Data from: Density-dependent effects of a widespread invasive herbivore on tree survival and biomass during reforestation

Reforestation has been widely adopted as a solution to multiple global change issues. However, the role of herbivory by invasive species in the restoration of grassland to forest has received little attention. We conducted a field experiment to investigate the impacts of a widespread invasive mammalian herbivore, the European rabbit (Oryctolagus cuniculus), on trees planted in a landscape-scale reforestation program in south-eastern Australia. Three native tree species were planted inside and outside rabbit-proof exclosures within 10 experimental units, and a random half of the units were subjected to intensive and sustained rabbit control for the remainder of the experiment. Quarterly survival of trees, and total aboveground biomass at the conclusion of the experiment, were estimated using hierarchical Bayesian models. Control substantially reduced rabbit densities on the five treatment units relative to the five non-treatment units. Survival of trees planted outside exclosures was highest at lowest rabbit density and declined non-linearly with increasing rabbit density for all three tree species, but even very low rabbit densities had strong negative effects on the survival of trees outside exclosures. There was a three-way interaction between tree height, being outside an exclosure, and rabbit density. Smaller trees planted outside exclosures always had substantially lower overall survival than trees of the same height planted inside exclosures, and the magnitude of the difference increased with increasing rabbit density. Increasing rabbit densities reduced the overall survival of increasingly taller trees of all three species planted outside exclosures. The aboveground biomass of trees surviving outside exclosures was significantly greater in treatment units compared with non-treatment units for all three species. The combined effects of differential survival and accumulation of aboveground biomass led to higher biomasses inside exclosures relative to outside exclosures just 21 months after planting. The aboveground biomass of trees planted outside exclosures declined with increasing rabbit density, and was effectively zero when rabbit densities exceeded 100 active warren entrances/ha. These results demonstrate that invasive herbivores can rapidly arrest the conversion of grassland to forest. Invasive herbivores such as the European rabbit may need to be completely excluded in order to maximize the benefits of reforestation.重新造林（reforestation）已被广泛用作应对诸多全球变化问题的解决方案。然而，入侵物种的植食作用在草原向森林的恢复过程中所扮演的角色，却极少受到关注。本研究开展了一项野外实验，以探究一种广泛分布的入侵性哺乳类植食动物——欧洲兔（Oryctolagus cuniculus）——对澳大利亚东南部一项景观尺度重新造林计划中栽植树木的影响。研究在10个实验单元内的防兔围栏（rabbit-proof exclosures）内外均栽植了3种本土树种，并将其中随机一半的实验单元在实验剩余阶段实施高强度、持续性的兔类防控措施。本研究采用分层贝叶斯模型（hierarchical Bayesian models），对树木的季度存活率以及实验结束时的总地上生物量进行了估算。相较于5个未实施防控的对照单元，防控措施显著降低了5个处理单元内的兔类种群密度。对于全部3个树种而言，栽植于围栏外的树木存活率在兔类密度最低时达到最高，并随兔类密度升高呈非线性下降趋势；但即便兔类密度极低，仍会对围栏外树木的存活率产生强烈的负面影响。树木树高、是否处于围栏外以及兔类密度三者间存在三阶交互效应。栽植于围栏外的矮小树木，其整体存活率始终显著低于相同树高且栽植于围栏内的树木，且两者间的存活率差异幅度随兔类密度升高而增大。兔类密度升高会降低所有3个树种中、栽植于围栏外的高大树木的整体存活率。对于全部3个树种而言，存活于围栏外的树木的地上生物量，在处理单元中均显著高于未实施防控的对照单元。存活率差异与地上生物量积累的综合效应，使得栽植仅21个月后的围栏内生物量显著高于围栏外。栽植于围栏外的树木地上生物量随兔类密度升高而下降，当兔类密度超过100个活跃兔穴入口/公顷时，其地上生物量几乎为零。上述研究结果表明，入侵性植食动物能够快速阻滞草原向森林的转化过程。为最大化重新造林的效益，诸如欧洲兔这类入侵性植食动物可能需要被完全排除在造林区域之外。