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）通过两种途径威胁东部铁杉（Tsuga canadensis）：直接致死途径为侵染宿主后引发落叶，间接致死途径则为预防性采伐。铁杉衰退后植被更新的成效，取决于幼苗幼树的先期更新、种子扩散以及种群补充。本研究旨在探究两个问题：(1) 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；(2) 基础弹道种子扩散模型在有林冠和无林冠区域中对实测数据的拟合效果。本研究的实测数据采集自哈佛森林（Harvard Forest）内的种子掉落与种子雨实验。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影响；基础弹道模型所预测的扩散距离数值，与实测的扩散数据拟合效果不佳。该模型拟合效果较差的原因在于，其无法解释罕见的长距离扩散事件。若要对非局域性种子扩散进行建模，需要构建引入更多参数的复杂模型。