3.1. Forest Composition and Structure
On state forest lands, the greatest area of forest cover is black spruce-white spruce-broadleaf forest, and white spruce-broadleaf forest (36% and 30% of timberland, respectively;
Table 2,
Table 3,
Table 4 and
Table 5). Extensive areas of mixed black spruce forest occur on cold soils underlain by permafrost. Permafrost dominated sites, because of their low forest productivity, are generally not harvested [
32,
33]. However, wood biomass harvesting, which harvests trees for energy generation, is expanding. Alaska is unique in that trees are harvested exclusively for energy generation, unlike other regions where harvest residues are used for such purposes. Wood biomass harvesting could potentially utilize black spruce material, which conventionally is too small to be profitable, and harvesting of this type may expand in the future. In contrast, white spruce is the most productive stand type in central Interior Alaska (
Table 2,
Table 3,
Table 4 and
Table 5), except for balsam poplar, which covers a small area in floodplains [
34]. Pure white spruce (20%) and mixed white spruce and broadleaf types (37%) contain the greatest wood volume on state forest lands (57%;
Table 2,
Table 3,
Table 4 and
Table 5). As a result, during the period of analysis, most timber harvesting occurred of white spruce types.
On other forest lands (only including inventory of Toghotthele lands; see
Section 2.2), pure or mixed white spruce forest covers the greatest area (69%), and broadleaf forest covers about one fourth of the land. This is because the land was selected mainly for productive forest lands for wood production. Wood volume on other forest lands is mostly composed of pure white spruce (36%), mixed white spruce and broadleaf (45%), and broadleaf types (18%).
The landscape of the Alaska boreal forest is shaped mainly by wildfire disturbances. Between 1943 and 2017, 42% of total lands in Interior Alaska burned at least once [
30]. This figure does not exclude non-forest lands, indicating the proportion of area burned within the boreal forest is higher than 42%. More than half of those areas within the fire perimeter burned twice or more in the 75-year period. Average fire return interval is estimated between 50–200 years [
35,
36]. As a result, forest stands rarely reach 200 years (
Figure 2 and
Figure 3). About 60% of timberland on state forest lands is reproduction (due to any type of disturbance, including timber harvesting;
Table 2,
Table 3,
Table 4 and
Table 5) and 20% of timberland stands are under 10 years old (
Figure 3).
3.2. Types of Forest Harvest Management Used in the Study Area
Clearcutting and selection cutting (18% and 39% of total area harvested, respectively) were two major harvesting methods used in the study area. Clearcutting is supposed to remove all stems, large or small, in the stand; however, in the study area, even if the harvesting method is prescribed as clearcutting, loggers may leave undesirable stems, such as small diameter trees and broadleaf stems. In the study area, selection cutting was in general a high grading method that removed the best trees. In the study area, large white spruce was predominantly harvested for the selection cutting (approximately 70%–80% of selection cutting). Salvage logging (13% of total area harvested) was another major forest management practice applied in the study area, and was used mainly after the Rosie Creek Fire in 1983 that burned 2677 ha of state forest lands.
Assisted regeneration following a harvest is a common forest management practice in many boreal regions. In the study region, the two most common post-harvest regeneration practices were site preparation and planting of seedlings. Depth of organic layer is one of the most important factors determining post-disturbance natural regeneration in Interior Alaska [
37], and removal of the organic layer (which naturally happens when fire burns the organic layer) promotes establishment of new vegetation. Site preparation exposes a mineral substrate that many species require for successful germination, while reducing the remaining vegetation that competes with tree regeneration [
37,
38]. Species competing with white spruce, especially
Calamagrostis canadensis, spread rapidly by below-ground rhizomes after disturbance [
39]. White spruce, on the other hand, regenerates predominantly from seed, and grows slower than most other early successional tree species [
40]. As a result, removing the organic layer and below-ground rhizomes of competitive species helps enhance white spruce regeneration. In addition, white spruce regeneration is limited by other factors [
40], including sporadic seed production [
41,
42,
43] and dispersal ability [
44]. As a result, site preparation and planting white spruce seedlings, when applied on state forest lands, are often used in combination. Despite these limitations in regeneration, post-harvest regeneration management has been applied at a very limited scale, mainly due to limited funding. In Alaska, artificial reforestation is required when natural regeneration is not adequate to meet the state stocking standard (450 stems per acre), and the AKDOF is responsible for the regeneration survey and, when necessary, reforestation. The AKDOF is also responsible for the wildfire management of 62 million hectares of lands, and most forest management resources are used for wildfire management (about 95% of funding) [
45]. As a result, forest harvest management needs to be economically self-sustained. Timber harvest revenue is marginal, especially when demand is low, and forest resource management in general is expensive, mainly due to the large area and limited access. A recent study, however, found that natural regeneration is adequate after white spruce harvesting without reforestation [
46,
47]. It is also important to point out that natural regeneration may take longer to regenerate forest stands than artificial reforestation, but depending on the management goal, natural regeneration may be more suitable than artificial reforestation (e.g., when the goal is to create wildlife habitat).
The predominant method for site preparation in Interior Alaska has been mechanical site preparation using heavy equipment. Prescribed burning and the application of herbicide have been limited to experimental purposes during the study period in central Interior Alaska [
48], unlike many other forest regions [
49,
50]. Planting of white spruce seedlings has been the predominant method for artificial reforestation. Some introduced species were planted experimentally at a very limited scale, including lodgepole pine (
Pinus contorta var.
latifolia Engelm. Dougl. ex Loud), Siberian larch (
Larix sibirica Ledeb.), and Scotch pine (
Pinus sylvestris L.) in the Fairbanks area. Out of the total 3223 ha planted in the area, 74 ha (2.3%) were planted with introduced species exclusively, and 162 ha (5%) were planted with mixed white spruce and introduced species.
On state forest lands, site preparation and artificial reforestation increased in the mid-1980s and 1990s, when total harvest area increased particularly because of the large burned forest area harvested after the Rosie Creek Fire and the associated salvage and sanitation logging [
51]. Almost half the area that received any post-harvest regeneration practice was salvage logged, and about one third of the total harvested area was clearcut, which was used mostly during the period of greatest harvest activity. Site preparation used only 18% of the harvested area (
Table 6). Artificial/assisted reforestation has been used on about half of harvested area (
Table 6).
Site preparation on other forest lands was only used before 1980, with a total of 31% of the harvested area (
Table 6). Only 13% of the area harvested on other forest lands was planted with white spruce seedlings (
Table 6). Unlike state forest lands, site preparation on other forest lands was used alone (without planting) in the majority of cases.
3.3. Transition of Forest Harvest Management
Harvested area and volume to date in Interior Alaska boreal forest since the late 1960s are small, particularly considering the vast total area and large total timber volume of the forest (
Table 1,
Table 2,
Table 3,
Table 4 and
Table 5). The total area harvested on state forest lands from the start of record collection in 1972 to 2012 is about 9435 ha out of 871,263 ha of total timberland on state forest lands (
Table 6), or 1.1%. Harvest activity on state forest lands was continuous from the early 1970s, with great variability among years (
Figure 4).
On state forest lands, annual area and volume harvested were quite low from 1972 until the early 1980s. In the mid-1980s, harvest area and volume gradually increased until the early 1990s in response to salvage and sanitation harvesting following a large fire in 1983 that burned 3500 ha of state forest lands [
51]. The priority after this fire was to salvage trees killed or damaged, and to recoup valuable timber before decay and prevent the spread of insect outbreaks from injured to healthy trees [
51]. A total of approximately 1200 ha was salvage logged due to the fire. Across all state forest lands, the total white spruce fuelwood volume harvested in the last 40 years is only 78,050 m
3, and nearly half of that volume came from salvage logging in the late-1980s and 1990s.
In the 1990s, as salvage logging from the 1983 fire was being completed, clearcutting increased rapidly in response to increased demand for spruce sawlogs in the Asian market (
Figure 4a) [
14]. In many boreal regions, homogenous forest (e.g., even-aged and/or single-species) created by extensive clearcutting and planting is subject to management efforts, such as intensive planting, to restore heterogeneous forest structures and a diversity of species habitats [
8]. Partial cutting is one of the management practices used to restore forest diversity [
9]. In the late 1990s, such concerns, along with a decreasing demand due to the downturn of wood product demand in the Asian market, resulted in a shift in the major harvesting method from clearcutting to partial cutting on state forest lands (
Figure 4a). Overall harvest area and volume also decreased and remained low for the rest of the study period.
On other forest lands, harvest activity has occurred sporadically, with a few peaks over the last few decades (
Figure 4b) when timber harvesting has been economically viable. During the period of analysis, harvest activities on other forest lands were lower than on state forest lands (
Table 6). However, the volume harvested on other forest lands contains errors, because the volume was only recorded in board feet, which measures the volume of processed boards and not the volume of the entire log. The greatest area of harvesting occurred on Toghotthele lands (
Table 6). Harvested area on other forest lands peaked in 1969, 1976, and 1979, but harvest volumes were not recorded for 1976 and 1981.
3.4. Perspectives on Sustainable Forest Management
A comparison of harvest intensities to forest growth provides a perspective on the relative degree or magnitude of utilization of wood volume. Throughout the period of our analysis, the mean annual volume harvested of white spruce and especially birch and aspen were lower than the annual volume growth of white spruce sawlogs, birch, and aspen types, respectively, on the entire timberlands on state forest lands (
Table 7). Harvest volume of all white spruce, birch, and aspen were four to five times greater in the Fairbanks area than other areas, but even there harvested volumes were 64%, 5%, and 1% of annual volume growth for white spruce, birch, and aspen, respectively.
White spruce sawlogs were the major harvested product category, accounting for about 90% of harvested volume on state forest lands. A total of 961,967 m
3 white spruce sawlogs were harvested from state forest lands during the 1972–2012 period, which is 37% of the total volume growth of white spruce sawlog types estimated for the time period. The overall mean volume of annual white spruce harvested is 23,463 m
3, with great variability among years (
Figure 4a). Although the overall mean annual harvested volume of white spruce sawlogs was lower than the annual growth (
Table 7), the harvested volume of white spruce sawlogs exceeded the annual volume growth in some years, especially during the 1990s. In the Fairbanks management area, annual harvested volume surpassed the mean annual growth rate in 8 out of 41 years, which is much greater than other management areas. The number of years exceeding the mean annual volume growth are 1, 1, and 2, for Kantishna, Delta, and Tok management areas, respectively.
Birch and aspen are minor harvested species compared to white spruce in the study area. A total of 93,804 m
3 birch and 8607 m
3 of aspen were harvested from state forest lands during the study period. However, birch harvest volume has increased in the most recent years (
Figure 4a), reflecting increased interest in wood biomass energy [
20]. Wood biomass is renewable energy that can mitigate climate change as long as the net carbon emissions of the wood energy harvest (i.e., carbon emission less carbon sequestered during regeneration) are less than those of energy generated by fossil fuels and the biomass harvest does not cause a reduction in long-term forest productivity. Wood biomass can also stimulate local economies, especially in rural Alaska, by decreasing dependence on imported fuel and creating local employment. Based strictly on the perspective of the relationship of volume harvested versus growth across the analysis area as a whole, the current low level of birch and aspen harvesting suggests that birch and aspen harvested for biomass energy can be significantly expanded in our analysis area.
It is important to note that the harvest activities we describe in this study area mostly produced the first rotation under management. The second-growth generated after those harvests have not yet reached a mature stage or even the age classes projected for short rotations. Although further monitoring is essential, this early investigation is useful to determine whether management to date is within the intensity that can sustain both ecological and economical values of the boreal forest.
This initial compilation of harvest volume, which we report here, is underestimated because we were only able to compile records on volume harvested for about 90% of the total area harvested. Records of volume harvested are missing for many units in the Delta (26% of the total area harvested) and Tok (72% of the total area harvested) areas. The Tok management area has overall lower standing volumes per area than the other areas (48 m3·ha−1 versus 68–79 m3·ha−1, respectively). We did not estimate the missing harvest volume data because the variability and lack of information on harvesting methods and harvested volume by species would introduce high variability into the estimate.
3.5. Accessibility
Access is one of the major constraints of forest harvest management in Interior Alaska [
14]. Even though vast forest land has remained unharvested, most areas are not road accessible. On state forest lands, 1010 out of 1128 (or 90%) of harvest units were within 1 km of a road, and 99% of harvest units were within 4 km of a road. This is not a surprising result, as access roads are essential for timber harvesting. An exception is ice road (frozen river) access during winter, which was not included as a “road” in our analysis. However, only 15% of the area on the state forest lands was located within 1 km of any type of road, including winter roads (not ice roads), and over 60% of the area was located more than 4 km from existing an road network. This apparently caused a concentration of harvest activities on the small road-accessible portion of state forest lands. Building roads is expensive and accessible forest areas will likely remain limited for the foreseeable future. As a result, sustainability of the accessible forest areas needs to be assessed.
Harvest intensity relative to the growth (annual volume harvested divided by annual volume growth) was highest in the most accessible forest areas (within 1 km of a road), except in the Delta management area, in which the highest concentration occurred in the areas within 2 km of a road (
Table 8). Even though the intensity was highest in the areas near a road, only in the Kantishna management area did the annual volume harvested exceeded the annual volume growth in those areas (
Table 8). Harvest intensity was higher in the Fairbanks and Kantishna management areas (
Table 8), partly due to more complete records in those areas than Delta and Tok. However, during some years, the volume harvested exceeded the annual volume growth in all of the management areas (
Table 8). The numbers of years that exceeded the annual growth were greater in the areas nearer a road, and were much greater in the Fairbanks management area than other management areas (
Table 8).
The volume growth used in this study was underestimated, because we did not include any pole timber types, which contain a lot of sawlogs and are likely to be harvestable. Obviously, it is essential to have better growth estimates for more accurate and precise planning of sustainable forest harvest management.