Mangrove forest growth in the Fly River delta, Papua New Guinea

During September 1991 and January 1993, permanent plots were established in the three major mangrove forest types in the Fly River delta to follow the growth rates of marked trees. A total of 13 plots were established in Nypa forests on Puruta, Aibinio and Wabuda Islands in September 1991. In January 1993, 5 plots were established in Rhizophora-Bruguiera forests on Umuda Island and 5 plots in Sonneratia-Avicennia forests on small islets east of Sumagi Island.The size of plots varied with tree densities in each forest type and ranged from 50 to 500 m². The diameter of each tree in the plot was measured at breast height (DBH = 1.5 m above the sediment). In the Nypa plots all emergent tree species other than Nypa were similarly measured and tagged. For Nypa, the plot was surveyed using an angle-count cruising method to obtain size frequency distribution of Nypa fronds (the palm has a creeping trunk with upright fronds) and their density per hectare. All fronds of at least four trees in each of 12 plots were measured (DBH) and tagged (13-19 fronds per plot).Size-frequencies and densities of trees and Nypa fronds in plots were converted to biomass per hectare using known allometric relationships between DBH and weight for trees or a measured relationship for Nypa based on harvesting and weighing fronds in the Fly Delta.All Nypa plots were remeasured 12 months after establishment in September 1992. Significant turnover and growth of Nypa fronds was observed. For the two canopy emergent tree species in the Nypa forests, Heritiera littoralis and Xylocarpus granatum, only Xylocarpus granatum showed significant trunk growth within this period. The plots in the other forest types were not remeasured at this time. However, these plots were revisited in February/March 1995.Trunk mass increments for individual Xylocarpus granatum trees were calculated by subtracting mass at time zero (calculated from DBH measurements and allometric relationships between DBH and weight) from mass at t = 1 yr. For Nypa, the same procedure was followed for fronds present at t= 0 and also for fronds that were produced during the year. For Xylocarpus granatum, mass increments were summed for all trees in a plot to obtain annual trunk and branch wood production per unit area. For Nypa, annual mass increments per tree were multiplied by tree densities to obtain production on an areal basis. Permanent plots were established to check the validity of previous estimates of mangrove forest production in the Fly River delta. Earlier estimates were based on a rapid survey technique, which is thought to underestimate productivity. Determining the growth rates of marked trees in these plots allows for a more accurate estimation of the production of trunk and branch wood in the major forest types. A larger number of Nypa plots were chosen to examine the potential for aging stem sections of Nypa for investigations of recent metal histories. This research project was a component of a larger project "The influence of fluvial discharge on the pelagic and benthic ecology and biogeochemistry of the Fly Delta and Gulf of Papua". It follows on from an earlier project "The influence of freshwater and detrital export from the Fly River system on adjacent pelagic and benthic systems".Rhizophora-Bruguiera forests dominate the seaward end of the Fly Delta and Nypa palm forms extensive forests in the mid intertidal zone in the mid- to low-salinity regions. Sonneratia-Avicennia forests occurr on accreting mud banks throughout the delta, but Sonneratia lanceolata is most abundant in the near-freshwater regions of the upper delta and extended some 240 km upstream from the sea.The rapid survey techniques used in the 1989/90 surveys are described in:Robertson AI, Daniel PA, Dixon P (1991) Mangrove forest structure and productivity in the Fly River estuary, Papua New Guinea, Marine Biology 111: 147-155. This publication is also referred to in the metadata record:Mangrove forest structure and productivity in the Fly River estuary, Papua New Guinea1991年9月至1993年1月期间，研究人员于弗莱河三角洲的三种主要红树林类型中设立永久样地，以追踪标记树木的生长速率。1991年9月，在普鲁塔岛、艾比尼奥岛与瓦布达岛的水椰（Nypa）林中，共设立13个样地。1993年1月，研究人员在乌穆达岛的红树-木榄（Rhizophora-Bruguiera）林中设立5个样地，同时在苏马吉岛东侧的小型岛礁上的海桑-白骨壤（Sonneratia-Avicennia）林中设立5个样地。样地面积随各林型的树木密度有所差异，范围为50至500平方米。 对样地内每棵树木，均在胸高位置（胸高直径DBH=距沉积物表面1.5米处）测量其直径。在水椰样地中，除水椰外的所有出现的乔木物种均采用相同方式测量并标记。对于水椰，研究人员采用角计数抽样法开展样地调查，以获取水椰叶丛（该棕榈植物具有匍匐茎与直立叶丛）的大小频率分布及其每公顷的密度。在12个样地中，至少选取4棵水椰树的全部叶丛测量DBH并标记（每个样地对应13至19片叶丛）。 通过已建立的树木DBH与生物量间的异速生长关系，或是基于弗莱河三角洲水椰叶丛采收称重得到的实测关系，将样地内乔木与水椰叶丛的大小频率、密度转换为每公顷生物量。 所有水椰样地均于1992年9月，即设立满12个月后开展复测。期间观测到水椰叶丛存在显著的更新与生长。对于水椰林中的两种冠层乔木银叶树（Heritiera littoralis）与木果楝（Xylocarpus granatum），仅木果楝在此期间表现出显著的树干生长。其余林型的样地未在此阶段复测，但研究团队于1995年2月/3月重新到访了这些样地。 单株木果楝的树干生物量增量通过将t=1年时的生物量减去初始时刻（t=0）的生物量得到，初始生物量由DBH测量值与DBH-生物量异速关系计算得出。对于水椰，该计算方法同时适用于t=0时已存在的叶丛，以及当年新生的叶丛。对于木果楝，将样地内所有树木的生物量增量求和，即可得到单位面积的年度树干与枝条木材生产量。对于水椰，将单株年度生物量增量乘以树木密度，即可得到单位面积的生产量。 设立该永久样地，旨在验证此前弗莱河三角洲红树林生产力估算结果的有效性。此前的估算基于快速调查技术，该方法被认为会低估生产力。通过测定这些样地内标记树木的生长速率，能够更准确地估算主要林型的树干与枝条木材生产量。此外，设置更多数量的水椰样地，用于探究通过水椰茎段年代分析获取近期金属历史记录的可能性。 本研究项目是大型项目“河流径流对弗莱三角洲与巴布亚湾浮游与底栖生态及生物地球化学的影响”的组成部分，承接自早期项目“弗莱河系统淡水与碎屑输出对邻近浮游与底栖系统的影响”。 红树-木榄林分布于弗莱三角洲的向海端，水椰棕榈则在中低盐度区域的潮间带中部形成广阔林分。海桑-白骨壤林分布于三角洲各处的淤积泥滩，但披针叶海桑（Sonneratia lanceolata）在三角洲上游的近淡水区域最为丰富，其分布范围从入海口向上游延伸约240公里。 1989/1990年调查中使用的快速调查技术详见文献：Robertson AI, Daniel PA, Dixon P (1991) 巴布亚新几内亚弗莱河河口的红树林结构与生产力，《Marine Biology》111: 147-155。该文献也在元数据记录中被引用：《巴布亚新几内亚弗莱河河口的红树林结构与生产力》。