Geography Reference
In-Depth Information
versely, abundant seed rain can contribute little to the upward movement of treeline
where seed dispersal is limited and seed viability is low (Cuevas 2000).
In addition to exceeding a minimum threshold in seed rain, seed dispersal, and
seed viability, germination and seedling survival at treeline require suitable germination
sites. Such “safe sites” include geomorphic features such as boulders and terrace risers
that decrease surface wind velocities, increase winter snow cover and summer soil
moisture, and provide additional heating through the irradiance of long-wave radiation
(e.g., Resler 2006). Once established, these seedlings eventually develop tree canopies
that increase the area of favorable microhabitat, improving subsequent regeneration
success. This sequence of tree establishment and growth—microrelief feature-seedling
regeneration-canopy development—is strongly suggestive of a positive feedback pro-
cess that explains the spatial pattern of many local treelines (Alftine and Malanson
2004; Bekker 2005; Resler 2006).
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