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.
In Iceland planting of Siberian larch, the most substantial non-native species in Iceland, began in 1938 (Sigurdsson et al., 2005). Long-term studies indicate 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.
In the Southern French Prealps, a dense forest cover, primarily comprising Austrian black pines, was replanted during the mid-19th century, on a previously degraded steep land surface. Previous studies have shown that this reforestation strongly modified the local geomorphic and hydrological processes (leading to a decrease in physical erosion and surface runoff). In contrast, very little is known about the impact of this ~150 year old afforestation on local nutrient cycling.
This project aims to understand and quantify the behaviour of the micro-nutrients including Zn, Fe, Mg and Sr 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 will document the radiogenic stable isotope variations in soils, pore waters and trees in forest stands of different age in Iceland (Hallormstadur and Skorradalur) and France (Laval and Brusquet). With the aims of determining: (1) the main sources of nutrient 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. soil waters, leaves, bark, soils); (3) How these nutrient-proxy isotope and elemental compositions evolve with time, following afforestation.