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Spatially discontinuous relationships between salt marsh invasion and mangrove forest fragmentation

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Abstract

Rapid and large-scale biological invasion results in widespread biodiversity loss and degradation of essential ecosystem services, especially in mangrove forests. Recent evidence suggests that the establishment and dispersal of invasive species may be exacerbated in a fragmented landscape, but the influence of mangrove fragmentation on coastal biological invasion at the landscape scale remains largely unknown. Here, using a derived 10-m-resolution coastal wetland map of the southeastern coast of China, we examine the relationship between the fragmentation of mangrove forests and the magnitude of salt marsh invasion and quantify the geographical variation in this relationship across a climatic gradient. Our results show that mangrove forests with small sizes, large edge proportions, and regular boundary shapes tend to suffer more serious salt marsh invasions than mangrove forests with large sizes, small edges and irregular boundary shapes, indicating a positive correlation between mangrove fragmentation and the magnitude of invasion. In particular, these fragmentation-invasion relationships are shown to be more intensive in the subtropics than in the tropics. Our findings provide the first spatially explicit evidence of the relationships between mangrove fragmentation and biological invasion on a landscape scale and highlight an urgent need for conservation and management actions to improve mangrove connectivity, which will increase the resistance to invasions, especially for small subtropical mangrove forests.

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Within the context of climate change, there is a pressing need to better understand the ecological implications of changes in the frequency and intensity of climate extremes. Along subtropical coasts, less frequent and warmer freeze events are expected to permit freeze-sensitive mangrove forests to expand poleward and displace freeze-tolerant salt marshes. Here, our aim was to better understand the drivers of poleward mangrove migration by quantifying spatiotemporal patterns in mangrove range expansion and contraction across land-ocean temperature gradients. Our work was conducted in a freeze-sensitive mangrove-marsh transition zone that spans a land-ocean temperature gradient in one of the world's most wetland-rich regions (Mississippi River Deltaic Plain; Louisiana, USA). We used historical air temperature data (1893-2014), alternative future climate scenarios, and coastal wetland coverage data (1978-2011) to investigate spatiotemporal fluctuations and climate-wetland linkages. Our analyses indicate that changes in mangrove coverage have been controlled primarily by extreme freeze events (i.e., air temperatures below a threshold zone of -6.3 to -7.6 °C). We expect that in the past 121 years, mangrove range expansion and contraction has occurred across land-ocean temperature gradients. Mangrove resistance, resilience, and dominance were all highest in areas closer to the ocean where temperature extremes were buffered by large expanses of water and saturated soil. Under climate change, these areas will likely serve as local hotspots for mangrove dispersal, growth, range expansion, and displacement of salt marsh. Collectively, our results show that the frequency and intensity of freeze events across land-ocean temperature gradients greatly influences spatiotemporal patterns of range expansion and contraction of freeze-sensitive mangroves. We expect that, along subtropical coasts, similar processes govern the distribution and abundance of other freeze-sensitive organisms. In broad terms, our findings can be used to better understand and anticipate the ecological effects of changing winter climate extremes, especially within the transition zone between tropical and temperate climates. This article is protected by copyright. All rights reserved.
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Invasive plant species can have a strong negative impact on the resident native species, likely imposing new selective pressures on them. Altered selective pressures may result in evolutionary changes in some native species, reducing competitive exclusion and allowing for coexistence with the invader. Native genotypes that are able to coexist with strong invaders may represent a valuable resource for management efforts. A better understanding of the conditions under which native species are more, or less, likely to adapt to an invader is necessary to incorporate these eco-evolutionary dynamics into management strategies. We propose that the spatial structure of invasion, in particular the size and isolation of invaded patches, is one factor which can influence the evolutionary responses of native species through modifying gene flow and the strength of selection. We present a conceptual model in which large, dense, and well-connected patches result in a greater likelihood of native species adaptation. We also identify characteristics of the interacting species that may influence the evolutionary response of native species to invasion and outline potential management implications. Identifying areas of rapid evolutionary change may offer one additional tool to managers in their effort to conserve biodiversity in the face of invasion.
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We assessed the prevalence of alien species as a driver of recent extinctions in five major taxa (plants, amphibians, reptiles, birds and mammals), using data from the IUCN Red List. Our results show that alien species are the second most common threat associated with species that have gone completely extinct from these taxa since AD 1500. Aliens are the most common threat associated with extinctions in three of the five taxa analysed, and for vertebrate extinctions overall. © 2016 The Author(s) Published by the Royal Society. All rights reserved.
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Carbon stock estimates based on land cover type are critical for informing climate change assessment and landscape management, but field and theoretical evidence indicates that forest fragmentation reduces the amount of carbon stored at forest edges. Here, using remotely sensed pantropical biomass and land cover data sets, we estimate that biomass within the first 500 m of the forest edge is on average 25% lower than in forest interiors and that reductions of 10% extend to 1.5 km from the forest edge. These findings suggest that IPCC Tier 1 methods overestimate carbon stocks in tropical forests by nearly 10%. Proper accounting for degradation at forest edges will inform better landscape and forest management and policies, as well as the assessment of carbon stocks at landscape and national levels.
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1.Phenotypic variation plays an important role in successful plant invasions. The spread of invasive species over large geographic ranges may be facilitated if plants can match their phenotype to local abiotic conditions. Spartina alterniflora, native to the United States, was introduced into China in 1979 and has spread over 19ᵒ of latitude along the eastern coast of China.2.We studied patterns in vegetative growth and sexual reproduction of S. alterniflora at 22 sites at 11 geographic locations over a latitudinal gradient of ~2000 km from Tanggu (39.05 °N, high latitude) to Leizhou (20.90 °N, low latitude) in China. We further evaluated the basis of phenotypic differences by growing plants from across the range in a common garden for 2 growing seasons.3.We found distinct latitudinal clines in plant height, shoot density, and sexual reproduction across latitude. Some traits exhibited linear relationships with latitude; others exhibited hump-shaped relationships. We identified correlations between plant traits and abiotic conditions such as mean annual temperature, growing degree days, tidal range, and soil nitrogen content. However, geographic variation in all but one trait disappeared in the common garden, indicating that variation largely due to phenotypic plasticity. Only a slight tendency for latitudinal variation in seed set persisted for two years in the common garden, suggesting that plants may be evolving genetic clines for this trait.4.Synthesis. The rapid spread of Spartina alterniflora (S. alterniflora) in China has probably been facilitated by phenotypic plasticity in growth and reproductive traits. We found little evidence for the evolution of genetic clines in China, even though these exist for some traits in the native range. The considerable variation among clones, within provenances, that persisted in the common garden suggests a potential for the evolution of geographic clines in the future. Low fecundity of low latitude S. alterniflora populations in China might result in a slower spread at low latitudes, but S. alterniflora is likely to continue to spread rapidly at high latitudes in China and into the Korean peninsula.This article is protected by copyright. All rights reserved.
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Due to their position at the land-sea interface, coastal wetlands are vulnerable to many aspects of climate change. However, climate change vulnerability assessments for coastal wetlands generally focus solely on sea-level rise without considering the effects of other facets of climate change. Across the globe and in all ecosystems, macroclimatic drivers (e.g., temperature and rainfall regimes) greatly influence ecosystem structure and function. Macroclimatic drivers have been the focus of climate-change related threat evaluations for terrestrial ecosystems, but largely ignored for coastal wetlands. In some coastal wetlands, changing macroclimatic conditions are expected to result in foundation plant species replacement, which would affect the supply of certain ecosystem goods and services and could affect ecosystem resilience. As examples, we highlight several ecological transition zones where small changes in macroclimatic conditions would result in comparatively large changes in coastal wetland ecosystem structure and function. Our intent in this communication is not to minimize the importance of sea-level rise. Rather, our overarching aim is to illustrate the need to also consider macroclimatic drivers within vulnerability assessments for coastal wetlands. This article is protected by copyright. All rights reserved. This article is protected by copyright. All rights reserved.
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We conducted an analysis of global forest cover to reveal that 70% of remaining forest is within 1 km of the forest’s edge, subject to the degrading effects of fragmentation. A synthesis of fragmentation experiments spanning multiple biomes and scales, five continents, and 35 years demonstrates that habitat fragmentation reduces biodiversity by 13 to 75% and impairs key ecosystem functions by decreasing biomass and altering nutrient cycles. Effects are greatest in the smallest and most isolated fragments, and they magnify with the passage of time. These findings indicate an urgent need for conservation and restoration measures to improve landscape connectivity, which will reduce extinction rates and help maintain ecosystem services.
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The El Nino/Southern Oscillation is Earth's most prominent source of interannual climate variability, alternating irregularly between El Nino and La Nina, and resulting in global disruption of weather patterns, ecosystems, fisheries and agriculture(1-5). The 1998-1999 extreme La Nina event that followed the 1997-1998 extreme El Nino event(6) switched extreme El Nino-induced severe droughts to devastating floods in western Pacific countries, and vice versa in the southwestern United States(4,7). During extreme La Nina events, cold sea surface conditions develop in the central Pacific(8,9), creating an enhanced temperature gradient from the Maritime continent to the central Pacific. Recent studies have revealed robust changes in El Nino characteristics in response to simulated future greenhouse warming(10-12), but how La Nina will change remains unclear. Here we present climate modelling evidence, from simulations conducted for the Coupled Model Intercomparison Project phase 5 (ref. 13), for a near doubling in the frequency of future extreme La Nina events, from one in every 23 years to one in every 13 years. This occurs because projected faster mean warming of the Maritime continent than the central Pacific, enhanced upper ocean vertical temperature gradients, and increased frequency of extreme El Nino events are conducive to development of the extreme La Nina events. Approximately 75% of the increase occurs in years following extreme El Nino events, thus projecting more frequent swings between opposite extremes from one year to the next.
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DeYoe, H.; Lonard, R.I.; Judd, F.W.; Stalter, R., and Feller, I., 2020. Biological flora of the tropical and subtropical intertidal zone: Literature review for Rhizophora mangle L. Journal of Coastal Research, 36(4), 857–884. Coconut Creek (Florida), ISSN 0749-0208. Rhizophora mangle L. is a tropical and subtropical mangrove species that occurs as a dominant tree species in the intertidal zone of low-energy shorelines. Rhizophora mangle plays an important role in coastal zones as habitat for a wide range of organisms of intertidal food webs, as a natural barrier to coastal erosion, and as carbon sequestration. A review of mangrove literature has been performed, but a review specifically on red mangroves has not. The approach was to cover a broad range of topics with a focus on topics that have seen significant work since the 1970s. This review includes a brief introduction to red mangroves and then focuses on the following topics: biogeography, habitats and zonation, geomorphological interactions, taxonomy, histology, anatomy, physiological ecology, productivity, biomass, litter, reproduction, population biology, plant communities, interactions with other species, impacts of storms, reforestation, remote sensing, modelling, and economic importance.
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Introduced plants provide a unique opportunity to examine how plants respond through plasticity and adaptation to changing climates. We compared plants of Spartina alterniflora from the native (United States, 27–43°N) and introduced (China, 19–40°N) ranges. In the field and greenhouse, aboveground productivity of Chinese plants was greater than that of North American plants. Aboveground biomass in the field declined with increasing latitude in the native range, a pattern that persisted in the greenhouse, indicating a genetic basis. Aboveground biomass in the field displayed hump‐shaped relationships with latitude in China, but this pattern disappeared in field and greenhouse common gardens, indicating phenotypic plasticity. Relationships in both geographic regions were explained by temperature, which is probably the underlying environmental factor affecting aboveground biomass. S. alterniflora has evolved greater biomass in China, but in the four decades since it was introduced, it has not yet evolved the genetic cline in biomass seen in its native range. By working at lower latitudes in the introduced range than have been sampled in the native range, we identified an optimum temperature in the introduced range above which aboveground productivity decreases.
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Smooth cordgrass (Spartina alterniflora) was introduced to China in December 1979 to buffer against tides and to accelerate coastal wetland accretion. Since then, its propagation and natural dispersal have allowed this exotic plant to rapidly expand throughout coastal China with generally negative ecological effects. In 2003 S. alterniflora was labeled as an invasive plant in China, and it now covers ~50,000 ha. In this review, we first summarize the mechanisms of spread and spatial distribution of S. alterniflora, and how its physiological characteristics and strong adaptability to the available niche space in China's wetlands have enabled its spread and competition with native plants. Then we review the effects of S. alterniflora on ecosystem function in terms of habitat conversion and the alteration of biodiversity, soil carbon flux and sequestration, and various processes of nutrient regulation. We conclude that we need a long-term and context-dependent perspective, in order to maximize the benefits and minimize the costs of S. alterniflora within each of China's unique provinces.
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Aim The identification of environmental variables associated with geographical range limits can shed light on the ecological and physiological constraints imposed by abiotic stresses. Mangrove forest is a coastal ecosystem restricted to the tropics and subtropics. However, the factors that influence fine‐scale assemblage composition of mangroves remain an open question. This study aimed to identify range‐limiting climate factor(s) associated with species composition that are likely to have influenced the evolution of mangroves. Location China. Time period 1950s–2013. Major taxa studied True mangrove species. Methods We collected species presence–absence data across 70 sites spread between 18 and 27°N in China to analyse the latitudinal diversity gradient in relationship to 25 climate variables. To examine the change in community composition along the diversity gradient, we analysed two components of beta diversity: turnover‐resultant dissimilarity and nestedness‐resultant dissimilarity. We used non‐metric multidimensional scaling to identify crucial climate factors associated with variation in mangrove assemblage composition. We also applied variance partitioning among taxonomic ranks to detect taxonomic signal in climatic tolerance of mangroves. Results First, total species richness decreases with increasing latitude. Second, variation in mangrove assemblage composition is significantly correlated with winter temperature, exhibiting a nested subset structure in species composition and a gradual decrease in the number of thermophilic species with increasing latitude. Finally, mangrove species show a genus‐level signal in tolerance of winter temperature, whereas most other climate factors vary primarily among species within genera. Main conclusions Winter temperature appears to constrain the current distributions of mangrove species, and tolerance of low air temperature is evolutionarily conservative at the genus level. These ecological and phylogenetic patterns collectively suggest that physiological constraints related to cold tolerance structure the nested pattern with respect to the latitude of mangrove assemblages at a regional scale.
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Plant invasion is a major threat to natural ecosystems, and mangrove forests are among the most threatened ecosystems in the world. However, since mangrove species primarily occur in the saline and intertidal environment that is inhospitable for most terrestrial and freshwater plants, it is commonly assumed that mangrove forests are resilient to plant invasion. Still, many salt tolerant aquatic and terrestrial plants as well as epiphytes are found to invade the mangrove forests, and we know little about those invasive plants, their functional traits, invasion patterns and pathways and their ecological consequences. In a survey of global literature, we found a total of 57 plants reportedly invasive in the world's mangrove forests. These plants possessed the traits of salinity tolerance, tolerance to anaerobic condition, high fecundity and rapid growth. About 19% of invasive plants were anthropogenically introduced for coastal land stabilization, and the rests were accidental introduction. Invaders were found to colonize along the forest edges or forest interior, but mostly in the raised lands. That is, the presence of diversified microhabitats such as raised land and intertidal mudflat might help both halophytic and non-halophytic plants to invade the mangrove forests. Some invaders (30%) were transient, but many (70%) could persist for a longer time; and these species could modify habitat conditions, impede natural regeneration of mangroves and disrupt their faunal assemblage. Together, plant invasion in mangrove forests is much more widespread and problematic than commonly perceived, underscoring the need for the integration of invasive plant management strategy into mangrove forest management.
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Salt marsh and mangrove have been recognized as being among the most valuable ecosystem types globally in terms of their supply of ecosystem services and support for human livelihoods. These coastal ecosystems are also susceptible to the impacts of climate change and rising sea levels, with evidence of global shifts in the distribution of mangroves, including encroachment into salt marshes. The encroachment of woody mangrove shrubs and trees into herbaceous salt marshes may represent a substantial change in ecosystem structure, although resulting impacts on ecosystem functions and service provisions are largely unknown. In this review, we assess changes in ecosystem services associated with mangrove encroachment. While there is quantitative evidence to suggest that mangrove encroachment may enhance carbon storage and the capacity of a wetland to increase surface elevation in response to sea-level rise, for most services there has been no direct assessment of encroachment impact. On the basis of current understanding of ecosystem structure and function, we theorize that mangrove encroachment may increase nutrient storage and improve storm protection, but cause declines in habitat availability for fauna requiring open vegetation structure (such as migratory birds and foraging bats) as well as the recreational and cultural activities associated with this fauna (e.g., birdwatching and/or hunting). Changes to provisional services such as fisheries productivity and cultural services are likely to be site specific and dependent on the species involved. We discuss the need for explicit experimental testing of the effects of encroachment on ecosystem services in order to address key knowledge gaps, and present an overview of the options available to coastal resource managers during a time of environmental change.
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Mangrove forests and saltmarshes are recognized for their roles in wave and current attenuation, although a comparison of in situ observations between woody and herbaceous plants is needed in order to understand the different mechanisms of bio-physical interaction within coastal wetlands. The aim of our study was to compare the mechanisms of flow reduction and energy dissipation by mangrove trees and saltmarsh grass in a subtropical area where tidal currents dominate. Fieldwork was conducted to measure the hydrodynamic processes occurring at the boundaries between a bare mudflat and vegetated tidal flat, as the flow transitions from a bare mudflat to either mangrove or saltmarsh. Synchronous ADV measurements at three sites revealed that the mangrove was more effective than the saltmarsh grass at flow reduction. In addition, a considerable rotation in flow direction was observed as the flow entered the mangrove trees, while rotation was considerably less pronounced within the saltmarsh edge. The mechanism for this difference was explained through a combination of changes in drag force and eddy viscosity over the two vegetation types. Although overall the mangrove was observed to dissipate energy more effectively than the saltmarsh, the relative efficiency of the vegetation at dissipating turbulent energy was found to vary with the maximum water level of tidal cycle. When the maximum water level remained below the mangrove canopy bottom (‘bio-line’), the energy dissipation ability of the mangrove was relatively low, a result of the presence of rigid, sparse trunks rather than denser saltmarsh grass found near the bed; when the maximum water level was sufficiently high to reach the mangrove canopy, the ability of the mangrove to dissipate energy was significantly increased, becoming more effective than the saltmarsh grass. This article is protected by copyright. All rights reserved.
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The mangrove forests of Southeast Asia are highly biodiverse and provide multiple ecosystem services upon which millions of people depend. Mangroves enhance fisheries and coastal protection, and store among the highest densities of carbon of any ecosystem globally. Mangrove forests have experienced extensive deforestation owing to global demand for commodities, and previous studies have identified the expansion of aquaculture as largely responsible. The proportional conversion of mangroves to different land use types has not been systematically quantified across Southeast Asia, however, particularly in recent years. In this study we apply a combined geographic information system and remote sensing method to quantify the key proximate drivers (i.e., replacement land uses) of mangrove deforestation in Southeast Asia between 2000 and 2012. Mangrove forests were lost at an average rate of 0.18% per year, which is lower than previously published estimates. In total, more than 100,000 ha of mangroves were removed during the study period, with aquaculture accounting for 30% of this total forest change. The rapid expansion of rice agriculture in Myanmar, and the sustained conversion of mangroves to oil palm plantations in Malaysia and Indonesia, are identified as additional increasing and under-recognized threats to mangrove ecosystems. Our study highlights frontiers of mangrove deforestation in the border states of Myanmar, on Borneo, and in Indonesian Papua. To implement policies that conserve mangrove forests across Southeast Asia, it is essential to consider the national and subnational variation in the land uses that follow deforestation.
Article
Rates of in-situ decomposition and nutrient (organic C, N and P) release of leaf litter were seasonally compared among three planted Kandelia obovata mangrove forests (K12, K24 and K48 with forest ages of 12, 24 and 48 years, respectively) and one natural mature K. obovata forest (KM) in Jiulongjiang Estuary, China. The average values of half-time (T50) of leaf litter decomposition in spring, summer, autumn and winter were 29.8, 18.7, 23.9 and 47.4d, respectively. Decomposition rates were lower in the older forests (with T50 values of 30.1 and 31.1d averaged by all seasons in K48 and KM, respectively) than in the younger ones (with T50 values 29.8 and 28.8d averaged by all seasons in K12 and K24, respectively), especially in summer and autumn. The annual mean T50 of nutrient release of leaf litter during decomposition followed an order of KM > K48 > K24 > K12. During leaf litter decomposition, C releases were very similar to dry weight losses, while N releases were slower and P releases were much faster than dry weight losses. With the development of restored mangrove forests, decomposition and nutrient release of leaf litter became slow, which may increase the chance of leaf litter being exported into the surrounding waters.
Article
Seed predation is an important ecological process that affects the abundance, diversity and distribution of plant species, and it is known to be influenced by defaunation and forest fragmentation. Most studies on seed predation in human-modified landscapes do not take into account the different spatial scales in which this process operates. In this study, we evaluated how variables at three distinct spatial scales affected the seed predation of a palm that provides a keystone resource to the frugivore community, the queen palm (Syagrus romanzoffiana). Thirteen landscapes that vary in forest cover, number of fragments and patch sizes were sampled in the Brazilian Atlantic forest. We also evaluated the contribution of the three main groups of seed predators: squirrels, terrestrial rodents and invertebrates. Our results indicate that seed predation is more affected by fragment and local variables than by landscape influences. In addition, the size of the fragment, its shape and the distance from the nearest forest edge were the main predictors of the proportion of predated seeds. Moreover, the two main seed predators (squirrels and invertebrates) responded to the same fragment and local variables. Because most of the Atlantic forest consists of small fragments, we expect that the seed predation of this keystone palm should be high in most of its distribution, with potential consequences for the frugivore community.This article is protected by copyright. All rights reserved.
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The regression equation, y=α + βρx, is very useful for representing the relation between y and x, when y tends asymptotically to a limit as x tends to infinity. Examples of such a relation are found in the growth of an animal approaching maturity and in the yield of a crop as function of quantity of fertiliser. By simple transformation, other asymptotic regression formulae, such as Gompertz's and the logistic, can be brought to the above form. The arithmetic labour of making a least-squares adjustment has proved so great that few research-workers appear to have attempted it, with a consequence that unsatisfactory and very inefficient methods have generally been adopted instead. It is therefore of immediate practical value to rationalise and reduce the arithmetic labour of finding efficient estimates of the three parameters. It is shown that the covariance matrix, which is used in Fisher's process of estimation, is effectively a function of ρ only. Tables are provided of the terms of this matrix for five, six and seven equally spaced values of x, these being the numbers of levels of x which will prove most useful in experimentation. Worked examples are given to show how, with the help of these tables, the arithmetic labour need no longer be considered an obstacle.
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We demonstrate the use of piecewise regression as a statistical technique to model ecological thresholds. Recommended procedures for analysis are illustrated with a case study examining the width of edge effects in two understory plant communities. Piecewise regression models are ‘‘broken-stick’’ models, where two or more lines are joined at unknown points, called ‘‘breakpoints.’’ Breakpoints can be used as estimates of thresholds and are used here to determine the width of edge effects. We compare a sharp-transition model with three models incorporating smooth transitions: the hyperbolic-tangent, benthyperbola, and bent-cable models. We also calculate three types of confidence intervals for the breakpoint estimate: an interval based on the computed standard error of the estimate from the fitting procedure, an empirical bootstrap confidence interval, and a confidence interval derived from an inverted F test.We recommend use of the inverted F test confidence interval when sample sizes are large, and cautious use of bootstrapped confidence intervals when sample sizes are smaller. Our analysis demonstrates the need for a careful study of the likelihood surface when fitting and interpreting the results from piecewise-regression models.
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Continental tropical forests are thought to be resistant to alien plant invasion due to a lack of disturbance, or low propagule pressure from introduced species. We assessed the importance of disturbance and edge effects by surveying areas of submontane and lowland forest of Amani Nature Reserve in the East Usambara mountains, Tanzania. These areas are in the vicinity of Amani Botanic Garden (ABG)-a propagule source for many alien plant species. We surveyed three edges in the vicinity of the ABG plantations, using plots interspersed along multiple 250 m transects. Survey plots were either in secondary or seminatural forest, representing a difference in past disturbance). Alien plant species richness and abundance declined with increasing distance from forest edges, indicating that edge effects were important. In addition, the effect of distance on richness and abundance of alien species as adults was much smaller in seminatural than secondary forest, emphasizing that invasion of seminatural forest is less likely to occur. Abundance and occurrence of individual species showed broadly similar declines with increasing distance from the forest edge, and lower abundance in seminatural compared to secondary forest. Alien species were dominant in 15 percent of plots surveyed. As 28 percent of the Amani nature reserve forest is within 250 m of an edge, the importance of disturbance and edges could make a potentially large proportion of the forest vulnerable to alien species invasion.