Skogsstatistisk årsbok 2003.pdf - Skogsstyrelsen

SUMMARYStandard error estimates for some importantNational Forest Inventory results are found in the1999 edition of the Statistical Yearbook of Forestry(tables 3.10, 3.11 and 3.12). The relativestandard errors of estimates of forest area andgrowing stock in the entire country are slightlyabove 0.5 per cent. Other errors (non-samplingerrors) are difficult to quantify, but are important.Regular check measurements indicate that thetotal volume of growing stock is underestimatedby about 1 per cent.Swedish forestry is characterised by multipleuse of forest land. According to the Forestry Actall forest land should be managed with environmentalconsideration. In some areas environmentalprotection restricts wood production. Such restrictionshave to be considered both in practicalmanagement as well as in calculations of potentialcut. Guidance for such issues can be derived fromNational Forest Inventory observations (table 3.9).The table shows the area of forest land where otherland use than wood production reduces the yieldof wood.4 Forest damageThe defoliation of Scots pine in the southern and middle parts of Sweden continues toincrease slightly in 2002 (note: applicable for thinning stands and stands mature for finalfelling). The main cause of increasing defoliation is probably last year’s fungi attacks (mainlyGremmeniella abietina).In young forests, significant damage is caused by moose. From 1997 to 2001, 566,000hectares experienced moderate to severe grazing, out of which 139,000 hectares was severelydamaged.Defoliation is calculated as the proportionalestimation of a tree’s absent needle mass. Toensure accuracy the methods are continuouslycalibrated. Results are normally expressed as themean defoliation of sample trees or as the proportionof trees within a certain defoliation class. InSweden up to 20 percent defoliation is considerednormal, while higher defoliation indicates stressor damage.Results are based on the National ForestInventory (NFI) and observation plots run by theSwedish Forest Administration. Some of the plotsare also part of ICP-Forests, levels I and II.In 1984 the NFI began assessing defoliation onpermanent and temporary plots. Since 1995 aseparate forest condition inventory monitors morethan 700 permanent plots on an annual basis.Sixty percent of the results since 1995 are basedon tree data from the annual inventory. Theremaining material is from the ordinary forestinventory (figure 4.2, and tables 4.1 and 4.2). Thehigher between-year variations before 1995 isprobably due to the fact that tree samples weredifferent each year.Beginning in 1995, the Forest Administrationestablished 223 permanent plots for intensivemonitoring. Over a three year period plot siteswere chosen objectively, bearing in mind criteriasuch as: tree species, stand age, soil type, windexposure, pollution sources and homogeneity. Atypical coniferous plot consists of middle age trees(40–60 years old) in homogenous stands. Plotdensity increases from the north to the south,reflecting the main deposition gradient for sulphurand nitrogen (table 4.3 and 4.4).The grazing damage from moose is monitoredby the NFI and the results are presented as fiveyearaverages (table 4.5).300