Effects of Fire and Stand Structure on Grass Phenology in a Ponderosa Pine Forest

Abstract

-Phenological patterns of four abundant understory grasses were followed for 2 yr after prescribed burning in an Arizona Pinus ponderosa forest that consisted of a mosaic of patches, each dominated by only one of three possible overstory strata. Although there were phenological differences between the unburned sawtimber patches (120 stems ha-' and 63 cm average diameter at breast height (dbh)) and the unburned pole patches (1730 stems ha-' and 15 cm average dbh), these differences were smaller than those between burned patches and were not statistically significant. Patches dominated by the third type of overstory stratum, saplings, were not included in this study. Responses were species-specific. Festuca arizonica and Muhlenbergia montana both failed to flower the 1 st yr after burning in both sawtimber and pole patches, a statistically significant difference from unburned patches. Both species resumed flowering to varying degrees in the 2nd yr. In the 2nd yr, a significantly higher percentage of F. arizonica plants flowered in the burned sawtimber patches than in burned pole patches, whereas unburned patches were more similar to one another. More than half (58-75%) of M. montana plants failed to flower in the 2nd yr regardless of treatment or type of patch. In contrast, Poa fendleriana and Sitanion hystrix showed no statistically significant phenological response to burning in either sawtimber or pole patches. These patterns are discussed in relation to previously reported changes in the biomass and density of these species following burning in the same area.