Soil Heating in Fire (SheFire): A model and measurement method for estimating soil heating and effects during wildland fires.

Fire has transformative effects on soil biological, chemical, and physical properties in terrestrial ecosystems around the world. While methods for estimating fire characteristics and associated effects aboveground have progressed in recent decades, there remain major challenges in characterizing soil heating and associated effects belowground. Overcoming these challenges is crucial for understanding how fire influences soil carbon storage, biogeochemical cycling, and ecosystem recovery. In this paper, we present a novel framework for characterizing belowground heating and effects. The framework includes (1) an open-source model to estimate fire-driven soil heating, cooling, and the biotic effects of heating across depths and over time (Soil Heating in Fire model; SheFire) and (2) a simple field method for recording soil temperatures at multiple depths using self-contained temperature sensor and data loggers (i.e., iButtons), installed along a wooden stake inserted into the soil (i.e., an iStake). The iStake overcomes many logistical challenges associated with obtaining temperature profiles using thermocouples. Heating measurements provide inputs to the SheFire model, and modeled soil heating can then be used to derive ecosystem response functions, such as heating effects on microorganisms and tissues. To validate SheFire estimates, we conducted a burn table experiment using iStakes to record temperatures that were in turn used to fit the SheFire model. We then compared SheFire predicted temperatures against measured temperatures at other soil depths. To benchmark iStake measurements against those recorded by thermocouples, we co-located both types of sensors in the burn table experiment. We found that SheFire demonstrated skill in interpolating and extrapolating soil temperatures, with the largest errors occurring at the shallowest depths. We also found that iButton sensors are comparable to thermocouples for recording soil temperatures during fires. Finally, we present a case study using iStakes and SheFire to estimate in situ soil heating during a prescribed fire and demonstrate how observed heating regimes would influence seed and tree root vascular cambium survival at different soil depths. This measurement-modeling framework provides a cutting-edge approach for describing soil temperature regimes (i.e., soil heating) through a soil profile and predicting biological responses.

Prosopis glandulosa persistence is facilitated by differential protection of buds during low- and high-energy fires.

Rangelands worldwide have experienced significant shifts from grass-dominated to woody-plant dominated states over the past century. In North America, these shifts are largely driven by overgrazing and landscape-scale fire suppression. Such shifts reduce productivity for livestock, can have broad-scale impacts to biodiversity, and are often difficult to reverse. Restoring grass dominance often involves restoring fire as an ecological process. However, many resprouting woody plants persist following disturbance, including fire, by resprouting from protected buds, rendering fire ineffective for reducing resprouting woody plant density. Recent research has shown that extreme fire (high-energy fires during periods of water stress) may reduce resprouting capacity. This previous research did not examine whether high-energy fires alone would be sufficient to cause mortality. We created an experimental framework for assessing the "buds-protection-resources" hypothesis of resprouting persistence under different fire energies. In July-August 2018 we exposed 48 individuals of a dominant resprouting woody plant in the region, honey mesquite (Prosopis glandulosa), to two levels of fire energy (high and low) and root crown exposure (exposed vs unexposed) and evaluated resprouting capacity. We censused basal and epicormic resprouts for two years following treatment. Water stress was moderate for several months leading up to fires but low in subsequent years. Epicormic and basal buds were somewhat protected from low- and high-energy fire. However, epicormic buds were protected in very few mesquites subjected to high-energy fires. High-energy fires decreased survival, caused loss of apical dominance, and left residual dead stems, which may increase chances of mortality from future fires. Basal resprout numbers were reduced by high-energy fires, which may have additional implications for long-term mesquite survival. While the buds, protection, and resources components of resprouter persistence all played a role in resprouting, high-energy fire decreased mesquite survival and reduced resprouting. This suggests that high-energy fires affect persistence mechanisms to different extents than low-energy fires. In addition, high-energy fires during normal rainfall can have negative impacts on resprouting capacity; water stress is not a necessary precursor to honey mesquite mortality from high-energy fire.

Grass bud responses to fire in a semiarid savanna system.

Increasingly, land managers have attempted to use extreme prescribed fire as a method to address woody plant encroachment in savanna ecosystems. The effect that these fires have on herbaceous vegetation is poorly understood. We experimentally examined immediate (<24 hr) bud response of two dominant graminoids, a C3 caespitose grass, Nassella leucotricha, and a C4 stoloniferous grass, Hilaria belangeri, following fires of varying energy (J/m2) in a semiarid savanna in the Edwards Plateau ecoregion of Texas. Treatments included high- and low-energy fires determined by contrasting fuel loading and a no burn (control) treatment. Belowground axillary buds were counted and their activities classified to determine immediate effects of fire energy on bud activity, dormancy, and mortality. High-energy burns resulted in immediate mortality of N. leucotricha and H. belangeri buds (p < .05). Active buds decreased following high-energy and low-energy burns for both species (p < .05). In contrast, bud activity, dormancy, and mortality remained constant in the control. In the high-energy treatment, 100% (n = 24) of N. leucotricha individuals resprouted while only 25% (n = 24) of H. belangeri individuals resprouted (p < .0001) 3 weeks following treatment application. Bud depths differed between species and may account for this divergence, with average bud depths for N. leucotricha 1.3 cm deeper than H. belangeri (p < .0001). Synthesis and applications: Our results suggest that fire energy directly affects bud activity and mortality through soil heating for these two species. It is imperative to understand how fire energy impacts the bud banks of grasses to better predict grass response to increased use of extreme prescribed fire in land management.

Effects of pyric herbivory on prairie-chicken (Tympanuchus spp) habitat.

The reduction and simplification of grasslands has led to the decline of numerous species of grassland fauna, particularly grassland-obligate birds. Prairie-chickens (Tympanuchus spp.) are an example of obligate grassland birds that have declined throughout most of their distribution and are species of conservation concern. Pyric herbivory has been suggested as a land management strategy for enhancing prairie-chicken habitat and stabilizing declining population trends. We assessed differences in vegetation structure created by pyric herbivory compared to fire-only treatments to determine whether pyric herbivory increased habitat heterogeneity for prairie-chickens, spatially or temporally. Our study was performed at four sites in the southern Great Plains, all within the current or historic distribution of either lesser (T. pallidicinctus), greater (T. cupido), or Attwater's (T. cupido attwateri) prairie-chickens. Key vegetation characteristics of grass cover and vegetation height in pyric herbivory and fire-only treatments were within the recommended range of values for prairie-chickens during their distinct life history stages. However, patches managed via pyric herbivory provided approximately 5% more forb cover than fire-only treatments for almost 30 months post-fire. Additionally, pyric herbivory extended the length of time bare ground was present after fires. Pyric herbivory also reduced vegetation height and biomass, with mean vegetation height in pyric herbivory treatments lagging behind fire-only treatments by approximately 15 months. Canopy cover in fire-only treatments exceeded levels recommended for prairie-chicken young within 12 months post-fire. However, canopy cover in pyric herbivory treatments never exceeded the maximum recommended levels. Overall, it appears that pyric herbivory improves vegetation characteristics reported as critical to prairie-chicken reproduction. Based on our results, we suggest pyric herbivory as a viable management technique to promote prairie-chicken habitat in the southern Great Plains, while still accommodating livestock production.

Vegetation changes associated with a population irruption by Roosevelt elk.

Interactions between large herbivores and their food supply are central to the study of population dynamics. We assessed temporal and spatial patterns in meadow plant biomass over a 23-year period for meadow complexes that were spatially linked to three distinct populations of Roosevelt elk (Cervus elaphus roosevelti) in northwestern California. Our objectives were to determine whether the plant community exhibited a tolerant or resistant response when elk population growth became irruptive. Plant biomass for the three meadow complexes inhabited by the elk populations was measured using Normalized Difference Vegetation Index (NDVI), which was derived from Landsat 5 Thematic Mapper imagery. Elk populations exhibited different patterns of growth through the time series, whereby one population underwent a complete four-stage irruptive growth pattern while the other two did not. Temporal changes in NDVI for the meadow complex used by the irruptive population suggested a decline in forage biomass during the end of the dry season and a temporal decline in spatial variation of NDVI at the peak of plant biomass in May. Conversely, no such patterns were detected in the meadow complexes inhabited by the nonirruptive populations. Our findings suggest that the meadow complex used by the irruptive elk population may have undergone changes in plant community composition favoring plants that were resistant to elk grazing.