Aim: 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. Location: California. Methods: 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. Results: 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. Main conclusions: 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. © 2013 John Wiley & Sons Ltd.

Climate, physical landscapes, and biota interact to generate heterogeneous hydrologic conditions in space and over time, which are reflected in spatial patterns of species distributions. As these species distributions respond to rapid climate change, microrefugia may support local species persistence in the face of deteriorating climatic suitability. Recent focus on temperature as a determinant of microrefugia insufficiently accounts for the importance of hydrologic processes and changing water availability with changing climate. Where water scarcity is a major limitation now or under future climates, hydrologic microrefugia are likely to prove essential for species persistence, particularly for sessile species and plants. Zones of high relative water availability - mesic microenvironments - are generated by a wide array of hydrologic processes, and may be loosely coupled to climatic processes and therefore buffered from climate change. Here, we review the mechanisms that generate mesic microenvironments and their likely robustness in the face of climate change. We argue that mesic microenvironments will act as species-specific refugia only if the nature and space/time variability in water availability are compatible with the ecological requirements of a target species. We illustrate this argument with case studies drawn from California oak woodland ecosystems. We posit that identification of hydrologic refugia could form a cornerstone of climate-cognizant conservation strategies, but that this would require improved understanding of climate change effects on key hydrologic processes, including frequently cryptic processes such as groundwater flow.