Epsilon Archive for Student Projects

Abstract

During the 20th century a significant proportion (about 15 %) of the peat-covered wetlands in Sweden were drained to improve forest productivity. Area wise, most of the forest drainage was done in northern Sweden, but relatively speaking a larger part of the wetlands have been drained in the southern parts of the country. Most often, the forest drainage resulted in increased forest growth, but failures were not unusual, and harmful effects, e.g. on species richness, have been a common cause for draining. Ditch cleaning is necessary for maintaining the ditches in good condition. Ditches in sensitive environments, and ditches in stands where the drainage did not give the intended effect should however not be maintained. Instead such areas could be set aside for nature conservation.

Södra is the biggest forest owner association in the country with almost half the total acreage of southern Sweden associated to the organisation. During January 2005 and 2007 south Sweden was struck by storms, and consequently a large amount of Södra's members were affected. Harvesters and forwarders passed over ditches while working up felled trees, and considerable reforestation areas suddenly called for measures. Harvestings on wetlands often give rise to rising groundwater levels, as a consequence of the discontinued stand water uptake. Remedial drainage, can compensate for this during the first years after harvesting.

During the early autumn 2007 about 2170 hectares of wooded land, distributed on 38 properties associated to Södra, were inventoried. The length and status of existing ditches were measured and reforestation areas over 0,5 hectares from the past tree years, most of them initiated by storm, were visited with the aim of estimating the need of ditch maintenance and remedial drainage. The inventory resulted in data on the status of 74 km ditches and 44 reforestation areas. In total, 34 % of the ditch length was made up of well-functioning drains, whereas 43 % of the drains needed cleaning. About 16 % of the ditch length, including the ditch and nearest surroundings, was given a status of "new natural state". Sites with this status can not be cleaned without a licence from the County Board. The remaining 7 % should not be cleaned due to nature conservations aspects. The field visits on the clear-cut areas showed that 8 % of the inventoried area needed remedial drainage

The conclusions of the study were; Ditch cleaning was neglected in southern Sweden. No correlation could be found between the extent of recommended ditch cleaning and the land owners forest management plan (updated or old). Neither was this recommendation correlated to whether or not the property was certified according to PEFC, nor did properties with a larger forested area show ditches in better conditions.
The present regeneration area where remedial drainage had been done or recommended seemed relatively small compared to the extent of remedial drainage carried out in the 1980's.

En betydande del, ca 15 %, av landets torvtäckta marker har dikats i syfte att öka skogens produktion. Omfattningen har varit störst i de norra delarna av landet, men relativt sett har en större andel mark avvattnats i landets södra delar. Avledningen av vattnet har i allmänhet lett till skogliga produktionsförbättringar. I vissa fall har dock dikningen inte gett avsedd effekt och även många artrika och värdefulla miljöer påverkats av dikning. För att dikenas avvattnande funktion skall bibehållas krävs underhåll i form av rensning. Diken i känsliga miljöer med höga naturvärden eller ståndorter där markavvattningen ej gett avsedd effekt bör dock undantas från rensning. Södra är den största skogsägareföreningen i landet med knappt halva skogsmarksarealen i Götaland ansluten till föreningen. Under 2005 och 2007 drabbades södra Sverige och således många av Södras medlemmar av stormarna Gudrun och Per. Förutom att upparbetningen av det stormfällda virket innebar passager över diken, tillkom även en betydande areal hyggen. Under hösten 2007 inventerades 2172 hektar produktiv skogsmark ansluten till Södra, fördelat på 38 fastigheter inom 3 olika skogsbruksområden. Syftet med inventeringen var att bedöma befintliga dikens behov av rensning och särskilja den andel som ej bör underhållas på grund av naturhänsyn. Hyggen uppkomna efter den 1 augusti 2004 med en areal om minst 0,5 hektar besöktes för att bedöma skyddsdikningsbehov. Påträffade diken registrerades och delades in i fyra kategorier beroende på om de fungerade, ej fungerade, om nytt naturtillstånd ansågs ha inträtt, eller om diket ej borde rensas av naturvårdsskäl. Inventeringen visade att 34 % av dikeslängden utgörs av fungerande diken, 43 % är i behov av rensning, nytt naturtillstånd fanns på 16 % av dikeslängden, och att resterande 7 % ej bör rensas av naturvårdsskäl. Av inventerad hyggesareal bedömdes 8 % vara i behov av skydds-dikning.