Epsilon Archive for Student Projects

Abstract

Ökad efterfrågan på virke har lett till ett ökat intresse för och användande av olika produktionshöjande åtgärder. En sådan åtgärd är dikesrensning som innebär att befintliga diken eller dikessystem rensas för att de skall bibehålla eller återfå sin ursprungligt avvattnande och produktionshöjande funktion. Sveaskog avser att öka omfattningen av dikesrensning. Därför behövs en kartläggning av behovet på deras marker. På samma gång behöver produktionseffekterna och livslängden på dikena utrönas.

Studien genomfördes via en studie av befintlig litteratur om skogsproduktionseffekter efter dikesrensning och dikning samt förfall och livslängd hos diken. Vidare kartlades behovet av dikesrensning på Sveaskogs marker via uppgifter från Riksskogstaxeringen (2003 - 2007).

Tidigare studier har visat tillväxtökningar efter dikesrensning från knappt 0,2 till omkring 1,0 m³ per hektar och år. Variationen beror på de studerade beståndens egenskaper, det geografiska läget samt metodiken i de olika studierna. Tillväxtökningen är måttlig de första fem åren och kulminerar vanligtvis ett till tre decennier senare. Skogstillväxten begränsas av grundvattennivån som i sin tur kan sänkas genom dikning eller dikesrensning. Dikets funktion påverkar således grundvattendjupet. Vidare korrelerar virkesförrådet starkt med grundvattennivån. Ett ökat virkesförråd innebär en ökad avdunstning som till viss del kan kompensera en bristande funktion hos diken.

Förfallet hos diken orsakas bland annat av nedrasat material och erosion, hopsjunkning av torv, inväxande vegetation och förnafall. Förfallet kan dessutom påskyndas av tjäle, felaktigt grävda diken eller dikessystem samt åverkan vid drivningsarbetet. Hastigheten på förfallet beror framförallt på torvtäckets tjocklek, markens fuktighet, torvens nedbrytningsgrad, jordart, vattnets hastighet och kvantitet, frostförhållanden och dikessläntens lutning samt till viss del dikningsmetod. Den ungefärliga livslängden på ett dike eller rensningsintervallet är enligt tidigare erfarenheter och studier mellan 20 och 30 år, men variationen är relativt stor.

På Sveaskogs produktiva skogsmarksareal utgör den dikade arealen 9 procent. På den andelen har dikena på 16 procent av arealen klassats med ej fungerande diken. Begränsningar från FSC gör att endast 14 procent av arealen kan bli aktuellt för rensning (45 000 ha). Den största arealen med ej fungerande diken återfinns i de två sydligaste marknadsområdena.

Summary (eng)

Increased demand of wood has led to increased use of several silvicultural measures that improve stand growth. Such a measure is ditch cleaning, which means that existing ditches or ditch networks are cleared and restored. The purpose of this is to retain or regain the original drainage capacity of the ditches. Sveaskog aims at increasing their ditch cleaning efforts and an analysis of the total cleaning need is therefore required. More specifically, assessments of the stand growth effects following ditch cleaning and the life span of functional ditches are needed.

The thesis work was carried out as a literature study on stand growth responses related to drainage, collapse and lifespan of ditches. The estimated need of ditch cleaning in Sveaskog's forests was based on data from the Swedish National Forest Inventory (2003-2007).

Earlier studies have shown increased annual stand volume increment of about 0.2 - 1.0 m³ per hectare following ditch cleaning. The variation is due to stand characteristics and spatial features, but also the methodology used in the different studies. The first five years after ditch cleaning the increment increase is moderate and usually culminate one to three decades later. The stand growth is limited by an unacceptable high ground water level, which can be lowered by drainage and ditch cleaning. Consequently, the condition of the ditch affects the level of the ground water table. There is also a strong correlation between stand volume and ground water level. Increased stand volume means increased transpiration and interception. This can partly compensate for poor ditch condition.

The function of ditches deteriorates due to collapse of ditch brinks, erosion, peat subsidence, and obstacles formed by vegetation or litter. In addition, frost, poorly planned drainage systems, bad excavation and damage caused by harvesting operations can decrease the condition of the ditch. The pace of ditch deterioration is affected by peat thickness, water content in soil, peat humification, type of soil, rate of water, frost, slope of ditch brink, and to some extent the drainage method used. According to previous studies and experience the approximate service life of a ditch is about 20 to 30 years, but the variation is great.

The drained areas constitute about nine per cent of Sveaskogs forests, of which 16 per cent is characterized by non-functioning ditches. When the FSC rules are applied only 14 per cent can be cleaned (45 000 ha). Areas with non-functioning ditches are more abundant on Sveaskog's holdings in the southern parts of Sweden.

Increased demand of wood has led to increased use of several silvicultural measures that improve stand growth. Such a measure is ditch cleaning, which means that existing ditches or ditch networks are cleared and restored. The purpose of this is to retain or regain the original drainage capacity of the ditches. Sveaskog aims at increasing their ditch cleaning efforts and an analysis of the total cleaning need is therefore required. More specifically, assessments of the stand growth effects following ditch cleaning and the life span of functional ditches are needed. The thesis work was carried out as a literature study on stand growth responses related to drainage, collapse and lifespan of ditches. The estimated need of ditch cleaning in Sveaskog’s forests was based on data from the Swedish National Forest Inventory (2003-2007). Earlier studies have shown increased annual stand volume increment of about 0.2 - 1.0 m³ per hectare following ditch cleaning. The variation is due to stand characteristics and spatial features, but also the methodology used in the different studies. The first five years after ditch cleaning the increment increase is moderate and usually culminate one to three decades later. The stand growth is limited by an unacceptable high ground water level, which can be lowered by drainage and ditch cleaning. Consequently, the condition of the ditch affects the level of the ground water table. There is also a strong correlation between stand volume and ground water level. Increased stand volume means increased transpiration and interception. This can partly compensate for poor ditch condition. The function of ditches deteriorates due to collapse of ditch brinks, erosion, peat subsidence, and obstacles formed by vegetation or litter. In addition, frost, poorly planned drainage systems, bad excavation and damage caused by harvesting operations can decrease the condition of the ditch. The pace of ditch deterioration is affected by peat thickness, water content in soil, peat humification, type of soil, rate of water, frost, slope of ditch brink, and to some extent the drainage method used. According to previous studies and experience the approximate service life of a ditch is about 20 to 30 years, but the variation is great.
The drained areas constitute about nine per cent of Sveaskogs forests, of which 16 per cent is characterized by non-functioning ditches. When the FSC rules are applied only 14 per cent can be cleaned (45 000 ha). Areas with non-functioning ditches are more abundant on Sveaskog’s holdings in the southern parts of Sweden.