Interactive effects of large herbivores and climate on California oak seedling outcomes

Abstract

The perpetuation of tree populations is dependent on successful seedling establishment, which is in turn controlled by biotic and climatic factors. California’s endemic oak species face limited recruitment and shrinking ranges due to anthropogenic climate change and land use, both of which can negatively impact germination and seedling survival. In this study, we investigated Quercus seed germination and seedling establishment under three large herbivore regimes (no large herbivores, wild herbivores, and both wild herbivores and cattle) across three sites representing present, near-future, and far-future projections for climate in California’s montane oak woodlands. Seed germination and living seedling abundance were highest in the intermediate and mesic climate levels, where total exclosures resulted in the best outcomes for seedlings. In contrast, in the arid level, the best large herbivore regime for establishment consisted of wild herbivores and no cattle. Seedling mortality was highest in the most arid climate level, and within that level, mortality was reduced by over 25% in both the wildlife and wildlife plus cattle treatments compared to the total exclosures. Our results suggest that increasing aridity will limit oak establishment and modulate the impact of large herbivores on oak seedlings, with wild large herbivores becoming a beneficial factor for seedling abundance, and both wild herbivores and cattle reducing mortality. Dynamic and contextually informed management of large herbivore populations and other restoration actions will be necessary to ensure the sustainability of California’s oak populations.