Needle Biomass Turnover Rate in Scots Pine Stands of Different Ages

Abstract

Understanding needle biomass turnover rates in Scots pine (Pinus sylvestris L.) stands is crucial for modelling forest ecosystem dynamics and nutrient cycling. This study examined needle litterfall and biomass turnover in Scots pine stands of varying ages in temperate forests (western Poland). The research focused on determining how stand age affects needle biomass, litterfall and the associated turnover rates. Data were collected from 20 Scots pine stands aged 26 to 90 years, and needle litterfall was measured and analysed in relation to stand characteristics such as age, density and biomass. The average annual needle litter production of the sampled Scots pine stands was 2008 kg·ha⁻¹·year⁻¹, similar to the values previously reported for this tree species in other temperate forests in Europe. The average needle biomass turnover rate for sampled Scots pine stands was 23.4%. We could not support the hypothesis that this parameter depended on the age of the Scots pine stand. The needle biomass turnover rate showed a positive correlation with crown length and a negative correlation with stand density due to the very weak correlations; however, further research is needed to confirm these relationships. Despite this, the parameter can be used to estimate needle litterfall and can be applicable to conditions corresponding to those of temperate forests in Central and Western Europe. This study also highlights the need for further research on needle biomass turnover in temperate forests to improve the accuracy of carbon and nutrient cycling models. This work contributes to a deeper understanding of the role of needle litterfall in maintaining soil fertility and forest productivity, offering insights into sustainable forest management and conservation strategies.

1. Introduction

Forests play a crucial role in the global carbon cycle, serving as significant reservoirs of biomass [1,2,3]. One of the most important components of forests is needle biomass, which participates in the process of photosynthesis and the cycling of nutrients and organic matter. As trees grow and mature, the proportions of tree biomass (root/stem/leaves) and the dynamics of their litterfall change, impacting the entire forest ecosystem [4,5]. The process of litterfall is particularly important for understanding the life cycle of coniferous forests and their impact on the environment [6,7,8]. Litterfall is the primary pathway for the transfer of nutrients from vegetation to the soil, forming a crucial link between producers and decomposers [9]. Litterfall also maintains soil fertility, the most important resource of organic matter [10,11]. Furthermore, litterfall produces an organic layer that protects the soil from extreme temperature and moisture changes, prevents soil erosion, hosts microbial communities, enhances organic matter mineralisation and improves soil physical and chemical properties such as water availability, infiltration and nutrient absorption [12,13,14]. Litterfall is directly linked to aboveground net primary productivity [13], making it a key parameter for measuring, modelling and predicting forest ecosystem dynamics [15].

Scots pine (Pinus sylvestris L.) is among the most important forest tree species in Europe and Asia, both in terms of range and ecological significance [16]. Its ability to adapt to different environmental conditions makes it an ideal model species for studying needle biomass dynamics [17]. Understanding the process of litterfall in Scots pine stands is crucial to identifying this species’ role in the global carbon cycle [18].

Needles are the most significant contributors to total litterfall, accounting for approximately 70%–85% of Scots pine stands [19,20,21]. Moreover, the correlation between needle and total litterfall is highly significant [22,23,24]. The amount of needle litterfall is influenced by various ecological factors and stand characteristics, including species composition [24,25], stand age [20,23], stand density [26], mean diameter at breast height (DBH) [27], basal area [28,29], aboveground biomass [6,9], volume increment [30], site index [15,27], crown closure and canopy openness [14,29]. Additionally, needle litterfall is affected by habitat conditions, such as water and nutrient availability [22,26], temperature [31,32], soil properties [33,34], latitude and elevation [23,27], edaphic conditions and anthropogenic disturbances [14,35].

The foliar biomass turnover rate (commonly defined as the ratio between measured foliar litterfall and modelled foliar biomass) is crucial for estimating biomass input in soil carbon models and understanding nutrient cycling in forest ecosystems [18,36]. Knowing the turnover rate for different tree species across various geographical regions enhances model accuracy. Factors influencing this rate need to be identified to refine these models further. While studies of needle litterfall have focused mainly on boreal forests [6,37,38], there is a scarcity of data on the temperate zone, especially in Central and Eastern Europe. Needle biomass turnover rates were reported for Scots pine in Finland [9,37,39] and for Norway spruce (Picea abies (L.) H. Karst) in Finland and Sweden [36,39].

In this study, we are concerned with the needle biomass turnover rate in Scots pine stands of different ages. Stand age is the most important factor affecting productivity and biomass increase dynamics [40]. As trees age, their physiological needs and resource allocation strategies change. Young trees invest more resources in rapid growth and biomass production, while older trees focus on maintaining stability and survival [41,42,43]. The relationship between the needle biomass turnover rate and tree age has not yet been studied.

The aim of this study was to determine the rate of needle biomass turnover in Scots pine stands of different ages in the temperate zone. Special attention was also given to assessing the impact of other accompanying characteristics of the stands (e.g., stand density, mean DBH, basal area, volume, biomass) on the rate of needle biomass turnover.

Based on the existing literature and preliminary observations, we proposed the following hypothesis: the rate of needle biomass turnover is correlated with stand age and is higher in younger Scots pine stands compared to older stands. This assumption was based on the more intense growth and greater metabolic activity of younger trees in comparison to older trees—differences that directly impact the rate of needle turnover and, consequently, the amount of biomass deposited on the soil surface.

2. Materials and Methods

2.1. Study Area

The study was carried out in 20 sample plots (Figure 1,

Table 1. The main characteristics of the sample plots established in 20 Scots pine stands.

Sample Plot	Stand Age (years)	Stand Density (tree·ha−1)	Quadratic Mean Diameter (cm)	Stand Height (m)	Mean Crown Length (m)	Basal Area (m2·ha−1)	Stand Volume (m3·ha−1)	Crown Cover (%)	Site Index (m)
1	26	5500	8.6	9.0	3.4	31.8	37	93	30.7
2	27	2900	10.9	7.1	3.8	22.2	78	89	32.2
3	32	2650	11.3	12.4	3.5	26.8	150	75	23.3
4	41	2281	13.8	15.2	4.5	34.2	246	79	32.4
5	42	1457	14.4	13.0	4.4	24.1	145	58	23.2
6	45	1750	15.7	15.5	4.4	33.7	244	79	29.4
7	46	1556	13.5	13.3	5.0	31.2	224	76	28.3
8	47	794	21.1	17.0	6.6	27.8	221	67	30.6
9	48	1331	18.4	19.1	5.4	35.5	320	83	34.0
10	48	1850	14.5	14.0	4.1	30.7	201	67	24.7
11	51	2066	13.5	13.3	4.7	29.8	195	85	27.0
12	51	862	21.3	19.7	5.6	30.9	281	68	31.2
13	53	1375	14.9	14.4	4.5	24.1	162	64	25.5
14	60	1750	14.8	11.3	3.9	30.3	178	73	22.5
15	67	1387	15.6	15.6	4.4	29.1	211	55	26.3
16	74	763	21.2	18.5	5.4	26.9	230	54	24.0
17	80	870	26.9	22.3	6.8	49.6	508	71	27.7
18	88	681	24.1	19.0	5.8	31.0	270	77	29.8
19	89	719	25.6	21.4	6.0	37.1	366	74	25.5
20	90	634	26.1	21.9	6.2	34.0	339	61	24.8

) in semi-natural, planted Scots pine (Pinus sylvestris L.) stands of different ages. The sample plots were located in western Poland (15.9°–16.6° E, 52.2°–52.9° N) across four forest districts (Oborniki, Sieraków, Krucz and Wolsztyn).

The study area features nutrient-poor habitats on podzolic soils, with Scots pine as the dominant tree species. These trees mostly form uniform stands with a small admixture of other species, like silver birch (Betula pendula Roth). These forests are managed by the State Forests National Forest Holding according to a forest management plan designed for multiple ecosystem services, such as wood and non-wood products, climate regulation, soil protection, water supply, recreation and biodiversity conservation. Standard management practices applied in these forests include tree planting, natural regeneration, a rotation period of about 100 yrs and thinning once per decade [44].

The forests are situated in a temperate zone at a low elevation of 100–140 m above sea level. The area receives an average annual rainfall of 580 mm, has an average temperature of 8.2 °C and experiences a growing season lasting 200–220 d [45].

2.2. Sample Plot Selection

The sampled stands had to meet specific criteria. Firstly, the proportion of Scots pine in the stands had to be above 90%. This high percentage ensured the dominance of the target species in the research areas. Additionally, each stand had to have a compartment area exceeding 1 ha. This size requirement was necessary to provide a sufficiently large and representative sample for the study. Furthermore, the selected stands had not undergone thinning since 2016. This criterion was important to ensure that recent forest management activities did not influence the study results. The stands were also chosen based on the low presence of folivorous insects in the preceding three years to minimise the potential confounding effects of insect damage on the study outcomes. The selection process began with an extensive review of forest management records to identify potential stands that met the initial criteria. Following this, field inspections were conducted to verify the conditions on-site. The combination of desk-based review and field verification ensured that the selected stands were suitable for the research objectives. During the field inspections, the exact locations of the sample plots were determined within each stand. Care was taken to avoid edge effects by placing plots well within the stand boundaries. Each plot was carefully marked and mapped for consistent monitoring throughout the study period. The rigorous selection process for the sample plots ensured that the study was based on representative and comparable stands.

2.3. Tree Measurements

Tree measurements were conducted from July to September 2018. In each stand, a grid of squares with sides of 25 m was established. The intersections of the grid lines were numbered from 1 to 16, and from these, eight points were randomly selected to serve as the centres of circular sample plots (Figure 2). The size of the sample plots varied according to the age of the stands. In particular, for 21–40-year-old stands, the plots were 0.01 ha in area; for 41–60, 0.02 ha; for 61–80, 0.03 ha; and for 81–100, 0.04 ha.

Each sample plot was thoroughly assessed for several parameters. All trees were measured for DBH using a DP II calliper (Haglöf Sweden, Langsle, Sweden) with an accuracy of 1 mm. Tree heights and crown base heights were measured using a Vertex IV hypsometer (Haglöf Sweden, Langsle, Sweden) with an accuracy of 10 cm, which enabled the length of the crown to be determined. Canopy cover percentage was estimated in each plot by evaluating the crown projection area.

This detailed measurement process allowed for the calculation of several key stand characteristics (Table 1), including the stand density (number of trees per hectare), the quadratic mean diameter and Lorey’s height (defined as the mean height of trees weighted by their basal area). Additionally, the mean crown length, basal area and stand volume were computed. The stand volume was calculated using empirical equations developed by Bruchwald [46]. Furthermore, the site index at 100 yrs was determined based on the methodology of Socha et al. [47]. Other important parameters, such as crown cover, tree biomass, needle biomass and needle mass fraction, were also calculated for each plot but after sample collection and laboratory procedures had been conducted.

2.4. Sample Collection and Laboratory Procedures

One plastic tray was placed in each of the eight sample plots within the 20 analysed stands to collect fallen needles, totalling 160 trays. Each tray was square-shaped, with sides of 50 cm and 5 cm high sidewalls. These trays were placed randomly along the perimeter of a circle with a radius of 2 m from the centre of each circular plot. The random placement was achieved using a random number generator to select an angle from 0 to 360° relative to due north.

The empty trays were set out on 1 September 2018. Subsequently, at the beginning of each month for an entire year, only the fallen needles were collected from the trays and stored in appropriately labelled paper envelopes. The monthly collection schedule minimised decomposition and other changes in the needle mass. Using labelled envelopes helped in maintaining the identity and source of each sample accurately. After a year, the collected needles from each sample plot were dried in ovens at 65 °C with forced air circulation. The drying process continued until a constant dry mass was achieved [48]. The needle mass was then weighed using a precision scale with an accuracy of 0.001 g. The dry mass of the needles collected throughout the year from each stand was then converted to a per-hectare basis and expressed in kilograms. This systematic collection and processing ensured accurate measurement of needle litterfall.

2.5. Data Analyses

In each stand, measurements of the diameter at breast height (DBH) and the height of all trees were taken, which enabled the calculation of aboveground biomass (kg·ha⁻¹) and needle biomass (kg·ha⁻¹). Both parameters were estimated using allometric equations proposed by Socha and Wężyk [49].

The aboveground biomass of each tree ($B_{ab}$) is calculated as follows:

$$B_{ab}=0.0677·h^{0.5474}·{\left(d^2\right)}^{1.0361}$$

(1)

The dry biomass of needles ($B_n$) is calculated as follows:

$$B_n=0.231644·h^{-0.54952}·d^{1.716144}$$

(2)

where d is the diameter at breast height and h is the height of the tree.

Subsequently, the needle mass fraction (%) was calculated as the ratio of needle biomass to total aboveground biomass. The annual needle litterfall (kg·ha⁻¹·yr⁻¹) was determined based on the sum of the dry mass of needles collected over the year. The needle biomass turnover rate (%) was calculated as the ratio of the annual needle litterfall (obtained from field measurements) to the total needle biomass (obtained from allometric equations), multiplied by 100%. An alternative method for determining the needle biomass turnover rate is based on the inverse number of needle cohorts of trees (proportion of needles shed annually) [39]. This method requires field observations of the number of needle cohorts on standing trees, which was not performed in this study.

Then, using the linear Pearson’s correlation coefficient, the relationships between the needle biomass turnover rate and all of the abovementioned stand characteristics were determined.

3. Results

This study analysed the annual needle biomass turnover rate in 20 stands of Scots pine between 26 and 90 years of age. Table 1 presents detailed parameters of these stands, including tree density (634–5500 trees·ha⁻¹), mean diameter at breast height (8.6–26.1 cm), stand height (7.1–22.3 m), average crown length (3.4–6.8 m), basal area (22.2–49.6 m²·ha⁻¹), stand volume (37–508 m³·ha⁻¹), crown closure (54%–93%) and site index (22.5–34.0 m).

Table 2. Dry mass of different tree parts, needle litterfall and needle biomass turnover rate of 20 sampled Scots pine stands.

Sample Plot	Stand Age (years)	Aboveground Biomass (kg·ha−1)	Needle Biomass (kg·ha−1)	Needle Mass Fraction (%)	Needle Litterfall (kg·ha−1·yr−1)	Needle Biomass Turnover Rate (%)
1	26	109,873	14,722	13.4	2310	15.7
2	27	85,011	9003	10.6	2390	26.5
3	32	151,008	12,715	8.4	2230	17.5
4	41	163,567	10,347	6.3	2400	23.2
5	42	104,154	8031	7.7	1610	20.0
6	45	161,911	9896	6.1	2400	24.3
7	46	155,298	8808	5.7	2070	23.5
8	47	166,209	6359	3.8	1640	25.8
9	48	194,939	8926	4.6	2410	27.0
10	48	139,089	9695	7.0	1610	16.6
11	51	136,424	9434	6.9	2610	27.7
12	51	172,025	7299	4.2	1910	26.2
13	53	110,903	7474	6.7	1870	25.0
14	60	128,600	10,262	8.0	1910	18.6
15	67	140,008	8156	5.8	1350	16.6
16	74	144,814	6613	4.6	1620	24.5
17	80	300,744	10,268	3.4	1920	18.7
18	88	169,988	7208	4.2	1800	25.0
19	89	219,392	7987	3.6	1920	24.0
20	90	202,319	7210	3.6	2190	30.4
Average		157,814	9021	6.2	2008	22.8

presents details of the aboveground biomass and needle litterfall in the examined stands. Aboveground biomass (kg·ha⁻¹) was lowest in the youngest stand (27-year-old stands: 85,011 kg·ha⁻¹ aboveground biomass) and highest in one of the oldest stands (80-year-old stand: 300,744 kg·ha⁻¹ aboveground biomass). Needle biomass (kg·ha⁻¹) was highest in the youngest stands (27-year-old stand: 14,722 kg·ha⁻¹) and lowest in the middle-aged stand (47-year-old stand: 6359 kg·ha⁻¹). In this case, there was no clear trend with age. The fraction of needle mass (%) was highest in the youngest stands (26-year-old stand: 13.4%) and lowest in one of the oldest stands (80-year-old stand: 3.4%). A trend of systematic decline with age was thus established for this parameter. In contrast, annual needle litterfall (kg·ha⁻¹·yr⁻¹) did not show a clear trend with stand age. The smallest annual needle litterfall was recorded in the 67-year-old stand (plot 15: 1350 kg·ha⁻¹·yr⁻¹), while the largest was in the 51-year-old stand (plot 11: 2610 kg·ha⁻¹·yr⁻¹), although the results achieved accumulated around an average of about 2000 kg·ha⁻¹·yr⁻¹. The needle turnover rate (%) ranged from 15.7% in the youngest stand (26-year-old) to 30.4% in the oldest stand (90-year-old); however, no specific trend could be noticed here either.

Figure 3 presents a correlation matrix for various parameters of the examined stands. Most of the correlations obtained are well established in stand studies and are well known or are the result of autocorrelation or calculations. The needle turnover rate was positively correlated with average crown length (r = 0.4953, p = 0.0264) and negatively correlated with tree density (r = −0.4557, p = 0.0435), needle biomass (r = −0.6199, p = 0.0035) and needle mass fraction (r = −0.4625, p = 0.0400), as shown in detail in Figure 4. No significant correlation was found with stand age, mean diameter at breast height, stand height, basal area, stand volume, crown closure, aboveground biomass, needle biomass, needle mass fraction or site index (

Table 3. Pearson’s correlation coefficients for needle biomass turnover rate and selected stand parameters of 20 Scots pine sampled stands.

Variable	Needle Biomass Turnover Rate
	r	p
Stand age	0.2456	0.2966
Quadratic mean diameter	0.3951	0.0847
Stand height	0.3623	0.1165
Crown length	0.4953 *	0.0264
Stand density	−0.4557 *	0.0435
Basal area	−0.0864	0.7174
Stand volume	0.2548	0.2784
Crown cover	0.0557	0.8156
Aboveground biomass	0.1204	0.6132
Needle biomass	−0.6199 **	0.0035
Needle mass fraction	−0.4625 *	0.0400
Site index	0.3419	0.1401

Significance level: * p < 0.05; ** p < 0.01.

).

4. Discussion

The annual needle litterfall for coniferous forests varies considerably between locations [29]. In European pine species, litterfall sharply decreases from Southern to Northern Europe [15,23,50]. The decrease in needle litterfall with increasing latitude has been confirmed by some meta-analyses and synthesis studies [7,22]. In addition, climate variables have been shown to play an important role in influencing litterfall dynamics [11,27]. The best correlation has been obtained with temperature, precipitation and evapotranspiration [6,7,32]. Moreover, extreme weather events could have a significant influence on litter production [51]. Therefore, all of the above findings justify the need for continuous research into needle litterfall in various parts of the world.

The annual needle litter production observed in the present study ranged from 1350 to 2610 kg·ha⁻¹·year⁻¹, similar to the values reported for Scots pine stands in other temperate parts of Europe [22,23,32]. Our result was greater than that obtained in northern Fennoscandia, where needle litter production ranged from 490 to 653 kg·ha⁻¹·year⁻¹ [23,31]. However, it was lower than the 6604 kg·ha⁻¹·year⁻¹ reported in France [29].

The needle biomass turnover rate, defined as the proportion of the needle litterfall to the total needle biomass of a particular stand, indicates what proportion of needles fall annually and is strongly correlated with needle longevity [36,39]. Therefore, the estimation of the foliar biomass turnover rate for coniferous forests could be based on an inverse number of needle cohorts [37,52]. The number of needle cohorts for Scots pine shows a linear relationship with latitude [50,53,54], and the same trend was confirmed for the turnover rate of needle biomass [39,55].

The average needle biomass turnover rate for Scots pine in western Poland determined in the present study was 23.4%. As expected (due to the relationship with latitude), this was slightly higher than the rate found in southern Finland, at 21%, and markedly higher than that in northern Finland, at 10% [37]. The turnover rate of needle biomass obtained from the 20 sampled stands ranged from 15.7% to 30.4%, which was within the range reported in Finland, from 4% to 52% [9,37,39]. However, similar studies on Scots pine in other parts of Europe have not been reported. Therefore, our results could be applicable to conditions corresponding to those of temperate forests in Central and Western Europe. The needle biomass turnover rate can be used to estimate needle litterfall and, consequently, to model biomass and nutrient cycling in forest ecosystems [39]. This is an important practical aspect of the study findings.

The 20 sampled Scots pine stands aged 26 to 90 years did not show a correlation of needle biomass turnover rate with age. This finding does not support our research hypothesis. In contrast, the needle biomass turnover rate showed a positive correlation with crown length and a negative correlation with stand density. This may be relevant to better fitting biomass cycling models. However, due to the very weak correlations, further research is needed to confirm these relationships. We found no other studies showing correlations of needle biomass turnover rate with various stand parameters, except the well-known dependence on location and associated climatic conditions [9,39,54]. Nevertheless, needle longevity, strongly linked to needle biomass turnover rate, showed a relationship with site fertility [56], site index [57], growth [58], leaf area [59] and production efficiency [57,60].

5. Conclusions

• The average annual needle litter production of the 20 sampled Scots pine stands was 2008 kg·ha⁻¹·year⁻¹, similar to the values reported for this tree species in other temperate forests in Europe.
• The average needle biomass turnover rate for the sampled Scots pine stands aged 26 to 90 years was 23.4%. This parameter can be used to estimate needle litterfall and may be applicable to conditions corresponding to those of temperate forests in Central and Western Europe.
• We could not support the hypothesis that the needle biomass turnover rate for Scots pine depends on the age of the stand.
• The needle biomass turnover rate showed a positive correlation with crown length and a negative correlation with stand density. However, due to the very weak correlations, further research is needed to confirm these relationships.