Early Post-Typhoon Effects on the Restoration Trajectory of Planted Mangroves: Implications for Forest Development and Macrofaunal Communities

The restoration trajectory patterns of community-managed mangrove plantations composed of different progressing ages (6-yr, 8-yr, 10-yr, 11-yr, 12-yr, 17-yr, 18-yr and 50-yr) were investigated, to: 1) evaluate recovery of vegetation; 2) assess habitat functionality; 3) identify indicator parameters for restoration success; and 4) evaluate the effects of typhoon. Using a space- for-time (SFT) substitution approach, the recovery trajectories of planted mangroves were evaluated and compared with the natural mangroves (of unknown ages) as reference trajectory endpoints for restoration. Mangrove vegetation structure and biomass, soil characteristics, the mollusc assemblage, and the nekton community were periodically measured from December 2008 to March 2010. There was a rapid decline in tree density (y = 14223e-0.14x; r2 = 0.77) and exponential increase in biomass (t ha-1, y = 6.24e0.16x; r2 = 0.98) with increasing mangrove stand age that tended to stabilize in the 11-yr old stands. Based on vegetation development trajectories and Principal Component Analyses, stands were grouped and classified as: young (6- and 8-yr), intermediate (10-, 11- and 12-yr), mature (17-, 18- and 50-yr) and natural (unknown ages). I estimated that the planted mangroves may match the vegetation structure and biomass of mature, natural mangroves within 25 years. The soil characteristics were associated with the changes in mangrove vegetation and similarly followed the vegetation developmental stage groupings. The organic matter and nitrogen contents and redox had a positive correlation, while soil temperature had negative correlation, with mangrove stand age. The strong correlation of these four variables with stand age suggests that they can serve as useful restoration indicators. The changes in vegetation and soil with mangrove stand age provided food and suitable habitat that facilitated mollusc colonization. There was a clear colonization pattern through shifts in species composition, abundance, and biomass with respect to the age of mangrove stands and with faunal type (from infauna to epifauna and arboreal fauna) suggesting species preference for different levels of habitat and food quality as mangrove forests progressed through different stages. Indicator species can be inferred based on the shifts in species dominance with developmental stage of the stands. The species Cerithidea cingulata dominated in young stands, while Nerita polita thrived in intermediate stands, and N. planospira dominated in mature and natural mangroves. The nekton community, as a whole, was not clearly associated with vegetation or soil variables as forests developed. However, removal of the fish from the analysis, and when considering only the crabs and shrimp, yielded a significant association with mangrove stand age, suggesting these fauna are sensitive to forest development. These results also imply that site-specific conditions (i.e. proximity to reef, elevation) exert more influence on the fish fauna than does the age state of the mangrove forest. The development trajectory pattern of the experimental forests was, however, disrupted by the occurrence of a strong tropical typhoon (International Codename: Chan-hom) that hit Lingayen Gulf on 9th May 2009. The typhoon did not affect younger plantations, but had more pronounced damaging effects in the older plantations. Tree density and biomass were severely reduced by 45- 60% in 11- and 18-yr plantations. Nine months after the typhoon, tree mortality continued to be observed. There were abrupt changes in soil characteristics. Most notably, there was a 20-40% reduction in organic matter content and 5-100C increase in soil temperature. The reduction in forest cover and biomass compounded by changes in soil conditions contributed to the decline in the abundance, biomass and species richness of mollusc assemblage. The nekton community, as a whole, was not affected by the typhoon. However, 2.5-mo after the typhoon, there was a reduction in the abundance and biomass of crabs and shrimp. This further strengthens the proposition that crabs and shrimp are better indicators of mangrove restoration than the fish fauna. In summary, the typhoon effectively stalled forest recovery trajectory by at least five to ten years