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.

Citation

Reemts, C.M., Picinich, C. and Sperry, J.H., 2024. Crown fires remove a fire-sensitive canopy dominant from oak-juniper woodlands: results from long-term monitoring of wildfires. Fire Ecology, 20(1), p.73. https://doi.org/10.1186/s42408-024-00311-w