Shrub communities, spatial patterns, and shrub-mediated tree mortality following reintroduced fire in Yosemite National Park, California, USA

Shrubs contribute to the forest fuel load; their distribution is important to tree mortality and regeneration, and vertebrate occupancy. We used a method new to fire ecology—extensive continuous mapping of trees and shrub patches within a single large (25.6 ha) study site—to identify changes in shrub area, biomass, and spatial pattern due to fire reintroduction by a backfire following a century of fire exclusion in lower montane forests of the Sierra Nevada, California, USA. We examined whether trees in close proximity to shrubs prior to fire experienced higher mortality rates than trees in areas without shrubs. We calculated shrub biomass using demography subplots and existing allometric equations, and we developed new equations for beaked hazel (Corylus cornuta ssp. californica [A. de Candolle] E. Murray) from full dissection of 50 stems. Fire decreased shrub patch area from 15.1% to 0.9%, reduced live shrub biomass from 3.49 Mg ha^-1 to 0.27 Mg ha^-1, and consumed 4.41 Mg ha^-1 of living and dead shrubs. Distinct (non-overlapping) shrub patches decreased from 47 ha^-1 to 6 ha^-1. The mean distance between shrub patches increased 135%. Distances between montane chaparral patches increased 285%, compared to a 54% increase in distances between riparian shrub patches and an increase of 267% between generalist shrub patches. Fire-related tree mortality within shrub patches was marginally lower (67.6% versus 71.8%), showing a contrasting effect of shrubs on tree mortality between this forest ecosystem and chaparral-dominated ecosystems in which most trees are killed by fire.