Is the central-marginal hypothesis a general rule? Evidence from three distributions of an expanding mangrove species, Avicennia germinans (L.) L

Defining Populations and Predicting Future Suitable Niche Space in the Geographically Disjunct, Narrowly Endemic Leafy Prairie-Clover (Dalea foliosa; Fabaceae)

Conservation actions for rare species are often based on estimates of population size and number, which are challenging to capture in natural systems. Instead, many definitions of populations rely on arbitrarily defined distances between occurrences, which is not necessarily biologically meaningful despite having utility from a conservation management perspective. Here, we introduce a case study using the narrowly endemic and highly geographically disjunct leafy prairie-clover (Dalea foliosa), for which we use nuclear microsatellite loci to assess the current delimitations of populations and management units across its entire known range. We model future potential suitable niche space for the species to assess how currently defined populations could fare under predicted changes in climate over the next 50 years. Our results indicate that genetic variation within the species is extremely limited, particularly so in the distal portions of its range (Illinois and Alabama). Within the core of its range (Tennessee), genetic structure is not consistent with populations as currently defined. Our models indicate that predicted suitable niche space may only marginally overlap with the geology associated with this species (limestone glades and dolomite prairies) by 2070. Additional studies are needed to evaluate the extent to which populations are ecologically adapted to local environments and what role this could play in future translocation efforts.

Extent of intraspecific trait variability in ecologically central and marginal populations of a dominant alpine plant across European mountains

BACKGROUND AND AIMS

Studying trait variability and restricted gene flow between populations of species can reveal species dynamics. Peripheral populations commonly exhibit lower genetic diversity and trait variability due to isolation and ecological marginality, unlike central populations experiencing gene flow and optimal conditions. This study focused on Carex curvula, the dominant species in alpine acidic meadows of European mountain regions. The species is sparser in dry areas such as the Pyrenees and Balkans, compared to the Central-Eastern Alps and Carpathians. We hypothesized that distinct population groups could be identified based on their mean functional trait values and their correlation with the environment; we predicted that ecologically marginal populations would have stronger trait correlations, lower within-population trait variability (intraspecific trait variability, ITV) and lower genetic diversity than populations of optimal habitats.

METHODS

Sampling was conducted in 34 populations that spanned the entire distribution range of C. curvula. We used hierarchical clustering to identify emergent functional groups of populations, defined by combinations of multiple traits associated with nutrient economy and drought tolerance (e.g. specific leaf area, anatomy). We contrasted the geographical distribution of these groups in relation to environment and genetic structure. We compared pairwise trait relationships, within-population trait variation (ITV) and neutral genetic diversity between groups.

KEY RESULTS

Our study identified emergent functional groups of populations. Those in the southernmost ranges, specifically the Pyrenees and Balkan region, showed drought-tolerant trait syndromes and correlated with indicators of limited water availability. While we noted a decline in population genetic diversity, we did not observe any significant changes in ITV in ecologically marginal (peripheral) populations.

CONCLUSIONS

Our research exemplifies the relationship between ecological marginality and geographical peripherality, which in this case study is linked to genetic depauperation but not to reduced ITV. Understanding these relationships is crucial for understanding the biogeographical factors shaping trait variation.

Thermal acclimation of leaf respiration is consistent in tropical and subtropical populations of two mangrove species

Populations from different climates often show unique growth responses to temperature, reflecting temperature adaptation. Yet, whether populations from different climates differ in physiological temperature acclimation remains unclear. Here, we test whether populations from differing thermal environments exhibit different growth responses to temperature and differences in temperature acclimation of leaf respiration. We grew tropical and subtropical populations of two mangrove species (Avicennia germinans and Rhizophora mangle) under ambient and experimentally warmed conditions in a common garden at the species' northern range limit. We quantified growth and temperature responses of leaf respiration (R) at seven time points over ~10 months. Warming increased productivity of tropical populations more than subtropical populations, reflecting a higher temperature optimum for growth. In both species, R measured at 25 °C declined as seasonal temperatures increased, demonstrating thermal acclimation. Contrary to our expectations, acclimation of R was consistent across populations and temperature treatments. However, populations differed in adjusting the temperature sensitivity of R (Q10) to seasonal temperatures. Following a freeze event, tropical Avicennia showed greater freeze damage than subtropical Avicennia, while both Rhizophora populations appeared equally susceptible. We found evidence of temperature adaptation at the whole-plant scale but little evidence for population differences in thermal acclimation of leaf physiology. Studies that examine potential costs and benefits of thermal acclimation in an evolutionary context may provide new insights into limits of thermal acclimation.

Oceanographic connectivity explains the intra-specific diversity of mangrove forests at global scales

The distribution of mangrove intra-specific biodiversity can be structured by historical demographic processes that enhance or limit effective population sizes. Oceanographic connectivity (OC) may further structure intra-specific biodiversity by preserving or diluting the genetic signatures of historical changes. Despite its relevance for biogeography and evolution, the role of oceanographic connectivity in structuring the distribution of mangrove's genetic diversity has not been addressed at global scale. Here we ask whether connectivity mediated by ocean currents explains the intra-specific diversity of mangroves. A comprehensive dataset of population genetic differentiation was compiled from the literature. Multigenerational connectivity and population centrality indices were estimated with biophysical modeling coupled with network analyses. The variability explained in genetic differentiation was tested with competitive regression models built upon classical isolation-by-distance (IBD) models considering geographic distance. We show that oceanographic connectivity can explain the genetic differentiation of mangrove populations regardless of the species, region, and genetic marker (significant regression models in 95% of cases, with an average R-square of 0.44 ± 0.23 and Person's correlation of 0.65 ± 0.17), systematically improving IBD models. Centrality indices, providing information on important stepping-stone sites between biogeographic regions, were also important in explaining differentiation (R-square improvement of 0.06 ± 0.07, up to 0.42). We further show that ocean currents produce skewed dispersal kernels for mangroves, highlighting the role of rare long-distance dispersal events responsible for historical settlements. Overall, we demonstrate the role of oceanographic connectivity in structuring mangrove intra-specific diversity. Our findings are critical for mangroves' biogeography and evolution, but also for management strategies considering climate change and genetic biodiversity conservation.

Genetic variability and the ecology of geographic range: A test of the central-marginal hypothesis in Australian scincid lizards

For many species, both local abundance and regional occupancy are highest near the centre of their geographic distributions. One hypothesis for this pattern is that niche suitability declines with increasing distance from a species geographic centre, such that populations near range margins are characterized by reduced density and increased patchiness. In these smaller edge populations, genetic drift is more powerful, leading to the loss of genetic diversity. This simple verbal model has been formalized as the central-marginal hypothesis, which predicts that core populations should have greater genetic diversity than edge populations. Here, we tested the central-marginal hypothesis using a genomic data set of 25 species-level taxa of Australian scincid lizards in the genera Ctenotus and Lerista. A majority of taxa in our data set showed range-wide patterns of genetic variation consistent with central-marginal hypothesis, and eight of 25 taxa showed significantly greater genetic diversity in the centre of their range. We then explored biological, historical, and methodological factors that might predict which taxa support the central-marginal hypothesis. We found that taxa with the strongest evidence for range expansion were the least likely to follow predictions of the central-marginal hypothesis. The majority of these taxa had range expansions that originated at the range edge, which led to a gradient of decreasing genetic diversity from the range edge to the core, contrary to the central-marginal hypothesis.

Glacial vicariance and oceanic circulation shape population structure of the coastal legume Derris trifoliata in the Indo-West Pacific

PREMISE

The phylogeography of coastal plant species is shaped by contemporary and historical biogeographic processes. In this study, we aim to decipher the phylogeography of Derris trifoliata, a woody legume of relatively recent origin and wide distribution, in coastal areas in the Indo-West Pacific (IWP) region.

METHODS

Genetic diversity and population structure were assessed by analyzing six nuclear and three chloroplast DNA sequences from 30 populations across the species' range. Phylogeography was inferred by estimating gene flow, divergence time, historical population size changes, and historical habitat suitability using paleoclimatic niche modeling.

RESULTS

High genetic diversity was observed at the species level. The populations of three oceanic regions included in this study (i.e., Indian Ocean, South China Sea, and Pacific Ocean) formed distinct clades and likely diverged during the late Pleistocene. Potential barriers to gene flow were identified, including the Sunda and Sahul shelves, geographic distance, and current patterns of oceanic circulation. Analysis of changes in population size supported the bottleneck model, which was strengthened by estimates of habitat suitability across paleoclimatic conditions.

CONCLUSIONS

The once widespread distribution of D. trifoliata was fragmented by changes in climatic suitability and biogeographic barriers that arose following sea-level changes during the Pleistocene. In addition, contemporary patterns of oceanic circulation and geographic distance between populations appear to maintain genetic differentiation across its distribution in the IWP.

Population genomics study for the conservation management of the endangered shrub Abeliophyllum distichum

Natural monuments are IUCN Category III protected areas that play an important role in biodiversity conservation as they provide species refuge and allow species migration. Despite their status, natural monuments are often confined to cultural and fragmented landscapes due to anthropogenic land-use demands. In this population genomic study, we surveyed 11 populations of the endemic shrub Abeliophyllum distichum Nakai (Oleaceae), including five natural monument habitats, covering its range-wide distribution in South Korea. Using 2,254 SNPs as markers, our results showed a mean expected heterozygosity (He) of 0.319, with populations in the central distribution showing significantly higher He than those at the periphery. There was no significant heterozygote deficiency and inbreeding among studied populations overall (FIS = −0.098), except for a single natural monument population (GS-NM147). Population structure and differentiation was moderate to high (FST = 0.196), while recent gene flow between populations appeared weak, which can be attributed to the fragmented distribution and the outcrossing mating system of the heterostylous plant. Based on these findings, we provide suggestions for the population conservation and management of this endangered species.

Reduction of Genetic Variation When Far From the Niche Centroid: Prediction for Mangrove Species

The niche-centroid hypothesis states that populations that are distributed near the centroid of the species' ecological niche will have higher fitness-related attributes, such as population abundance and genetic diversity than populations near the edges of the niche. Empirical evidence based on abundance and, more recently, genetic diversity data support this hypothesis. However, there are few studies that test this hypothesis in coastal species, such as mangroves. Here, we focused on the black mangrove Avicennia germinans. We combined ecological, heterozygosity, and allelic richness information from 1,419 individuals distributed in 40 populations with three main goals: (1) test the relationship between distance to the niche centroid and genetic diversity, (2) determine the set of environmental variables that best explain heterozygosity and allelic richness, and (3) predict the spatial variation in genetic diversity throughout most of the species' natural geographic range. We found a strong correlation between the distance to the niche centroid and both observed heterozygosity (Ho; ρ2 = 0.67 P < 0.05) and expected heterozygosity (He; ρ2 = 0.65, P < 0.05). The niche variables that best explained geographic variation in genetic diversity were soil type and precipitation seasonality. This suggests that these environmental variables influence mangrove growth and establishment, indirectly impacting standing genetic variation. We also predicted the spatial heterozygosity of A. germinans across its natural geographic range in the Americas using regression model coefficients. They showed significant power in predicting the observed data (R2 = 0.65 for Ho; R2 = 0.60 for He), even when we considered independent data sets (R2= 0.28 for Ho; R2 = 0.25 for He). Using this approach, several genetic diversity estimates can be implemented and may take advantage of population genomics to improve genetic diversity predictions. We conclude that the level of genetic diversity in A. germinans is in agreement with expectations of the niche-centroid hypothesis, namely that the highest heterozygosity and allelic richness (the basic genetic units for adaptation) are higher at locations of high environmental suitability. This shows that this approach is a potentially powerful tool in the conservation and management of this species, including for modelling changes in the face of climate change.

Temperature acclimation of leaf respiration differs between marsh and mangrove vegetation in a coastal wetland ecotone

Temperature acclimation of leaf respiration (R) is an important determinant of ecosystem responses to temperature and the magnitude of temperature-CO₂ feedbacks as climate warms. Yet, the extent to which temperature acclimation of R exhibits a common pattern across different growth conditions, ecosystems, and plant functional types remains unclear. Here, we measured the short-term temperature response of R at six time points over a 10-month period in two coastal wetland species (Avicennia germinans [C₃ mangrove] and Spartina alterniflora [C₄ marsh grass]) growing under ambient and experimentally warmed temperatures at two sites in a marsh-mangrove ecotone. Leaf nitrogen (N) was determined on a subsample of leaves to explore potential coupling of R and N. We hypothesized that both species would reduce R at 25°C (R²⁵ ) and the short-term temperature sensitivity of R (Q₁₀ ) as air temperature (T_air ) increased across seasons, but the decline would be stronger in Avicennia than in Spartina. For each species, we hypothesized that seasonal temperature acclimation of R would be equivalent in plants grown under ambient and warmed temperatures, demonstrating convergent acclimation. Surprisingly, Avicennia generally increased R²⁵ with increasing growth temperature, although the Q₁₀ declined as seasonal temperatures increased and did so consistently across sites and treatments. Weak temperature acclimation resulted in reduced homeostasis of R in Avicennia. Spartina reduced R²⁵ and the Q₁₀ as seasonal temperatures increased. In Spartina, seasonal temperature acclimation was largely consistent across sites and treatments resulting in greater respiratory homeostasis. We conclude that co-occurring coastal wetland species may show contrasting patterns of respiratory temperature acclimation. Nonetheless, leaf N scaled positively with R²⁵ in both species, highlighting the importance of leaf N in predicting respiratory capacity across a range of growth temperatures. The patterns of respiratory temperature acclimation shown here may improve the predictions of temperature controls of CO₂ fluxes in coastal wetlands.

Rules of Plant Species Ranges: Applications for Conservation Strategies

Earth is changing rapidly and so are many plant species’ ranges. Here, we synthesize eco-evolutionary patterns found in plant range studies and how knowledge of species ranges can inform our understanding of species conservation in the face of global change. We discuss whether general biogeographic “rules” are reliable and how they can be used to develop adaptive conservation strategies of native plant species across their ranges. Rules considered include (1) factors that set species range limits and promote range shifts; (2) the impact of biotic interactions on species range limits; (3) patterns of abundance and adaptive properties across species ranges; (4) patterns of gene flow and their implications for genetic rescue, and (5) the relationship between range size and conservation risk. We conclude by summarizing and evaluating potential species range rules to inform future conservation and management decisions. We also outline areas of research to better understand the adaptive capacity of plants under environmental change and the properties that govern species ranges. We advise conservationists to extend their work to specifically consider peripheral and novel populations, with a particular emphasis on small ranges. Finally, we call for a global effort to identify, synthesize, and analyze prevailing patterns or rules in ecology to help speed conservation efforts.

Unexpected population genetic structure in two closely related euphaeid damselflies from the Yaeyama and Taiwan Islands (Odonata: Euphaeidae)

In general, population genetics theory predicts that a fragmented smaller population will contain relatively less genetic diversity than a larger population, and so will have a higher rate of genetic fixation due to random genetic drift or inbreeding. However, in this study, having analysed the genetic structure of the mitochondrial DNA COI region between two closely related euphaeid damselflies, we obtained unexpected results which contradict the theoretically expected patterns. Despite their geographical proximity, Euphaea yayeyamana was clearly genetically isolated on Ishigaki and Iriomote Islands, and no haplotype crossovers were observed. Even within each island, several diverse haplotypes were observed, indicating a significantly high haplotype intra-island diversity. However, the genetic diversity within Taiwan’s population of Euphaea formosa was significantly lower than that within either Ishigaki or Iriomote Island, even though Taiwan is significantly larger, with high mountain ranges that reach c. 4000 m a.s.l. and an abundance of habitats, all factors that should contribute to high genetic diversity. The current low diversity status for Taiwan’s population may be due to genetic bottleneck effects. In contrast, despite the very small population sizes of Ishigaki and Iriomote Islands coupled with the effects of glacial and interglacial geological events, they have maintained markedly high genetic diversity.

Evidence for the genetic similarity rule at an expanding mangrove range limit

PREMISE

Host-plant genetic variation can shape associated communities of organisms. These community-genetic effects include (1) genetically similar hosts harboring similar associated communities (i.e., the genetic similarity rule) and (2) host-plant heterozygosity increasing associated community diversity. Community-genetic effects are predicted to be less prominent in plant systems with limited genetic variation, such as those at distributional range limits. Yet, empirical evidence from such systems is limited.

METHODS

We sampled a natural population of a mangrove foundation species (Avicennia germinans) at an expanding range limit in Florida, USA. We measured genetic variation within and among 40 host trees with 24 nuclear microsatellite loci and characterized their foliar endophytic fungal communities with internal transcribed spacer (ITS1) gene amplicon sequencing. We evaluated relationships among host-tree genetic variation, host-tree spatial location, and the associated fungal communities.

RESULTS

Genetic diversity was low across all host trees (mean: 2.6 alleles per locus) and associated fungal communities were relatively homogeneous (five sequence variants represented 78% of all reads). We found (1) genetically similar host trees harbored similar fungal communities, with no detectable effect of interhost geographic distance. (2) Host-tree heterozygosity had no detectable effect, while host-tree absolute spatial location affected community alpha diversity.

CONCLUSIONS

This research supports the genetic similarity rule within a range limit population and helps broaden the current scope of community genetics theory by demonstrating that community-genetic effects can occur even at expanding distributional limits where host-plant genetic variation may be limited. Our findings also provide the first documentation of community-genetic effects in a natural mangrove system.

Cold Wave-Induced Reductions in NDII and ChlRE for North-Western Pacific Mangroves Varies with Latitude and Climate History

Mangrove forests growing at the poleward edges of their geographic distribution are occasionally subject to freezing (<0 °C) and cold wave (>0 °C) events. Cold wave effects on mangrove trees are well documented and adaptation to cold stress has been reported for local mangrove populations in the North Atlantic. However, there is less understanding of effects of cold waves on mangroves in the northern Pacific, especially at the regional scale. Moreover, it is unclear if cold tolerant mangrove species of North Asia display variation in resistance to cold temperatures across their geographic distribution. Using a cold wave event that occurred in January 2021, we evaluated the effects of low temperatures on vegetation index (VI) change (relative to a recent five-year baseline) for mangrove forests dominated by Kandelia obovata (Rhizophoraceae) and Avicennia marina (Acanthaceaee) at the northern edge of their geographical range. We used two VIs derived from Sentinel-2 imagery as indicators for canopy health: the normalized difference infrared index (NDII) and the chlorophyll red-edge index (ChlRE), which reflect forest canopy water content and chlorophyll concentration, respectively. We isolated the cold wave effects on the forest canopy from phenology (i.e., cold wave induced deviation from a five-year baseline) and used multiple linear regression to identify significant climatic predictors for the response of mangrove forest canopy VI change to low temperatures. For areas where the cold wave resulted in temperatures <10 °C, immediate decreases in both VIs were observed, and the VI difference relative to the baseline was generally greater at 30-days after the cold wave than when temperatures initially recovered to baseline values, showing a slight delay in VI response to cold wave-induced canopy damage. Furthermore, the two VIs did not respond consistently suggesting that cold-temperature induced changes in mangrove canopy chlorophyll and water content are affected independently or subject to differing physiological controls. Our results confirm that local baseline (i.e., recent past) climate predicts canopy resistance to cold wave damage across K. obovata stands in the northern Pacific, and in congruence with findings from New World mangroves, they imply geographic variation in mangrove leaf physiological resistance to cold for Northern Pacific mangroves.

Tropicalization of temperate ecosystems in North America: The northward range expansion of tropical organisms in response to warming winter temperatures

Tropicalization is a term used to describe the transformation of temperate ecosystems by poleward-moving tropical organisms in response to warming temperatures. In North America, decreases in the frequency and intensity of extreme winter cold events are expected to allow the poleward range expansion of many cold-sensitive tropical organisms, sometimes at the expense of temperate organisms. Although ecologists have long noted the critical ecological role of winter cold temperature extremes in tropical-temperate transition zones, the ecological effects of extreme cold events have been understudied, and the influence of warming winter temperatures has too often been left out of climate change vulnerability assessments. Here, we examine the influence of extreme cold events on the northward range limits of a diverse group of tropical organisms, including terrestrial plants, coastal wetland plants, coastal fishes, sea turtles, terrestrial reptiles, amphibians, manatees, and insects. For these organisms, extreme cold events can lead to major physiological damage or landscape-scale mass mortality. Conversely, the absence of extreme cold events can foster population growth, range expansion, and ecological regime shifts. We discuss the effects of warming winters on species and ecosystems in tropical-temperate transition zones. In the 21st century, climate change-induced decreases in the frequency and intensity of extreme cold events are expected to facilitate the poleward range expansion of many tropical species. Our review highlights critical knowledge gaps for advancing understanding of the ecological implications of the tropicalization of temperate ecosystems in North America.

Mangrove growth response to experimental warming is greatest near the range limit in northeast Florida

Shrubs are invading into grasslands around the world, but we don't yet know how these shrubs will fare in a warmer future. In ecotonal coastal wetland ecosystems, woody mangroves are encroaching into herbaceous salt marshes owing to changes in temperature, precipitation, and sediment dynamics. Increasing mangrove biomass in wetlands often increases carbon storage, which is high in these productive ecosystems, but little is known about how mangrove growth will change in response to warming. To address this knowledge gap, we deployed warming experiments at three coastal wetland sites along a latitudinal gradient in northeast Florida where Avicennia germinans, black mangroves, are encroaching into salt marshes. We achieved air temperature warming (+1.6°C during the day) at all three sites and measured stem elongation, canopy height and area changes, and leaf and node number. After 2 yr of warming, we found that mangrove growth rate in height increased due to warming. Warming increased stem elongation by 130% over unwarmed control plots after 1 yr at the northern site. Mangrove growth in canopy area did not respond to warming. Site differences in growth rate were pronounced, and mangrove growth in both height and area were lowest at the northern site, despite greater impacts of warming at that site. We also found that area-based relative growth rate was five times higher across all treatments than height-based relative growth rate, indicating that mangroves are growing wider rather than taller in these ecotonal environments. Our findings indicate that the growth effect of experimental warming depends on site characteristics and growth parameter measured. We also propose that differential mangrove growth across the three sites may be driven by biotic factors such as the identity of the salt marsh species into which mangroves are encroaching. Our results suggest that, as seen in other ecosystems, wetland plants may respond most strongly to warming at their poleward range edge.

When ecological marginality is not geographically peripheral: exploring genetic predictions of the centre-periphery hypothesis in the endemic plant Lilium pomponium

Background

Quantifying variation of genetic traits over the geographical range of species is crucial for understanding the factors driving their range dynamics. The center-periphery hypothesis postulates, and many studies support, the idea that genetic diversity decreases and genetic differentiation increases toward the geographical periphery due to population isolation. The effects of environmental marginality on genetic variation has however received much less attention.

Methods

We tested the concordance between geographical and environmental gradients and the genetic predictions of center-periphery hypothesis for endemic Lilium pomponium in the southern Alps.

Results

We found little evidence for concordance between genetic variation and both geographical and environmental gradients. Although the prediction of increased differentiation at range limits is met, genetic diversity does not decrease towards the geographical periphery. Increased differentiation among peripheral populations, that are not ecologically marginal, may be explained by a decrease in habitat availability that reduces population connectivity. In contrast, a decrease of genetic diversity along environmental but not geographical gradients may be due to the presence of low quality habitats in the different parts of the range of a species that reduce effective population size or increase environmental constraints. As a result, environmental factors may affect population dynamics irrespective of distance from the geographical center of the range. In such situations of discordance between geographical and environmental gradients, the predictions of decreasing genetic diversity and increasing differentiation toward the geographical periphery may not be respected.

Effects of environmental heterogeneity on phenotypic variation of the endemic plant Lilium pomponium in the Maritime and Ligurian Alps

Geographical limits of species’ distributions are assumed to be coincident with ecological margins, although this assumption might not always be true. Indeed, harsh environments such as Alpine and Mediterranean ecosystems may favour high phenotypic variability among populations, especially those in peripheral sites. Floral traits are often found to be less variable and less affected by environmental heterogeneity than vegetative traits because variation in the former may have negative effects on fitness. For this reason, it is important to quantify variation in floral traits and plant fecundity in study range limits. The objective of the study is to examine phenotypic variation and differences in reproduction in endemic Lilium pomponium in the Maritime and Ligurian Alps in relation to environmental variation across its distribution range. In this species, marginal climatic populations occur both in the peripheral and central geographical locations of the distribution range; hence, geographical and ecological gradients are not concordant. Floral trait variation is related to local environmental conditions with an array of interactions among resource availability, potential pollen limitation and population size that are differentially related to floral traits. Contrary to the general expectation, all central and peripheral populations had similar, moderate seed production with each group limited by different factors acting on different stages of the life-history strategy. Our results are in line with the idea that general expectations are confirmed only when its assumptions are met and that the differences in pollination environment along an environmental gradient may not be the main determinant of the distribution limit.

Hurricanes overcome migration lag and shape intraspecific genetic variation beyond a poleward mangrove range limit

Expansion of many tree species lags behind climate change projections. Extreme storms can rapidly overcome this lag, especially for coastal species, but how will storm-driven expansion shape intraspecific genetic variation? Do storms provide recruits only from the nearest sources, or from more distant sources? Answers to these questions have ecological and evolutionary implications, but empirical evidence is absent from the literature. In 2017, Hurricane Irma provided an opportunity to address this knowledge gap at the northern range limit of the neotropical black mangrove (Avicennia germinans) on the Atlantic coast of Florida, USA. We observed massive post-hurricane increases in beach-stranded A. germinans propagules at, and past, this species' present day range margin when compared to a previously surveyed nonhurricane year. Yet, propagule dispersal does not guarantee subsequent establishment and reproductive success (i.e., effective dispersal). We also evaluated prior effective dispersal along this coastline with isolated A. germinans trees identified beyond the most northern established population. We used 12 nuclear microsatellite loci to genotype 896 hurricane-driven drift propagules from nine sites and 10 isolated trees from four sites, determined their sources of origin, and estimated dispersal distances. Almost all drift propagules and all isolated trees came from the nearest sources. This research suggests that hurricanes are a prerequisite for poleward range expansion of a coastal tree species and that storms can shape the expanding gene pool by providing almost exclusively range-margin genotypes. These insights and empirical estimates of hurricane-driven dispersal distances should improve our ability to forecast distributional shifts of coastal species.

Is the central-marginal hypothesis a general rule? Evidence from three distributions of an expanding mangrove species, Avicennia germinans (L.) L

The central-marginal hypothesis (CMH) posits that range margins exhibit less genetic diversity and greater inter-population genetic differentiation compared to range cores. CMH predictions are based on long-held "abundant-centre" assumptions of a decline in ecological conditions and abundances towards range margins. Although much empirical research has confirmed CMH, exceptions remain almost as common. We contend that mangroves provide a model system to test CMH that alleviates common confounding factors and may help clarify this lack of consensus. Here, we document changes in black mangrove (Avicennia germinans) population genetics with 12 nuclear microsatellite loci along three replicate coastlines in the United States (only two of three conform to underlying "abundant-centre" assumptions). We then test an implicit prediction of CMH (reduced genetic diversity may constrain adaptation at range margins) by measuring functional traits of leaves associated with cold tolerance, the climatic factor that controls these mangrove distributional limits. CMH predictions were confirmed only along the coastlines that conform to "abundant-centre" assumptions and, in contrast to theory, range margin A. germinans exhibited functional traits consistent with greater cold tolerance compared to range cores. These findings support previous accounts that CMH may not be a general rule across species and that reduced neutral genetic diversity at range margins may not be a constraint to shifts in functional trait variation along climatic gradients.