Reemts and Hansen 2008




Ashe juniper, a critical component of golden-cheeked warbler habitat, is a slow-growing species that may require decades to recover following a crown fire.

Due to the paucity of available golden-cheeked warbler habitat, crown fires in mature oak-juniper woodlands should be avoided.

In this study, researchers investigated plant community changes over nearly a decade following a crown fire in mature oak-Ashe juniper woodlands (Quercus spp.-Juniperus ashei) of central Texas. Of particular interest was determining the length of time needed for reestablishment of Ashe juniper trees, a required habitat component for the federally endangered golden-cheeked warbler (Dendroica chrysoparia), and how it varied across three locally common soil types: Evant silty clay (Evant); Eckrant-rock outcrop complex (Eckrant); and Real-rock outcrop complex (Real).

The study occurred on the Fort Hood Military Reservation, located at the intersection of the Cross Timbers Southern Tallgrass Prairie and the Edwards Plateau ecoregions. A limestone area with steep-sided mesas and wide valleys, this area historically supported communities ranging from mixed-grass prairies to floodplain forests, including oak shrublands, oak savannas, and oak juniper woodlands. The latter hosts one of the largest known breeding populations of the golden-cheeked warbler.

In the region’s oak-juniper woodlands, Ashe juniper can co-occur with live oak (Q. fusiformis), post oak (Q. stellata), blackjack oak (Q. marilandica), shin oak (Q. sinuata var. breviloba) and Texas red oak (Q. buckleyi). Unlike oaks, Ashe juniper does not resprout after being top-killed by fire. Communities vary according to slope, moisture levels, fire frequency, and especially the soil type. Post oak dominates the deeper Evant soil type; shin oak dominates the shallow, flat Eckrant soil type; and Texas red oak dominates the sloped Real soil type.

In February 1996, following a year-long drought and during a period of high temperatures and strong winds, military exercises at Fort Hood sparked grass fires that spread into adjoining woodlands and burned for more than two weeks. Crown fires in these oak-juniper woodlands included more than 2,000 ha of golden cheeked warbler habitat, and affected more than 4,000 ha of woodlands overall.

Background: unburned Ashe juniper-oak woodland (Photo: Charlotte Reemts). Inset: golden-cheeked warbler (Photo: Gil Eckrich).

Within a few months after the fire, permanent vegetation monitoring transects were established in burned areas of former golden-cheeked warbler habitat, randomly located on all three soil types. Control transects were placed in unburned mature oak-juniper woodlands on the same soils. Data were collected annually from 1996 to 2002, and in 2005.

Stem density and basal area were calculated for four categories of woody plants: seedlings, including resprouts (<0.3 m tall), shrubs (>0.3 but <1.8 m tall), saplings (>1.8 m tall but < 5.0 cm dbh) and trees (>5.0 cm dbh). In the tudy’s final year, stem density and basal area were calculated by species for Ashe uniper, post oak, shin oak, and Texas red ak.

Researchers observed low relative bundance of woody species and basal reas in the burned areas. Densities of Ashe juniper in burned areas were very low throughout the study period. Oak dominance in burned areas, measured as a percentage of basal area, was significantly lower than in unburned areas across all soil types. However, by study’s end in 2005, burned areas became more similar to unburned ones, except for a lack of Ashe juniper. High numbers of oak stump sprouts suggest that burned areas will develop into woodlands in the future.

Seedling density in burned areas increased rapidly between 1999 and 2005, at which time it was about the same between burned and unburned areas. In 1996, shrub density was similar between burned and unburned areas. In burned areas it increased during the first few years, but then remained constant until declining during the study’s last year. Sapling density in burned areas was low at the start (<800 stems/ha), but increased greatly within a year (1,800 – 5,000 stems/ha). By 2005, sapling density was higher in burned than unburned areas. Tree density on burned areas was low until 2005, but tree regrowth on burned areas varied by soil type and species.

Ashe juniper seedling density was considerably lower compared to unburned sites for the entire period and across all soil types (in 2005, 76 ± 267/seedlings/ha, burned, vs. 6,192 ± 389 seedlings/ha, unburned). Similarly, density of shrub-size stems for this species was also lower than on unburned areas, except on Evant soils. No stems had reached the tree size class by the end of the study.

Post oak seedling density was significantly lower in burned than in unburned areas throughout the study period. Early in the study, post oak shrub density was high in burned areas, but declined by 2005 as resprouted stems moved into the sapling class. Density of tree-sized stems was very low (51 ± 67 trees/ha) at that time.

Shin oak seedling density rebounded to levels found in unburned areas during the first growing season for Evant and Real soils, but not for Eckrant soils, where the species is dominant. Shrubs and saplings of this species, by 2005, were of similar densities in both burned and unburned areas. However, at that time none of the regrowth had reached the tree class.

Texas red oak recovered faster than post oak and shin oak. Seedling density remained lower on Real soils (where it is a dominant) throughout the study, likely because most stems were in taller size classes. As with post oak, Texas red oak shrub density was high in early years, but declined as shrub-sized stems entered the sapling class. By 2005, sapling density was higher in burned areas than unburned. At that time, tree-size stem density had also reached levels found in unburned areas, but with low basal area.

Because fire-sensitive Ashe juniper commonly invades open land when fire is suppressed, authors were surprised that its abundance was low even 10 years after the fire, and the few junipers present were small and short. However, a lack of juniper regeneration after a crown fire had been previously observed in this region. Slow colonization was attributed to: the large extent of the crown fire; timing of the fire (February), which killed newly sprouted seedlings; juniper seeds’ sensitivity to fire; lack of relict juniper trees nearby to provide seeds in subsequent years; lack of trees of suitable size to provide roosts for birds that deposit seeds; and competition from resprouting species, particularly oaks.

Abundance of golden-cheeked warblers greatly diminished in the burned areas following the 1996 fire. However, locally, golden-cheeked warbler overall population did not decrease, presumably because the birds relocated to suitable habitat in nearby unburned areas.

Authors predict repopulating warblers in the burned area will take several decades due to the slow growth of Ashe juniper, and they recommend avoiding crown fires in mature oak-juniper woodlands where golden-cheeked warblers currently reside, due to the fragmented and overall reduced levels of available habitat for these birds.

Stem densities in four vegetation strata on three soils. Seedlings were <0.3 m in height, shrubs were between 0.3 and 1.8 m in height, saplings were > 1.8 m tall but <5.0 cm dbh and trees were > 5.0 cm dbh. Evant soils are dominated by post oak (Quercus stellata), Eckrant soils are dominated by the shrubby shin oak (Q. sinuate var. breviloba) and Real soils are dominated by Texas red oak (Q. buckleyi). For unburned areas (open symbols), only data from 2005 are shown.

Chart is reprinted by permission of Forest Ecology and Management 255 (2008) 1057-1066

Charlotte M. Reemts and Laura L. Hansen, Forest Ecology and Management 255 (2008) 1057–1066

The Oak Woodlands and Forests Fire Consortium seeks to provide fire science to resource managers, land owners and the public about the use, application, and effects of fire in the region.

This research brief was funded by The Joint Fire Science Program.