Attributes of blowdown patches from a severe wind event in the Southern Rocky Mountains,USA

There is increasing interest in large, infrequent, natural disturbances and how they affect ecosystems. Attributes of patches produced by some natural disturbances, such as blowdowns, have seldom been measured. We measured attributes of patches produced by a large blowdown (over 10 000 ha) in northern Colorado, USA in 1997. The blowdown produced 402, 655, or 756 patches, based on three different concepts of a blowdown patch. An inverse-J relationship shows that most patches are small in size (<200 ha), while few are large. Most patches have a high percentage of blown-down trees (>50% down). Blowdown patches are highly variable in their size, perimeter length, and distance to the nearest patch. The blowdown patches are larger and have more complex shapes than patches in the surrounding forest. Mean size of blowdown patches (25 ha) may be smaller than those of crown fires in a nearby forest, but similar total areas may be affected. About 75% of the blowdown area is within 125 m of a forest not blown down, so natural tree regeneration should not be a problem. About 16,400 ha of mature spruce-fir forest is susceptible to first-year attack by spruce beetles, as this forest is within the expected dispersal distance (1.2 km) from blowdown patches where beetle reproduction is favored. Timber harvest patches differ from blowdown patches in size and distance to nearest patch. It also may be inappropriate to mimic forest blowdown patches using timber harvesting in this region, due to the rare occurrence of large blowdowns, their spatial restriction, and other factors.