Shelterwood systems are applied in southern British Columbia to facilitate forest regeneration and to achieve visual and watershed management objectives. Although designed to regenerate even-aged stands, they provide opportunities to establish regeneration and achieve greenup of initially harvested areas, before the final removal cut. Group and patch shelterwood systems may be desired to maintain or recreate naturally patchy forests and can be easier to harvest and can be more amenable for creating conditions that favour regeneration of moderately tolerant to intolerant tree species than uniform shelterwood systems. Group shelterwood systems can also be less susceptible to windthrow.
Due to patterns in light, air temperature, soil temperature, and soil moisture, establishment and growth of regeneration is influenced by both the size of the openings and proximity to the edge of the uncut stand (Coates 2000; Huggard and Vyse 2002). Light levels increase as opening size increases and south sides of openings are typically influenced by shade from adjacent stands, resulting in lower amounts of direct sunlight near the southern edge compared to the northern edge (Messier 1996; Coates 2000; Delong et al. 2000; Spittlehouse et al. 2004). Light levels also generally increase under the intact stand located to the north of newly created openings. Snow may persist longer in the spring near the south edges of gaps compared to north edges (Huggard and Vyse 2002; Spittlehouse et al. 2004). Air and soil temperatures tend to increase with opening size and are higher at the north edge, adjacent to the edge of the stand, than at the south edge due to higher energy inputs associated with higher levels of direct sunlight (Huggard and Vyse 2002; Grey et al. 2002; Spittlehouse et al. 2004). Air and soil temperatures may also increase for a short distance under the intact stand to the north of large openings. Numerous studies indicate reductions in soil moisture, compared to that at the center of large openings, that extend up to 1 tree length into openings from the adjacent stand (Coates 2000; Gray et al. 2002; Spittlehouse et al. 2004). Few studies have documented effects of openings on nutrient availability in western forests. However, Huggard and Vyse (2002) report a significant increase in soil nitrate and a decrease in soil potassium in openings, compared to under the adjacent forest, and a rapid change between the forest and gap condition.
Seasonal patterns in resource availability can influence growth patterns and tree morphology. Year to year variations in these seasonal patterns can also result in differences in growth responses and in morphological features such as height:diameter ratio. Some studies suggest that an understanding of seasonal patterns of resource availability may be useful in understanding the effects of competition and other limiting factors on tree growth (Kimberley and Richardson 2004). Due to the multiple environmental factors influencing growth in small openings, an understanding of the seasonal dynamics and interacting effects of major environmental factors on physiology and growth of trees may lead to a better understanding of the effects of these factors. Understanding limiting factors to tree survival and growth and the effects of silvicultural practices on these limiting factors is widely recognized as a key to successful silviculture prescriptions.
Light, air temperature, soil temperature and soil moisture gradients across openings and differences in requirements and tolerances of tree species result in variation in which species grow well in and dominate particular environments. Studies in other parts of B.C. show clear patterns of tree growth in relation to light levels, and size and location in gaps which generally reflect the shade tolerance of the species. At Date Creek, near Smithers, Coates (2000) reports that the five species examined (western redcedar, western hemlock, subalpine fir, hybrid ...
Hossain, Kazi L., Comeau, Phillip G.. 2009. Growth of 10 tree species in relation to location and microclimatic gradients in a strip shelterwood. Forest Investment Account (FIA) - Forest Science Program. Forest Investment Account Report. FIA2009MR392
Topic: FLNRORD Research Program
Keywords: Forest, Investment, Account, (FIA), British, Columbia
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