Long-Term Impacts of Nurse Plants on Evergreen and Deciduous Oak Saplings in Northern California, Research Data

Throughout California, oak savannas exhibit poor recruitment of saplings in natural populations. To prevent the decline of California oak (Quercus spp.) populations, management practices need to elucidate the drivers of recruitment limitations. Previous research suggests that nurse plants enhance the establishment of oaks by buffering them from stressors such as insolation and herbivory. However, their long-term effects on sapling performance remain poorly known. Here we explore the differential impacts of nurse plants on the growth and distribution of sympatric evergreen and deciduous oak species. We compared the saplings found underneath and around nurse plants in two environments with contrasting herbivore densities in 2015 and 2023. The nurse plants and oaks observed in 2015 were located and evaluated after eight years. We uncovered that beneficiary oaks protected by nurse plants had greater upward growth compared to those exposed in the open. While recruitment probability increases with nurse plant size, deciduous species require larger nurse plants compared to evergreen oaks for effective facilitation. Additionally, changes in herbivory stress shifted the distribution of beneficiaries underneath the nurse plant, with high stress favoring recruits closer to the nurse plant’s center. We conclude that nurse plants are only beneficial to the long-term growth of oak saplings under high herbivory scenarios. As nurse plants are not as effective facilitators for deciduous species, compared to evergreen ones, further research devoted to management solutions for deciduous oak species is warranted.

加利福尼亚州全境的橡树稀树草原（oak savannas）天然种群中，栎树幼苗更新状况普遍不佳。为遏制加州栎属（Quercus spp.）种群的衰退，相关管理实践亟需阐明其幼苗更新限制的核心驱动因素。过往研究表明，护伴植物（nurse plant）可通过为栎树缓冲光照、植食取食等胁迫因子，提升其定植成功率。然而，护伴植物对幼苗生长表现的长期效应仍有待系统厘清。
本研究旨在探究护伴植物对同域分布的常绿栎与落叶栎物种生长及分布的差异化影响。研究于2015年与2023年，在植食动物密度差异显著的两类生境中，对比了护伴植物下方及周边的栎树幼苗情况，并于8年后对2015年记录的护伴植物及栎树个体进行了重新定位与评估。
研究发现，受护伴植物庇护的受益栎树，其向上生长量显著高于裸露开阔生境中的个体。尽管幼苗更新概率随护伴植物体型增大而提升，但落叶栎物种需依赖更大体型的护伴植物，才能获得有效的促进作用。此外，植食胁迫程度的变化会改变受益幼苗在护伴植物下方的分布格局：当胁迫程度较高时，幼苗更倾向于分布在靠近护伴植物中心的区域。
本研究得出结论：仅在植食胁迫程度较高的情境下，护伴植物才对栎树幼苗的长期生长具有促进作用。相较于常绿栎物种，护伴植物对落叶栎物种的促进效果相对有限，因此亟需开展针对落叶栎物种的管理解决方案相关研究。