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Nutrient removal under different harvesting scenarios for larch plantations in northeast China: Implications for nutrient conservation and management
Forest Ecology and Management 400 (2017) 150–158
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Nutrient removal under different harvesting scenarios for larch plantations in northeast China: Implications for nutrient conservation and management Tao Yan a,b,c, Jiaojun Zhu a,b,⇑, Kai Yang a,b,⇑, Lizhong Yu a,b, Jinxin Zhang a,b a b c
CAS Key Laboratory of Forest Ecology and Management, Institute of Applied Ecology, Shenyang 110016, China Qingyuan Forest CERN, Chinese Academy of Sciences, Shenyang 110016, China University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 12 April 2017 Received in revised form 1 June 2017 Accepted 2 June 2017
Keywords: Larix spp. Stand age Nutrient removal Residue management Tree harvesting
a b s t r a c t Larch (Larix spp.) is a dominant timber species in northeast China. However, compared with the adjacent secondary forests, the soil nutrient conditions in the 40-year-old larch plantations have significantly deteriorated. Moreover, large quantities of nutrients are removed from sites when larch plantations are harvested, leading to further depletion of soil nutrients. Therefore, it is essential to assess nutrient removal under different harvesting scenarios to improve nutrient management. In this study, we quantified biomass and nutrient (including macro-nutrients: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg), and micro-nutrients: iron (Fe), manganese (Mn), copper (Cu) and zinc (Zn)) accumulation in above- and below-ground components along a chronosequence (10-, 21-, 34and 55-year-old, respectively) of larch plantations in northeast China. Our results showed that the root/shoot ratio was 0.23, 0.12, 0.18 and 0.24 in the 10-, 21-, 34- and 55-year-old stands, respectively, with an average of 0.19. For each larch tree thinned in the 10- and 21-year-old stands or harvested in the 34- and 55-year-old stands, 173.6, 1197.3, 1878.1 and 4230.7 g of nutrient elements were removed from the sites, respectively. If the leaves, branches, bark and roots remained at the sites, nutrient removal was reduced by 89, 69, 59 and 44% in the 10-, 21-, 34-, and 55-year-old stands, respectively. Branches and leaves contained the largest proportion of nutrients in the 10- and 21-year-old stands, and should thus remain at the sites to avoid further nutrient removal during early thinning. In contrast, debarking stems was a feasible practice when clear-cutting the 34- and 55-year-old stands because bark contained large amounts of nutrients (especially N, P and K). Substantial proportions of the macro-nutrient N and the micro-nutrient Zn were accumulated in the stems, and thus, these two elements would become depleted after harvesting. Except for the stems, macro-nutrients (N, P and K) were mainly stored in the leaves of the 10- and 21-year-old stands and the bark and roots of the 34- and 55-year-old stands, By contrast, macro-nutrients (Ca and Mg) and micro-nutrients (Fe, Mn, Cu and Zn) were primarily concentrated in the branches and roots of all tested stands. Therefore, nutrient loss may be avoided by leaving the harvested components containing specific nutrients at the sites. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Plantation forests are increasing worldwide (FAO, 2010). In China, the current area occupied by plantation forests has reached 6.9 107 ha and accounts for 36% of the total national forest area (Chinese Ministry of Forestry, 2014). Larch (Larix spp., mainly including L. olgensis, L. principis-rupprechtii, and L. kaempferi) is
⇑ Corresponding authors at: Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China. E-mail addresses: [email protected] (J. Zhu), [email protected] (K. Yang). http://dx.doi.org/10.1016/j.foreco.2017.06.004 0378-1127/Ó 2017 Elsevier B.V. All rights reserved.
the dominant timber species of plantation forests in northeast China (Zhu et al., 2008, 2010; Mason and Zhu, 2014) and has been planted because secondary forests cannot meet increasing timber demands (Yang et al., 2013). The area of larch plantations in northeast China has reached 2.61 106 ha and accounts for 85% of the total area of larch plantations in China (Chinese Ministry of Forestry, 2014). Larch plantations provide a large proportion of the timber supply and carbon (C) sequestration (Yan et al., 2014; Gao et al., 2016). However, compared with adjacent secondary forests, the soil nutrients of ca. 40-year-old larch plantations have dramatically declined (e.g., 30% of soil available nutrients) due to the
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single species composition and mono-silviculture systems used in these plantations (Yang et al., 2013). Wang et al. (2014) found evidence of nutrient depletion in the deep-layer soils of larch plantations in northeast China. Larch plantations are frequently subjected to different harvesting types (e.g., thinning and clear-cutting) and scenarios (e.g., stem-only harvesting and whole-tree harvesting), which can remove large quantities of nutrients with biomass from the sites and thus lead to further depletion of soil nutrients and greater negative effects on long-term productivity (Yan et al., 2014; Gómez-García et al., 2016). Therefore, it is imperative to maintain the soil nutrients of the larch plantations to meet the growing demands for timber supply, bio-energy production and C sequestration. The biomass of plantations has been of critical importance for economic and ecological requirements (e.g., Egnell, 2011), and the pattern of biomass allocation between above- and belowground plant components affects plant growth, biogeochemical cycling and ecosystem function (Peichl and Arain, 2006; Peri et al., 2010; Uri et al., 2014). Although root biomass accounts for a substantial proportion of the total forest biomass, the belowground components (i.e., entire root systems) are rarely evaluated due to the great complexity of extracting roots (Augusto et al., 2015; Addo-Danso et al., 2016). Therefore, biomass and/or nutrient accumulation in roots have often been neglected or roughly estimated from root/shoot (R/S) ratios (Mokany et al., 2006; Uri et al., 2014). Such neglect or rough estimation of root systems may lead to considerable uncertainties in determining biomass and nutrient accumulations (Peichl and Arain, 2006; Luo et al., 2012). In stem-only harvesting (SOH), only the stems are harvested, and the logging residues (leaves, branches and treetops) remain on-site. By contrast, whole-tree harvesting (WTH) removes both stems and logging residues from a site (Palviainen and Finér, 2012; Merilä et al., 2014; Nieminen et al., 2016). Moreover, root removal is becoming increasingly prevalent because of the growing demands for biomass as a renewable energy source (Augusto et al., 2015; Uri et al., 2015). Obviously, different harvest scenarios (e.g., SOH, WTH, and WTH + root harvesting) remove different amounts of nutrients along with biomass harvesting (Merilä et al., 2014; Nieminen et al., 2016). For example, Strömgren et al. (2013) reported that the combined harvesting of stems and slash would cause 2–4 times more N removal than conventional SOH. Therefore, the knowledge of nutrient accumulation and allocation to different components is essential for nutrient management and for the optimization of harvesting. Consequently, appropriate tree harvest management is of great importance for the sustainability of site productivity in plantations (Merilä et al., 2014; Achat et al., 2015). Biomass allocation and nutrient concentrations of different components exhibit age-related changes (Peri et al., 2006, 2010). Thus, knowledge of the development of both above- and belowground biomass over the life cycle of larch plantations is imperative for the accurate quantification of biomass and nutrient accumulation, which will allow for more reasonable decisions in terms of forest management. Although a few studies have reported the biomass and nutrient accumulation patterns in larch plantations, these studies were not based on a destructive sampling methodology, especially including the below-ground components, along a chronosequence. In this study, we quantified the accumulation and distribution patterns of biomass and nutrients (i.e., macro-nutrients: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg); micro-nutrients: iron (Fe), manganese (Mn), copper (Cu) and zinc (Zn)) in both above- and below-ground components of larch plantations along a chronosequence (10-, 21-, 34- and 55year-old) in northeast China. The objectives of the present study
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were as follows: (i) to assess the nutrient removal coupled with biomass under seven harvesting scenarios (stem-only harvesting, stem-wood without bark harvesting, stem + branch harvesting, stem + root harvesting, stem + branch + root harvesting, WTH, and WTH + root harvesting); and (ii) to determine the optimum harvesting rotation for larch plantations to guide the nutrient conservation and management of larch plantations. 2. Materials and methods 2.1. Study site This study was conducted at the Qingyuan Forest CERN (Chinese Ecosystem Research Network), Chinese Academy of Sciences, in the mountainous region of Liaoning Province, China (latitude 41°510 N, longitude 124°540 E, elevation 500–1100 m above sea level). The region has a continental monsoon climate, with a humid and rainy summer and a cold and dry winter. The mean annual air temperature varies from 3.9 to 5.4 °C and the minimum and maximum air temperatures are 7.6 and 36.5 °C in January and July, respectively. The annual precipitation ranges from 700 to 850 mm, of which 80% occurs from June to August. The mean annual frost-free period is 130 days, with an early frost in October and late frost in April (Zhu et al., 2007). Since the 1960s, patches of secondary forests have been cleared and replaced with larch plantations, thereby forming a mosaiked larch plantation-secondary forest landscape (Yang et al., 2013; Mason and Zhu, 2014). Larix spp. (mainly including L. olgensis, L. principis-rupprechtii, and L. kaempferi) have been widely planted since then and have become the most important commercial timber species in northeast China (Mason and Zhu, 2014), representing 65% of the conifer plantations in these areas (Wang et al., 2006). Larch plantations mainly consist of Euonymus alatus, Acanthopanax senticosus, Ribes mandshuricum and Rhamnus davurica in the shrub layer and Paris verticillata, Carex siderosticta and C. callitrichos in the herbaceous layer. 2.2. Sampling and measurements Four stands of larch plantations in a chronosequence (10-, 21-, 34- and 55-year-old) were randomly selected within the Qingyuan Forest CERN. Each stand was monospecific and even-aged. The average tree heights were 7.2, 18.2, 21.3 and 23.8 m, the average diameters at breast height (DBHs) were 6.3, 15.3, 17.3 and 25.6 cm, and the average tree densities were 4725, 1967, 1292 and 500 trees ha1 for the 10-, 21-, 34- and 55-year-old stands, respectively. All four stands shared similar environmental conditions, soil types and previous land uses, varying only in the age of the plantations. Specifically, the distance between any two stands was less than 2.3 km to reduce the influences from site conditions (Sun et al., 2016). Thus, all the four stands had similar climate and micro-environmental conditions. Besides, all the four stands were located at the well-drained middle slope position, with slopes ranging from 13 to 17° and elevations ranging from 462 to 615 m above sea level, and thus similar topographical conditions. The soils of the four stands were typical brown forest soils, which were classified as Udalfs according to the United States Department of Agriculture soil taxonomy (second edition), with 25.6% sand, 51.2% silt, and 23.2% clay on average (Yang et al., 2013), and soil depth was 50–60 cm. Additionally, all the four aged larch plantations were in their first rotation, and were developed by replacing the secondary forests. Therefore, the conditions were appropriate for our chronosequence study. Three plots (20 m 20 m) were randomly established within each stand. The DBH and tree height of all individual trees in each plot were
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measured. The individual trees in each plot were divided into five DBH classes based on the DBH distribution, which represented the entire range of the diameter distribution (usually 2-cm intervals per class for the 21-, 34- and 55-year-old stands and 1-cm interval per class for the 10-year-old stand). Five healthy individuals with different DBHs were selected within each plot and destructively harvested. Field work for harvesting the trees was carried in August 2015 when the biomass was at its peak and the nutrient concentrations were stable (Yan et al., 2016). The selected sample trees were cut as close as possible to the ground. After felling, the total tree height and DBH were accurately measured and then the stem was then cut into 1-m sections. A stem disc (2 cm wide) from each section was taken as a stem subsample. In the middle of each section, 10 cm of annular stem bark was stripped off as a stem bark subsample (Yan et al., 2014). The live crown was divided into three layers (upper, middle and lower) (Peichl and Arain, 2006; Uri et al., 2014). All branches were cut from the stem, and all leaves attached to the branches were clipped. Using a combined method involving a pulley device and manual digging, the entire root system was carefully excavated to a depth of approximately 1 m in a circular plot centered on the stump of the selected tree (Hart et al., 2003; Peri et al., 2008) because larch is a shallow-rooted (0–60 cm) tree species (Wang et al., 2014). In this study, the root system was considered the stump and its roots (Uri et al., 2015). After the attached soil was carefully cleaned off, the roots were weighed separately (King et al., 2007; Lima et al., 2012). Tree age was estimated by counting the rings of the basal stem disc (Peri et al., 2008). In total, 60 sample trees (15 trees from each of the four tested stands) were destructively harvested. The fresh weight of all components was obtained in the field with a portable electronic balance (±1 g). Subsamples of different components were transported to the laboratory and oven-dried at 65 °C to a constant weight (±0.01 g) for moisture determination, and two to nine dried subsamples of each component were taken to determine nutrient concentrations (the bark was separated from the stem disc for stem nutrient analysis) (Peri et al., 2006; Nunes et al., 2013). The biomasses of the different components were obtained by multiplying the fresh weight by the respective moisture content (the ratio of dry weight to fresh weight). The nutrient accumulation in each component was calculated by multiplying the nutrient concentration by its respective biomass. The total biomass and nutrient accumulation of each individual tree were calculated by summing the different components. We selected seven main harvesting scenarios: SOH – B: (stemwood without bark harvesting); SOH: (stem-only harvesting); SOH + Br: (stem + branch harvesting); SOH + R: (stem + root harvesting); SOH + Br + R: (stem + branch + root harvesting); WTH: (whole-tree harvesting); and WTH + R: (WTH + root harvesting) (Achat et al., 2015).
2.3. Chemical analysis All subsamples of different components were ground using a mechanical mill to pass through an 80-mesh sieve. The nitrogen (N) concentration was measured using a CN analyzer (Elementary Vario EL III, Germany) and expressed as mg N mass per 100 mg dry mass (%). Phosphorus (P) was measured using the molybdenum blue colorimetric method after sample digestion in sulfuric acidhydrogen peroxide. K, Ca and Mg were measured by flame photometry using an Atomic Absorption Spectrophotometer (Aanalyst 800, USA). Fe, Mn, Cu and Zn were measured using an Inductively Coupled Plasma Mass Spectrometer (NexION300X, USA) after sampling digestion in nitric acid-perchloric acid (Hopmans and Elms, 2009; Yan et al., 2016).
2.4. Statistical analysis The normality of the data was checked using the KolmogorovSmirnov test and the homogeneity of variances was tested using the Levene’s test. Since there was no true replication for each stand age in our larch plantation chronosequence, we did not apply any statistical analysis to compare the differences among the four age classes (Bond-Lamberty et al., 2006; Peichl and Arain, 2006). The least significant differences (LSD) test was used to perform post hoc multiple comparisons for biomass yield or nutrient removal under different harvesting scenarios within each age stand (Varik et al., 2013). The significance level was set at P 0.05. All statistical analyses were performed using SPSS 13.0 (SPSS Inc., Chicago, IL, USA) for Windows. 3. Results 3.1. Biomass accumulation and R/S ratio patterns The total biomass at the tree level was 10.41, 112.13, 174.94 and 399.19 kg tree1 in the 10-, 21-, 34- and 55-year-old stands, respectively. The total biomass at the stand level was 49.37, 218.26, 227.54 and 201.58 t ha1 in the 10-, 21-, 34- and 55year-old stands, respectively (Table 1). Stems comprised the largest amount (i.e., 38.80, 60.30, 66.31 and 67.27% in the 10-, 21-, 34- and 55-year-old stands, respectively) of the total tree biomass, followed by the roots (i.e., 17.96, 11.17, 14.58 and 19.33% in the 10-, 21-, 34- and 55-year-old stands, respectively). In decreasing order, the biomass was contained in the branches (i.e., 19.88, 15.62, 8.19 and 5.27% in the 10-, 21-, 34- and 55-year-old stands, respectively), bark (i.e., 9.13, 9.60, 9.20 and 6.78% in the 10-, 21-, 34- and 55-year-old stands, respectively) and leaves (i.e., 14.22, 3.31, 1.71 and 1.35% in the 10-, 21-, 34- and 55-year-old stands, respectively) (Table 1; Fig. 1). Belowground (root) to aboveground biomass ratio (R/S ratio) was 0.23, 0.12, 0.18 and 0.24 in the 10-, 21-, 34- and 55-year-old stands, respectively, and with a mean R/S ratio of 0.19 across the four tested stands (Fig. 2). 3.2. Nutrient concentration and accumulation of different components The concentrations of macro-nutrients decreased in the order of N > Ca > K > P > Mg; for micro-nutrients, the order was Fe > Mn > Zn > Cu (Table 2). Generally, the stems had the lowest concentrations of both macro- and micro-nutrients, the leaves had the highest macro-nutrient concentrations, and the bark had the highest micro-nutrient concentrations (Table 2). The total nutrient accumulations in the above- and belowground components were 173.57, 1197.25, 1878.08 and 4320.67 g tree1 in the 10-, 21-, 34- and 55-year-old stands, respectively. The partial totals for macro- and micro-nutrients were 168.03 and 5.54, 1160.56 and 36.69, 1814.92 and 63.16, and 4170.84 and 149.83 g tree1 in the 10-, 21-, 34- and 55year-old stands, respectively (Table 3). Generally, nutrient accumulation exhibited the following order (from greater to lower abundance) in different components and stands: N > Ca > K > P > Mg > Fe > Mn > Zn > Cu (Table 3). Nutrients were mainly accumulated in the leaves and branches of the 10-year-old stand (66%), the stems and branches of the 21year-old stand (57.8%), the stems and roots of the 34- and 55-yearold stands (64.7 and 74.8%, respectively) (Table 3). Generally, except for the stems, macro-nutrients (e.g., N, P and K) were mainly contained in the leaves of the 10- and 21-year-old stands and in the bark and roots of the 34- and 55-year-old stands,
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T. Yan et al. / Forest Ecology and Management 400 (2017) 150–158 Table 1 Biomass accumulation in different components at tree and stand levels for four larch stands. Component
Stand age (years) 10
21
34
55
Tree level (kg tree1)
Stem Branch Bark Leaf Root Total
4.04 2.07 0.95 1.48 1.87 10.41
67.61 17.52 10.76 3.71 12.53 112.13
116.01 14.32 16.10 3.00 25.51 174.94
268.52 21.05 27.08 5.38 77.16 399.19
Stand level (t ha1)
Stem Branch Bark Leaf Root Total
19.23 9.77 4.49 6.97 8.88 49.37
132.52 33.21 21.04 7.22 24.27 218.26
150.85 18.65 20.99 3.90 33.16 227.54
135.66 10.56 13.63 2.71 39.01 201.58
3.3. Biomass and nutrient removal under different harvesting scenarios
Fig. 1. Partitioning of tree biomass among different components in the 10-, 21-, 34and 55-year-old stands of larch plantations.
Compared with SOH, WTH increased biomass yields by 71, 27, 13 and 9%, and the corresponding nutrient removals increased by 311, 90, 36 and 17% in the 10-, 21-, 34- and 55-year-old stands, respectively (Fig. 3). For SOH + Br, the biomass yields increased by 41, 22, 11 and 7%, and the corresponding nutrient removals increased by 146, 62, 24 and 9% in the 10-, 21-, 34- and 55-yearold stands, respectively (Fig. 3). For SOH - B, the biomass yields decreased by 19, 14, 12 and 9%, and the corresponding nutrient removals decreased by 46, 30, 27 and 19% in the 10-, 21-, 34and 55-year-old stands, respectively (Fig. 3). For SOH + R, the biomass yields increased by 37, 16, 19 and 26%, and the corresponding nutrient removals increased by 60, 38, 42 and 27% in the 10-, 21-, 34- and 55-year-old stands, respectively (Fig. 3). For WTH + R, the biomass yields increased by 109, 43, 32 and 35%, and the corresponding nutrient removals increased by 371, 128, 79 and 44% in the 10-, 21-, 34- and 55-year-old stands, respectively (Fig. 3). For SOH + Br + R, the biomass yields increased by 79, 38, 30 and 33%, and the corresponding nutrient removals increased by 206, 99, 67 and 36% in the 10-, 21-, 34- and 55year-old stands, respectively (Fig. 3). Generally, when nutrient removal under the WTH + R scenario was regarded as 100%, the proportion of nutrient removal under all harvesting scenarios was lower than the proportion of biomass yield (except Cu in the 21-year-old stand and Zn in the 34- and 55year-old stands). Large proportions of the macro-nutrient N and the micro-nutrient Zn were accumulated in the stems, and especially for the 34- and 55-year-old stands (Table 3, Figs. 4 and 5). In contrast, relatively small proportions of the macro-nutrient P and the micro-nutrient Fe were removed under different harvesting scenarios (Figs. 4 and 5).
4. Discussion 4.1. R/S ratio patterns Fig. 2. Root/shoot ratios in the 10-, 21-, 34- and 55-year-old stands of larch plantations. The green broken line represents the mean root/shoot ratio (0.19) across the stands in our present study, the blue broken line represents the mean root/shoot ratio (0.20) of larch plantations in northeast China (Wang et al., 2008), and the red broken line represents the mean root/shoot ratio of Larix forests (0.25, including both natural and plantation forests) in China (Luo et al., 2012). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
whereas macro-nutrients (e.g., Ca and Mg) and micro-nutrients (e.g., Fe, Mn, Cu and Zn) were primarily concentrated in the branches and roots of all tested stands (Table 3).
R/S ratio plays an important role in estimating biomass and/or carbon at large scale (Mokany et al., 2006; Wang et al., 2008). The relative higher R/S ratio in the initial stages after afforestation (e.g., 10 years) may be related to the higher competition of trees resulting from the high stem density (King et al., 2007). Moreover, a greater proportion of root biomass would improve water and nutrient uptake (Peichl and Arain, 2006; Peri and Lasagno, 2010). Consequently, this mechanism would ensure the establishment of younger trees at an early growth stage (Peri et al., 2006). Thereafter, the decrease of the R/S ratio (e.g., before 21 years) is mainly
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Table 2 Nutrient concentrations in the different components of four larch stands. Nutrient
Stand age (years)
Stem
Branch
Bark
Leaf
Root
)
10 21 34 55
2.46 3.69 5.02 5.06
7.58 3.94 3.56 4.10
8.20 7.90 8.33 6.20
21.82 21.70 23.47 25.11
6.37 5.24 5.47 3.41
P (g kg1)
10 21 34 55
0.24 0.13 0.07 0.13
0.64 0.38 0.36 0.29
1.02 0.62 0.71 0.45
2.41 2.08 1.45 1.48
0.92 0.59 0.39 0.27
K (g kg1)
10 21 34 55
0.90 0.50 0.40 0.41
2.74 1.54 1.69 1.31
2.93 2.32 2.19 1.69
4.62 6.26 5.92 5.27
2.41 2.91 2.24 0.71
Ca (g kg1)
10 21 34 55
1.15 1.14 1.28 3.58
14.16 12.31 12.89 7.01
5.73 3.72 5.78 11.61
11.81 10.30 11.56 10.58
2.13 9.01 13.06 5.22
Mg (g kg1)
10 21 34 55
0.20 0.11 0.15 0.20
0.50 0.48 0.55 0.55
0.62 0.40 0.41 0.53
0.83 0.71 0.82 0.77
0.48 0.46 0.39 0.14
Fe (mg kg1)
10 21 34 55
140.74 41.58 25.94 32.11
209.03 242.79 222.31 145.47
144.42 209.30 152.14 217.15
327.96 410.68 479.14 420.10
2377.43 2257.72 1847.38 805.94
Mn (mg kg1)
10 21 34 55
48.34 24.96 39.38 47.25
217.46 134.14 272.52 268.37
278.26 181.82 392.82 393.12
292.83 115.83 218.86 119.25
235.49 96.62 119.95 50.22
Cu (mg kg1)
10 21 34 55
3.00 6.01 2.00 2.80
4.46 4.26 8.46 6.10
8.38 7.85 5.48 3.76
3.64 3.89 3.72 5.24
6.10 4.85 5.48 2.87
Zn (mg kg1)
10 21 34 55
10.83 19.23 42.26 122.01
61.95 40.22 42.52 49.26
51.09 32.48 57.88 40.96
18.97 14.45 20.08 27.72
46.92 40.38 45.61 27.00
1
N (g kg
Table 3 Nutrient accumulation (g tree1) in the different components of four larch stands. Stand age (years)
Component
N
P
K
Ca
Mg
Fe
Mn
Cu
Zn
Total
Percentage (%)
10
Stem Branch Bark Leaf Root Total
9.83 15.18 7.67 31.05 10.39 74.12
0.86 1.24 0.93 3.48 1.49 8.00
3.03 4.90 2.32 6.17 3.77 20.19
4.33 30.24 5.16 18.18 3.73 61.64
0.80 0.98 0.55 1.21 0.53 4.07
0.68 0.50 0.15 0.49 1.79 3.61
0.19 0.48 0.26 0.31 0.35 1.59
0.011 0.008 0.007 0.005 0.007 0.038
0.04 0.12 0.04 0.03 0.07 0.30
19.77 53.65 17.09 60.93 22.13 173.57
11.4 30.9 9.8 35.1 12.8 100
21
Stem Branch Bark Leaf Root Total
247.95 63.18 82.75 78.41 47.99 520.28
7.21 5.68 6.06 7.41 5.45 31.81
26.17 23.20 20.99 21.81 26.67 118.84
74.79 216.35 39.28 36.13 97.99 464.54
6.37 8.34 3.93 2.59 3.86 25.09
2.92 3.98 2.39 1.40 16.03 26.72
1.35 2.17 1.69 0.36 0.79 6.36
0.50 0.07 0.08 0.01 0.04 0.70
1.35 0.70 0.33 0.05 0.48 2.91
368.61 323.67 157.50 148.17 199.30 1197.25
30.8 27.0 13.2 12.4 16.6 100
34
Stem Branch Bark Leaf Root Total
575.17 46.65 134.59 70.23 102.06 928.70
6.62 4.55 10.49 4.43 6.22 32.31
38.96 21.89 31.76 16.76 41.52 150.89
126.32 168.31 87.56 31.60 254.06 667.85
13.49 7.54 5.72 2.24 6.18 35.17
2.82 2.97 2.77 1.28 30.05 39.89
3.03 3.49 5.26 0.55 2.12 14.45
0.20 0.13 0.09 0.01 0.09 0.52
5.58 0.65 0.89 0.06 1.12 8.30
772.19 256.18 279.13 127.16 443.42 1878.08
41.1 13.6 14.9 6.8 23.6 100
55
Stem Branch Bark Leaf Root Total
1311.66 80.25 161.40 133.16 259.19 1945.66
31.54 5.76 11.82 7.97 19.82 76.91
84.75 24.51 42.92 28.05 59.10 239.33
892.32 138.58 337.02 60.51 397.04 1825.47
44.78 10.96 13.41 4.08 10.24 83.47
8.40 2.95 5.66 2.12 64.22 83.35
9.12 5.35 9.75 0.73 3.76 28.71
0.69 0.13 0.11 0.03 0.23 1.19
32.29 1.07 1.04 0.15 2.03 36.58
2415.55 269.56 583.13 236.80 815.63 4320.67
55.9 6.2 13.5 5.5 18.9 100
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the growing individuals (Peichl and Arain, 2007). Our results supported the optimal partitioning theory (OPT) which considered that plants would preferentially allocate more biomass to the organs that can acquire the most limiting resources (e.g., nutrient, water and light) (Bloom et al., 1985; Luo et al., 2012). The mean value of the R/S ratio across the larch plantation chronosequence was 0.19 (Fig. 2), which was almost identical to the mean R/S ratio (0.20) reported for larch plantations in northeast China (Wang et al., 2008) but was lower than that of Larix forests (0.25, including both natural and plantation forests) in China (Luo et al., 2012). This discrepancy could be explained by the fact that the practice of thinning in plantations promotes the relative growth of the above-ground components (Coletta et al., 2015), whereas management without thinning is implemented in natural forests. 4.2. Nutrient removal under different harvesting scenarios and implications
Fig. 3. Biomass yields (kg tree1; left, light gray columns) and nutrient removal (g tree1; right, gray columns) of the 10- (A), 21- (B), 34- (C) and 55-year-old (D) stands under different harvesting scenarios. Different letters in the columns (light gray or gray) indicate significant differences among different harvesting scenarios within each stand at P < 0.05. Mean value ± within stand standard error (n = 3).
driven by the rapid growth of above-ground components after early thinning (Gómez-García et al., 2016). This may be because thinning could significantly promote the growth of the aboveground components during early stages (Han et al., 2014). Furthermore, the increasing R/S ratio in older stands (e.g., 34- and 55-year-old) is probably due to the fact that a relatively larger proportion of biomass would be allocated to roots to support
On average, 173.6, 1197.3, 1878.1 and 4230.7 g of nutrient elements would be removed from each entire larch individual in each site by thinning in the 10- and 21-year-old stands or harvesting in the 34- and 55-year-old stands, respectively. If the leaves, branches, bark and roots could be retained on the sites, the nutrient removal would be reduced by 89, 69, 59 and 44% for the 10-, 21-, 34- and 55-year-old stands, respectively (Table 3). Although the biomass yields under other harvesting scenarios (e.g., WTH, stems plus branches and leaves harvesting; SOH + Br, stems plus branches harvesting) were higher than that of SOH, the corresponding nutrient removals increased more intensively (Figs. 3–5). Compared with SOH, WTH and SOH + Br can increase biomass yields by 30 and 20%, respectively, but the nutrient removals increased by 114 and 60%, respectively (Figs. 3–5). These results suggested that the most nutrient-rich components (i.e., leaves and branches) should not exported from the sites after harvesting (Achat et al., 2015; Augusto et al., 2015), especially during the early thinning of the 10- and 21-year-old stands. Compared with SOH, SOH – B decreased biomass yield by 14%, but the nutrient removal decreased by 31% (Figs. 3–5). This result indicated that debarking the stem after harvesting would have substantially reduced the nutrient loss (Hart et al., 2003; Peri et al., 2006), especially during the final clear-cutting in the 34- and 55-year-old stands. Therefore, the retention of logging residues (e.g., branches, leaves and roots) should be a critical consideration for optimizing the long-term productivity in plantation management (Kumaraswamy et al., 2014). Such logging residues would have less effect on biomass yield but would have more effect on nutrient loss. Root harvesting is becoming increasingly common due to the increasing demand for biomass as a renewable energy source (Merilä et al., 2014; Uri et al., 2015). Compared with SOH, SOH + R (stem plus roots harvesting), SOH + Br + R (stem plus branches and roots harvesting), and WTH + R (whole-tree plus roots harvesting) increased biomass yields by 25, 45 and 55%, respectively, but the average nutrient removal increased by 42, 102 and 156%, respectively (Figs. 3–5). These results indicated that root harvesting removes considerable amounts of nutrients. However, when considering both biomass yield and nutrient removal, SOH + R was a more feasible harvesting option compared with the other harvesting scenarios. Our results agreed with those of Iwald et al. (2013) and Nieminen et al. (2016), who reported that root harvesting is a better alternative than logging residue harvesting if the aim is to maximize biomass harvesting but minimize nutrient loss. With the exception of stems, macro-nutrients (N, P and K) were mainly stored in the leaves of the 10- and 21-year-old stands and in the bark and roots of the 34- and 55-year-old stands. Whereas
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Fig. 4. Relative removal of biomass yields and specific macro-nutrients (N, P, K, Ca and Mg) from the 10- (A), 21- (B), 34- (C) and 55-year-old (D) stands, respectively, under different harvesting scenarios.
Fig. 5. Relative removal of biomass yields and specific micro-nutrients (Fe, Mn, Cu and Zn) from the 10- (A), 21- (B), 34- (C) and 55-year-old (D) stands, respectively, under different harvesting scenarios.
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macro-nutrients (Ca and Mg) and micro-nutrients (Fe, Mn, Cu and Zn) were primarily concentrated in the branches and roots of all four tested stands (Table 3), which were mainly driven by the high specific nutrient concentration and biomass allocation pattern (Fig. 1; Tables 1 and 2). Therefore, nutrient loss may be avoided by leaving the harvested components that contain specific nutrients on-site (Pyttel et al., 2015; Nieminen et al., 2016). For example, keeping the leaves on-site would mainly reduce N, P and K loss, whereas leaving the branches would mainly reduce Ca, Mg, Fe, Mn, Cu and Zn loss during the early thinning of 10- and 21year-old stands. Debarking the stem after harvesting in the 34and 55-year-old stands would mainly reduce the loss of N, P and K. However, substantial proportions of the macro-nutrient N and the micro-nutrient Zn accumulated in the stems, a trend that became more obvious along the chronosequence (Table 3, Figs. 4 and 5). These results indicated that the depletion of N and Zn occurred after harvesting even though the leaves, barks, branches and roots were left on-site. Therefore, special attention should be paid to N and Zn in order to maintain long-term productivity of larch plantations. For instance, prolonging the harvest rotation would be a feasible management practice for minimizing N and Zn depletion. We have assessed nutrient removal under different harvesting scenarios along the larch plantation chronosequence. However, the main limitation of our findings is the lack of replications of each stand age, thus we did not apply any statistical analysis to directly compare the differences among the four age classes (Bond-Lamberty et al., 2006; Peichl and Arain, 2006). Therefore, further evidence for the dependence of biomass and nutrient accumulation on stand age will require data from replicated chronosequence studies in the next research work. 5. Conclusions We investigated the above- and below-ground nutrient accumulation and allocation patterns in 10-, 21-, 34- and 55-year-old stands of larch plantations in northeast China. On average, 173.6, 1197.3, 1878.1 and 4230.7 g of nutrient elements would be removed from each larch individual in each site by thinning in the 10- and 21-year-old stands or harvesting in the 34- and 55year-old stands, respectively. If the leaves, branches, bark and roots could be retained at the site when thinning the 10- and 21- stands or harvesting the 34- and 55- stands, the nutrient removal would be reduced by 89, 69, 59 and 44%, respectively. To prevent the increased depletion of soil nutrients, we recommend that residues (i.e., leaves and branches) should be left at the site when thinning the 10- and 21-year-old stands to avoid increased depletion of soil nutrients. Keeping the leaves on-site would mainly reduce N, P and K loss, whereas the remaining branches would mainly reduce Ca, Mg, Fe, Mn, Cu and Zn loss at these stages. As large proportions of the macro-nutrient N and the micro-nutrient Zn were accumulated in stems, these two elements become depleted after harvesting even if the other components were retained on-site. Because large amounts of nutrients were accumulate in the bark of 34and 55-year-old stands, debarking the stems on-site after harvesting would be a preferred method of mitigating the loss of nutrients, particularly macro-nutrients (e.g., N, P and K). Acknowledgments We thank Ms. Fan-peng Zeng, Mr. Wen-kai Jin and Mr. De-an Zheng for their help with field measurements and Ms. Peng Jiang and Ms. Huan-huan Song for their laboratory assistance. Furthermore, we thank Prof. M. Altaf Arain from McMaster University in Canada for his helpful suggestions on the statistical analysis of
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Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco
Nutrient removal under different harvesting scenarios for larch plantations in northeast China: Implications for nutrient conservation and management Tao Yan a,b,c, Jiaojun Zhu a,b,⇑, Kai Yang a,b,⇑, Lizhong Yu a,b, Jinxin Zhang a,b a b c
CAS Key Laboratory of Forest Ecology and Management, Institute of Applied Ecology, Shenyang 110016, China Qingyuan Forest CERN, Chinese Academy of Sciences, Shenyang 110016, China University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 12 April 2017 Received in revised form 1 June 2017 Accepted 2 June 2017
Keywords: Larix spp. Stand age Nutrient removal Residue management Tree harvesting
a b s t r a c t Larch (Larix spp.) is a dominant timber species in northeast China. However, compared with the adjacent secondary forests, the soil nutrient conditions in the 40-year-old larch plantations have significantly deteriorated. Moreover, large quantities of nutrients are removed from sites when larch plantations are harvested, leading to further depletion of soil nutrients. Therefore, it is essential to assess nutrient removal under different harvesting scenarios to improve nutrient management. In this study, we quantified biomass and nutrient (including macro-nutrients: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg), and micro-nutrients: iron (Fe), manganese (Mn), copper (Cu) and zinc (Zn)) accumulation in above- and below-ground components along a chronosequence (10-, 21-, 34and 55-year-old, respectively) of larch plantations in northeast China. Our results showed that the root/shoot ratio was 0.23, 0.12, 0.18 and 0.24 in the 10-, 21-, 34- and 55-year-old stands, respectively, with an average of 0.19. For each larch tree thinned in the 10- and 21-year-old stands or harvested in the 34- and 55-year-old stands, 173.6, 1197.3, 1878.1 and 4230.7 g of nutrient elements were removed from the sites, respectively. If the leaves, branches, bark and roots remained at the sites, nutrient removal was reduced by 89, 69, 59 and 44% in the 10-, 21-, 34-, and 55-year-old stands, respectively. Branches and leaves contained the largest proportion of nutrients in the 10- and 21-year-old stands, and should thus remain at the sites to avoid further nutrient removal during early thinning. In contrast, debarking stems was a feasible practice when clear-cutting the 34- and 55-year-old stands because bark contained large amounts of nutrients (especially N, P and K). Substantial proportions of the macro-nutrient N and the micro-nutrient Zn were accumulated in the stems, and thus, these two elements would become depleted after harvesting. Except for the stems, macro-nutrients (N, P and K) were mainly stored in the leaves of the 10- and 21-year-old stands and the bark and roots of the 34- and 55-year-old stands, By contrast, macro-nutrients (Ca and Mg) and micro-nutrients (Fe, Mn, Cu and Zn) were primarily concentrated in the branches and roots of all tested stands. Therefore, nutrient loss may be avoided by leaving the harvested components containing specific nutrients at the sites. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Plantation forests are increasing worldwide (FAO, 2010). In China, the current area occupied by plantation forests has reached 6.9 107 ha and accounts for 36% of the total national forest area (Chinese Ministry of Forestry, 2014). Larch (Larix spp., mainly including L. olgensis, L. principis-rupprechtii, and L. kaempferi) is
⇑ Corresponding authors at: Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China. E-mail addresses: [email protected] (J. Zhu), [email protected] (K. Yang). http://dx.doi.org/10.1016/j.foreco.2017.06.004 0378-1127/Ó 2017 Elsevier B.V. All rights reserved.
the dominant timber species of plantation forests in northeast China (Zhu et al., 2008, 2010; Mason and Zhu, 2014) and has been planted because secondary forests cannot meet increasing timber demands (Yang et al., 2013). The area of larch plantations in northeast China has reached 2.61 106 ha and accounts for 85% of the total area of larch plantations in China (Chinese Ministry of Forestry, 2014). Larch plantations provide a large proportion of the timber supply and carbon (C) sequestration (Yan et al., 2014; Gao et al., 2016). However, compared with adjacent secondary forests, the soil nutrients of ca. 40-year-old larch plantations have dramatically declined (e.g., 30% of soil available nutrients) due to the
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single species composition and mono-silviculture systems used in these plantations (Yang et al., 2013). Wang et al. (2014) found evidence of nutrient depletion in the deep-layer soils of larch plantations in northeast China. Larch plantations are frequently subjected to different harvesting types (e.g., thinning and clear-cutting) and scenarios (e.g., stem-only harvesting and whole-tree harvesting), which can remove large quantities of nutrients with biomass from the sites and thus lead to further depletion of soil nutrients and greater negative effects on long-term productivity (Yan et al., 2014; Gómez-García et al., 2016). Therefore, it is imperative to maintain the soil nutrients of the larch plantations to meet the growing demands for timber supply, bio-energy production and C sequestration. The biomass of plantations has been of critical importance for economic and ecological requirements (e.g., Egnell, 2011), and the pattern of biomass allocation between above- and belowground plant components affects plant growth, biogeochemical cycling and ecosystem function (Peichl and Arain, 2006; Peri et al., 2010; Uri et al., 2014). Although root biomass accounts for a substantial proportion of the total forest biomass, the belowground components (i.e., entire root systems) are rarely evaluated due to the great complexity of extracting roots (Augusto et al., 2015; Addo-Danso et al., 2016). Therefore, biomass and/or nutrient accumulation in roots have often been neglected or roughly estimated from root/shoot (R/S) ratios (Mokany et al., 2006; Uri et al., 2014). Such neglect or rough estimation of root systems may lead to considerable uncertainties in determining biomass and nutrient accumulations (Peichl and Arain, 2006; Luo et al., 2012). In stem-only harvesting (SOH), only the stems are harvested, and the logging residues (leaves, branches and treetops) remain on-site. By contrast, whole-tree harvesting (WTH) removes both stems and logging residues from a site (Palviainen and Finér, 2012; Merilä et al., 2014; Nieminen et al., 2016). Moreover, root removal is becoming increasingly prevalent because of the growing demands for biomass as a renewable energy source (Augusto et al., 2015; Uri et al., 2015). Obviously, different harvest scenarios (e.g., SOH, WTH, and WTH + root harvesting) remove different amounts of nutrients along with biomass harvesting (Merilä et al., 2014; Nieminen et al., 2016). For example, Strömgren et al. (2013) reported that the combined harvesting of stems and slash would cause 2–4 times more N removal than conventional SOH. Therefore, the knowledge of nutrient accumulation and allocation to different components is essential for nutrient management and for the optimization of harvesting. Consequently, appropriate tree harvest management is of great importance for the sustainability of site productivity in plantations (Merilä et al., 2014; Achat et al., 2015). Biomass allocation and nutrient concentrations of different components exhibit age-related changes (Peri et al., 2006, 2010). Thus, knowledge of the development of both above- and belowground biomass over the life cycle of larch plantations is imperative for the accurate quantification of biomass and nutrient accumulation, which will allow for more reasonable decisions in terms of forest management. Although a few studies have reported the biomass and nutrient accumulation patterns in larch plantations, these studies were not based on a destructive sampling methodology, especially including the below-ground components, along a chronosequence. In this study, we quantified the accumulation and distribution patterns of biomass and nutrients (i.e., macro-nutrients: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg); micro-nutrients: iron (Fe), manganese (Mn), copper (Cu) and zinc (Zn)) in both above- and below-ground components of larch plantations along a chronosequence (10-, 21-, 34- and 55year-old) in northeast China. The objectives of the present study
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were as follows: (i) to assess the nutrient removal coupled with biomass under seven harvesting scenarios (stem-only harvesting, stem-wood without bark harvesting, stem + branch harvesting, stem + root harvesting, stem + branch + root harvesting, WTH, and WTH + root harvesting); and (ii) to determine the optimum harvesting rotation for larch plantations to guide the nutrient conservation and management of larch plantations. 2. Materials and methods 2.1. Study site This study was conducted at the Qingyuan Forest CERN (Chinese Ecosystem Research Network), Chinese Academy of Sciences, in the mountainous region of Liaoning Province, China (latitude 41°510 N, longitude 124°540 E, elevation 500–1100 m above sea level). The region has a continental monsoon climate, with a humid and rainy summer and a cold and dry winter. The mean annual air temperature varies from 3.9 to 5.4 °C and the minimum and maximum air temperatures are 7.6 and 36.5 °C in January and July, respectively. The annual precipitation ranges from 700 to 850 mm, of which 80% occurs from June to August. The mean annual frost-free period is 130 days, with an early frost in October and late frost in April (Zhu et al., 2007). Since the 1960s, patches of secondary forests have been cleared and replaced with larch plantations, thereby forming a mosaiked larch plantation-secondary forest landscape (Yang et al., 2013; Mason and Zhu, 2014). Larix spp. (mainly including L. olgensis, L. principis-rupprechtii, and L. kaempferi) have been widely planted since then and have become the most important commercial timber species in northeast China (Mason and Zhu, 2014), representing 65% of the conifer plantations in these areas (Wang et al., 2006). Larch plantations mainly consist of Euonymus alatus, Acanthopanax senticosus, Ribes mandshuricum and Rhamnus davurica in the shrub layer and Paris verticillata, Carex siderosticta and C. callitrichos in the herbaceous layer. 2.2. Sampling and measurements Four stands of larch plantations in a chronosequence (10-, 21-, 34- and 55-year-old) were randomly selected within the Qingyuan Forest CERN. Each stand was monospecific and even-aged. The average tree heights were 7.2, 18.2, 21.3 and 23.8 m, the average diameters at breast height (DBHs) were 6.3, 15.3, 17.3 and 25.6 cm, and the average tree densities were 4725, 1967, 1292 and 500 trees ha1 for the 10-, 21-, 34- and 55-year-old stands, respectively. All four stands shared similar environmental conditions, soil types and previous land uses, varying only in the age of the plantations. Specifically, the distance between any two stands was less than 2.3 km to reduce the influences from site conditions (Sun et al., 2016). Thus, all the four stands had similar climate and micro-environmental conditions. Besides, all the four stands were located at the well-drained middle slope position, with slopes ranging from 13 to 17° and elevations ranging from 462 to 615 m above sea level, and thus similar topographical conditions. The soils of the four stands were typical brown forest soils, which were classified as Udalfs according to the United States Department of Agriculture soil taxonomy (second edition), with 25.6% sand, 51.2% silt, and 23.2% clay on average (Yang et al., 2013), and soil depth was 50–60 cm. Additionally, all the four aged larch plantations were in their first rotation, and were developed by replacing the secondary forests. Therefore, the conditions were appropriate for our chronosequence study. Three plots (20 m 20 m) were randomly established within each stand. The DBH and tree height of all individual trees in each plot were
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measured. The individual trees in each plot were divided into five DBH classes based on the DBH distribution, which represented the entire range of the diameter distribution (usually 2-cm intervals per class for the 21-, 34- and 55-year-old stands and 1-cm interval per class for the 10-year-old stand). Five healthy individuals with different DBHs were selected within each plot and destructively harvested. Field work for harvesting the trees was carried in August 2015 when the biomass was at its peak and the nutrient concentrations were stable (Yan et al., 2016). The selected sample trees were cut as close as possible to the ground. After felling, the total tree height and DBH were accurately measured and then the stem was then cut into 1-m sections. A stem disc (2 cm wide) from each section was taken as a stem subsample. In the middle of each section, 10 cm of annular stem bark was stripped off as a stem bark subsample (Yan et al., 2014). The live crown was divided into three layers (upper, middle and lower) (Peichl and Arain, 2006; Uri et al., 2014). All branches were cut from the stem, and all leaves attached to the branches were clipped. Using a combined method involving a pulley device and manual digging, the entire root system was carefully excavated to a depth of approximately 1 m in a circular plot centered on the stump of the selected tree (Hart et al., 2003; Peri et al., 2008) because larch is a shallow-rooted (0–60 cm) tree species (Wang et al., 2014). In this study, the root system was considered the stump and its roots (Uri et al., 2015). After the attached soil was carefully cleaned off, the roots were weighed separately (King et al., 2007; Lima et al., 2012). Tree age was estimated by counting the rings of the basal stem disc (Peri et al., 2008). In total, 60 sample trees (15 trees from each of the four tested stands) were destructively harvested. The fresh weight of all components was obtained in the field with a portable electronic balance (±1 g). Subsamples of different components were transported to the laboratory and oven-dried at 65 °C to a constant weight (±0.01 g) for moisture determination, and two to nine dried subsamples of each component were taken to determine nutrient concentrations (the bark was separated from the stem disc for stem nutrient analysis) (Peri et al., 2006; Nunes et al., 2013). The biomasses of the different components were obtained by multiplying the fresh weight by the respective moisture content (the ratio of dry weight to fresh weight). The nutrient accumulation in each component was calculated by multiplying the nutrient concentration by its respective biomass. The total biomass and nutrient accumulation of each individual tree were calculated by summing the different components. We selected seven main harvesting scenarios: SOH – B: (stemwood without bark harvesting); SOH: (stem-only harvesting); SOH + Br: (stem + branch harvesting); SOH + R: (stem + root harvesting); SOH + Br + R: (stem + branch + root harvesting); WTH: (whole-tree harvesting); and WTH + R: (WTH + root harvesting) (Achat et al., 2015).
2.3. Chemical analysis All subsamples of different components were ground using a mechanical mill to pass through an 80-mesh sieve. The nitrogen (N) concentration was measured using a CN analyzer (Elementary Vario EL III, Germany) and expressed as mg N mass per 100 mg dry mass (%). Phosphorus (P) was measured using the molybdenum blue colorimetric method after sample digestion in sulfuric acidhydrogen peroxide. K, Ca and Mg were measured by flame photometry using an Atomic Absorption Spectrophotometer (Aanalyst 800, USA). Fe, Mn, Cu and Zn were measured using an Inductively Coupled Plasma Mass Spectrometer (NexION300X, USA) after sampling digestion in nitric acid-perchloric acid (Hopmans and Elms, 2009; Yan et al., 2016).
2.4. Statistical analysis The normality of the data was checked using the KolmogorovSmirnov test and the homogeneity of variances was tested using the Levene’s test. Since there was no true replication for each stand age in our larch plantation chronosequence, we did not apply any statistical analysis to compare the differences among the four age classes (Bond-Lamberty et al., 2006; Peichl and Arain, 2006). The least significant differences (LSD) test was used to perform post hoc multiple comparisons for biomass yield or nutrient removal under different harvesting scenarios within each age stand (Varik et al., 2013). The significance level was set at P 0.05. All statistical analyses were performed using SPSS 13.0 (SPSS Inc., Chicago, IL, USA) for Windows. 3. Results 3.1. Biomass accumulation and R/S ratio patterns The total biomass at the tree level was 10.41, 112.13, 174.94 and 399.19 kg tree1 in the 10-, 21-, 34- and 55-year-old stands, respectively. The total biomass at the stand level was 49.37, 218.26, 227.54 and 201.58 t ha1 in the 10-, 21-, 34- and 55year-old stands, respectively (Table 1). Stems comprised the largest amount (i.e., 38.80, 60.30, 66.31 and 67.27% in the 10-, 21-, 34- and 55-year-old stands, respectively) of the total tree biomass, followed by the roots (i.e., 17.96, 11.17, 14.58 and 19.33% in the 10-, 21-, 34- and 55-year-old stands, respectively). In decreasing order, the biomass was contained in the branches (i.e., 19.88, 15.62, 8.19 and 5.27% in the 10-, 21-, 34- and 55-year-old stands, respectively), bark (i.e., 9.13, 9.60, 9.20 and 6.78% in the 10-, 21-, 34- and 55-year-old stands, respectively) and leaves (i.e., 14.22, 3.31, 1.71 and 1.35% in the 10-, 21-, 34- and 55-year-old stands, respectively) (Table 1; Fig. 1). Belowground (root) to aboveground biomass ratio (R/S ratio) was 0.23, 0.12, 0.18 and 0.24 in the 10-, 21-, 34- and 55-year-old stands, respectively, and with a mean R/S ratio of 0.19 across the four tested stands (Fig. 2). 3.2. Nutrient concentration and accumulation of different components The concentrations of macro-nutrients decreased in the order of N > Ca > K > P > Mg; for micro-nutrients, the order was Fe > Mn > Zn > Cu (Table 2). Generally, the stems had the lowest concentrations of both macro- and micro-nutrients, the leaves had the highest macro-nutrient concentrations, and the bark had the highest micro-nutrient concentrations (Table 2). The total nutrient accumulations in the above- and belowground components were 173.57, 1197.25, 1878.08 and 4320.67 g tree1 in the 10-, 21-, 34- and 55-year-old stands, respectively. The partial totals for macro- and micro-nutrients were 168.03 and 5.54, 1160.56 and 36.69, 1814.92 and 63.16, and 4170.84 and 149.83 g tree1 in the 10-, 21-, 34- and 55year-old stands, respectively (Table 3). Generally, nutrient accumulation exhibited the following order (from greater to lower abundance) in different components and stands: N > Ca > K > P > Mg > Fe > Mn > Zn > Cu (Table 3). Nutrients were mainly accumulated in the leaves and branches of the 10-year-old stand (66%), the stems and branches of the 21year-old stand (57.8%), the stems and roots of the 34- and 55-yearold stands (64.7 and 74.8%, respectively) (Table 3). Generally, except for the stems, macro-nutrients (e.g., N, P and K) were mainly contained in the leaves of the 10- and 21-year-old stands and in the bark and roots of the 34- and 55-year-old stands,
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T. Yan et al. / Forest Ecology and Management 400 (2017) 150–158 Table 1 Biomass accumulation in different components at tree and stand levels for four larch stands. Component
Stand age (years) 10
21
34
55
Tree level (kg tree1)
Stem Branch Bark Leaf Root Total
4.04 2.07 0.95 1.48 1.87 10.41
67.61 17.52 10.76 3.71 12.53 112.13
116.01 14.32 16.10 3.00 25.51 174.94
268.52 21.05 27.08 5.38 77.16 399.19
Stand level (t ha1)
Stem Branch Bark Leaf Root Total
19.23 9.77 4.49 6.97 8.88 49.37
132.52 33.21 21.04 7.22 24.27 218.26
150.85 18.65 20.99 3.90 33.16 227.54
135.66 10.56 13.63 2.71 39.01 201.58
3.3. Biomass and nutrient removal under different harvesting scenarios
Fig. 1. Partitioning of tree biomass among different components in the 10-, 21-, 34and 55-year-old stands of larch plantations.
Compared with SOH, WTH increased biomass yields by 71, 27, 13 and 9%, and the corresponding nutrient removals increased by 311, 90, 36 and 17% in the 10-, 21-, 34- and 55-year-old stands, respectively (Fig. 3). For SOH + Br, the biomass yields increased by 41, 22, 11 and 7%, and the corresponding nutrient removals increased by 146, 62, 24 and 9% in the 10-, 21-, 34- and 55-yearold stands, respectively (Fig. 3). For SOH - B, the biomass yields decreased by 19, 14, 12 and 9%, and the corresponding nutrient removals decreased by 46, 30, 27 and 19% in the 10-, 21-, 34and 55-year-old stands, respectively (Fig. 3). For SOH + R, the biomass yields increased by 37, 16, 19 and 26%, and the corresponding nutrient removals increased by 60, 38, 42 and 27% in the 10-, 21-, 34- and 55-year-old stands, respectively (Fig. 3). For WTH + R, the biomass yields increased by 109, 43, 32 and 35%, and the corresponding nutrient removals increased by 371, 128, 79 and 44% in the 10-, 21-, 34- and 55-year-old stands, respectively (Fig. 3). For SOH + Br + R, the biomass yields increased by 79, 38, 30 and 33%, and the corresponding nutrient removals increased by 206, 99, 67 and 36% in the 10-, 21-, 34- and 55year-old stands, respectively (Fig. 3). Generally, when nutrient removal under the WTH + R scenario was regarded as 100%, the proportion of nutrient removal under all harvesting scenarios was lower than the proportion of biomass yield (except Cu in the 21-year-old stand and Zn in the 34- and 55year-old stands). Large proportions of the macro-nutrient N and the micro-nutrient Zn were accumulated in the stems, and especially for the 34- and 55-year-old stands (Table 3, Figs. 4 and 5). In contrast, relatively small proportions of the macro-nutrient P and the micro-nutrient Fe were removed under different harvesting scenarios (Figs. 4 and 5).
4. Discussion 4.1. R/S ratio patterns Fig. 2. Root/shoot ratios in the 10-, 21-, 34- and 55-year-old stands of larch plantations. The green broken line represents the mean root/shoot ratio (0.19) across the stands in our present study, the blue broken line represents the mean root/shoot ratio (0.20) of larch plantations in northeast China (Wang et al., 2008), and the red broken line represents the mean root/shoot ratio of Larix forests (0.25, including both natural and plantation forests) in China (Luo et al., 2012). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
whereas macro-nutrients (e.g., Ca and Mg) and micro-nutrients (e.g., Fe, Mn, Cu and Zn) were primarily concentrated in the branches and roots of all tested stands (Table 3).
R/S ratio plays an important role in estimating biomass and/or carbon at large scale (Mokany et al., 2006; Wang et al., 2008). The relative higher R/S ratio in the initial stages after afforestation (e.g., 10 years) may be related to the higher competition of trees resulting from the high stem density (King et al., 2007). Moreover, a greater proportion of root biomass would improve water and nutrient uptake (Peichl and Arain, 2006; Peri and Lasagno, 2010). Consequently, this mechanism would ensure the establishment of younger trees at an early growth stage (Peri et al., 2006). Thereafter, the decrease of the R/S ratio (e.g., before 21 years) is mainly
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Table 2 Nutrient concentrations in the different components of four larch stands. Nutrient
Stand age (years)
Stem
Branch
Bark
Leaf
Root
)
10 21 34 55
2.46 3.69 5.02 5.06
7.58 3.94 3.56 4.10
8.20 7.90 8.33 6.20
21.82 21.70 23.47 25.11
6.37 5.24 5.47 3.41
P (g kg1)
10 21 34 55
0.24 0.13 0.07 0.13
0.64 0.38 0.36 0.29
1.02 0.62 0.71 0.45
2.41 2.08 1.45 1.48
0.92 0.59 0.39 0.27
K (g kg1)
10 21 34 55
0.90 0.50 0.40 0.41
2.74 1.54 1.69 1.31
2.93 2.32 2.19 1.69
4.62 6.26 5.92 5.27
2.41 2.91 2.24 0.71
Ca (g kg1)
10 21 34 55
1.15 1.14 1.28 3.58
14.16 12.31 12.89 7.01
5.73 3.72 5.78 11.61
11.81 10.30 11.56 10.58
2.13 9.01 13.06 5.22
Mg (g kg1)
10 21 34 55
0.20 0.11 0.15 0.20
0.50 0.48 0.55 0.55
0.62 0.40 0.41 0.53
0.83 0.71 0.82 0.77
0.48 0.46 0.39 0.14
Fe (mg kg1)
10 21 34 55
140.74 41.58 25.94 32.11
209.03 242.79 222.31 145.47
144.42 209.30 152.14 217.15
327.96 410.68 479.14 420.10
2377.43 2257.72 1847.38 805.94
Mn (mg kg1)
10 21 34 55
48.34 24.96 39.38 47.25
217.46 134.14 272.52 268.37
278.26 181.82 392.82 393.12
292.83 115.83 218.86 119.25
235.49 96.62 119.95 50.22
Cu (mg kg1)
10 21 34 55
3.00 6.01 2.00 2.80
4.46 4.26 8.46 6.10
8.38 7.85 5.48 3.76
3.64 3.89 3.72 5.24
6.10 4.85 5.48 2.87
Zn (mg kg1)
10 21 34 55
10.83 19.23 42.26 122.01
61.95 40.22 42.52 49.26
51.09 32.48 57.88 40.96
18.97 14.45 20.08 27.72
46.92 40.38 45.61 27.00
1
N (g kg
Table 3 Nutrient accumulation (g tree1) in the different components of four larch stands. Stand age (years)
Component
N
P
K
Ca
Mg
Fe
Mn
Cu
Zn
Total
Percentage (%)
10
Stem Branch Bark Leaf Root Total
9.83 15.18 7.67 31.05 10.39 74.12
0.86 1.24 0.93 3.48 1.49 8.00
3.03 4.90 2.32 6.17 3.77 20.19
4.33 30.24 5.16 18.18 3.73 61.64
0.80 0.98 0.55 1.21 0.53 4.07
0.68 0.50 0.15 0.49 1.79 3.61
0.19 0.48 0.26 0.31 0.35 1.59
0.011 0.008 0.007 0.005 0.007 0.038
0.04 0.12 0.04 0.03 0.07 0.30
19.77 53.65 17.09 60.93 22.13 173.57
11.4 30.9 9.8 35.1 12.8 100
21
Stem Branch Bark Leaf Root Total
247.95 63.18 82.75 78.41 47.99 520.28
7.21 5.68 6.06 7.41 5.45 31.81
26.17 23.20 20.99 21.81 26.67 118.84
74.79 216.35 39.28 36.13 97.99 464.54
6.37 8.34 3.93 2.59 3.86 25.09
2.92 3.98 2.39 1.40 16.03 26.72
1.35 2.17 1.69 0.36 0.79 6.36
0.50 0.07 0.08 0.01 0.04 0.70
1.35 0.70 0.33 0.05 0.48 2.91
368.61 323.67 157.50 148.17 199.30 1197.25
30.8 27.0 13.2 12.4 16.6 100
34
Stem Branch Bark Leaf Root Total
575.17 46.65 134.59 70.23 102.06 928.70
6.62 4.55 10.49 4.43 6.22 32.31
38.96 21.89 31.76 16.76 41.52 150.89
126.32 168.31 87.56 31.60 254.06 667.85
13.49 7.54 5.72 2.24 6.18 35.17
2.82 2.97 2.77 1.28 30.05 39.89
3.03 3.49 5.26 0.55 2.12 14.45
0.20 0.13 0.09 0.01 0.09 0.52
5.58 0.65 0.89 0.06 1.12 8.30
772.19 256.18 279.13 127.16 443.42 1878.08
41.1 13.6 14.9 6.8 23.6 100
55
Stem Branch Bark Leaf Root Total
1311.66 80.25 161.40 133.16 259.19 1945.66
31.54 5.76 11.82 7.97 19.82 76.91
84.75 24.51 42.92 28.05 59.10 239.33
892.32 138.58 337.02 60.51 397.04 1825.47
44.78 10.96 13.41 4.08 10.24 83.47
8.40 2.95 5.66 2.12 64.22 83.35
9.12 5.35 9.75 0.73 3.76 28.71
0.69 0.13 0.11 0.03 0.23 1.19
32.29 1.07 1.04 0.15 2.03 36.58
2415.55 269.56 583.13 236.80 815.63 4320.67
55.9 6.2 13.5 5.5 18.9 100
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the growing individuals (Peichl and Arain, 2007). Our results supported the optimal partitioning theory (OPT) which considered that plants would preferentially allocate more biomass to the organs that can acquire the most limiting resources (e.g., nutrient, water and light) (Bloom et al., 1985; Luo et al., 2012). The mean value of the R/S ratio across the larch plantation chronosequence was 0.19 (Fig. 2), which was almost identical to the mean R/S ratio (0.20) reported for larch plantations in northeast China (Wang et al., 2008) but was lower than that of Larix forests (0.25, including both natural and plantation forests) in China (Luo et al., 2012). This discrepancy could be explained by the fact that the practice of thinning in plantations promotes the relative growth of the above-ground components (Coletta et al., 2015), whereas management without thinning is implemented in natural forests. 4.2. Nutrient removal under different harvesting scenarios and implications
Fig. 3. Biomass yields (kg tree1; left, light gray columns) and nutrient removal (g tree1; right, gray columns) of the 10- (A), 21- (B), 34- (C) and 55-year-old (D) stands under different harvesting scenarios. Different letters in the columns (light gray or gray) indicate significant differences among different harvesting scenarios within each stand at P < 0.05. Mean value ± within stand standard error (n = 3).
driven by the rapid growth of above-ground components after early thinning (Gómez-García et al., 2016). This may be because thinning could significantly promote the growth of the aboveground components during early stages (Han et al., 2014). Furthermore, the increasing R/S ratio in older stands (e.g., 34- and 55-year-old) is probably due to the fact that a relatively larger proportion of biomass would be allocated to roots to support
On average, 173.6, 1197.3, 1878.1 and 4230.7 g of nutrient elements would be removed from each entire larch individual in each site by thinning in the 10- and 21-year-old stands or harvesting in the 34- and 55-year-old stands, respectively. If the leaves, branches, bark and roots could be retained on the sites, the nutrient removal would be reduced by 89, 69, 59 and 44% for the 10-, 21-, 34- and 55-year-old stands, respectively (Table 3). Although the biomass yields under other harvesting scenarios (e.g., WTH, stems plus branches and leaves harvesting; SOH + Br, stems plus branches harvesting) were higher than that of SOH, the corresponding nutrient removals increased more intensively (Figs. 3–5). Compared with SOH, WTH and SOH + Br can increase biomass yields by 30 and 20%, respectively, but the nutrient removals increased by 114 and 60%, respectively (Figs. 3–5). These results suggested that the most nutrient-rich components (i.e., leaves and branches) should not exported from the sites after harvesting (Achat et al., 2015; Augusto et al., 2015), especially during the early thinning of the 10- and 21-year-old stands. Compared with SOH, SOH – B decreased biomass yield by 14%, but the nutrient removal decreased by 31% (Figs. 3–5). This result indicated that debarking the stem after harvesting would have substantially reduced the nutrient loss (Hart et al., 2003; Peri et al., 2006), especially during the final clear-cutting in the 34- and 55-year-old stands. Therefore, the retention of logging residues (e.g., branches, leaves and roots) should be a critical consideration for optimizing the long-term productivity in plantation management (Kumaraswamy et al., 2014). Such logging residues would have less effect on biomass yield but would have more effect on nutrient loss. Root harvesting is becoming increasingly common due to the increasing demand for biomass as a renewable energy source (Merilä et al., 2014; Uri et al., 2015). Compared with SOH, SOH + R (stem plus roots harvesting), SOH + Br + R (stem plus branches and roots harvesting), and WTH + R (whole-tree plus roots harvesting) increased biomass yields by 25, 45 and 55%, respectively, but the average nutrient removal increased by 42, 102 and 156%, respectively (Figs. 3–5). These results indicated that root harvesting removes considerable amounts of nutrients. However, when considering both biomass yield and nutrient removal, SOH + R was a more feasible harvesting option compared with the other harvesting scenarios. Our results agreed with those of Iwald et al. (2013) and Nieminen et al. (2016), who reported that root harvesting is a better alternative than logging residue harvesting if the aim is to maximize biomass harvesting but minimize nutrient loss. With the exception of stems, macro-nutrients (N, P and K) were mainly stored in the leaves of the 10- and 21-year-old stands and in the bark and roots of the 34- and 55-year-old stands. Whereas
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Fig. 4. Relative removal of biomass yields and specific macro-nutrients (N, P, K, Ca and Mg) from the 10- (A), 21- (B), 34- (C) and 55-year-old (D) stands, respectively, under different harvesting scenarios.
Fig. 5. Relative removal of biomass yields and specific micro-nutrients (Fe, Mn, Cu and Zn) from the 10- (A), 21- (B), 34- (C) and 55-year-old (D) stands, respectively, under different harvesting scenarios.
T. Yan et al. / Forest Ecology and Management 400 (2017) 150–158
macro-nutrients (Ca and Mg) and micro-nutrients (Fe, Mn, Cu and Zn) were primarily concentrated in the branches and roots of all four tested stands (Table 3), which were mainly driven by the high specific nutrient concentration and biomass allocation pattern (Fig. 1; Tables 1 and 2). Therefore, nutrient loss may be avoided by leaving the harvested components that contain specific nutrients on-site (Pyttel et al., 2015; Nieminen et al., 2016). For example, keeping the leaves on-site would mainly reduce N, P and K loss, whereas leaving the branches would mainly reduce Ca, Mg, Fe, Mn, Cu and Zn loss during the early thinning of 10- and 21year-old stands. Debarking the stem after harvesting in the 34and 55-year-old stands would mainly reduce the loss of N, P and K. However, substantial proportions of the macro-nutrient N and the micro-nutrient Zn accumulated in the stems, a trend that became more obvious along the chronosequence (Table 3, Figs. 4 and 5). These results indicated that the depletion of N and Zn occurred after harvesting even though the leaves, barks, branches and roots were left on-site. Therefore, special attention should be paid to N and Zn in order to maintain long-term productivity of larch plantations. For instance, prolonging the harvest rotation would be a feasible management practice for minimizing N and Zn depletion. We have assessed nutrient removal under different harvesting scenarios along the larch plantation chronosequence. However, the main limitation of our findings is the lack of replications of each stand age, thus we did not apply any statistical analysis to directly compare the differences among the four age classes (Bond-Lamberty et al., 2006; Peichl and Arain, 2006). Therefore, further evidence for the dependence of biomass and nutrient accumulation on stand age will require data from replicated chronosequence studies in the next research work. 5. Conclusions We investigated the above- and below-ground nutrient accumulation and allocation patterns in 10-, 21-, 34- and 55-year-old stands of larch plantations in northeast China. On average, 173.6, 1197.3, 1878.1 and 4230.7 g of nutrient elements would be removed from each larch individual in each site by thinning in the 10- and 21-year-old stands or harvesting in the 34- and 55year-old stands, respectively. If the leaves, branches, bark and roots could be retained at the site when thinning the 10- and 21- stands or harvesting the 34- and 55- stands, the nutrient removal would be reduced by 89, 69, 59 and 44%, respectively. To prevent the increased depletion of soil nutrients, we recommend that residues (i.e., leaves and branches) should be left at the site when thinning the 10- and 21-year-old stands to avoid increased depletion of soil nutrients. Keeping the leaves on-site would mainly reduce N, P and K loss, whereas the remaining branches would mainly reduce Ca, Mg, Fe, Mn, Cu and Zn loss at these stages. As large proportions of the macro-nutrient N and the micro-nutrient Zn were accumulated in stems, these two elements become depleted after harvesting even if the other components were retained on-site. Because large amounts of nutrients were accumulate in the bark of 34and 55-year-old stands, debarking the stems on-site after harvesting would be a preferred method of mitigating the loss of nutrients, particularly macro-nutrients (e.g., N, P and K). Acknowledgments We thank Ms. Fan-peng Zeng, Mr. Wen-kai Jin and Mr. De-an Zheng for their help with field measurements and Ms. Peng Jiang and Ms. Huan-huan Song for their laboratory assistance. Furthermore, we thank Prof. M. Altaf Arain from McMaster University in Canada for his helpful suggestions on the statistical analysis of
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