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Huanglongbing (HLB), referred to as citrus greening disease, is a bacterial disease impacting citrus production worldwide and is fatal to young trees and mature trees of certain varieties. In some areas, the disease is devastating the citrus industry. A successful solution to HLB will be measured in economics: citrus growers need treatments that improve tree health, fruit production, and most importantly, economic yield. The profitability of citrus groves is the ultimate metric that truly matters when searching for solutions to HLB. Scientific approaches used in the laboratory, greenhouse, or field trials are critical to the discovery of those solutions and to estimate the likelihood of success of a treatment aimed at commercialization. Researchers and the citrus industry use a number of proxy evaluations of potential HLB solutions; understanding the strengths and limitations of each assay, as well as how best to compare different assays, is critical for decision-making to advance therapies into field trials and commercialization. This perspective aims to help the reader compare and understand the limitations of different proxy evaluation systems based on the treatment and evaluation under consideration. The researcher must determine the suitability of one or more of these metrics to identify treatments and predict the usefulness of these treatments in having an eventual impact on citrus production and HLB mitigation. As therapies advance to field trials in the next few years, a reevaluation of these metrics will be useful to guide future research efforts on strategies to mitigate HLB and vascular bacteria] pathogens in other perennial crops. [ABSTRACT FROM AUTHOR]

Neotropical black mangrove (Avicennia germinans) is expanding poleward into temperate salt marsh along Atlantic Florida, USA, with field evidence of trait shifts consistent with greater cold tolerance within range margin populations. However, whether these shifts have a genetic basis remains unanswered. To address this gap, we measured multiple phenotypic traits of twenty A. germinans maternal cohorts from areas in both the Atlantic Florida range core and margin in a 2-year greenhouse common garden with annual temperatures analogous to range margin conditions. Compared to those from the range core, range margin cohorts survived in greater numbers, established (i.e., produced first true leaves) more quickly, and were less stressed under winter temperatures. Range margin cohorts were not taller, but invested more into lateral growth and biomass accumulation that presumably reflects adaptation to their colder and open-canopy environment. Range margin cohorts also exhibited leaf traits consistent with greater resource acquisition that may compensate for a shorter growing season and reduced light quality at higher latitude. Our results suggest that genetically based phenotypic differences better enable these range margin mangroves to thrive within their stressful environment and may facilitate further poleward expansion. An improved understanding of adaptive trait variation within ecologically important mangrove foundation species should also help inform coastal restoration initiatives. [ABSTRACT FROM AUTHOR]

Mangroves form coastal tropical forests in the intertidal zone and are an important component of shoreline protection. In comparison to other tropical forests, mangrove stands are thought to have relatively low genetic diversity with population genetic structure gradually increasing with distance along a coastline. We conducted genetic analyses of mangrove forests across a range of spatial scales; within a 400 m2 parcel comprising 181 Rhizophora mangle (red mangrove) trees, and across four sites ranging from 6–115 km apart in Honduras. In total, we successfully genotyped 269 R. mangle trees, using a panel of 677 SNPs developed with 2b-RAD methodology. Within the 400 m2 parcel, we found two distinct clusters with high levels of genetic differentiation (FST = 0.355), corresponding to trees primarily located on the seaward fringe and trees growing deeper into the forest. In contrast, there was limited genetic differentiation (FST = 0.027–0.105) across the sites at a larger scale, which had been predominantly sampled along the seaward fringe. Within the 400 m2 parcel, the cluster closest to the seaward fringe exhibited low genetic differentiation (FST = 0.014–0.043) with the other Honduran sites, but the cluster further into the forest was highly differentiated from them (FST = 0.326–0.414). These findings contradict the perception that genetic structure within mangroves forests occurs mainly along a coastline and highlights that there is greater genetic structure at fine spatial scales. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Loss of plant biodiversity can result in reduced abundance and diversity of associated species with implications for ecosystem functioning. In ecosystems low in plant species diversity, such as Neotropical mangrove forests, it is thought that genetic diversity within the dominant plant species could play an important role in shaping associated communities. Here, we used a manipulative field experiment to study the effects of maternal genotypic identity and genetic diversity of the red mangrove Rhizophora mangle on the composition and richness of associated soil bacterial communities. Using terminal restriction fragment length polymorphism (T‐RFLP) community fingerprinting, we found that bacterial community composition differed among R. mangle maternal genotypes but not with genetic diversity. Bacterial taxa richness, total soil nitrogen, and total soil carbon were not significantly affected by maternal genotypic identity or genetic diversity of R. mangle. Our findings show that genotype selection in reforestation projects could influence soil bacterial community composition. Further research is needed to determine what impact these bacterial community differences might have on ecosystem processes, such as carbon and nitrogen cycling. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Aim We evaluated underlying mechanisms and genetic effects of climate-driven range expansion of Rhizophora mangle L., a coastal foundation species, along both West ( WFL) and East ( EFL) Florida, USA. Location Eight sites encompassing the entire Florida R. mangle range at a regional scale. Methods We characterized R. mangle population genetic structure with a combination of genetic analyses using seven microsatellite loci and model-based propagule transport. We tested hypotheses on the genetic effects of range expansion along both WFL and EFL. Finally, we compared WFL and EFL range edges and assessed potential factors shaping observed differences. Results Regional-scale Florida R. mangle genetic structure is shaped in part by the non-independent effects of geographical distance and ocean currents that drive asymmetric propagule transport from WFL to EFL. WFL conformed to theoretical expectations of range expansion, with pronounced divergence at the range edge, whereas EFL deviated from expectations. Significant differences in diversity and differentiation at the WFL and EFL range edges were attributed to differences in migration rates, population size and founder effects. Main conclusions Contrasting genetic landscapes at the WFL and EFL range edges are in part the product of variation in ocean circulation and demographic history. These underlying mechanisms may have potential ecological and evolutionary consequences that need to be addressed with further empirical research. [ABSTRACT FROM AUTHOR]

PREMISE OF THE STUDY: The Last Glacial Maximum (LGM) was a period of massive range contraction. Post-LGM, water-dispersed coastal species, including the red mangrove (Rhizophora mangle), expanded poleward as propagules were transported by ocean currents. We assessed postglacial marine expansion pathways for ft. mangle within the Caribbean Basin and Florida. METHODS: Six microsatellite loci were used to genotype 237 individuals from nine ft. mangle populations in the Caribbean, Florida, and Northwest Africa. We evaluated genetic variation, population structure, gene flow along alternative post-LGM expansion pathways to Florida, and potential long-distance dispersal (LDD) from West Africa to Caribbean islands. KEY RESULTS: These ft. mangle populations had substantial genetic structure (FST = 0.37, P < 0.0001) with three discrete population clusters (Caribbean mainland, Caribbean islands, and Florida). Genetic connectivity along the mainland pathway (Caribbean mainland to Florida) vs. limited gene dispersal along the Antilles Island pathway (Caribbean islands to Florida) supported Florida recolonization from Caribbean mainland sources. Genetic similarity of Northwest Africa and two Caribbean islands provided evidence for trans-Atlantic LDD. We did not find a pattern of decreasing genetic diversity with latitude. CONCLUSIONS: We outline a complex expansion history for ft. mangle, with discrete pathways of recolonization for Florida and Caribbean islands. Contrary to expectation, connectivity to putative Caribbean mainland refugial populations via ocean currents, and not latitude, appears to dictate genetic diversity within Caribbean island and Florida ft. mangle. These findings provide a framework for further investigation of additional water-dispersed neotropical species, and insights for management initiatives. [ABSTRACT FROM AUTHOR]

Mangrove ecosystems in the Caribbean are frequently exposed to hurricanes, leading to structural and regenerative change that elicit calls for recovery action. For those mangroves unaffected by human modifications, recovery can occur naturally. Indeed, observable natural recovery after hurricanes is the genesis of the "disturbance adaptation" classification for mangroves; while structural legacies exist, unaltered stands often regenerate and persist. However, among the >7,000 islands, islets, and cays that make up the Caribbean archipelago, coastal alterations to support development affect mechanisms for regeneration, sediment distribution, tidal water conveyance, and intertidal mangrove transgression, imposing sometimes insurmountable barriers to natural post‐hurricane recovery. We use a case study approach to suggest that actions to facilitate recovery of mangroves on Caribbean islands (and similar settings globally) may be more effective when focusing on ameliorating pre‐existing anthropogenic stressors. Actions to clean debris, collect mangrove propagules, and plant seedlings are noble endeavors, but can be costly and fall short of achieving recovery goals in isolation without careful consideration of pre‐hurricane stress. We update a procedural framework that considers six steps to implementing "Ecological Mangrove Restoration" (EMR), and we apply them specifically to hurricane recovery. If followed, EMR may expedite actions by suggesting immediate damage assessment focused on hydrogeomorphic mangrove type, hydrology, and previous anthropogenic (or natural) influence. Application of EMR may help to improve mangrove recovery success following catastrophic storms, and reduce guesswork, delays, and monetary inefficiencies. [ABSTRACT FROM AUTHOR]

Climate-driven range expansion of ecosystem-defining foundation species can have wide-reaching ecological consequences. Expansion may also result in mating system changes in these foundation species because of the ecological characteristics of range margins, such as greater conspecific isolation and reduced pollinator availability. It is important to understand how mating systems may change during expansion due to their direct influence on intraspecific genetic and demographic dynamics. Here, we used 12 microsatellite loci to genotype progeny arrays of the neotropical black mangrove (Avicennia germinans) at six collection sites (n = 23 maternal trees; 1,612 genotyped propagules) along a latitudinal gradient towards a northern distributional limit on the Atlantic coast of Florida, USA (27.56–30.01oN), where mangroves have expanded into salt marsh over the past several decades. We assessed mating system variation at three spatial scales. First, at the species-distribution level, published outcrossing rates for tropical conspecifics were more than two times higher than those for subtropical Florida A. germinans , consistent with reductions in pollinator diversity and in mangrove abundance with latitude. Second, at the population level, Florida outcrossing rates did not systematically decline towards the northern range limit, but instead, a more open pollen-dispersal neighbourhood at the transition from mangrove to salt marsh dominance may elevate outcrossing until conspecific abundances become too low towards the range limit. Third, at the individual level, outcrossing increased as conspecific cover increased at the Florida range margin, consistent with density-dependent plastic shifts in mating system. These findings suggest that ecological structure influences the A. germinans mating system at varying spatial scales. Further research needs to evaluate the effect of A. germinans mating system variation on the survival and fitness of offspring and on the extent of population-level local adaptation at expanding distributional limits. [ABSTRACT FROM AUTHOR]