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.

Figure 1.

Mean (±s.e.) annual sediment accretion (open diamonds) and elevation (filled circles) rates measured for five treatments of mangrove seedling density over 28 months at Palakuda, Sri Lanka.

Figure 2.

Percentage mortality of A. marina seedlings after growing for six months at four different densities and in four different blocks; block 4 was at the lowest tidal level (close to the forest fringe) and block 1 was the highest. Filled diamond, block 1; open square, block 2; filled triangle, block 3; crosses, block 4.

Figure 3.

Mean (±s.e.) aboveground dry weight in species mix treatments after 4 years of growth. A, A. marina; B, B. gymnorrhiza; C, C. tagal; all other treatments are possible combinations of these three species.

Figure 4.

Mean (±s.e.) number of mangrove wildings in species mix treatments after 3 years of growth. A, A. marina; B, B. gymnorrhiza; C, C. tagal; all other treatments are combinations of these species and an unplanted control (Cont).

Figure 5.

Relationships between aboveground biomass (kg dry weight) and (a) sediment water content (% by weight in top 1 cm) in 2007 and (b) sediment temperature (1 cm depth) in 2008. Regression equations were (a) per cent moisture = 15.9 + 0.000274 (biomass), R² = 0.27, p = 0.001. (b) temp = 33.8–0.000091 (biomass), R² = 0.64, p < 0.001.