Gap dynamics: Difference between revisions

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==Forest gaps and forest regeneration==

Until recently, forest regeneration practices in North America have largely followed an agricultural model, with research concentrated on techniques for establishing and promoting early growth of planted stock after [[clearcutting]],<ref name="clea">Cleary, B.D.; Greaves, R.D.; Hermann, R.K. (Compilers and Eds.). 1978. Regenerating Oregon’s Forests. Oregon State Univ. Exten. Serv., Corvallis OR. 287 p.</ref><ref name="laven">Lavender, D.P.; Parish, R.; Johnson, C.M.; Montgomery, G.; Vyse, A.; Willis, R.A.; Winston, D. (Eds.). 1990. Regenerating British Columbia’s Forests. Univ. B.C. Press, Vancouver BC. 372 p.</ref><ref name="wag">Wagner, R.G.; Columbo, S.J. (eds.). 2001. Regenerating the Canadian forest: Principles and practice for Ontario. Fitzhenry & Whiteside, Markham, Ont.</ref> followed by studies of growth and yield emphasizing single-species growth uninfluenced by overstorey canopy. Coates (2000)<ref name="coat1">Coates, K.D. 2000. Conifer seedling response to northern temperate forest gaps. For. Ecol. Manage. 127 (1–3):249–269.</ref> questioned this approach and proposed a shift to a more ecologically and socially based approach able to accommodate greater diversity in managed stands. Predictive models of forest regeneration and growth that take account of variable levels of canopy retention will be needed as the complexity of managed forest stands increases (Coates 2000).<ref name="coat1" />

Tree regeneration occurring inside canopy gaps after disturbance has been studied widely (Bazzaz and Pickett 1980, Platt and Strong 1989).<ref name="bazz">Bazzaz, F.A.; Pickett, S.T.A. 1980. Physiological ecology of tropical succession: A comparative review. Annu. Rev. Ecol. Syst. 11:287–310.</ref><ref name="pla">Platt, W.J.; Strong, D.R. 1989. Special feature: gaps in forest ecology. Ecology 70:535–576.</ref> Studies of gap dynamics have contributed much to an understanding of the role of small-scale disturbance in forest ecosystems, but they have been little used by foresters to predict tree responses following partial cutting (Coates and Burton 1997).<ref name="coat2">Coates, K.D.; Burton, P.J. 1997. A gap-based approach for development of silvicultural systems to address ecosystem management objectives. For. Ecol. Manage. 99:337–354.</ref>

In high-latitude northern forests, position inside a gap can have a pronounced effect on resource levels (e.g., light availability) and microclimate conditions (e.g., soil temperature), especially along the north–south axis. Such variation must inevitably affect the amount and growth of regeneration; but relying solely on natural regeneration to separate the effects of gap size and position is problematic (Coates 2000).<ref name="coat1" /> Among the many factors affecting seedling establishment following canopy disturbance are parent tree proximity and abundance, seedbed substrate, presence of seed consumers and dispersers, and climatic and microclimatic variability. Planted trees can be used to avoid many of the stochastic events surrounding natural seedling establishment.

Gradients of canopy influence can be created by partial cutting, and tree growth responses within gaps of various sizes and configurations, as well as within the adjacent forest matrix can form a basis for tree species selection. Hybrid spruce (the complex of white spruce, Sitka spruce, and occasionally Engelmann spruce) was one of several [[Pinophyta|coniferous]] species used in a study in the Moist Cold subzone of the Interior Cedar–Hemlock zone in northwestern British Columbia. A total of 109 gaps were selected from a population of openings created by logging within each light and heavy partial cutting treatment in stands averaging 30 m in canopy height; 76 gaps were less than 1000 m², 33 were between 1000 m² and 5000 m². Canopy gap size was calculated as the area of an ellipse, the major axis of which was the longest line that could be run from canopy edge to canopy edge inside the gap, and the minor axis was the longest line that could be run from canopy edge perpendicular to the long line. [[Seedling]]s were planted in gaps and in the undisturbed and clearcut treatment units. There were strong and consistent trends in growth response among the seedlings as gap size increased. In all species, growth increased rapidly from small single-tree gaps to about 1000 m², but thereafter, there was little change up to 5000 m². Tree size and current growth rates for all species were highest in full open conditions. In large and medium gaps (300–1000 m²), the largest trees of all species occurred in the middle gap position, with little difference between the sunny north and shady south positions, lodgepole pine excepted. The light advantage expected off the north end of higher-latitude gaps was not a benefit for tree growth, suggesting that below-ground effects of canopy edge trees have an important influence of seedling growth in these forests (Coates 2000).<ref name="coat1" />

In a study near Chapleau, Ontario (Groot 1992, Groot et al. 1997),<ref name="groot1">Groot, A. 1992. Small forest openings to promote the establishment and growth of white spruce in boreal mixedwood stands. Draft NODA proposal, with comments by R.F. Sutton.</ref><ref name="groot3">Groot, A.; Carlson, D.W.; Fleming, R.L.; Wood, J.E. 1997. Small openings in trembling aspen forest: microclimate and regeneration of white spruce and trembling aspen. Nat. Resour. Can., Can. For. Serv., Sault Ste. Marie ON, NODA/NFP Tech Rep. TR-47. 25 p.</ref> openings were created in 40-year-old aspen and monitored to determine their influence on outplanted white spruce seedling development. Circular openings 9 m and 18 m in diameter, 9 m and 18 m wide east–west strips, and a 100 m × 150 m clearcut were planted and spot-seeded. The variation in solar radiation, air temperature, and soil temperature among the strips and plots was almost as great as the variation between the clearcut and intact forest. Solar radiation during the first growing season varied from 18% of the above-canopy values within the uncut stand to 68% values at the center of the 18 m strip. Near the edges of the strips, solar radiation was about 40% of the above-canopy along the south and 70% to 80% along the north. Stomatal conductance in white spruce seedlings declined generally from more sheltered to more exposed environments, correlating best with increased vapor pressure deficit (VPD). Without vegetation control, position in openings had little effect on the growth of planted white spruce; regrowth of lesser vegetation isolated seedlings from the microclimatic effects of [[Canopy (biology)|overstorey treatment]]. Seedling diameters were independent of environment, while height growth was only slightly greater in environments having more light. With vegetation control, white spruce diameter and height were greatest in the center of the strips, even though there was less light there than along the north edge of the strips. Moisture stress may have accounted for that result.

==Primary succession==