Long-term spatiotemporal dynamics in a mountain birch (Betula pubescens ssp. czerepanovii) forest in south-east Norway.

Mountain birch forest covers large areas in Eurasia, and their ecological resilience provides important ecosystem services to human societies. This study describes long-term stand dynamics based on permanent plots in the upper mountain birch belt in SE Norway. We also present forest line changes over a period of 70 years. Inventories were conducted in 1931, 1953, and 2007. Overall, there were small changes from 1931 up to 1953 followed by a marked increase in biomass and dominant height of mountain birch throughout the period from 1953 to 2007. In addition, the biomass of spruce (Picea abies) and the number of plots with spruce present doubled. The high mortality rate of larger birch stems and large recruitment by sprouting since the 1960s reveal recurrent rejuvenation events after the earlier outbreak of the autumnal moth (Epirrita autumnata). Our results demonstrate both a high stem turnover in mountain birch and a great ability to recover after disturbances. This trend is interpreted as regrowth after a moth attack, but also long-term and time-lagged responses due to slightly improved growth conditions. An advance of the mountain birch forest line by 0.71 m year-1 from 1937 to 2007 was documented, resulting in a total reduction of the alpine area by 12%. Most of the changes in the forest line seem to have taken place after 1960. Regarding silviculture methods in mountain birch, a dimension cutting of larger birch trees with a cutting interval of c. 60 years seems to be a sustainable alternative for mimicking natural processes.

Biotic threats for 23 major non-native tree species in Europe.

For non-native tree species with an origin outside of Europe a detailed compilation of enemy species including the severity of their attack is lacking up to now. We collected information on native and non-native species attacking non-native trees, i.e. type, extent and time of first observation of damage for 23 important non-native trees in 27 European countries. Our database includes about 2300 synthesised attack records (synthesised per biotic threat, tree and country) from over 800 species. Insects (49%) and fungi (45%) are the main observed biotic threats, but also arachnids, bacteria including phytoplasmas, mammals, nematodes, plants and viruses have been recorded. This information will be valuable to identify patterns and drivers of attacks, and trees with a lower current health risk to be considered for planting. In addition, our database will provide a baseline to which future impacts on non-native tree species could be compared with and thus will allow to analyse temporal trends of impacts.

Gap formation and dynamics after long-term steady state in an old-growth Picea abies stand in Norway: Above- and belowground interactions.

Stand dynamics and the gap initiation prior to gap formation are not well-understood because of its long-term nature and the scarcity of late-successional stands. Reconstruction of such disturbance is normally based on historical records and dendroecological methods. We investigated gap initiation and formation at the fine-scale stand level in the old-growth reserve of Karlshaugen in Norway. Given its long-term conservation history, and thorough mapping in permanent marked plots with spatially referenced trees, it provides an opportunity to present stand development before, during, and after gap formation. Late-successional decline in biomass was recorded after more than 50 years of close to steady state. Gaps in the canopy were mainly created by large old trees that had been killed by spruce bark beetles. Snapping by wind was the main reason for treefall. Long-term dominance of Norway spruce excluded downy birch and Scots pine from the stand. Comparisons of the forest floor soil properties between the gap and nongap area showed significantly higher concentrations of plant available Ca within the gap area. Plant root simulator (PRS™) probes showed significantly higher supply rates for Ca and Mg, but significantly lower K for the gap compared to the nongap area. Soil water from the gap area had significantly higher C:N ratios compared to the nongap area. Fine-scale variation with increasing distance to logs indicated that CWD is important for leaking of DOC and Ca. Our long-term study from Karlshaugen documents gap dynamics after more than 50 years of steady state and a multiscale disturbance regime in an old-growth forest. The observed disturbance dynamic caused higher aboveground and belowground heterogeneity in plots, coarse woody debris, and nutrients. Our study of the nutrient levels of the forest floor suggest that natural gaps of old-growth forest provide a long-lasting biogeochemical feedback system particularly with respect to Ca and probably also N. Norway spruce trees near the gap edge responded with high plasticity to reduced competition, showing the importance of the edge zone as hot spots for establishing heterogeneity, but also the potential for carbon sequestration in old-growth forest.