Mechanically-created gaps promote flowering and seed set of rare Penstemon personatus: Disentangling canopy opening from ground disturbance

Abstract Fire exclusion from mixed conifer forests of northern California has changed the growing environment for understory plants, including the rare Penstemon personatus D.D. Keck, a rhizomatous perennial species endemic to the northern Sierra Nevada. We asked whether increased light availability and/or ground disturbance associated with mechanical thinning would change density or promote flowering of P. personatus, with particular interest in the effects of 0.2 ha gaps, where tree removal created canopy openings that may be similar to those that characterized stands under mixed-severity fire regimes. We found evidence of trade-offs at small spatial scales, with both total and flowering stem density decreasing in plots with increased ground disturbance, but a strong positive flowering stem density response in plots where tree removal increased solar exposure. One year post-treatment, solar exposure was positively correlated with flower density, seeds per capsule, and predicted seed set. At the treatment scale, there were no significant differences in total stem density among treatments or years, however flowering stem density was significantly higher in gaps two years post-treatment. The gap treatment was also strongly associated with higher flower densities, and a predicted mean rank seed set (365 seeds m−2) that was 6 times greater than seed set within the thinning treatment (53 seeds m−2), contrasting sharply with a near total lack of seed set in control plots. This study emphasizes the importance of canopy gaps to this rare species, with local negative effects of ground disturbance from mechanical thinning offset by treatment-level promotion of flowering and seed set.