Decomposition of fallen timber in mangrove forests at Missionary Bay, Hinchinbrook Island, north Queensland

Between December 1986 and August 1987, surveys were made of wood detritus from the floor of two stands of mixed Rhizophora spp. mangrove forest in Missionary Bay, Hinchinbrook Island. The mature forest stand consisted of a mixture of Rhizophora stylosa, Rhizophora apiculata, Rhizophora x lamarkii and Bruguiera gymnorhiza of heights between 10 and 15m. The younger forest stand consisted of a mix of even sized Rhizophora stylosa and Rhizophora apiculata of heights between 15 and 20m.Surveys were conducted along 6 randomly placed 50m straight line transects within each forest. Observers moved along each transect, recording the length and diameter of each dead trunk, prop root, branch, and twig which lay within 1m of either side of the transect line. Each piece of fallen dead wood was assigned to one of three decay classes: moderate (M), very (V), and extremely (E), which was assessed on the ease of penetration with a pointed stick.In the laboratory, samples from each type and decay class of fallen wood were used to determine dry weight to volume ratios for converting wood volume (calculated by assuming each piece of wood was a cylinder) to mass equivalents. The total mass of trunks, prop roots, branches, and twigs was then calculated for each transect.Subsamples of wood from which all fauna and walls of shipworm tubes had been removed was dried at 80°C for 4 weeks, ground to a powder and analysed for carbon and nitrogen using a Leco 600 CHN analyzer.In the field, numbered bundles of twigs and branches from a freshly felled Rhizophora apiculata were weighed and layed out randomly on the forest floor in the low intertidal zone to measure rates of decomposition. Five bundles were retained for calculation of fresh weight to dry weight conversion factors. Four or five bundles were sampled after .2, 1.0, 1.5, 1.8 and 2.3 years in the field. Decomposition rates of trunks and large prop roots were determined from trees that had been felled for research purposes between 1977 and 1986 and trees that had been felled during a cyclone in 1971 at Magnetic Island. The length and radius of each log was measured, weighed. If partially decomposed, the original diameter was estimated. A subsample fo each was taken for CHN analysis. This research was undertaken to provide measurements of the standing stock and decomposition rates of wood detritus on the floor of mangrove forests in a tropical system. The role of dead wood in detrital dynamics was assessed by comparing the annual flux of wood detritus in the mangrove system with previous estimates for litter (leaves, stipules and flowers).

1986年12月至1987年8月期间，研究人员对澳大利亚欣钦布鲁克岛（Hinchinbrook Island）传教士湾（Missionary Bay）两片红树属（Rhizophora spp.）混生红树林林床的木质碎屑开展了系统性调查。其中成熟林分由株高10至15米的红海榄（Rhizophora stylosa）、尖叶红海榄（Rhizophora apiculata）、杂交红树（Rhizophora × lamarkii）与木榄（Bruguiera gymnorhiza）混合组成；幼林分则由株高15至20米、株型均匀的红海榄与尖叶红海榄混交构成。

每个林分内均布设6条随机分布的50米直线样带，调查人员沿样带行进，记录样带两侧1米范围内所有枯树干、支柱根、枝条及细枝的长度与直径。每一块枯落木质残体被划分为三个腐烂等级：中度（M）、重度（V）与极度（E），分级依据为尖杆刺入残体的难易程度。

实验室阶段，研究人员针对不同类型与腐烂等级的木质残体样本，测定其干重与体积的比值，以此将通过圆柱模型计算得到的木材体积转换为质量当量，随后据此计算每条样带内枯树干、支柱根、枝条及细枝的总质量。

将去除所有动物群落及船蛆管腔壁的木质亚样本置于80℃环境下干燥4周，研磨成粉末后，使用LECO 600碳氢氮分析仪（Leco 600 CHN analyzer）测定其碳、氮元素含量。

野外分解实验中，研究人员将取自新鲜砍伐的尖叶红海榄的带编号细枝与枝条捆束称重后，随机布设至潮间带低地区的林床中，以测定木质残体的分解速率。其中5捆样本留存，用于计算鲜重与干重的转换系数；分别在野外放置0.2年、1.0年、1.5年、1.8年及2.3年后，对4至5捆样本进行采样回收。

树干与大型支柱根的分解速率则通过两类伐倒木测定：一类为1977年至1986年间为科研目的伐倒的林木，另一类为1971年磁岛（Magnetic Island）飓风期间倒伏的林木。研究人员测量每根原木的长度与半径并称重，若残体已发生部分腐烂，则估算其初始直径；同时采集每个样本的亚样本开展碳氢氮元素分析。

本研究旨在测定热带红树林林床内木质碎屑的现存量与分解速率，通过对比该红树林系统内木质碎屑的年通量与此前针对凋落物（叶片、托叶及花）的估算结果，评估枯木在碎屑动态中的生态作用。