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The effects of periodic gales on Nothofagus (southern beech) stand devel- opment and landscape dynamics were studied in a 10.4-km2 study area in the Sierra de las Pinturas, part of the Andes in Argentine Tierra del Fuego. We reconstructed blowdown history (patch sizes, dates of origin, return intervals) since the late 1700s to assess how periodic large-scale wind disturbance influenced landscape pattern in a relatively simple system. Most previous studies have focused on single storms in more diverse forests and in landscapes influenced by several disturbance types and human activities. Boundaries of post-blowdown stands were digitized from aerial photographs and analyzed within a geo- graphic information system. Ages of blowdowns and return intervals were determined from scars, growth releases, and maximum tree ages. Discrete blowdown patches (0.1 to >100 ha) covered two-thirds of the study area and ranged from 19 to -200 yr of age, with 20- 30 yr between major events. The meteorology of these storms was unknown, but we suspect they were caused by intense low-pressure systems originating in Antarctica. The mean return interval for 34 sites was 145 yr, with a range 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 (diameter 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 suggested that forests located on valleys parallel to the wind, windward side slopes, and possibly upper leeward slopes were most vulnerable to blowdown, but few landscape positions escaped being hit by repeated storms. Return intervals were not sig- nificantly related to slope, elevation, or aspect; but surprisingly, shorter return intervals were associated with deeper soils. The landscape pattern of blowdown and recovery shifted over time because of variation among individual storms and because a small proportion of old-growth stands were converted to blowdowns and vice versa. Browsing by guanacos (Lama guanaco), a large native camelid, has severely inhibited tree regeneration during the past 75 yr in small blowdowns and the perimeters of larger ones, converting some stands to open meadows and incipient alpine communities. In the relatively simple Noth- ofagus forests of Tierra del Fuego, periodic gales are the main determinants of forest structure and pattern across a range of scales from small patches to entire landscapes. |
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