Assessing spatial and temporal variations in surface soil moisture in fire-disturbed black spruce forests in Interior Alaska using spaceborne synthetic aperture radar imagery — Implications for post-fire tree recruitment

Abstract

Recent studies [Bourgeau-Chavez, L.L., Kasischke, E.S., Riordan, K., Brunzell, S.M., Nolan, M., Hyer, E.J., Slawski, J.J., Medvecz, M., Walters, T., and Ames, S. (in press). Remote monitoring of spatial and temporal surface soil moisture in fire disturbed boreal forest ecosystems with ERS SAR imagery. Int. J. Rem. Sens.] demonstrated that ERS SAR imagery can be used to estimate surface soil moisture in recently burned black spruce forests in interior Alaska. We used this relationship to analyze the intra- and inter-annual variations surface soil moisture in two burned black spruce forests in Alaska. The results of this study showed distinct seasonal and longer-term trends in soil moisture in the two sites, with the site that burned in 1994 having higher soil moisture than the site that burned in 1999. The differences in soil moisture between the sites were related to landscape-scale variations in soil drainage and seasonal permafrost thawing. Finally, we found that the 1999 site had dramatically lower levels of tree recruitment (both aspen and black spruce) than the 1994 site as a result of the lower soil moisture levels. These results show that the ERS SAR and similar systems can be used to monitor a site characteristic that is important to understanding changes in the ecosystem community structure that result from variations in climate and the fire regime in the boreal region.

Introduction

The 2004 and 2005 Alaskan fire seasons represented this state's highest and third highest burned area since 1950 (2.71 and 1.78 × 10⁶ ha, respectively), with most of the fire-affected areas located in the interior boreal forest region. Alaska's burned area during the decade between 1996 and 2005 (7.58 × 10⁶ ha) represents 1/3 of the total burned area recorded since 1950. The burned area recorded over the past decade is equivalent to 17% of the boreal forest region in Interior Alaska, while that in 2004 and 2005 represents 10%. The increased fire activity in Alaska is consistent with the longer-term trends throughout the entire North American boreal forest region, where the average annual burned area has nearly tripled, from an average of 1.11 × 10⁶ ha during the 1960s to 3.04 × 10⁶ ha during the 2000s, most likely as the result of climate warming (Gillett et al., 2004, Kasischke and Turetsky, 2006).

The terrestrial ecosystems of the boreal region are a significant reservoir of carbon because of the development of deep organic layers in peatlands occupying poorly-drained sites and the presence of permafrost under many forests (Gorham, 1991, Harden et al., 2000, Kasischke and Stocks, 2000). In addition, variations in the consumption of surface organic layers during fires in permafrost forests directly affects the patterns of post-fire successions (Landhaeusser and Wein, 1993, Johnstone and Kasischke, 2005, Johnstone and Chapin, 2006) and post-fire soil respiration (Bergner et al., 2004). Monitoring and understanding the impacts of climate change and the recent increases in burned area on these ecosystems presents an important challenge to the scientific community (Kasischke et al., 1995a, Turetsky et al., 2002, Turetsky et al., 2004).

Because of the remote location of the boreal forest and the large extent of fires that are common to this region, satellite-remote sensing systems are becoming an increasingly important tool for monitoring this biome's land surface. Using coarse resolution (8 by 8 km) data from the AVHRR system, numerous scientists have noted an overall increase in vegetation greenness in the boreal region during the 1980s and 1990s (Myneni et al., 1997, Tucker et al., 2001, Zhou et al., 2001). While the observed increases in vegetation greenness are consistent with the warming that has occurred in this region (Lucht et al., 2002), other sources have been found, including earlier melting of snow (Dye & Tucker, 2003) and regeneration of vegetation following fires (Kasischke and French, 1997, Hicke et al., 2003). In addition, inter-annual variations in NDVI greenness have shown to be correlated with large-scale atmospheric circulation, such as the Arctic Oscillation (Vicente-Serrano et al., 2006). Throughout the Russian boreal forest, satellite observations provide the only means for reliable burned area information (Sukhinin et al., 2004). Satellite imagery have been also been demonstrated to be useful for mapping relative patterns of damage resulting from fires (Michalek et al., 2000, Isaev et al., 2002, Epting et al., 2005).

French et al. (1996) and Bourgeau-Chavez et al. (2007) showed that the backscatter measurements collected by spaceborne synthetic aperture radar (SAR) systems can be used to estimate soil moisture in recently burned forests, a capability that may prove to be important in understanding patterns of post-fire vegetation recovery in boreal forests. Here we present the results of a study with the goal of using ERS-1 and ERS-2 C-band (5.7 cm wavelength) SAR data to: (1) analyze the spatial and temporal variations patterns of soil moisture in recently disturbed black spruce forests; (2) relate these patterns to variations in climate and site characteristics; and (3) explore the relations between soil moisture patterns and post-fire tree recruitment.