窄冠黑杨窄冠黑白杨的选育（Ⅱ）木材性质的研究

研究证明所育成的窄冠黑杨、窄冠黑白杨心材的比例小，颜色浅，有利于提高制浆的白度，木材的基本密度随树高而增大，木材纤雏长度与树高的关系不大，但随年龄的增大而增长，纤维长度5年生时基本稳定，可达1mm。因此，为提高制浆质量，以5年生以上采伐利用为好，为提高树木的经济价值，最好10年生采伐利用，4m以下做胶合板材，4m以上制浆造纸。     

TWO-STORIED HIGH FOREST

The system has been adopted at Dartington with Japanese larch(Larix leptolepis), and it has been suggested that it worksthere because Japanese larch grows fast. It is argued in thispaper, however, that the system is best adapted to rather slowlygrowing trees, and the reason for its use with Japanese larchis that this species is characterized by a rapid height growthbut a small current annual increment, so that, unless the treesare isolated, they make extremely narrow annual rings. WithDouglas fir (Pseudotsuga taxifolia) and Sitka spruce (Piceasitcbensis) the annual rings might become too broad if thistreatment were applied, except at a rather advanced age. Nevertheless,the system has certain other advantages and, to attain these,it may be useful to adopt it more extensively, even if the annualrings become broader than we like.     

THE EFFECT OF RATE OF GROWTH (RING WIDTH) ON THE DENSITY OF SOFTWOODS

The relation between rate of growth and density has been examinedin fairly wide-ringed Douglas fir (Pseudotsugo taxifolia) andCorsican pine (Pinus nigra var. calabrica) grown in the southof England. A comparison between wood formed early in the lifeof the tree and wood of similar ring width formed later hasshown that the latter is invariably denser, indicating thatthe higher density of older wood is due to an age effect ratherthan to diminishing ring width. It is concluded that after acertain age rapid growth can produce wood of reasonably highdensity. This confirms that Turnbull's findings on widely spacedexotic softwoods grown in South Africa apply, with reservations,to trees grown in this country under conditions where rapidgrowth can be maintained.     

A Possible Indicator of Shake in Oak: Relationship between Flushing Dates and Vessel Sizes

Previous work by Savill (1986) has shown that oak trees (Quercusrobur and Q. petraea) with large earlywood vessels appear tohave a much greater predisposition to shake than trees withsmaller vessels. The aim of the investigation described herewas to determine whether there are any externally visible characteristicscorrelated with vessel size, so that shake-prone trees can berecognized in the field. It was found that trees that flushlatest within a population tend to have the biggest vessels.Possible reasons for this correlation are discussed briefly.The finding has the practical value that oaks which are predisposedto shake can be marked at flushing time and removed in thinningoperations.     

History of Peridermium Stem Rust of Scots Pine (Pinus sylvestris L.) in Thetford Forest, East Anglia

‘Peridermium’ stem rust (Peridermium pini (Pers.)Lev.) has been present in Thetford forest for at least 40 years,but has only become a major problem during the last decade.Survey data are presented which show that there has been a dramaticincrease in the disease in crops of Scots pine between 1964,when with less than one per cent of the trees showed symptoms,and 1979, when the figure was 10 per cent. The disease has apparentlyspread outwards from a central focus in the middle of the forest.In four plots the proportion of trees with visible symptomshas increased in five years from an average of 28 per cent to46 per cent. However, only 1–2 per cent of the trees havedied annually, and it appears that many trees with ‘deadtops’ may survive for long periods. The situation in Thetfordseems to contrast with that in north-east Scotland, where limiteddata suggest there has been no appreciable increase in diseaseincidence during recent years. There are only two records ofthe disease on Corsican pine (P. nigra var. maritima (Ait.)Melville) in Thetford.     

An Analysis of Peridermium Stem Rust of Scots Pine in Thetford Forest in 1984 and 1985

The status of stem rust (Peridermium pini or (Pers.) Lev.) inScots pine (Pinus sylvestris L.) in the central block of Thetfordforest, East Anglia was determined by means of detailed analysisof freshly felled trees. A total of 935 trees was examined,these comprising c. 100 from each of nine compartments plantedbetween 1922 and 1933. Lesions in all stages of development were found; from thosein their first year of sporulation on young twigs to those whichhad been established for many years on the trunk and main branches.Death of cambium occurred typically on lesions on shoots thatwere more than five years of age. Downward growth of the funguswas estimated to average between 4.7 and 5.6 cm per year, whenbased on the length of sporulating tissue on young twigs, and6.3 cm per year when based on cambial killing in trunk lesions.The average rate of circumferential killing of trunk lesionsaveraged 4.3 crn per year (i.e. 2.2 cm in each direction). Therewas no evidence that wounds provided a significant infectioncourt. A strong association was found between the presence of the ‘purplemould’ (Tuberculina maxima) and an absence of sporulationby the rust fungus. However, there was no evidence for lesioninactivation by T. maxima. Data are presented on the distribution of disease between sitesand between trees. Five per cent of the total sample of treeswere dead and an additional 12 per cent had crown symptoms associatedwith girdling trunk lesions. Among the 775 trees with predominantlyhealthy crowns, it is estimated that 9 per cent would have diedwithin 5 years, 15 per cent within 10 years and 22 per centwithin 20 years. By contrast 66 per cent had no disease at all,probably for genetic reasons. It is concluded that, althoughthe disease situation is serious, there is no requirement fora major change to be made in the felling programme for the pre-warScots pine in Thetford.     

A Reappraisal of the Importance of Cronartium ribicola on Pinus strobus in Great Britain

Because of the possibility of serious infection by Cronartiumribicola (White pine blister rust), Pinus strobus, the Weymouthpine, is now very rarely planted in British forests. An assessment,in 1962, in a number of experimental forest plots of P. strobusestablished in 1953 and 1954 showed that initial infection ofthe trees while growing in the nursery may be an important factorin causing later damage and losses in the forest. The resultsalso indicated that the risk of infection in plantation cropsas a result of spore dispersal from black currants in the localityis much lower than is generally thought. It is suggested thatif P. strobus is raised in nurseries which are relatively isolatedfrom currants and if protective chemical spraying against thedisease in the nursery is carried out, the incidence of infectionin plantations at least half a mile from the nearest currantsis likely to be low. The disease cannot spread within a pinecrop by direct transmission from tree to tree.     

THINNING GRADES BASED ON THE THICKNESS OF ANNUAL RINGS

This paper propounds a new method of calculating numerical thinningschedules for conifers which are designed to give, on the average,any reasonable pattern of ring width at breast height in the100 largest trees in a plantation. It starts from the ForestryCommission's Revised yield Tables for Conifers in Great Britainand is based on the principle that, if the current annual incrementin a plantation is unaffected by the grade of thinning, thering widths must be inversely proportional to the bole areaper acre, i.e. the total area of cambium on the main stems ofthe trees. Sources of error in the method of calculation arediscussed, and it is considered that such errors are not soserious as to endanger the use of the method as a means of calculatingapproximate thinning schedules. As an approximate yield tablecan be constructed for each thinning grade, the method can beused to estimate the relative costs of growing trees of variousdiameters with various ring width patterns.     

Root anchorage and stem strength of black spruce (Picea mariana) trees in regular and irregular stands

A tree winching experiment was conducted, simulating wind actionand resulting damage, in order to assess mechanical resistanceof black spruce (Picea mariana (Mill.) B.S.P.) trees in standsof regular and irregular structures. The critical bending moment(M_c) of winched trees was determined and the relationship betweenM_c and tree and stand characteristics was investigated throughstatistical analyses. Stem mass (SW), average spacing betweentrees (S) and height:diameter ratio (H/d.b.h.) were initiallyselected by a screening procedure to predict M_c. Potential differencesbetween stand structures were tested in mixed models using differentsubsets of the data. Included fixed effects varied between thedifferent models. SW was the most useful and significant variablein all models and H/d.b.h. was significant only when snappedtrees were included in the analysis. When decayed samples wereexcluded, resistance to uprooting was higher in irregular stands.Decay seemed to play an important role in irregular stands andshould be investigated further. Since no difference was observedin the relationship between stem mass and critical turning momentbetween distant sites, relationships should be applicable acrosswide regions. This study provides some of the basic relationshipsrequired to model windthrow risk in irregular stands. However,the effects of stand structure on wind load for individual treeswill also need to be considered.     

The effect of heavy or 'free growth' thinning on oak (Quercus petraea and Q. robur)

Free growth is a type of heavy thinning which aims to maximizediameter increment and produce valuable timber on a relativelyshort rotation. An experiment is described which investigatedthe application of free growth to a stand of oak {Quercus petraeaand Q. robur) planted in 1930. At age 58 free growth thinninghad resulted in a mean diameter at breast height of 39.0 cmwith an estimated mean tree volume of 0.98 m³, compared with29.3 cm and 0.52 m³ for equivalent crown thinned trees. Discountedcash flow calculations showed that free growth thinning of oakcould be justified using a 3 per cent discount rate assumingthat the increased intensity of pruning results in a large proportionof veneer quality timber; an independent assessment indicatedthis may be possible. Free growth thinning of oak is not a commonpractice in British broadleaved silviculture probably becauseof the cost of controlling epicormic shoots. It is suggestedthat it may be more appropriate to other species such as ash,sycamore and wild cherry.     

The Ecology of Armillaria mellea Vahl (ex Fries) in the Indigenous and Exotic Woodlands of Rhodesia

A. mellea is the cause of severe killing in young plantationsof Pinus elliottii in Rhodesia. P. patula is more resistantthough it may act as a carrier of the fungus; its resistancemay be overcome when planted in conditions of high inoculumconcentration such as clear-felled areas of P. elliottii whichhave a previous disease history. The source of infection isthe stumps of felled indigenous trees. Subsequent spread inthe pine is from tree to tree by root contact. The rate of increasein the incidence of the disease up to thirteen years from plantinghas been plotted. An initially logarithmic increase of killingshows a significant decline when the plantation is eight toten years of age. Viable basidiospores of A. mellea are presentin the air spora but little evidence of their importance inthe spread of A. mellea could be found.     

Stem deformation in young plantations of black spruce (Picea mariana (Mill.) B.S.P.) and jack pine (Pinus banksiana Lamb.) in the boreal forest of Quebec,Canada

Stem deformation has often been observed in young black spruce (Picea mariana (Mill.) B.S.P.) and jack pine (Pinus banksiana Lamb.) plantations. Whenever important stem deformations are observed at the time of harvesting, timber value is negatively affected especially during the wood transformation process. The present work was undertaken to quantify and qualify the importance of stem deformation of black spruce and jack pine in the boreal forest of central Quebec at the stand and tree levels. In 30 black spruce and jack pine plantations, approximately 22% of spruce trees and 27% of pine trees exhibited stem deformation. The proportion of deformed trees was higher in the youngest plantations and decreased with the age of the plantations. Stem deformation caused the formation of compression wood which is another factor that can reduce the value of wood products. Thirty-nine black spruces and 34 jack pines were analysed at the tree level. On average, compression wood represented 14% and 20% of stem volume in 7- and 10-year old black spruce plantations, respectively. These proportions ranged from 18% in the youngest jack pine plantation to 26% in the oldest one. Stems of both species classified as normal contained a lower volume of compression wood than stems classified as deformed or very deformed. Annual percentages of compression wood and annual shoot length increased significantly with tree age (p < 0.0001 for both variables). Statistically significant correlations were also found between the range of displacement of the stem and the percentage of compression wood. The fewer number of trees with deformed stems in older plantations combined with high compression wood formation suggests that, over time, a deformed tree can become normal and straight in appearance.     

A Note on the Dimensions of Shoots and Roots of Planting Stock

Published figures suggest that in Pinus and Picea spp. plantingstock raised in nurseries the top/root ratio increases fromthe first year onwards. Transplanting checks growth in heightthe more severely the longer the time that has elapsed sincethe last transplantation, but growth in weight and diameteris much less affected, so that the shoot of a transplant isstouter and heavier than that of a seedling of the same height.On the other hand, growth of the root, both in length and weight,is encouraged, so that the top/root ratio of a transplant ismuch less than that of a seedling of the same age. In seedlings of broadleaved trees top/root ratio is less thanin Pinus and Picea, and usually decreases for 2 or 3 years. The top/root ratio and dimensions of the plants are also subjectto some individual variation which might be either inherentor due to external circumstances, including espacement. Informationon such variation is too scanty and inconsistent to supportany generalizations, except that diameter of shoot increaseswith increasing espacement.     

Timber Improvement: I. A Study of the Variation in Wood Characteristics in Young Sitka spruce

As part of a programme aimed at improving Sitka spruce timber(Picea sitchensis (Bong.) Carr.), a study has been made of thevariation in some wood characteristics of significance in affectingtechnological behaviour, and the effect on these characteristicswhen trees of outstanding form and vigour are selected. It wasfound that:

1. There is a similar pattern of within-tree variationin density,tracheid length, and grain inclination for bothselected andaverage, final crop trees.
2. There is a very appreciablevariation in these timber characteristicsbetween trees at comparableages, indicating the potential forimprovement by selection.
3. There is a reduction in density and some increase in graininclinationbut no difference in tracheid length in vigorouscompared withaverage-growth trees at comparable ages.
4. Thereis no evidence that selection for above average densityhasan adverse effect on tracheid length; it tends to favourtreeshaving a high minimum early wood density and, to a limitedextent,wood with narrow cells.

     

Beech in Wessex - a Perspective on Present Health and Silviculture

The condition of beech (Fagus sylvatica L) growing in thirtywoods in the south of England is described. In older woods,up to 20 per cent of trees were killed or seriously damagedby the summer drought of 1976. Most surviving trees have recoveredfully. Beech snap disease, associated with the fungus Nectria coccineais widespread in younger stands, especially those also affectedby lime-induced chlorosis, beech scale (Cryptococcus fagisuga)or severe competition. Nevertheless, sufficient trees surviveto provide a choice of final crop. In the long term, beech doesnot seem likely to die out through drought or disease. Trees of almost all ages and conditions exhibit a tendency tomultiple forking. The principle remedy is to keep trees in closedcanopy until a 6 to 8 m clean bole has been obtained. Beechresponds well to heavy thinning at this stage. The long term future of beech woodlands depends on timber valuesand on support for their ecological and amenity value. Silviculturally,beech is the best adapted species for the shallowest calcareoussoils, especially on the Jurassic limestone.     

Dieback Disease of Pines with Special Reference to Corsican Pine Pinus nigra var. Calabrica Schn: III. Mycological Factors

The fungus Brunchorstia pinea Karst. v. H. is found to be consistentlyassociated with early stages of dieback disease in a numberof pine species. Inoculation of buds with conidial suspensions of the funguscauses typical disease symptoms to develop. The proportion ofsuccessful inoculations depends on the time of year, the locationof the tree, and its physiological condition. Vigorous treesare able to avoid infection, while weaker trees are very susceptibleto attack. It is suggested that though infection depends on low host resistance,Brunchorstia must be considered a primary pathogen since inall probability trees would survive and produce a crop wereit not for the activities of the fungus.     

Productivity of Closely-spaced Young Poplar on Agricultural Soils in Britain

Recent ideas on ‘silage’ and ‘fuel’forestry call for more information on the total harvestablewoody dry matter produced by hardwoods grown at very close spacingsin fertile soils and coppiced every few years. Yields of oven-driedstems and branches (S and B) are presented here for Populustrichocarpa Torr. and Gray, clone ‘Fritzi Pauley’.Plantings in Bedfordshire at 21 600 trees ha^–1 had a meanannual increment (M.A.I._SB) of 5.2 t ha^–1 y^–1 overfive years, and plantings in the Cambridgeshire fens at 1480trees ha^–1 produced 4.8 t ha^–1 y^–1 over sixyears. Fan-shaped spacing experiments, established in Midlothianby inserting cuttings through black polythene into nursery soilwith added fertilizers, gave 4.6 t ha^–1 y^–1 at theend of the first year and about 7 t ha^–1 y^–1 oneyear after coppicing, but only with over 250 000 stems ha^–1producing closed canopies with leaf area indices of about 4.Similar spacing experiments planted without fertilizer on farmlandin Gloucestershire, Suffolk, Argyll and Midlothian gave averageM.A.I._SB values of 6.5–7.0 t ha^–1 y^–1 afterthree years with over 25 000 trees ha^–1 and similar valuesafter five years with over 10 000 trees ha^–1. Peak currentannual increments (C.A.I._SB) averaged 10–12 t ha^–1y^–1. The maximum M.A.I._SB, attained in Gloucestershire,was 10.0 t ha^–1 y^–1 at age 5 with over 20 000 treesha^–1, with maximum C.A.I._SB values of about 14 t ha^–1y^–1 at age 4; M.A.I._SB values of about 11.5 t ha^–1y^–1 were anticipated at this site by age 6–8. Equivalentstem volumes are given. As expected, trees subjected to competitionaccumulated greater proportions of their woody biomass in stemsrather than branches. Biomass yields of fully-stocked young hardwood stands are independentof planting density. In Britain, M.A.I._SB values of 6–8t ha^–1 y^–1 can be obtained over 1 or 25 years byplanting 250 000 or 2000 trees ha^–1, using vigorous Populusspp, Salix spp or Nothofagus procera on good sites. Advantages and problems of ‘silage’ forestry arediscussed, and it is considered that hardwood fuel coppicescould not meet more than about 2% of national energy needs. The reciprocals of individual tree weights were linearly relatedto planting density.     

A Study on the Mode of Infection of Pinus sylvestris L. by Peridermium pint (Pers.) Lev.

Inoculations by Peridermium pini (Pers) Lev. aeciospores ofscalpel wounds 2 mm deep on current year's Pinus sylvestrisL. shoots gave more numerous infections than shallower wounds.Some lesions resulting from these infections developed pycnidialdroplets and a few also produced aecial blisters within thefirst year after infection. Infection of P. sylvestris by P.pini can take place by direct stomatal penetration. This wasobserved only inside the needle sheaths of trees with previousnatural infections and was not observed on healthy trees. Thehyphae that developed from these penetrations passed intercellularlythrough the mesophyll to the vascular tissue where they proliferated.     

Effect of variable retention cutting on the relationship between growth of coarse roots and stem of Picea mariana

Silviculture heading for structural heterogeneity creates many single trees standing at stand margins, inner edges or in remnant tree groups. As they played just a minor role in the age class forest, the growth behaviour of strongly released trees is rather unexplored. Here we show how retention cutting, presently spreading in the boreal of Québec province, affects stem and coarse root growth of remained single black spruce [Picea mariana (Mill.) Britton]. Increment cores from roots and stems of 125 trees show that retention cutting triggers coarse root growth of the remaining trees. Compared with reference trees retention trees accelerate root in relation to stem growth. Mean and variability of the root–stem allometry significantly rise after retention cutting. The found acceleration of root in relation to stem growth means mechanical stabilisation of the retention trees and corroborates the retention cutting method. Evaluation of silvicultural treatments can be incomplete and misleading as long as they are just based on aboveground reactions and neglect root growth.