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Long-term forest recovery processes following a large, mixed severity fire in ponderosa pine ecosystems of the Black Hills, South Dakota, USA




Wudtke, Benjamin J., author
Smith, Frederick W., advisor
Battaglia, Mike A., committee member
Vaske, Jerry J., committee member

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Understanding the pattern and timing of ecological recovery following wildfires in western forests has become critical as the area burned has dramatically increased in the past decade. This is especially important in ponderosa pine forests, where mixed severity fires lead to a complicated landscape mosaic of initial fire effects and patterns of recovery. I compared forest structural change in relation to initial fire severity, 10-years following the Jasper Fire of 2000 in ponderosa pine forests of the Black Hills of South Dakota. The Jasper Fire burned ~34,000 ha as a mixed severity fire with 25% low, 48% moderate and 27% high severity. I measured 43 sites within and adjacent to the Jasper fire perimeter which represented unburned areas and areas burned at low, moderate and high severity. These sites were established in 2001, immediately following the Jasper fire, and were measured annually for the first 5 years. My work constitutes the 10-year re-measurement of these sites. I assessed forest recovery as accumulation of forest floor biomass, seedling regeneration, snag dynamics, tree survivorship and growth of surviving trees. Stand density has remained constant for unburned sites at ~25 m2 ha-1 since the fire, but has continued to decline in low and moderate sites which were at 18 and 9 m2 ha-1. After 10 years, high severity sites had 63 Mg ha-1 of forest floor woody debris and were significantly greater than other burned sites and unburned sites where biomass was ~14 Mg ha-1. Approximately 80% of forest floor biomass on high severity sites was coarse woody material (>7.6 cm). There was no difference in fine material (<2.5 cm) between burned and unburned sites. The difference in coarse woody debris was due to the near complete fall of snags on high severity sites where 87% of fire-killed trees have now fallen. Litter on low severity sites was similar to unburned sites but is still significantly lower on moderate and high severity sites. Duff remains ~85 to 99% less on burned sites compared to unburned sites. Regeneration was substantial on unburned, low, and moderate severity sites. Unburned sites averaged ~6,000 seedlings ha-1, while low and moderate severity sites averaged ~1,200 seedlings ha-1. Regeneration was sparse on high severity sites and averaged 28 seedlings ha-1, likely attributed to factors limiting seed availability. Regeneration in the first 4-5 years was low on burned sites, when the Palmer Drought Severity index averaged. Since 2007, the amount of surviving seedlings substantially increased, when the drought index averaged 3.5. Tree growth measured as basal area increment for the 50 largest diameter trees per severity was significantly reduced on sites for 2001 through 2006 compared to five-year pre-fire growth. The only significant difference between severities during that time was in 2002 when moderate severity sites had higher relative growth. Growth for all sites increased in 2007 through 2010 and tree growth on moderate severity sites was significantly greater than unburned and low severity. The persistent drought from 2001 - 2006 had a more pronounced effect on tree growth than any fire effects. Forest recovery following mixed severity wildfire is strongly influenced by initial fire effects and postfire climate. Low severity areas are similar to unburned areas in nearly all aspects of stand structure. Overstory density was substantially reduced in moderate severity areas but had increased tree growth and seedling regeneration produced a new cohort of trees that will likely lead to the development of a multi-aged forest. Regeneration in high severity areas continued to be slow, and the persistence of a sparsely treed woodland forest, or cover type conversion in some instances, is likely. Importantly, many processes of recovery have accelerated in the past 3 years, as the persistent severe drought conditions of 2001 - 2007 have subsided.


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South Dakota


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