incongruence in diversity–disturbance relationships (hereafter DDRs) between aboveground vegetation and soil seedbank, the process that causes the difference remains unclear. This incongruence between the two DDRs could result in the decrease in the source of recovery of aboveground vegetation following disturbances being overlooked when only aboveground vegetation is surveyed. Here, we verified a process that species turnover across the disturbance gradient causes the incongruence. Based on a vegetation and seedbank survey, we examined DDRs and species turnover of aboveground vegetation and seedbank along disturbance duration (i.e., excluding years of ungulate grazing). The degree of species turnover was considerably greater in aboveground vegetation than in seedbank; thus, the degree of species turnover along a disturbance gradient caused the difference in DDR between aboveground vegetation and seedbank.

     

Predicting Plant Diversity Response to Disturbance: Applicability of the Intermediate Disturbance Hypothesis and Mass Ratio Hypothesis

Predicting the relationships between disturbance, biodiversity and productivity of ecosystems continue to preoccupy ecologists and resource managers. Two hypotheses underpin many of the discussions. The Intermediate Disturbance Hypothesis (IDH), which proposes that biodiversity peaks at intermediate levels of disturbance, is often extended to predict that productivity follows the same response pattern. The Mass Ratio Hypothesis (MRH) proposes that the biological traits of the dominant species are the critical drivers of ecosystem function (e.g., productivity) and that these species increase in biomass rapidly after disturbance then stabilize. As a consequence, species diversity first peaks then declines after disturbance as a few species dominate the site. Both provide a conceptual link among disturbance, species diversity and productivity (an index of ecosystem function). We assessed the current state of empirical support for these two hypotheses with a literature survey and determined if their conformance is related to ecosystem type or site productivity. Conformance of IDH reported in past reviews (considering all ecosystems) ranged from 16 to 21%. This contrasts with our finding that in terrestrial ecosystems conformance to IDH was 46% (22 of 48 studies), 17% studies reported non-compliance, and 23% reported inconclusive results. Most studies explained their results with respect to IDH or MRH. Only two studies were specifically designed to test the validity of IDH or MRH. We conclude that (i) the IDH is mostly applicable to predict species diversity response to disturbance in upland sites of medium to high productivity and the MRH is applicable to organic sites of low productivity; (ii) there is a critical need for more studies specifically designed to test these hypotheses in natural ecosystems using common protocols; and (iii) enhanced understanding of these models will add value in refining management policies and in the selection of meaningful diversity indicators of sustainability.     

Evaluating the role of fire disturbance in structuring small reptile communities in temperate forests

Fire is an integral disturbance shaping forest community dynamics over large scales. However, understanding the relationship between fire induced habitat disturbance and biodiversity remain equivocal. Ecological theories including the intermediate disturbance hypothesis (IDH) and the habitat accommodation model (HAM) offer predictive frameworks that could explain faunal responses to fire disturbances. We used an 80 year post-fire chronosequence to investigate small reptile community responses to fires in temperate forests across 74 sites. First, we evaluated if changes in species richness, abundance and evenness post-fire followed trends of prior predictions, including the IDH. Second, using competing models of fine scale habitat elements we evaluated the specific ways which fire influenced small reptiles. Third, we evaluated support for the HAM by examining compositional changes of reptile community post-fire. Relative abundance was positively correlated to age post-fire while richness and evenness showed no associations. The abundance trend was as expected based on the prior prediction of sustained population increase post-disturbance, but the trend for richness contradicted the prediction of highest diversity at intermediate levels of disturbance (according to IDH). Abundance changes were driven mainly by changes in overstorey, ground layer, and shelter, while richness and evenness did not associate with any vegetation parameter. Community composition was not strongly correlated to age since fire, thus support for the HAM was weak. Overall, in this ecosystem, frequent fire disturbances can be detrimental to small reptiles. Future studies utilizing approaches based on species traits could enhance our understanding of biodiversity patterns post-disturbance.     

Legacy effects of anthropogenic disturbances modulate dynamics in the world's coral reefs

Rapidly changing conditions alter disturbance patterns, highlighting the need to better understand how the transition from pulse disturbances to more persistent stress will impact ecosystem dynamics. We conducted a global analysis of the impacts of 11 types of disturbances on reef integrity using the rate of change of coral cover as a measure of damage. Then, we evaluated how the magnitude of the damage due to thermal stress, cyclones, and diseases varied among tropical Atlantic and Indo-Pacific reefs and whether the cumulative impact of thermal stress and cyclones was able to modulate the responses of reefs to future events. We found that reef damage largely depends on the condition of a reef before a disturbance, disturbance intensity, and biogeographic region, regardless of the type of disturbance. Changes in coral cover after thermal stress events were largely influenced by the cumulative stress of past disturbances and did not depend on disturbance intensity or initial coral cover, which suggests that an ecological memory is present within coral communities. In contrast, the effect of cyclones (and likely other physical impacts) was primarily modulated by the initial reef condition and did not appear to be influenced by previous impacts. Our findings also underscore that coral reefs can recover if stressful conditions decrease, yet the lack of action to reduce anthropogenic impacts and greenhouse gas emissions continues to trigger reef degradation. We uphold that evidence-based strategies can guide managers to make better decisions to prepare for future disturbances.     

Forest disturbance and recovery in Peruvian Amazonia

Amazonian forests function as biomass and biodiversity reservoirs, contributing to climate change mitigation. While they continuously experience disturbance, the effect that disturbances have on biomass and biodiversity over time has not yet been assessed at a large scale. Here, we evaluate the degree of recent forest disturbance in Peruvian Amazonia and the effects that disturbance, environmental conditions and human use have on biomass and biodiversity in disturbed forests. We integrate tree-level data on aboveground biomass (AGB) and species richness from 1840 forest plots from Peru's National Forest Inventory with remotely sensed monitoring of forest change dynamics, based on disturbances detected from Landsat-derived Normalized Difference Moisture Index time series. Our results show a clear negative effect of disturbance intensity tree species richness. This effect was also observed on AGB and species richness recovery values towards undisturbed levels, as well as on the recovery of species composition towards undisturbed levels. Time since disturbance had a larger effect on AGB than on species richness. While time since disturbance has a positive effect on AGB, unexpectedly we found a small negative effect of time since disturbance on species richness. We estimate that roughly 15% of Peruvian Amazonian forests have experienced disturbance at least once since 1984, and that, following disturbance, have been increasing in AGB at a rate of 4.7 Mg ha⁻¹ year⁻¹ during the first 20 years. Furthermore, the positive effect of surrounding forest cover was evident for both AGB and its recovery towards undisturbed levels, as well as for species richness. There was a negative effect of forest accessibility on the recovery of species composition towards undisturbed levels. Moving forward, we recommend that forest-based climate change mitigation endeavours consider forest disturbance through the integration of forest inventory data with remote sensing methods.     

Forest response to chronic hurricane disturbance in coastal New England

Question: Hurricanes and cyclones cause a wide range of damage to coastal forests worldwide. Most of these storms are not catastrophic in ecological terms, but forest responses to storms of moderate intensities are poorly understood. In regions with a high frequency of moderate hurricanes, how does variation in disturbance intensity affect the magnitude of ecological responses? Location: Naushon Island, Massachusetts, USA. Methods: We use historical records and dendroecological methods to characterize establishment and growth of Fagus grandifolia, Quercus alba, and Quercus velutina in response to seven non‐catastrophic hurricanes of varying intensity, and a major logging event, relative to baseline conditions, over the past 150 years. Our aim was to document variation in the magnitude of responses to known disturbance events of varying intensity, and to determine whether tree growth after moderate hurricanes differs from growth during periods of no disturbance. Results: Forest harvesting in 1824‐1827 had a strong impact on forest composition and growth. Since then, the study region has been characterized by little harvesting but frequent hurricanes. However, only one of the seven storms examined caused substantial increases in growth and new establishment for the dominant species; most moderate disturbances had minimal impact on growth and regeneration dynamics. We also document highly variable responses among species to individual storms, including substantial growth decreases that may not be detected by standard analytical approaches. Conclusions: Our results caution against the use of simple metrics such as wind speed to predict forest response to specific hurricanes, and highlight the importance of individual disturbance events in controlling long‐term forest dynamics, even in regions characterized by high disturbance frequency. In addition, we show that standard approaches to reconstructing disturbance history based on increases in radial growth and pulses of tree establishment are likely to underestimate the frequency of moderate disturbances.     

Issues affecting the measurement of disturbance response patterns in herbaceous vegetation – A test of the intermediate disturbance hypothesis

The 'intermediate disturbance hypothesis' (IDH) predicts maximum species diversity at intermediate levels of disturbance. Disturbance levels are measured by intensity or frequency of disturbance or by the time since disturbance. The IDH has been tested over a wide range of studies and communities with results either accepting or rejecting the hypothesis. The objectives of this study were to determine how observed disturbance response patterns for the same herbaceous plant community are influenced by modelling techniques, particularly in relation to the expression of disturbance gradients, choice of species diversity indices, and time of assessment since disturbance. Response patterns were examined using a multi-factorial disturbance experiment involving grazing (absent and present); fire (absent and present); soil cultivation (none, 5 and 20 cm); and amendment (none, fertiliser, and fertiliser plus clover seeds). Generalized linear models with a b-spline function were used to define response patterns for five different disturbance gradients over a 24-month period using three indices of diversity. Seven basic disturbance response models were recognized ranging from the classical IDH bell-shaped pattern through increasing or decreasing trends, to a no-change model with increasing levels of disturbance. Only 6.7% of all models were consistent with the IDH. The no-change model was found in nearly half the cases investigated and increased in occurrence with time since disturbance. The choice of the disturbance intensity gradient, the species diversity index used and the time of assessment after disturbance significantly influenced the frequency of occurrence of the disturbance response models observed. Consequently, the responses of vegetation to disturbance gradients show many patterns depending on how they are defined and modelled rather than the simple bell-shaped curve as predicted by the IDH, but the ecological mechanisms supporting some of these patterns need to be further investigated.