4.1. Remote Sensing Indicators of Soil Properties
Of the remote sensing indices considered, we found relationships of ground-based data with elevation and slope from the DEM, with the first principal component of the HV polarization from the ALOS-PALSAR time series, and with multiple indices from Landsat.
We expected to find stronger relationships of soil moisture with the SAR data because microwave sensors measure the dielectric constant of soils, which is directly related to the soil water content [47
]. Promising relationships between post-fire soil moisture and SAR backscatter time series have been found elsewhere using the C-band radar signal [41
]. The greater wavelength of the L-band radar signal of ALOS-PALSAR allows it to more readily penetrate the vegetation canopy and enter deeper into the soil surface. However, in this study there was a several-year time lag between the SAR data acquisition and our field data collection, which may have weakened the strength of the associations. Considering the sensitivity of the radar signal to soil moisture, and the strong relationships between surface moisture and permafrost properties, the use of SAR data has been identified as a high research priority for permafrost mapping [48
Soil temperature, soil moisture, and plant community composition were related to numerous Landsat-derived indices, including dNBR, NDVI, NDII6, and NDII. dNBR has been used as an indicator of burn severity with inconsistent effectiveness in the boreal forest [49
]. We found a weak association of dNBR with organic layer thickness, but strong associations with post-fire soil temperature, moisture, and vegetation composition, site characteristics that are strongly impacted by burn severity. NDVI, a measure of vegetation phytomass or health, was negatively associated with soil temperature, positively associated with soil moisture, and negatively correlated with the primary axis of plant community variation. The inverse relationship between NDVI and dNBR, and the direction of the relationships with soil properties and plant community composition, together suggest that higher NDVI values were due to lower burn severity. Within the early successional environment of this study area, high NDVI values indicated a high proportion of unburned patches or resprouting vegetation typical of low severity burns [35
], though in other cases high NDVI values could indicate increases in colonizing vegetation in response to high severity burns. We expect the relationship between NDVI and soil temperature to evolve over time with plant succession. Indeed, an inverse relationship between NDVI and permafrost presence was found at a broad regional scale [28
], likely because warm soils tend to support productive vegetation types such as deciduous forests.
The infrared indices (NDII6 and NDII7), which incorporate both the NIR and SWIR bands, were the remote sensing metrics most highly correlated with all measured soil properties and plant communities. Vegetation reflects highly in the NIR region, and water (on the soil surface or within plant leaves) absorbs electromagnetic radiation in the SWIR region. High NIR reflectance and low SWIR reflectance were indicative of cold soils with high water content. NDII7 was the strongest indicator of surface soil moisture, soil temperature at depth, and organic layer thickness, and was therefore chosen for regression-based spatial modeling of soil properties and more detailed analyses. NDII was also found to be closely related to permafrost presence at the regional scale, presumably because of its sensitivity to water content of vegetation or soils [28
]. NDII is sensitive to leaf water content and is commonly considered a vegetation index [50
], although it is influenced by soils when not obscured by vegetation cover, as is the case after fire disturbance [52
]. Therefore, surface soil moisture, which strongly influences SWIR reflectance [53
], and vegetation properties are both likely to contribute to NDII in our post-fire study area.
We analyzed seasonal reflectance patterns over several years to determine the extent to which soil moisture versus vegetation properties influenced NDII. Sites with and without near-surface permafrost had the same seasonal patterns of NDII7, NIR, and SWIR reflectance, although reflectance values varied in magnitude. Our interpretation is that the sites with colder soils had greater NIR reflectance due to higher vegetation cover resulting from lower burn severity, and lower SWIR reflectance due to higher water content in plant leaves or the soil surface.
The seasonal surface soil moisture dynamics in this region are driven by spring snowmelt and the seasonal thawing of frozen ground. Surface moisture is typically highest at the time of spring snowmelt and gradually declines through the summer as thaw depth increases [54
]. If SWIR were sensitive predominantly to surface soil moisture, we would expect to see SWIR reflectance increase from springtime to fall. Instead, SWIR reflectance decreased from springtime through the summer, and slightly increased in fall. The wide variations in precipitation from 2013–2015, with 2014 a summer of a record-breaking high rainfall, would have influenced surface moisture, yet the seasonal reflectance patterns observed at our field sites were generally consistent each year [55
]. The seasonal patterns of NDII7 and NIR reflectance were, however, consistent with expected plant phenology. As vegetation grows and leaves expand from springtime through the summer we see increased NDII7 and NIR reflectance, and subsequent declines in fall as leaves senesce. Across our sites, it therefore appears that the relationship between NDII7 and soil moisture was predominantly an indirect relationship driven by variations in vegetation properties, which reflect burn severity and co-vary with soil properties.
Within severe burns, which have a greater proportion of soil exposure relative to plant cover, direct relationships between SWIR reflectance and surface soil moisture are more likely, however. Our time-series of surface soil moisture from a field monitoring site in a severe burn had a strong negative relationship with SWIR reflectance, suggesting a sensitivity of SWIR reflectance to surface soil moisture. We conclude that NDII7 performed well at predicting subsurface properties because it is an integrative index sensitive both to vegetation properties and surface moisture, both of which are correlated with soil moisture and temperature at depths beyond the reach of optical sensors.
4.2. Controls over Permafrost Distribution and Drainage Class
It has been well documented that the loss of the insulating surface organic layer of soil through wildfire can initiate rapid permafrost degradation [6
]. The level of degradation is directly related to the thickness of the surface organic layer remaining after disturbance, and thus to burn severity. As expected, an inverse relationship between surface organic layer thickness and soil temperature at 1 m depth was found, demonstrating the positive influence of burn severity over permafrost degradation at our study sites. The strong negative relationship we found between shallow soil moisture (upper 20 cm) and deep temperature (1 m) supports our conjecture that permafrost thawing after fire would result in increased drainage and subsequently drier surface soils in this rocky upland environment [18
], since the permafrost table, impermeable to water, strongly controls water table depth, hydrology, and soil moisture regimes [14
Topography also influenced soil properties, as elevation and slope were both positively correlated with soil temperature and negatively with soil moisture. The association of upper slopes with warmer and drier soils and flatter bottomlands with wetter and colder soils is consistent with our understanding of topographic controls over soil properties [57
]. Elevation influences microclimate primarily because of the pronounced wintertime temperature inversions, which cause mean annual air temperatures of lowlands to be significantly lower than adjacent hilltops [6
]. Because of the low solar angles at high latitudes, slope and aspect are especially important in mediating solar input, affecting air temperatures, evaporation, and snowmelt [58
]. Slope further influences the soil environment by controlling the mass flow of water. Mineral soil stratigraphy, soil texture, and loess thickness also tend to co-vary with topography due to differences in material redistribution, typically resulting in thicker deposits of fine-grained soils in lower positions [58
]. The accumulation of thick organic layers in cold and poorly drained landscape positions reinforces cold and wet soil conditions through the development of a shallow permafrost table and through resilience to deep burning and subsequent permafrost degradation [11
According to our mapping results, 37% of the study area lacked permafrost in the upper meter. Whereas permafrost-free areas occurred on hilltops and upper slopes, they also occurred in lower bottomland landscape positions, where permafrost conditions would be most expected. The inconsistency of topographic controls on permafrost presence suggests that other factors also strongly influence the spatial pattern of permafrost distribution after disturbance. The consistent relationships between burn severity class and mapped permafrost presence provide compelling evidence that burn severity was a key control over the spatial distribution of permafrost. Nearly 90% of the high-severity burn area lacked near-surface permafrost, whereas permafrost persisted across ~80% of the low-severity burn. Some of the deviations from the expected topographic control over permafrost distribution and burn severity could be the result of temporally dynamic factors that influence fire behavior, such as previous precipitation affecting fuel moisture, wind speed/direction, humidity, etc. [62
]; or to variation in vegetation and the properties of the surface organic layer, which influence the vulnerability to deep burning [63
Topography and burn severity both influence the post-fire distribution of permafrost, which in turn impacts soil moisture regimes and landscape hydrology. The prevalent water tracks in our study area are evidence of surface flow due to permafrost. The presence of near-surface permafrost restricts subsurface infiltration, limiting flow to the shallow active layer, and resulting in active layer soils that are wet to saturated [64
]. Permafrost-dominated watersheds are thus characterized by flashy near-surface flow, with high peaks of stream discharge responsive to precipitation, and low base flow [65
]. As permafrost thaws in a watershed, the water holding capacity of soils increases and flow paths deepen, which cause higher base flow and less responsiveness to precipitation [32
Burn severity is also tightly linked to vegetation, with the post-fire plant community reflecting the level of disturbance and the pre-fire vegetation [34
]. Our ordination analysis shows that most of the variation in plant communities occurred along the first axis, which we interpret as a gradient of burn severity due to its close relationships to surface organic layer thickness and dNBR, as well as to patterns of plant species cover. This axis was strongly associated with soil temperature at 1 m depth, and to a lesser extent with surface soil moisture. Low shrub communities were the dominant post-fire vegetation type and encompassed a wide range of permafrost and moisture conditions. Extensive cover of dwarf shrubs (e.g., Vaccinium vitis-idaea
), low shrubs (e.g., Rhododendron tomentosum
), sedge tussocks (Eriophorum vaginatum
spp., and Rubus chamaemorus
were indicative of low burn severity and colder, wetter soils. Over the broader interior Alaskan region, these particular species have been found to comprise the early successional vegetation of low-severity burns as remnants or resprouts from surviving vegetation, whereas colonizing species regenerating by seed dominate in high severity burns [34
]. Likewise, we found high cover of grasses and colonizing mosses (e.g., Ceratodon purpureus
) to be associated with high burn severity and warmer, drier soils. Elevation was positively correlated with the primary axis of plant communities, suggesting a topographic control over burn severity and species composition. Topographic impacts on soil moisture likely interacted with pre-fire vegetation to influence burn severity, creating a range of post-fire early successional communities. These communities were closely related to the infrared remote sensing indices. The strong associations between post-fire vegetation, soil properties, and remote sensing indices enabled us to indirectly map subsurface properties.