Produção de fitomassa e acúmulo de nutrientes por plantas de cobertura no cerrado piauiense

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Forest Ecology and Management 161 (2002) 169±179

Biomass production and nutrient accumulation in short-rotation grey alder (Alnus incana (L.) Moench) plantation on abandoned agricultural land Veiko Uria,*, Hardi Tullusa, Krista LoÄhmusb a

Institute of Silviculture, Estonian Agricultural University, Kreutzwaldi 5, 51014 Tartu, Estonia b Institute of Geography, University of Tartu, Vanemuise 46, 51014 Tartu, Estonia Received 10 July 2000; received in revised form 26 January 2001; accepted 5 February 2001

Abstract In 1999, the area of abandoned agricultural land in Estonia formed 223,000 ha which is partly perspective for afforestation with grey alder, the most rapidly growing indigenous tree species. The production and nutrient (NPK) accumulation of a grey alder short-rotation plantation on former agricultural land was investigated. The production of above-ground biomass was estimated during 5 years after the establishment of the stand. In the ®fth year after planting, the biomass of the above-ground part of the plantation was 15.9 t DM ha 1 and above-ground biomass production was 6.4 t DM ha 1 per year. In the fourth year after planting, the biomass of the below-ground part was 2.7 t DM ha 1, which accounted for 18% of total biomass. The biomass of the nodules was estimated at 169  76 kg DM ha 1 and the biomass of ®ne roots (d < 2 mm) at 550  105 kg DM ha 1. Of ®ne roots 73.8% and all nodules were located in the upper 0±20 cm soil layer. In the fourth year after establishment, the amount of nitrogen introduced in the soil through leaf litter was 59.6 kg ha 1. As the site represented abandoned farm land, the pool of total soil nitrogen was relatively large (2.64 t ha 1 in the 0±20 cm layer). However, concerning phosphorus, the positive effect of grey alders was signi®cant. While in the ®rst year after planting the concentration of lactate soluble phosphorus in the 0±20 cm layer was 22.53 mg kg 1, then in the following years it was more than twice as high, varying from 47.2 to 59.7 mg kg 1. After the ®fth year the amount of nitrogen accumulated in aboveground biomass was 147.1 kg ha 1, that of phosphorus 21.3 kg ha 1 and that of potassium 42.6 kg ha 1. Most of accumulated nitrogen and potassium (38.9 and 37.6%, respectively) was located in the leaves and most of phosphorus (61.0%) in the wood. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Alnus incana; Above-ground biomass; Production; Below-ground biomass; Nutrient accumulation; Soil properties

1. Introduction During the last decade the economic situation has changed drastically in Estonia: agricultural production has declined, as a result of which 223,000 ha of *

Corresponding author. Tel.: ‡372-31-3111; fax: ‡372-31-3156. E-mail address: [email protected] (V. Uri).

abandoned agricultural land have come into existence (Meiner, 1999). The prices of fossil fuel have risen considerably, which has urged researchers to seek possibilities for a far more extensive use of wood and peat in energetics and heat management. Issues related to the establishment and use of energy forests have been placed on the agenda. In Finland and Sweden, most studies have addressed the use of willows, birches and grey alders as energy forests both on

0378-1127/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 1 ) 0 0 4 7 8 - 9


V. Uri et al. / Forest Ecology and Management 161 (2002) 169±179

mineral and organic soil (Granhall, 1982; Elowsson and Rytter, 1988; SÏlapokas, 1991; Saarsalmi, 1995; HytoÈnen et al., 1995; Rytter, 1996), where willows are generally the most rapidly growing species (Saarsalmi, 1995; HytoÈnen, 1996). In Estonia, the ®rst energy willow plantations were established during 1993±1995 in co-operation with the Swedish University of Agricultural Sciences (Koppel et al., 1996). Although grey alder has commonly not so high production capacity as some willow species and clones, it has some essential advantages which make it a promising species for short-rotation forestry. It grows rapidly, is symbiotically N2-®xing by the actinomycete Frankia, and has only a few pests and diseases. Decomposition of alder litter improves soil properties. After cutting, a new alder generation emerges by coppicing from root system, and arti®cial reforestation of clearcuts is not needed. Grey alder seedlings withstand direct sunlight and frost. In younger age, grey alder grows faster than other local tree species, the current annual increment being 7.57 m3 ha 1. However, growth slows down earlier; maturity is attained at the age of 40±50 years. Harvesting can be performed with ordinary equipment without special harvesting machinery. Natural grey alder stands are widespread in Estonia, forming 4.2% of the whole forested area, while in private forests they account for 11.1%. Alder forests are also typical riparian ecosystems in Europe, which can retain and transform nutrient ¯uxes from adjacent intensively exploited territories. Therefore, riparian alder stands are commonly evaluated as buffer zones for protecting waterbodies against pollution (Mander et al., 1995, 1997; LoÄhmus et al., 1996). Up to the present, grey alder has not been cultivated in Estonia. The ®rst experimental plantation on former agricultural land was established in 1995 with the aim to study the possibilities of cultivating this tree species as short-rotation forest as well as its impact on soil fertility on abandoned agricultural land. The main aims of the present study were:  to analyse the dynamics of the above-ground biomass and production of short-rotation grey alder plantation after establishment;  to estimate nutrient (NPK) allocation, demand and accumulation in the above-ground part of grey alder plantation;

 to estimate biomass and nutrient (NPK) content in the below-ground part of grey alder plantation;  to find out the impact of grey alder plantation on soil nutrient status on abandoned agricultural land. 2. Material and methods 2.1. Plantation The study based on an experimental area located in the southeastern part of Estonia, PoÄlva county, 58830 N and 278120 E. According to the data of the VoÄru metereological station, which is the closest to the experimental area, mean annual temperature is 6 8C, mean amount of precipitation is 653 mm and the mean length of the vegetation period is 191 days. The experimental plantation was established on abandoned farm land in spring 1995. The soil was classi®ed as Planosol (according to FAO classi®cation). Before the establishment of the experimental plantation this area had been out of agricultural use for 2 years. The soil was not prepared before planting. About 1±2-year-old transplants of generative and vegetative natural origin were used for planting. Only seedlings of generative origin were included in the present study. Survival and growth of the planting stock of different origin is described previously (Uri and Tullus, 1999). The total area of the plantation was 0.08 ha. Planting arrangement 0:7 m  1:0 m was employed. The plantation was established with a high primary density proceeding from the high density of natural grey alder stands and considering the experience of energy forestry in Nordic countries, where a primary density of up to 40,000 trees/ha was used (Elowsson and Rytter, 1988; Saarsalmi, 1995). There was no done plantation tending. 2.2. Estimation of above-ground biomass and production The above-ground biomass of the culture was estimated annually in August when leaf mass was the largest; dimension analysis (Bormann and Gordon, 1984; LoÄhmus et al., 1996) was used. Using a random procedure based on height or diameter distribution, seven model trees per plantation were felled (except in the ®rst year of the plantation when one average model

V. Uri et al. / Forest Ecology and Management 161 (2002) 169±179

tree was selected for biomass determination). The diameter at root collar was measured in 1995 and 1996 and the diameter at breast height (1.3 m) was used from 1997. The stem was divided into sections according to annual height increment. In sections, living branches were divided into fractions: leaves, current year shoots, older shoots and dead branches were separated. From every fraction, a subsample was taken for estimation of dry matter content as well as for chemical analysis. The samples were dried at 70 8C until constant weight and weighed to 0.01 g. The share of the wood and bark of stems was determined. The dry mass of different fractions was calculated for each model tree by multiplying respective fresh mass by the dry matter ratio. In the ®rst year after planting, because of the small dimensions of the planting stock, the above-ground biomass of the plantation was calculated roughly on the basis of an average model tree. To estimate the above-ground biomass of the plantation in the following years, regression equations of form (1) were used (Uri and Tullus, 1999): y ˆ axb ;


where y is the above-ground biomass of model tree, x the independent variable, and a and b are parameter estimates. The parameter estimates of the regression equation are presented in Table 1. The masses of different fractions were calculated using the percentage distribution of the fractions obtained on the basis of model trees. The annual production of stemwood, bark and branches was calculated as the difference between the masses of the respective fractions for the studied year and for the previous year. Table 1 Parameter estimates of the regression equation (1) for estimation of the above-ground biomass of grey alder (g)a Year






1996 1997 1998 1999


3.059 3.791 2.503 2.682

1.406 1.252 1.655 1.564

0.991 0.960 0.996 0.969

0.12 0.17 0.06 0.19


Level of signi®cance P < 0:0001 in all cases, D is the diameter (cm), H the height (m), r2 the coef®cient of determination, and S.E.E. is the standard error of estimate.


2.3. Estimation of below-ground biomass The below-ground biomass of the culture was estimated in October 1998, i.e. in the fourth year of planting. Two methods were used: excavation of the root system of model trees for estimation of the biomass of the stump and coarse roots (d  2 mm), and soil coring for estimation of the biomass of ®ne roots (d < 2 mm) and nodules. Excavation of root system is suitable for estimation of the mass of thicker fractions (Vogt and Persson, 1991) but not for estimation of ®ne roots (d < 2 mm) and nodules, since their losses in excavation are considerable, amounting to almost 2/3 of mass in the case of ®ne roots (LoÄhmus and Oja, 1983; LoÄhmus et al., 1991). In the study area, the root systems of three model trees were investigated. The trees were divided into ®ve diameter classes and the root system of a randomly selected tree from minimum, maximum and average diameter classes was excavated. The excavated root systems were washed, placed in plastic bags and separated into fractions in the laboratory on the following day: 2 mm  d < 5 mm, 5 mm  d < 10 mm, d > 10 mm and the stump. For determination of the dry matter ratio a subsample was separated from each fraction, and dried samples were weighed to 0.01 g. The dry mass of each fraction was calculated as described in Section 2.2. The below-ground biomass (d  2 mm) of the stand was estimated by two different methods: 1. on the basis of the above-ground biomass and average share of the root system of the biomass of model trees; 2. using allometric relationship between breast height diameter and the mass of root system. The same results were obtained with the use of either method, while the calculated mass did not include the fraction of ®ne roots (d < 2 mm) and nodules. Root coring was used to estimate the biomass of nodules and d < 2 mm roots (Vogt and Persson, 1991). Twenty soil cores were taken randomly over the whole plantation with a cylindrical soil auger. To avoid compression of soil layers, the internal diameter of the upper part of the auger was 1.6 mm larger than the diameter of the cutting edge (1 48 mm). The soil cores were divided into four 10 cm layers to a depth of


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40 cm, placed in polyethylene bags and kept in a refrigerator until processing. Roots and nodules were washed out of the soil cores during a week after sampling. Washed-out samples were preserved in plastic bags in a refrigerator at 5 8C. Further, the fractions of ®ne roots and nodules were separated under the binoculars and cleaned from soil particles. Dead nodules were separated as well. The samples were dried and weighed to 0.005 g. The vertical biomass distributions of ®ne roots and nodules were estimated as the means of 20 cores. As nodules were found no deeper than 20 cm and the amount of ®ne roots deeper than 40 cm was negligible, soil core data were used for estimation of the biomass of nodules and ®ne roots/ha, summing up the averages of successive 10 cm soil layers from soil cores. 2.4. Estimation of nutrient accumulation The concentrations of main nutrients (NPK) were determined annually by tree sections from different fractions of model trees. In 1997±1998, the concentrations of nutrients were analysed both for tree sections and for trees of different growth classes. The impact of sections and growth classes on nutrient concentrations was analysed using ANOVA. As the concentration of NPK varied dynamically in different fractions, NPK accumulation was treated as the content of nutrients in the above-ground parts of trees in the period of maximum leaf mass, i.e. in August. The contents of NPK in different fractions of model trees were calculated as weighted averages considering the share of a particular section in the biomass of the respective fraction of the whole tree as well as the concentration of nutrients in the respective fraction of this tree section. Both the pool and annual demand of nutrients in the above-ground part of the plantation were calculated; the biomass or annual increment of a fraction was multiplied by the respective nutrient concentration. In 1998, the NPK pools were estimated also in the below-ground part of the stand. To estimate autumn retranslocation of N and P, fallen leaves were gathered every year, dried to constant weight (70 8C) and the concentrations of NP were determined. In 1998, 10 litter traps (collecting area 0.25 m2) were placed in the experimental area, and litter was sampled once a fortnight (during leaf fall once a week). The amount of NP introduced in the soil

in 1998 and 1999 was calculated by multiplying leaf litter amount by the respective nutrient concentration of samples taken from litter traps. The amount of branch litter was negligible. This allowed to estimate the ¯ux of litter and through it also the real amount of nutrients which reached the soil with litter. In 1995±1997, the amount of nitrogen and phosphorus introduced in the soil with leaf litter was estimated from total leaf mass and litter NP concentrations. A correction coef®cient (relative difference between total leaf mass and mass of leaf litter) was used in calculations. According to the correction coef®cient, leaf litter mass formed 87.7% of total leaf mass in 1998. In the experimental area, soil investigations were carried out annually with the aim to estimate the effect of alders on soil fertility. From 10 points over the whole area, successive soil samples were taken to a depth of 50 cm (layers: 0±10, 10±20, 20±30, 30±40 and 40±50 cm). The subsamples of a layer were merged in a composite soil sample. In the fourth year after planting soil samples were taken also from the control area (adjacent abandoned ®eld). Sampling procedure was analogous to that used in the experimental area. Soil bulk density was determined to evaluate the pool of soil nutrients. 2.5. Laboratory analysis Plant samples were analysed for total Kjeldahl nitrogen, total Kjeldahl phosphorus, potassium and ash concentration. Block digestion and steam distillation methods were used for testing the plant material for nitrogen concentration (Tecator AN 300). Digest by ¯ow injection analysis was used for testing the plant material for Kjeldahl phosphorus concentration (Tecator ASTN 133/94). To test the plant material for potassium, ¯ame photometric method was employed. Kjeldahl nitrogen, available phosphorus and available potassium were determined from composite soil samples, pH and the content of organic matter were determined as well. For testing soil samples for nitrogen according to Kjeldahl, Tecator ASN 3313 was used. Determination of available phosphorus in the soil was performed by ¯ow injection analysis (ammonium lactate extractable), with the use of Tecator ASTN 9/84. Available potassium was determined from the same solution by ¯ame photometric

V. Uri et al. / Forest Ecology and Management 161 (2002) 169±179

method. Analyses were performed at the Biochemistry Laboratory of the Estonian Agricultural University. 2.6. Statistical methods Normality of variables was checked by w2 test, the Kolmogorov±Smirnov test was used for small samples. To analyse the effect of qualitative factors on response variables, one-way ANOVA was applied. When data did not follow a normal distribution, or when there occurred inhomogeneity of group variance the nonparametric Kruskal±Wallis analysis of variance was used. Linear and allometric models were used for estimating relationships. In all cases the level of signi®cance P < 0:05 was accepted. 3. Results


®rst year after planting, the above-ground biomass was 0.37 t DM ha 1 and by the autumn of the ®fth year, 15.86 t DM ha 1 (Fig. 1); current annual biomass production was 6.38 t DM ha 1 (Table 2). The potential ef®ciency of the leaf mass of the stand in different years (annual total production per unit of leaf mass) varied from 2.15 to 5.44. 3.2. Below-ground biomass Using the average share of the root systems of model trees, below-ground (d  2 mm) mass was calculated as 2.01 t DM ha 1. By using regression equation (2), compiled on the basis of model trees data, the mass of the coarse fractions of the below-ground part was calculated as 1.97 t DM ha 1. y ˆ 8:083D 2:819 ;

P < 0:001;

3.1. Above-ground biomass and production

r ˆ 0:999; S:E:E: ˆ 0:05;

Despite the competition of herbaceous plants, the survival rate of grey alders after the ®rst growing season was high (94.1%), which favoured their further growth and development. Naturally regenerated seedlings displayed the highest growth percentage (Uri and Tullus, 1999). The dry matter ratio for biomass fractions varied in different years; the content of dry matter in the stemwood varied from 51 to 61%, in the leaves from 39 to 50% and in the primary growth of branches from 41 to 47% in the 5-year period. By the autumn of the

where y is the mass of the d  2 mm below-ground part of tree (g), and D is the breast height diameter of tree. When the obtained relationship was applied to excavated model trees, the difference between the predicted and empirical masses was 0.4±1.5%. For ®ne d < 2 mm roots, obtained from soil cores, the distribution of rooting density (g m 2) in the upper soil layers (0±10 and 10±20 cm) was close to a normal distribution (P ˆ 0:50 and 0.52). However, in the third (20±30 cm) and fourth (30±40 cm) layers it differed


Fig. 1. The dynamics of the above-ground biomass in grey alder plantation.



V. Uri et al. / Forest Ecology and Management 161 (2002) 169±179

Table 2 Production (t DM ha


per year) of the above-ground part of grey alder plantation during 5 years after establishment 1995a





Stem Bark Wood Primary growth of branches Secondary growth of branches Leaves

0.09 0.03 0.06 0.06 ± 0.13

1.53 0.31 1.22 0.42 0.05 0.45

3.19 0.54 2.65 0.84 0.53 1.24

3.25 0.18 3.07 0.82 0.70 1.97

3.76 0.74 3.02 0.49 0.62 1.51








Stem biomass for 1995 includes also the secondary growth of branches.

from a normal distribution (P ˆ 0:04 and 0.01, respectively). There appeared no statistically signi®cant differences between the rooting densities of samples taken from ®ve different parts of the plantation (P ˆ 0:29). The average bio- and necromass of the nodules, calculated on the basis of soil cores, was 169  76 and 34  18 kg ha 1, respectively. Analysis of the vertical distribution of the living nodules revealed that 80.6% of them were located in the upper 10 cm soil layer and the total biomass of the nodules was contained in the layer with a depth of 0±20 cm. Of the dead nodules 98.4% were located in the uppermost 10 cm soil layer and the remaining 1.6% in the next layer (Fig. 2). The mass of the ®ne roots (d < 2 mm) was estimated at 550  105 kg DM ha 1 of which approximately half (47.5%) were located in the upper 10 cm soil layer and 73.8% were located in the upper 20 cm soil layer.

Fig. 2. The vertical distribution of ®ne roots (d < 2 mm) and nodules.

Summation of the coarse fraction and the fraction of the ®ne roots (d < 2 mm) and nodules yielded 2.68 t DM ha 1 for the biomass of the below-ground part (1998). As the above-ground mass of the crop in 1998 was 12.27 t DM ha 1, of which the stems accounted 55%, the branches 14% and the leaves 13%, the below-ground part of the crop accounted for 18% of total biomass. 3.3. Nutrient accumulation 3.3.1. Leaves In the third year after planting (1997) the concentration of nitrogen was higher in the leaves from the upper crown sections (P < 0:05). Such a tendency was not revealed in 1998. In 1998, the concentration of nitrogen in the leaves was signi®cantly higher than in 1997 (P < 0:05). In both years, phosphorus and potassium concentrations in the leaves did not depend on the height of sections, but the concentration of potassium in the leaves was signi®cantly lower in 1997 than in 1998. In the case of phosphorus, there occurred no signi®cant difference between different years. NPK concentrations in the leaves of trees from different height classes did not reveal signi®cant differences (P > 0:05), only the concentration of leaf nitrogen from suppressed trees was higher in 1998 (P < 0:05). 3.3.2. Primary growth of branches The concentration of nitrogen, phosphorus and potassium in the current year shoots did not reveal signi®cant differences between tree height sections. Comparison of NPK in the samples of 1997 and 1998 did not yield a signi®cant difference in nitrogen

V. Uri et al. / Forest Ecology and Management 161 (2002) 169±179

and phosphorus concentrations. The concentration of potassium was higher in 1998, while there was no difference between trees from different growth classes. 3.3.3. Stemwood There was no difference between the NPK concentrations of the stemwood in 1997 and 1998. The relative height of the sample and social status of tree had no signi®cant effect on NPK concentrations. 3.3.4. Stembark Nitrogen and phosphorus concentrations in the stembark were not different in 1997 and 1998; the difference was revealed in the concentration of potassium (in 1998 average potassium concentration in the bark being higher compared with 1997). The concentrations of N and P in the bark were not different in different tree sections, except in 1998 when potassium concentration in the lower part of the stem was signi®cantly higher. In the bark of trees from different growth classes, NPK concentration did not reveal any differences.


However, annual NPK concentrations revealed signi®cant differences in different fractions. Mean NPK concentrations in the fractions decreased as follows: leaves > stembark; current year shoots > branches > stemwood: The annual amount of accumulated nutrients increased with increase in biomass (Table 3). During the ®rst year after planting 10.7% of the whole aboveground nutrient pool had accumulated in the wood, while in the ®fth year, owing to changes in biomass allocation, this percentage was already 44.1. The respective percentages of nitrogen accumulated in the leaves were 78 and 38.8%. At the end of the ®rst growing season, 7.3% of phosphorus had accumulated in the wood and 75.6% in the leaves, while in 1999 the respective values were 61.0 and 14.2%. The same trend applied also to potassium: the share of accumulated potassium increased in the wood and decreased in the leaves.

Table 3 NPK accumulation and demand (kg ha 1) in the above-ground part of grey alder plantation during 5 years after establishment Stems Wood

Branches Bark

Secondary growth



Primary growth

Annual demand

1995 N P K

0.57 0.08 0.22

0.66 0.05 0.17

±a ±a ±a

0.57 0.05 0.19

3.61 0.31 1.67

5.41 0.49 2.25

4.68 0.41 2.02

1996 N P K

7.59 0.73 2.90

5.90 0.99 2.02

1.06 0.07 0.39

4.39 0.55 2.66

15.88 2.45 5.11

34.82 4.79 13.08

32.74 4.56 12.32

1997 N P K

16.32 1.96 5.44

14.15 0.91 3.08

5.20 0.46 1.70

9.92 0.99 2.93

41.4 2.45 13.40

86.99 6.77 26.55

81.29 5.69 23.22

1998 N P K

26.30 6.08 13.36

16.83 1.76 4.55

10.67 1.41 4.08

10.61 1.56 3.77

77.62 5.52 23.76

142.03 16.33 49.52

108.07 10.75 36.24

1999 N P K

36.63 13.02 11.09

28.26 2.37 7.79

18.17 1.85 5.48

6.99 1.07 2.20

57.06 3.03 15.99

147.11 21.34 42.55

92.28 9.55 26.42


Stem biomass for 1995 includes also the secondary growth of branches.


V. Uri et al. / Forest Ecology and Management 161 (2002) 169±179

Table 4 Nutrient concentration in the upper 20 cm soil layer in grey alder plantation

1995 1996 1997 1998 1999

N (mg kg 1)

P (mg kg 1)

K (mg kg 1)

Organic matter (%)


1050 1010 1050 1040 1150

22.53 59.69 51.96 51.19 47.20

212.53 139.00 170.27 170.34 201.37

2.40 2.77 2.51 2.65 2.68

5.93 5.68 5.47 5.41 5.72

Below-ground biomass formed approximately one®fth (18.0%) of total biomass and the distribution of nutrients (NPK) between the above-ground and belowground parts of the stand was similar to biomass allocation. In the below-ground part, accumulated nitrogen formed 18.6%, phosphorus 21.2% and potassium 20.5% of the corresponding nutrient pool. The highest nitrogen concentration occurred in the nodules, 17.7 g kg 1, and in the ®ne roots, 14 g kg 1; in the coarse roots (d > 2 mm) and the stump, nitrogen concentrations were 9.7 and 5.9 g kg 1, respectively. By the August of the fourth year 168.5 kg ha 1 of nitrogen, 19.8 kg ha 1 of phosphorus and 59.7 kg ha 1 of potassium had accumulated in total biomass. Considering the total amount of accumulated NPK, annual demand formed a considerable part of it (Table 3). For rough estimation of the amount of nitrogen introduced in the soil by leaf litter, NP concentration was analysed in green and fallen leaves. The average relative difference between the nitrogen concentrations of leaf litter and green leaves in the autumn of the ®rst, second, third, and fourth years after planting, was: 1.5, 11.0, 3.1 and 11.3%, respectively. In 1998, the difference between the leaf mass, determined on the basis of model trees, and the mass of leaf litter, determined with the use of litter traps, was 12.3% (1.97 t ha 1 and 1.72 kg ha 1, respectively). In 1999, the difference between the mass of leaves and the mass of litter was considerably larger (1.51 and 1.14 t ha 1, respectively), which can be explained primarily by the high activity of leaf grazing species and by drought in¯icted damage of leaves. Retranslocation of phosphorus from the leaves decreased with the ageing of the stand: the mean relative difference between the phosphorus concentrations of leaf litter and green leaves in the autumn of the ®rst, second, third, and fourth years after planting was 30.7, 15.4, 5.34 and 2.81%, respectively.

Fig. 3. The amount of nitrogen introduced in the soil with leaf litter.

3.3.5. Soil There were revealed no signi®cant trends either in the dynamics of the concentrations of soil Kjeldahl nitrogen, potassium and organic matter or pH in 1995±1999 (Table 4). During the ®rst ®ve growing years approximately 145 kg ha 1 of nitrogen reached the soil with leaf litter (Fig. 3). The amount of available phosphorus in the soil underwent remarkable changes. When in 1995 the concentration of lactate soluble phosphorus in the upper 20 cm soil layer was determined at 22.53 mg kg 1, then in the following year the concentration of phosphorus increased more than twice and remained at a stable level. The pool of available phosphorus in the upper 50 cm soil layer was 89.3 kg ha 1 in the year of the plantation's establishment and 287 kg ha 1 in the following year. 4. Discussion Although literature data indicate that grey alder suffers from competition with herbaceous plants during the ®rst growing years on abandoned agricultural land (Saarsalmi, 1995), germination and increment of plants in our experimental plantation in the

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®rst year after establishment were good in spite of the lack of weed control. Such young plantations are rarely described in the literature. According to Rytter (1996), in the ®rst year after harvesting, the mass of shoots sprouting from root suckers may amount to 1 t ha 1. In Estonia, the above-ground biomass of a 6-year-old grey alder stand growing in the most favourable site type (Aegopodium site type) was estimated at 50.9 t DM ha 1 and its annual aboveground production at 14.8 t DM ha 1 (Tullus et al., 1998); the biomass production of a 10-year-old natural grey alder stand amounted to 9.6 t DM ha 1 per year and biomass up to 60 t DM ha 1 (Tullus et al., 1998). According to Saarsalmi (1995), in fertilised experimental plantations the biomass production (without leaves) of the above-ground part of a 5-year-old plantation can be 5.8±6.1 t ha 1 per year and total above-ground biomass, 31 t ha 1. In fertilised and irrigated grey alder experimental plantations on organic soil the annual increment of above-ground biomass in a 4-year-old crop was estimated at 8 t DM ha 1 and in a 5-year-old crop, approximately at 12 t DM ha 1 (Rytter, 1996). The above-ground production of the investigated plantation was smaller than the respective results obtained for fertilised and in some cases irrigated grey alder plantations. In our stand no weed control, fertilisation or irrigation was carried out. Overbark stem production in the ®fth year after establishment (3.8 t ha 1) (Table 2) was higher than the mean increment of natural grey alder stands of the same age, growing in fertile site types in Estonia (3.1 t ha 1). If weather had been more favourable (late severe frost in spring damaged the trees, the vegetation period was dry), the annual increment of the last year would probably have been larger, in so far as the maximum value of annual current increment for grey alder is attained at the age of 6±8 (10) years (Rytter, 1996; Tullus et al., 1998), and the age of plants in the experimental plantation was 6±7 years. Comparison of the share of different fractions in total biomass revealed continuous increase in the share of the stemwood and decrease in the share of the leaves and primary growth of branches with the ageing of the stand (Fig. 1). According to studies made on grey alder stands in Estonia, the below-ground part of a 14-year-old stand accounted for 19.4% of total mass and that of a 40-year-old stand, 16.6% of total mass (LoÄhmus


et al., 1996). In Sweden, however, the below-ground part of a 6-year-old grey alder plantation on fertilised organic soil formed 23% of the total biomass of the stand (Rytter, 1989). The nodules accounted approximately for 6% (169  76 kg ha 1) of the total mass of the below-ground part. The obtained result was in good accordance with the results of similar studies. Bormann and Gordon (1984) found that the average mass of the nodules in a 5-year-old Alnus rubra was 146 kg ha 1. According to Rytter (1989), more than 90% of the total mass of the nodules on intensively managed organic soil were contained in the upper soil layer (0±6 cm). Our study con®rmed the tendency of the highest nodule and ®ne root concentration in the uppermost 10 cm soil layer (Fig. 2). The study con®rmed that excavation of root system is not a suitable method for determination of the masses of ®ne roots and nodules (LoÄhmus and Oja, 1983; LoÄhmus et al., 1991). The mass of ®ne roots determined with this method was 76% smaller and the mass of nodules 82% smaller than the respective masses determined by soil coring. In Europe, the concentration of nitrogen in alder leaves is usually in the range of 20±30 g kg 1 (Mikola, 1958). On fertilised and irrigated organic soils, leaf nitrogen concentrations have been estimated at 30± 40 g kg 1 (Rytter, 1990), on fertilised mineral soils, at 33±36 g kg 1 (Saarsalmi, 1995). In Estonian natural riparian grey alder stands, mean leaf nitrogen concentration ranges from 32 to 33 g kg 1 (LoÄhmus et al., 1996). In our plantation, average leaf nitrogen concentration ranged from 27 to 39 g kg 1, which are relatively high values. The nitrogen concentration of the leaves in the ®rst year after planting was low (27:2  1:4 g kg 1), which was probably related to the effect of planting, since the transplants had to recover and adapt to new conditions. When in majority of deciduous trees the bulk of nutrients are transported from the leaves to the other tree fractions before defoliation, then in grey alder leaves the concentration of nitrogen is high also immediately before defoliation. Leaves rich in nitrogen fall and decompose on the ground. Leaf litter of grey alder is more readily degradable than that of willow (SÏlapokas and Granhall, 1991). Since the amount of nitrogen introduced in the soil with leaves can vary signi®cantly depending on growth conditions, age of the stand, etc., then the


V. Uri et al. / Forest Ecology and Management 161 (2002) 169±179

results presented in the literature differ a great deal: Tarrant and Trappe (1971) have found that 12±300 kg ha 1 of nitrogen originating from alder litter can be added to the soil annually. Rytter has established that the amount of nitrogen which reaches the soil with leaf litter in a 3-year-old grey alder stand, growing on organic soil, is 46±66 kg ha 1, while in a 6-year-old stand the respective amount is 97±125 kg ha 1 (Rytter et al., 1989). Turner reports 60±80 kg ha 1 of nitrogen in annual litterfall from 34-year-old red alder stands (Turner et al., 1976). In Estonia, the annual amount of nitrogen added to the soil with litter in natural riparian grey alder forests has been estimated at 71±76 kg ha 1 (LoÄhmus et al., 1996). In the present study the amount of nitrogen added to the soil with litter in the fourth growing season was 60 kg ha 1 and in the ®fth growing season 33 kg ha 1. Decrease in the amount of nitrogen in the last year (1999) is related to unfavourable weather conditions in the growing season. Since the pool of total nitrogen in the upper 20 cm soil section was large (2.64 t ha 1), then the effect of alders on enrichment of the soil with nitrogen was not re¯ected in the increase in total nitrogen concentration. The studied plantation was characterised by a decrease in the difference between the phosphorus concentrations of the green leaves and leaf litter with the ageing of the stand. At the same time, there were revealed changes in the content of available soil phosphorus. Increase in phosphorus retranslocation in the leaves in 1999 was obviously due to unfavourable weather conditions. In natural grey alder stands, phosphorus retranslocation before leaf fall was higher than in the plantation, reaching 50±60% (LoÄhmus et al., 1996). Comparison of the content of available phosphorus in the soil at the time of the establishment of the plantation and at the end of the second growing season revealed that it had more than redoubled in the upper soil layers. Literature data too point to the in¯uence of alders on soil phosphorus content, which can be attributed to microbiological activity as well as to the activity of mycorrhiza (Ingestad, 1987; Giardina et al., 1995; Arveby and Granhall, 1998) or by root secretion which contains phosphatase enzymes (Giardina et al., 1995). The amount of phosphorus added to the soil with leaf litter was 0.3 kg ha 1 in 1995 and 0.7 kg ha 1 in 1996, which is negligible compared with the respective increase in available soil phosphorus. The

effect of alders on the content of soil phosphorus is evidenced also by the fact that in the fourth year after establishment the pool of available phosphorus in the upper 30 cm soil layer of the plantation was 171 kg ha 1, while in the adjacent area (abandoned agricultural land) it was only 118 kg ha 1. This phenomenon can be partly ascribed to the activity of mycorrhiza. In Estonia, ectomycorrhiza is abundant on grey alders in natural stands (Tullus et al., 1998) and it occurred in the investigated plantation too. Mycorrhizas play an important role in mineral nutrition of trees (Persson, 1983), namely in assimilation of phosphorus. Due to the balanced nutrition of grey alder its phosphorus demand is higher than that of other non-®xing tree species. According to Ingestad (1987) the optimum N:K:P ratio for grey alder is 100:50:18, respectively. In the studied plantation the NKP ratio for the leaves was determined in the year of establishment (1995) and in the following (1996) year. This ratio was (100:46:9) in the ®rst year and (100:32:15) in the second year. Thus, the share of phosphorus in the leaves increased during 1 year. Nitrogen ®xation process requires energy and carbon (Ingestad, 1980), and since adenosine triphosphate (ATP) is the energy carrier in ®xation reaction, a supply of phosphorus is essential in nitrogen ®xation (Sprent, 1979). Low soil phosphorus content was the cause of retarded root growth, nodulation and nitrogen ®xation in red alder (DeBell and Radwan, 1984). In fertilisation experiments phosphorus has been shown to be the only nutrient which increases the growth of alder (HytoÈnen et al., 1995). Changes in soil available potassium and organic matter as well as in pH in different years were not statistically signi®cant, mainly due to small litter ¯ux compared with the pool of soil organic matter. It appears that both phosphorus and potassium, but not nitrogen, were limiting nutrients in our case. 5. Conclusions The fact that the survival and growth of plants in the experimental plantation were good, shows that grey alder is a perspective tree species for afforestation of abandoned agricultural lands, both in the environmental and economic aspects. Soil improvement with respect to available phosphorus was noted already

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