Facilitative and competitive effects of a N-fixing shrub on white fir saplings

In Sierra Nevada forests, shrubs are considered strong soil moisture competitors with regenerating trees, reducing seedling establishment, and slowing growth. Recent studies, however, suggest that in some circumstances shrubs can facilitate tree establishment and growth by modifying harsh microclimate conditions; increasing acquisition of water, carbon, and/or nutrients via shared mycorrhizal connections; or enhancing soil fertility, particularly under nitrogen-fixing shrubs such as Ceanothus spp. We examined the establishment dates and growth rates and patterns of white fir saplings growing in greenleaf manzanita, whitethorn ceanothus, and bare patches to examine whether establishment was correlated with past wet years, whether saplings growing in ceanothus had nitrogen-enriched foliage or faster growth rates than in the other two patches, and whether saplings in shrub patches experienced competition for light.Sapling establishment was not correlated with high precipitation or heavy snowpack years, suggesting shade-tolerant saplings do not need wet years to become established. Soils under ceanothus were nitrogen enriched, but white fir sapling foliage did not have higher nitrogen concentrations and saplings did not grow faster in ceanothus than in the other two patches. Because growth rates of saplings were comparable in all patch types examined despite significantly different edaphic and abiotic conditions, we inferred that the various competitive and facilitative interactions affecting tree growth are in net balance across the patch types examined. However, competition for light is important—a significant percentage of growth release events occurred after saplings emerged above their host shrubs. Where shrubs are present, shade-tolerant species (i.e., white fir) are favored over drought-tolerant (pine) species. Our results may help interpret changes in understory conditions that are contributing to mixed conifer's compositional shift toward more shade-tolerant species after a century of fire-suppression.