Bonanza Creek LTER: Shrub, Seedling and Sapling Density from 1975 to Present in the Bonanza Creek Experimental Forest near Fairbanks, Alaska

This study is a survey of the density of tall shrubs, saplings, and seedlings at Bonanza Creek LTER Control Plots. Densities have been measured at some sites since 1975 while others began in the mid 1980's. In 2006 methodology was changed in attempt to better capture the spatial variability of shrub and tree species within the research sites. The current method, presented here, uses a 2 meter wide transect along two site boundaries covering 220m2. All seedlings, saplings, and tall shrubs >1m (regardless of species) are counted by species and size class along the transect. The previous method used 4m2 circular plots at 20 points on a 10mx10m grid within the control plot for an area of 80m2. The circular method measured all seedlings, saplings, and a certain set of "tall shrubs", regardless of height. The height requirement creates an issue when comparing the old and new methods for early successional stands. For Alnus and Salix this is likely only a problem in the FP0 sites as they are the only sites with a high density of shrubs shorter than 1m. Shrub community changes at these early successional sites should be analyzed using percent cover data. At the later successional sites most Alnus and Salix shrubs are greater than 1m so the transect method likely does not underrepresent density. The metadata and raw data from the circular method can be found in <a href="http://www.lter.uaf.edu/data/data-detail/id/175"> Vegetation Plots of the Bonanza Creek LTER Control Plots: Species Count (1975 - 2004) </a>.

本研究针对博南扎克里克长期生态研究（Long Term Ecological Research, LTER）对照样地内的高大灌木、幼树及幼苗的种群密度开展调查。部分样地自1975年起便启动了密度监测工作，其余样地的监测则始于20世纪80年代中期。2006年，研究方法进行了优化调整，以更好地捕捉研究样地内灌木与树木物种的空间异质性。本文采用的现行调查方法为：沿样地的两条边界设置宽度为2米的样带，样带总覆盖面积达220平方米。沿该样带，所有高度大于1米的幼苗、幼树及高大灌木（无物种限制）均按物种及大小级完成计数统计。此前采用的调查方法为：在对照样地内的10米×10米网格的20个采样点上设置面积为4平方米的圆形样方，总采样面积为80平方米。圆形样方法会对所有幼苗、幼树以及特定类别的“高大灌木”进行统计，不限制植株高度。高度限定条件使得新旧两种调查方法在早期演替林分的对比分析中存在偏差。对于桤木属（Alnus）和柳属（Salix）物种而言，该偏差仅可能出现在FP0样地中——因为该样地是唯一存在大量高度低于1米灌木的样地。因此，针对此类早期演替样地的灌木群落变化，应采用盖度百分比数据开展分析。而在晚期演替样地中，多数桤木属与柳属灌木的高度均大于1米，因此样带法大概率不会低估种群密度。圆形样方法的元数据与原始数据可通过以下链接查阅：<a href="http://www.lter.uaf.edu/data/data-detail/id/175">博南扎克里克LTER对照样地植被样方：物种计数（1975-2004）</a>