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How silvicultural systems and management methods guide forest regeneration using coastal Douglas-fir… Cai, Qinyu Jun 21, 2013

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  UNIVERSITY OF BRITISH COLUMBIA How Silvicultural Systems and Management Methods Guide Forest Regeneration Using Coastal Douglas-fir as an Example        FRST 497 graduating essay submitted as part of the Bachelor of Science in Forestry degree at the University of British Columbia     Qinyu Cai June 21, 2013     Page | 1  Abstract Forest regeneration needs to be applied under the direction of silvicultural prescriptions and the context of silvicultural systems. Management interventions should be implemented if the current or expected stand conditions are unfavourable for target species to achieve management objectives. To illustrate these concepts, this paper uses coastal Douglas-fir (Pseudotsuga menziesii var. menziesii) as the target regeneration species, with the objectives of maximizing stand production and wood quality. Firstly, the ideal stand conditions for coastal Douglas-fir growth are discussed in this paper. Then functions of silvicultural systems and management methods are introduced. During the regeneration prescription process, the silvicultural system should firstly be determined. The stand should be site-prepared before regeneration if microsite conditions are unfavourable to coastal Douglas-fir growth. Different strategies are applied based on natural or artificial regeneration method. After sowing, or planting, animal damage and vegetation competition should be reduced to provide free growth condition to seedlings.  Key words: Forest regeneration, Coastal Douglas-fir, Silvicultural system, Management methods       Page | 2 TABLE OF CONTENS Abstract ............................................................................................................................................1 Introduction ......................................................................................................................................5 Introduction to Coastal Douglas-fir ..............................................................................................5 What Are Silvicultural Systems ...................................................................................................6 General Requirements for Coastal Douglas-Fir Growth ..................................................................7 Seed supply ...................................................................................................................................7 Seedbed ........................................................................................................................................8 Seedling Establishment Environment ...........................................................................................9 Introduction to Silvicultural Systems .............................................................................................11 Clearcut System ..........................................................................................................................11 Patch Cut System ........................................................................................................................12 Seed Tree System .......................................................................................................................12 Shelterwood System ...................................................................................................................12 Selection System ........................................................................................................................14 Retention System ........................................................................................................................14 Forest Management Methods .........................................................................................................14 Nursery .......................................................................................................................................15 Site Preparation ..........................................................................................................................15 Fertilization ................................................................................................................................16 Planting Strategies ......................................................................................................................16 Implementation for coastal Douglas-fir regeneration .....................................................................17 Pre-regeneration .........................................................................................................................17 Regeneration ...............................................................................................................................20 Post-regeneration ........................................................................................................................21 Conclusions ....................................................................................................................................21   Page | 3 References ......................................................................................................................................23                  Page | 4 LIST OF FIGURES Figure 1:  BC Coastal Western Hemlock (CWH) and Coastal Douglas-fir (CDF) Zones ................................................................................................................... 6 Figure 2: Monthly average precipitation and temperature of Vancouver ................... 9 Figure 3: The strip shelterwood system .................................................................... 13 Figure 4: Decision making chart ............................................................................... 22    LIST OF TABLES Table 1: Relation between ground cover and first year seedling mortality .............. 10            Page | 5 Introduction When foresters intend to efficiently regenerate forests, they require an understanding of what growth conditions are suitable for target species to meet management objectives and how silvicultural systems and treatments modify site and stand conditions. The problem is how these two parts are connected to each other. Foresters need to aeesee what conditions are required for the growth of target species and what conditions silvicultural systems create for successful regeneration. The purpose of this paper is to provide suggestions for forest regeneration. This paper uses coastal Douglas-fir (Pseudotsuga menziesii var. menziesii) as an example to explore what silvicultural systems and methods can be used to achieve successful regeneration of this species in the British Columbia Coastal Western Hemlock (CWH) and Coastal Douglas-fir (CDF) Biogeoclimatic Zones. This paper reviews some relevant literature, describes the ideal growth conditions of coastal Douglas-fir, and recommends several silvicultural systems and methods that could benefit regeneration. Introduction to Coastal Douglas-fir Coastal Douglas-fir is native to North America. It is widely distributed in western coastal regions of Canada and the U.S., from British Columbia to California (NRCS, 2006; Figure 1). Coastal Douglas-fir as a commercial wood is highly valuable for building and constriction purposes. The wood is extremely hard, with relatively high bending and shear strength. The large dimension of old growth trees allows for the production of big pieces of lumber for heavy loading purposes as (Green et al, 1999). With all these characteristics, coastal Douglas-fir is a common species that lumber companies prefer for regeneration in coastal regions.   Page | 6  Figure 1:  BC Coastal Western Hemlock (CWH) and Coastal Douglas-fir (CDF) Zones (Izard, 1986)  What Are Silvicultural Systems Silvicultural systems are a key component of prescriptions for forest regeneration (BC Ministry of Forests, 1999). Silvicultural systems are stand-level long-term management plans for sustainable forest regeneration and production of desire goods, services and features. According to the unique stand conditions and objectives for a particular site, different systems of silviculture can be applied. A suitable system can make achieve management objectives for timber by providing optimal yield for the present stand and ideal stand conditions for future regeneration. Like an instruction manual, the silvicultural system guides the tending, harvesting and re-establishing phases of stand management (Helms, 1998). The particular system applied to an individual stand is based on the management objectives, species composition, and site conditions.    Page | 7 General Requirements for Coastal Douglas-Fir Growth In many cases of British Columbia, the goals for forest management are to maximize the stand yield and wood quality and provide long-term sustainable harvests (Forest Practices Branch, 2003). To meet these goals, certain stand conditions need to be met in terms of the requirements of tree growth. There are three major factors that will determine the forest regeneration: seed supply, seedbed and environment. These factors can be individually modified by silvicultural systems and management methods. So in the following paragraphs, stand conditions like soil condition, temperature, evaporation, and solar radiation will be discussed. Seed supply Both natural and artificial regeneration rely on high quality seed supply by parent trees. For natural regeneration, the quality of seeds depends on the properties of parent trees. If the parent tree is well adapted to stand environments and natural disturbance agents, it has better chance to pass relevant genetic materials to next generations through seeds. The quantity of seed is also important to natural regeneration. The cone production of coastal Douglas-fir is periodic. An observation in Western Washington and Oregon from 1909 to 1941 showed the average time between two abundant cone crops is 5 years, with minimum 2-year interval and maximum 11-year interval (Isaac, 1943). The periodicity of specific stand may variable. It needs long term observation to predict the abundant years. For direct seeding, high quality seeds are also preferred to achieve high survival rate and good wood quality of offspring. The seedfall distribution is another factor that should be considered during natural regeneration. Experiment did by Isaac, L.A. (1943) showed the dispersal distance of coastal Douglas-fir seeds. The seeds were released at tree heights and caught by traps   Page | 8 placed on the open ground. The results showed that about 55% of the seeds were caught within 61 meter (200 feet) from tree edge. About 95% of the seeds were caught within 244 meter (800 feet). It means that if leave an opening more than 244 meter away from the forest edge, there are little chance for forest succession. The distance between two edges should be calculated based on the seed production level, target forest density, and seed survival rate on target stand. Seedbed Seeds need a suitable medium for germination and further growth. When using direct seeding to regenerate coastal Douglas-fir, seeds prefer mineral soil seedbed that contains maritime sandstone and shale (Sharpe, n.d.). For the soil texture, regeneration is more successful on fine-textured than coarse-textured soil (Strothmann & Roy, 1984). Coastal Douglas-fir grows on clay loams also, but requires good soil drainage. Overly wet soil or flooding is not tolerated (NRCS, 2006).  Douglas-fir is tolerant of slight acidic soil, and the ideal soil pH range for growth is 5-7.5 (NRCS, 2006). The soil should contain adequate nitrogen, magnesium, phosphorus, potassium, and calcium. Direct seeding is an inexpensive way for Douglas-fir regeneration, especially for large areas. A study of Douglas-fir direct seeding in western Washington showed the germination rate and causes of seed loss and damage (Lawrence & Rediske, 1962). The overall seedling survival rate was 12.7% in the survived the first year. Other than those destroyed by animal activities and other undesirable environmental conditions, the lost seeds were attributed to molds (19.5%), fungi (9.1%), and insects (11.1%). The results indicate that the organisms in soil affect the germination and survival rates of direct seeding.   Page | 9 Seedling Establishment Environment Planting of seedlings is a more common way for establishing coastal Douglas-fir. Seedling establishment is an important phase of coastal Douglas-fir planting, because during this stage, the seedling is trying to get established and adapting the site environment. The seedling is vulnerable to the severe environment. By arriving at the site in good condition, the survival rate of the seedlings can be increased. A study had been done by Leo A. Isaac about the regeneration of coastal Douglas-fir at the Wind River Experimental Forest, Washington since 1924, where has the similar climate and geological environment with British Columbia coastal Douglas-fir regions (Isaac, 1938). The seedlings showed heat damage when the soil temperature was over 50?C during their first few weeks? establishment. Low temperature, which causes frost, can also cause severe damage to the young seedlings. During the study, when the air temperature dropped down to -2?C and the soil temperature was -5?C, frost happened and caused heavy losses. In terms of environmental of coastal Douglas-fir, the precipitation during summer is low and temperature is high in BC coastal regions. For example, in Vancouver, south coast of BC, the average daily maximum temperature increases since spring and reaches the peak in August at 22?C. The average precipitation decreases from March and is lowest in August at 50mm/month (Figure 2.).  Figure 2: Monthly average precipitation and temperature of Vancouver (Source: hellobc.com)   Page | 10 The coastal regions have a dry and warm climate through the growing season (April to October). As the temperature is high and the rainfall is low, the relation between evaporation rates and solar radiation is critical to the seedling establishment. Without sufficient water supply and exposed in sunlight would cause the seedlings wilt. So ground cover will benefit to the establishment of the seedlings as the roots are not well set underground. Table 1. shows the first year mortality of seedlings in the Wind River 6-year observations. With the average mortality 93% for no cover seedlings, dense shade reduced the mortality by nearly a half (43%) (Table 1.).      Amount of ground cover   Year   None Medium Dense Total 1928   92% 70% 20% 73% 1929  94% 86% 29% 86% 1930  100% 97% 80% 95% 1931  90% 57% 31% 66% 1932  91% 84% 24% 87% 1933   92% 80% 52% 84% Average   93% 79% 43% 83% Table 1: Relation between ground cover and first year seedling mortality (Isaac, 1943)  In spite of this need for shading during establishment on warm dry sites, coastal Douglas-fir is a shade-intolerant species. During the growing season, it needs enough overhead sunlight for competing with ground vegetation and other shade tolerant species like hemlock and red cedar. The seedlings under heavy canopy will soon die out and the   Page | 11 seedlings with moderate shade maintain a slow growth rate. Once the overhead shade was removed, the seedling recovered (Frothingham, 1909). At least 50 percent of overhead opening is recommended for the seedlings in their first growing season (Isaac, 1943). To sum up, the requirements of coastal Douglas-fir regeneration which can be modified by silvicultural systems include soil texture, drainage, pH, nutrient, and organism; range of stand level temperature; and solar radiation and evaporation influenced by ground shade and overstory canopy.  Introduction to Silvicultural Systems Silviculture is the combination of science and art used to regenerate and grow forests. A complete forest rotation includes harvesting, site preparation, regeneration, and stand tending. Silvicultural systems have different functions that can be used to deal with problems during these phases and to achieve specific stand structure characteristics. There are five main silvicultural systems used in forest management. Clearcut System The clearcut system is a harvest method where all trees in the target stand are cut at the designated rotation age. Clear-cutting creates even-aged stand by harvesting the whole stand in a single cut and promptly regenerating with new seedlings. The clearcut system is easy for application and very popular around the world. As clear cutting creates even-aged stand, it is suitable for managing pure timber stand with shade-intolerant species (Forest Practices Branch, 2003). The shape of the cutting areas can be irregular blocks or strips.   Page | 12 Patch Cut System The patch cut system also harvests all trees in the single cut. The difference between patch cut and clear cut systems is the size of the harvest area. If the harvest area is no more than one hectare, it is called patch cut system (Forest Practices Branch, 2003). Otherwise, it?s clear cut system. The small openings created by patch cutting are managed individually, which allows more site specific treatments to the target stands than in clearcut. Seed Tree System In the seed tree system, most of the trees in the stand are removed, except those with desired phenotypes. These remaining trees will provide good quality seeds for natural regeneration. For the purpose of maximize stand yield and wood quality, the seed trees should be prolific producers of seeds, healthy, disease resistant, windfirm, and with a good live crown ratio. The seed trees can be left individually or as a small group. The seed tree system relies on natural regeneration. Artificial regeneration can be applied when the natural regeneration doesn?t meet the ideal stock density or species composition (Forest Practices Branch, 2003). Shelterwood System In the shelterwood system some original trees are retained as shelter trees. These shelter trees provide cover for the next generation. The overstory canopy can intercept the sunlight and the deep roots prevent soil erosion. Shelter trees also act as seed trees that provide seed sources for the regeneration. When the new seedlings are established and the shelter is no longer necessary, the shelter trees will be removed for the uninhibited growth of new generation. There are a variety of shelterwood patterns (Forest Practices Branch, 2003).   Page | 13 ? Uniform shelterwood, means the left shelter trees are evenly distributed through the stand. ? Group shelterwood, is several cohorts of shelter trees remained together. It creates gaps between each groups ? Strip shelterwood, is left shelter trees within a strip. At the next harvest time, the next strip is also harvested using shelterwood system. Keep doing so during the following harvest period until the entire stand is harvested. The trees in the stand will have the gradually distributed ages (Figure 3.). ? Irregular shelterwood, is a classification based on timing. It has longer removal timing than the traditional shelterwood system. It creates a divers stand structures. ? Natural shelterwood, creates opening for the natural regenerated understory trees by removing overstory trees. It can only be used to the natural unmanaged stands. ? Nurse-tree shelterwood, intends to make a two-story, mix-species stand (Forest Practices Branch, 2003). The overstory is dominated by shade intolerant species which provides shade for shade tolerant species in the understory.  Figure 3: The strip shelterwood system (BC Ministry of Forests, 1999)   Page | 14 Selection System The selection system is the only uneven-aged system that truly creates uneven-aged stands. In this system part of the trees are harvested and/or thinned to provide opening for younger generations. By doing so, it creates consecutive age classes. Based on the selection methods, there are several variations for selection system (Forest Practices Branch, 1999). ? Single tree selection, selects same amount of individual trees in each size class. The purpose of doing so is to maintain stable stand structures. ? Group tree selection, selects a group of trees with same age class and creates opening for other age classes. ? Strip selection, is to harvest trees with in a strip. The strips should be perpendicular to the wind direction and the harvest sequence of the strips should be against the wind direction. The purpose of doing so is to prevent windthrow. Retention System In the retention system, more than half of the cutblock is within one tree height range of the retained trees (Forest Practices Branch, 2003). The trees can be retained individually or in groups throughout the stand. The purposes of the retention system are to reduce the influent of harvest and maintain forest structural diversity.  Forest Management Methods Within these silvicultural systems, there are also a few methods which are commonly applied in forest management activities. By directly modifying the stand   Page | 15 conditions, these methods create or maintain ideal environment which fits the management requirements. The paragraphs below list several common methods that can be applied during forest regeneration processes. Nursery Nursery is where seeds are specially selected and tested, and propagated to seedlings. There are two ways of planting seedlings: bareroot and container seedling. In bareroot seedling, seeds are sown to nicely prepared seedbed. Water and nutrients supply are controlled by growers. After intensive farming, seedlings are lifted, packed, and stored (Landis, 2001). In container nurseries, the seeds are placed in small containers. Seeds are typically germinated and grew in greenhouses. The environmental conditions are controlled by the grower to improve the survival rate of seeds, the growth rate, and the size, form and phenology of the resulting seedlings. Different conditions are given to seedlings to obtain various morphology characters (Landis, 2001). The key characteristics of stock size are seedling height and root collar diameter. Stocks with different height and diameter are produced by providing different growth conditions. Customers can purchase seedlings that meet their requirements based on the height: diameter ratio which leads to the best establishment of plantations on particular sites. Site Preparation Site preparation is the direct way to modify microsite conditions. It includes removing part or the entire forest floor, removing some vegetation, and modifying soil structure and ground profile. These methods can change the quantity and distribution of logging waste, reduce vegetation competition, improve solar radiation, and change soil nutrition, temperature and drainage (University of Florida, 2009). There are three major treatments of site preparation: mechanical, chemical and prescribed fire.   Page | 16 ? Mechanical treatment Mechanical treatment is using heavy equipment like excavator, skidder, crawler and forwarder to chop and remove ground vegetation and change soil structure and microtopography (University of Florida, 2009). ? Chemical treatment Chemical treatment uses herbicides to control pests and competing vegetation. By killing the pests and competitors, it can increase the survival and growth rates of target species (University of Florida, 2009). ? Prescribed fire Prescribed fire is a controlled burning of trees, logging waste, shrubs and brushed. By burning the vegetation, it can create space for new growth plantation and maintain the habitats for those organisms that depend on natural fire. Fertilization Fertilization is adding insufficient organic or inorganic material which vegetation needed for growth into seedbed to increase merchantable yield and value of forests (Forest Practices Code, 1995). The variety and quantity of fertilizer can be determined after evaluating species, stock density, site quality, forest health condition, stand age, and soil nutrient availability (Hanley et al, 2006). Fertilization helps trees grow better and faster. As a result, the timber yield increases. Planting Strategies A planting plan just indicates the target species, timing, and the location of management activities. Planting strategies show how the planting should be done.   Page | 17 Planting strategies include reduction of physical damage, selection of planting spots and pattern, determination of suitable planting density, and timing (Mitchell et al, 1990). The goal is to maximize the stock growing potential and give the best chance for the stocks. There are two ways to plant seeds or seedlings: manually and mechanically. Generally, Planting manually is time-consuming (costly) but results in high survival rates. Mechanical planting is faster (cheaper) than manually but leads lower survival rates. The method selection depends on the terrain of management areas. If the stand is too steep or too rough, manual way may be the only option.  Implementation for coastal Douglas-fir regeneration The coastal Douglas-fir regeneration requires a set of processes. These processes can be divided into three time periods: pre-regeneration, regeneration, and post-regeneration phase. Pre-regeneration In pre-regeneration phase, the goal for regeneration activities is to adjust stand environment to which meets coastal Douglas-fir growth conditions. These activities include silvicultural systems selection and site preparation. Before foresters decide to use which silvicultural system to target forest, they need to figure out if they want natural or artificial regeneration. There are lots of factors that affect the selection of natural or artificial regeneration, which are beyond the scope of this paper. Once the regeneration method is decided, relative silvicultural system can be applied.   Page | 18 For natural regeneration stand management, seed tree system is commonly used. 8 coastal Douglas-fir seed trees per acre retained singly or in group can regenerate most of the stand after logging in a 10-year period (Isaac, 1943). To improve the coastal Douglas-fir yield and quality, the retained trees should be mature, strong and dominant coastal Douglas-fir, with good growth form, prolific in recent years, and immune to local diseases and insects. The density of seed trees is based on the productivity and survival rate of parent trees, germinating rate of seeds, and ideal stand density. As the seed tree system remains trees with desired morphological features, the genetic codes of the phenotypes can be passed through seeds to future generations. The retention system can also be used for natural regeneration. D?Anjou (2002) did a study in Roberts Creek Study Forest, British Columbia. Compared with the unlogged stand, the study showed an increased Douglas-fir cone crop in dispersed retention treatment during the 8-year study period. There were two harvest entries (3-7 years interval), the first retained a density of 75-95 stem/ha and the second 20-30 stem/ha. However, the retained trees should be removed once the seed germinated and start to get established, otherwise shade-tolerance species could become the dominated species among the next generation. Shelterwood systems can provide protective cover for seedlings when the site conditions are unfavourable to coastal Douglas-fir. Williamson (1973) used shelterwood system in a high elevation area of the Oregon Cascades in severe site conditions. The results showed that retaining 25 to 50 percent of the original basal area with site preparation leaded to satisfactory stocking. Shelterwood can prevent frost, excessive insolation, and maintain surface temperature (Curtis et al, 1998).  For artificial regeneration, clearcut system is the dominant system in the coastal Douglas-fir region. As coastal Douglas-fir is a shade-intolerant species, clearcutting is a   Page | 19 suitable silvicultural system for its regeneration on sites without severe conditions. Clearcuts can maximize sunlight and open ground. Open ground helps the soil heat up more quickly in the spring. Also, the clearcut system is easy to apply and economical during regeneration activities (Forest Practices Branch, 2008). Clearcutting has significant effects to control the spread of Douglas-fir dwarf mistletoe, pests like defoliators and bark beetles, and pathogens like wood and root decay fungi. Clearcut system provides easy access for site preparation and manual or mechanical sowing seeds and planting seedlings. After the silvicultural system is determined and harvest activities have been done, the logging site may need to be modified to fulfill coastal Douglas-fir regeneration conditions. Removing the logging slash can help to create a uniform seedbed and improve access for machines and planters. The logging slash can be removed by methods like machine-pile-and-burn, which using bulldozer with brush blade to pile the slash and later burned (Piatek et al, 2003). Logging slash and vegetation can also be removed by prescribed fire if the stand slope is too steep for machines. Burning requires knowledge of the behaviour and consequences of fire, so how to burn should be carefully decided (Strothmann, 1984). Prescribed fire can also adjust the soil pH value (Smith, 1970). As coastal Douglas-fir prefers soil pH range 5-7.5, the ashes after combustion can reduce the acidity of acid soil. The availability of nutrients like nitrogen and phosphorus of soil will also be improved after burning. The soil can also be manipulated for the better regeneration of Coastal Douglas-fir. Mechanical scarification is the method that exposes mineral soil by a tractor with a blade, so the rooting is easier for new-established seedlings (Piatek et al, 2003). Mounding is another method to improve the microsite condition. Mounds are plowed in rows by plow.   Page | 20 The seeds/seedlings can be planted on the top of the mound, where the soil drains faster (University of Florida, 2009).  Regeneration Direct seeding of coastal Douglas-fir is becoming less common nowadays as the uncontrollability due to the damage from small mammals and birds and irregular spacing of stocks. However it is still an option during coastal Douglas-fir forest management. The most efficient way to sow seeds in large areas is aerially broadcast seeds on mechanically prepared or clean burned sites (Strothmann, 1984). The seeds should be treated to reduce animal damage. The coat of the seeds is often treated with repellent. Compared to direst seeding, planting seedlings is more reliable. In the nursery, appropriate moisture, temperature, and solar conditions are provided to the seed germination and seedling establishment. Seedlings with different height and root diameter are offered to the market. Foresters can do experiments to find out the best seedling height and root diameter which are optimal for particular site types. Then they can order the seedling from nurseries with required seedling morphology. Big coastal Douglas-fir seedlings are more expensive but are more likely to survive (Piatek et al, 2003). Seedlings should be carefully handled and stored from nursery to planting site to maintain the vigour. The planting strategy can also improve survival rates of seedlings. Planters should put roots in right position when planting seedlings. The planting spot selection is critical to seedling establishment. Places like the top of mounds are usually preferred because they are well drained and accumulate heat fast. Planting density and pattern are determined based on stand aspect, slope, and capacity. Spot fertilization is also an option for seedling to outcompete undesirable vegetation.   Page | 21 Post-regeneration After the seeds germinated and seedling established, the new generation needs an unencumbered growth condition.  There are three aspects that can be managed: vegetation competition, animal damage, and crown closure condition. For vegetation competition, mechanical and chemical methods mentioned in site preparation should be applied during several years after regeneration phase if vegetation competition occurs. Understory vegetation is a competitor to coastal Douglas-fir seedlings. Ground vegetation may intercept sunlight and consume nutrients. Ares et al. (2007) showed that five-year control of competing vegetation by herbicide improved coastal Douglas-fir growth in coastal Washington Animal damage could also present after the planting. For example, deer browsing is a common problem for seedling. Placing fences, netting, and polyethylene sleeves can be used to set barriers to these animals (Strothmann, 1984). Using bigger seedlings is another way to reduce the animal damage.  Conclusions To regenerate coastal Douglas-fir with good quality and high yield, foresters should take advantages of different silvicultural systems and management methods to adjust stand conditions which don?t match coastal Douglas-fir growing habitat. The decision making chart below (Figure 4.) concludes what silvicultural systems and methods can be applied to deal with problems occurred in different stages of regenerate coastal Douglas-fir. By following this chart, the regeneration activities can be applied in right order and factors which related to improve coastal Douglas-fir regeneration can be   Page | 22 considered. However, more studies should be done in the future to find out whether or how the results regeneration activities can be affected by stand history like fire, windthrow, pathogens, and pest.  Figure 4: Decision making chart   Page | 23 References Ares, A., Terry, T., Harrington, C., Devine, W., Peter, D., & Bailey, J. (2007). Biomass removal, soil compaction, and vegetation control effects on five-year growth of Douglas-fir in coastal Washington. Forest Science, 53(5), 600-610. Curtis, R. O., DeBell, D. S., Harrington, C. A., Lavender, D. P., St.Clair, J. B., Tappeiner, J. C., & Walstad, J. D. (1998). Silviculture for multiple objectives in the Douglas-fir region. (p. 45). U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. (PNW-GTR-435). D?Anjou, B. (2002). Roberts Creek Study Forest: Harvesting, windthrow and conifer regeneration within alternative silvicultural systems in Douglas-fir dominated forests on the Sunshine Coast. BC Ministry of Forests. Forest Service. Vancouver Forest Region. (TR-018). Forest Practices Code. (1995). Forest fertilization guidebook. Minister of Forests, Retrieved from: http://www.for.gov.bc.ca/tasb/legsregs/fpc/fpcguide/fert/ferttoc.htm#top Forest Practices Code. (1995). Pruning guidebook. Minister of Forests. Retrieved from: http://www.for.gov.bc.ca/tasb/legsregs/fpc/fpcguide/pruning/pruntoc.htm Forest Practices Branch. BC Ministry of Forests. (1999). Introduction to silvicultural systems. (2nd ed.). Retrieved from: http://www.for.gov.bc.ca/hfp/training/00014/index.htm Forest Practices Branch. British Columbia Ministry of Forests. (2003). Silvicultural systems handbook for British Columbia. (Part 2.1, p. 5, 7, 9). For. Pract. Br., BC. Min. For., Victoria, BC. Retrieved from:   Page | 24 http://www.for.gov.bc.ca/hfp/publications/00085/silvsystemshdbk-web.pdf Forest Practices Branch. BC Ministry of Forests. (2008). Silvicultural systems in British Columbia: clearcutting, seed tree, shelterwood, and selection. Retrieved from: http://www.for.gov.bc.ca/hfp/publications/00205/ Frothingham, E. H. (1909). Douglas fir: a study of the pacific coast and rocky mountain forms. (p. 38). U.S. Forest Service. Green, D. W., Winandy, J. E., & Kretschmann, D. E. (1999).Mechanical properties of wood. (p. 2). Hanley, D. P., Chappell, H. N., & Nadelhoffer, E. H. (2006). Fertilizing coastal Douglas-fir forests. (p. 8). University of Washington. Retrieved from: http://cru.cahe.wsu.edu/CEPublications/eb1800/eb1800.pdf Hellobc.com. Monthly average precipitation and temperature of Vancouver. (n.d.). Vancouver, coast & mountains climate & weather. [Web Graphic]. Retrieved from?http://www.hellobc.com/vancouver-coast-mountains/Climate-weather.aspx Helms, J. A. (1998). The dictionary of forestry. (p. 210). Society of American Foresters: Bethesda, US (MD). Isaac, L. A. (1938). Factors affecting establishment of Douglas-fir seedlings. U.S. Department of Agriculture. Isaac, L. A. (1943). Reproductive habits of Douglas-fir. (p. 28-34, 40, 69, 96). Washington, D.C.: U.S. Forest Service. Izard, D. Tree book: Learning to recognize trees of British Columbia. (p. 176). Canadian Forest Service.   Page | 25 Landis, T. (2001). Introduction to forest nursery management. USDA Forest Service. Retrieved from?http://www.forestryimages.org/series/viewseries.cfm?ser=31 Lawrence, W. H., & Rediske, J. H. (1962). Fate of Sown Douglas-fir seed. Forest Science, 8(3), 210-218. Mitchell, W. K., Dunsworth, G., Simpson, D. G., & Vyse, A. Ministry of Forests, BC. (1990). Regenerating British Columbia?s forests. (Chap. 4, p. 237-246). Vancouver: University of British Columbia Press. Retrieved from: http://www.for.gov.bc.ca/hfd/pubs/docs/mr/mr063.pdf NRCS, PLANTS Database, United States Department of Agriculture. (2006). Distribution: Pseudotsuga menziesii (mirb.) franco var. menziesii. Piatek. K.B., Harrington, C.A., & DeBell, D.S. (2003). Site preparation effects on 20 year survival and growth of Douglas-fir (pseudotsuga menziesii) and on selected soil properties. (p). Western Journal of Applied Forestry, 18(1). Retrieved from http://www.fs.fed.us/pnw/olympia/silv/publications/opt/467_PiatekEtal2003.pdf Sharpe, K. (n.d.). Douglas-fir habitat requirements. Retrieved from: http://www.ehow.com/info_8123249_douglas-fir-habitat-requirements.html Smith, D.W. 1970. Concentrations of soil nutrients before and after fire. Can. J. Soil. Sci. 50, 17-29. Strothmann, R. O., & Roy, D. F.  U.S. Department of Agriculture, Pacific Southwest Forest and Range Experiment Station, Forest Service. (1984). Regeneration of Douglas-fir in the Klamath Mountains region, California and Oregon (Gen. Tech. Rep. PSW-81.). Retrieved from: http://www.fs.fed.us/psw/publications/documents/psw_gtr081/psw_gtr081.pdf   Page | 26 Thienen, F. V. Minister of Forests, Lands and Natural Resource Operations, Research, Innovation and Knowledge Management Branch. (2007).Commercial thinning. Retrieved from website: http://www.for.gov.bc.ca/hre/standman/trtct.htm University of Florida. (2009). Site preparation. Retrieved from: http://www.sfrc.ufl.edu/extension/florida_forestry_information/forest_management/site_preparation.html Williamson, R.L. (1973). Results of shelterwood harvesting of Douglas-fir in the Cascades of western Oregon.(p. 13). Res. Pap. PNW-161. Portland, OR. U.S. Department of Agriculture, Forest Service, Pacific Northwest Forest and Range Experiment Station. 

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