Recruitment after fires is a vital part of renewal in mixed-conifer forests for many dominant species, and areas where initial recruitment is poor can suffer regeneration failures that last for decades. There is evidence from observational analyses that post-fire recruitment is influenced by weather patterns, particularly the amount of precipitation in the years immediately following a fire. In addition, conifer regeneration can be strongly affected by interspecific interactions with shrubs, which can grow quickly after fires into suppressive canopy. As a controlled test of the impacts of winter precipitation on conifer recruitment and conifer-shrub competitive interactions, we set up a manipulative experiment in the northern Sierra Nevada, within the area burned by the King Fire in 2014. We crossed a competition treatment (shrub removal) with manipulated drought (precipitation reduction by rain-out shelters) and planted 1200 Pinus ponderosa and Abies concolor seeds in 16 plots to answer three questions: 1. How does post-fire drought affect conifer recruitment? 2. What are the effects of drought on conifer-shrub interactions? 3. How do these effects differ by conifer species? We analyzed emergence, survival, and growth using generalized linear mixed-effects models.
The drought treatment had 17% fewer seedlings emerge, but those that did emerge were 23% taller and had 33% larger canopies (height and width p < 0.005), and stayed larger throughout the growing season. Emergence of P. ponderosa seedlings was more negatively affected by shrubs than emergence of A. concolor (p = 0.06). Conifer seedlings in plots with shrubs present were 9% taller at the beginning of the growing season (p = 0.03), but 29% shorter by the end of the growing season (p = 0.02). We hypothesize that this patterns may be due to initial competitive pressure to rise above shading, and a faster depletion of available soil moisture stopping seedling growth. In plots with shrubs, survival of seedlings was positively related to proximity to the nearest shrub (p = 0.02), which may be driven by evapotranspiration or temperature. Our results are consistent with observational analyses of post-fire drought and shrubs decreasing conifer regeneration, and further break down how these drivers may affect conifer recruitment. With global change projections predicting increasing forest area burned and increasing drought stress for California regions, forest recruitment may become increasingly tied to post-fire climate, and understanding these effects will be vital to protect forest communities.