Regional patterns of recruitment success and failure in two endemic California oaks

Oak woodlands and savannas are key defining landscapes in the California Floristic Province, making up almost a quarter of the region's forests and woodlands. Two endemic Californian oak species, valley oak (Quercus lobata Nee) and blue oak ( Quercus douglasii Hook. & Arn.), are widely considered at risk of decline from persistent recruitment failures in the last century. However, decades of research have produced no definitive conclusion about the existence, extent, or causes of this `regeneration problem'. Underlying causes of perceived recruitment failure are unclear and could include drivers at distribution-wide to local scales including climate and atmospheric changes, habitat fragmentation, altered herbivore populations, changing fire regimes, exotic plant and animal invasions, livestock grazing, and soil conditions altered by past land uses. We performed meta-analyses of existing stand-scale data from the published and grey literatures on seedling and sapling recruitment in blue and valley oaks throughout each of their distributions. We sought to evaluate whether distribution-wide regeneration `problems' exist for either species and to assess what factors correlate with distribution-wide recruitment patterns. Nearly 80% of sites surveyed for blue oaks but fewer than 50% of sites surveyed for valley oaks contained some evidence of seedling or sapling recruitment. A majority of sites surveyed for both species appear to have insufficient recruitment to replace adult populations, though further demographic work would be required to quantify minimum replacement recruitment rates. Reserve sites were seven times more likely than non-reserve sites to contain valley oak populations with evidence of recruitment. Blue oak recruitment patterns were weakly related to climate and geographical factors and strongly variable at subregional scales. We suggest several lines of additional research that could fill gaps in the existing literature and clarify the patterns emerging from this analysis.