Soil changes and tree growth in intensively managed Pinus radiata in northern Spain

Soil changes and tree growth in intensively managed Pinus radiata in northern Spain

Forest Ecology and Management 196 (2004) 393–404 Soil changes and tree growth in intensively managed Pinus radiata in northern Spain Agustı´n Merinoa...

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Forest Ecology and Management 196 (2004) 393–404

Soil changes and tree growth in intensively managed Pinus radiata in northern Spain Agustı´n Merinoa,*, Alejandro Ferna´ndez-Lo´peza, Fernando Solla-Gullo´na, Jose´ Miguel Edesob a

Escuela Polite´cnica Superior, Universidad de Santiago de Compostela, E-27002 Lugo, Spain b Escuela de Topografı´a, Universidad del Paı´s Vasco, E-01006 Vitoria-Gasteiz, Spain Received 10 June 2003; received in revised form 18 February 2004; accepted 2 April 2004

Abstract In intensively managed forest plantations, clear-cutting and intensive site preparation may have negative effects on fundamental soil processes that determine the sustainability of these systems. This study was conducted to evaluate the effects of highly mechanized site preparation—involving the removal of organic residues (humus layer and slash)—on soil properties and tree growth in radiata pine plantations growing on steep forest land in northern Spain. In spite of the high risk of erosion, this type of management is carried out without any consideration for soil conservation. These plantations are growing on acidic soils with low reserves of available nutrients and therefore nutrient deficiencies may occur in the medium or long term. Site preparation caused moderate to considerable soil compaction and the effects were still pronounced after 9 years. As a consequence of accelerated erosion and soil disturbance, significant depletion of soil organic matter, Ca and Mg available nutrients, cation exchange capacity and base saturation were observed after removal of logging residues; in the most severe cases the effects were still detected 9 years after site preparation. Availability of Ca to trees was also reduced. Nine years after site preparation, scalped and ploughed soils also showed decreased microbial biomass, lower microbial activity, and less aerobic and anaerobic mineralizable N. Tree growth was significantly reduced where the logging residues and humus layer were removed, especially when this was followed by deep ploughing. During the 9 years of the study, the mean annual increment of timber volume decreased from 8.4 to 2.4 m3 ha1. We conclude that leaving logging residues on site avoids soil compaction, favours nutrient uptake in trees and maintains soil biological properties. On the basis of these results, the retention of logging residues and reduction of soil perturbation during site preparation are highly recommended. # 2004 Elsevier B.V. All rights reserved. Keywords: Pinus radiata; Forest nutrition; Soil management; SOM mineralization

1. Introduction Sustainability of forest plantations, in terms of longterm production and maintenance of site quality, is one *

Corresponding author. Tel.: þ34-982-252231; fax: þ34-982-241835. E-mail address: [email protected] (A. Merino).

of the main objectives of silviculture. However, concerns have been raised about the sustainability of intensively managed forests, such as plantations, following identification of a range of possible impacts of clearcutting and intensive site preparation techniques on soil properties that determine productivity (Nambiar, 1996). Management of logging residues and soil disturbance during site preparation alter soil functions

0378-1127/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2004.04.002

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associated with forest sustainability, such as structure, aeration, water retention capacity, and can also disrupt certain biological processes involved in nutrient availability (Ross et al., 1995; Smethurst and Nambiar, 1990). Because of the large quantity of nutrients removed, whole-tree harvesting can lead to decreased soil available reserves of limiting nutrients (Nambiar, 1996; Johnson and Todd, 1998; Olsson et al., 2000). In addition to the nutrient export with biomass, nutrient losses by leaching can increase significantly after clear-cutting. Nutrient losses may be especially severe in tropical and subtropical regions, where fast-growing forest species grow on strongly acidic soils with low nutrient capital (Hopmans et al., 1993; Fo¨ lster and Khanna, 1997). Soil micro-organisms affect growth and vitality of trees by release or uptake of plant nutrients during decomposition of organic matter. One of the methods used to assess the rate and progress of soil reclamation is the monitoring of soil microbial characteristics and specifically, microbial biomass and organic matter mineralization. These parameters can provide an indication of changes in total soil organic matter long before changes in total soil C and N can be reliably detected. In northern Spain, because of the strongly acidic nature of the soils, the main factor limiting the growth of Pinus radiata plantations is the availability of P, Mg and Ca (Sa´ nchez-Rodrı´guez et al., 2002; Za´ s and Serrada, 2003). The results of previous studies (Ouro et al., 2001; Merino et al., 2003) suggest that the nutritional stability of these systems is dependent on the supply of nutrients through decomposition of logging residues. Therefore, successive rotations involving the export of large amounts of limiting nutrients by removal of logging residues can lead to a reduction in production. The radiata pine plantations of this mountainous region are managed in short rotations, usually of 25–35 years. After clear-felling, the conventional method of site preparation consists of the partial removal of logging residues followed by down-slope subsoiling. However, a highly mechanized technique, which consists of pushing the logging residues and the humus layer away from the site, has been also employed in the last 15–20 years. This technique exposes large amounts of mineral soil, especially if it is followed by down-slope deep ploughing. In spite of the potential for mass erosion, this technique is

applied on steep slopes without the use of any soil conservation practices and, as a consequence, severe erosion occurs after periods of intense precipitation (Olarieta et al., 1999; Edeso et al., 1999). The technique is used as a way of reducing the amount of logging residues, allowing rapid establishment of new plantations after harvesting. Brushing or other methods that conserve organic matter are difficult because of the steep slopes. Although acute depletion of organic matter and available nutrients has been reported (Merino et al., 1998; Pe´ rez-Batallo´ n et al., 2001), until present there has been no record of the effects of such practices on tree growth. This study was undertaken to assess the changes in soil properties and forest growth during the 9 years following the application of different site preparation techniques. The results obtained from 21 stands of P. radiata subjected to different kinds of logging residue management and soil tillage are discussed. For this, the changes in soil properties, nutrient foliar concentrations and tree growth were studied throughout the 9-year period.

2. Materials and methods 2.1. Site description The stands under study were located in the provinces of Bizkaia and Gipuzkoa (N Spain), the part of Spain with the largest area of radiata pine plantations. The terrain is mountainous, with the gradients of the slopes often exceeding 35%. The climate is Temperate Subtropic with humid winter. Mean annual precipitation for all the sites varies between 1400 and 1800 mm, distributed evenly throughout the year. The average annual temperature at the different sites is, respectively, 11 and 14 8C, and the mean frost-free period is 9 months. The soils are developed on argillaceous sediments interbedded with sandstone. According to the FAO–UNESCO System (1998), the soils described in non-harvested areas are Dystric or Gleyic cambisols. They have a fine texture, moderate–high organic matter content, are strongly acidic and show low hydraulic conductivity. The clay minerals consist mainly of mica and kaolinite (Merino et al., 1991). The regimes of soil humidity and temperature in the two soils are Udic and Mesic, respectively.

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2.2. Silvicultural treatments evaluated For this study, a total of 21 stands of P. radiata were selected, seven for each of the three treatments carried out. In all cases, clear-cutting, site preparation and planting of seedlings (at 3 m intervals) were carried out in the winter of 1992–1993. The plantations selected for study were managed by forestry companies in the region and all were established on previous radiata pine plantations. None of the plantations received any fertilizer and understory vegetation was not cleared. The local practices evaluated were as follows: (a) Stem-only harvesting (C stands). Commercialsized boles and bark were removed. Branches and needles were cut and left on the ground and were sometimes partially burnt. No mechanized operations were used. (b) Scalping (S stands). All aboveground organic residues, including residual vegetation and the humus layer, were pushed to the side of the site with a bulldozer working down-slope. Organic residues displaced by this operation accumulated outside of the site instead of being redistributed within the site, as is the case with usual windrowing operations. Seedlings were planted after pitting or a down-slope ripping, with moderate soil disturbance. (c) Scalping plus down-slope ripping (SP stands). The treatment was similar to the previous one, except that seedlings were planted after deep ploughing. For this the mineral topsoil was thoroughly mixed with the subsoil to a depth of 40–50 cm, and the subsoil was exposed over more than 90% of the surface area. 2.3. Sampling and analysis of soil and needles In each stand, soil and foliar sampling as well as tree growth measurements were carried out within two subplots, measuring 30 m  30 m, set up in two representative areas of the stand. Soil samples were collected in 1993, 1997 and 2001. Samples from the upper mineral horizon (15 cm) were collected at random at nine points and gathered to form three bulked samples. Samples for determination of bulk density were taken at another four points within each stand,

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using a brass core (40 mm long, 55 mm inner diameter). Soil samples for chemical analysis were airdried and sieved with a 2 mm screen. The pH was measured in H2O and 0.1 M KCl (soil:solution ratio 1:2.5) with a glass electrode. Total C, N and S were analysed with a LECO Elemental Analyzer. Organic matter was calculated as the amount of C measured, multiplied by 1.72. Exchangeable cations were extracted with unbuffered 1N NH4Cl. Effective cation exchange capacity (eCEC) was determined as the sum of exchangeable Al, Ca, Mg, K and K. Available P, Mn, Fe, Cu, Ni and Zn were extracted using the Mehlich 3 procedure (Mehlich, 1984). The elements in these two extracts were analysed by atomic absorption spectrophotometry. Undamaged, full-sized needles of the current season’s growth were sampled from the upper third of the unshaded crowns of all trees (minimum 30) in the subplot. Two sets of samples were made with the material collected and each was treated separately in the laboratory. The samples were oven-dried (65 8C) to a constant weight, milled (0.25 mm) and extracted with H2SO4/H2O2 (Jones et al., 1991). K, Ca, Mg, Mn, Fe, Cd, Cr, Cu, Ni and Zn in the leaf extracts were analysed by atomic absorption spectrophotometry, whereas P was determined photometrically, by the ascorbic acid method. Nitrogen and S in needles were analysed in solid milled material using a LECO Analyzer. Microbial biomass C (MB-C) and N (MB-N) were measured following the method of fumigation of soil samples with ethanol-free chloroform vapour (Brookes et al., 1985; Vance et al., 1987). The soil was extracted with 0.5 M K2SO4. Organic C was determined in the extracts by digestion with K2Cr2O7 and titration with (NH4)2FeSO4, whereas total N was analysed by Kjeldahl digestion. The differences in C and N in fumigated and non-fumigated samples were calculated and they were converted to biomass C and N by multiplying by 2.64 and 0.54, respectively. In each case, C and N were extracted in triplicate and determined in each extract in duplicate. Soil respiration in the samples collected from different sites was measured following the alkali absorption technique (Witkamp, 1966). For this, 70 g of fresh soil (three samples of each soil) was placed in a glass container (1 L capacity) and the moisture content was adjusted to 60%. The samples were incubated at 25 8C for 10 days to allow the respiration to settle and then

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the respiration, in terms of changes in CO2 produced, was measured. The results were expressed as mg CO2C produced g1 h1, i.e. as the average rate of CO2 produced during the entire period of incubation. The microbial metabolic quotient (qCO2), an index used to evaluate the efficiency of the soil microbial community in utilizing the substrate, and which can be used to assess the process of soil development or degradation (Isam and Domsch, 1988), was calculated from the soil respiration rate and microbial biomass C (Anderson and Domsch, 1989). Duplicate soil subsamples were extracted with 2 M KCl before and after the incubation and the N mineralization capacity was calculated as the difference between the values obtained before and after incubation. Anaerobic mineralizable N was determined by the method of Waring and Bremner (1964). Three samples from each site were placed under water and maintained at a constant temperature of 40 8C for 7 days. The NH4þ present before and after incubation was extracted with 2 M KCl. The initial concentration of NH4þ was subtracted to calculate mineralizable N. 2.4. Tree measurements To test the effects of site preparation over time, the diameter and height were measured in all trees within the subplots in the autumns of 1997, 1999 and 2001. In 1997, stem volumes (at 4 years age) were estimated from ground diameter and height measurements, assuming the stem to have a conical shape. In the other 2 years, diameter at breast height (dbh) was determined and the ratio of the volume of timber to 7.5 cm thin-end stem diameter was calculated by using the nationally developed stem volume equation (DGCN, 2000). In each plot, site index values (reference age at 20 years) were obtained from curves constructed by Espinel et al. (1997), who applied data of P. radiata trees from the region to the model of Bailey and Clutter (1974). 2.5. Data analysis The effects of site preparation techniques on growth, soil properties and foliar nutrient concentrations were analysed by analysis of variance. Data were assessed for homogeneity and normality. The distribution of soil preparation techniques was considered to

be random, therefore an unbalanced design with two crossed factors of three and two levels was considered, and a general linear model was used for the analysis. Changes over time were tested by analysis of repeated measurements. The relationships among growth indices, soil parameters and foliar levels of nutrients were analysed by Pearson’s correlation, stepwise linear and quadratic regression analyses. The GLM, CORR and REG procedures of the SAS statistical package (SAS Institute, 1990) were used.

3. Results 3.1. Soil physical and chemical properties The bulk density values increased significantly in the plots where scalping and down-slope ploughing (SP plots) was carried out (Table 1) and the effect was still apparent after 9 years. In 2001, several these plots showed bulk density values close to 1.55 g cm3, which indicated severe compaction. There was also a strong correlation between bulk density values and soil organic matter contents (r ¼ 0:80, P < 0:001). Scalping also led to severe depletion of soil organic matter and total N and S contents (Table 1). When scalping was followed by ploughing (SP), an initial decrease in soil organic matter of 50% was found and gains were rather low (in two cases concentrations remained as low as 1.8%). Soil available P was always very low, usually <3 mg kg1 and there were no significant differences among treatments (Table 1). Scalping resulted in initial large depletions of exchangeable Ca, Mg and base saturation. In the SP stands recovery of these parameters was slow. Thus, recovery of Mg concentrations in these soils was not apparent until 4 years later, and recovery of the Ca contents were even slower. In spite of these effects on base cations, no significant changes in soil pH were found as a consequence of the site preparation technique. No significant effect attributable to site preparation was found for any of the other elements analysed (Fe, Mn, Cu and Zn, data not showed). 3.2. Soil biological properties Soil biological properties (Table 2) were determined in October 2001, 9 years after site preparation

A. Merino et al. / Forest Ecology and Management 196 (2004) 393–404 Table 1 Selected properties of the upper mineral soils in the Pine radiata stands subjected to different site preparation techniques Treatmenta

1993

1997

2001

3

BD (g cm ) C S SP

1.2 aA 1.17 aB 1.34 bA

1.2 aA 1.19 aB 1.36 bA

1.15 aA 1.12 aA 1.34 bA

OM (%) C S SP

5.3 bA 4.1 abA 2.6 aA

4.5 bA 4.5 bAB 2.6 aA

4.2 bA 4.6 bB 2.9 aA

N (mg kg1) C S SP

1.8 bA 1.6 abA 1.06 aA

1.52 bA 1.59 bA 0.76 aA

1.42 bA 1.54 bA 0.96 aA

S (mg kg1) C S SP

0.78 0.64 0.53

0.71 0.68 0.60

0.62 0.66 0.54

Ca (cmolc kg1) C S SP

2.28 aA 1.34 bA 0.64 cB

1.8 aA 1.95 aA 0.41 bB

2.15 aA 2.45 aB 1.61 aA

Mg (cmolc kg1) C S SP

2.0 aA 0.75 bB 0.92 abB

2.02 aA 1.9 aA 1.2 aAB

2.03 aA 1.5 aA 1.88 aA

K (cmolc kg1) C S SP

0.36 aB 0.27 aB 0.26 aA

0.51 aA 0.51 aA 0.29 aA

0.2 aB 0.33 aAB 0.23 aA

Al (cmolc kg1) C S SP

6.54 aA 5.97 aA 6.17 aA

4.7 aA 4.8 aA 4.3 aB

4.73 aA 3.63 aA 4.24 aB

Base saturation (cmolc kg1) C 38.9 a S 28.7 ab SP 18.6 b

50.8 a 41.4 a 21.1 b

35.3 43.1 30.2

Significantly different treatment means are indicated by different lowercase letters (a < b < c), whereas significantly different means among years are indicated by different capital letters (A < B < C), Tukey–Kramer’s test, P < 0:05. a C: stem-only harvesting; S: scalped; SP: scalping plus downslope ripping.

and plantation establishment. The ANOVA revealed a lower significant (P < 0:01) soil MB-C in the scalped and ploughed soils (SP). The MB-C was significantly

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related to soil total C and N (r ¼ 0:80 and 0.81, respectively, P < 0:001). In the C and S stands the mean value for the ratio of MB-C:total organic C was approximately 2.3%. The corresponding mean for SP soils was slightly lower, although the differences were not significantly different. The scalped stands without ploughing (S) showed similar values of MB-N to those with logging residues on the surface (C). Similarly to the MB-C, the lowest levels corresponded to the most intensively managed soils, for which values were frequently lower than 20 mg kg1. The MB-N was significantly related to soil total C and N (r ¼ 0:75 and 0.72, respectively, P < 0:01). The mean percentage of MB-N with respect to total N was 2.8%, with no significant effect attributable to site preparation technique. The correlation between MB-C and MB-N was linear (r ¼ 0:84, P < 0:001). The ratio of MB-C:MB-N varied between 9 and 26. The SP stands showed slightly lower ratios, although the mean values did not differ significantly from the others. The CO2 released (Table 2) was significantly related to soil total C (r ¼ 0:74, P < 0:01), MB-C (r ¼ 0:71, P < 0:01) and MB-N (r ¼ 0:63, P < 0:01) contents. The lowest rates (P < 0:05) of soil respiration were found in the SP soils. The metabolic quotient (qCO2) (the quantity of CO2-C produced per unit of microbial biomass C per unit time) varied between 0.9 and 4.7 g CO2-C g1 MB-C h1. Although some disturbed soils (SP) showed higher levels, the averages for different treatments did not differ significantly. About 80% of inorganic N was found as NH4þ-N, whereas the concentration of NO3-N was usually less than 1 mg kg1 (Table 2). The SP soils showed significantly lower inorganic N concentrations. The N mineralized during 10 days of aerobic incubation (between 0.1 and 15 mg kg1) was significantly related to total C and N (r ¼ 0:48 and 0.55, P < 0:01), MB-C and MB-N (r ¼ 0:51 and 0.57, P < 0:01). Most of the N mineralized (at least 80%) was present as NH4þ, which indicates the low nitrifying potential of these soils. Anaerobic N mineralization was closely correlated with aerobic mineralization (r ¼ 0:78, P < 0:01), total C and N (r ¼ 0:66 and 0.70, P < 0:01), microbial C and N (r ¼ 0:60 and 0.73, P < 0:01). Aerobic and anaerobic mineralizable N were both lower after in scalping and deep ploughing (SP) than in conventionally harvested

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Table 2 Soil microbiological properties in P. radiata plantations subjected to different site preparation techniques, measured 9 years after plantation Parameters 1

MB-C (mg g soil) MB-N (mg g1 soil) MB-C:MB-N MB-C:organic C (%) MB-N:total soil N (%) Soil respiration (mg CO2-C g1 soil h1) qCO2 (mg CO2-C g1 MB-C h1) NH4þ-N (mg N kg1) NO3-N (mg N kg1) Aerobic mineralizable N Anaerobic mineralizable N

Stem-only harvesting (C)

Scalping (S)

Scalping and ploughing

529.1 a 37.9 a 13.5 2.3 2.9 0.83 a 1.8 6.5 a 0.7 16.4 a 39.1 a

579.2 a 41.7 a 14.0 2.3 2.8 1.05 a 1.9 6.6 a 0.9 17.4 a 42.3 a

303.0 b 26.3 b 10.7 1.7 3.0 0.65 b 2.6 3.4 b 0.4 12.9 b 23.9 b

soils (C), but no difference was observed after scalping without ploughing (S). 3.3. Foliar nutrient levels Statistical analysis did not reveal any significant effects of site preparation on foliar N concentrations. From 1999 onwards none of the plantations showed foliar N concentrations lower than the limit for possible deficiency (12 mg g1; Will, 1985). In 1997, however, foliar N levels lower than 11 mg g1 were found in three scalped and ploughed soils. Foliar N level correlated with soil aerobic and anaerobic mineralizable N (r ¼ 0:55 and 0.70, P < 0:05, respectively), whereas no relationship was found with total N. In 1997, average foliar S concentrations were consistently lower in the SP stands (Fig. 1). All the stands under study showed P foliar concentrations below the critical level of 1.2 mg g1, at which growth is potentially reduced (Will, 1985). Foliar P were slightly lower in the SP stands, although the differences were not significant. Foliar P levels were slightly correlated with soil available P (r ¼ 0:35, P < 0:05) and pH (r ¼ 0:49, P < 0:05). Satisfactory foliar Ca concentrations (>1 mg g1) were observed in all stands. The analysis revealed significantly lower Ca concentrations in the SP plantations. Some of the plots showed levels within the marginal ranges for Mg (0.7–1 mg g1) or K (3– 5 mg g1; Will, 1985). No changes attributable to site preparation were observed for these elements (Fig. 1). Significant correlations between concentrations in soils and needles were found for K (r ¼ 0:46,

P < 0:05), Ca (r ¼ 0:64, P < 0:01) and Mg (r ¼ 0:82, P < 0:05). Satisfactory foliar concentrations of Fe and Zn (higher than 40 and 10 mg kg1, respectively) were observed in all plantations studied. Foliar levels of Zn and, to a lesser extent, Fe were significantly lower after scalping and ploughing (SC plots) than in the stemonly harvested soils (C plots). Some stands showed foliar Cu levels lower than 4 mg kg1, considered as the limit for possible deficiency. 3.4. Tree growth In conventionally harvested stands (C) the mean height was more than 9 m at 9 years. During the first 4 four years (1997) no significant differences among treatments were detected (Table 3). At 7 years (1999), tree growth (dbh and mean height) was significantly lower in treatments where logging residues and humus layer were removed (S and SP plots), especially when this was followed by deep ploughing (SP). After 9 years (2001) the differences in tree height in conventionally harvested plots and whole-tree harvested soils were 2.9 m (S plots) and 3.5 m (SP plots). In some ploughed plots the mean tree heights were less than 4 m. A combination of differences in total height, dbh and survival resulted in a dramatic reduction of timber volume by over 300%. At 20 years of age the calculated site indices were 23.9, 19.8 and 19.9 m for C, S and SP plots, respectively. Tree growth was significantly correlated with the foliar concentrations of N, Ca, N/P ratio and Zn (r ¼ 0:65, 0.62, 0.64 and 0.69, P < 0:01), and also

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Fig. 1. Element concentrations in P. radiata needles sampled 4 years (1997), 7 years (1999) and 9 years (2001) after site preparation and planting. Significantly different treatment means are indicated by different lowercase letters, Tukey–Kramer’s test, P < 0:05.

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Table 3 Growth and productivity in response to different site preparation treatments Treatmenta

1997

Height (m) C S SP

2.44 a 1.57 a 1.75 a

6.98 a 4.57 b 4.40 b

9.23 a 6.32 b 5.77 b

DBH (cm) C S SP

5.05b a 3.22b a 4.70b a

9.4 a 6.3 b 5.67 b

12.21 a 8.34 b 7.47 b

25.17 a 7.55 b 6.36 b

76.46 a 27.11 b 21.86 b

Stem volume (m3 ha1) C 8.67 a S 2.58 b SP 3.74 b

1999

2001

Significantly different treatment means are indicated by different lowercase letters, Tukey–Kramer’s test, P < 0:05. a C: stem-only harvesting; S: scalped; SP: scalping plus downslope ripping. b Ground diameter.

with foliar levels of S and P (r ¼ 0:37 and 0.33, P < 0:05). There were also significant correlations between tree growth and certain soil properties, such as available P and Zn (r ¼ 0:60 and 0.62, P < 0:01), as well as coarse sand content (r ¼ 0:60, P < 0:01). Although some plantations showed low foliar concentrations of Mg, K and Cu, tree growth was not significantly related to these element and tree growth. The stepwise multiple regression revealed that foliar N and Ca accounted for up to 53% of the growth rates, whereas the coarse sand content and the available P accounted for 48% of the productivity. Despite the changes during intensive site preparation, soil bulk density and organic matter content were not significantly related to tree growth. However, the lowest growths were found in soils with bulk densities higher than 1.5 g cm3.

4. Discussion 4.1. Soil properties and forest nutrient status Scalping and ploughing exposed the denser subsoil, which increased the bulk densities in the most intensively managed soils. Compaction by heavy equipment in these soils was probably also favoured by their

fine texture and by the lack of organic residues, which would have alleviated this effect. During the first 4 years a dense crust of up to 4 mm in thickness developed in some of the highly disturbed soils, probably as a result of the exposure of bare soil to raindrops, an effect also described by Mwendera and Feyen (1994). The slow rates of recovery of bulk density are consistent with those seen in other studies of highly disturbed forest soils (Rab, 1996; Froehlich et al., 1986). The bulk density levels were high enough to impede root elongation (Froehlich et al., 1986; Skinner et al., 1989), which delayed the establishment of the pioneer vegetation. This, along with reduced water infiltration, lack of protective litter cover and the higher soil erodibility all contributed to the high rates of soil loss recorded at these sites (Edeso et al., 1999; Olarieta et al., 1999). The decrease in soil organic matter, total N, total S and exchangeable Ca and Mg in the most disturbed soils were brought about by the exposure of less-fertile subsoil as a consequence of the scalping by bulldozer and the ploughing, and also because of the accelerated erosion that occurred. In the case of organic matter and total N, additional losses may have occurred as a result of the increased decomposition and mineralization brought about by the increased soil temperature and humidity which has been shown to occur after harvesting (Pe´ rez-Batallo´ n et al., 2001). Organic matter mineralization is also favoured by the effect of mixing soil horizons (Salonius, 1983). The low rate of soil organic matter and total N recovery in scalped and ploughed soils occurred due to the lack of organic residues from decomposing slash and humus layer (35 and 42 Mg ha1, respectively), as well as the scarce colonization by understory vegetation. Excessive foliar N levels may lead to underdevelopment of lateral shoots and to stem deformations (Turner and Lambert, 1986). However, the negative relationship between foliar and tree height, which has also been described in previous studies (Sa´ nchezRodrı´guez et al., 2002), was probably related to the very small differences in N levels in a range at which it is not a limiting factor. The positive relation between foliar P concentration and tree growth shows that this element is one of the nutrients that most limits tree growth, as pointed out in previous studies (Sa´ nchez-Rodrı´guez et al., 2002; Virgel Mentxaka, 2002; Za´ s and Serrada, 2003). The

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mass balance calculations (Ouro et al., 2001) revealed that stem-only harvesting and whole-tree harvesting removed 17 and 76%, respectively, of the P pool of these systems (vegetation, humus layer and soil available P pool). This suggests that the available reserves of this element may be decreased in the long term by whole-tree harvesting and humus layer removal. However, in spite of the large amount of P export, no changes in soil and foliar concentrations were found, probably because most of the P taken up by trees during the forest rotation is derived from soil nutrient turnover through mineralization of organic P (McLaughlin, 1996), which does not necessarily imply a higher level of soil extractable P. In accordance with the decrease in soil available Ca after severe site preparation, lower Ca foliar concentrations were observed. Similar results have been recorded elsewhere (Nambiar, 1996; Johnson and Todd, 1998; Olsson et al., 2000). The greater effect of site preparation on Ca than on Mg and K may have been due to more Ca being removed by tree biomass than was present in the soil available reserves. Thus, the removal of stem and logging residues and humus layer implies the export of 53% of the Ca capital of the system (vegetation, humus layer and available Ca pool), whereas the corresponding values for Mg and K were 20 and 18%, respectively (Ouro et al., 2001). The results of this study show that the initial negative effects on exchangeable Ca and Mg were partially recovered throughout the 9 years, through the inputs from mineral weathering, bulk deposition and decomposition of organic residues. Unlike Ca and Mg, soil and foliar K contents were not affected by site preparation treatment. The different behaviour of these elements, also observed by Olsson et al. (2000), may be due to the faster release of K from litter and logging residues, as revealed in litterbag studies (Ouro et al., 2001). This source, along with K inputs by weathering (estimates are not available), atmosphere and litterfall (7 and 10 kg ha1 per year, respectively; Gonza´ lez-Arias et al., 2000, 1998) may be sufficient to supply the annual tree requirements of this species in northern Spain (47 kg ha1; Ouro et al., 2001). The lower organic matter may have led to the low Fe and Zn concentrations observed in soil and needles because the availabilities of these elements are closely linked to soil organic matter contents.

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The decrease in tree growth can be partially attributed to the depletions of Ca and P in soils and needles, which were observed after the intensive site preparation. Decreased productivities following removal of slash and litter have also been found for radiata pine (Nambiar, 1996; Smith et al., 2000), as well as for other coniferous (Gomez et al., 2002) or eucalypt species (Fo¨ lster and Khanna, 1997; Merino et al., 2003). The most important alterations observed in soils— the increase in bulk density and the decrease in organic matter—were not significantly related to tree growth. It is well known that soil compaction reduces soil aeration/drainage and water retention and increases mechanical impedance, all of which have a major influence on the productivity of different species of the genus Pinus (Froehlich et al., 1986; Skinner et al., 1989). However, the effect of compaction on tree growth is dependent on soil texture (higher in loamy and clayey soils) and water regime (Gomez et al., 2002). The lack of effect of soil compaction on tree growth may be due to the fact that a high level of plantavailable water is maintained during the period of higher growth (average rainfall in summer >150 mm), independently of the effect of compaction on soil porosity. In this aspect, as well as in others, it should be considered that, although all stands showed rather similar properties, there were differences in initial soil conditions as well as in the treatments practised. In addition, it should also be considered that the sampling may not have been sufficient to characterize the high heterogeneity of this parameter in such a complex system. 4.2. Soil biological properties The MB-C and MB-N, as well as soil respiration values, found in the conventionally managed soils were within the range observed for other forest systems in temperate regions (Dı´az-Ravin˜ a et al., 1988; Ohtonen et al., 1992). There were no differences in MB-C or MB-N following logging residue removal, which is consistent with the findings of other studies (Ross et al., 1995; Pe´ rez-Batallo´ n et al., 2001). The low MB-C per unit soil organic C found in the highly disturbed soil may have been caused by the lower input and less diversified organic substrate in

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the highly disturbed soil (Anderson and Domsch, 1989). The lower microbial C:N ratios in these soils may also reflect a qualitative change in the composition of soil microflora. According to Campbell et al. (1991), this effect may indicate the presence of relatively less fungi than of bacteria or actinomycetes. The higher values microbial metabolic quotient (qCO2) in the most disturbed soils may also indicate a decline in fungal dominance and lower efficiency of substrate utilization by the soil microbial community (Sakamoto and Oba, 1994). The relatively low N mineralization rates found in these soils may be due to their fine texture, which provides greater stability for the organic matter present, because of a higher resistance to microbial attack and poor aeration (Anderson and Paul, 1984). Aerobic incubation showed that ammonification predominated over nitrification, which is characteristic of acid forest soils because of the low NH4þ concentrations and low initial population of nitrifying bacteria (Acea and Carballas, 1990). The lower inorganic N and mineralizable N in the scalped and ploughed stands, coinciding with significant depletion of total N and microbial N, are partially consistent with the depletions in these parameters found after the removal of logging and forest floor in the studies of Smethurst and Nambiar (1990) and Ross et al. (1995).

5. Conclusions The productivity of radiata pine plantations in northern Spain is dependent on conservative management of soil organic matter and nutrients. This study showed that intensive site preparation, involving residue removal and soil tillage, led to a decrease in tree productivity, from 8.4 to 2.4 m3 ha1 per year, over the 9 years of the study. Differences in tree growth were mainly attributed to long-term deleterious effects of the intensive site preparation methods on nutrient concentrations in soils and foliage, as well as soil compaction and biological properties. These effects were caused by removal of organic residue, soil disturbance—produced by the bulldozer work and soil tillage—and the accelerated erosion that occurred during the first year after site preparation. Harvesting and site preparation practices that conserve organic matter and the addition of fertilizer are strongly

recommended, whereas that removal of whole logging residues and the humus layer should be avoided.

Acknowledgements Funding for this research was provided by the Environmental Department of the Spanish Basque Government. We are grateful to Dr. Marı´a Jose Gonza´ lez Amucha´ stegi and Dr. Orbange Ormaetxea for their assistance in the fieldwork. Thanks to Dr. Christine Francis for revising the English grammar of the text.

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