Northern Canadian boreal forests provide a broad range of ecosystem services including carbon sequestration, habitat and biodiversity, air and water purification, and regional and global climate regulation. These forests also provide numerous resources for the Forest Industry in Canada such as black spruce (
Picea mariana (Mill.) BSP.) wood products, one of the dominant species [
1]. Boreal northern black spruce forests are characterized by the development of thick organic layers in regions prone to paludification such as the interior of Alaska, the Canadian Hudson Bay-James Bay lowlands, and Labrador. Paludification is a natural process where organic material accumulates on the ground surface over time, resulting in higher soil moisture levels and elevated water tables [
2,
3]. These conditions alter forest succession dynamics and favor the invasion of Sphagnum moss species [
4,
5,
6].
Black spruce trees are well adapted to paludified sites because of the development of an adventitious root system,
i.e., the continuous upward development of roots at the base of the stem [
7,
8]. This capacity of developing adventitious roots constitutes an advantage for the species, allowing it to grow in wet environments since roots damaged by flooding can be replaced by new roots growing near the surface of the organic layer upwards on the stem in drier and more favourable conditions. At maturity, the root system of black spruce trees is often exclusively adventitious [
7,
8], which means that initial roots fail to develop and disappear with time. Because of this continuous development of adventitious roots, it is rather difficult to obtain a tree’s true age, since the root collar of trees (root/shoot interface) and initial growth rings are located under the roots and sometimes even completely missing [
7]. This phenomenon has also been observed in white spruce (
Picea glauca (Moench) Voss; [
9,
10], balsam fir (
Abies balsamea (L.) Mill.; [
11]) and Norway spruce (
Picea abies (L.) Karst.; [
12]).
From the perspective of sustainable forest management, erroneous age determination can lead to incorrect interpretations of forest dynamics [
13,
14,
15,
16,
17]. For instance, natural disturbance studies relating pulses of tree establishment to specific events or climate variables need accurate tree ages to correctly interpret data [
18,
19]. A recent study done in northern Quebec’s black spruce forests showed that 80% of naturally-established commercial stands had been misclassified (underestimated) in terms of age classes, a fundamental indicator of sustainable forest management in Canada [
15]. Age underestimation leads to overestimation of forest productivity [
20], which is traditionally estimated using the site index (SI) and is the average height of dominant and co-dominant trees at reference age, usually 50 years [
21]. SI is widely used in empirical growth and yield models to calculate sustainable allowable harvest levels of commercial species (e.g., [
22]). To correctly illustrate the productive potential of a site, suppressed trees or periods of very slow initial growth need to be avoided for SI measurement; this is why age at breast height or at 1 m, it is usually taken as the zero height level [
21,
22]. Age-correction index can then be used to estimate the time needed for trees to reach breast height or 1 m [
22]. Problems in stand dynamics interpretations occur when age adjustments wrongly assume that variability between trees is small, that specific site conditions have little effect on early growth rates, or that growth rates of seedlings are a good reflection of early growth rates of mature trees (sapling method) [
23]. For example, the age correction for black spruce from Pothier and Savard [
22] would be up to seven years, while DesRochers and Gagnon [
7], Vasiliauskas and Chen [
20] and Parisien
et al. [
24] found age underestimations up to 19, 18, and 26 years, respectively. Unlike Pothier and Savard [
22] that used the sapling method, the other three studies [
7,
20,
24] evaluated the actual time it took trees to reach 1 m. Moreover, because adventitious roots continue developing over time with the continuous accumulation of organic matter and growth of mosses [
8], age underestimation of black spruce trees probably increases with the organic layer thickness (OLT). This would require specific age correction indices for sites with different OLT [
24]. However, it is difficult to build age correction tools when the extent of the age under-estimation is unknown. The objective of this study was thus to estimate aging errors that are done with standard ring counts (at 1 m or at ground level) of black spruce trees in relation to the organic layer thickness. We hypothesized that age underestimations would increase with the thickness of the organic layer. Aging error was evaluated on 81 trees growing in sites with different levels of organic matter accumulation by aging trees at 1 m and ground level (0 m), and comparing it to real age by cross-dating growth rings down into the stump to the root collar (root/shoot interface). To our knowledge, no previous study has examined the effect of OLT on aging error of black spruce trees.