Abstract
Large, old trees store a considerable part of the total carbon in old-growth forests; however, their influence on long-term climate change mitigation capacity remains unclear. Damage caused by wind and insect outbreaks can rapidly kill trees, and these natural events are expected to increase with climate change, thereby reducing the long-term carbon storage capacity of old-growth stands. The aim was to estimate the importance of large, old trees in carbon storage at the stand level and the potential risks for the carbon stock if they die. The data were collected from Scots pine and Norway spruce stands on mineral soils, of which 44 were old-growth stands, and 47 were mature stands two times younger than the former. Stand-level carbon storage in tree biomass was investigated; the presence of deadwood was analyzed. The carbon stock changes were calculated by excluding the 1-15 largest trees per sample plot. The results showed that the largest trees in old-growth coniferous stands hold a disproportionally large (~ 50% of the total tree biomass carbon pool) share of carbon compared to mature stands, ~ 50% and ~ 10%, respectively. A reduction of the tree biomass carbon pool by approximately 50% required a removal of around 30% more trees in mature than in old-growth stands. A relatively low proportion of deadwood from the total stand volume (~ 20%) suggests a limited single-tree mortality and vulnerable mitigation potential in the assessed old-growth stands. Nevertheless, continuous single-tree mortality due to ageing or windthrow re-shapes forest structure, and raises the potential risk of the stands to maintain and/or further increase substantial tree biomass-linked carbon pools.