BLOWDOWN HISTORY AND LANDSCAPE PATTERNS IN THE ANDES OF TIERRA-DEL-FUEGO, ARGENTINA

The effects of periodic gales on Nothofagus (southern beech) stand dev elopment and landscape dynamics were studied in a 10.4-km(2) study are a in the Sierra de las Pinturas, part of the Andes in Argentine Tierra del Fuego. We reconstructed blowdown history (patch sizes, dates of o rigin, return intervals) since the late 1700s to assess how periodic l arge-scale wind disturbance influenced landscape pattern in a relative ly simple system. Most previous studies have focused on single storms in more diverse forests and in landscapes influenced by several distur bance types and human activities. Boundaries of post-blowdown stands w ere digitized from aerial photographs and analyzed within a geographic information system. Ages of blowdowns and return intervals were deter mined from scars, growth releases, and maximum tree ages. Discrete blo wdown patches (0.1 to >100 ha) covered two-thirds of the study area an d ranged from 19 to approximate to 200 yr of age, with 20-30 yr betwee n major events. The meteorology of these storms was unknown, but we su spect they were caused by intense low-pressure systems originating in Antarctica. The mean return interval for 34 sites was 145 yr, with a r ange of 103-218 yr. Based on treefall size distributions, most stands blown over in the past 100 yr were even-aged, with mean tree dbh (diam eter at breast height) between 20 and 32 cm. Aerial photographs taken in 1970 were used for before-and-after comparisons of a 1972 blowdown. Seventy-one percent of the area blown over in 1972 was even-aged, and 35% of the boundaries from this storm exactly coincided with previous stand boundaries. Damage patterns from storms in 1924 and 1972 sugges ted that forests located on valleys parallel to the wind, windward sid e slopes, and possibly upper leeward slopes were most vulnerable to bl owdown, but few landscape positions escaped being hit by repeated stor ms. Return intervals were not significantly related to slope, elevatio n, or aspect; but surprisingly, shorter return intervals were associat ed with deeper soils. The landscape pattern of blowdown and recovery s hifted over time because of variation among individual storms and beca use a small proportion of old-growth stands were converted to blowdown s and vice versa. Browsing by guanacos (Lama guanaco), a large native camelid, has severely inhibited tree regeneration during the past 75 y r in small blowdowns and the perimeters of larger ones, converting som e stands to open meadows and incipient alpine communities. In the rela tively simple Nothofagus forests of Tierra del Fuego, periodic gales a re the main determinants of forest structure and pattern across a rang e of scales from small patches to entire landscapes.