Kolefnisbinding og vöxtur mismunandi skógargerða í þremur skógum á Suðvesturlandi Heiðmörk, Nesjavöllum og Ölfusvatni

Abstract

Eitt af mikilvægum hlutverkum skógræktar er að binda koldíoxíð (CO2) úr andrúmslofti og geyma sem kolefni (C) í lífmassa og jarðvegi til lengri tíma. Aukin umræða og kröfur um notkun ábyrgra aðferða til þess að meta kolefnisbindingu skóga var aðal kveikjan að þessari rannsókn. Markmið þessa verkefnis var að i) lýsa og aðlaga aðferðafræði til að meta magn kolefnis og kolefnisbindingu í lífmassa trjáa í afmörkuðum skóglendum (einstaka skógajörðum) á áreiðanlegan hátt, ii) framkvæma úttektir og mælingar með þeirri aðferðafræði á þremur skógræktarsvæðum á suðvesturhorninu, það er á Heiðmörk, Ölfusvatni og Nesjvöllum og að lokum iii) bera saman niðurstöðurnar á kolefnisbindingu þessara svæða eftir svæðum, aldri skóga og ríkjandi trjátegundum. Úttektin var framkvæmd árið 2017. Skógarnir voru fyrst kortlagðir og flokkaðir með tilliti til trjátegunda og aldurs. Notast var við flokkað slembival til þess að leggja út mælifleti í skógarflokka. Skógmælingar og árhringjamælingar voru gerðar á hverjum fleti sem nægðu til að meta aldur ríkjandi trjáa, kolefnisforða í lífmassa trjáa árið 2017 og 2012, og þar með árlega kolefnisbindingu síðustu fimm árin. Niðurstöðurnar voru að flatarmál flestra skóganna breyttist nokkuð við endurkortlagningu, en þar munaði mestu um náttúrulega birkiskóga sem höfðu aukið útbreiðslusvæði sitt eða ekki verið teknir með í fyrri úttektum. Gerður var samanburður á aldri trjáa miðað við framkvæmdarskráningu og árhringjagreiningu, en þá kom ljós vanmat með árhringjagreiningu og var bætt við 3 árum við niðurstöður úr árhringjagreiningu þar sem aldur byggði aðeins á þeim. Heildarmat á kolefnisforða í lífmassa trjágróðurs (sem bundið CO2) í Heiðmörk, Ölfusvatni og Nesjavöllum var 101.406 (±23.535), 2.585 (±1290) og 1.752 (±875) tonn CO2 árið 2017. Þessi kolefnisforði skiptist á milli ólíkra skógargerða á eftirfarandi hátt: Í Heiðmörk 15%, 13%, 11% og 61% á milli náttúrulegra birkiskóga, blandskóga, barrskóga undir 5 metra hæð og barrskóga yfir 5 metra hæð; Á Ölfusvatni 93% og 7% á milli barrskóga

An important function of forests is to sequester carbon dioxide (CO2) from the atmosphere and store it as carbon (C) in biomass and soil for a long time. This is a contribution to the mitigation of manmade greenhouse gas emissions and climate change. The increasing debate and demand for the use of responsible methods to estimate the carbon sequestration of forests was the main trigger for this study. The aim of this project was to i) describe and adapt a methodology to reliably estimate the amount of carbon stock and carbon sequestration in demarcated woodlands (individual forests), ii) carry out assessments and measurements with that methodology in three forests in the south west of Iceland, in Heidmork, Ölfusvatn and Nesjavellir and finally iii) compare the results on the carbon sequestration of these areas by area, age of forests and dominant tree species. The assessment was carried out in 2017. The forests were first mapped and stratified by tree species and age. Stratified random sampling was used to assign measurement plots to forest strata. Forest measurements and dendrochronological measurements were carried out on each plot and were sufficient to estimate the age of the dominant trees, the carbon stock in tree biomass for the years 2017 and 2012, and thus the annual carbon sequestration for the last five years. The results were that the forested area of most of the forests changed somewhat by remapping them, the largest changes were in natural birch forests that had expanded their distribution area or were not included in previous surveys. A comparsion was made of trees based on recorded data on tree planting and dendrochronological age dating that revealed an underestimation of age with the denrochronological data and 3 years were added to the age of trees that had been age estimated with dendrochronological methods. The total estimate of carbon stock in the biomass of trees (as fixed CO2) In Heidmork, Ölfusvatn and Nesjavellir was 101.406 (±23.535), 2.585 (±1290) and 1.752 (±875) tons of CO2 in 2017. These carbon stocks are divided between different forest strata as follows: In Heidmork 15%, 13%, 11% and 61% between natural birch forests, mixed forests, coniferous forests below 5 meter height and coniferous forests above 5 meter height; In Ölfusvatn 93% and 7% between coniferous forests below 5 meter height and planted birch forests; In Nesjavellir 35% and 65% between planted and natural forests. v The current (2012-2017) annual rate of carbon sequestration in tree biomass wae estimated to be 7.749 (±1.742), 300 (±123) and 148 (±74) tons CO2 annually for the forests in Heidmork, Ölfusvatn and Nesjavellir. There was no significant difference in the growth potential of the forests in these three areas so the data from the different areas were combined for further analysis. The avarage age of birch forests in this comparison was 25 years, compared to 30 years for all coniferous forests. On avarage these forest classes produced 0.1 and 2.0 m3 volume of wood per ha and year, the current annual carbon sequestration was 0.7 tons of CO2 per ha and year and 9.8 tons of CO2 per ha and year the difference was significant in both cases. The avarage annual growth was significantly lower in both forest types than the current annual growth or 33% in the birch and 36% in the coniferous forests. That suggests that the avarage annual growth rate (and the rate of carbon sequestration in biomass) will increase in both types of forest in the future. To exclude the influence of different ages a comparison was made between conifer and birch forests that were in the age classes 15-25 years and 40-60 years. All variables related to tree size, wood volume or carbon sequestration were significantly higher in the coniferous forests. The results showed that the stratification of forests and the quality of mapping are of great importance for the quality and accuracy of forest inventorys and and assessments of carbon sequestration in forests. The capacity of different types of forests for growth and carbon sequestration is different by species, the age of the forest and land use history. It is important to understand which elements in forests describes the difference in the best way for assessing the carbon stock and carbon sequestration. Using stratified random sampling in forest inventorys reduces the number measurement plots in forests needed for the assessment. Neither birch nor conifer forests showed any clear signs that annual carbon sequestration was decreasing during the first six decades of the forest development.