Forests play a key role in the carbon cycle, acting as both sources and sinks of carbon. Forests sequester carbon by capturing atmospheric carbon dioxide and transforming it into biomass through photosynthesis. Sequestered carbon then accumulates in the form of biomass, litter and as carbon in forest soils. Afforestation, the transformation of non-forested lands to forest plantations, has the potential to enhance carbon sequestration and increase carbon stocks, and is seen as a viable way of mitigating greenhouse gas emissions. But trees take up considerable amounts of nutrients from soils; so repeated harvesting of this biomass or planting in low nutrient soils may impair long-term soil productivity.
It has been estimated that some 30% of Iceland was covered with forests before the time of settlement (c. AD 874), but by the 1890’s this had fallen to less than 1% (Sigurdsson et al., 2005). With the decline of forests, soil erosion increased, causing the loss of valuable fertile land. Soil protection and carbon sequestration are therefore the main aims of afforestation in Iceland. Formal afforestation started in 1899, and more recently the Regional Afforestation Project Act (no 56/1999) aimed to afforest 5% of the area in Iceland below 400 m above sea-level by 2040 (no 56/1999).
Planting of Siberian larch, the most substantial non-native species in Iceland, began in 1938 (Sigurdsson et al., 2005). Few long-term studies have been conducted on changes in soil nutrients after afforestation in Iceland, but the little work that has been done indicates that some macronutrients show an increase in bioavailability with forest maturity (Sigurdsson et al., 2005, Ritter, 2007) but the sources and pathways of nutrient delivery remain poorly understood.
This project aims to understand and quantify the behaviour of the micro-nutrients Zn, Fe, Cu and Ni in soils and trees accompanying afforestation. These essential trace elements are integral to forest growth and reproduction, and where their concentrations are limited this leads to physiological stress and external symptoms such as stunting. The sources of these micro-nutrients and how they are used by the trees can be traced using their stable isotope composition.
For example, Zn in soils is largely controlled by bedrock mineralogy, litter recycling and aeolian deposition (Alloway, 2008), each with a distinct isotope composition (Viers et al., 2007; Moynier et al., 2009). Zinc uptake by plants also involves isotope fractionation as does translocation within the plant itself. Typically, the heavy isotopes of Zn are preferentially absorbed to root cell walls (so, roots are enriched in heavy Zn isotopes). Zinc isotopes also show a preferential aerial migration of light isotopes during transport by the xylem from root to shoot, attributed to cross-membrane diffusion and Zn binding to cell walls (e.g. Caldelas & Weiss, 2016). Our preliminary results for Siberian larch (Larix sibirica) from forest stands planted up to 63 years ago at Hallormstadur in east Iceland, show systematic Zn stable isotope variations between soils, pore waters and trees that evolve with stand age, indicating substantial changes in the source and utilisation of the nutrients over time.
This project aims to use Fe, Zn, Cu and Ni isotope and elemental concentrations to determine: (1) the main sources of micronutrient supply and removal to soils and trees; (2) the mechanisms that control uptake and translocation of these elements in trees and other vegetation, and how these impact the nutrient concentrations in the different parts of the forest environment (e.g. leaves, bark, soils); (3) How these micro-nutrient isotope and elemental compositions evolve with time, following afforestation.
Click on an image to expand
Fig. 1 Pore water sampling at Hallormstadur
Fig. 2 Zn isotope composition against age of forest stand (years) showing temporal variations following afforestation.