Data from: Environmental predictors of woody plant encroachment in calcareous fens are modified by biotic and abiotic land-use legacies

Woody plant encroachment in grasslands is a world-wide concern. Assertions that abiotic stress reduction facilitates encroachment are not universally supported. To devise restoration and management strategies, the ability of stress reduction, in the context of co-occurring biotic land-use legacies, needs to be assessed to predict shrub cover. We determined whether legacy-induced reduction in Carex stricta (a potential facilitator of shrub encroachment and attractor of animal dispersers) and an increase in herbaceous invaders (potential competitors) altered the ability of stress reduction to predict woody plant cover in ploughed and never-ploughed plots within 11 Wisconsin (USA) calcareous fens. We hypothesized that drier conditions (low saturation stress) and higher nutrient availability associated with ploughing would be associated with high woody plant cover; that higher cover of herbaceous invaders associated with ploughing would predict low woody plant cover, and C. stricta cover predicts high woody plant cover in saturated, never-ploughed plots. Restricted maximum-likelihood (REML) multiple regression was used to determine the importance of interactions among ploughing, saturation, nutrients and the two biotic legacies in predicting woody richness and cover, as well as cover by dispersal syndromes and individual species. Unexpectedly, encroachment by animal-dispersed species was higher in never-ploughed plots, while wind-dispersed species had no relationship with ploughing. Animal-dispersed species cover decreased with saturation stress in never-ploughed plots, only. Carex cover was associated with increased encroachment by animal-dispersed species, mainly in never-ploughed highly saturated plots. Herbaceous-invasive cover was associated with decreased encroachment. Wind-dispersed species cover declined with increasing saturation stress, with responses varying among species. Synthesis and applications. The results suggest that the impacts of stress reduction on encroachment are not uniform and are modified by biotic legacies. We suggest that impacts of restoring stressful abiotic conditions on encroachment may not be known until the impacts on competitors and facilitators are understood.

草原木本植物侵占是全球广受关注的生态问题。认为非生物胁迫（abiotic stress）缓解可促进木本植物侵占的观点，并未得到普遍证实。为制定草地修复与管理策略，需在共存的土地利用生物遗留效应（biotic land-use legacies）背景下，评估胁迫缓解对灌木盖度的预测能力。
本研究旨在明确：由遗留效应介导的紧序薹草（Carex stricta，一种潜在的木本植物侵占促进剂与动物传播媒介吸引物）种群缩减，以及草本入侵种（herbaceous invaders，潜在竞争者）的数量扩张，是否会改变胁迫缓解对美国威斯康星州11处钙质沼泽（calcareous fens）内翻耕样地与未翻耕样地中木本植物盖度的预测能力。本研究提出如下假说：与翻耕操作相关的干燥环境（低饱和胁迫（saturation stress））及较高养分有效性，将对应更高的木本植物盖度；翻耕引发的草本入侵种高盖度，可预测较低的木本植物盖度；而在饱和且未翻耕的样地中，紧序薹草盖度则将对应更高的木本植物盖度。
本研究采用受限最大似然（Restricted Maximum-Likelihood, REML）多元回归法，分析翻耕、饱和胁迫、养分水平与两种生物遗留效应之间的交互作用，在预测木本植物物种丰富度与盖度、传播策略类群（dispersal syndromes）盖度及单个物种盖度时的重要性。
出乎意料的是，动物传播物种的侵占程度在未翻耕样地中更高，而风传播物种的侵占程度与翻耕操作并无显著关联。仅在未翻耕样地中，动物传播物种的盖度随饱和胁迫加剧而显著降低。紧序薹草盖度与动物传播物种的侵占程度提升呈显著正相关，这一关联主要出现在未翻耕的高饱和胁迫样地中。草本入侵种盖度则与木本植物侵占程度降低显著相关。风传播物种的盖度随饱和胁迫加剧而下降，且不同物种的响应模式存在显著差异。
综合分析与实践应用。研究结果表明，胁迫缓解对木本植物侵占的影响并非统一，而是会受到生物遗留效应的调控。本研究提示，若未明确胁迫缓解对竞争者与促进剂的影响，则难以准确预判恢复胁迫性非生物环境对木本植物侵占的实际效应。