Old unmanaged forests are commonly assumed to be carbon neutral; however, there is still a lack of reference studies available to increase the recognition of carbon stock changes in these forests. Studies of old forest carbon storage from hemiboreal regions are very rare compared to temperate and boreal forests in Europe; therefore, the aim of this study was to quantify the carbon stock in hemiboreal over-mature (167–213 years) Norway spruce (Picea abies (L.) Karst.) stands. To explore the total ecosystem carbon pool, the carbon stock of tree biomass, deadwood, and soil in unmanaged (for at least the last 40 years) spruce stands was calculated and compared between different forest site types on dry, wet, and drained mineral soils. Total carbon stock of hemiboreal over-mature spruce stands ranged from 164.8 Mg C ha⁻¹ to 386.7 Mg C ha⁻¹, and 238.5 Mg C ha⁻¹ on average, with no significant differences (p > 0.05) between the forest site types. The carbon stock of tree biomass was significantly affected by the basal area of the upper tree layer (p < 0.0001) and the interaction between the forest site type and proportion of spruce in the stand composition (p = 0.002). Tree biomass was the dominant carbon pool, followed by soil and deadwood in over-mature spruce stands.

Old-growth forests are widely recognised for the benefits they provide for biodiversity; however, a more comprehensive understanding of their role in climate change mitigation must still be established to find the optimal balance between different forest ecosystem services at a national or regional scale. Very few studies have assessed carbon pools in old-growth Scots pine (Pinus sylvestris L.)-dominated boreal forests, and none have been conducted in hemiboreal forests. Therefore, we assessed the carbon storage of the living tree biomass, deadwood, forest floor (soil organic horizon, including all litter and decomposed wood), and mineral soil in 25 hemiboreal old-growth (163–218 years) unmanaged Scots pine stands in Latvia. The studied stands were without known records of any major natural or human-made disturbance in the visible past. Our results show, that the total ecosystem carbon pool (excluding ground vegetation) was 291.2 ± 54.2 Mg C ha⁻¹, which was primarily composed of living tree biomass (59%), followed by mineral soil (31%), deadwood (5%), and the forest floor (5%). Within the studied stand age group, the total carbon pool remained stable; however, interchanges among the carbon pools, i.e., living biomass and laying deadwood, did occur.

For the last three decades, the area of old-growth forest stands in Europe has continued to increase as has their importance in achieving forest-related policy goals. This has triggered an increase in research interest in old-growth forests, both from climate change mitigation and biodiversity protection perspectives. However, carbon stock in old-growth stands in European hemiboreal forests had not been systematically studied. Therefore, in this article, we characterize differences in carbon stock between mature and old-growth stands on fertile mineral soils in hemiboreal Latvia to contribute to the understanding of carbon storage changes under different management strategies for forest lands. Carbon stock varied significantly both between old-growth stands of different dominant tree species and between mature (1.9–2.3 times younger) and old-growth stages of the same dominant species in forest stands. The carbon stock of tree biomass and deadwood was larger in old-growth stands, but their mean annual carbon stock change was significantly lower than in mature stands (27% to 47% depending on dominant tree species). Old-growth stands can serve as carbon reservoirs in areas with limited natural disturbances; however, for maintenance of climate neutrality, it is essential to expand the area of managed stands with larger annual carbon stock increase.Study Implications: Forest ecosystems play a major role in regulation of global climate: They can store high quantities of carbon and also can gain or lose it rather quickly. For the last three decades, the area of old-growth forest stands in Europe has continued to increase as has their importance in achieving forest-related policy goals. Old-growth forests can represent the baseline carbon-carrying capacity in particular conditions. Therefore, we characterized differences in carbon stock between mature and old-growth stands on fertile mineral soils in hemiboreal Latvia to contribute to the understanding of carbon storage and for planning forest management activities.

Organic soils store a large amount of carbon stock, but they are also a large source of greenhouse gas emissions in a forest. Results of previous studies do not provide whole-country representative data of carbon stock in drained fertile organic soil forests in Europe, as the effects of stand age and dominant tree species are significant. Moreover, the growing role of old-growth stands has triggered interest in empirical data about drained organic soils. These data might serve as a reference of theoretical carbon carrying capacity that could be achieved in hemiboreal Latvia. We aimed to characterize tree biomass and deadwood carbon pools in coniferous old-growth stands on fertile, drained organic soils. Seven old-growth Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) dominated stands (131–174 years) were measured. Both groups of stands had similar carbon stocks, reaching 167 and 154 t C ha⁻¹ in tree biomass and 11 and 10 t C ha⁻¹ in deadwood, respectively. A large variation in deadwood carbon storage was found across sample plots, ranging from 0.6 to 26.6 t C ha⁻¹. Dead standing trees and downed logs store a great share of the total deadwood carbon, 5 and 4 t C ha⁻¹, respectively. Significantly less carbon was stored in dead standing trees with broken tops (1 t C ha⁻¹). Further assessment of soil carbon stock and fluxes is ongoing to reduce uncertainty in the soil carbon evaluation of old-growth stands in the context of climate change mitigation targets in a hemiboreal region.

The increasing interest in carbon budget estimation and the growing use of woody biomass in bioenergy production raises the necessity for precise estimates of belowground biomass and soil carbon pools in forest ecosystems, particularly in terms of changes in the age structure of forests. The aim of this study was to estimate the belowground biomass of young (< 40 years) stands of Scots pine (Pinus sylvestris L.) in Latvia. The biomass of small roots (diameter 2-20 mm), coarse roots (diameter > 20 mm), and stumps of 39 trees from eight stands growing on dry, nutrient-poor mineral soils was measured and compared to the aboveground variables of sampled trees. The results revealed that stumps, small roots, and coarse roots comprised 43%, 35% and 22%, respectively, of the belowground biomass of young Scots pines. The proportion of belowground biomass over the total tree biomass was age-dependent, ranging from 33% to 17% for 8-year and 40-year old trees, respectively. Aboveground tree variables were significantly correlated with the belowground biomass, being stemwood volume and basal area the best predictors (R2 = 0.86-0.98, relative errors = 26-43%) of the belowground biomass components. Accordingly, the developed models produced more accurate estimates compared to previous models for the region, thus reducing the uncertainty in determining the carbon budget for belowground biomass. Still, an analysis of a more comprehensive dataset is needed to account for the effect of the social status of trees, as well as the within- and between-stand variation.

Birch (Betula pendula Roth, Betula pubescens Ehrh.) and European aspen (Populus tremula L.) stands dominate the deciduous forests of Northern Europe. Due to increasing forest protections, more deciduous stands will reach the old-growth stage. Thus, data on the carbon storage potential in such areas are essential. We aimed to establish a benchmark for carbon stocks of the main carbon pools in old-growth deciduous hemiboreal stands. Carbon pools were calculated from measurements in forty old-growth (104–148 years-old) deciduous stands in forests on fertile mineral soil. The carbon stock in these stands is distributed across tree biomass (~ 60%), mineral soil (~ 30%), the forest floor (~ 5%), and deadwood (~ 4%). Living tree biomass and deadwood carbon pools were closely associated with stand parameters: dominant tree species, standing volume and stand density. As the stand ages and tree dieback occurs, the significance of individual large trees to maintain high density and standing volume, thus also the carbon stock of the stand, rises. Reliance on a small number of large trees makes the carbon storage in old-growth stands fragile and easily affected by natural disturbances. It happens at an earlier age for species with a relatively short life span, like birch and aspen. Our data from stands with the limited recent influence of such disturbances provide a benchmark for carbon storage potential in old deciduous stands. Keywords: carbon pools, tree biomass, deadwood, forest floor, soil, birch, European aspen