Crown fires remove a fire-sensitive canopy dominant from oak-juniper woodlands: results from long-term monitoring of wildfires

Abstract

Background In central Texas, re-sprouting oaks (Quercus spp.) co-occur with non-resprouting Ashe juniper (Juniperus ashei) in a mosaic of fire-dependent (oak savanna) and fire-sensitive (oak-juniper woodland) habitats. The region’s mature woodlands are the only nesting habitat for the endangered golden-cheeked warbler (Setophaga chrysoparia). We studied long-term recovery of woodland structure and species composition after single and repeated crown fires on three soil types (mesa, slope, and deep savanna soils). Results On once-burned sites, density and basal area of non-juniper trees (all woody species except juniper) reached or exceeded unburned levels after 14–24 years, indicating successful recruitment (24 years vs unburned, mesa: 481 ± 254 vs 155 ± 137 stems ha−1, 2 ± 1 vs 1 ± 2 m2 ha−1; slope: 910 ± 330 vs 251 ± 103 stems ha−1, 5 ± 2 vs 3 ± 2 m2 ha−1). Ashe juniper, however, remained mostly absent from burned woodlands (juniper tree density, 24 years vs unburned, mesa: 6 ± 10 vs 691 ± 410 stems ha−1; slope: 20 ± 17 vs 731 ± 183 stems ha−1) and total basal area was 47–87% lower than in unburned areas. In formerly fire-suppressed savannas, non-juniper tree density exceeded unburned levels and juniper density recovered the most (24 years vs unburned, non-juniper: 679 ± 250 vs 251 ± 103 stems ha−1; juniper: deep: 50 ± 71 vs 317 ± 297 stems ha−1). Juniper trees were still absent from twice-burned sites in year 11 and understory density was recovering more slowly, at least on slopes (one fire: 224 ± 206 stems ha−1; two fires: 26 ± 47 stems ha−1). Juniper recovery was correlated with distance to the wildfire perimeter, suggesting that regeneration is limited in part by dispersal. Conclusions We found successful recruitment of resprouting hardwood species after one and two crown fires, likely due to the low deer densities at Fort Cavazos. In fire-suppressed oak savannas, a single crown fire did not restore savanna structure and Ashe juniper is slowly re-establishing. Long-term restoration of a savanna on these soils will require additional treatments, like repeated prescribed fire. In oak-juniper woodlands, crown fires removed the fire-sensitive Ashe juniper from canopy co-dominance for decades, making the woodlands unsuitable as habitat for golden-cheeked warblers. Given the long-term consequences of crown fires for golden-cheeked warbler habitat, existing mature oak-juniper woodlands should be protected from crown fire.