Article Carbon Pools in Old-Growth Scots Pine Stands in Hemiboreal Latvia 1, 1 2 1 1 Laura K, enin¯ , a *, Ieva Jaunslaviete ,Lıga¯ Liepa , Daiga Zute and Aris¯ Jansons 1 Latvian State Forest Research Institute Silava, R¯ıgas street 111, LV-2169 Salaspils, Latvia; [email protected] (I.J.); [email protected] (D.Z.); [email protected] (A.J.)¯ 2 Department of Silviculture, Forest faculty, Latvia University of Life Sciences and Technologies, Liela 2, LV-3001 Jelgava, Latvia; [email protected] * Correspondence: [email protected]; Tel.: +371-26-172-965 Received: 1 August 2019; Accepted: 16 October 2019; Published: 16 October 2019 Abstract: 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. Keywords: Pinus sylvestris; deadwood; above- and below-ground tree biomass; forest floor; mineral soil; semi-natural forest; over-mature forest 1. Introduction Carbon sequestration and storage is one of the essential ecosystem services provided by forests, which has great potential to mitigate climate change [1,2]. Boreal forests store approximately one-third of global terrestrial carbon and therefore are highly important in this context [3]. The carbon sequestration in forests is affected by climate [4], soil [5], and natural disturbance [6], along with forest stand characteristics, such as tree species composition [7], age [8,9], and silviculture practices [10–13]. The total ecosystem carbon (TEC) is stored in different pools (above- and below-ground tree biomass, deadwood, forest floor, and soil), and the dynamics of these pools depend on forest development stage [14]. Generally, the TEC pool increases with stand age and is closely related to site productivity [4,5]. Dynamics of TEC pools can be abruptly altered following stand-replacing natural or human-made disturbances. Following windthrow or fire events, a large proportion of live tree biomass is turned into deadwood, while following clear-cut, a large proportion of live biomass is removed from the site [15,16]. The old-growth forests are important from ecosystem functioning and biodiversity aspects [17]. The long and intensive exploitation of European forests has heavily shaped the distribution, structure, and composition of these ecosystems. In Europe, old-growth forests with natural structures are rare, Forests 2019, 10, 911; doi:10.3390/f10100911 www.mdpi.com/journal/forests Forests 2019, 10, 911 2 of 10 mainly preserved in distant areas, where forest management operations have been unprofitable due to challenging terrain or low productivity [17,18]. Globally, old-growth boreal and temperate forests serve as a major carbon sink [4]. In some specific conditions for a certain period, old-growth forests might become carbon neutral [19] or even a carbon source, when net carbon balance becomes negative [20]. In Europe, several studies have investigated ecosystem carbon pools in stands dominated by old-growth Norway spruce (Picea abies (L.) Karst.) [14,16,21,22]. These studies show that live tree biomass and soil are the largest carbon 1 pools [14,16,21,22]. Among these studies, the lowest TEC (175 Mg ha− ) was estimated in old-growth 1 boreal forests [16], the highest TEC (> 400 Mg ha− ) in Central European forests [14,22], and moderate 1 estimates (240 Mg C ha− ) in hemiboreal forests [21], implying that TEC pools change according to latitudinal position. Similar observations are reported from southern boreal forests in Canada, where 1 depending on site productivity the TEC pools ranged from 120 to 725 Mg C ha− [23]. The soil organic carbon (SOC) pool in old-growth forests can increase over time [24] or remain rather stable [14,22], which likely depends on soil and climate conditions, as well as on forest stand characteristics. The deadwood carbon pool in old-growth forests is related to tree morality rates, and generally increases with stand age [22]. Forest floor carbon pools are usually one the of the smallest TEC pools, closely related to site productivity and decomposition rates [14,16,22]. The dominant tree species is a significant factor influencing TEC dynamics [25]. Scots pine (Pinus sylvestris L.) is a long-living common tree species in Northern Europe [26]. Yet, knowledge of TEC pools in old-growth Scots pine stands is scarce, as the majority of studies have assessed TEC in old-growth Norway spruce stands [14,16,21,22]. Therefore, the aim of our study was to assess the main carbon pools (living tree biomass, deadwood, forest floor, and mineral soil) in hemiboreal old-growth (>160 years) Scots pine forests in Latvia. Gained insight into the TEC of old-growth forests could be used to develop balanced management strategies for protected forests as well as to maximise climate change mitigation through adaptive forest management strategies [1,27,28]. Studies have shown that in old-growth Norway spruce-dominated stands, the total carbon pool remained stable between stand ages of 116 to 145 years [22]. Considering the low annual growth rates of Scots pine trees older than 100 years [29] and the absence of evidence of recent natural or human-made disturbances in studied stands, we hypothesised that the TEC pool of old growth Scots pine forests would not change in stands aged between 163 and 218 years. 2. Materials and Methods 2.1. Study Area Latvia is located in the hemiboreal region within a moderately cool and moist climate [26]. According to data from the Latvian Environment, Geology and Meteorology Centre, the mean annual temperature is +5 C, ranging from around 5 C in January to +17 C in July. The mean annual ◦ − ◦ ◦ precipitation is 550–850 mm; about 500 mm falls during the growing season (April to October). According to the National Forest Inventory data, Scots pine is the second-most common tree species in Latvia, occurring in 874,000 ha or 26% of the total forest area. The largest proportion of Scots pine grows on sandy (19%) or sandy loam (37%) soils. The sampling plots were established in the Hylocomniosa forest site type, which is characterised by medium-rich sandy loam soils with a normal moisture regime [30]. In this forest site type, 36% of the area is occupied by pine-dominated forests. In Latvia, the majority (85%) of the forests older than 120 years are dominated by Scots pine. 2.2. Fieldwork The methodology used for the fieldwork and the data analysis is the same as was published in 2018 by K, eni¯ n, a et al. [21], where old-growth Norway spruce stands were analysed. In total, 25 stands were stratified from the set of potential areas using the following criteria: age > 160 years, dominance of Scots pine (> 50% out of the basal area), location (> 5 km from villages and > 1 km from Forests 2019, 10, 911 3 of 10 roads), and no documented information or field observations indicating past management activities Forests 2019, 10, x FOR PEER REVIEW 3 of 10 (e.g., stumps), and growing in mesotrophic soil conditions (Hylocomniosa forest type) in protected areas in Latvia (Figure1). The mean stand age ( 95% confidence interval) in our study was 179 7 years, Latvia (Figure 1). The mean stand age (± ±95% confidence interval) in our study was 179 ± 7 years, ranging between 163 and 218 years. Figure 1. Location of the sampled old-growth Scots pine-d pine-dominatedominated stands in the territory of Latvia. Six toto eight eight circular circular sampling sampling plots plots of 500 of m 2 500(143 m sample2 (143 plots sample in total) plots were in placedtotal) systematicallywere placed insystematically each stand. in In each these stand. sampling In these plots, sampling the tree plot speciess, the and tree diameter species and at breast diameter height at breast (DBH) height were recorded(DBH) were for allrecorded the living for and all standingthe living dead and treesstanding of DBH dead6.1 trees cm of and DBH for lying≥ 6.1deadwood cm and for of lying DBH ≥ deadwood14.1 cm. Inof theDBH centre ≥ 14.1 of cm. the In large the centre sampling of the plots large (500 sampling m2), smaller plots subplots(500 m2), (25smaller m2) weresubplots placed, (25 ≥ wherem2) were living placed, trees where of DBH living 2.1 totrees 6.0 of cm DBH and laying2.1 to 6.0 deadwood cm and laying of DBH deadwood 6.1 to 14.0 of cm DBH were 6.1 recorded. to 14.0 cm wereThe recorded. tree height was measured for three to five living trees of each species in each canopy layer and forThe all tree snags height (dead was trees measured with broken for three tops).