Tree Seed Dispersal in Hemlock Removal Experiment at Harvard Forest 2005

Throughout the Northeast, the hemlock woolly adelgid (Adelges tsugae) threatens eastern hemlock (Tsuga canadensis) through direct mortality resulting from infestation followed by defoliation and indirect mortality in the form of pre-emptive logging. The efficacy of regeneration of vegetation following hemlock decline depends upon advance regeneration of seedlings and saplings, seed dispersal, and recruitment. This study investigated (1) whether the basic parameters of height of release and wind velocity affected seed dispersal distance and (2) the fit of a basic ballistic model of seed dispersal to empirical data in areas both with and without canopies. Empirical data was collected from seed dropping and seed rain experiments at Harvard Forest. Height and wind velocity only affected seed dispersal distance in open areas. Predicted values of dispersal distance generated by the basic ballistic model did not provide a good fit to observed dispersal data. Poor fits of the ballistic model to the data were due to the model’s inability to account for rare, long distance dispersal events. More complex models with additional parameters are necessary to model Non-localized seed dispersal.在北美东北部地区，铁杉球蚜（Adelges tsugae）通过侵染后引发落叶导致东部铁杉（Tsuga canadensis）直接死亡，同时以预防性采伐的形式造成间接死亡，对东部铁杉构成严重威胁。铁杉衰退后植被恢复的成效，取决于幼苗幼树的先期更新、种子扩散以及种群补充三个核心环节。本研究开展了两项探究：其一，释放高度与风速这两个基础参数是否会对种子扩散距离产生影响；其二，在有林冠和无林冠的区域，基础弹道种子扩散模型对实测数据的拟合效果如何。本研究的实测数据采自哈佛大学森林（Harvard Forest）开展的种子掉落与种子雨实验。研究结果显示，仅在开阔区域中，释放高度与风速才会对种子扩散距离产生显著影响。基础弹道模型生成的扩散距离预测值与实测扩散数据的拟合效果欠佳，其原因在于该模型无法涵盖罕见的长距离扩散事件。若要模拟非局域性种子扩散过程，亟需引入额外参数的更为复杂的模型。