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Effects of Fire Exclusion on Previously Fire-Managed Semiarid Savanna Ecosystem
Rangeland Ecology & Management xxx (xxxx) xxx
Contents lists available at ScienceDirect
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Effects of Fire Exclusion on Previously Fire-Managed Semiarid Savanna Ecosystem Heath D. Starns*, Charles A. Taylor 1, Nick E. Garza, Douglas R. Tolleson Texas A&M AgriLife Research, Sonora Research Station, Sonora, TX 76950, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 June 2019 Received in revised form 13 September 2019 Accepted 20 September 2019
Long-term (> 100 yr) fire exclusion is associated with numerous ecological consequences in grasslands and savannas, including transitions into shrub- or tree-dominated systems. Several studies have reported differences in woody vegetation after multiple fires among burned and unburned rangelands, but none have reported the impacts of fire exclusion after a period of fire management. We evaluated effects of fire exclusion on herbaceous and woody canopy cover and herbaceous biomass in semiarid savanna of southwest Texas in pastures with known burn histories. Pastures were burned in summer and winter in 1994, 2000, and 2006, followed by 11 yr of fire exclusion. Between 2006 and 2017, woody subcanopy increased (5e21%) in all treatments while overstory canopy remained unchanged. Herbaceous cover decreased (5e18%) in all treatments but remained higher in burned treatments. From 2006 to 2017, herbaceous biomass declined in all treatments by > 650 kg$ha1 and was not statistically different among treatments. These trends support other research demonstrating the importance of historical mean fire return interval in maintaining grasslands and savannas. © 2019 The Society for Range Management. Published by Elsevier Inc. All rights reserved.
Key Words: Edwards Plateau herbaceous biomass long-term effects rangeland Texas woody encroachment
Introduction Fire is an important driver in the evolutionary history of grasslands and savannas, and frequency and seasonality of fires historically interacted with climate and herbivory to influence vegetation structure and composition in both biomes (Axelrod 1985; Bond and Keeley 2005; Beerling and Osborne 2006; Govender et al. 2006). Both biomes have declined in area as a result of human activity and have been identified as having high risk of biodiversity loss (Hoekstra et al. 2005). Fire exclusion has contributed to conversion of millions of hectares of these biomes to shrublands and woodlands (Bragg and Hulbert 1976; Fuhlendorf and Smeins 1997; Briggs et al. 2005; Archer et al. 2017). In recent decades, prescribed fire has increased in acceptance as a method for maintaining grasslands and savannas and reversing woody plant encroachment (Twidwell et al. 2013). Fire regime (e.g., seasonality, frequency, and intensity) plays an important role in determining the ecological response of these landscapes to fire (Engle and Bidwell 2001; Taylor et al. 2012; Weir and Scasta 2017;
* Correspondence: Heath D. Starns, Texas A&M AgriLife Research, Sonora Research Station, Sonora, TX 76950, USA. E-mail address: [email protected] (H.D. Starns). 1 Retired from Texas A&M AgriLife Research, Sonora Research Station, Sonora, TX 76950, USA.
Dickson et al. 2019). Season of fire affects vegetation structure and composition, with summer burns exhibiting more negative impacts on woody plants in temperate grasslands than burns during the dormant season (Taylor et al. 2012; Weir and Scasta 2017). The effects of fire frequency (mean fire return interval [MFRI]) have been studied extensively in grasslands, and evidence suggests woody species can reach a height at which they escape the influence of fire if MFRI exceeds a specific threshold (Briggs et al. 2005; Taylor et al. 2012; Knuckey et al. 2016; Ratajczak et al. 2016; Aslan et al. 2018). Although several experiments have reported cumulative effects of multiple fires, none have followed fire reintroduction with a subsequent period of fire exclusion. Taylor et al. (2012) reported on vegetative communities resulting from multiple fires during summer and winter in an ungrazed Texas semiarid savanna. Our objective was to evaluate changes in these plant communities following 11 subsequent yr without fire. Methods Our research was conducted on the Texas A&M AgriLife Research Station (lat 31 N, long 100 W; hereafter SRS) 56 km south of Sonora, Texas. Precipitation is bimodal and highly variable (range 156 mme1 054 mm, median 557 mm per SRS records), mostly occurring in spring and autumn. The SRS features gently rolling
https://doi.org/10.1016/j.rama.2019.09.006 1550-7424/© 2019 The Society for Range Management. Published by Elsevier Inc. All rights reserved.
Please cite this article as: Starns, H.D et al., Effects of Fire Exclusion on Previously Fire-Managed Semiarid Savanna Ecosystem, Rangeland Ecology & Management, https://doi.org/10.1016/j.rama.2019.09.006
H.D. Starns et al. / Rangeland Ecology & Management xxx (xxxx) xxx
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topography (3e4% slopes) representative of the Edwards Plateau ecological region of Texas, and soils are generally shallow (15e30 cm deep). Vegetation is primarily savanna with clusters of woody species interspersed within a matrix of midgrasses and shortgrasses. A 12-ha study pasture was fenced in 1994 and equally divided into six 2-ha treatment units. Before 1994, the site had been unburned since 1916 and grazed by sheep at light and heavy stocking rates (SRS records, unpublished). Since 1994, all livestock were excluded but wildlife were present. In 2006, three unique vegetative communities existed as a result of seasonal differences in fire effects (Taylor et al. 2012). Summer burned units consisted of predominately live oak (Quercus virginiana) overstory and an understory woody plant community free of sacahuista (Nolina texana) and Ashe juniper (Juniperus asheii) (Taylor et al. 2012). Unburned units had more overstory canopy than burned treatments in 2006, which was dominated by a live oakAshe juniper mix (Taylor et al. 2012). Winter burned units had an overstory dominated by live oak with an understory of sacahuista and prickly pear (Opuntia spp.) and without Ashe juniper (Taylor et al. 2012). After 2006, fire was excluded from all treatments for a period of ~12 yr due to drought and lack of adequate fine fuel, resulting in a single “skipped” fire based on the estimated historic MFRI of ~6 yr (Frost 1998; Stambaugh et al. 2014). Before the winter fire season of 20172018, we resampled transects reported on by Taylor et al. (2012) to evaluate effects of the “skipped” fire cycle on the vegetative communities and aboveground herbaceous biomass. Vegetation Sampling Cover of woody overstory (> 1.5 m), woody subcanopy (< 1.5 m), and herbaceous canopy was sampled in each experimental unit along 10 permanent 50-m transects (n ¼ 6 units 10 transects ¼ 60). In NovemberDecember of 2017, cover of each species within each canopy layer was recorded using the line intercept method (Floyd and Anderson 1987). Species-level cover data were summed to quantify total cover of each canopy layer. Three quadrats (0.25 m2) were clipped along each transect and dried at 45 C to a stable weight to estimate herbaceous biomass. Because the prior study determined fire was not a primary driver of herbaceous community composition (Taylor et al. 2012), we did not evaluate frequencies of herbaceous species for the present study. Analysis To evaluate cover of the various canopy layers, we created linear models using year and burn season as fixed effects. Significant terms were further evaluated using pair-wise comparisons where appropriate. Herbaceous biomass was recorded periodically throughout the initial study, so we included those data for analysis of effects of burn season on herbaceous biomass. We created a linear mixed-effects model to evaluate herbaceous biomass using burn season and sampling year as fixed effects. Because herbaceous biomass was clipped during different months across years, we included sampling month as a random factor in that analysis. All analyses were performed using the R environment for statistical computing (R Core Team 2017). Results Year was a significant predictor of changes in total cover of herbaceous and woody subcanopy plants (P < 0.001), but no significant change in overstory canopy was observed (Fig. 1A; P ¼ 0.846). Subcanopy cover increased in all treatments from 2006 to 2017 (see Fig. 1B), with the greatest increase in unburned treatments (11.1732.34%). In contrast, herbaceous cover decreased in
Figure 1. Mean cover of A, woody overstory, B, woody subcanopy, and C, herbaceous canopy cover from 2006 to 2017, expressed as mean percentage of transect length. In all panels, solid lines represent control (unburned) treatments, long dashed lines represent winter burned treatments, and short dashed lines represent summer burned treatments. Shaded areas indicate 95% confidence interval.
all treatments (see Fig. 1C). The control treatment showed the largest decrease in herbaceous cover (95.9477.83%). Season of burn was a significant predictor of differences in overstory, understory, and herbaceous cover among treatments (Table 1). Total overstory canopy cover was lower in summer treatments than unburned and winter treatments (P < 0.001). Overstory did not differ statistically between unburned and winter treatments (P ¼ 0.066). Woody subcanopy cover followed a pattern similar to overstory in all treatments. Summer burned treatments had significantly less subcanopy than unburned (P < 0.01), but winter burned treatments were not statistically different than summer burned (P ¼ 0.065) and unburned (P ¼ 0.54) treatments (see Table 1). Herbaceous cover was lower in unburned treatments than in summer or winter burned treatments (P < 0.001), which did not differ (P ¼ 0.74). Year was also a significant predictor of changes in herbaceous biomass (P < 0.001). Herbaceous biomass was similar in all treatments before implementation of fire in 1994 (Fig. 2). While biomass did not differ between winter burned and unburned treatments from 1994 to 2017, biomass in summer burned treatments was greater than in other treatments in 2006 (P < 0.01). However, after 11 yr of fire exclusion, herbaceous biomass in the summer burned treatment was not different than winter burned and unburned treatments (P > 0.9). Further, herbaceous biomass in 2017 was significantly lower in all treatments than in 1994 or 2006 (P < 0.05).
Please cite this article as: Starns, H.D et al., Effects of Fire Exclusion on Previously Fire-Managed Semiarid Savanna Ecosystem, Rangeland Ecology & Management, https://doi.org/10.1016/j.rama.2019.09.006
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Table 1 Pair-wise comparisons of differences in means of herbaceous canopy cover, overstory (> 1.5 m) canopy cover, understory (< 1.5 m) canopy cover, and herbaceous biomass in a previously fire-managed ecosystem after 11 yr of fire exclusion. Bold text indicates statistically significant results. Canopy type
Comparison
Estimate
Lower bound
Upper bound
P¼
Overstory
Control winter Control summer Winter summer Control winter Control summer Winter summer Control winter Control summer Winter summer Control winter Control summer Winter summer
4.039 11.000 6.961 1.910 5.921 4.011 4.430 4.992 0.562 2.407 14.837 ¡12.429
8.286 15.246 2.714 6.108 10.119 0.187 2.647 3.209 2.345 1.106 11.337 ¡15.933
0.208 6.753 11.207 2.288 1.723 8.209 6.213 6.775 1.221 5.921 18.337 ¡8.926
0.066 < 0.001 < 0.001 0.535 < 0.01 0.065 < 0.001 < 0.001 0.741 0.243 < 0.001 < 0.001
Understory
Herbaceous
Herb.Biomass
Overstory of summer burned treatments remained dominated primarily by live oak. However, Pinchot’s juniper was able to reestablish in the overstory in 2017, whereas in 2006 it was only present in the subcanopy in summer treatments. In 2017 overstory of winter treatments remained primarily a mixture of live oak and Ashe juniper. Subcanopy species sacahuista and Ashe juniper were not present in summer treatments in 2006 but had returned by 2017. Similarly, several shrub species were noted in the subcanopy of winter treatments in 2017 that had been eliminated from detection by repeated fires. Discussion Our objective was to evaluate effects of one “skipped” prescribed fire on woody and herbaceous canopy in a fire-dependent ecosystem previously managed using fire. Our results indicate that woody subcanopy increases rapidly when fire is removed from this semiarid savanna. Legacy effects of previous fire treatments were still apparent via differences observed in canopy cover of woody understory and overstory plants, suggesting impacts of summer fires have greater longevity. Our results are in agreement with other studies of fire exclusion (Briggs et al. 2005; Joubert et al. 2012) and fire season (Briggs et al. 2005; Taylor et al. 2012; Weir and Scasta 2017).
The lack of change of overstory in all treatments between 2006 and 2017 probably stems from a combination of two factors. First, previous burns created significant differences in overstory canopy among treatments in 2006. In control treatments, overstory canopy was already high in 2006, leaving little opportunity for a significant increase. Second, our results suggest that subcanopy species in general did not have sufficient time between burns to transition into overstory in burned treatments. The increase in subcanopy cover is consistent with other reports that fire differentially impacts shrubs of smaller sizes (Freeman et al. 2017). The decrease in herbaceous cover in control treatments is unsurprising in light of the increase in total woody plant (overstory þ subcanopy) cover. Overstory canopy is known to reduce herbaceous biomass in temperate savannas (Haworth and McPherson 1994), and leaf litter is negatively correlated with herbaceous vegetation in woodlands (Sydes and Grime 1981). Although we did not directly evaluate leaf litter as a component of ground cover, it was noted as a large component in woody-dominated areas of control treatments. Furthermore, the increase in woody plant cover inevitably increased shading, which also suppresses grass growth beneath woody canopy in savannas (Charles-Dominique et al. 2018). The positive effect of summer fires on herbaceous biomass observed in our study is probably in part due to the lack of woody plants and associated shading (Charles-Dominique et al. 2018).
Figure 2. Mean herbaceous biomass (kg$ha1) of control (solid line/circles), summer burned (dotted line/triangles), and winter burned (dashed line/squares) treatments during the period of fire management (19942006) and a subsequent period of fire exclusion (20062017). Error bars represent one standard deviation from the mean.
Please cite this article as: Starns, H.D et al., Effects of Fire Exclusion on Previously Fire-Managed Semiarid Savanna Ecosystem, Rangeland Ecology & Management, https://doi.org/10.1016/j.rama.2019.09.006
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However, the rapid decline in herbaceous biomass following fire exclusion did not correspond with an equally rapid increase in woody cover, suggesting possible influence by factors outside the scope of our study. Although rainfall is known to have significant impacts on herbaceous biomass, it is unlikely to have contributed to the decrease in herbaceous biomass from 2006 to 2017 since 2017 had higher precipitation than 2006. It should be noted that the presence of axis deer (Axis axis), a non-native large ungulate, was first noted on SRS around 2008. Axis deer are primarily grazers (Henke et al. 1988) and were not excluded from the study area. Thus, the invasion of axis deer may have impacted herbaceous biomass in a manner similar to livestock grazing. Our results support findings of other studies that fire exclusion facilitates woody encroachment into savanna ecosystems (Beckage et al. 2011; Taylor et al. 2012). In this semiarid savanna, reintroduction of fire at the estimated 6-yr MFRI during summer had substantial positive effects on herbaceous biomass. In stark contrast, re-exclusion of fire for 11 yr was associated with a sharp decline in herbaceous biomass. Our results highlight the importance of maintaining fire according to the approximate MFRI and season for a given region. Acknowledgments The authors would like to thank Robert Moen, Dr. Erika Campbell, and numerous other technicians who assisted in data collection from 1994 through 2017, in addition to the associate editor and two anonymous reviewers. References Archer, S.R., Andersen, E.M., Predick, K.I., Schwinning, S., Steidl, R.J., Woods, S.R., 2017. Woody plant encroachment: causes and consequences. Rangeland Systems. Springer, New York, NY, USA, pp. 25e84. Aslan, C.E., Samberg, L., Dickson, B.G., Gray, M.E., 2018. Management thresholds stemming from altered fire dynamics in present-day arid and semi-arid environments. Journal of Environmental Management 227, 87e94. Axelrod, D.I., 1985. Rise of the grassland biome, central North-America. Botanical Review 51, 163e201. Beckage, B., Gross, L.J., Platt, W.J., 2011. Grass feedbacks on fire stabilize savannas. Ecological Modelling 222, 2227e2233. Beerling, D.J., Osborne, C.P., 2006. The origin of the savanna biome. Global Change Biology 12, 2023e2031. Bond, W.J., Keeley, J.E., 2005. Fire as a global 'herbivore': the ecology and evolution of flammable ecosystems. Trends in Ecology & Evolution 20, 387e394. Bragg, T.B., Hulbert, L.C., 1976. Woody plant invasion of unburned Kansas bluestem prairie. Journal of Range Management 29, 19e24.
Briggs, J.M., Knapp, A.K., Blair, J.M., Heisler, J.L., Hoch, G.A., Lett, M.S., McCarron, J.K., 2005. An ecosystem in transition. Causes and consequences of the conversion of mesic grassland to shrubland. BioScience 55, 243e254. Charles-Dominique, T., Midgley, G.F., Tomlinson, K.W., Bond, W.J., 2018. Steal the light: shade vs fire adapted vegetation in forest-savanna mosaics. New Phytologist 218, 1419e1429. Dickson, T.L., Hayes, B.A., Bragg, T.B., 2019. Effects of 34 years of experimentally manipulated burn seasons and frequencies on prairie plant composition. Rangeland Ecology & Management 72, 82e91. Engle, D.M., Bidwell, T.G., 2001. The response of central North American prairies to seasonal fire. Journal of Range Management 54, 2e10. Floyd, D.A., Anderson, J.E., 1987. A comparison of 3 methods for estimating plant cover. Journal of Ecology 75, 221e228. Freeman, M.E., Vesk, P.A., Murphy, B.P., Cook, G.D., Richards, A.E., Williams, R.J., 2017. Defining the fire trap: extension of the persistence equilibrium model in mesic savannas. Australian Ecology 42, 890e899. Frost, C.C., 1998. Presettlement fire frequency regimes of the United States: a first approximation. Fire in ecosystem management: shifting the paradigm from suppression to prescription. Tall Timbers Fire Ecology Conference Proceedings 70e81. Fuhlendorf, S.D., Smeins, F.E., 1997. Long-term vegetation dynamics mediated by herbivores, weather and fire in a Juniperus-Quercus savanna. Journal of Vegetation Science 8, 819e828. Govender, N., Trollope, W.S.W., Van Wilgen, B.W., 2006. The effect of fire season, fire frequency, rainfall and management on fire intensity in savanna vegetation in South Africa. Journal of Applied Ecology 43, 748e758. Haworth, K., McPherson, G.R., 1994. Effects of Quercus-emoryi on herbaceous vegetation in a semiarid savanna. Vegetatio 112, 153e159. Henke, S.E., Demarais, S., Pfister, J.A., 1988. Digestive capacity and diets of whitetailed deer and exotic ruminants. Journal of Wildlife Management 52, 595e598. Hoekstra, J.M., Boucher, T.M., Ricketts, T.H., Roberts, C., 2005. Confronting a biome crisis: global disparities of habitat loss and protection. Ecology Letters 8, 23e29. Joubert, D.F., Smit, G.N., Hoffman, M.T., 2012. The role of fire in preventing transitions from a grass dominated state to a bush thickened state in arid savannas. Journal of Arid Environments 87, 1e7. Knuckey, C.G., Van Etten, E.J.B., Doherty, T.S., 2016. Effects of long-term fire exclusion and frequent fire on plant community composition: a case study from semi-arid shrublands. Austral Ecology 41, 964e975. R Core Team [computer program]. 2017. R: a language and environment for statistical computing. Vienna, Austria. Ratajczak, Z., Briggs, J.M., Goodin, D.G., Luo, L., Mohler, R.L., Nippert, J.B., Obermeyer, B., 2016. Assessing the potential for transitions from tallgrass prairie to woodlands: are we operating beyond critical fire thresholds? Rangeland Ecology & Management 69, 280e287. Stambaugh, M.C., Sparks, J.C., Abadir, E.R., 2014. Historical pyrogeography of Texas, USA. Fire Ecology 10, 72e89. Sydes, C., Grime, J.P., 1981. Effects of tree leaf litter on herbaceous vegetation in deciduous woodland. 1. Field investigations. Journal of Ecology 69, 237e248. Taylor, C.A., Twidwell, D., Garza, N.E., Rosser, C., Hoffman, J.K., Brooks, T.D., 2012. Long-term effects of fire, livestock herbivory removal, and weather variability in Texas semiarid savanna. Rangeland Ecology & Management 65, 21e30. Twidwell, D., Rogers, W.E., Fuhlendorf, S.D., Wonkka, C.L., Engle, D.M., Weir, J.R., Kreuter, U.P., Taylor, C.A., 2013. The rising Great Plains fire campaign: citizens' response to woody plant encroachment. Frontiers in Ecology and the Environment 11, E64eE71. Weir, J.R., Scasta, J.D., 2017. Vegetation responses to season of fire in tallgrass prairie: a 13-year case study. Fire Ecology 13, 137e142.
Please cite this article as: Starns, H.D et al., Effects of Fire Exclusion on Previously Fire-Managed Semiarid Savanna Ecosystem, Rangeland Ecology & Management, https://doi.org/10.1016/j.rama.2019.09.006
Contents lists available at ScienceDirect
Rangeland Ecology & Management journal homepage: http://www.elsevier.com/locate/rama
Effects of Fire Exclusion on Previously Fire-Managed Semiarid Savanna Ecosystem Heath D. Starns*, Charles A. Taylor 1, Nick E. Garza, Douglas R. Tolleson Texas A&M AgriLife Research, Sonora Research Station, Sonora, TX 76950, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 June 2019 Received in revised form 13 September 2019 Accepted 20 September 2019
Long-term (> 100 yr) fire exclusion is associated with numerous ecological consequences in grasslands and savannas, including transitions into shrub- or tree-dominated systems. Several studies have reported differences in woody vegetation after multiple fires among burned and unburned rangelands, but none have reported the impacts of fire exclusion after a period of fire management. We evaluated effects of fire exclusion on herbaceous and woody canopy cover and herbaceous biomass in semiarid savanna of southwest Texas in pastures with known burn histories. Pastures were burned in summer and winter in 1994, 2000, and 2006, followed by 11 yr of fire exclusion. Between 2006 and 2017, woody subcanopy increased (5e21%) in all treatments while overstory canopy remained unchanged. Herbaceous cover decreased (5e18%) in all treatments but remained higher in burned treatments. From 2006 to 2017, herbaceous biomass declined in all treatments by > 650 kg$ha1 and was not statistically different among treatments. These trends support other research demonstrating the importance of historical mean fire return interval in maintaining grasslands and savannas. © 2019 The Society for Range Management. Published by Elsevier Inc. All rights reserved.
Key Words: Edwards Plateau herbaceous biomass long-term effects rangeland Texas woody encroachment
Introduction Fire is an important driver in the evolutionary history of grasslands and savannas, and frequency and seasonality of fires historically interacted with climate and herbivory to influence vegetation structure and composition in both biomes (Axelrod 1985; Bond and Keeley 2005; Beerling and Osborne 2006; Govender et al. 2006). Both biomes have declined in area as a result of human activity and have been identified as having high risk of biodiversity loss (Hoekstra et al. 2005). Fire exclusion has contributed to conversion of millions of hectares of these biomes to shrublands and woodlands (Bragg and Hulbert 1976; Fuhlendorf and Smeins 1997; Briggs et al. 2005; Archer et al. 2017). In recent decades, prescribed fire has increased in acceptance as a method for maintaining grasslands and savannas and reversing woody plant encroachment (Twidwell et al. 2013). Fire regime (e.g., seasonality, frequency, and intensity) plays an important role in determining the ecological response of these landscapes to fire (Engle and Bidwell 2001; Taylor et al. 2012; Weir and Scasta 2017;
* Correspondence: Heath D. Starns, Texas A&M AgriLife Research, Sonora Research Station, Sonora, TX 76950, USA. E-mail address: [email protected] (H.D. Starns). 1 Retired from Texas A&M AgriLife Research, Sonora Research Station, Sonora, TX 76950, USA.
Dickson et al. 2019). Season of fire affects vegetation structure and composition, with summer burns exhibiting more negative impacts on woody plants in temperate grasslands than burns during the dormant season (Taylor et al. 2012; Weir and Scasta 2017). The effects of fire frequency (mean fire return interval [MFRI]) have been studied extensively in grasslands, and evidence suggests woody species can reach a height at which they escape the influence of fire if MFRI exceeds a specific threshold (Briggs et al. 2005; Taylor et al. 2012; Knuckey et al. 2016; Ratajczak et al. 2016; Aslan et al. 2018). Although several experiments have reported cumulative effects of multiple fires, none have followed fire reintroduction with a subsequent period of fire exclusion. Taylor et al. (2012) reported on vegetative communities resulting from multiple fires during summer and winter in an ungrazed Texas semiarid savanna. Our objective was to evaluate changes in these plant communities following 11 subsequent yr without fire. Methods Our research was conducted on the Texas A&M AgriLife Research Station (lat 31 N, long 100 W; hereafter SRS) 56 km south of Sonora, Texas. Precipitation is bimodal and highly variable (range 156 mme1 054 mm, median 557 mm per SRS records), mostly occurring in spring and autumn. The SRS features gently rolling
https://doi.org/10.1016/j.rama.2019.09.006 1550-7424/© 2019 The Society for Range Management. Published by Elsevier Inc. All rights reserved.
Please cite this article as: Starns, H.D et al., Effects of Fire Exclusion on Previously Fire-Managed Semiarid Savanna Ecosystem, Rangeland Ecology & Management, https://doi.org/10.1016/j.rama.2019.09.006
H.D. Starns et al. / Rangeland Ecology & Management xxx (xxxx) xxx
2
topography (3e4% slopes) representative of the Edwards Plateau ecological region of Texas, and soils are generally shallow (15e30 cm deep). Vegetation is primarily savanna with clusters of woody species interspersed within a matrix of midgrasses and shortgrasses. A 12-ha study pasture was fenced in 1994 and equally divided into six 2-ha treatment units. Before 1994, the site had been unburned since 1916 and grazed by sheep at light and heavy stocking rates (SRS records, unpublished). Since 1994, all livestock were excluded but wildlife were present. In 2006, three unique vegetative communities existed as a result of seasonal differences in fire effects (Taylor et al. 2012). Summer burned units consisted of predominately live oak (Quercus virginiana) overstory and an understory woody plant community free of sacahuista (Nolina texana) and Ashe juniper (Juniperus asheii) (Taylor et al. 2012). Unburned units had more overstory canopy than burned treatments in 2006, which was dominated by a live oakAshe juniper mix (Taylor et al. 2012). Winter burned units had an overstory dominated by live oak with an understory of sacahuista and prickly pear (Opuntia spp.) and without Ashe juniper (Taylor et al. 2012). After 2006, fire was excluded from all treatments for a period of ~12 yr due to drought and lack of adequate fine fuel, resulting in a single “skipped” fire based on the estimated historic MFRI of ~6 yr (Frost 1998; Stambaugh et al. 2014). Before the winter fire season of 20172018, we resampled transects reported on by Taylor et al. (2012) to evaluate effects of the “skipped” fire cycle on the vegetative communities and aboveground herbaceous biomass. Vegetation Sampling Cover of woody overstory (> 1.5 m), woody subcanopy (< 1.5 m), and herbaceous canopy was sampled in each experimental unit along 10 permanent 50-m transects (n ¼ 6 units 10 transects ¼ 60). In NovemberDecember of 2017, cover of each species within each canopy layer was recorded using the line intercept method (Floyd and Anderson 1987). Species-level cover data were summed to quantify total cover of each canopy layer. Three quadrats (0.25 m2) were clipped along each transect and dried at 45 C to a stable weight to estimate herbaceous biomass. Because the prior study determined fire was not a primary driver of herbaceous community composition (Taylor et al. 2012), we did not evaluate frequencies of herbaceous species for the present study. Analysis To evaluate cover of the various canopy layers, we created linear models using year and burn season as fixed effects. Significant terms were further evaluated using pair-wise comparisons where appropriate. Herbaceous biomass was recorded periodically throughout the initial study, so we included those data for analysis of effects of burn season on herbaceous biomass. We created a linear mixed-effects model to evaluate herbaceous biomass using burn season and sampling year as fixed effects. Because herbaceous biomass was clipped during different months across years, we included sampling month as a random factor in that analysis. All analyses were performed using the R environment for statistical computing (R Core Team 2017). Results Year was a significant predictor of changes in total cover of herbaceous and woody subcanopy plants (P < 0.001), but no significant change in overstory canopy was observed (Fig. 1A; P ¼ 0.846). Subcanopy cover increased in all treatments from 2006 to 2017 (see Fig. 1B), with the greatest increase in unburned treatments (11.1732.34%). In contrast, herbaceous cover decreased in
Figure 1. Mean cover of A, woody overstory, B, woody subcanopy, and C, herbaceous canopy cover from 2006 to 2017, expressed as mean percentage of transect length. In all panels, solid lines represent control (unburned) treatments, long dashed lines represent winter burned treatments, and short dashed lines represent summer burned treatments. Shaded areas indicate 95% confidence interval.
all treatments (see Fig. 1C). The control treatment showed the largest decrease in herbaceous cover (95.9477.83%). Season of burn was a significant predictor of differences in overstory, understory, and herbaceous cover among treatments (Table 1). Total overstory canopy cover was lower in summer treatments than unburned and winter treatments (P < 0.001). Overstory did not differ statistically between unburned and winter treatments (P ¼ 0.066). Woody subcanopy cover followed a pattern similar to overstory in all treatments. Summer burned treatments had significantly less subcanopy than unburned (P < 0.01), but winter burned treatments were not statistically different than summer burned (P ¼ 0.065) and unburned (P ¼ 0.54) treatments (see Table 1). Herbaceous cover was lower in unburned treatments than in summer or winter burned treatments (P < 0.001), which did not differ (P ¼ 0.74). Year was also a significant predictor of changes in herbaceous biomass (P < 0.001). Herbaceous biomass was similar in all treatments before implementation of fire in 1994 (Fig. 2). While biomass did not differ between winter burned and unburned treatments from 1994 to 2017, biomass in summer burned treatments was greater than in other treatments in 2006 (P < 0.01). However, after 11 yr of fire exclusion, herbaceous biomass in the summer burned treatment was not different than winter burned and unburned treatments (P > 0.9). Further, herbaceous biomass in 2017 was significantly lower in all treatments than in 1994 or 2006 (P < 0.05).
Please cite this article as: Starns, H.D et al., Effects of Fire Exclusion on Previously Fire-Managed Semiarid Savanna Ecosystem, Rangeland Ecology & Management, https://doi.org/10.1016/j.rama.2019.09.006
H.D. Starns et al. / Rangeland Ecology & Management xxx (xxxx) xxx
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Table 1 Pair-wise comparisons of differences in means of herbaceous canopy cover, overstory (> 1.5 m) canopy cover, understory (< 1.5 m) canopy cover, and herbaceous biomass in a previously fire-managed ecosystem after 11 yr of fire exclusion. Bold text indicates statistically significant results. Canopy type
Comparison
Estimate
Lower bound
Upper bound
P¼
Overstory
Control winter Control summer Winter summer Control winter Control summer Winter summer Control winter Control summer Winter summer Control winter Control summer Winter summer
4.039 11.000 6.961 1.910 5.921 4.011 4.430 4.992 0.562 2.407 14.837 ¡12.429
8.286 15.246 2.714 6.108 10.119 0.187 2.647 3.209 2.345 1.106 11.337 ¡15.933
0.208 6.753 11.207 2.288 1.723 8.209 6.213 6.775 1.221 5.921 18.337 ¡8.926
0.066 < 0.001 < 0.001 0.535 < 0.01 0.065 < 0.001 < 0.001 0.741 0.243 < 0.001 < 0.001
Understory
Herbaceous
Herb.Biomass
Overstory of summer burned treatments remained dominated primarily by live oak. However, Pinchot’s juniper was able to reestablish in the overstory in 2017, whereas in 2006 it was only present in the subcanopy in summer treatments. In 2017 overstory of winter treatments remained primarily a mixture of live oak and Ashe juniper. Subcanopy species sacahuista and Ashe juniper were not present in summer treatments in 2006 but had returned by 2017. Similarly, several shrub species were noted in the subcanopy of winter treatments in 2017 that had been eliminated from detection by repeated fires. Discussion Our objective was to evaluate effects of one “skipped” prescribed fire on woody and herbaceous canopy in a fire-dependent ecosystem previously managed using fire. Our results indicate that woody subcanopy increases rapidly when fire is removed from this semiarid savanna. Legacy effects of previous fire treatments were still apparent via differences observed in canopy cover of woody understory and overstory plants, suggesting impacts of summer fires have greater longevity. Our results are in agreement with other studies of fire exclusion (Briggs et al. 2005; Joubert et al. 2012) and fire season (Briggs et al. 2005; Taylor et al. 2012; Weir and Scasta 2017).
The lack of change of overstory in all treatments between 2006 and 2017 probably stems from a combination of two factors. First, previous burns created significant differences in overstory canopy among treatments in 2006. In control treatments, overstory canopy was already high in 2006, leaving little opportunity for a significant increase. Second, our results suggest that subcanopy species in general did not have sufficient time between burns to transition into overstory in burned treatments. The increase in subcanopy cover is consistent with other reports that fire differentially impacts shrubs of smaller sizes (Freeman et al. 2017). The decrease in herbaceous cover in control treatments is unsurprising in light of the increase in total woody plant (overstory þ subcanopy) cover. Overstory canopy is known to reduce herbaceous biomass in temperate savannas (Haworth and McPherson 1994), and leaf litter is negatively correlated with herbaceous vegetation in woodlands (Sydes and Grime 1981). Although we did not directly evaluate leaf litter as a component of ground cover, it was noted as a large component in woody-dominated areas of control treatments. Furthermore, the increase in woody plant cover inevitably increased shading, which also suppresses grass growth beneath woody canopy in savannas (Charles-Dominique et al. 2018). The positive effect of summer fires on herbaceous biomass observed in our study is probably in part due to the lack of woody plants and associated shading (Charles-Dominique et al. 2018).
Figure 2. Mean herbaceous biomass (kg$ha1) of control (solid line/circles), summer burned (dotted line/triangles), and winter burned (dashed line/squares) treatments during the period of fire management (19942006) and a subsequent period of fire exclusion (20062017). Error bars represent one standard deviation from the mean.
Please cite this article as: Starns, H.D et al., Effects of Fire Exclusion on Previously Fire-Managed Semiarid Savanna Ecosystem, Rangeland Ecology & Management, https://doi.org/10.1016/j.rama.2019.09.006
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However, the rapid decline in herbaceous biomass following fire exclusion did not correspond with an equally rapid increase in woody cover, suggesting possible influence by factors outside the scope of our study. Although rainfall is known to have significant impacts on herbaceous biomass, it is unlikely to have contributed to the decrease in herbaceous biomass from 2006 to 2017 since 2017 had higher precipitation than 2006. It should be noted that the presence of axis deer (Axis axis), a non-native large ungulate, was first noted on SRS around 2008. Axis deer are primarily grazers (Henke et al. 1988) and were not excluded from the study area. Thus, the invasion of axis deer may have impacted herbaceous biomass in a manner similar to livestock grazing. Our results support findings of other studies that fire exclusion facilitates woody encroachment into savanna ecosystems (Beckage et al. 2011; Taylor et al. 2012). In this semiarid savanna, reintroduction of fire at the estimated 6-yr MFRI during summer had substantial positive effects on herbaceous biomass. In stark contrast, re-exclusion of fire for 11 yr was associated with a sharp decline in herbaceous biomass. Our results highlight the importance of maintaining fire according to the approximate MFRI and season for a given region. Acknowledgments The authors would like to thank Robert Moen, Dr. Erika Campbell, and numerous other technicians who assisted in data collection from 1994 through 2017, in addition to the associate editor and two anonymous reviewers. References Archer, S.R., Andersen, E.M., Predick, K.I., Schwinning, S., Steidl, R.J., Woods, S.R., 2017. Woody plant encroachment: causes and consequences. Rangeland Systems. Springer, New York, NY, USA, pp. 25e84. Aslan, C.E., Samberg, L., Dickson, B.G., Gray, M.E., 2018. Management thresholds stemming from altered fire dynamics in present-day arid and semi-arid environments. Journal of Environmental Management 227, 87e94. Axelrod, D.I., 1985. Rise of the grassland biome, central North-America. Botanical Review 51, 163e201. Beckage, B., Gross, L.J., Platt, W.J., 2011. Grass feedbacks on fire stabilize savannas. Ecological Modelling 222, 2227e2233. Beerling, D.J., Osborne, C.P., 2006. The origin of the savanna biome. Global Change Biology 12, 2023e2031. Bond, W.J., Keeley, J.E., 2005. Fire as a global 'herbivore': the ecology and evolution of flammable ecosystems. Trends in Ecology & Evolution 20, 387e394. Bragg, T.B., Hulbert, L.C., 1976. Woody plant invasion of unburned Kansas bluestem prairie. Journal of Range Management 29, 19e24.
Briggs, J.M., Knapp, A.K., Blair, J.M., Heisler, J.L., Hoch, G.A., Lett, M.S., McCarron, J.K., 2005. An ecosystem in transition. Causes and consequences of the conversion of mesic grassland to shrubland. BioScience 55, 243e254. Charles-Dominique, T., Midgley, G.F., Tomlinson, K.W., Bond, W.J., 2018. Steal the light: shade vs fire adapted vegetation in forest-savanna mosaics. New Phytologist 218, 1419e1429. Dickson, T.L., Hayes, B.A., Bragg, T.B., 2019. Effects of 34 years of experimentally manipulated burn seasons and frequencies on prairie plant composition. Rangeland Ecology & Management 72, 82e91. Engle, D.M., Bidwell, T.G., 2001. The response of central North American prairies to seasonal fire. Journal of Range Management 54, 2e10. Floyd, D.A., Anderson, J.E., 1987. A comparison of 3 methods for estimating plant cover. Journal of Ecology 75, 221e228. Freeman, M.E., Vesk, P.A., Murphy, B.P., Cook, G.D., Richards, A.E., Williams, R.J., 2017. Defining the fire trap: extension of the persistence equilibrium model in mesic savannas. Australian Ecology 42, 890e899. Frost, C.C., 1998. Presettlement fire frequency regimes of the United States: a first approximation. Fire in ecosystem management: shifting the paradigm from suppression to prescription. Tall Timbers Fire Ecology Conference Proceedings 70e81. Fuhlendorf, S.D., Smeins, F.E., 1997. Long-term vegetation dynamics mediated by herbivores, weather and fire in a Juniperus-Quercus savanna. Journal of Vegetation Science 8, 819e828. Govender, N., Trollope, W.S.W., Van Wilgen, B.W., 2006. The effect of fire season, fire frequency, rainfall and management on fire intensity in savanna vegetation in South Africa. Journal of Applied Ecology 43, 748e758. Haworth, K., McPherson, G.R., 1994. Effects of Quercus-emoryi on herbaceous vegetation in a semiarid savanna. Vegetatio 112, 153e159. Henke, S.E., Demarais, S., Pfister, J.A., 1988. Digestive capacity and diets of whitetailed deer and exotic ruminants. Journal of Wildlife Management 52, 595e598. Hoekstra, J.M., Boucher, T.M., Ricketts, T.H., Roberts, C., 2005. Confronting a biome crisis: global disparities of habitat loss and protection. Ecology Letters 8, 23e29. Joubert, D.F., Smit, G.N., Hoffman, M.T., 2012. The role of fire in preventing transitions from a grass dominated state to a bush thickened state in arid savannas. Journal of Arid Environments 87, 1e7. Knuckey, C.G., Van Etten, E.J.B., Doherty, T.S., 2016. Effects of long-term fire exclusion and frequent fire on plant community composition: a case study from semi-arid shrublands. Austral Ecology 41, 964e975. R Core Team [computer program]. 2017. R: a language and environment for statistical computing. Vienna, Austria. Ratajczak, Z., Briggs, J.M., Goodin, D.G., Luo, L., Mohler, R.L., Nippert, J.B., Obermeyer, B., 2016. Assessing the potential for transitions from tallgrass prairie to woodlands: are we operating beyond critical fire thresholds? Rangeland Ecology & Management 69, 280e287. Stambaugh, M.C., Sparks, J.C., Abadir, E.R., 2014. Historical pyrogeography of Texas, USA. Fire Ecology 10, 72e89. Sydes, C., Grime, J.P., 1981. Effects of tree leaf litter on herbaceous vegetation in deciduous woodland. 1. Field investigations. Journal of Ecology 69, 237e248. Taylor, C.A., Twidwell, D., Garza, N.E., Rosser, C., Hoffman, J.K., Brooks, T.D., 2012. Long-term effects of fire, livestock herbivory removal, and weather variability in Texas semiarid savanna. Rangeland Ecology & Management 65, 21e30. Twidwell, D., Rogers, W.E., Fuhlendorf, S.D., Wonkka, C.L., Engle, D.M., Weir, J.R., Kreuter, U.P., Taylor, C.A., 2013. The rising Great Plains fire campaign: citizens' response to woody plant encroachment. Frontiers in Ecology and the Environment 11, E64eE71. Weir, J.R., Scasta, J.D., 2017. Vegetation responses to season of fire in tallgrass prairie: a 13-year case study. Fire Ecology 13, 137e142.
Please cite this article as: Starns, H.D et al., Effects of Fire Exclusion on Previously Fire-Managed Semiarid Savanna Ecosystem, Rangeland Ecology & Management, https://doi.org/10.1016/j.rama.2019.09.006