Data from: Threshold dynamics in plant succession after tree planting in agricultural riparian zones

Trajectories of plant communities can be described by different models of plant succession. While a Clementsian (gradual continuum model) or Gleasonian approach (relay floristics model) has traditionally been used to inform restoration outcomes, alternative succession models developed recently may better represent restoration trajectories. The threshold dynamics succession model, which predicts an abrupt species turnover after an environmental threshold is crossed, has never been used in a restoration context. This model might, however, better describe shifts in plant competitive ranking and facilitation interactions during species turnover.
Fifty-three riparian zones, planted with trees 3–17 years prior to sampling, and 14 natural riparian forests were studied in two agricultural watersheds of south-eastern Québec (Canada). The cover of vegetation strata was assessed at the site scale, and the cover of plant species was estimated in a total of 784 1-m2 plots. Canopy cover was measured stereoscopically for each plot.
As revealed by Principal Response Curves and broken stick models, herbaceous species composition was stable during the first 12–13 years after tree planting, but then abruptly shifted. This two-step pattern in species turnover followed the increase in canopy cover after tree planting. Once canopy cover passed a threshold of ca 40%, plant succession started and led to the re-establishment of forest communities 17 years after planting.
Following herbaceous species turnover, the cover of ecological groups changed significantly towards covers of natural riparian forests: shade-tolerant species generally increased, while light-demanding and non-native species decreased. Vegetation structure was also significantly affected by tree planting: tree and shrub cover increased, while monocot cover decreased.
Synthesis and applications. Tree planting efficiently restored herbaceous forest communities in riparian zones by inducing a species turnover mediated by light availability corresponding to the threshold dynamics model in plant succession. Fostering and monitoring canopy closure in tree-planted riparian zones should improve restoration success and the design of alternative strategies. The innovative statistical approach of this study aiming to identify succession patterns and their associated theoretical models can guide future restoration in any type of ecosystem around the world to bridge the gap between science and management.

植物群落的演替轨迹可通过多种植物演替模型进行刻画。传统上，克莱门茨演替模型（Clementsian，即连续渐变模型）或格莱松演替方法（Gleasonian，即区系更替模型）常被用于指导生态修复目标的设定，而近年来提出的新型演替模型或能更精准地反映植物群落的修复轨迹。其中，阈值动态演替模型（threshold dynamics succession model）可预测当环境阈值被突破后，物种组成会发生突发性更替，但该模型此前从未应用于修复生态学研究场景。不过，该模型或能更贴切地描述物种更替过程中，植物竞争等级变化以及促进（facilitation）交互关系的演变。

本研究在加拿大魁北克东南部的两个农业流域内开展，共调查了53处于采样前3至17年完成乔木种植的河岸带，以及14处天然河岸林。在样地尺度上评估了植被层（vegetation strata）盖度，并在总计784个1平方米样方中估算了各植物物种的盖度。同时采用立体测量法测定了每个样方的林冠盖度（canopy cover）。

主响应曲线（Principal Response Curves）与断棍模型（broken stick models）分析结果显示，乔木种植后的前12至13年，草本植物群落组成保持稳定，但随后发生了突发性转变。这种两阶段的物种更替模式与乔木种植后林冠盖度的增加过程相契合。当林冠盖度突破约40%的阈值后，植物演替正式启动，并在种植17年后促使森林群落得以重建。

在草本植物物种更替完成后，植物生态类群（ecological groups）的盖度向天然河岸林的盖度特征发生了显著变化：耐阴物种盖度普遍上升，而喜光物种与外来非本土物种的盖度则呈下降趋势。乔木种植还对植被结构产生了显著影响：乔木与灌木盖度增加，而单子叶植物（monocot）盖度降低。

研究总结与应用启示。本研究表明，通过诱导由光照有效性介导的物种更替——这一过程契合植物演替的阈值动态模型——乔木种植可高效修复河岸带的草本森林群落。在人工种植乔木的河岸带中，促进并监测林冠闭合过程，或能提升修复成功率，并为优化修复策略提供指导。本研究采用的创新性统计方法，可用于识别演替模式及其对应的理论模型，该方法能够为全球各类生态系统的未来修复工作提供指引，弥合生态学研究与生态管理之间的鸿沟。