Genetically-based adaptive trait shifts at an expanding mangrove range margin

Abstract

ABSTRACTMany species are expanding beyond their distributional range margins in response to a warming planet. Due to marginal environmental conditions and novel selection pressures, range margins may foster unique genetic adaptations that can better enable species to thrive under the extreme climatic conditions at and beyond their current distributional limits. Neotropical black mangrove (Avicennia germinans) is expanding poleward into temperate salt marsh along Atlantic Florida, USA, with field evidence of adaptive trait shifts within range-margin A. germinans populations. However, whether these adaptive shifts have a genetic basis remains to be answered.We monitored twenty A. germinans maternal cohorts from areas in both the Atlantic Florida range core and margin in a greenhouse common garden with annual temperatures analogous to range-margin conditions. We measured variation in a series of phenotypic traits starting at initial planting of field-collected propagules and continuing until two years development.Maternal cohorts from the Atlantic Florida range margin consistently outperformed those from the range core throughout the experiment. Range-margin cohorts survived in greater numbers, established faster, and were less stressed under winter chilling and sub-zero temperatures that are often reached at the Atlantic range margin, but not within the range core. Range-margin cohorts did not grow taller, but instead 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.Synthesis. We confirmed that there is a genetic basis to adaptive trait shifts towards an expanding mangrove range margin. 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 in the future. In addition, our documentation of adaptive trait variation among maternal cohorts of an ecologically-important mangrove foundation species, quantitative data that is lacking for mangroves, should help inform mangrove restoration initiatives.