Regional and temporal patterns of natural recruitment in a California endemic oak and a possible ‘research reserve effect’
The perceived absence of young cohorts in many long-lived trees, particularly Quercus species, has raised concerns about their long-term viability. While there is a robust body of literature on valley oak (Quercus lobata) planting experiments, conducted mainly on research reserves, an assessment of natural recruitment across a range of climate and land use types has been lacking. At a regional scale over time, we explored patterns of natural recruitment in this endemic California oak reported to be experiencing persistent recruitment failure.
We conducted a regional-scale study, including dendroecology and historical resurveys, field surveys and published planting experiments to elucidate the prevalence, timing, distribution and relation to environmental drivers of valley oak sapling recruitment.
We detected a substantial increase in the presence of recruitment over time at 10 sites originally surveyed between 20 and 40 years ago (sites with sapling recruitment increased from 10% to 70%), potentially related to a corresponding period of relatively wetter years. We found saplings recruiting from a range of years in a variety of land management regimes, and surprisingly, sites designated as research reserves had lower recruitment than other sites. Sapling recruitment did not appear to be synchronous or episodic.
Our results indicate that sapling recruitment failure is not a persistent condition across our sites, and long-term conservation prospects for the species may be better than previously reported. Research reserves had lower natural sapling recruitment and lower seedling survival in experimental controls than other sites, and our findings highlight the importance of field data from sites with a representative range of climate and land use regimes.