Strong seedling recruitment does not limit mangrove vulnerability to harvest

Abstract

Ecological studies on the impacts of timber harvesting contributed to inform sustainable management strategies of tropical forests. However, these studies rely strongly on two major assumptions: (i) strong seedlings recruitment predispose for positive population dynamics, and (ii) more adult trees is a guarantee for a strong reproductive capability of the population. These assumptions are applied without accounting for the life history of the harvested species. Here, we revisit these assumptions in light of the life history theory, which predicts different responses of short-and long-lived species to perturbation. We predict that harvesting adults, rather than reducing seedling recruitment of longlived species, would have greater impact on population dynamics. We tested our prediction on three mangrove species in South Africa. First, we reconstructed the projection matrices for three mangrove species in the Mngazana Estuary of Eastern Cape province of South Africa. Next, we simulated a range of harvest intensities for different life stages and explored how harvesting influences population dynamics. Finally, we examined the age-specific mortality trajectories for all three species. We found that populations of all three species were closer to their stable stage distribution. Contrary to popular belief, we found that changes in seedling recruitment will have minimal effects on mangrove longterm population dynamics, a finding consistent with the life history theory. However, contrary to expectation, simulating harvest of adults had minimal effect on population dynamics. This is due to low reproductive values for these late stages. Our analysis of age-specific mortality trajectories further provided evidence for positive senescence particularly for Avicennia which was the least resilient to harvest. Our findings cast doubt on the traditional forest management strategies that rely strongly on seedling density as a metric of sustainability and forbid unquestionably harvesting large individuals. We call for caution while generalizing forest management strategies irrespective of the life history of the species at hand.

Spatial and temporal analysis for mangrove community healthiness in Liki Island, Papua-Indonesia

“…The number of seedlings in this area was greater than other studies in a threatened protected forest in South Sumatra ranged from 100-800 seedling/ha [37]. The high number of recruitments in mangrove forests has indicated that the forest was IOP Publishing doi:10.1088/1755-1315/944/1/012017 5 in stable condition, while it was still limited to harvest mangrove wood [38]. Sapling stand-level had a gradually increasing trend from landward to seaward zone, at 200 to 400 sapling/ha, respectively.…”

Population Structure of an African Cycad: Fire May Stimulate the Coning Phenology of Encephalartos lanatus (Zamiaceae) and Also Predispose Its Cones to Damage

“…The reason is that the traditional belief in the reverse J-growth trajectory as indicative of positive growth does not take into consideration the life history of the species at hand. Such consideration is critical since long-and shortlived species respond differently to perturbations ( [64]; see also Gaoue and Yessoufou [65]). This is because long-lived species, unlike short-lived ones, invest heavily in long-term survival strategies [66].…”

“…This is because long-lived species, unlike short-lived ones, invest heavily in long-term survival strategies [66]. According to life history theory, the survival of reproductive adults drives long-term population growth for long-lived species [64,65,67,68]. As such, low proportions of early life stages, such as those we found in the present study for a long-lived species like E. lanatus, would have limited impacts on the long-term population dynamics of this cycad species (see [65]).…”

Forest Structure and Projections of Avicennia germinans (L.) L. at Three Levels of Perturbation in a Southwestern Gulf of Mexico Mangrove

“…Mangrove species breed repeatedly, and trees from different generations form part of the structure of the forest, yet recruitment alone is not enough to maintain or regenerate a mangrove forest because mortality from size class I to size class II was large at all the observed areas. Climate extremes (cold and warm) and herbivory contribute to seedling mortality [50,51] and the loss at smaller size classes has a strong influence on larger classes [52]. Larger trees suffer senescence and contribute less to recruits, thus conserving larger trees while harvesting relatively young trees was not a good management practice [53].…”