The carbon sink provided by plantation forests and their products in Britain

The rate of accumulation of carbon in forest plantations inBritain is estimated using the record of forest planting since1925 and a model that calculates the flow of carbon from theatmosphere to trees, litter, soil, wood products and back tothe atmosphere. It is assumed that all trees planted so farhave the carbon accumulation characteristics of P. sitchensis,Yield Class 14 m³ ha^-1 a^-1, but that future planting could includeF. sylvatica Yield Class 6 and Populus Yield Class 12. It isfurther assumed that conifer plantings increase surface litter,but not soil organic matter, whereas broadleaved tree plantings(on mineral soils) increase both. Because the current forest estate is relatively young, it isestimated to be accumulating about 2.5 million tonnes of carbonper year (1990), and to be still increasing in carbon density(tonnes C ha^-1). In order to maintain this rate of carbon removalfrom the atmosphere, planting would need to continue at a rateof 25–30 thousand ha of conifers or (theoretically) 10thousand ha of poplars per year (on good mineral soils). Itis noted that 2.5 million tonnes C is about 1.5 percent of theUK carbon emission, and may be similar to the natural carbonsink in Britain represented by wetlands and rivers.     

Establishing forest tree species on peatland in a reflooded area of the Huleh Valley, Israel

The study evaluates the establishment performance of four treespecies (Ficus carica, Fraxinus syriaca, Salix acmophylla, Platanusorientalis) 3 years after planting in the peatland in the HulehValley. To develop good tree establishment and avoid uprooting,two types of soils were tested as alternatives for the rhizosphere.For each tree 6 m³ of soil (2 x 2 x 1.5 m) were replaced withriver bed alluvium or terra-rossa, in which the tree was planted.Control trees were planted directly in the peat. Changing therhizosphere soil type to river bed alluvium significantly improvedthe establishment and performance of Salix acmophylla and Fraxinussyriaca (survival, height, diameter and canopy shade area).Ficus carica was not affected by changing the soil in the rhizosphere,while Platanus orientalis was affected mainly in canopy shadearea. Terra-rossa improved establishment only of Salix acmophylla;it did not affect the other species.     

Factors influencing tree species diversity and Betula alleghaniensis establishment in silvicultural openings

We examined the short-term effects of group-selection harvestingwith seed tree retention on the diversity, abundance and establishmentof tree seedlings in a northern hardwood forest in the UpperPeninsula of Michigan (49 openings, 20 closed canopy referencesites). Three opening sizes were examined – opening radius0.5 x canopy height (267 ± 62 m², n = 16), 0.75 x canopyheight (642 ± 85 m², n = 17) and 1.0 x canopy height(1192 ± 155 m², n = 16) (canopy height = 22 m). A singleyellow birch (Betula alleghaniensis Britt.) seed tree was retainedin the centre of each opening. Tree seedling density was significantlyhigher in the largest group-selection opening than at the closedcanopy reference sites (P < 0.05), the main factor for thiswas the increased proportion of yellow birch, red maple (Acerrubrum L.) and other minor species. Nevertheless, yellow birchwas still a minor component of the developing gap cohort, comprising5.9 per cent of the seedlings and 1.1 per cent of the saplings.Within openings, microsite variables, such as per cent coversof bare soil and coarse woody debris, were the best predictorsof yellow birch occurrence and density. Our results suggestthat microsite limitations and competing vegetation may greatlyreduce the efficacy of openings for ensuring the maintenanceof mid-tolerants.     

Regeneration and gene transformation systems of Robinia pseudoacacia ‘Idaho’ mediated by Agrobacterium tumefaciens

Robinia pseudoacacia ‘Idaho’ is one of several multi-purpose trees used in ornamental, soil and water conservation, fodder and nectar sources. Plant abiotic stress tolerance transformed by genes could meet the requirements for reclamation of arid or alkalid lands and vegetation restoration. For this paper, we studied the effects of auxin and cytokine on Idaho locust in vitro regeneration and the establishment of gene transformation systems for plants mediated by Agrobacterium tumefaciens. Results showed that the ratios of cytokinin and auxin were the major factors affecting adventitious bud differentiation on a MS medium; the concentration of 0.5 mg&#8226;L^–1 6-BA benefitted callus proliferation and 0.25 mg&#8226;L^–1 IBA promoted shoot rooting; however, a higher IBA concentration will inhibit rooting. The most effective antitoxin for screening transgenic Idaho locust shoots was G418 and the most sensitive concentra-tion of it was 8 mg&#8226;L^–1.     

Agroforestry systems: sources of sinks of greenhouse gases?

The prominent role of forestry and agroforestry systems in the flux and long-term storage of carbon (C) in the terrestrial biosphere has increased global interest in these land-use options to stabilize greenhouse gas (GHG) emissions. Preliminary assessments suggest that some agroforestry systems (e.g., agrosilvicultural) can be CO₂ sinks and temporarily store C, while other systems (e.g., ruminant-based silvopastoral systems) are probably sources of GHG (e.g., CH₄).Agroforestry systems can be significant sources of GHG emissions, especially at low latitudes. Practices such as tillage, burning, manuring, chemical fertilization, and frequent disturbance can lead to emission of CO₂, CH₄, and N₂O from soils and vegetation to the atmosphere. Establishment and management of agroforestry systems incompatible with prevailing edaphic and climatic conditions can accelerate soil GHG emissions. Non-sustainable agroforestry systems are quickly degraded, and woody and herbaceous crops can become significant GHG sources. Silvopastoral systems can result in soil compaction and erosion with significant loss of labile C and N compounds to the atmosphere. Ruminant-based silvopastoral systems and rice paddy agrisilvicultural systems are well documented sources of CH₄ which significantly contribute to the global CH₄ budget.Early assessments of national and global terrestrial CO₂ sinks reveal two primary beneficial attributes of agroforestry systems: 1) direct near-term C storage (decades to centuries) in trees and soils, and, 2) potential to offset immediate GHG emissions associated with deforestation and subsequent shifting agriculture. Within the tropical latitudes, it is estimated that one ha of sustainable agroforestry can provide goods and services which potentially offset 5–20 ha of deforestation. At a global scale, agroforestry systems could potentially be established on 585–1275×10⁶ ha of technically suitable land, and these systems could store 12–228 (median 95) Mg C ha^–1 under current climate and edaphic conditions.The US Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged.     

Rates of Spread of Some Wind-driven Fires

This paper shows that head fires spread in cribs and in controlledburns in gorse and heather (in the New Forest) at similar rates,R, given by where is the bulk density of the burnt fuel and U is the wind speed in m/s. Although the values of U were similar, the values of differed by more than an order of magnitude betweenthe two types of fire. The constant of proportionality is roughly0.07 kg/m³ for the wildland fuels and 0.05 kg/m³ for cribs.The similarity of the relationships is perhaps more significantthan the agreement between the coefficients, which appears closerthan it really is since the cribs were drier than the wildlandfuels. Assuming a wind speed of 10 mile/h the constant for theGreat Fire of London (1666) was also about 0.05 kg/m³. A comparison between the flames in the controlled burns anda partly theoretical correlation of flame lengths from burningcribs has been partly successful and the duration of flamingof a piece of fuel has been empirically correlated with moisturecontent, though the general effects of moisture on the rateof spread are not satisfactorily accounted for. The reason forthis and other anomalies is probably the relatively few datafor wildland head fires (ten in all) and the fact that theirfuel composition was not uniform. The author uses many of theresults of the work described in an earlier paper in Forestry(Thomas, 1967) to derive heat balances for head fires and todiscuss the theoretical aspects of the above expression.     

Carbon pools and sequestration in forest ecosystems in Britain

British vegetation is estimated to contain 113.8 million tC,80 per cent of which is in forests and woodlands (91.9 milliontC). Sitka spruce plantations, although covering 21.4 per centof the forest/woodland area, contain only 8.2 per cent of theforest/woodland carbon, because the plantations are young andhave an average of only 14.1 tC ha^–1. Broadleaved woodlandsin Britain have an average of 61.9 tC ha^–1 and contain46.8 per cent of the total carbon in all vegetation. A breakdownis given of the carbon density (tC ha^–1) and content ofdifferent tree species. A carbon density map of Britain highlightsthe concentration of carbon in the broadleaved woodlands insouthern England and in the large conifer plantations in southernScotland and northern England. Carbon storage in the trees, products, litter and soil can beevaluated in terms of long-term equilibrium storage or short-termrate of storage. These two components vary among forest typesin Britain and globally. Plantations harvested at the time ofmaximum mean annual increment (MAI) will not store as much carbonas mature, old-growth forests on the same site unless they havelong-lasting products and/or are very fast growing. Maximumequilibrium carbon storage is generally achieved by harvestingat the time of maximum MAI when the lifetime of products exceedsthe time to maximum MAI. Undisturbed peatlands sequester CO₂and emit CH₄, and may be greenhouse neutral. When peatlandsare drained and planted with trees, they stop emitting CH₄ andstore carbon in the trees, forest litter, forest soil and woodproducts. However, these greenhouse gas ‘gains’are offset by the oxidation to CO₂ of the peat, and the gainsare exceeded by CO₂ losses when 20–40 cm depth of peathas been oxidized. Forests in Britain are currently sequestering1.5–1.7 million tC a^–1 in trees, 0.3–0.5 tCa^–1 in litter and 0.5 million tC a^–1 in wood products,totalling about 2.5 million tC, equivalent to about 1.5 percent of the carbon currently emitted by burning fossil fuelsin the UK. In order to maintain the current forest carbon sink,the forest area needs to continue to expand at about 25 000ha a^–1 of upland conifers or 10 000 ha a^–1 of poplarson good land.     

Light Use Efficiency and Woody Biomass Production of Poplar and Willow

Stands of clonal Salix viminalis (in 1985) and Populus trichocarpa(in 1986) were grown for one growing season from cuttings incontainers, at 0.3 m spacing, supplied with trickle irrigationand nutrients. Woody biomass production (B_w) in the first yearwas analysed as the product of the proportion of dry matterpartitioned to wood ({macron}), the seasonal mean efficiencywith which intercepted light was used to produce dry matter({macron}), the mean fraction of incident light interceptedby the canopies (f{macron}), and the amount of incoming solarradiation over the season (A). Thus, Bw = {macron}f{macron}{macron}A. For Salix, B_w=10 t ha^–1y^–1, while for Populus, B_w= 5 t ha^–1y^–1, mainly because of differences in{macron} and f{macron}. The Populus partitioned more dry matterto roots (and correspondingly less to stems) and interceptedless light over the growing season. The Salix and Populus cloneshad surprisingly similar ({macron}) values, namely 1. 58 and1. 50 g MJ^–1, respectively (based on total dry matterand total solar radiation), which are very like the {macron}values measured on C₃ agricultural crops in Britain. Also, theSalix and Populus clones produced canopies with similar lightextinction coefficients and hence similar relationships betweenfractional interception and leaf area index.     

Overstorey density influence on the height of Picea abies regeneration in northern Sweden

The study aimed specifically at investigating if canopy opennesswas a better predictor of the height growth of Norway spruce(Picea abies (L.) Karst.) advance regeneration than overstoreybasal area or overstorey standing volume. In 1990, a field experimentwith 3 x 2 factorial design and two replications (blocks) wasestablished in an uneven-aged Norway spruce forest. Plots hada net plot area of 30 x 30 m, each with a 10-m-wide treatedbuffer zone. Three overstorey density levels retained approximately15, 40 and 70 per cent of the pre-harvest overstorey standingvolume and were allotted to the plots. Two types of thinningthat harvested smaller trees or harvested larger trees wererandomly allocated to each pair of overstorey density plots.In mid-June 2000, canopy openness was estimated from hemisphericalphotographs taken at five marked points in the centre of eachof the plots at 0.9 m from ground to the top of the ‘fish-eye’camera lens. Regression results showed that canopy opennesswas a better predictor of height increments of spruce seedlings(0.1< height < 0.5 m), saplings (0.5 height < 2.0m), and small trees (height 2.0 m, diameter at 1.3 m height< 5 cm) than with overstorey basal area (m² ha^–1) oroverstorey standing volume (m³ ha^–1). The height incrementof the spruce advance regeneration was not significantly correlatedto stand basal area or to standing volume. Overstorey basalarea in the net plots was significantly negative (P 0.05) withmean canopy openness estimates, and the r² value was 0.40. Resultsindicated that basal area was not linearly related to canopyopenness as it increased, which might explain the lack of predictivepower of retained basal area on spruce regeneration height indense stands in boreal Sweden.     

Conifer Plantations on Drained Peatlands in Britain: a Net Gain or Loss of Carbon?

It is estimated that British peatlands, excluding lowland fens,contain about 3000 million tonnes of carbon, 76 per cent ofwhich is in deep peats (> 45 cm deep) and 9 per cent of whichhas been drained and planted with trees. Undisturbed peatlands emit CH₄ but accumulate CO₂-derived carbon.The net greenhouse effect may be near zero. Peatland drainagevirtually stops methane emission and increases CO₂-carbon lossthrough aerobic decomposition, but can also increase CO₂-carbonfixation by the peatland vegetation partly through microbialmineralization of nitrogen, resulting in either a net loss orgain in CO₂-carbon. Planting conifer forests leads to an accumulation of CO₂-derivedcarbon in the trees, wood products, litter and forest soil upto equilibrium values, totalling about 16.7 kg C m^–2 forPicea sitchensis, Yield Class 12. Deep and shallow peats inthe British uplands contain about 0.47 and 0.80 kg C m^–2per centimetre depth, respectively. Thus, the 16.7 kg C m^–2that is stored by P. sitchensis (Yield Class 12) is equivalentto the carbon stored in about 35.5 cm of deep peat or 20.9 cmof shallow peat. If forests are planted on peats substantiallydeeper than this, there could be a net loss of CO₂-carbon inthe long term. Scenarios are presented for the time course of CO₂-carbon gainand loss when peatlands are drained and planted with conifers.If CO₂ loss rates from drained peats are 50–100 g C m^–2a^–1 there is likely to be increased carbon storage inthe whole system for at least three rotations; but if CO₂ lossrates are 200–300 g C m ^–2 a^–1 increased storagemay be restricted to the first rotation, after which there isa net loss of carbon.     

Soils, Forests, and Pastures in South Wales

Marginal upland soils may be used for forestry or sheep grazing.In the State Forests of South Wales a study of selective plantingshows the greatest volume yield is generally obtained from Sitkaspruce (Picea sitchensis) planted in ridge-crest soils. Yieldsfrom Sitka spruce on ridge-flank soils are reduced by the presenceof Ericaceae on upper slopes and the dryness of the soils onlower slopes. Contrary to this trend, the nutritional valueof the pasture for sheep is lowest on ridge crests and increasesdownslope, being greatest on the Agrostis pastures of lowerslopes. Further, reclamation is most economically accomplishedon ridge flanks where they are not too steep. Although profitcomparisons are very difficult to make, ridge slopes might bemore usefully allocated to sheep grazing and ridge crests toSitka spruce forests. To be economically viable, a forest mustbe large, but within any new forestry acquisitions the largerareas occupied by slope soils might be reclaimed or utilizedas sheep pastures.     

Litter production, seasonal pattern and nutrient return in seven natural forests compared with a plantation in southern China

The amount and pattern of litterfall and its nutrient returnwere studied in seven natural forests of Schima superba Gardn.and Champ. (SCS), Castanopsis fabri Hance (CAF), Tsoongiodendronodorum Chun (TSO), Cinnamomum chekiangense Nakai (CIC), Altingiagracilipes Hemsl. (ALG), Castanopsis carlesii (Hemsl.) Hayata(CAC) and Pinus massoniana D. Don (PIM), and compared with thatof an adjacent 29-year-old plantation of Chinese fir (Cunninghamialanceolata Lamb.) (CUL) in Jianou, Fujian, China. Mean annualtotal litterfall over 3 years of observations varied from 4.63Mg ha^–1 in the CUL to 8.85 Mg ha^–1 in the PIM; ofthis litterfall, the leaf contribution ranged from 62 to 73per cent. Litterfall in the CAF, ALG and CAC showed an unimodaldistribution pattern, while for the five other forests, thelitterfall pattern was multi-peak. The rank order of the eightforests, according to nutrient return mass with the exceptionof P, was different from the order when rank was according tototal mass of litterfall. The highest annual N, K and Ca returnsfrom total litterfall were noticed in the TSO, the CAF and theCUL, respectively. The amounts of P and Mg potentially returnedto the soil were the highest in the PIM. The leaf fraction providedgreater potential returns of N, P, K, Ca and Mg to the soilthan other litter fractions. The results of this study demonstratethat natural forests have a greater capability for maintainingsite productivity than the monoculture coniferous plantation,due to higher amount of above-ground litter coupled with greaternutrient returns; therefore conservation of natural forestsis recommended as a practical measure in forest management torealize sustainable development of forestry in mountainous areasof southern China.     

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.     

Estimating net primary productivity of Chinese pine forests based on forest inventory data

Forest inventory data (FID) include forest resources informationat large spatial scale and long temporal scale. They are importantdata sources for estimating forest net primary productivity(NPP) and carbon budget at landscape and regional scales. Inthis study, more than 100 datasets of biomass, volume, NPP andstand age for Chinese pine forests (Pinus tabulaeformis) fromthe literature were synthesized to develop regression equationsbetween biomass and volume, and between NPP and biomass as wellas stand age. Using these regression equations and the fourthFID surveyed by the Forestry Ministry China from 1989 to 1993,NPP values of Chinese pine forests were estimated. The meanNPP of Chinese pine forests was 4.35 Mg ha^–1 yr^–1.NPP varied widely among provinces, ranging from 1.5 (Neimenggu)to 13.73 Mg ha^–1 yr^–1 (Guizhou). Total NPP of Chinesepine was 10.87 Tg yr^–1 (1 Tg = 10¹² g). NPP values ofChinese pine forests were not distributed evenly across differentprovinces in China. This study may be useful not only for estimatingforest carbon of other forest types but also for evaluatingterrestrial carbon balance at regional and global levels.     

Carbon storage and sequestration in the forests of Northern Ireland

The rate of accumulation of carbon in forests and woodlandsin Northern Ireland was estimated using the record of forestplanting since 1900 and a model that calculated the flow ofcarbon from the atmosphere to trees, litter, soil, wood productsand back to the atmosphere. It was assumed that all coniferforests had the carbon accumulation characteristics of Piceasitchensis, and upper and lower estimates of carbon storagewere calculated assuming Yield Class 16 m³ha^–1 a^–1unthinned and Yield Class 14 m³ ha^–1 a^–1 thinned.Broadleaved woodlands were assume to have the carbon accumulationcharacteristics of Fagus sylvatica, Yield Class 6 m³ha^–1a^–1. Northern Ireland currently has about 78 300 ha offorest, 83 per cent of which is coniferous, 77 per cent state-owned,mostly planted since 1945, with peak planting in 1960–1975.In 1990, conifer forests contained 3–4 MtC (trees + litter)and broadleaved wdlands contained about 0.8 MtC (trees + litter+ new forest soil). In 1990, conifer forests were sequestering0.15–0.20 MtC a^–1 and broadleaved woodlands about0.025 MtC a^–1. To maintain these sink sizes, new coniferforests need to be planted at 1500–2000 ha a^–1,and new broadleaved woodland at100–150 ha a^–1 inaddition to full restocking. Current carbon sequestration byNorthern Ireland forests represents around 6.5–8.2 percent of the total for UK forests and is greater per hectar thanin Britain because the average forest age is younger in NorthernIreland     

Some Aspects of the Growth and Spread of Fire in the Open

This paper describes some recent experimental and theoreticalwork on the growth and spread of fire in the open and discussessome examples of field data in terms of the theoretical calculationspresented. The lengths of flames from laboratory fires have been relatedto the size and rate of burning of the fuel by formulae derivedfrom a simplified dimensional analysis. The effects of a windblowing across a long fuel bed on the length and orientationof flames are also described. The scaling laws for flame heightsuggest that in the horizontal spread of fire, heat transferfrom the flames above the fuel bed is important primarily withshallow fuel beds. It is suggested that the main effect of a wind on crib firesis aerodynamic. The wind deflects the advancing fire front fromthe vertical, but perpendicular to this deflected front therate of spread of fire, at least for cribs, is roughly the sameas in still air. However, a theory of spread allowing for heattransfer through the fuel bed and radiation from the flamespredicts that there can be a stable ‘fast‘ spreadas well as a ‘slow’ spread. In ‘fast’spread the flames are thick and control the spread. In ‘slow’ spread radiant heat transfer from theburning zone is usually responsible for the spread. The flamesare thin and of low emissivity. The most important factors determiningthe rate of ‘slow’ spread R are ^pb the bulk densityof the fuel bed and ø the deflection from the verticalof the front of burning fuel which varies with wind speed. Rpb cos ø is approximately constant over a wide rangeof conditions with an order of magnitude of 5–10 mg cm^–2s^–1.     

The tolerance of young trees to applications of clopyralid alone and in mixture with foliar-acting herbicides

The selective herbicide clopyralid is often used to controlcompeting Cirsium arvense in newly planted woodlands. When appliedas an overall spray at different dates in the spring (at 0.2kg acid equivalent (a.e.) ha^–1) to 10 tree species (Fraxinusexcelsior, Prunus avium, Quercus robur, Acer pseudoplatanus,Populus x canadensis cv. ‘Ghoy’, Pseudotsuga menziesii,Pinus nigra ssp. laricio, Larix kaempferi, Picea abies and Piceasitchensis) it did not reduce survival, and had little effecton growth. However, some species showed distortion of the youngestsprayed leaves or needles for several weeks after treatment,particularly F. excelsior, L. kaempferi and P. x canadensis.Sequential applications of clopyralid (first at 0.1 kg a.e.ha^–1 followed by 0.2 kg a.e. ha^–1 after 3 weeks),which are often required to control C. arvense, did not leadto increased leaf damage or growth reduction. Mixtures of clopyralidwith selective graminicides (cycloxydim at 0.45 kg active ingredient(a.i.) ha^–1; fluazifop-p-butyl at 0.38 kg a.i. ha^–1and propaquizafop at 0.15 kg a.i. ha^–1) did not causesignificant adverse effects on survival or growth of any species.If herbicides are required to control mixed stands of susceptibleproblem weeds such as C. arvense and grasses which are overtoppingyoung trees, these herbicide mixtures, applied as overall sprays,are less likely to cause damage to trees than attempts to usedirected applications of broad-spectrum foliar-acting herbicides.     

Comparison between empirical and theoretical biomass allometric models and statistical implications for stem volume predictions

Comparisons between empirical and theoretical allometric modelsfor estimating tree biomass and the statistical caveats attachedto empirical stem volume equations are presented in this paper.First, the elastic and stress similarity models, derived fromfirst biomechanical principles, as well as predictions obtainedfrom geometric similitude, were validated against allometricequations that relate dry above-ground tree biomass M to stemdiameter D. In addition, a recent geometric model which predictsthat M D^8/3 was also validated against a pooled dataset whichconsisted of 764 M-D pairs compiled from empirical studies conductedthroughout the globe and for several tree species. Moreover,59 empirical equations which relate M to D were selected froma European database to validate the aforementioned theoreticalmodels. The analysis indicated that the biomechanical and thegeometric models failed to describe the shape in M-D allometryfor the empirical datasets. Finally, the multicollinearity problem,which is directly related to the reliability of the predictions,was analysed for stem volume equations (V). In total, 23 empiricalmodels based on the six-parameter formula V = a + bD + cD² +dD³ + eH + fD²H were used in order to pinpoint the dependencybetween the parameters. It is illustrated that parameters a,b and c are highly related to each other, and parameter e isalso related to parameter f. It is concluded that the interrelationshipbetween D and stem height (H) could be one of the reasons forthis dependency and scepticism should be placed in the reliabilityof V estimates derived from these models.     

Managing birch woodlands for the production of quality timber

Interest in silver birch (Betula pendula Roth) and downy birch(Betula pubescens Ehrh.) has greatly increased in recent yearspartly as a result of pressures to restore and expand nativewoodlands but also due to renewed interest in birch as a treecapable of producing quality timber. Despite the many advantagesof birch as a commercial timber tree—ease of establishment,fast growth on good sites, high value timber and a short rotation,it has a poor reputation in Britain largely as a result of thepoor form of the existing, mainly unmanaged resource. The followingpoints need to be considered if stands of quality birch treesare to be produced in an economical timescale. (1) Sites: silverbirch needs good sites that are relatively well drained withlight mineral soils. Downy birch does well on moist to wet sites.(2) Regeneration: natural regeneration through a shelterwoodis the preferred system of regenerating birch as some overheadprotection is beneficial to germination success. About 20–40seed trees should be left per hectare. Good ground preparationand control of grazing are essential. The vast majority of seedlingsare recruited in the first year of the regeneration cycle thereforeplanting should be considered if the initial regeneration successis poor. Direct seeding is also a successful method of regeneration.Birch readily regenerates naturally into suitably prepared openareas next to existing birch woods but these should not be toobig, e.g. gaps or strips 20–60 m wide have been suggestedin the literature. (3) Maintenance: density of regenerationneeds to be reduced to about 2500–3000 stems ha^–1by the time the trees are about 3–6 m tall. Birch seedlingsmust always be taller than the competing vegetation. (4) Thinning:thinning should begin when the mean height of the stand is about8–10 m. At this point at least half the number of treesshould be removed with the emphasis on retaining dominants andco-dominants of good form. The aim is to maintain about halfthe height of the tree as living crown to sustain a high rateof growth. Additional thinnings will be required at intervalsof 5 to 7 years and final thinning should leave around 300–500stems ha^–1. (5) Rotation: a rotation of 40–50 yearsis possible on good sites and perhaps 50–55 years on lessfavourable sites.     

Thinning reduces soil carbon dioxide but not methane flux from southwestern USA ponderosa pine forests

Forest soils are important components of the global carbon cycle because they both store and release carbon. Carbon dioxide is released from soil to the atmosphere as a result of plant root and microbial respiration. Additionally, soils in dry forests are often sinks of methane from the atmosphere. Both carbon dioxide and methane are greenhouse gases whose increasing concentration in the atmosphere contributes to climate warming. Thinning treatments are being implemented in ponderosa pine forests across the southwestern United States to restore historic forest structure and reduce the risk of severe wildfire. This study addresses how thinning alters fluxes of carbon dioxide and methane in ponderosa pine forest soils within one year of management and examines mechanisms of change. Carbon dioxide and methane fluxes, soil temperature, soil water content, forest floor mass, root mass, understory plant biomass, and soil microbial biomass carbon were measured before and after the implementation of a thinning and in an unthinned forest. Carbon dioxide efflux from soil decreased as a result of thinning in two of three summer months. Average summer carbon dioxide efflux declined by an average of 34 mg C m⁻² hr⁻¹ in the first year after thinning. Methane oxidation did not change in response to thinning. Thinning had no significant short-term effect on total forest floor mass, total root biomass, or microbial biomass carbon in the mineral soil. Understory plant biomass increased after thinning. Thinning increased carbon available for decomposition by killing tree roots, but our results suggest that thinning reduced carbon dioxide emissions from the soil because the reduction in belowground autotrophic respiration was larger than the stimulation of heterotrophic respiration. Methane oxidation was probably not affected by thinning because thinning did not alter the forest floor mass enough to affect methane diffusion from the atmosphere into the soil.