Department of Ecology, Evolution, and Behavior, University of Minnesota, 100 Ecology Building, 1987 Upper Buford Circle, Saint Paul, Minnesota, 55108, USA.
Northern Research Station, USDA Forest Service, 180 Canfield Street, Morgantown, West Virginia, 26505, USA.
Ecology. 2017 Sep;98(9):2322-2332. doi: 10.1002/ecy.1928.
Nitrogen (N) additions have decreased species richness (S) in hardwood forest herbaceous layers, yet the functional mechanisms for these decreases have not been explicitly evaluated. We tested two hypothesized mechanisms, random species loss (RSL) and non-random species loss (NRSL), in the hardwood forest herbaceous layer of a long-term, plot-scale, fertilization experiment in the central Appalachian Mountains, USA. Using a random thinning algorithm, we simulated changes in species densities under RSL and compared the simulated densities to the observed densities among N-fertilized (+N), N-fertilized and limed (+N+L), and reference (REF) plots in regenerating forest stands. We found a lower S in the +N treatment across all survey years and determined that the reduction in S was a function of NRSL. Furthermore, non-random effects were observed in certain species, as they occurred at densities that were either higher or lower than expected due to RSL. Differential advantages were also observed among species between +N and +N+L treatments, suggesting that species responded to either the fertilization or acidification effects of N, though no consistent pattern emerged. Species nitrophily status was not a useful trait for predicting specific species losses, but was a significant factor when averaged across all treatments and sampling years. Our results provide strong evidence that declines in S in the forest herbaceous layer under N fertilization are due largely to NRSL and not simply a function of species rarity.
氮（N）添加减少了硬木森林草本层的物种丰富度（S），但这些减少的功能机制尚未得到明确评估。我们在美国阿巴拉契亚山脉中部的一个长期、基于斑块尺度的施肥实验中，对硬木森林草本层中的两种假设机制（随机物种损失（RSL）和非随机物种损失（NRSL））进行了测试。我们使用随机稀疏算法，模拟了 RSL 下物种密度的变化，并将模拟密度与再生林分中 N 施肥（+N）、N 施肥和石灰处理（+N+L）和对照（REF）斑块的观测密度进行了比较。我们发现+N 处理在所有调查年份的 S 值都较低，并确定 S 的减少是 NRSL 的函数。此外，在某些物种中观察到了非随机效应，因为它们的密度高于或低于由于 RSL 而预期的密度。在+N 和+N+L 处理之间，物种之间也观察到了差异优势，这表明物种对 N 的施肥或酸化效应有反应，尽管没有出现一致的模式。物种的氮营养状态不是预测特定物种损失的有用特征，但在所有处理和采样年份的平均值中是一个重要因素。我们的结果提供了强有力的证据，表明 N 施肥下森林草本层 S 的下降主要是由于 NRSL，而不仅仅是物种稀有性的函数。
