Forests currently face numerous stressors, raising questions about processes of forest recovery as well as the role of humans in stimulating recovery by planting trees that might not otherwise regenerate. Theoretically, planted trees can also provide a seed source for further recruitment once the planted trees become reproductive, acting as “nucleation” sites; however, it is unclear whether changing site conditions over time (e.g., through the growth of competitors like woody shrubs) influences establishment in the long term, even if seed availability increases. We tested the nucleation concept in a system where shrub competition is known to influence tree establishment and growth, performing an observational study of sites within and close to newly reproductive planted stands in yellow-pine (YP) and mixed-conifer ecosystems in the Sierra Nevada, California. We surveyed and then modeled both seedling occurrence and density as a function of distance to seed sources, competing woody vegetation, and other environmental characteristics. We found that proximity to a planted stand was associated with an increase in the probability of YP seedlings (species more likely to originate from the planted stand) from 0.33 at 35 m from the planted stand to 0.56 directly adjacent to the stand and 0.65 within the stand. However, we found no significant effect of proximity on YP seedling density. This proximity effect suggests that seed availability continues to be a driver of recruitment several decades postwildfire, though other processes may influence the expected density of recruits. Proxies for competitive pressure (shrub volume and shrub cover) were not significant, suggesting that competing vegetation did not have a major influence on recruitment. Though seedling presence and density appeared to be independent of shrub impacts, we did find that shrubs were significantly taller than seedlings. Therefore, we suggest that shrubs may not limit seedling establishment, but they may negatively affect seedlings' ability to grow and serve as a seed source for further recruitment and forest expansion. Altogether, we find that planting may provide a statistically significant but small role in driving recruitment outside of the planted site.

中文翻译：

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高灌木竞争下的核位点和森林恢复

森林目前面临着许多压力因素，引发了人们对森林恢复过程以及人类通过种植可能无法再生的树木来刺激恢复的作用的质疑。从理论上讲，一旦种植的树木成为可繁殖的，作为“成核”场所，种植的树木也可以为进一步招募提供种子来源；然而，尚不清楚随着时间的推移改变场地条件（例如，通过木本灌木等竞争者的生长）是否会影响长期的建立，即使种子可用性增加。我们在已知灌木竞争影响树木建立和生长的系统中测试了成核概念，对内华达山脉黄松 (YP) 和混合针叶树生态系统中新繁殖种植林内和附近的地点进行了观察研究, 加州。我们调查了幼苗发生率和密度，然后将其建模为到种子源的距离、竞争性木本植被和其他环境特征的函数。我们发现靠近种植林与 YP 幼苗（更可能起源于种植林的物种）的概率从距种植林 35 m 处的 0.33 增加到直接邻近林分的 0.56 和在种植林内的 0.65 相关。站立。然而，我们发现接近度对 YP 幼苗密度没有显着影响。这种邻近效应表明，在野火后的几十年里，种子可用性仍然是招募的驱动因素，尽管其他过程可能会影响预期的招募密度。竞争压力（灌木体积和灌木覆盖）的代理指标不显着，这表明竞争性植被对招募没有重大影响。虽然幼苗的存在和密度似乎与灌木的影响无关，但我们确实发现灌木明显高于幼苗。因此，我们建议灌木可能不会限制幼苗的建立，但它们可能会对幼苗的生长能力产生负面影响，并作为进一步补充和森林扩张的种子来源。总而言之，我们发现种植在推动种植地点以外的招募方面可能具有统计学意义，但作用很小。但它们可能会对幼苗的生长能力和作为进一步补充和森林扩张的种子来源的能力产生负面影响。总而言之，我们发现种植在推动种植地点以外的招募方面可能具有统计学意义，但作用很小。但它们可能会对幼苗的生长能力和作为进一步补充和森林扩张的种子来源的能力产生负面影响。总而言之，我们发现种植在推动种植地点以外的招募方面可能具有统计学意义，但作用很小。