The alpine forest represents a transitional zone separating the alpine tundra and subalpine forest communities. This treeline ecotone is also the highest altitude at which trees are found to occur, although the exact environmental factors and mechanisms limiting this occurrence are just beginning to be unraveled. These treelines are composed of evergreen conifer species most often, although deciduous conifers and broadleaf species also occur, as well as evergreen broadleaves at lower latitudes. There is also a strong
Table 3 The importance of ecological facilitation for seedling establishment, growth, and survival in the alpine forest
Biotic: Inanimate (rocks, dead wood, microtopography) Abiotic: Plant structure (clustering), intraspecific and interspecific facilitation of microsites
Snow burial - prevents ice crystal abrasion and desiccation; warmer and less extreme diurnal temperature differences; no excessive sunlight exposure Clustering at the shoot-to-landscape scale - increased snow deposition and burial Flagging - prevents damage from snow loading and rime ice accumulation Summer Less sky exposure Day: Less sunlight and cooler temperatures Night: Higher minimum temperatures and less LTP; less dew and frost accumulation Less wind exposure - warmer needles in sun
Less sun sky exposure due to burial and mutual shading Day: Less sunlight for photosynthesis and lower temperatures Less wind exposure Day: Warmer temperatures and greater transpiration Night: Colder minimum temperatures and greater LTP
Inanimate, intraspecific, interspecific, and structural facilitation can all generate protective snow burial, as well as amelioration of subsequent growth limitation factors within and just above associated ground cover. LTP represents low-temperature photoinhibition of photosynthetic carbon gain.
correlation between higher latitudes and a lower treeline altitude, as well as with more continental versus maritime mountains. Ecological facilitation of seedling microsites by inanimate structures and microtopography, along with intra- and interspecific facilitation, is a fundamental property of timberline migration up or down the mountain and, thus, the formation of new subalpine forests at a different altitude. This facilitation of microsites involves environmental parameters such as avoidance of wind exposure, wind /snow abrasion, and exposure to sunlight and the cold nighttime sky. In addition, the ability to survive in exposed microsites appears coupled to developmental capabilities for forming krummholz and flagged forms that enable wind protection, including adequate snow collection and burial to prevent damage from the abiotic environment. As new seedling and sapling cover increases, facilitation of growth processes by microclimate amelioration leads to the ultimate growth of trees to a forest-tree stature, culminating in the protective environment of a new subalpine forest.
See also: Alpine Ecosystems and the High-Elevation Treeline; Boreal Forest; Edge Effect; Organismal Ecophysiology; Wind Effects.
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