Wetlands (2010) 30:1077–1084 DOI 10.1007/s13157-010-0114-y ARTICLE Micronesian Mangrove Forest Structure and Tree Responses to a Severe Typhoon J. Boone Kauffman & Thomas G. Cole Received: 30 November 2009 /Accepted: 7 October 2010 /Published online: 12 November 2010 # Society of Wetland Scientists 2010 Abstract Tropical cyclones are common disturbances that Keywords Allometric equations . Disturbance ecology. have strong effects on mangrove composition and structure. Plant adaptations . Total above-ground biomass . Tropical Because there are numerous ecosystem services provided wetlands . Typhoon impacts by mangroves, it is important to understand their adapta- tions and responses to these climatic events. In April 2004, Typhoon Sudal, a category 3–4 cyclone, passed over the Introduction state of Yap, Federated States of Micronesia. For four months following the typhoon we measured forest struc- Mangroves are important wetland ecosystems throughout ture, above-ground biomass, tree mortality and response in the tropical and subtropical coastal areas of the world six mangroves. The sites were dominated by species (Mitsch and Gosselink 2000), providing a variety of common in mangroves throughout the Indo-Pacific— ecosystem services such as sources of food (fishes, crabs, Sonneratia alba, Brugueira gymnorrhiza, and Rhizophora etc.), wood, and protection from extreme events such as apiculata. Total above-ground biomass (TAGB) of man- cyclones and tsunamis (Giesen et al. 2006). Tropical grove forests ranged from 211–573 Mg ha-1. Tree mortality cyclones (typhoons) affect ecosystem structure in tropical ranged from 6% to 32% among stands. Adaptations and coastal ecosystems in many islands of the western Pacific. responses to the typhoon varied by species, as well as by In order to persist in an environment, the biota must possess geographic location. Sonneratia alba had a higher frequen- adaptations that facilitate survival of such severe natural cy of mainstems broken (26%), but was the only species disturbances. that vigorously sprouted from dormant basal or epicormic Most studies on the impacts of tropical cyclones on tissues. Standing live trees accounted for 80–95% of ecosystems have been conducted in the southern United TAGB, suggesting that adaptations of mangrove trees can States (Tilmant et al. 1994, Davis et al. 2004), the Caribbean facilitate the persistence of an intact forest structure (Sherman et al. 2001,Ostertagetal.2003), and Central following typhoons of this intensity. Climatic changes such America (Fenner 1991,Cahoonetal.2003 ). Despite the fact as sea level rise and increased severity of cyclonic events that typhoons are major, natural disturbances in the western could alter this relationship. Pacific (Schneider 1967), few studies have focused on the ecological and physical effects of typhoons on Pacific islands. Over the past 40 years, an average of ≈ 18 destructive J. B. Kauffman (*) typhoons have passed through the region annually (Furze Northern Research Station, USDA Forest Service, and Preble 2003). Furthermore, based on a range of models, 271 Mast Road Durham, New Hampshire 03824, USA it is likely that future tropical cyclones will become more e-mail: [email protected] intense, with greater peak wind speeds and more heavy precipitation associated with ongoing increases of tropical T. G. Cole sea surface temperatures (IPCC 2007). Institute of Pacific Islands Forestry, USDA Forest Service, 60 Nowelo Street, Increased typhoon intensity associated with climate Hilo, HI 96720, USA change could have severe negative consequences on Pacific 1078 Wetlands (2010) 30:1077–1084 islands because land areas are typically small and very were Qamun, Qaaf and Tabinifiy (windward), and Ruunuw, isolated from continents and other islands which would Qatliw and Maaq (leeward). Wind speeds during the affect recolonization rates and processes. Because of their typhoon were lower on leeward sites than the windward importance as natural disturbances and potential interac- sites (Prior and Guard 2005). tions with anthropogenic perturbations of island ecosys- tems, it is important to better understand how Pacific Island ecosystems are affected by, and respond to typhoons. Our Methods understanding of disturbance impacts and mangrove forest values is further limited by a lack of adequate allometric In each of the six sites, we sampled forest structure and equations to predict aboveground biomass of trees which wood debris mass in five 6-m radius (0.0113 ha) circular may have trunk diameters exceeding 1m (Saenger and plots established every 50 m along a 200-m transect. The Snedaker 1993, Komiyama et al. 2008). initial starting point of each transect was ~10 m from the The objectives of this study were to examine the effects forest/seagrass ecotone. Transects were established perpen- of a severe typhoon on mangrove composition and structure dicular to the forest ecotone and at least 100 m away from in Micronesia, and to describe the short-term responses of the ecotones of other coastal communities (e.g., rock the dominant mangrove tree species to the typhoon. In shoreline without mangrove). The start point and transect order to describe mangrove forest structure it was necessary direction was selected prior to entering the mangrove stand to develop robust allometric equations to determine minimizing any a priori knowledge of stand composition or aboveground mangrove tree biomass for species of this condition. region. We hypothesized that mangrove forest mortality and The diameter at 1.3 m in height (dbh) and the species of structural effects would vary among different stands due to all trees that were rooted in the plots were measured. If the differences in their location on the island, and their trees were multi-stemmed, each individual stem was differences in species composition measured. From these data we calculated site characteristics of the stands (species composition, basal area, stand density, and tree biomass). Tree biomass was determined Study Area using species-specific allometric equations where diameter was the independent variable (Table 1). These equations The study was conducted in Yap, the westernmost state of were developed for this study from mangrove data collected the Federated States of Micronesia (9o33′ N, 138o09′ E). from the Micronesian islands of Pohnpei, Yap, Kosrae, and Yap is composed of four islands separated only by Palau by Cole et al. (1999). The maximum diameters of mangrove-lined channels. The total land area is about trees included in these equations were 132 cm for 9700 ha. Mangrove forests comprise about 12% of the Bruguiera gymnorrhiza (L.) Lamk, 323 cm for Sonneratia land area. The climate in Yap is humid, warm and tropical, alba J.E. Smith, and 60 cm for Rhizophora apiculata Bl. with an average annual temperature of 27°C and an The total number of trees included in these equations were average rainfall of 3000 mm y-1. The heaviest rains occur 327, 346, and 191, respectively (Cole et al. 1999). Most between June and October, while the months of November previous published equations are limited by smaller sample to May are drier with strong east to north-east tradewinds. sizes (<104) and a maximum diameter of 49 cm On 9 April 2004, Typhoon Sudal passed over the State (Komiyama et al. 2008). Prop root biomass for Rhizophora of Yap, Federated States of Micronesia. The duration of the apiculata, and leaf biomass for all species were calculated extreme weather event was approximately five hours. The from published equations (Table 1, Clough and Scott 1989). storm over Yap had wind speeds ranging from 119 to 151 The effects of the typhoon and the response of each km h-1 along the north shore (leeward side), 155–161 km h-1 individual tree in each plot were recorded. Methods of on the eastern shore, and 177–209 km h-1 on the southern ascertaining tree damage were similar to those of Franklin shore (windward side) (Prior and Guard 2005). The typhoon et al. (2004). We first recorded if the tree was alive or dead. hit Yap during a high, spring tide and as a result many of the We also examined each tree for the following categorical lowland areas were submerged under 2–4 m of water or (yes/no) types of damage: (1) crown damage (loss of leaves damaged by wave action (Mark Lander, University of Guam, and fine stems); (2) snapped (main stems broken off by the pers. comm.). typhoon and the height at which they were broken); (3) In August 2004, we sampled six different mangrove wind-thrown—either uprooted by the typhoon or leaning forests dispersed throughout the perimeter of the islands (when the tree was uprooted but leaning on other trees); and that constitute Yap. Three of the sites were located on the (4) no discernable damage to the tree. Tree responses windward (southern and eastern) coasts and three were included: (1) crown refoliated (tree canopy has produced located on the leeward (north-western) coasts. These sites new foliage); (2) epicormic sprouts (new stems and Wetlands (2010) 30:1077–1084 1079 Table 1 Equations to determine components of total above-ground We partitioned the downed wood into size classes based biomass (TAGB) of mangrove forests in Yap, Federated States of on their diameter. The diameter classes and equations used Micronesia to partition woody debris were the same as those of Species and plant part Equation Kauffman et al. (1995)andGuildetal.(1998)for determination of woody debris in upland tropical forests: 3 a Live wood volume (m ) 0–0.64 cm diam., 0.65–2.54 cm diam., 2.55–7.5 cm diam. 2.50 2 Bruguiera gymnorrhiza Vwood=0.0000754(D) (R =0.84) and ≥7.6 cm diam. For wood particles >7.5 cm diam. we 2.10 2 Sonneratia alba Vwood=0.0003841(D) (R =0.78) further separated them into sound and rotten classes. 2.64 2 Rhizophora spp. Vwood=0.0000695(D) (R =0.96) Lengths of the sampling plane varied among the woody b Live wood biomass (kg) AGBwood=Vwood×Sg×1000 debris size classes: 2 m for wood particles ≤0.64 cm diam., c Leaf biomass (kg) 5 m for woody debris 0.65 to 2.54 cm diam., 10 m for d ¼ ðÞÀ1:1679þ1:4914ÂlogðÞ D Bruguiera gymnorrhiza AGBleaf 10 woody debris 2.55–7.5 cm diam., and 12 m for the coarse ¼ ðÞÀ1:8571þ2:1072ÂlogðÞ D Rhizophora spp.