Читать книгу North American Agroforestry - Группа авторов - Страница 75

When wind encounters the edge of a forest, some of the air is deflected over the canopy for a distance of up to 20 tree heights (Cionco, 1985; Fritschen, 1985). If the forest occurs as a narrow strip, this deflection of air creates a protected zone to the leeward in which wind speed is reduced, wind‐related stresses such as desiccation are decreased, and snow deposition may increase.

This modification of microclimate is essential to the maintenance of ribbon forests (Billings, 1969; Peet, 1988) and is a fascinating feature of subalpine regions in the Rocky Mountains. Ribbon forests are arranged as alternating parallel strips of forest and moist alpine meadow oriented perpendicular to the prevailing winds. Snow accumulation to the lee of each forest strip inhibits seedling establishment, while tree growth rates at the far edge of each drift are increased by water from snowmelt and protection from desiccation by winter winds. Thus, the pattern and spacing of forest strips is determined by the effect of tree canopy structure on windspeed and snow deposition.

Ribbon forests are a classic model for one of the most common temperate agroforestry practices, windbreaks. Farm windbreaks are linear groups of trees that provide a sheltered microclimate for leeward fields. The extent and degree of shelter depends on the structural characteristics of the windbreak such as height, density, and orientation, and these can be manipulated to meet particular management goals (e.g., odor control). Dense windbreaks result in deposition of snow in drifts close to the leeward edge and act as living snow fences. More porous windbreaks cause snow to be distributed more evenly across the leeward field, a preferable situation if soil moisture conservation or protection of winter wheat from desiccation is the goal (Brandle & Finch, 1991; Mize, Brandle, Schoenberger, & Bentrup, 2008).