The Silviculture of Hybrid Poplar Plantations

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  The objective of this extension note isto provide general information on hy-brid poplar management, highlightingkey areas and topics to consider whenestablishing a poplar plantation. Introduction As demand for wood products andfibre increases, forest managers facethe challenge of growing more woodon less land. Shorter rotations and di-versification of products may helpmitigate this pressure.Hybrid poplars grow quickly. Therotation age ranges from 10 to 26 yearsin British Columbia depending on anumber of factors, including climate,site, and management regime. Themore intensive the management, theshorter the rotation.Hybrid poplar is used primarily forpulp, but can also be used for lumberand plywood. Markets for the latterproducts are developing and need to be encouraged. Non-wood uses include ornamental plantings, soil sta-bilization, and conservation. In BritishColumbia, hybrid poplar plantationsare being grown in the interior, andon the south coast, mid-coast, andVancouver Island. Site Selection Hybrid poplars have a high demandfor moisture and nutrients. Poplarsprefer moist, well-drained sites, andgrow well on fluvial floodplains where water and nutrients are abundant. They  are also tolerant of periodic flooding,especially in the winter when they arenot actively growing (Figure 1). Bestgrowth and the shortest rotationsoccur on the south coast of BritishColumbia, where there is a long grow-ing season and ample rainfall. Extension Note 47 The Silviculture of Hybrid PoplarPlantations M ARCH 2000 Keith D. ThomasPhilip G. ComeauKevin R. BrownB.C. Ministry of ForestsResearch BranchP.O. Box 9519, Stn. Prov. Govt. Victoria, BC V8W 9C2(250) Ministry of Forests Research Program   Salmon left in poplar plantationfollowing periodic flooding of a river, near Crofton, B.C.   The colder and drier the climate,the longer the rotation age. However,given adequate water and nutrientsthrough irrigation, poplar grows wellon coarse soils and in drier climates inBritish Columbia. Frost and wintercold are factors to consider becausethey can severely damage hybridpoplar trees. Frost-tolerant hybridsshould be grown in these colder areas. Site Preparation Site preparation is critical to success-ful establishment of hybrid poplarplantations, and has a significant impact on the productivity and subsequent rotation age. In general,productivity increases with site prepa-ration by reducing competition fromnon-crop species during the establish-ment phase of the plantation.The intensity of site preparationdepends on several factors, includingthe zoning of the land (agriculture orforestry), accessibility, topography,soil type, and drainage. The zoningwill determine the appropriate type of site preparation. For example, landclassed as forest will be subject to dif-ferent regulations than agriculturalland, as well as to different environ-mental constraints with respect to soildisturbance. Flat and accessible siteshave the greatest number of optionsfor mechanical site preparation, andremote and/or difficult topography the least. Soil type (clay, loam, orsand) and drainage determine the typeof equipment to be used (if any), andthe appropriate time of year for sitepreparation. In general, these factorsdetermine the choice of managementregime. Management Regime On the south and mid-coast of BritishColumbia, rotation ages are 10–26 years, depending on the managementregime and site productivity. Poplarare grown from unrooted stem orbranch cuttings (45 cm long, with abasal diameter of 10–20 mm) or un-rooted whips (cuttings 1.0–2.0 m long,with a basal diameter of 10–20 mm).These cuttings and whips are harvest-ed and processed during January andFebruary and placed in cold storageuntil planting in March and April. It iscommon practice to obtain whipsfrom nurseries. However, cuttings andwhips can be obtained from existingstands or during harvesting opera-tions.Cuttings are hand-planted with thebasal end inserted deeply into the soil,leaving about 20 cm showing abovethe soil surface. A planting bar (dib-ble) is typically used to create plantingholes. When long whips are used, they are planted to a depth of at least40 cm. Whips and cuttings are gener-ally planted at 3 m or wider spacing,with total densities ranging from 600to 1100 trees/ha. During the first year,poplar will grow about 1.5 m in height,and from 3 to 5 m for each of the fol-lowing 5 years. The target piece size isusually 32–40 cm diameter at breastheight (Figure 2), and 24 m long witha 10-cm top. Short rotation intensive culture(  ) High-intensity management of hybridpoplar is often referred to as “ShortRotation Intensive Culture (  ).”In south coastal British Columbia, arealistic rotation age for this type of management is 10–12 years with yieldsof approximately 300–400 m 3 /ha.Sites suitable for this regime are pro-ductive land with easy terrain andaccess that are close to urban centresand mill facilities (e.g., lower FraserValley, Vancouver Island, parts of southern interior).This management regime involves pre-planting site preparation to remove competing vegetation. In order tomaximize tree growth and reduce voledamage, vegetation management is re-quired for the first two to fourgrowing seasons, or until canopy clo-sure. There are several tools availableto the poplar manager that can beused alone or in combination. Theseinclude cultivation (disking, furrow-ing), herbicides (e.g., glyphosate),mulches (e.g., polyethylene mats), andfertilization. Fertilizers are usually ap-plied during the third growing season.In the western United States,  plantations are irrigated with nutrientsolutions on a regular basis and haverotation ages of 5–8 years. Medium-intensity management Medium-intensity management is best suited to productive lands thathave easy terrain, but that may haveconstrained access. For example, theislands at the confluence of theHarrison and Fraser rivers have limit-   Eight-year-old hybrid poplar (TxD) near Kilby, B.C.  ed access during part of the year whenwater flow is high, but, during periodsof low flow, vehicular access to someislands is possible.This form of management is lessintensive than  and has a rota-tion age of 15–20 years with potential yields of 300–400 m 3 /ha. The only realdifference between the two is theamount of vegetation managementduring the first 3 years. There is usual-ly some form of pre-planting sitepreparation to reduce weed competi-tion during the first growing seasonand then some basic vegetation con-trol (mowing) or selected vegetationremoval around trees (1 m 2 area). Themain purpose of vegetation manage-ment in this regime is to reduce treegirdling by voles and to reduce com-petition from non-crop vegetation.Fertilization, if done, is usually withlow-cost or free organic wastes such as manure, fish compost, or biosolids(Figure 3). Low-intensity management The low-intensity managementregime is best suited to sites that havedifficult terrain and/or access (e.g.,mid-coast, Bute and Kingcome inlets)(Figure 4). This type of managementis often used on forest land that wasclearcut. As a result, larger whips areused so that post-planting vegetationcontrol is unnecessary. This is becausethe return from the crop is usually notrealized for 25–30 years followingplanting, and, as a result, forest man-agers need to minimize the number of entries prior to harvest. Potential yields for this management regimecould range from 250 to 450 m 3 /ha.There is usually no site preparationnor weed control, and inputs prior toharvest are kept to an absolute mini-mum. Clonal Selection Hybrid poplars are crosses betweendifferent species of poplar. There aremany crosses, but Populus trichocarpa  x P. deltoides  , often referred to as TxD(TD cross), and Populus trichocarpa  x  P. maximowiczii  , often referred to as TxM (TM cross), are two of themore common ones used in BritishColumbia. Within each cross there aremany clones that have been selectedfor various growth characteristics, in-cluding frost hardiness, diseaseresistance, drought resistance, stemform, and branching. Some geneticwork is currently under way to createpoplar clones that are resistant toglyphosate herbicide.    Biosolids being applied to a 4-year-old medium-intensity hybrid poplar planta-tion on an island at the con fl  uence of the Harrison and Fraser rivers, BritishColumbia.   Newly planted hybrid poplar plantation in Kingcome Inlet, British Columbia.  Planting material (Figure 5) isavailable from a number of sourceswithin British Columbia. It is best touse only those clones that have beenproven adaptable to the area. For ex-ample, in areas where Venturia  spp.(shepherd’s crook) may be an issue,planting clones resistant to this funguswould be beneficial. Companies andforest managers who are currently growing poplar in these areas shouldbe consulted in order to benefit fromtheir experience. Vegetation Management Hybrid poplars are sensitive to com-petition from non-crop vegetation,especially grasses, for resources otherthan light.  and medium-intensi-ty culture often require vegetationmanagement for the first 3–4 years, oruntil canopy closure. Vegetation man-agement improves tree growth andminimizes damage from rodents, suchas voles, which girdle the root collar(Figure 6).However, in low-intensity manage-ment regimes, vegetation manage-ment may not be an option due to siteconstraints or site access. In these in-stances it is recommended that larger(> 1.5 m) whips be planted to give thepoplars additional competitive advan-tage.Vegetation management tools in-clude manual weeding, mowing,mechanical cultivation, herbicide, andmulching. Numerous studies havedemonstrated the benefits of vegeta-tion management and its positiveeffect on poplar growth. Manual weeding  Manual weeding is labour-intensiveand expensive. It is best suited tosmall, privately managed plantationswhere forest managers do not chargefor their labour, or to plantationswhere there is proven damage fromvoles and/or competition from non-crop vegetation, and where it is notpossible to get cultivation equipmenton site or to apply herbicides. Mechanical cultivation Mechanical cultivation (disking, fur-rowing) between plantation rowsprovides some control of competingvegetation, and tree growth is greaterthan in non-cultivated areas. A study by Thomas et al. (2000) found thatthe vegetation remaining when theland was cultivated in only one direc-tion significantly reduced tree volume(Figure 7).Due to physical limitations of culti-vation machinery and the need tominimize tree damage, the area im-mediately around each tree is leftuncultivated and the surroundingvegetation competes with poplarwhips for soil moisture and nutrients.In addition, mechanical cultivationcan be constrained by terrain and may cause soil compaction of finer-tex-tured soils. Another consideration isthat the shallow feeder roots of thepoplar can be severed with each passof the cultivator, which can have anegative effect on tree growth. Onsites with easy access and terrain, cultivation costs are approximately $100/ha per treatment. It is commonpractice to cultivate twice yearly     Nursery stock being grown inthe Fraser Valley, BritishColumbia.   Four-year-old hybrid poplar girdled by voles.
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