Variation in propagule mass and its effect on carbon assimilation and seedling growth of red mangrove ( Rhizophora mangle ) in Florida, USA

Short-distance barriers affect genetic variability of Rhizophora mangle L. in the Yucatan Peninsula

The environmental variability at local scale results in different physiognomic types of mangrove forest. However, this variability has never been considered in studies of mangrove genetic variability. This study analyzed the genetic and morphological variability and structure of Rhizophora mangle at regional and local scales in the Yucatan Peninsula. Thirteen mangrove populations (eight scrub and five tall), located in seven sites, were sampled, and their morphological variability and relationship with the availability of phosphorus and salinity were analyzed. The diversity and genetic structure were estimated at different hierarchical levels with nine microsatellites, also Bayesian inference and Principal Coordinates Analysis were used. We found a great morphological variability of R. mangle that responded to local environmental variability and not to the precipitation gradient of the peninsula. The genetic diversity found in the peninsula was greater than that reported for other populations in Mexico and was grouped into two regions: the Gulf of Mexico and the Caribbean Sea. At a local scale, tall and scrub mangroves had significant genetic differentiation suggesting that ecological barriers promote genetic differentiation within sites. These results need to be considered in future population genetic studies and for mangrove management and conservation.

Facilitating your replacement? Ecosystem engineer legacy affects establishment success of an expanding competitor

Interactions with resident species can affect the rate that expanding species invade novel areas. These interactions can be antagonistic (biotic resistance), where resident species hinder invasive establishment, or facilitative (biotic assistance), where residents promote invasive establishment. The predominance of resistance or assistance could vary with the abiotic context. We examined how the effects of a resident ecosystem engineer interact with abiotic stress to resist or assist the establishment of an expanding competitor. In Florida salt marshes, native cordgrass, Spartina alterniflora, is an influential ecosystem engineer that, when dead, exerts a legacy effect by forming persistent wrack patches. We examined how the legacy effect of Spartina wrack varies with spatial context and abiotic conditions to influence establishment of the northward-expanding black mangrove, Avicennia germinans. Field surveys documented that Spartina wrack and Avicennia propagules co-occur in the high intertidal zone, and we conducted two outdoor mesocosm experiments to investigate this association. Wrack positively affected propagule establishment when propagules were beneath wrack, but negatively affected establishment when propagules were above wrack. The abiotic tidal regime influences the magnitude of wrack effects by controlling ambient moisture, and the positive and negative effects of wrack were stronger in low moisture conditions that simulated desiccation stress during harsh neap tides. Thus, the same resident engineer can either resist or assist an expanding competitor and the magnitude of these effects depends on abiotic conditions. We propose that under harsh conditions, there is greater scope for an engineer's mediating influence to affect associated species, both positively and negatively.

Precocious reproduction increases at the leading edge of a mangrove range expansion

Climate change‐driven shifts in species ranges are ongoing and expected to increase. However, life‐history traits may interact with climate to influence species ranges, potentially accelerating or slowing range shifts in response to climate change. Tropical mangroves have expanded their ranges poleward in the last three decades. Here, we report on a shift at the range edge in life‐history traits related to reproduction and dispersal. With a common garden experiment and field observations, we show that Rhizophora mangle individuals from northern populations reproduce at a younger age than those from southern populations. In a common garden at the northern range limit, 38% of individuals from the northernmost population were reproductive by age 2, but less than 10% of individuals from the southernmost population were reproductive by the same age, with intermediate amounts of reproduction from intermediate latitudes. Field observations show a similar pattern of younger reproductive individuals toward the northern range limit. We also demonstrate a shift toward larger propagule size in populations at the leading range edge, which may aid seedling growth. The substantial increase in precocious reproduction at the leading edge of the R. mangle range could accelerate population growth and hasten the expansion of mangroves into salt marshes.

Strong positive growth responses to salinity by Ceriops tagal, a commonly occurring mangrove of the Gujarat coast of India

BACKGROUND AND AIMS

Mangroves of Western Gujarat (India) are subject to die-back. Salinity intolerance is one possible cause, especially in young plants. We therefore quantified the extent to which young plants of one widely occurring mangrove species (Ceriops tagal) tolerate high salt in terms of establishment, growth, water status, proline content and mineral accumulation.

METHODOLOGY

In a greenhouse study, juvenile plants were established from mature propagules over 40 days in soil containing added NaCl, raising soil water salinity to 0.2, 2.5, 5.1, 7.7, 10.3, 12.6, 15.4, 17.9, 20.5 and 23.0 ppt (w/v). Growth and physiological characteristics were monitored over the subsequent 6 months.

PRINCIPAL RESULTS

Despite a negative relationship between the percentage of young plant establishment and salt concentration (50 % loss at 22.3 ppt), the remaining plants proved highly tolerant. Growth, in dry weight, was significantly promoted by low salinity, which is optimal at 12.6 ppt. Water content, leaf expansion and dry matter accumulation in tissues followed a similar optimum curve with leaf area being doubled at 12.6 ppt NaCl. Salinity >12.6 and <23 ppt inhibited plant growth, but never to below control levels. Root:shoot dry weight ratios were slightly reduced by salinity (maximum 19 %), but the water potential of roots, leaves and stems became more negative as salinity increases while proline increases in all tissues. The concentration of Na increased, whereas concentrations of K, Ca, N and P decreased and that of Mg remained stable.

CONCLUSIONS

Ceriops tagal has a remarkably high degree of salinity tolerance, and shows an optimal growth when soil water salinity is 12.6 ppt. Salinity tolerance is linked to an adaptive regulation of hydration and ionic content. The cause of localized die-back along the coastal region of Gujarat is thus unlikely to be a primary outcome of salinity stress although amendments with Ca and K, and perhaps proline, may help protect against extreme salinity.

The Ecology and Physiology of Viviparous and Recalcitrant Seeds

▪ Abstract  Understanding seed physiology is central to reconstructing how angiosperms have evolved, to characterizing dormancy and germination regimes shared by suites of species, and to devising sound strategies for seed bank conservation, agriculture, and forestry. While species with dormant seeds have received the lion's share of attention, hundreds of plant species exhibit no seed dormancy and germinate either viviparously on the parent plant or shortly after release. Embryos of these recalcitrant and viviparous species cannot tolerate the maturation drying that is usually prerequisite to dormancy; such desiccation intolerance creates challenges for storing and preserving such embryos. I review the physiology, morphology, and ecology of these desiccation-intolerant, nondormant lineages. Differences in the production and function of plant hormones are implicated in the occurrence of recalcitrance and vivipary in plant families. Plant hormones are key regulators of seed physiology and simultaneously coordinate responses of the seedling and mature plant to their environment. Desiccation-intolerant embryos occur most commonly among species of wet or flooded environments and have evolved multiple times in disparate lineages. Natural selection in wetland environments simply may not eliminate these seed types or may select for changes in hormone physiology that simultaneously affect both maternal and embryonic tissues. Integrative data from ecological, genetic, and physiological studies are needed to elucidate evolutionary origins and maintenance of reproductive strategies in organisms.