Variations in fire regimes can shape vegetation structure and species composition on local scales as in the case of mixed-severity fires through landscape scales Turner and Romme Vegetation response to a particular fire depends on fire severity, climatic conditions e. Oak-juniper Quercus spp.
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The dominant juniper species include both seeders and resprouters Adams Many junipers can be top-killed by fire when small Noel and Fowler , but large individuals often survive prescribed fires Ansley and Rasmussen Recolonization by seeding junipers can be slow in woodlands Reemts and Hansen , but more rapid in grassland and shrublands Ansley and Rasmussen The effects of wildfires on species composition in oak-juniper woodlands depend on the fire return interval, fire season, and regeneration strategies of the dominant species. In communities co-dominated by seeding juniper species, extensive, high-severity fires can shift dominance to resprouting oak species Floyd et al.
Repeated summer prescribed fires can also reduce or eliminate junipers from a community Ewing et al. Some oak species can survive high-frequency fires, at least in the short term Harrell et al. The fire regime and historic extent of oak-Ashe juniper Juniperus ashei Buchholz woodlands in central Texas is debated e.
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Oak-juniper woodlands in this area include a number of different communities, ranging from oak savannas in various stages of juniper encroachment to mature woodlands, which generally occur on shallower soils Diamond Historically, woodland cover within the region varied, but likely occupied around half of the overall landscape and most of the slopes Wills , Diamond and True , Murray et al. Fire regime ranged from low-severity, frequent fires in the savanna communities, to high-severity, infrequent crown fires in the woodlands Bray , Gehlbach , Diamond We focused on woodland communities on shallow soils, rather than on savannas.
Most hardwood species in the woodlands, including all oaks, resprout vigorously after being top-killed by fire Reemts and Hansen A few species, such as prairie sumac Rhus lanceolata [A. Gray] Britton , colonize burned areas primarily from seed, but can also resprout after fire Rasmussen and Wright Ashe juniper does not resprout and, while small individuals can be killed by relatively low-intensity fires, larger individuals are killed only by high-intensity fires Bryant et al. Ashe juniper seeds are dispersed by birds and, to a lesser extent, some mammals, and dispersal is relatively inefficient Chavez-Ramirez and Slack , The woodland communities can successfully regenerate after a single fire, although regaining the original structure may take decades Huss , Gehlbach However, little is known about the effects of multiple wildfires on vegetation structure or species composition.
We examined the response of oak-juniper woodlands to two overlapping wildfires and on Fort Hood Military Reservation, Texas, USA, to determine how repeated fires influence regeneration. Because most of the woody species resprout and because of the relatively long interval 14 years between fires, we predicted that vegetation structure and species composition after the second wildfire would be similar to the response after the first wildfire. Common vegetation communities include oak-Ashe juniper woodlands and oak shrublands on slopes and mesa tops.
Grasslands and riparian woodlands are found in the valleys and on rolling lowlands; savannas dominated by Texas live oak Quercus fusiformis Small. Military training requirements preclude cross-fencing, allowing cattle to roam freely.
Only part of the wildfire and only those transects analyzed for this study are shown. On 21 February , military training activities started three grassland fires. Due to hot, dry, and windy conditions, the fires moved into adjacent woodlands where they burned mostly as crown fires. By the time the fires were controlled on 7 March , they had burned more than ha of woodlands. The second fire, which burned from 3 to 12 April , was smaller ha; ha overlapping with first fire.
Both fires were preceded by above-average rainfall 11 to 13 months before the fire; 14 to 16 and 8 to 10 months before the fire; Figure 2. The fire occurred during a four-month period of below-average rainfall; rainfall during March to April was above average. Total monthly rainfall cm for the three-year period around the time of the two wildfires based on the closest weather stations in Bell and Coryell counties, Texas, USA. Average monthly rainfall from to was 7. Following the first fire, we randomly located permanent transects in moderately to severely burned oak-juniper woodlands Figure 1.
While the fire history of these nonburned woodlands is not known, all are present in aerial imagery taken in and appear similar in maturity to the woodlands subsequently burned in Tree skeletons in the burned areas were also of similar size to trees in the nonburned areas. We sampled transects for a full description, see Reemts and Hansen ; for this analysis, we focused on the 19 transects that burned in both fires 8 on mesas and 11 on slopes and on the nonburned transects 20 on mesas and 11 on slopes.
On Fort Hood, the vegetation on this soil type includes bastard oak Quercus sinuata Walter var. This soil series is in the Steep Adobe ecological site and supports a more diverse woodland, co-dominated by Buckley oak Q. In mesic canyons, Ashe juniper dominance decreases and additional species, such as bigtooth maple Acer grandidentatum Nutt. Transects were m long and followed either a random bearing for level sites or were perpendicular to the slope i.
From to , we re-randomized the selected plots every year.
Edwards Plateau Vegetation: Plant Ecological Studies in Central Texas
Starting in , we used the plot locations from the first sampling year on each transect. We visually estimated burn severity for substrate and vegetation in July to August for the first fire and April for the second fire, using definitions from National Park Service fire monitoring methods National Park Service For substrate, plots were classified as severely burned if litter was completely consumed and mineral soil was visible; moderately burned if litter was mostly consumed leaving coarse ash; lightly burned if litter was partially consumed or charred; and scorched if litter was partially blackened.
For vegetation, all plant parts except major stems were consumed in severely burned plots; foliage, twigs, and small stems were consumed in moderately burned plots; foliage and smaller twigs were partially to completely consumed in lightly burned plots; and foliage was scorched but still attached to supporting twigs in scorched plots. We did not distinguish between resprouts from top-killed individuals and regeneration from seed.
Stems that split above the root crown were counted as one. Saplings were divided into five dbh classes: 0 cm to 0. From to , we recorded the stem density of trees in 5 cm or 10 cm dbh classes.
Starting in , we recorded dbh to the nearest 0. If saplings or trees branched above the root crown, only the largest stem was measured and counted. In the study area, Ashe juniper is the species that most commonly has multiple branches rather than multiple stems; Ashe juniper is therefore somewhat under-represented in the basal area data. While we sampled multiple transects in the burned areas, these transects represent sub-samples of the two wildfire events and we lack true replication. However, our data are still useful for comparing vegetation responses to these two fires Wester and provide an opportunity to document a poorly studied phenomenon i.
We compared burn severity between the and fires using a chi-square analysis SAS 9. We used plots as sampling units because fire intensity was patchy and fire severity within a single transect often spanned three or even four burn severity categories. We compared burn severity substrate and vegetation separately between years within topographic position; we also compared substrate and vegetation burn severity within a year and topographic position.
Because the plot locations within a transect varied among years, we averaged plot-level values for each transect. For each analysis, we selected an appropriate covariance structure using the adjusted Akaike Information Criterion AICc. Throughout this study, year 0 is defined as the year of the fire and We also conducted a repeated measures analysis comparing density and basal area in the once-burned and twice-burned sites through time years 0 to 2. Significance tests were adjusted to account for multiple comparisons using the simulated distribution of the maximum of a multivariate t random vector SAS Institute To examine changes in species composition, we used nonmetric multidimensional scaling with a Sorenson distance measure PCORD 6.
For each transect, we summed average basal area for the shrub, sapling, and tree layers. Shrubs were included because many transects did not have saplings or trees in the first few years after fire.
Shrub dbh was not recorded, but most shrubs woody stems between 0. For the purposes of this analysis, we converted shrub density into basal area by assuming that the average shrub dbh was 0. We used data from the following years: once-burned transects from not all transects were sampled in and ; twice-burned transects from to ; and nonburned transects from and We eliminated species with fewer than three occurrences from the analysis and applied a square-root transformation.
A two-dimensional ordination was chosen based on a Monte Carlo permutation test and scree plot. Understory stem density did not differ between mesas and slopes in any year. Averaged data from nonburned transects collected in , , and are shown for comparison. For the ordination, final stress was Ordination indicated that species composition changed immediately after fire, but became more similar to nonburned areas over time Figure 5.
Species composition after the second fire was similar to that after the first fire, but twice-burned transects were grouped lower along Axis 2 than once-burned transects. Species composition in nonburned areas did not change between and Axis 1 was negatively correlated with basal area of juniper nonburned areas had much higher basal area of juniper, Table 6. Basal area of bastard oak, Texas ash, Buckley oak, and Texas live oak were also negatively correlated with Axis 1, while poison ivy Toxicodendron radicans [L.
Axis 2 was positively correlated with basal area of Buckley oak. Nonmetric multi-dimensional scaling ordination of species basal area shrubs, saplings, and trees in burned and nonburned transects on Fort Hood Military Reservation Texas, USA. Contrary to our predictions, vegetation structure differed after the second fire: initial understory stem density was much higher in than after the first fire in Species with higher density after the second fire included resprouting trees e.
The dramatic increase in stem density could be a response to the repeated fires: some woody plants gradually lose the ability to resprout as they increase in size Babb , Vesk and Westoby , Franklin et al. Because the trees burned in the second fire were much younger and smaller than those burned in the first fire based on the size of the tree skeletons , the increase in stem density could be a more vigorous resprouting response.
In Oklahoma, for example, sand shinnery oak Quercus havardii Rydb. Similarly, Gambel oak Quercus gambelii Nutt.