Evaluating Post-Fire Vegetation Recovery in North American Mixed Prairie Using Remote Sensing Approaches ()
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Various biotic and abiotic factors can modify the composition and structure of grasslands . Of all those factors, the impact of fire is the most controversial due to its complex nature. There exists a large body of literature studying fire’s effect on grassland ecosystems considering fire severity, fire frequency, and fire season (for example,     ). Though general conclusions can be reached regarding fire’s impact, there are conflicting results within literature possibly due to different localities of grasslands ecosystems as well as limited resources available to most existing investigations.
Fire has been a significant ecological process on this planet for a very long time, and its impact on terrestrial ecosystems is well documented . Some examples of its impact being: 1) the shaping of the global biome distribution, 2) maintaining the structure and function of fire-prone communities, 3) acting as an evolutionary force, 4) being employed as one of the first tools by humans to re-shape the world. Fire has a significant impact on flora and fauna in the grassland ecosystem, in both direct and indirect ways. Direct effects include mortality of individuals, which are short-term. Indirect effects, such as species composition and changes in habitat, are long-term. Indirect impacts are not as easy to observe and evaluate but are usually more important than short-term effects given the scale of the impact on the ecosystem. Fire, coupling with other numerous factors, i.e. topography, soil, fauna (insects, herbivores), together with herbaceous plants  , can restrict the encroachment of woody plants (trees, shrubs) and release nutrients bound up in organic matter, accelerating the rate of decomposition in the soil, so as to maintain the establishment and the stability of grasslands   . Descriptive studies   show that fire occurrences decrease herbaceous production for one to three years. Meanwhile, the herbaceous response is influenced strongly by precipitation. Fire also reduces woody plant cover and promotes herbaceous dicotyledons. Plant species in semi-arid grasslands are more likely to be subject to fire season and frequency rather than fire behaviour .
The reestablishment of periodic fires is fundamental to the ecological restoration of grasslands on the Great Plains . The significance of the reestablishment of fire regimes has been recognized as fundamental to the ecological restoration of grasslands . However, current understanding of fire’s effect on the grassland ecosystem still needs to be improved. Based on historical materials about the Canadian prairies, Rannie  reviewed the history of wildfires and identified a “grass fire era” from the late 18th century to the late 19th century with much enhanced anthropogenic grassland fires. Moreover, Wakimoto et al.  did similar work but with more effort in determining and reconstructing historical fire regimes and fire behaviours in the northern mixed-prairie through reviewing and summarizing available resources such as 1) diaries and reports of travellers, 2) photographic records, 3) oral histories of First Nations, 4) related research literature. According to their investigation, evidence of past fire regimes mostly relies on historical accounts of early travellers. However, these written and oral accounts vary in detail and quality, and records are quite limited. General conclusions about historical fire frequency can also be reached based on rates of fuel accumulation and woody plant invasion , as well as charcoal remains from lake sediment cores . In regions of grasslands with trees, tree fire scars can also be used to study historical fire regime. Furthermore, tree-ring studies help establish drought cycles and their duration, which in turn provide information about historical climate, fuel loading, and potential fires. However, this remains a challenge if woody plants are scarce; because grasses and forbs do not carry fire scars and growth ring patterns .
There are two major fire types: wildfire and prescribed fire. Wildfires provide first-hand data for grasslands fire studies, such as . By compiling the wildfires which occurred in the region (most of them in late summer), Kruger  investigated fire’s immediate and long-term effect on forage species as well as other range plant species on the northern mixed prairie. A typical methodology can be found in  . By surveying the vegetation and soil properties at burned sites and its adjacent unburned sites, the hypothesis of no differences across sites can be tested with statistical analysis (e.g. t-test). Fire’s long-term impact can also be analyzed with field surveys of longer periods or intervals, e.g. <5 years, >5 years, 10 years, 15 years  . However, such wildfire studies have obvious limitations. Besides expensive sampling effort, no systemic evaluation of fire regime can be conducted, and it is impossible to know when and where wildfires will occur . A large body of literature is dedicated to controlled burning, or prescribed fires, including academic as well as applied researches on various types of grasslands on the Great Plains (e.g.   -  ). Moreover, detailed long-term ecological researches have been designed and implemented, such as the famous Long-Term Ecological Research program (or LTER, more details available from    ), to understand the fundamental mechanisms of fire’s effect on grasslands ecosystems. A good review of the ecological effects of prescribed fires can be obtained from  with their central region covering the northern mixed-prairie. Prescribed fires are effective in identifying the basic principles of fire through extrapolation, synthetization and generalization . Based on the established understanding of ecological processes and mechanisms, ecological models can be developed to simulate vegetation dynamics and quantify various fire regimes, such as the LANDFIRE model from the US Department of Agriculture  to study fire regimes of different ecosystems including part of the mixed prairie. Though prescribed fire experiments and fire modelling have significantly improved our understanding of fire’s impact, both are resource-consuming and not readily portable to different locales.
Various approaches have been provided to evaluate fire effects in grassland. The USDA conducted a comprehensive review  on the ecological effects of fire, aiming to provide well-rounded information for range managers to effectively use prescribed fire as a management tool. Part of their investigation covers the Canadian prairie in GNP. In general, they found that post-fire response of organisms depends on the complex interactions between a myriad of factors, including time of prescribed burning, historical fire regime, phenological stage of the organisms, fire severity of different burn seasons, climatic variation within or across burn seasons. Research from the Great Plains (such as  ) indicates that fire generally decreases shrub coverage and increases the cover of graminoid species as well as the percentage of live vegetation. In contrast, lack of fires from active suppression causes direct woody species encroachment, such as big sagebrush, ponderosa pine and Douglas fir . Ford and Johnson  reported that in general, grass cover recovered quickly from the fire treatment, and the long-term effect of fire was neutral. Burning during the dormant-season had little effect on grass cover when the burning site was revisited and sampled after as little as two months from the time of the prescribed fire. Meanwhile, burning during the growing season seems to negatively impact grass cover for up to two years after the fire. Studies    on the effects of the seasonality of fire (spring fire) on buffalograss and blue grama indicated mixed results, over a time frame of three months to 16 years. Often, early-spring burns (March) produce neutral or positive responses; and late-spring burns (May) produce negative results. Whereas fall burns led to more yield than did spring burns. Negative, neutral and positive responses to fire were evident in both season-long grazed areas , and areas protected from domestic livestock grazing . Shortgrass prairie ecosystem recovers relatively quickly from fire disturbance. Vegetation cover, arthropod, and mammal species abundance treated with dormant-season fire recovered in approximately two months and showed no significant difference from untreated communities. By studying vegetation response (grass cover) to different timings of fires (dormant season versus growing season), Ford and Johnson  concluded that in the short-term, burning during the growing-season appears to reduce fire severity but exerts greater impact on grass communities (opposite for soil crusts) compared to burning during the dormant season. Dormant-season fire in the shortgrass steppe is less damaging to grass communities (opposite for soil crusts) than growing-season fire. Wakimoto et al.  used vegetation similarity values to quantitatively measure the similarity between vegetation cover types between burned and unburned sites. For grassland and shrub-land, vegetation similarity values are consistently and significantly different across treatments, indicating the fact that burned communities cannot return to the unburned status even after 10 years’ plant succession. Grassland sites however, do not show a significant difference either short-term or long-term, i.e. 1 - 2, 3 - 5, 6 - 10, >10 years. Wakimoto et al.  also found that fire affected the vegetation structure of 62% of surveyed sites. Such structure change tended to happen on sites with shrubs. Shrubs are more susceptible to fire mortality, with some shrub species being especially sensitive to fire. They found that shrubs were killed entirely at some shrubland sites, converting the vegetation cover from pine-shrubland-grass to wheatgrass-needlegrass. No such change occurred at wheatgrass-grama-needlegrass sites.
Fire showed different effects on various major grassland species. Kruger  found that burned sites showed more cover of blue grama, sandberg bluegrass and green needlegrass one year post-fire. Such an increase became less obvious 2 - 5 years post-fire. After 6 - 15 years post-fire, these species showed mixed results; with some slightly higher and some slightly lower. Meanwhile, unburned grassland and shrubland had more undesirable species compared with burned sites. After 15 years post-fire, burned and unburned sites showed little difference when compared with each other. Comprehensive and detailed species’ response to fire can be accessed at  as well as the USDA’s Fire Effects Information System .
Obviously, as an important ecological factor, fire has been studied extensively in tallgrass, shortgrass as well as mix-grass prairie in the central and southern parts of the North American Great Plains    . For different grasslands, the influence of fire may contribute to different vegetation responses. For example, Oesterheld et al.  showed contrasting productivity responses for subhumid versus semiarid grasslands. Most research on grassland fires deals with particular local landscapes and ecosystems. We must be cautious when interpreting the results from different researchers on the impact of fire on the grassland ecosystem, because Ford and Johnson  confirmed that impact of fire varies for different types of plants according to their active growth season, with C4 plants least vulnerable to the dormant-season fire and most vulnerable to the growing season fire. And research findings from different localities can vary significantly due to differences in historical and current prescribed fire regimes . Guo et al.  found that aboveground dry biomass, plant moisture, and dominant species together with plant forms are different for cool seasons and warm seasons on tallgrass prairie. With unique flora and fauna composition, the Canadian northern mixed prairie can be significantly different from other grasslands. However, little is known about fire effects on the semiarid mixed-grass prairie in Canada . There is a lack of knowledge about the pre-settlement fire regime  and under-standing of fire effects on the dynamics of that ecosystem, especially the plant communities  .
All previous research on vegetation responses to fire in this region are based on the short-term investigation (less than a year, see   ). However, grassland communities have evolutionary adaptations, showing variation in population recovery dynamics from fire season, frequency and behaviour . As a result, fire regime should be studied in more consistently and reliably, at longer terms and finer temporal resolutions. Long-term field studies with climate disturbances suggested that short-term ecosystem responses are usually opposite to long-term responses . Vegetation recovery is critically important because it plays a significant role in maintaining the grassland ecosystem through its influence in runoff, soil moisture, spatial distribution of erosion-deposition, nutrients as well as other biophysical activities .
Quantifying long-term grassland recovery trajectory after fire is important for ecosystem sustainability. Remote sensing provides new ways to conduct grassland fire studies. The contribution of remote sensing in fire studies has been recognized since the 1970s  for forest fire monitoring and control efforts. Performance of various satellite sensors have been tested in fire studies including ERS-1, GOES, DMSP, AVHRR, Landsat (a review can be retrieved at , SPOT , and MODIS  ). LTER investigated the sensitivity of the Landsat product to distinguish burned from unburned grasslands with positive results . Remote sensing based approaches have obvious advantages compared to conventional field surveys by providing timely and cost-effective imagery at various scales. Related studies have confirmed the robust performance in the sensitivity of remote sensing data in capturing the spectral characteristics of wildfires to study their occurrence, size and severity   . Besides visible bands (sensitive to blackened, charred vegetation), other bands are also proved effective, including NIR  (sensitive to green vegetation) and SWIR bands (sensitive to moisture content)  . Furthermore, various vegetation indices and specifically burn indices have been designed for fire studies   . Time series of burn patches can be mapped with remote sensing approaches . Also Yang et al.  and Lu et al.  have demonstrated the feasibility of using remote sensing in evaluating grassland fires in the northern mixed prairie. However, a comprehensive examination of fire effects on the ecosystem is needed to further our understanding about ecosystem dynamics, especially vegetation responses to fire. This research intends to investigate the vegetation recovery trajectory of the C3 dominated northern mixed grassland from a spring fire event, focusing on a longer historical perspective with remote sensing approaches. Field data were collected before the burn and five growing seasons following the burn. Various spectrum bands, vegetation and fire indices developed for the Landsat product are tested for their capacity in distinguishing burned and unburned areas as well as studying the long-term vegetation recovery trajectory.
By investigating a wildfire that took place on April 27th, 2013 with the help of a time series of remote sensing data, this study tries to understand fire’s immediate and long-term effects on the northern mixed-prairie. Specifically, there are two major research objectives:
・ To evaluate fire’s effect on the northern mixed-prairie using field survey data;
・ To investigate the application of remote sensing approaches in grasslands post-fire recovery study.
o to verify the feasibility of remote sensing in the fire study using the hyperspectral measurement from the field survey;
o to test the performance of satellite remote sensing products together with the ground surveyed data derived from Objective 1;
o to identify the grassland post-fire recovery trajectory with the most suitable remote sensing satellite products.
2. Materials and Methods
2.1. Study Area
The study area is located in the west block of Grasslands National Park (GNP), on the border of Saskatchewan (CA) and Montana (US) (Figure 1). It is situated in the prairie ecozone and has been providing habitats for a rich diversity of flora and fauna that have evolved in a highly dynamic environment which includes grazing, prairie wildfires, soil erosion, drought, and flooding .
Grasslands are characterized by rapid growth and slow decomposition rates due to the chemical and physical composition of the plants . The decomposition of aboveground materials by microorganisms is limited in grassland and can be accelerated post-fire due to a higher temperature and more available nutrients . Fire plays a significant role in the mixed prairie. This makes fire the primary decomposition agent and important nutrient cycler of the grasslands ecosystem . A survey  indicated that naturally caused fires (lightning) are relatively common in the grassland here, with one year in six years having abundant fuel and suitable weather conditions to encourage fire occurrence. Through evolutionary history, grasslands ecosystems were maintained in conditions appropriate for their productivity and biodiversity in nature, by self-organizing all the biotic and abiotic components into equilibrium . However, most disturbances were removed since the human settlement in the 19th century. Wildfire was actively suppressed, resulting in near-total fire extinction and major shifts in ecosystem structure and function, with 80% of the native prairie lost forever . In light of this, GNP was founded in 1988 with its mission to preserve the still standing pristine mixed-grass prairie in North America  .
The area is semi-arid, with annual precipitation between 300 and 330 mm, and average temperature ranging from 28˚C in the summer to −22˚C in the winter. The dry air, strong sunshine and high winds result in evaporation up two times the moisture gained from precipitation, encouraging wildfire occurrence . Poorly distributed precipitation patterns and frequent drought are typical in the mixed-prairie. Wind prevails in all seasons, with velocities exceeding most other parts of the continent. Arctic air mass forms northerly winds that drive blizzards across this region in winter, whereas in summer winds are hot and dry, resulting in parched and dusty prairie . The dominant landscape of the west block in GNP is rolling uplands and river valley, with elevation varying from 770 m to 900 m above sea level. The nearly level to slightly rolling topography encourages dry winds to carry wildfires and spread it quickly, with few obstacles to slow down or stop the fire’s progress  . The park is dominated by grasses (family of Poaceae) and non-graminoid herbaceous plant called forbs, with few trees and some shrubs along the river valley (Figure 1). Both cool season species and warm season species are found on this prairie, with the former showing dominance . Cool season grasses start growing in the spring as soon as the temperature rises. They become mature and then are dormant in the summer
Figure 1. The study area is located at the West Block of Grasslands National Park, Canada. Background of thi
Conflicts of Interest
The authors declare no conflicts of interest regarding the publication of this paper.
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