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.在北美东北部区域，铁杉球蚜（Adelges tsugae）对东部铁杉（Tsuga canadensis）构成严重威胁：该虫害通过侵染寄主引发落叶，直接造成寄主死亡；而预防性采伐则会引发间接死亡。铁杉种群衰退后的植被更新成效，取决于先期萌生的实生苗与幼树、种子扩散以及种群补充这三大关键要素。本研究围绕两个核心科学问题展开探究：其一，种子释放高度与风速这两项基础参数是否会对种子扩散距离产生影响；其二，在有林冠覆盖与无林冠覆盖的区域中，基础弹道模型（ballistic model）对种子扩散实测数据的拟合效果如何。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。研究结果表明，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。基础弹道模型所预测的种子扩散距离数值，与实际观测得到的扩散数据拟合效果不佳。该模型拟合效果欠佳的根源在于，其无法涵盖罕见的长距离种子扩散事件。若要对非局域性种子扩散进行建模，需构建引入额外参数的更为复杂的模型。