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Citation for this synthesis:
Innes, Robin J. 2014. Fire regimes of Alaskan black cottonwood communities.
In: Fire Effects Information System, [Online].
U.S. Department of Agriculture, Forest Service,
Rocky Mountain Research Station, Missoula Fire Sciences Laboratory (Producer).
Available: www.fs.usda.gov/database/feis/fire_regimes/AK_black_cottonwood/all.html
[].
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The range of black cottonwood extends from Alaska, south through Yukon, British Columbia, and Alberta, the western, conterminous United States, and into Baja California Norte [7,19,20]. This review covers black cottonwood communities in Alaska. In Alaska, black cottonwood communities occur along the southern coast from the Alaska Peninsula to southeastern Alaska, including various islands [26,32,37,39]. As of this writing (2014), there was little published information on historical fire regimes in these communities. Information from Canada and the northwestern United States is included in this review to supplement the little information available from Alaska. For a list of Alaskan black cottonwood communities, see Appendix A. For a list of common plant species occurring in Alaskan black cottonwood communities, see Appendix B. For information on fire regimes in balsam poplar communities, see the Fire Regime Synthesis for Alaskan quaking aspen and balsam poplar.
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| Distribution of Alaskan black cottonwood communities based on the LANDFIRE Biophysical Settings (BpS) data layer [22]. Numbers indicate LANDFIRE map zones. Click on the map for a larger image and zoom in to see details. |
Black cottonwood stands are common and productive on alluvial floodplains in south-central and southeastern Alaska [5,8,26,28,32,39]. They occur on upper river terraces, large glacial moraines, and gentle hillslopes [5,8,18,26,28,39]. Black cottonwood is an early-seral species [7,10,15,16]. Along floodplains and recently deglaciated sites, alders are usually present throughout black cottonwood stand development; willows are common in early succession but become less common in mid- to late succession. Black cottonwood dominates or codominates many riparian floodplain communities in midsuccession and often occurs in mixed-conifer and mixed-hardwood stands with white spruce, Lutz spruce, Sitka spruce, mountain hemlock, western hemlock, balsam poplar, paper birch, and/or Kenai birch. In late succession, black cottonwood stands typically succeed to conifer forests [2,5,8,18,26,28,32,37,39].
Black cottonwood is frequently damaged by fire [1,15]. The seedlings and saplings are usually killed regardless of fire severity because of thin bark and shallow roots [6], and all trees may be killed or top-killed by high-severity fire. Mature black cottonwoods with thick bark may survive low- and moderate-severity fires [16,25]. Mature black cottonwood communities often have higher fuel loading than young stands, and mature black cottonwood trees have more heart rot than young trees. These factors can contribute to increased fire severity in mature stands [12].
Black cottonwood sprouts from lateral roots, the root crown, and/or the bole after top-kill by fire [12,16]. Rates of sprouting are highest if fire occurs when cottonwoods are dormant [12]. In floodplain cottonwood forests along the Oldman River in southern Alberta, Canada, 75% of cottonwood trees, most of which were black cottonwood, balsam poplar, and narrowleaf cottonwood, sprouted from stumps within 5 months of an April fire that occurred prior to green up [12]. However, summer is the peak fire season in south-central and southeastern Alaska [29], so fire is most likely in Alaskan black cottonwood stands after green up, when sprouting may be less than in spring. Black cottonwood also establishes from seed after fire [6,34,41].
Lightning-caused fires are uncommon in south-central and southeastern Alaska [17,24], and thus also in black cottonwood communities. It is unclear how human ignitions affected fire regimes in Alaskan black cottonwood communities historically. Fire might have been used by some Native Alaskan populations in southeastern Alaska to, for example, fell large trees or clear burial grounds, but the effects were likely local [17,38].
Compared with other disturbances, fire is relatively unimportant in Alaskan black cottonwood stands. Frequent flooding, shifting channels, and sediment deposition are the most common disturbances in Alaskan black cottonwood floodplain forests [5,23,26,28]. Avalanches, rock slides, and windthrow may be primary disturbances on other sites [8,26].
As of this writing (2014), fire frequency had not been reported in Alaskan black cottonwood communities. LANDFIRE did not model fire regime characteristics in Alaskan black cottonwood communities (fire regime group="NA") because experts did not consider fire an important disturbance in these communities (Appendix A). Parminter [30] observed no evidence of fire on alluvial balsam poplar-black cottonwood-white spruce floodplain forests in the Cassiar Timber Sale Area in British Columbia and stated that these communities "have not been influenced by fire to a significant degree, if at all". In contrast, fire may be a relatively common disturbance in black cottonwood and other riparian cottonwood communities in the Rocky Mountain region of Alberta [12,33]. For example, in a riparian black cottonwood grove along the Oldman River, 100-year-old black cottonwoods had up to 4 fire scars, suggesting low-severity surface fires at 20- to 30-year intervals [33].
Fire frequency in Alaskan black cottonwood communities may depend in part on fire frequency in adjacent communities. Arno [3] stated that black cottonwood forests along major rivers of the Pacific Northwest likely burned frequently because they were historically surrounded by communities with frequent fires, such as Pacific ponderosa pine savannas and sagebrush steppes [3]. In southeastern Alaska, however, many black cottonwood communities are adjacent to western hemlock or spruce-hemlock forests [21], where fire was historically very rare [27,37,40]. Thus, fire in southeastern Alaskan black cottonwood communities was also likely very rare.
Anecdotal information suggests that repeated fires at short intervals may exclude black cottonwood. Black cottonwood trees were killed and replaced by bunchgrasses in 4 locations after 2 fires that were 19 years apart in Glacier National Park, Montana (Singer 1975 cited in [4]).
Fire is typically less frequent and of lower severity in cottonwood riparian areas than adjacent upland areas because high soil and fuel moisture levels in these communities provide natural firebreaks [9,31,33,35]. A review stated that fires are of lower severity and occur less frequently in riparian areas than in surrounding uplands [9,31]. However, plant productivity can be high in black cottonwood communities, and under dry conditions, abundant fuels resulting from high productivity may result in higher fire severity in riparian areas relative to surrounding uplands [31].
Rivers in black cottonwood stands can also provide firebreaks [9,31,33,35]. Wide rivers provide more effective firebreaks than small rivers with greater canopy continuity, at least during mild fire weather [9,31,36]. Rivers and associated riparian vegetation may not provide effective firebreaks during extreme fire weather, especially strong winds [4,31,36].
Black cottonwood and other riparian communities consist of a mosaic of stands in varying successional stages [11,13,14,33]. Patchy vegetation and patchy fuels that result from the transport and concentration of debris on banks by floodwaters result in patchy fires in black cottonwood and other riparian areas [9,31].
As of this writing (2014), no information was available about fire size in Alaskan black cottonwood communities.
In Alaska, wind-driven fires beginning in upland communities may spread to adjacent black cottonwood forests, but as of 2014, no studies reported this. Gom and Rood [12] suggested that prairie fires would have periodically burned into riparian cottonwood woodlands in Alberta. In southeastern Alaska, however, many black cottonwood communities are adjacent to western hemlock or spruce-hemlock forests [21]. Because fire has been historically very rare in these communities [27,37,40], fire spread from these communities into adjacent black cottonwood communities was likely very rare.
| Appendix B. Common and scientific names of plant species. Follow the links to FEIS Species Reviews. | |
| Common name | Scientific name |
| Fern and fern allies | |
| horsetail | Equisetum spp. |
| lady fern | Athyrium filix-femina |
| oak fern | Gymnocarpium dryopteris |
| variegated scouring rush | Equisetum variegatum |
| Forbs | |
| common cow parsnip | Heracleum maximum |
| dwarf fireweed | Chamerion latifolium |
| fireweed | Chamerion angustifolium |
| purple sweetroot | Osmorhiza purpurea |
| small enchanter's nightshade | Circaea alpina |
| three-leaf foamflower | Tiarella trifoliata |
| western rattlesnake root | Prenanthes alata |
| Graminoids | |
| bluejoint reedgrass | Calamagrostis canadensis |
| Lapland reedgrass | Calamagrostis lapponica |
| Shrubs | |
| alder | Alnus spp. |
| Barclay's willow | Salix barclayi |
| devil's-club | Oplopanax horridus |
| Drummond's mountain-avens | Dryas drummondii |
| red elderberry | Sambucus racemosa |
| sagebrush | Artemisia spp. |
| salmonberry | Rubus spectabilis |
| Sitka alder | Alnus crispa subsp. sinuata |
| Sitka willow | Salix sitchensis |
| stink currant | Ribes bracteosum |
| willow | Salix spp. |
| Trees | |
| Alaska willow | Salix alaxensis |
| balsam poplar | Populus balsamifera subsp. balsamifera |
| black cottonwood | Populus balsamifera subsp. trichocarpa |
| cottonwood | Populus spp. |
| hemlock | Tsuga spp. |
| Kenai birch | Betula papyrifera var. kenaica |
| Lutz spruce | Picea × lutzii |
| mountain hemlock | Tsuga mertensiana |
| narrowleaf cottonwood | Populus angustifolia |
| paper birch | Betula papyrifera |
| Pacific ponderosa pine | Pinus ponderosa var. ponderosa |
| quaking aspen | Populus tremuloides |
| red alder | Alnus rubra |
| Sitka spruce | Picea sitchensis |
| spruce | Picea spp. |
| western hemlock | Tsuga heterophylla |
| white spruce | Picea glauca |
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Midground shows black cottonwoods along
Glacier Creek in Girdwood, Alaska, with Mount Alyeska in the background. Photo by Kate Mohatt, U.S. Forest Service.