Regional-scale studies of vegetation change in western North America have often focused on species ranges rather than vegetation structure. Studies of fire have attributed recent increases in annual area burned and fire size to a warming climate, but have minimized the issue of fire severity and left ignition frequency unaddressed. I combined satellite measurements with data from a network of 655 historical (1932-1936) and 208 modern (1988-1999) vegetation plots to analyze vegetation and fire in Yosemite National Park (3,027 km 2).
I examined the relationship between decreasing snowpack and the ignition and size of all 1,870 fires that occurred between 1984 and 2005. I quantified fire severity for the 103 fires >40 ha with Landsat-derived fire severity indices. Deeper snowpack exponentially decreases the number of lightning ignitions. The proportion of the landscape burned at higher severities increases with the log10 of annual area burned.
The density of large-diameter trees in coniferous forests declined 24 % in the twentieth century. Declines were highest in subalpine and upper montane forests. For Pinus ponderosa, P. jeffreyi, and P. lambertiana, decreases were greatest at lower elevations. For Quercus chrysolepis, Q. kelloggii, Calocedrus decurrens, P. albicaulis, and Pseudotsuga menziesii, increases were present at higher elevations. Large-diameter tree density for other species declined throughout their ranges. Pinus ponderosa forests that burned since 1936 retained most of their large-diameter P. ponderosa component, but those that remained unburned had similar densities of large-diameter trees of other species.
I used climatological averages, reconstructions of past climate, and climate projections to evaluate changes in climate-vegetation and climate-fire relationships. I calculated water balance using a Thornthwaite-type model. Changes in water balance from 1700 (“Little Ice Age”) to the present are small compared to expected changes from the present to 2050 (1.5 °C warmer). Decreased snowpack will increase lightning ignitions 19.1 % by 2050 contemporaneous with a 21.9 % increase in annual area burned at high severity. Decreases in snowpack and concomitant increases in summer drought and fire severity suggest that vegetation change may accelerate—future fires may be more severe, post-fire recovery may take longer, and large-diameter trees may exhibit increased mortality.
|Advisor:||Franklin, Jerry F.|
|School:||University of Washington|
|School Location:||United States -- Washington|
|Source:||DAI-B 69/09, Dissertation Abstracts International|
|Subjects:||Ecology, Physical geography, Forestry|
|Keywords:||Climate change, Fire, Lightning ignitions, Sierra Nevada, Vegetation, Yosemite National Park|
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