More frequent burning increases vulnerability of Alaskan boreal black spruce forests

This study was conducted in interior Alaska, which stretches from the Brooks Range in the north to the Alaska Range in the south and encompasses multiple topographic and permafrost gradients. Black spruce forests represent 45% of the land cover in the interior of Alaska and are the prevailing forest type in Alaskan (66% of all forests) (Turetsky et al 2011) and Canadain boreal forests (Amiro et al 2001). These forests occur primarily in areas with discontinuous permafrost (Ferrians 1998), which can greatly influence site drainage conditions and the organic soil layer thickness. However, some have suggested that climate-fire-vegetation interactions may lead to decreased black spruce forest cover and increased deciduous forest cover throughout this landscape (Kelly et al 2013), although the rate of change in response to recent changes in Alaska's fire regime is under debate (Barrett et al 2011, Mann et al 2012).

At the landscape scale, changes in fire frequency are analyzed as the FFI between fire events (Johnstone 2006, Johnstone and Chapin 2006). Field observations within black spruce stands which burned twice in the last 37–52 years (considered here to be intermediate-interval fire events) were investigated and compared with data previously collected in long-interval (with a FFI at least >70 years) fire events (Kane et al 2007, Kasischke et al 2008, Turetsky et al 2011). Measurements were collected in black spruce forest stands located in four intermediate-interval fire events throughout the interior of Alaska (figure 1) (the data for this study are available in Hoy et al 2016). In each of the intermediate-interval fire events, a fire first occurred in the 1950s or 1960s, with the sites burning again in the 2000s (table 1).

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For all intermediate-interval sites, data were collected in plots that were located in both burned stands and adjacent unburned stands of a similar age (figure 2). Unburned stands were located across multiple topographic positions and were dominated by live, intermediate-aged (∼37–52 years) black spruce trees (figure 2(b)). There were no live trees present in the intermediate-interval burned stands (figure 2(a)). One indicator used in plot selection was the presence of severely weathered and charred trunks of black spruce trees which had burned in the 1950s and 1960s and remained as either standing dead or fallen dead until the time of the second fire event. Plots were selected where the intermediate-interval burned stands had been dominated by black spruce trees both prior to the historic burn (in the 1950s and 1960s) and the more recent burn (in the 2000s).

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Individual plots were located within the four intermediate-interval fire events studied, consisting of an area at least 30 by 40 m on the same topographic position with similar fire severity. As site accessibility is an issue in Alaska, data collections were restricted to areas near highways and other roads accessible to field vehicles and hiking short distances (<2 km). All plots were located at least 100 m from paved roads to avoid local effects of the road on the natural environment and fire behavior. Following the design of previous studies (Kane et al 2007, Turetsky et al 2011), sample sites were located across different topographic positions (also referred to as landscape classes) representing a gradient of well-drained to poorly-drained areas including flat uplands (FU); south (S) facing backslopes; east and west facing backslopes (EW); north (N) facing backslopes; and flat lowlands (FL) (see table 2 for a list of sites by landscape class).

The organic layer sampling methods followed Kane et al (2007) and Kasischke et al (2008). Date of burn from the most recent fire event was established for each plot using MODIS hotspot data (Giglio et al 2006). In the intermediate-interval burned plots, the 21 soil depth measurements collected in each plot consisted of a measurement from the top of the remaining SOL following fire to the mineral soil layer, and a breakdown of soil type was noted (including the char, fibric, mesic and humic soil layers (Canadian Agricultural Services Coordinating Committee 1998)). The 21 adventitious root (AR) measurements were collected by measuring the distance from the top AR to the mineral soil using black spruce trees ≥2 m tall and believed to have burned during the most recent fire event (during the 2000s).

Unburned intermediate-aged black spruce stands adjacent to intermediate-interval burned sites, or located in black spruce stands of a similar age to the intermediate-interval fire events studied, were sampled in order to estimate pre-fire depth using ARs. Similar to the design used to make depth measurements in burned stands, SOL measurements were made in unburned stands from the top of the SOL to the mineral soil including a breakdown of soil type (moss, fibric, mesic and humic soil). As in the collection of AR data in burned stands, AR measurements were collected in the 7 unburned black spruce stands used in this study. These included the measurement of the top of the organic layer to the top AR, and from the top of the AR to the mineral soil. Using the relationship between the AR depth above the mineral soil and total organic layer depth in unburned stands, we were able to estimate pre-fire organic layer depths in burned stands following the approaches of Boby et al (2010) and Kasischke et al (2008).

SOC storage and losses were estimated using empirical relationships between organic horizon depth (cm) and SOC (kg m⁻²) developed by Turetsky et al (2011) from published soil data for Alaskan black spruce forests stratified by landscape (topographic) class. In burned stands, the estimated depth of burn derived from AR measurements less the char layer was used in the carbon loss analysis to account for ecosystem carbon retained in the soil as char or charcoal (Turetsky et al 2011).

Organic layer depths in intermediate-interval burned sites were analyzed using three measures of fire severity (Kasischke et al 2008): (1) depth of burn (or absolute depth reduction), the amount of organic matter which burned during the fire, (2) percent depth reduction (or relative depth reduction), the relative amount of organic matter removed during the fire when compared to pre-fire organic layer levels, and (3) the post-fire depth at the site following the intermediate-interval burn. The characteristics of the organic layer depths found in intermediate-interval burned sites were compared with those from the long-interval sites previously sampled by Turetsky et al (2011) using linear mixed effects models, which can account for any non-normality in the dataset as well as random variables (Pinheiro and Bates 2010). Fixed effects included in the models were FFI (Interval), landscape class (the five topographic positions described above), Julian date, and the interaction between interval and landscape class, while the random effect of fire identity was used to account for differences among fire events used in the analysis. Other fixed effects terms (such annual-area burned) which have been shown to be significant in past studies of long-interval fires in black spruce forests (Turetsky et al 2011) were not included in these models as there was a clear focus here to investigate potential factors with specific relevance to intermediate-interval burns. The influence of individual fire events on the models was investigated using the Cook's D and Covariance Ratios for both fixed effects and covariance parameters of the combined long- and intermediate-interval datasets. It was determined that the main results were robust enough to account for any variation due to individual fire events. The significance of the random effects was assessed through a Wald test.

Simple linear regression was used to analyze differences between pre- and post-fire organic layer depths, depth of burn and the date of burn in intermediate-interval burned stands and the relationship between AR depth to mineral soil and the total organic layer depth in unburned stands. All data are presented as means ± one standard error. The data from plots of the same landscape class located ∼2 km or less from one another were combined to form a single site for statistical analysis to account for potential spatial correlation between the plots.

An initial relationship was developed between the organic layer depth and the AR depth to mineral soil using only the sample points from the seven intermediate-aged unburned black spruce sites. Based on overlapping confidence intervals for the slope and intercept of each equation, this relationship did not vary significantly from the relationship published in Turetsky et al (2011) to relate AR data with total organic matter depth in mature black spruce stands (linear regression model from Turetsky et al (2011): F_1,317 = 3378, R² = 0.93, p = 0.0001, β₀ (y-intercept) = 4.56 ± 0.35, β₁ (slope) = 1.03 ± 0.02) (figure 3). As a result, this previously published relationship was used to estimate pre-fire depth in the intermediate-interval burned stands.

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Post-fire organic layer depths in intermediate-interval burned sites ranged from 0.4 ± 0.1 cm in a flat upland site to 10.1 ± 1.2 cm in a flat lowland site, with an average post-fire depth of 3.4 ± 0.5 cm. Fire had burned down to the mineral soil, leaving only a thin char layer behind, in 40.5% of all individual sample measurements across all plots. An average of 11.4 ± 0.5 cm of organic matter was consumed within the intermediate-interval burn sites (with a range of 7.4 ± 1.0–18.0 ± 1.2 cm), resulting in depth reduction of 78.9% ± 2.6% (table 2, figure 4). Pre-fire depth was found to explain 59.9% of the variation in post-fire depth across sites (F_1,27 = 40.39, R² = 0.5993, p < 0.0001, β₀ = −4.87 ± 1.3, β₁ = 0.54 ± 0.09; figure 5).

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Based on the spatial interpolation of date of burn observed from processing the MODIS hotspot data, 14 of the sites sampled for this study burned early in the fire season (between May and July), while 15 sites burned later in the fire season (after July 31st). Depth of burn, stratified by landscape class, did not vary greatly across early and late season intermediate-interval sites (table 2). The day in year that an intermediate-interval plot burned was not correlated with the variation in depth of burn (F_1,27 = 0.08, R² = 0.003, p = 0.78, β₀ = 12.94 ± 5.24, β₁ = −0.0073 ± 0.03; figure 6). However, an analysis of the relationship between depth of burn and Julian date for individual landscape classes (excluding N facing slopes as only one was sampled) showed that only depth of burn for EW slopes was significantly correlated (at p < 0.1) with Julian date (F_1,4 = 7.95, R² = 0.67, p = 0.048, β₀ = −4.55 ± 5.61, β₁ = 0.074 ± 0.027).

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Results of the linear mixed-effects model showed that depth of burn varied with interval and Julian date (p = 0.0608 and p = 0.0003, respectively), but did not vary with landscape class (p = 0.2773) (table 3). The models for the percent depth reduction and post-fire depth varied with interval length (p = 0.0042 and p = 0.0002, respectively), landscape class (p = 0.0019, p = <0.0001, respectively), and Julian Date (p = <0.0001 and p = 0.0018, respectively) (table 3). The organic layers of long-interval sites were ∼70% thicker at the time of burning than those found in the intermediate-interval sites. Post-burn, the organic layers were almost 65% thinner in intermediate-interval sites than in long-interval sites (table 2). Percent depth reduction was 16 percentage points more (∼25% greater) in intermediate-interval sites than in the long-interval sites used in this comparison (∼79% and ∼63% respectively, table 2). The interaction between interval and landscape class was significant in the depth of burn and post-fire depth model (p = 0.0348 and p = 0.0064, respectively), but was not significant for the percent depth reduction model (and p = 0.1289). Fire identity was a significant random effect in the depth of burn model (p = 0.0407), but not in the percent depth reduction and post-fire depth models (p = 0.1341 and p = 0.1619, respectively) (table 3).

Similar to trends in SOL depths, the post-fire SOL carbon stock was lower in intermediate-interval sites than in long-interval sites (table 4). In all landscape classes where we were able to sample both intermediate-interval and long-interval sites during the early and late fire seasons, post-fire SOL carbon stock was greater in the long-interval landscape class. There was also less variability in carbon stocks found across the intermediate-interval landscape classes than the long-interval landscape classes (table 4).

We used these data to estimate carbon losses associated with burning. Carbon losses in intermediate-interval stands ranged from 1.61 ± 0.33 kg C m⁻² in early-season EW slopes, to 3.03 kg C m⁻² in the early-season S slope and 2.99 ± 0.66 kg C m⁻² in the flat lowland late-season intermediate-interval sites sampled (table 4). Intermediate-interval early-season losses are slightly lower than those reported in Turetsky et al (2011) for the different landscape classes, however the intermediate-interval late-season values are considerably less than those previously reported (table 4).

An important question asked in this study is whether or not more frequent burning has increased the vulnerability of SOL in Alaskan black spruce forests. Increased SOL vulnerability would provide the conditions needed for a shift from the thick organic soil stable state found in black spruce forests to the thin organic soil stable state generally found in the deciduous stands of Alaskan boreal forests. We show that fire interval is an important control on the vulnerability of black spruce stands recovering from fire (all three mixed-effects models included interval as a significant fixed-effect term, table 3). Absolute depth reduction (depth of burn) is essential for understanding combustion rates and factors related to the fire itself (Turetsky et al 2011, Kasischke and Hoy 2012). Additionally, it is the relative depth reduction (percent depth reduction), and the post-fire depth, that are useful to discuss important post-fire impacts to the landscape, such as how fire will likely affect forest succession (Barrett et al 2011, Genet et al 2013) and the permafrost regime (Yoshikawa et al 2002, Yi et al 2009). The relative depth reduction is much greater in the intermediate-interval stands sampled, and both the post-fire depth and depth of burn are less, as compared to the long-interval sites previously examined (table 2). As mentioned above, 40.5% of all individual sample measurements across all intermediate-interval burned plots had burned down to the mineral soil. This lack of an insulating soil layer at the site is likely to result in the thick to thin organic layer domain shift discussed in Johnstone et al (2010a). In another study which assessed short-interval fires (14–15 year FFI), Brown and Johnstone (2011) found pre-fire depths in short-interval stands to range from 12.1 to 24.8 cm and post-fire depths similar to those found in the intermediate-interval stands studied here. The depth reduction reported by Brown and Johnstone in the short-interval plots they studied, at 75%, was similar to that reported here (78.9% ± 2.6%) across all intermediate-interval sites (table 2). Overall, these results indicate that an intermediate-interval fire could be an important mechanism for triggering a deciduous post-fire trajectory through creating a thin organic soil layer (Johnstone et al 2010a, Barrett et al 2011, Johnstone et al 2011).

While landscape position has been shown to be a significant factor in controlling depth of burn in long-interval burn stands (Kane et al 2007, Turetsky et al 2011), landscape class did not represent a significant control on depth of burn in the intermediate-interval sites assessed in this study (figure 4). All landscape classes burned relatively consistently across the intermediate-interval sites (table 2, figure 4). It is possible that many of the intermediate-interval sites sampled for this current study, while on different topographic positions, could have had similar drainage conditions due to properties intrinsic to stand development. While data on active layer thicknesses from regrowing, intermediate-interval black spruce stands are not available, Kasischke et al (2012) showed that in mature black spruce stands located across landscape position, active layer thickness increased as organic layer thickness decreased, while Kane et al (2007) demonstrated that organic soil moisture decreased as organic layer thickness decreased. These results indicate that the thinner organic layers found in the 37–52 year old, intermediate interval sites may not yet have had adequate time to redevelop the organic soil and permafrost layers common to mature black spruce forests. In sites with the same soil texture, thinner organic soil layers can lead to thicker active layers, which in turn result in drier conditions, even on traditionally poorly-drained, flat lowland sites. These drier conditions could result in similar levels of burning across topographic positions in intermediate-interval events, as evidenced by the frequent occurrence of fire burning down to the mineral soil in all the intermediate-interval sites studied. Additionally, while depth of burn has been found to vary seasonally in long-interval black spruce stands (Turetsky et al 2011), in the intermediate-interval sites studied here it was only in the EW slopes that were significantly correlated with Julian date (figure 6). This represents another possible indication that the intermediate-interval sites had similar drainage conditions throughout the fire season, leading to similar burn conditions whether or not a site was burned in early-season or late-season.

An almost equal number of sites burned during the early and late time periods of the fire season (14 early-season and 15 late-season sites). However we did not have replication in some combinations of topographic position by seasonality (i.e. there were no early-season fires on north facing slope sites and no late-season fires on south facing slope sites) and this could have affected our analysis. Turetsky et al (2011) found that depth of burn increased with time throughout the fire season in flat upland and all sloped sites, but not in flat lowland sites, which burned to the same degree throughout the growing season. In the intermediate-interval sites studied here, there was very little change in depth of burn throughout the growing season across the different landscape positions. These differences could be due to the difficulty in locating and accessing a wider range of sites in multiple landscape classes.

Post-fire organic soil carbon stocks averaged 0.51 ± 0.08 kg C m⁻² in the intermediate-interval sites studied here. These carbon stocks were much lower than have been previously reported. For example, C stocks of various FFI within boreal regions of Alaska and Canada have been shown to range from 2.07 to 5.74 kg C m⁻² (Kane et al 2007, Brown and Johnstone 2011, Turetsky et al 2011, Harden et al 2012). Carbon losses estimated in this study (table 4) are somewhat lower than those reported by Turetsky et al (2011), indicating that as less material was stored in intermediate-interval sites prior to a fire, less C was also consumed in the SOL during burning relative to long-interval sites. However, the results from this study clearly show that the more frequent burning of black spruce stands will result in further depletion of the large carbon reservoir that currently exists in black spruce forests.