Light thresholds for competitive reversals in Sierran conifers: Enhancing the restoration component of fuels-reduction canopy thinnings

Mid-elevation forests of the Sierra Nevada, California, have higher overstory tree-canopy cover now than over much of their history, leading to domination of the recruitment phase by shade-tolerant tree species such as white fir. Restoration of shade-intolerant species such as ponderosa pine can be enhanced by knowing how height growth (a component of competitive ability) varies with light. We measured 3-yr height growth of six conifer and one oak species growing along a 50-km transect spanning the main crest of the mountain range in the northern Sierra Nevada/southern Cascades. Naturally established seedlings <0.5 m tall were selected from a range of stand conditions, and light was quantified by fisheye-lens photography of the canopy. Likelihood methods were used to fit models of growth as a function of light for each species. Ponderosa pine and white fir both showed linear increases in height with light; variation explained ranged from r² = 0.14 – 0.41. Height growth curves crossed at an irradiance of 20 mol m^-2 d^-1 (~33% of full sun), providing a rule of thumb for minimum light levels at which ponderosa pine is likely to have a height growth advantage over co-occurring white fir. Height growth of the other species did not vary with light, except for sugar pine, whose light-dependent growth rate was similar to that of white fir. To restore historic species composition, canopy thinning projects should include microsites with sufficient light for shade-intolerant regeneration, if appropriate juvenile trees or seed sources are present.