Research Project Summary:
Forest floor and plant responses to experimental fires in an Alaskan black spruce/feather moss community
Unless otherwise indicated, the information in this Research Project Summary comes from the following papers:
Dyrness, C. T.; Norum, Rodney A. 1983. The effects of experimental fires on black spruce forest floors in interior Alaska. Canadian Journal of Forest Research. 13: 879-893.
Zasada, John C.; Norum, Rodney A.; Teutsch, Christian E.; Densmore, Roseann. 1987. Survival and growth of planted black spruce, alder, aspen and willow after fire on black spruce/feather moss sites in interior Alaska. The Forestry Chronicle. 63(2): 84-88.
Zasada, John C.; Norum, Rodney A.; Van Veldhuizen, Robert M.; Teutsch, Christian E. 1983. Artificial regeneration of trees and tall shrubs in experimentally burned upland black spruce/feather moss stands in Alaska. Canadian Journal of Forest Research. 13: 903-913.FIRE MANAGEMENT IMPLICATIONS:
Postfire vegetation growth varied with species and depth of the postfire forest floor. Across 3 years, total annual biomass production was greater on plots that burned at high severity than on plots that burned at low severity (58 g/m² vs. 37 g/m²). Species composition was markedly different on low-severity and high-severity plots. Sprouting plants, with their roots crowns or rhizomes in organic and/or mineral soil layers, dominated low-severity plots, while species with wind-blown seeds or spores dominated high-severity plots .
Artificial regeneration (seeds, cuttings, and seedlings) established best on high-severity plots. Mineral soil proved a better seedbed than feather mosses for 8 artificially sown tree and tall shrub species. For these woody species, no artificial regeneration survived into the 3rd postfire year. Hardwood cuttings were especially sensitive to depth of residual organic matter, only surviving on high-severity plots . Woody species on high-severity plots gained more height than those on plots that burned at lower severities (P<0.05 for survivorship and height variables) . Since this study was not replicated at other locations, the authors emphasized that the positive relationship of fire severity to height may be applicable to only these study plots, and they called for further studies on this relationship .
The authors concluded that overall, relatively severe burning apparently increased germination, early postfire establishment, and growth of tree and tall shrub seedlings on this site. Although organic substrates might favor germination, tree and tall shrub seedlings often fail to survive on organic substrates because seedlings can desiccate quickly in summer, resulting in seedling death. However, seedlings in organic soils may survive in summers with ample rain .
This study demonstrated that artificial regeneration of black spruce and hardwoods is feasible on burned black spruce/feather moss sites. Hardwood species generally grew faster than black spruce, and they will likely continue to increase site productivity and provide species diversity in this black spruce/feather moss community for years to come.
Viereck and others  conducted an earlier stud in the Washington Creek Fire Ecology Experimental Area on this black spruce/feather moss community. The study investigated the effects of prescribed fires of varying severities on the forest floor and on the short-term effect of fire on black spruce. See the Research Project Summary of their study for further information.
The study site was in the Washington Creek Fire Ecology Experimental Area, about 54 km north of Fairbanks, Alaska [8,9] (latitude 64.20 longitude -149.49 radius 10 km) .
The study site is classified in the following plant community and probably historically experienced this fire regime:
|Table 1. Fire regime information on the vegetation community studied in this Research Project Summary. Fire regime characteristics are based on the LANDFIRE Biophysical Settings data layer . This vegetation model was developed by local experts using literature and expert estimate as documented in the PDF file linked from the BioPhysical Settings Group name listed below.|
|Vegetation Community||Mean interval
|Fire severity*||Percent of fires|
|Western North American boreal black spruce wet-mesic slope woodland||76||>Replacement||46|
|Surface or low||0|
Replacement=Any fire that causes greater than 75% top removal of a vegetation-fuel type, resulting in general replacement of existing vegetation; may or may not cause a lethal effect on the plants.
Mixed=Any fire burning more than 5% of an area that does not qualify as a replacement, surface, or low-severity fire; includes mosaic and other fires that are intermediate in effects.
Surface or low=Any fire that causes less than 25% upper layer replacement and/or removal in a vegetation-fuel class but burns 5% or more of the area [1,4].
PREFIRE WEATHER AND FUELS:
|Table 2. Fire weather for 7 experimental fires in interior Alaskan black spruce/feather moss-lichen. Data are means .|
|Unit||Date of fire||Time||Temperature
|Wind direction||Rate of spread
Fuel moisture conditions were variable. Surface woody fuels were sparse. Downed woody fuels loads ranged from 4,484 to 22,364 kg/ha. About half of woody fuels were <3 inches (8 cm) in diameter .
|Table 3. Fuel conditions and tree densities for 7 experimental units in interior Alaskan black spruce/feather moss-lichen. Data are means .|
|Unit||Down, dead woody fuels loading (kg/ha)||Tree species||Density
|Upper moss||Lower moss||Upper duff||Lower duff|
|*Units 3 & 6 were burned at the same time, and data for their fuel moisture samples were pooled.|
|Table 4. Fire severity descriptions [7,9]|
|Unburned||plant parts green & unchanged|
|Scorched||moss & other plants yellow or brown but species identifiable|
|Low||lightly burned, plants charred but original form of mosses & twigs visible|
|Moderate||shallow ash layer present, organic layer partially consumed, parts of woody twigs remain|
|High||ash layer present, organic material in the soil consumed or nearly consumed, no discernible plant parts remain|
The objectives of these prescribed, experimental fires were to measure fire behavior, subsequent fire effects on the forest floor and vegetation , and postfire artificial and natural regeneration of trees and tall shrubs [8,9] on sites that are representative of upland black spruce/feather moss forests of interior Alaska .
Across burned units, the full range of fire severity classes shown in Table 4 was achieved. Almost half of the 13 ha either did not burn, was only scorched, or burned at low severity; the other half burned at moderate to high severity. On average, the fires consumed about half of the prefire forest floor. This mosaic of unburned patches and lightly to severely burned areas was "apparently typical of that encountered for black spruce-feather moss fires in interior Alaska", with patterns similar to those occurring in wildfires .
Fire behavior within the units ranged from slowly moving, mixed surface-and-ground fires to rapidly advancing crown fires. The forest floor (largely mosses and their decomposition products) primarily carried the fire; down, dead, woody fuels contributed little to fire behavior or subsequent fire effects. Surface fires determined the rate of spread. The crown fires were passive, with trees torching after surface fires had passed. Crowning flames reached 15 m or higher. Most of the units were completely covered by fire and burned down to glowing combustion within an hour of ignition. Fire severity ranged from unburned, scorched, or low-severity (most of Unit 1) to moderate- (most of Units 4, 5, and 7) and high-severity (most of Units 2, 3, and 6) (see Table 4). Units 2 and 3 burned "vigorously, with rapidly spreading surface fires and rapidly igniting crown fires". The fire could barely propagate on Unit 4, probably due to the high moisture content of surface fuels (see Table 3) .
|Table 5. Weather conditions and fire behavior during the experimental fires |
|Relative humidity (%)||42||33||30||54||42||33||36|
|Rate of spread (m/min)||0.4||1.1||1.2||0.3||4.0||no data||1.8|
|Flame length* (m)||0.6||0.8||1.1||0.2||0.7||0.9||0.7|
|*Of surface fires only.|
FIRE EFFECTS ON PLANT COMMUNITY AND FUELS:
The fire killed the entire overstory on some units and part of the overstory on others. Smoldering combustion of the feather moss and soil organic layers also occurred on some units .
Fire severity and consequent removal of the soil organic layer affected postfire responses of vegetation. After burning, forest floors comprised a small-scale mosaic of unburned, scorched, lightly burned, moderately burned, and heavily burned areas. Forest floor consumption was negatively correlated with the moisture content of lower moss and lower duff layers at the time of burning (P=0.05). From postfire years 1 to 3, total biomass production of vegetation was greatest on units that burned at high severity. In postfire year 3, biomass production averaged 110 g/m² and 175 g/m² on low- and high-severity units, respectively. In postfire year 3, units that burned at mostly low severity were dominated by sprouting species including pinegrass, bog blueberry, and bog Labrador tea. Severely burned units were colonized by species with wind-blown seeds or spores including fireweed, fire moss, and common liverwort. For severely burned plots, most aboveground biomass in postfire year 1 to 3 was mosses and liverworts, and almost none was shrubs or trees. Black spruce seedlings were mostly concentrated in high-severity units, but they were still small in postfire year 3 and had "inconsequential" biomass. Fireweed was mostly restricted to high-severity units in postfire years 1 and 2, reaching peak biomass in postfire year 2. It began colonizing less severely burned units in postfire year 3, but its total biomass was decreasing sharply by then. Neither fire moss nor common liverwort was apparent in the prefire vegetation. Fire moss biomass on high-severity units steadily increased during postfire years 1 to 3, while that of common liverwort remained steady, from 15 to 20 g/m², across 3 postfire years. Fire moss biomass increased from about 8 g/m² in postfire year 1 to 38 g/m² in postfire year 2 and almost 60 g/m² in postfire year 3. Pinegrass apparently began invading severely burned plots in postfire year 3.
Artificial regeneration sources were seeds, unrooted stem cuttings that were stored in a freezer prior to outplanting, and seedlings. In the fall after the prescribed fires and the subsequent spring and summer, plots on Units 1, 2, and 5 were hand-sown with local seed collected from 8 tree and tall shrub species native to the area: paper birch, black spruce, balsam poplar, quaking aspen, American green alder, Alaska willow, Bebb willow, and Scouler willow. Those units were selected for planting because they represented the full range of burning severities . Seeds were sown during the time of natural seed rain for each species: fall—about 2 months after the last fire—for American green alder, black spruce, and paper birch; and summer for balsam poplar, quaking aspen, and the willows. In addition to seed sowing, black spruce seedlings were outplanted. The seedlings were started in a nursery from local seed (3 months), then hardened off outside (3 months) prior to outplanting in September 1978 . Stem cuttings of Alaska willow and balsam popular were collected near the study sites and stored in a freezer until late spring planting. Natural regeneration of trees and tall shrubs was monitored on unplanted plots .
Rainfall at time of planting and during the 1st postfire growing season was below normal . Weather data for subsequent growing seasons were not provided.
Establishment and survivorship of trees and tall shrubs were highest on high-severity plots [8,9].
|Table 6. Mean cover (SD) and frequency of artificial tree and tall shrub regeneration by fire severity class, postfire year 3 |
|Burn severity class|
|Cover (%)||Frequency (%)||Cover (%)||Frequency (%)||Cover (%)||Frequency (%)|
|Scorched||23 (4)||70||6 (2)||45||27 (3)||98|
|Low severity||44 (6)||85||17 (4)||53||30 (4)||88|
|Moderate severity||10 (2)||70||13 (2)||78||11 (2)||85|
|High severity||15 (4)||40||53 (5)||85||25 (4)||75|
|Other (mostly dead trees)||8 (1)||85||11 (1)||90||7 (1)||70|
The units that burned at mostly moderate and high severity (Units 5 and 2, respectively) had highest establishment of both artificial and natural regeneration for all tree and tall shrub species. Among these species, only seeded Bebb willow occurred on low-severity plots, at a rate of 23 germinants, 5 first-year seedlings, and 0 third-year seedlings .
|Table 7. Total number of germinants and seedlings on moderate- and high-severity plots |
|Moderate severity||High severity||Moderate severity||High severity||Moderate severity||High severity|
|American green alder||13||160||0||83||0||65|
|willow (Salix spp.)||0||0||0||0||0||13|
Over half of artificially seeded germinants (all species) were apparent by 5 June 1979 (postfire year 1). Some black spruce seeds blew in from parent trees onto and established in artificially seeded plots. This natural black spruce regeneration established later than artificial black spruce regeneration, around 26 June 1979. Some summer-seeded willows germinated within 2 weeks of sowing; however, the earliest willow germinants had lowest 1st-year survivorship.
Survival of black spruce seedlings was significantly greater than that of hardwood species (P=0.05). Among hardwoods, no species showed consistently higher survivorship across units . First-year survivorship averaged 46% for American green alder, 60% for paper birch, and 80% for black spruce. Overall, 3rd-year survivorship was highest on high-severity plots. Third-year survivorship averaged 34% for American green alder and 43% for paper birch. Since black spruce establishment on seeded plots was bolstered by natural seed rain, 3rd-year survivorship of artificial regeneration could not be calculated .
By postfire year 6, height growth varied among species and units, but black spruce was always shortest and American green alder always tallest. Among tree and tall shrub species, height was least in Unit 1, which had the lowest overall fire severity . For black spruce, American green alder, and paper birch, seedling height was significantly greater on Unit 2—which burned at mostly high severity—than that on units 1 and 5. This was also true for Alaska willow and Bebb willow (P<0.05 for all species), but not Scouler willow [8,9]. For hardwoods, mean height of natural regeneration was always greater than that of planted seedlings, while mean height of planted black spruce seedlings was greater than that of natural seedlings. For hardwood seedlings, best survivorship and growth was on high-severity plots that had little to no residual soil organic matter and were colonized with fire moss, juniper haircap moss, and other mosses that are common in early postfire succession. Poor hardwood seedling growth was positively associated with residual soil organic matter (P=0.05). For example, height of grayleaf willow seedlings averaged 25 cm on low-severity and 135 cm on high-severity plots. The effects of soil organic matter depth were apparent even over small areas, so seedlings on adjacent low- and high-severity microsites showed vast differences in height. Overall survivorship of willow cuttings was poor. Alaska willow and balsam poplar cuttings survived only on high-severity plots .
|Table 8. Mean height (cm (SD)) of artificial regeneration, postfire years 3 and 6. For spruce, alder, aspen, and grayleaf willow, data for both hand-sown and planted regeneration are given. For planted seedlings, height at planting averaged 10 cm for spruce and 15 cm for alder, aspen, and grayleaf willow . For other species listed, all data are for regeneration from hand-sown seed only . Cells are blank where data were not available.|
|3rd year||6th year*||3rd year||6th year||3rd year||6th year|
|American green alder||sown: 5.0
|balsam poplar||not present||not present||sown: 2.4
|Bebb willow||not present||16.4
|black spruce||sown: 21.0
|Alaska willow||not present||not present||sown: 36.8 (21)||sown: 5.5
|planted: 5.5 (0.2)|
|paper birch||sown: 2.5
|quaking aspen||sown: 2.0||35.0
|*For postfire year 6, data were provided for planted regeneration only .|
|Table 9. Mean height (cm (SD)) of natural regeneration, postfire year 6 |
|American green alder||195.1 (31.5)||152.0 (47.2)|
|black spruce||none||17.0 (9.1)|
|grayleaf willow||243.0 (52.0)||40.2 (16.3)|
|Common name||Scientific name|
|Cladonia spp.||reindeer lichens|
|Nephroma arcticum||arctic kidney lichen|
|Peltigera aphthosa||green dog lichen|
|Ceratodon purpureus||fire moss|
|Hylocomium splendens||splendid feather moss|
|Marchantia polymorpha||common liverwort|
|Pleurozium schreberi||Schreber's moss|
|Polytrichum juniperinum||juniper haircap moss|
|Alnus viridis subsp. crispa||American green alder|
|Ledum groenlandicum||bog Labrador tea|
|Rosa acicularis||prickly rose|
|Salix alaxensis||Alaska willow|
|Salix bebbiana||Bebb willow|
|Salix glauca||grayleaf willow|
|Salix scouleriana||Scouler willow|
|Vaccinium uliginosum||bog blueberry|
|Vaccinium vitis-idaea||mountain cranberry|
|Betula papyrifera||paper birch|
|Picea mariana||black spruce|
|Populus balsamifera subsp. balsamifera||balsam poplar|
|Populus tremuloides||quaking aspen|
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7. Viereck, Leslie A.; Foote, Joan; Dyrness, C. T.; Van Cleve, Keith; Kane, Douglas; Seifert, Richard. 1979. Preliminary results of experimental fires in the black spruce type of interior Alaska. Res. Note PNW-332. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Forest and Range Experiment Station. 27 p. 
8. Zasada, John C.; Norum, Rodney A.; Teutsch, Christian E.; Densmore, Roseann. 1987. Survival and growth of planted black spruce, alder, aspen and willow after fire on black spruce/feather moss sites in interior Alaska. The Forestry Chronicle. 63(2): 84-88. 
9. Zasada, John C.; Norum, Rodney A.; Van Veldhuizen, Robert M.; Teutsch, Christian E. 1983. Artificial regeneration of trees and tall shrubs in experimentally burned upland black spruce/feather moss stands in Alaska. Canadian Journal of Forest Research. 13: 903-913.