Human population density is globally three times higher along the coasts than inland, and thus environmental impacts of human activities are greater in magnitude on coastal ecosystems such as beaches and dunes. Vulnerability assessment (the loss of capacity to return to the original dynamic state after system displacement) is thus necessary to evaluate the conservation status and determine the most relevant disturbance events. Twenty-six sites along 902 km of Gulf of Mexico coastline, varying in conservation status and sedimentary dynamics, were sampled. At each site a vulnerability index (VI) was calculated based on variables that described geomorphological condition, marine influence, aeolian influence, vegetation condition and human effects. Vulnerability was very variable along the coast and only 19% of the sampled locations (mostly in the central Gulf of Mexico) displayed low vulnerability. Cluster analyses of the values assigned to the checklists for each location grouped the studied sites into three, according to their VI values. Low vulnerability locations had abundant sediment supply and low human impact. Locations with medium to high VI were mostly affected by their natural geomorphological and marine features and had medium to intense human activities. Management strategies should consider the observed variability in vulnerability, the natural dynamics of these systems and the role of human activities and interests, in order to achieve adequate policies and establish well-informed priorities for integrated coastal zone management.
The role of plant polyphenols as defenses against insect herbivores is controversial. We combined correlative field studies across three geographic regions (Northern Mexico, Southern Mexico, and Costa Rica) with induction experiments under controlled conditions to search for candidate compounds that might play a defensive role in the foliage of the tropical oak, Quercus oleoides. We quantified leaf damage caused by four herbivore guilds (chewers, skeletonizers, leaf miners, and gall forming insects) and analyzed the content of 18 polyphenols (including hydrolyzable tannins, flavan-3-ols, and flavonol glycosides) in the same set of leaves using high performance liquid chromatography and mass spectrometry. Foliar damage ranged from two to eight percent per region, and nearly 90% of all the damage was caused by chewing herbivores. Damage due to chewing herbivores was positively correlated with acutissimin B, catechin, and catechin dimer, and damage by mining herbivores was positively correlated with mongolinin A. By contrast, gall presence was negatively correlated with vescalagin and acutissimin B. By using redundancy analysis, we searched for the combinations of polyphenols that were associated to natural herbivory: the combination of mongolinin A and acutissimin B had the highest association to herbivory. In a common garden experiment with oak saplings, artificial damage increased the content of acutissimin B, mongolinin A, and vescalagin, whereas the content of catechin decreased. Specific polyphenols, either individually or in combination, rather than total polyphenols, were associated with standing leaf damage in this tropical oak. Future studies aimed at understanding the ecological role of polyphenols can use similar correlative studies to identify candidate compounds that could be used individually and in biologically meaningful combinations in tests with herbivores and pathogens.
Salinity tolerance in plant species varies widely due to adaptation and acclimation processes at the cellular and whole-plant scales. In mangroves, extreme substrate salinity induces hydraulic failure and ion excess toxicity and reduces growth and survival, thus suggesting a potentially critical role for physiological acclimation to salinity. We tested the hypothesis that osmotic adjustment, a key type of plasticity that mitigates salinity shock, would take place in coordination with declines in whole-plant hydraulic conductance in a common garden experiment using saplings of three mangrove species with different salinity tolerances (Avicennia germinans L., Rhizophora mangle L. and Laguncularia racemosa (L.) C.F. Gaertn., ordered from higher to lower salinity tolerance). For each mangrove species, four salinity treatments (1, 10, 30 and 50 practical salinity units) were established and the time trajectories were determined for leaf osmotic potential (Ψ), stomatal conductance (g), whole-plant hydraulic conductance (K) and predawn disequilibrium between xylem and substrate water potentials (Ψ). We expected that, for all three species, salinity increments would result in coordinated declines in Ψ, g and K, and that the Ψ would increase with substrate salinity and time of exposure. In concordance with our predictions, reductions in substrate water potential promoted a coordinated decline in Ψ, g and K, whereas the Ψ increased substantially during the first 4 days but dissipated after 7 days, indicating a time lag for equilibration after a change in substratum salinity. Our results show that mangroves confront and partially ameliorate acute salinity stress via simultaneous reductions in Ψ, g and K, thus developing synergistic physiological responses at the cell and whole-plant scales.
In comparison with A. germinans, a tree species with wide girth and flare at the base, R. mangle supports a thinner stem of higher mechanical resistance that is stabilized by rhizophores resembling flying buttresses. This provides a unique strategy to increase tree slenderness and height in the typically unstable substrate on which the trees grow, at a site that is subject to frequent storms.
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