White spruce understory protection: From planning to growth and yield

Abstract

A large component of the boreal mixedwood forest is comprised of aspen and white spruce mixtures of varying proportions and ages. The slower growing white spruce usually starts as an understory component but will succeed to a white sprucedominated stand after aspen break-up. Since both species are utilized by the forest industry, one method of maximizing total yield is to protect the unmerchantable white spruce understory while harvesting the merchantable aspen overstory. Although some of the white spruce understory is lost when the machine corridors are harvested, future conifer yield is augmented by the accelerated growth of the protected spruce component, a result of increased light levels. In a 10 year trial comparing the growth of released versus control understory spruce, annual height growth, diameter growth and volume increment were 76%, 152% and 83% higher, respectively, for the released conifer compared to the control. In order to account for the yield implications in timber supply analysis, accurate forecasts of future stand development can only be obtained through the use of a forest growth model since long-term data are not available. The Mixedwood Growth Model (MGM) has a unique architecture that allows for the modeling of various strata in understory protection stands. This "multi-strata" modeling approach was used to forecast the combined yield of all the strata, including the impact of adjacent strata with regards to light availability. Operational examples of understory protection, data on white spruce release and aspen regeneration, as well as modeled volume forecasts are presented.Keywords: forest management, boreal mixedwood, harvesting systems, white spruce understory protection, white spruce release, growth and yield, Mixedwood Growth Model résumé Une grande partie des forêts mixtes boréales est formée de regroupements de tremble et d' épinette dans diverses proportions et d'âges variés. L' épinette blanche, à croissance plus lente, se présente d'abord en sous-étage, mais formera à terme un peuplement dominé par l' épinette blanche après le déclin du tremble. Étant donné que ces deux espèces sont utilisées par l'industrie forestière, il serait possible de maximiser le rendement total en protégeant le sous-étage d' épinette blanche, sans valeur marchande, lors de la récolte commerciale de la strate dominante de tremble. Même si une partie du sous-étage d' épinette blanche devait être détruite dans les sentiers aménagés pour la machinerie au moment de la récolte, le rendement futur des résineux en serait accru dû à la croissance accélérée de la strate d' épinettes protégée, en raison d'une plus forte luminosité. Dans un essai sur 10 ans destiné à comparer la croissance du sous-étage d' épinette dégagé par rapport au témoin, la croissance annuelle en hauteur, la croissance en diamètre et l'accroissement en volume ont été respectivement de 76 %, de 152 % et de 83 % plus élevés chez les épinettes dégagées comparativement au témoin. Pour tenir compte des implications qu'auront ces résult...

Effects of aerial strip spraying on mixedwood stand structure and tree growth

“…For each of the six blocks sampled, a tree list was prepared for initialization of the model and included DBH, height, and tree factor using data for all measured trees in both treated and untreated subplots. Simulations were run to age 120, with and without a commercial thinning (understory protection [Grover et al 2014]) to harvest 85% of the aspen at age 70. Multiple strata (strips) were simulated with lateral shading from adjacent strips (i.e., taller untreated strips of aspen-shaded treated spruce strips) included in the simulations for a simulated area of 100 m long (west to east) and 48 m wide (north to south).…”

Managed Mixtures of Aspen and White Spruce 21 to 25 Years after Establishment

“…Each subplot was first projected to age 60 using the same site index and model parameterization as above. At age 60, understory protection harvesting was simulated following a strip harvesting pattern (6 m wide extraction area, 6 m wide machine corridor, 6 m extraction area, 3 m residual strip) as suggested by Grover et al [54]. For any given area, the pattern results in an approximately 57% extraction area, 29% in machine corridor and 14% in residual strips.…”

“…For any given area, the pattern results in an approximately 57% extraction area, 29% in machine corridor and 14% in residual strips. At age 60, each subplot was harvested as either extraction area (only mature aspen were harvested) and machine corridor (all trees were removed and regenerated to aspen at 9267 st ha´1 [54] using the average top height at age 25 obtained from the actual plot data). The residual strip was left unharvested.…”

“…Yields for a simulated understory protection (two stage harvest) where aspen was harvested at age 60 while protecting the spruce component of the mixedwood [54,55], followed by a complete harvest of the released spruce, and regenerated aspen at year 90 (Table 5) were also estimated using MGM. Combined yields of over 500 m 3 ha´1 were predicted for the two most productive understory protection scenarios (Table 5).…”

“…This two stage harvesting system maximized the productive potential of both species. In our analysis, only a generalized harvest scenario was used in the MGM simulations, in contrast with the detailed site level analysis of Grover et al [54]. Our analysis is likely to overestimate both spruce and aspen growth since it does not incorporate spruce windthrow losses after the first harvest entry and the reduced aspen growth in portions of the harvest area due to shading by adjacent trees [54,71].…”