Acclimation of a dominant mangrove plant (Kandelia candel) to soil texture and its response to canopy shade

Kandelia candel, a dominant plant species in Hong Kong mangroves, had different growth strategies during its seedling stage for acclimation to various soil types. Leaves of the seedlings grown in sandy soils (coarse texture) were significantly thicker than those in loamy–sandy and silty soils (fine texture). However, leaf weight per unit leaf area of seedlings grown in sandy soils was similar to that in loamy-sandy soils and was 1.60 times that in silty soils. These indicate that K. candel in sandy soils had developed a more loose leaf structure and a xerophilous characteristic during its seedling stage for acclimation to the features of coarse textured soils, which were low water holding capacity and low mineral content. In addition, for young seedlings grown in sandy and loamy–sandy soils, more biomass was allocated to roots than in silty soils, indicating that K. candel seedlings in sandy soils developed stronger roots for anchorage and water absorption; another xerophilous characteristic to acclimate to loose and coarse textured soils. As the plants became more mature, no significant difference in leaf thickness of the saplings was found between loamy-sandy and sandy soils. In order to acclimate to the low water holding capacity in sandy soils, K. candel seedlings had lower physiological activities including lower chlorophyll content; lower activities of root oxidase, nitrate reductase, peroxidase and superoxide dismutase but higher malonaldehyde contents than those in silty soils. Canopy shade is an important factor affecting the growth and physiology of K. candel. The seedling grew worse under the mangrove canopy than that in the open gap, with smaller leaf area, specific leaf area, leaf number and biomass. The seedlings under the canopy eventually died and no saplings were found under canopy shade, implying K. candel is a shade intolerant species and canopy shade might force its newly recruited individuals to expand out of the forest.