Physiological ecology of Malaysian mangroves in response to sea level rise
Muhammad Nor, Siti Mariam
Mangroves are threatened by rising sea levels as a consequence of climate change or locally altered coastal hydrology. Prolonged submergence resulting from increased flooding regimes may negatively influence mangrove growth and survival, particularly at the seedling stage. Seedlings from two dominant Malaysian species, Bruguiera gymnorrhiza and Rhizophora apiculata, were exposed to differing durations of flooding in order to simulate a range of sea level rise conditions. This experiment was conducted in the glasshouse for 11 weeks with four flooding treatments: 6 hrs, 18 hrs, 24 hrs and 24 hrs (stagnant). Survival, growth, xylem anatomy and a suite of ecophysiological responses were monitored to quantify and understand plant response to varying degrees of inundation. The ecophysiological measurements comprised leaf chlorophyll content, chlorophyll fluorescence, maximum photosynthetic rate (Amax) and stomatal conductance (gs). Leaf carbohydrate reserves (non-structural carbohydrates, starch and sucrose) and leaf area were also quantified at the end of the experiment, after week 11 (chapter 3). Mangrove seedling survival was 100% under all flooding treatments. Longer flooding significantly increased the seedlings’ height and stem diameter but resulted in fewer leaves produced under long submergence. Morphological adaptations were observed during flooding treatment (i.e. lenticels structure on seedlings stem and adventitious root). Leaf physiological properties (chlorophyll content and chlorophyll fluorescence) exhibited significantly higher values under long submergence, although they showed period of stress during flooding experiment. Maximum photosynthesis rate and stomatal conductance remained higher during the early part of the flooding experiment and lower toward the end of flooding period. Flooding resulted in higher accumulation of total non-structural carbohydrates in B. gymnorrhiza than R. apiculata although the effect was not significant. Plant leaf area was significantly higher under the 24 hour (stagnant) treatment particularly in B. gymnorrhiza seedlings, implying the seedlings expanded their leaf area under long submergence. Overall, most of the response variables were not affected by the flooding treatment and in fact seedlings showed increased growth under high flooding. Seedlings exposed to longer submergence times may suffer from oxygen deficiency, particularly where submergence times approach 24 hrs (as simulated by the 24 hr ‘stagnant’ treatment). To examine how flooding treatments affected the plant stem, xylem anatomy was quantified in both mangrove species. Small segments of the plant apex (3 cm from the shoot tip) and plant stem (12-15 cm from the shoot tip) were examined using light and electron microscopy. Xylem vessel diameter, cell wall thickness, vessel density and vessel hydraulic diameter were quantified. Vessel diameter varied between plant sections and species. Vessel density and lumen area were significantly higher at the plant apex (p < 0.05, p < 0.001 respectively), but vessel hydraulic diameter was significantly higher at plant mid stem (p < 0.001). There were surprisingly few effects on vessel metrics in response to the flooding treatment: at the extreme treatment of 24 hrs flooding, there was marginally, but not significantly, less cell wall thickening in one species (R. apiculata); vessel lumen area increased to a small extent (p > 0.05), but vessel density did not vary among treatments. Overall, the data demonstrate significant differences in xylem anatomy associated with location on the plant (apex or stem), but very minor effects of flooding, at least following the 11 week experimental treatment (chapter 4). This study also investigated the carbon dynamics of a mangrove forest site in Malaysia. Standing stock was quantified along with above- and belowground production at a site on the east coast of the Kelantan delta, Malaysian peninsular during 2014-2016. The above- and below-ground standing biomass of mangrove trees were quantified: although above ground biomass is larger (276.54 t ha-1), the allocation below ground was significant (20.81 t ha-1), the two contributing to a total biomass of 297.35 t ha-1. Aboveground productivity was 4.81 t ha-1 y-1 and belowground productivity was 12.70 t ha-1 y-1, peaking seasonally during the monsoon period in March and December 2015. These values are among the highest recorded for mangrove forests globally and the data also suggested a rapid turnover time for roots, of approximately 19 months. Thus, although standing biomass is higher above ground, more productivity is allocated below ground. Fifty-five percent of the root biomass was found in the top 30 cm and 78% of the roots, in all soil layers, consisted of fine roots (< 3 mm diameter), making the fine root component a particularly important carbon pool in this ecosystem. A positive relationship was found between fine root biomass, sediment carbon and nitrogen content. Soil temperature, salinity and dissolved oxygen were also investigated in relation to belowground production (chapter 5). This study provides evidence that the seedlings of at least these two mangrove species can survive under flooded conditions. This may have occurred here because of high oxygen levels in the experimental conditions, suggesting that negative effects of enhanced flooding in the field could arise from oxygen starvation rather than direct effects of flooding. In addition, it showed very rapid belowground biomass accumulation and turnover in a natural mangrove forest.