Intra- and interspecific facilitation in mangroves may increase resilience to climate change threats

Abstract

Mangroves are intertidal ecosystems that are particularly vulnerable to climate change. At the low tidal limits of their range, they face swamping by rising sea levels; at the high tidal limits, they face increasing stress from desiccation and high salinity. Facilitation theory may help guide mangrove management and restoration in the face of these threats by suggesting how and when positive intra-and interspecific effects may occur: such effects are predicted in stressed environments such as the intertidal, but have yet to be shown among mangroves. Here, we report the results of a series of experiments at low and high tidal sites examining the effects of mangrove density and species mix on seedling survival and recruitment, and on the ability of mangroves to trap sediment and cause surface elevation change. Increasing density significantly increased the survival of seedlings of two different species at both high and low tidal sites, and enhanced sediment accretion and elevation at the low tidal site. Including Avicennia marina in species mixes enhanced total biomass at a degraded high tidal site. Increasing biomass led to changed microenvironments that allowed the recruitment and survival of different mangrove species, particularly Ceriops tagal.

Effects of species richness, identity and environmental variables on growth in planted mangroves in Kenya

“…Removal of the initial forest cover at the field site had led to salinisation of the sediment and a total lack of natural regeneration; under these conditions, it was necessary to plant nursery-reared trees to initiate restoration, and our results suggest that A. marina acts as a 'superplant' (Zedler et al 2001) in fostering ecosystem recovery. These results are consistent with studies on mangrove saplings which have reported Avicennia possessing pioneer traits especially with respect to aboveground development and the facilitation of wildling (saplings external to the plot) recruitment (McKee 1995, Huxham et al 2010, Friess et al 2012 and with work in some terrestrial grassland (Hooper & Dukes 2004, Hooper et al 2005, freshwater plant, microbial (Downing 2005, Bruno et al 2006) and salt marsh communities (Callaway et al 2003) that showed that productivity was driven by a species identity effect.…”

The effects of salinity on growth and survival of mangrove seedlings changes with age

“…Based on the rate of survival, growth performances and water conservation strategy, the genus Rhizophora shows a wide range of adaptive features hence it is suggested that Rhizophora is better used in mangrove planting under moderate conditions (although other genera, such as Avicennia, may be used as 'nurse species' under extreme salinity conditions; Huxham et al 2010). This study addresses the key point that mangrove seedlings commonly raised and maintained in nurseries are usually irrigated with fresh or brackish water depending on availability, without paying attention to the salinity of water, and seedlings of a few months age are transferred to the field which could result in early seedling mortality.…”

Climate change, biotic interactions and ecosystem services

“…They conclude that extra plant species should be added in order to conserve bumble-bee diversity and the extension of their field season, and they suggest the plant species that could be used. Huxham et al (2010) present an experiment which shows that biotic interactions between mangrove species may help guide mangrove management and restoration in the face of climate change. Mangroves provide a range of goods and services including coastal protection, fishery nursery habitat and carbon sequestration, yet they are particularly vulnerable to climate change.…”

“…In addition, theories predicting changes in other non-trophic interactions can be very useful. Facilitation theory among plant species is particularly relevant here, and Maestre et al (2010) and Huxham et al (2010) provide examples on how to use and test this theory with experiments, and on how it can inform management practices to ameliorate climate change impacts.…”