Long-term impacts of nurse plants on evergreen and deciduous oak saplings in Northern California

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 all naturally occurring saplings found underneath and around nurse plants in two environments with contrasting densities of herbivorous mule deer (Odocoileus hemionus) in 2015 and 2023. The nurse plants and oaks observed in 2015 were located and evaluated after eight years, along with all new oak saplings recruited between 2015 and 2023. We uncovered that beneficiary oaks protected by nurse plants had greater height growth compared to those exposed in the open. Additionally, increased herbivory stress is associated with a shift in the distribution of beneficiaries underneath the nurse plant, favoring recruitment closer to the nurse plant 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.