Influence of species richness and environmental context on early survival of replanted mangroves at Gazi bay, Kenya

Abstract

Mangrove reforestation projects often suffer from low sapling survival, especially after transplanting saplings from nurseries to reforestation areas. This may be due to the sediment conditions at the target site, the planting strategy or failure to reestablish ecosystem processes. We examined experimentally the influence of environmental context, species richness and identity, sapling height and position on sapling survival and environmental variables linked to ecosystem functioning at deforested sites in Gazi Bay, Kenya. At site 1, a high shore location, 32 plots (36 m 2 ) were planted with 8 treatments: all possible combinations of Avicennia marina, Bruguiera gymnorrhiza, and Ceriops tagal and an unplanted control (total: 3390 saplings; 4 plots/treatment). At site 2, a low shore location, the influence of sapling height, sapling position and sediment depth were tested by planting with 697 Sonneratia alba in a single monospecific plot (341 m 2 ). After *2 years, there were significant differences in survival among the three species at site 1 with Bruguiera gymnorrhiza recording the lowest survival rate (29%). Survival was correlated with salinity (a strong effect) and height above chart datum (a weaker effect) at site 1. Sapling position did not significantly affect survival at either site. There was thus no evidence that early survival of transplanted saplings is influenced by the species mix in which they are grown, or by their position in the plot. Rather the tolerance of individual species to salinity was the key to their survival at the high tidal site. Species mix also had no significant effects on environmental variables in the plots. The former presence of a species at a site does not guarantee it will succeed there again if environmental degradation has exceeded species' tolerance.

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

“…This inference is supported by a study of the restoration of hypersaline areas, in which transplanted seedlings older than five months survived under conditions that prevented the establishment of wild plants (Kirui et al 2008). Hence the present work supports field observations that seedlings beyond the critical age of early vulnerability to salt may be used successfully in restoring salinized areas.…”

“…Salinity is one of the most important factors influencing the health, distribution, growth and productivity of mangroves (Ball 2002). It often shows high spatial and temporal fluctuations, driven by inputs of fresh water and sea water, inundation, ground water seepage and evaporation (Naidoo 1985;Aziz & Khan 2001a), and has been shown to determine the survival and growth of planted mangrove seedlings in studies of mangrove restoration projects in a range of sites (Bosire et al 2008;Kirui et al 2008;Krauss et al 2008;Hoppe-Speer et al 2011).…”

“…The salinity range that mangrove plants are able to tolerate in the field varies according to the species and it depends on the ability to maintain high water use efficiency (Reef & Lovelock 2015). For example, Avicennia marina demonstrates extreme salt tolerance (Kirui et al 2008), Rhizophora mucronata shows relatively high salt tolerance whilst species such as Bruguiera sexangula, Sonneratia caseolaris, and Aegiceras corniculatum show moderate or low salt tolerance (Hogarth 2007;Jayatissa et al 2008). In addition to survival and growth, the effects of salinity on tissue water potential, physiological aspects like stomatal conductance and the role of inorganic ions in influencing mangrove seedling response to different salinity levels have all been reported (Smith & Snedaker 1995;Aziz & Khan 2001a;b;Reef & Lovelock 2015).…”

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

“…Mangroves may survive rising sea levels by migrating inland or by facilitating soil surface elevation increases (McLeod & Salm 2006;McKee et al 2007a); where neither is possible, they will drown out and die. Third, hypersaline conditions are common at the high-tidal limits of mangrove forests and salinity is often the main factor limiting growth and recovery following disturbance (Jayatissa et al 2008;Kirui et al 2008). All climate change scenarios predict increased temperatures, implying faster evaporation of seawater.…”

“…Managers must also protect high-quality forests by reducing human impacts, for example by developing alternative livelihoods for those dependent on mangrove exploitation (McLeod & Salm 2006). However, the direct planting of new trees is necessary in areas that cannot recover otherwise (Kairo et al 2001;Bosire et al 2008;Kirui et al 2008). It is likely to become more important if managers are to keep pace with the scale of changes predicted under most climate change scenarios; for example, it will be necessary to establish mangroves in previously unforested areas as current habitat disappears.…”