Optimizing sampling across methods improves the power of ecological monitoring data

Transect-based monitoring has long been a valuable tool in ecosystem monitoring. These transects are often used to measure multiple ecosystem attributes. The line-point intercept (LPI), vegetation height, and canopy gap intercept methods comprise a set of core methods, which provide indicators of ecosystem condition. However, users struggle to design a sampling strategy that optimizes the ability to detect ecological change using transect-based methods. We assessed the sensitivity of these core methods on a one-hectare plot to transect length, number, and sampling interval to determine: 1) minimum sampling required to describe ecosystem characteristics and detect change for each method and 2) optimal transect length and number for all three methods to make recommendations for future analyses and monitoring efforts. We used data from 13 National Wind Erosion Research Network locations spanning the western US, which included 151 measurements over time across five biomes. We found that longer and increased numbers of transects were more important for reducing sampling error than increased sample intensity along transects. For all methods and indicators across plots, three 100-m transects reduced sampling error so that indicator estimates fall within an 95% confidence interval of +/- 5% for canopy gap intercept and LPI-total foliar cover, +/- 5 cm for height and +/- two species for LPI-species counts. For the same criteria at 80% confidence intervals, two 100-m transects are needed. Site-scale inference was strongly affected by sample design, consequently our understanding of ecological dynamics may be influenced by sampling decisions.

横断面监测技术在生态系统监测中占据着长久以来的重要地位。此类横断面通常被用于测量多个生态系统属性。线路点截断法（LPI）、植被高度和冠层间隙截断法构成了一组核心方法，它们为生态系统状况提供了指示指标。然而，用户在制定能够优化使用横断面方法检测生态变化的采样策略方面面临挑战。本研究评估了这些核心方法对一个一公顷样地上的横断面长度、数量和采样间隔的敏感性，旨在确定：1）每种方法描述生态系统特征和检测变化的最低采样量；2）三种方法的最优横断面长度和数量，以期为未来的分析和监测工作提供建议。我们使用了来自美国西部13个国家风蚀研究网络地点的数据，这些数据涵盖了五个生物群落中的151个随时间变化的测量值。我们发现，相较于增加横断面上的样本强度，更长的横断面和增加的横断面数量对于减少采样误差更为重要。对于所有方法和指标，三个100米的横断面能够减少采样误差，使得冠层间隙截断和LPI总叶面积指标的估计值落在95%置信区间内的误差不超过±5%，高度指标不超过±5厘米，LPI物种计数指标不超过两种物种。对于80%置信区间的相同标准，则需要两个100米的横断面。地点规模的推断受到样本设计的影响极大，因此，我们对生态动态的理解可能受到采样决策的影响。