No abstract

Top dieback in 40–60 years old forest stands of Norway spruce [Picea abies (L.) Karst.] in southern Norway is supposed to be associated with climatic extremes. Our intention was to learn more about the processes related to top dieback and in particular about the plasticity of possible predisposing factors. We aimed at (i) developing proxies for P50 based on anatomical data assessed by SilviScan technology and (ii) testing these proxies for their plasticity regarding climate, in order to (iii) analyze annual variations of hydraulic proxies of healthy looking trees and trees with top dieback upon their impact on tree survival. At two sites we selected 10 tree pairs, i.e., one healthy looking tree and one tree with visual signs of dieback such as dry tops, needle shortening and needle yellowing (n = 40 trees). Vulnerability to cavitation (P50) of the main trunk was assessed in a selected sample set (n = 19) and we thereafter applied SilviScan technology to measure cell dimensions (lumen (b) and cell wall thickness (t)) in these specimen and in all 40 trees in tree rings formed between 1990 and 2010. In a first analysis step, we searched for anatomical proxies for P50. The set of potential proxies included hydraulic lumen diameters and wall reinforcement parameters based on mean, radial, and tangential tracheid diameters. The conduit wall reinforcement based on tangential hydraulic lumen diameters ((t/bht)2) was the best estimate for P50. It was thus possible to relate climatic extremes to the potential vulnerability of single annual rings. Trees with top dieback had significantly lower (t/bht)2 and wider tangential (hydraulic) lumen diameters some years before a period of water deficit (2005–2006). Radial (hydraulic) lumen diameters showed however no significant differences between both tree groups. (t/bht)2 was influenced by annual climate variability; strongest correlations were found with precipitation in September of the previous growing season: high precipitation in previous September resulted in more vulnerable annual rings in the next season. The results are discussed with respect to an “opportunistic behavior” and genetic predisposition to drought sensitivity.

No abstract

Northern landscapes accumulate carbon in vegetation and soils while rivers transport significant amounts of land-derived carbon to coastal areas. Here, we quantify carbon sources and sinks in main ecosystems (forests, peatlands, mountains, agricultural areas, lakes) in Norway for 1990-2008, and compare riverine carbon transport with terrestrial carbon accumulation in Norway's four major discharge areas. Mean annual carbon accumulation (6.0 ± 0.9 Tg C; 19 g C m-2) in terrestrial ecosystems balanced 40% of national greenhouse gas emissions. The area-normalized terrestrial sink strength declined in the following order (in g C m-2 yr-1): tree biomass (40±3) >peatlands (19±15) >forest soils (9±1) >>lakes (2±1) >mountains (0.5±0.3), while agricultural soils were sources of carbon (-36±74). The most precise estimate in the carbon balance was for tree biomass, because of the underlying forest inventory data. Poor data on land management and soil type in agricultural soils, and on (former) drainage and peatland type resulted in high uncertainty in carbon loss and uptake estimates in agricultural soils and peatlands, which impacted the uncertainty in total terrestrial carbon accumulation. Also, carbon losses from disturbance in organic soil types were poorly constrained. Riverine coastal inputs of land-derived organic carbon (OC) were 1.0 ± 0.1 Tg C yr-1 (3.0 g C m-2 yr-1), with highest area-specific riverine export in western (4.5 g C m-2 yr-1) and lowest (1.7 g C m-2 yr-1) in subarctic Norway. In west and middle Norway, river OC export was approximately equal to carbon accumulation in soils and wetlands, while it was 50% of soil and wetland carbon accumulation in southeast and subarctic Norway. Lateral aquatic transport of carbon is not explicitly accounted for in forest soil carbon accumulation estimates, although aquatic fluxes represent a climate-dependent carbon loss from soil carbon pools. The lack of methods that adequately account for lateral fluxes in carbon balances adds considerable uncertainty to soil carbon sink estimates. Climate warming and associated changes in precipitation may result in substantial alterations of terrestrial and aquatic carbon fluxes, with uncertain implications for the terrestrial carbon sink of northern landscapes.

Relationships between soil C and N stocks and soil formation, climate, and vegetation were investigated in a gridded database connected to the National Forest Inventory in Norway. For mineral soil orders, C and N stocks were estimated to be 11.1–19.3 kg C·m−2 and 0.41–0.78 kg N·m−2, respectively, declining in the following order: Gleysols > Podzols > Brunisols > Regosols. Organic peat-type soils stored, on average, 31.3 kg C·m−2 and 1.10 kg N·m−2, whereas shallow Organic folisols stored, on average, 10.2 kg C·m−2 and 0.34 kg N·m−2. For Norway’s 120 000 km2 of forest, the total of soil C stocks was estimated to be 1.83 Gt C, with a 95% CI of 1.71–1.95 Gt C. Podzolic soils comprise the largest soil group and store approximately 50% of the forest soil C. Sixty percent of the soil C stock in Podzolic soils was stored in the mineral soil, increasing with temperature and precipitation. Poorly drained soil types store approximately 47% of the total forest soil C in Norway. Soils with water saturation have large C stocks mainly in the forest floor, suggesting that they are more susceptible to forest management and environmental change. Soil C stocks under pine and spruce forests were similar, although pine forests had larger C stocks in the forest floor, while spruce forests had the highest C stocks in the mineral soil compartment. C stocks in the forest floor increase from dry to moist ground vegetation, while ground vegetation nutrient classes reflect better the C and N stocks in the mineral soil.