BEE VISITATION TO RARE PENSTEMON PERSONATUS VARIES WITH FOREST MECHANICAL THINNING INTENSITY

Small gaps were a reliable feature of mixed conifer forests in the northern Sierra Nevada mountains of California, USA, under historical low-severity, frequent fire regimes. Where fire has been excluded, mechanical treatments can also interrupt the tree canopy and increase light penetration to the forest understory. We evaluated the effects of mechanical thinning treatments at two intensities to the rare forb Penstemon personatus D.D.Keck (Plantaginaceae). We asked whether bee visitation to P. personatus was predicted by variation in light environment and flower production among three forest treatments: unthinned, mechanically thinned, and a gap treatment. At small spatial scales bee visitation increased with the number of available flowers, an effect that was amplified in full-sun conditions. Plots in gap treatments were overwhelmingly likely to occur in full sun during plot observations and were strongly correlated with higher numbers of flower visits, bee visitors, and bee morphotaxa richness relative to thinned or unthinned plots. Although plots in the thinned treatment had similar densities of available flowers as gap plots, they frequently remained shaded during plot observations and experienced bee visitation at levels similar to unthinned plots. Results support management that incorporates small gaps into forest thinning treatments to increase available light to the understory and promote both flower production and bee visitation for understory species such as P. personatus.