The community structure ofupland forests in interior Alaska results from secondary succession following fires (Viereck et al, 1983; Van Cleve et al, 1996; Chapin et al, 2006). Ecosystem processes in these upland areas are strongly influenced by the aspect or orientation (which regulates the amount of solar insolation reaching a site), elevation (average temperature decreases with increasing elevation) and slope position (Slaughter and Viereck, 1986). Because of the role of topography, north facing slopes as well as sites at higher elevations (above 650 m) and sites located in
Black Spruce forest mineral soil temperature (d = 5 cm)
0 30 60 90 120 1 50 180 210 240 270 300 330 360
Day of year
Figure 13.1 Seasonal soil temperature profiles collected from mature black spruce stands at an elevation of 650 m
Source: data courtesy of E. Kane, University of Alaska.
the bottoms of stream valleys are most susceptible to the formation of permafrost. The overall influence of permafrost on mineral soil temperature is illustrated in Figure 13.1. On the north-slope site, the mineral soil remains frozen for much of the year, thawing for only 120 days and reaching a maximum temperature of only 2°C. While the south-slope site only has an average temperature 2°C warmer, during the middle of the growing season, its maximum temperature is over 7°C.
For 10 to 60 years after the fire, deciduous forests (aspen and birch) dominate warmer and drier sites at lower elevations without permafrost; these are eventually succeeded by mature stands of white spruce 100 to 150 years after the fire. On cool, wet sites (often underlain by permafrost), deciduous tree species are unable to invade following fires, resulting in an initial domination (for 5 to 30 years after the fire) by herbs and shrubs that were present prior to the fire and are able to reproduce vegetatively. Black spruce seedlings that are established 1 to 10 years following a fire eventually grow into mature forest stands after 50 to 80 years.
Black spruce forests cover 50 to 60 per cent of the forested land of interior Alaska and are the most common forests that burn (see Figures 13.2a—c). Black spruce trees have relatively thin bark that does not provide protection from the heat t t
Note: (a) Black spruce-lichen forest with a 10—12 cm deep organic layer; (b) Black spruce-feathermoss forest with a 20—25 cm deep organic layer; (c) Black spruce-sphagnum moss forest with a 36—40 cm deep organic layer; and (d) Typical surface organic layer that sits on top of mineral soil in a black spruce-feathermoss forest. The top layer consists of live moss, which is underlain by the upper duff layer (light brown layer consisting of dead moss and fibric soil sublayers) and the lower duff layer (dark brown, consisting of mesic and humic soil sub-layers). Source: Photographs by E. Kasischke.
generated during fires (Johnson, 1992). Even surface fires with low to moderate intensity are hot enough to induce high rates of mortality in black spruce stands. As a result, most fires in these forests are stand-replacing events.
A key characteristic of black spruce stands in the North American boreal forest region is a deep layer of dead organic matter (see Figure 13.2d). The organic layer develops as a stand recovers after a fire because of the low rates of decomposition that occur on sites with cold and wet conditions (O'Neill et al, 2006). In mature stands, the organic layer depth in mature Alaskan black spruce forests averages 25 cm in depth (range of 7 to >40 cm) (see Figure 13.3, Plate 19). These surface organic layers represent a significant reservoir for terrestrial carbon, containing an average of 63 tC ha-1. During fires, substantial levels of the surface organic layers in black spruce forests are consumed, primarily through smoldering combustion.
The amount of organic material remaining after a fire is highly variable (see Figure 13.3) and depends on the moisture level of the ground-layer organic
Note: The burned and unburned forest stands where these data were collected were 70 to 300 years old.
Source: Plot based on unpublished data from E. Kasischke and M. Turetsky (Michigan State University).
material. While the moisture content of the upper portion of the surface organic layer is controlled by climatic variables (precipitation, air temperature, wind and solar insolation), the moisture content of the deeper layers just above mineral soil is regulated by site drainage characteristics (mineral soil texture, slope and seasonal thawing depth where permafrost is present). On average, the depths of the surface organic layers are reduced by 64 per cent during fires, and 54 per cent of the carbon is consumed. Because depth of soil thawing increases over the growing season, sites with permafrost are better drained during late-season fires. As a result, depth of burning is 30 per cent greater during late-season (after 1 August) fires compared to early-season fires, resulting in a 40 per cent greater amount of carbon consumed (see Figure 13.3)
When considering the surface organic layers in black spruce forests, two measures are needed to assess fire severity. The first measure is the depth of the organic material remaining after a fire. The second measure is the depth of the organic layer prior to the fire. Together, this information can be used to assess the degree to which a site has been altered by the fire (fire severity). In turn, fire severity affects ecosystem processes in three ways. First, it regulates key pathways for exchange of water and energy between the land surface and atmosphere. Second, it affects vegetative reproduction. And third, it alters key site characteristics that regulate seed germination and seedling growth.
The deep surface organic layer in black spruce forests facilitates the formation of permafrost (Yoshikawa et al, 2002). The moisture and water present in the organic layer decrease thermal conductance and reduce the amount of downward energy transfer during the warm summer months. While the organic layer also serves as an insulating blanket during the winter, it is less effective when the water present in this layer freezes and increases thermal conductance. As a result, there is a net energy loss in surfaces covered by organic layers, with the degree of cooling being proportional to the depth of the surface organic layer.
The region where the soil layer thaws by the end of the growing season in sites with permafrost is called the active layer, because it is within this soil volume where above freezing temperatures allow biological processes (such as decomposition, water and nutrient uptake by roots) to occur. In sites with permafrost, soil temperature and the depth of the active layer are inversely proportional to the depth of the surface organic layer. In black spruce-lichen forests with shallow organic layers (see Figure 13.2a), the active layer is 200-300 cm deep. In contrast, the active layer in black spruce-sphagnum moss forests (see Figure 13.2c) may only be 25-35 cm deep.
By reducing the depth of the organic layer, fires result in a net warming of the ground layer for the first two decades following a fire. Within 5 to 10 years following a fire, in black spruce-feathermoss forests (see Figure 13.2b) the depth of the active layer can increase by 100-150 cm in sites underlain by permafrost.
The effects of fire on soil moisture are more complex. In killing the overstory trees as well as consuming most of the understory vegetation, fires cause a net increase in soil water because they eliminate losses from evapotranspiration. Decreases in surface albedo result in a warming of the surface and increase in evaporation. In areas underlain by permafrost, the depth of seasonal thawing increases slowly for the first several years after a fire. The net result is that soils in burned areas are as wet or even wetter than in unburned areas for the first several years after a fire, and gradually become drier than unburned stands five to ten years following the fire as the increase in depth of seasonal thawing results in better drained soils (Kasischke and Johnstone, 2005). In sites without permafrost, the warming of the ground following fires may be accompanied by an immediate drying of soils even with the elimination of evapotranspiration.
Vegetative reproduction is a common strategy for plant species in fire-prone ecosystems because it allows for rapid re-establishment following disturbance (Zasada et al, 1983). Herbs and shrubs in black spruce forests reproduce following fire through sprouting from roots and stumps. This strategy is successful in sites with shallow to moderately deep burning of the organic layers, but fails when deep-burning fire consumes the underground vegetative material.
Figure 13.4 presents examples of variations in the patterns of vegetation recovery that have been observed in black spruce forests in interior Alaska that were collected at sites that burned during a 1994 fire (see Johnstone and Kasischke, 2005; Johnstone and Chapin, 2006). The depth of the organic layers in these black spruce-feathermoss forests (see Figure 13.2c) sites was 25 cm prior to the burn (see Figure 13.2d). The data from these plots illustrate how patterns of regrowth vary as a function of fire severity. In the site in Figure 13.4a, only the upper part of the organic layer was consumed by the fire, and vegetative reproduction at this site resulted in the re-establishment of herbs and shrubs that were present prior to the
Note: These photographs were taken in the summer of 2003 in areas that were adjacent to the unburned stand presented in Figure 13.2b. (a) Site with 15—20 cm of organic matter remaining after the fire; (b) Site with 4—6 cm of organic matter remaining after the fire; (c) Site with <1 cm of organic matter remaining after the fire. Source: Photographs by E. Kasischke.
fire. At this time, recruitment of black spruce was 4 seedlings m-2, a density that was 370 per cent of the stand density at the time of the fire. While aspen seedlings were present, their growth was low, with total aspen biomass being 9 g m-2.
The site in Figure 13.4b experienced a more severe fire than that in Figure 13.4a, with only 4-6 cm of organic matter remaining. The vegetation at this site was dominated by seedlings, with little or no vegetative reproduction. Aspen seedlings were dense in this stand (4 stems m-2), but their growth was modest (53 g m-2), with most stems being less than 1 m in height. Spruce seedlings were present in lower levels than in the lightly-burned stand, 1.7 stems m-2, which was 150 per cent of pre-burn stand density. Finally, the stand in Figure 13.4c experienced a severe burn, with less than 1 cm of organic matter left. The vegetation reproduction at this site was only from seedlings. While aspen density was low (2.2 stems m-2), the growth was vigorous (323 g m-2), with many stems being 2-3 m in height. Black spruce seedling density was low (0.5 stems m-2), being only 50 per cent of pre-burn stand density. In summary, while the lightly burned site in Figure 13.4a is following a regrowth trajectory that is common in black spruce forests, the severely burned site in Figure 13.4c is following a different trajectory. This site will most likely develop into a mature, dense aspen stand with a very low-density understory of black spruce. Permafrost will eventually result in poorly drained soils in the lightly burned site as mosses regrow, the depth of the organic layer increases, and the ground layer cools. In the severely burned site, however, the ground layer will continue to warm until permafrost disappears.
In addition to depth of burning, site microclimate and moisture conditions are important for post-fire tree recruitment. Figure 13.5 presents photographs collected in 2006 of sites that burned during a June 1999 fire. The stand in Figure 13.5a differed from the examples in Figure 13.4 because it contained a mature black spruce-lichen forest prior to the fire (see Figure 13.2a) and had a much shallower organic layer and a deeper active layer. The area burned in the 1999 fire is located in a region that experiences frequent periods of high wind from an adjacent mountain pass, which causes much higher rates of surface evaporation and transpiration. As a result of deeper active layers and high winds, the post fire soil moisture in the sites in this region were lower than the sites in the 1994 burn (Kasischke et al, 2007), and resulted in conditions where seeds could not germinate and seedlings experienced higher rates of mortality. Because of these dry conditions, there was very little recruitment of aspen in the severely burned site (see Figure 13.5a) (0.06 stems m-2 compared to 2.2 stems m-2 in the site that was severely burned in 1994). The recruitment of black spruce seedlings in the site with the deeper post-fire organic layer (see Figure 13.5b) was 0.7 stems m-2, which was only 15 per cent of the pre-burn stand density (compared to 4.4 stems m-2 and 300 per cent of pre-burn stand density in the lightly burned stand from the 1994 fire; see Figure 13.4a). This study illustrates that microclimate and soil moisture conditions at a site are also important in determining post-fire regeneration.
Note: These photographs were taken in the summer of 2006: (a) Black spruce-lichen stand with <1 cm of organic matter remaining after the fire (site adjacent to the unburned stand of Figure 13.2a, which had 10 cm of organic matter); (b) Black spruce-feathermoss stand with 10—12 cm of organic matter remaining after the fire (site had a 16 cm deep organic layer prior to the fire).
Source: Photographs by E. Kasischke.
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