Restoration of Rio Grande Bosque forests: Developing understory slash pile burn protocols that minimize cottonwood mortality

The Middle Rio Grande cottonwood gallery forests are threatened by high densities of the fire-prone woody exotic invasives, Tamarisk  and Elaeagnus angustifolia. Fish and Wildlife crews cut the woody invasives from cottonwood understory along the Rio Grande in the Sevilleta National Wildlife Refuge.  Large cut wood was removed and 4-inch-diameter wood and smaller slash were piled for burning.  Our study objective was to determine protocols for reducing fuels loads while minimizing fire-induced mortality of overstory cottonwood.  Temperature-sensitive paint strips and 22 temperature-recording data loggers were placed on cottonwood trucks to monitor trunk temperatures during the burn.   Pile burns were conducted in April and October of 2006. In April, fuel moisture levels were low, 7 % for litter and 4% for wood, and flame lengths exceeded 2 m.  The fire completely consumed both the slash piles and the litter layer, and cottonwood mortality was high.  The October burn was conducted when fuel moisture levels were 29% for litter and 14.5% for wood.  Fire was contained within the slash piles and did not consume the litter layer.  Flame lengths were 1-1.5 m. Cambial temperatures at the 3-ft height never exceeded the lethal threshold of 60 ºC.  Total cottonwood mortality from the two burns is currently being assessed.  Preliminary recommendations for slash pile burning include litter moisture levels of at least 20-25%, and long low slash piles of less than 0.75 m in height.  This combination should maintain flame lengths of approximately 0.5 m and result in minimal mortality of cottonwood overstory.