Soil fertility rehabilitation in young Pinus radiata D. Don. plantations from northern Spain after intensive site preparation

Forest Ecology and Management 116 (1999) 83±91

Soil fertility rehabilitation in young Pinus radiata D. Don. plantations from northern Spain after intensive site preparation A. Merinoa,*, J.M. Edesob a

Department of Soil Science and Agricultural Chemistry, Escuela PoliteÂcnica Superior, Universidad de Santiago de Compostela, E-27002 Lugo, Spain b Department of Mining, Metallurgy and Material Science, Universidad del PaõÂs Vasco, E-01006 Vitoria, Spain Received 19 December 1997; accepted 14 July 1998

Abstract Soil fertility and tree nutrition of young Pinus radiata were examined four years after harvesting, site preparation and planting in a hilly area of northern Spain. Conventional stem-only harvesting was compared with highly mechanized techniques applied in the region, such as (a) whole-tree harvesting and humus layer removal, and (b) whole-tree harvesting and humus layer removal followed by down-slope deep ploughing. For the study, 58 plantations of Pinus radiata were selected which were located at sites with similar climatic conditions, on slopes often exceeding 35% and on soils with similar properties, developed from argillite. Whole-tree harvesting negatively affected the physical properties and fertility in the rooting soil layer over a period of some years, because of the severe soil disturbance and accelerated erosion which took place. Soils, whose bulk density was increased at the time of site preparation, still showed values high enough to be restrictive to tree growth after a period of four years. A very low rate of recovery was observed for organic matter, total N and S and exchangeable Ca, whose contents remained very low even four years after site preparation. In these soils, exchangeable K increased over this time, most probably as a consequence of the rapid release of this element from logging residues and soil organic matter decomposition. Intensive site preparation also affected the nutrition and growth of young trees. The removal of organic components and deep soil disturbance led to de®ciencies of P, S and N in foliage, as well as potentially toxic levels of Mn, which were associated with symptoms of chlorosis, needle loss and poor growth. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Harvesting management; Acidi®cation; Bulk density; Organic matter; Pinus radiata; Forest nutrition; Tree growth

1. Introduction The exploitation of fast-growing tree species with high nutrient demand, in short rotations, involves the removal of large amounts of biomass and nutrients.

*Corresponding author. Tel.: +34-982-252231; fax: +34-982241835; e-mail: [email protected]

Since consecutive rotations can lead to depletions in soil reserves, the nutrient replenishment capacity is of special concern in these kind of forest systems (Madeira, 1989). Although inputs from the atmosphere and from mineral weathering make up important sources of nutrients, most of the annual nutrient requirements are supplied from the decomposition of organic residues in the soil (Waring and Schlesinger, 1985). In conventionally managed forests,

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A. Merino, J.M. Edeso / Forest Ecology and Management 116 (1999) 83±91

where the abundant logging residues left on site supply important amounts of nutrients, the biogeochemical recovery of the soil to its status prior to harvesting can be relatively rapid (Snyder and Harter, 1984; Tuttle et al., 1985). Some forest management practices, designed to reduce competing vegetation and to prepare a seed-bed suitable for the next rotation, include removal of logging residues and soil organic horizons. Since the humus layer plays an important role as a nutrient reservoir, as well as in regulating soil moisture, temperature and physical structure, such practices can result in extensive alterations to soil properties and nutrient dynamics and can reduce the ability of the system to restore the nutrient extracted during forest exploitation (Munson et al., 1993; Clayton and Kennedy, 1985; Madeira, 1989; Olsson et al., 1996). This latter aspect is particularly important for certain nutrients, such as N and P, whose supply by decomposition of logging residues and litter is the main source for tree nutrition. In acidic soils with low amounts of weatherable minerals, the removal of organic components may also affect the status of other elements, such as Ca, Mg or K, although mineral weathering can provide a substantial part of these nutrients even in highly weathered soils (Johnson et al., 1991; Zabowski et al., 1994). The changes in soil fertility as a consequence of biomass removal can result in substantial reductions in tree nutrition and productivity (Vitousek and Matson, 1985; Tuttle et al., 1985; Smith et al., 1994; Munson et al., 1993). In some cases, the deleterious effects in soils are expressed in terms of tree response in the second rotation (Tiarks and Haywood, 1996). Highly mechanized harvesting management can also adversely affect the physical properties of the soil. The traf®cking of heavy machinery and disturbance of soil usually results in soil compaction, lower porosity and, hence, aeration and decreased hydraulic conductivity, which can be restrictive to tree growth (Froehlich, 1979; Huang et al., 1996). Because of the higher runoff and alterations in soil organic matter content and structure, these practices increase the potential for soil erosion in hilly areas (Farrish et al., 1993; Lal, 1996b). In northern Spain, most plantations are made up of Pinus radiata D. Don, which is grown in 20±30 year rotations. Highly mechanized operations are usually employed to prepare harvested sites for planting. A

common technique involves down-slope, deep ploughing after the removal of the logging residues, stumps and humus layer of the soil. In a previous publication (Merino et al., 1998) deleterious changes in soil physical and chemical properties following intense site preparation were assessed and the process by which these alterations took place were discussed. It was hypothesized that, as a consequence of the removal of organic components and severe soil disturbance, the recovery from such alterations may take a long time and that seedling establishment and tree production may be affected in later plantations. The present study was undertaken to examine the soil properties four years after harvesting and to determine the repercussions of these silvicultural practices on nutrient uptake and growth. 2. Materials and methods 2.1. Site description and experiment design The present study was conducted in different Pinus radiata plantations located in the provinces of Bizkaia and Gipuzkoa, northern Spain. Site characteristics and soil properties were detailed in a previous paper (Merino et al., 1998). Brie¯y, it is a mountainous area with slopes often exceeding 35%. The average annual temperature varies between 118 and 148C at the different sites, and the average annual rainfall ranges between 1200 and 1800 mm. Soils under mature plantations are mostly classi®ed as Dystric Cambisols or Gleyic Cambisols. They are made up of two mineral horizons, A and B, under a humus layer of 2±4 cm depth. The soils are ®ne textured, have high bulk density, moderately high organic matter contents in the upper mineral horizon and are strongly acidic. Some of them are temporally anaerobic. The clay minerals consist mainly of mica and kaolinite. Fifty eight different Pinus radiata stands were selected. They were situated on homogeneous slopes often exceeding 35% and had a minimum area of 0.5 ha. Twenty four of these were mature pine plantations (F plots). The remainder were plantations in which, at the beginning of 1993, clear felling and seed-bed preparation were carried out. In all plots, subsequent plantations were established in the spring of the same year. In these harvested plots, site pre-

A. Merino, J.M. Edeso / Forest Ecology and Management 116 (1999) 83±91

paration was carried out using the three most-usual practices in the region: 1. Fifteen plots were conventionally managed with most of the logging residues left on the ground. The humus soil layer was not removed and soils underwent little disturbance (CH plots). 2. In another eight harvested plots, below-ground parts of the trees were extracted and logging residues and litter layer were removed (HR plots). Following this, about 80% of the mineral soil was exposed. 3. At 11 sites, in addition to the previous practices, down-slope deep ploughing (50 cm depth) was carried out (HRP plots). In this operation mineral topsoil was thoroughly mixed with subsoil to a depth of 50±60 cm, and the mineral subsoil exposed over more than 90% of the surface. 2.2. Soil sampling and analyses of soils and needles In all plots, soil sampling was performed in August 1993, 5±6 months after harvesting and site preparation, and following a period of intense precipitation. In March 1997, further samples were collected under the same conditions. In the mature plantations, sampling could not be repeated in 1997 as most of the trees had already been harvested. From each stand, nine samples of mineral soil were taken at random outside skid trails from the upper 15 cm, and three samples of equal mass were gathered to build up three composite samples. Soil samples were air-dried and sieved with a 2-mm screen before analysis. Samples for the determination of bulk density were taken at four points (in 1993) or three points (in 1997) of each plot using a brass core (40 mm long, 55 mm inside diameter), and were dried to constant mass at 1058C. The pH was measured in H2O and 0.1 M KCl (soil : solution ratio of 1 : 2.5) with a glass electrode. Total C, N and S were analyzed with a LECO Elemental Analyzer. Organic matter was read as C and then multiplied by 1.72. Available P was extracted using the Mehlich 3 procedure (Mehlich, 1984) and determined photometrically using the molybdenumblue-method. Exchangeable cations (Na, K, Ca, Mg, Al and Mn) were extracted with unbuffered 1N NH4Cl and analyzed by atomic absorption spectrophotometry. In the samples collected in 1993 exchangeable Mn

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was not determined. Exchangeable base cations were de®ned as the equivalent sum of Na, K, Ca and Mg. In March 1997, tree-growth measurements were carried out at the same time as foliage samples were collected. The diameter at the base and total height of 40 living trees were measured. A minimum of 30 trees per plots were sampled from the upper one-third of the crown and bulked to give two composite samples. These were oven-dried (60 8C) to a constant weight, milled (0.25 mm) and digested with H2SO4/H2O2. In leaf digestion, K, Ca, Mg, Mn and Al were analyzed by atomic absorption spectrophotometry, whereas P was determined photometrically by the molybdenumblue-method. Nitrogen and S in needles were analyzed in solid milled material using a LECO analyzer. 2.3. Statistical analysis The Tukey test for separation of media was used. Data were subjected to the Pearson correlation as well as regression analysis. Differences in the averages of soil parameters of samples collected from the same plots in 1993 and 1997 were analyzed by the pairedsample t test. 3. Results 3.1. Soil physical and chemical properties Bulk density increased following intense site preparation practices (HR and HRP soils, Fig. 1(a)). The effect was more acute in the soils where logging residues were removed and ploughing took place (HRP), where bulk density increased 17% compared to soils under mature plantation (F). Four years after site preparation (1997), most of these ploughed soils (HRP) had still higher bulk densities than the uncut areas (a mean of 1.16 g cmÿ3) and some showed values as high as 1.7 g cmÿ3. In the stem-only harvested soils (CH), or in whole-tree harvested soils without deep ploughing (HR), bulk density did not increase signi®cantly at the time of harvesting practices. However, four years after harvesting, bulk density increases of 15% (CH plots) and 19% (HR) were found in these harvested soils. Moreover, these disturbed soils remained bare and showed a super®cial crust.

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Fig. 1. (a) Means and standard errors of bulk density, (b) organic matter, (c) total N, (d) total S and(e) available P for the upper 15 cm depth of the soils under the radiata pine plantations or harvested soil after different harvesting managements. F, mature plantation. CH, stem-only harvested. HR, whole-tree harvested. HRP, whole-tree harvested and ploughing. Different letters denote significant differences at p<0.05 level. A>B>C, differences in soil properties of samples collected in 1993 (Tukey test); a>b>c, differences in soil properties of samples collected in 1997 (Tukey test). (*) Differences in soil properties between samples collected in 1993 and 1997 (paired-sample t test).

In comparison with the mature plantations, the upper layer of the whole-tree harvested soils (HR and HRP) showed decreases in soil organic matter at the time of site preparation (Fig. 1(b)). Depletions of 65% were recorded in the rooting layer of HRP soils, whereas the HR soils showed organic matter losses of 44%. Throughout the four years following the site preparation (1997), these sites showed no increase in organic matter, and even seemed to decrease slightly in HRP sites (Fig. 1(b)). A small increase was observed in the HR plots, although the paired-sample t test did not reveal any signi®cant difference. The total N contents decreased after intensive site preparation in HR and HRP soils; N levels were more than 40% less than in the mature plantation soils (Fig. 1(c)). Four years after site preparation, low levels of total N were still found in these soils. In HRP soils a decrease in total N seemed to take place over this time (also re¯ected in slight increases in the C/N ratio). However, this point was not con®rmed by the paired-sample t test. The total S contents in the soils where little soil disturbance took place (CH) remained similar to those of uncut plantations (F) (Fig. 1(d)). In HRP and HR sites, slash removal and ploughing led to large depletions in total S as compared to the original soils. The large losses in total S from the upper soil layer were not recovered over the four years after harvesting; on the contrary, soil S dropped slightly over this time. Available P levels were always very low, <4 mg kgÿ1, and no signi®cant changes were found between treatments (Fig. 1(e)). Soil pH was not signi®cantly modi®ed by any harvesting management (Fig. 2(a)). In 1997, the pH was consistently higher than at the time of site preparation in all types of plots, with values being higher than those of uncut soils (Fig. 2(a)). Site preparation resulted in large depletions in exchangeable Ca, especially in plots with intensive site preparation, where losses of 60% (HR soils) and 80% (HRP soils) took place. Four years later, the exchangeable Ca content increased slightly in HR soils (not signi®cant), whereas HRP soils still exhibited the low values found at the time of site preparation (Fig. 2(b)). Exchangeable Mg was also observed to decrease following site preparation in 1993 (HR sites) (Fig. 2(c)). As with Ca, a considerable increase in exchangeable Mg took place in these same soils,

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lowing site preparation, K was subjected to substantial increases in the less-disturbed soils (CH and HR plots), where contents of this element were even higher than those of uncut areas (F). In the ploughed soils (HRP), on the contrary, K levels remained at the levels found in 1993. Harvesting management did not have any immediate effect on exchangeable Al (Fig. 2(e)). In accordance with the increases in soil pH over the four years following harvesting, exchangeable Al was subjected to decreases in all treatments. This depletion, however, was only signi®cant in the most-disturbed soils (HRP). Exchangeable Mn was always low (0.08±0.15 cmolc kgÿ1) and changes attributable to harvesting management were not observed following site preparation or four years later (data not shown). 3.2. Foliage nutrient levels and tree growth

Fig. 2. pH and exchangeable cations in the 0±15 cm surface soil layer for mature radiata pine plantations and the three harvesting managements studied. (a) pH (H2O), (b) Ca, (c) Mg, (d) K and (e) Al. See explanation in Fig. 1.

whereas the most-disturbed soils maintained the low levels of four years before. None of the treatments signi®cantly modi®ed the original content of exchangeable K (Fig. 2(d)). Over the four years fol-

The needle concentration of elements for the young trees after different site preparation systems are given in Table 1. Foliage levels of S and P in current year needles were consistently lower in the stands, where intense site preparation practices involved logging residue removal and deep soil disturbance (HRP plots). Average foliage N content was not signi®cantly affected by harvesting or site preparation. The concentration of Mn in needles increased considerably in the most-disturbed soils. Foliage Ca and Mg increased slightly in the young trees of the most-disturbed soils (HRP plots), although the differences were not signi®cant. No changes attributable to harvesting or site preparation were found for the K or Al contents in needles. Total height and basal diameter of the trees for the three harvesting management practices studied are shown in Table 2. No signi®cant differences in tree growth were found for any of the treatments. The average height of trees of the HRP stands were higher, although not signi®cantly different from the others. 4. Discussion 4.1. Soil properties and nutrient replenishment Five-to-six months after harvesting and site preparation, notable modi®cations in some physical and

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Table 1 Concentration of nutrient elements (mg gÿ1 dry weight) in the needles of four-years old individuals of Pinus radiata growing in plantations with different systems of site preparation. Standard errors are shown in parenthesis. CH, stem-only harvesting; HR, whole-tree harvesting (including below-ground biomass) and humus removal; and HRP, whole-tree harvesting and deep ploughing. Significant differences in means are indicated with a different letter, a
N

S

P

K

Ca

Mg

Mn

Al

molar Ca/Al

CH

14.8a (0.07) 15.1a (0.04) 13.2a (0.08)

1.53a (0.09) 1.51ab (0.08) 1.26b (0.07)

2.02a (0.15) 1.92ab (0.17) 1.47b (0.14)

7.04a (0.57) 6.80a (0.61) 6.05a (0.26)

6.41a (0.91) 4.97a (0.36) 8.08a (1.90)

1.21a (0.17) 1.22a (0.09) 1.48a (0.15)

0.69a (0.07) 0.70a (0.12) 1.21b (0.44)

2.50a (0.36) 1.86a (0.37) 1.80a (0.20)

1.03a (0.17) 1.15a (0.15) 1.16a (0.17)

HR HRP

Table 2 Averages of basal diameter and total height in four-year old radiata pine trees. In each plot, data were calculated from measurements of 40 living individuals. See explanation in Table 1 Treatment

Basal diameter (cm)

Total height (m)

CH HR HRP

3.84 (0.52) 3.86 (0.23) 4.53 (0.7)

1.88 (0.23) 2.09 (0.2) 2.1 (0.32)

chemical properties of the soils were recognized. The effects were more severe in plots in which deep ploughing was carried out after whole-tree harvesting and litter layer removal. In these soils, large increases in bulk density were found which were attributed to the exposure of dense subsoil and to soil compaction by machinery. Decreases in organic matter, total N, total S and exchangeable Ca and Mg were also identi®ed in the rooting zone of the disturbed soils, which were produced by the mixing of upper and deeper horizons, increased soil erosion and accelerated cation leaching. These results are also reported in a previous paper by Merino et al. (1998). The present paper further discusses the rehabilitation of the soil properties four years after site preparation and repercussions of these silvicultural practices on nutrient status and growth of pines. In 1997, bulk density of the most-disturbed soils still had the high values found at the time of harvesting and site preparation in 1993. The bulk-density values found in the ploughed soils (HRP) were higher than 1.35 g cmÿ3, and in some cases 1.7 g cmÿ3. In some of these soils super®cial crusts of up to 4 mm in thickness were formed. High bulk densities can cause low porosity and low water in®ltration and the values

found here are suf®ciently large to cause an impedance to root elongation and, therefore, a reduction in plant growth (Froehlich, 1979; Rab, 1996). These ®ndings are consistent with other studies conducted in highly disturbed deforested areas and con®rm that soil may take years to recover from compaction (Froehlich et al., 1985; Madeira, 1989; Tiarks and Haywood, 1996; Lal, 1996a). The rates of recovery, however, are determined by soil properties and soilmoisture conditions at the time of site preparation. Thus, in some soils slight recovery after several years of soil disturbance has been recorded (Froehlich et al., 1985; Tuttle et al., 1985). Four years after site preparation, the most-disturbed soils still showed the low levels of organic matter, total N and S found in 1993. The low rate of recovery in the ploughed soils was presumably due to the reduced litter input and to the scarcity of understorey vegetation. In other areas, slow increases in soil organic matter and N have also been observed after harvesting, especially when much soil disturbance had taken place (Tuttle et al., 1985; Madeira, 1989). The removal of the canopy and the humus layer may have reduced the amount of N leached to the mineral soil, limiting the capacity of increase in N during this time. Thus, in a Mediterranean Pinus radiata plantation, Cortina et al. (1995) calculated an export of N from the humus layer of 20 kg N haÿ1 yearÿ1. According to these data the amount that could be transferred over four years to the mineral soil accounts for 2% of the total N contained in the upper 15 cm of the mineral soil of the mature pine plantations (3800 kg N haÿ1) and for 3% of the HRP soils (2400 kg haÿ1).

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Although not con®rmed by statistical analysis, N content seems to have decreased slightly in ploughed soils, compared to the 1993 levels. In some of the ploughed soils, the scarcity of vegetation may have favoured some N export by erosion and NOÿ 3 leaching during these years. Large losses of soil by erosion (ca. 80 Mg haÿ1) were recorded in these whole-tree harvested soils over the ®rst year after harvesting (Edeso et al., 1994) and this may also be important at later stages. According to different studies (Vitousek and Matson, 1985; Smethurst and Nambiar, 1995), NO3leaching can lead to considerable N losses in harvested areas even several years after harvesting. Four years after soil disturbance, the total S decreased in all deforested soils. These depletions, however, cannot be explained by soil mixing and suggest that SO2ÿ 4 leaching may have taken place over this time. The higher soil pH values may promote the desorption of the anion originated from mineralization. This pattern of behaviour has also been suggested by Krake and FernaÂndez (1993) in a study of the solution composition in recently harvested areas. Available P was always present at very low levels and no signi®cant change was found due to harvesting management. Decreases in available P following the removal of logging residues and litter layer, however, have been reported by other researchers (Mroz et al., 1985; Tuttle et al., 1985). With all treatments, pH increased compared to the values found four years before. Thus, in 1997, the pH values observed at all harvested sites were higher than those of uncut areas. The reason for this post-harvesting effect is not clear and is in contrast with the possible higher nitri®cation rates during this time. However, this has also been observed in other deforested areas (Snyder and Harter, 1984; Krake and FernaÂndez, 1993) and has been attributed to the release of soluble cations and OHÿ from the accelerated decomposition of logging residues and organic matter decomposition due to high soil temperature and moisture content. Owing to the low amount of weatherable minerals in these soils, mineral weathering may be much less important in supplying Ca, Mg or K, in comparison with decomposition of logging residues. The in¯uence of organic matter during the ®rst years after harvesting was evident for K. Thus, in the most-disturbed soils, the exchangeable pool of this element remained at the

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low values of 1993, whereas in stem-only harvested and in whole-tree harvested sites without ploughing this element increased considerably. In stem-only harvested soils, the increases in exchangeable K re¯ect the high release of this element during litter decay (Cortina and Vallejo, 1994), whereas in the whole-tree harvested soils without ploughing this effect was probably due to the mineralization of organic matter of the surface layer. In agreement with our results, Snyder and Harter (1984) found a rapid increase in exchangeable K after clearcutting, whereas Staaf and Olsson (1991) observed higher concentrations of K in the solutions of recently clear-felled soils. Organic residues also have a potential for supplying large amounts of Ca and Mg. According to Barraqueta and Basagoiti (1993), logging residues and litter layer in pine plantations of the area contain 17.7 kmolc Ca haÿ1 and 4.6 kmolc Mg haÿ1. These amounts account for 33 and 30% of the total soil storage of Ca and Mg, respectively (34 and 11 kmolc haÿ1 of Ca and Mg were measured in the mineral soil of mature plantations) (Merino et al., 1998). In spite of these high contents, no increases in Ca or Mg took place over the four years after harvesting, even in the plots where logging residues were left on site. Low replenishment rates of these nutrients have also been observed in harvested soils from Alabama (Tuttle et al., 1985) and may be due to their low release by biomass decomposition during the ®rst years after harvesting (Cortina and Vallejo, 1994). 4.2. Foliage nutrient levels and tree growth The foliage status of P and S of the new trees was reduced in the most-disturbed soils (HRP treatment). Foliage P was not correlated with available P in soils. However, coinciding with low soil levels of P (<0.8 mg kgÿ1), three plots ± where intense ploughing was carried out ± showed foliage levels below the critical levels of 1.2 mg gÿ1, where growth is potentially reduced (Will, 1985). In fact, the trees of these stands showed clear symptoms of chlorosis, needle loss and poor growth. Sulphur in needles was only slightly correlated to soil S content (rˆ0.51, p<0.01). In the three ploughed plots, S contents of the foliage were as low as 0.8 mg gÿ1. The average concentration of foliage N was not signi®cantly affected by any harvesting management.

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Nevertheless, in three stands with intense site preparation, N levels in needles were lower than the limit for possible de®ciency (12 mg gÿ1, Will, 1985). Nitrogen in needles was slightly correlated with soil organic matter and needle total N in soil (rˆ0.50, pˆ0.01 and rˆ0.43, pˆ0.03, respectively). Although the lowest N contents in foliage coincided with very low levels in the soil (<0.8 mg gÿ1), not all stands with low soil N had tree N status below the level of tree demand. This may have been due to the stimulation of N mineralization after harvesting and soil disturbance. Thus, the increased mineralization in the soil may have maintained the amount of available N during the ®rst years, even in soils where large depletions in total N occurred. Although exchangeable Mn was not altered by harvesting or site preparation, the Mn concentration in foliage increased considerably in plots where deep ploughing was carried out. The average Mn content in these young trees was above the threshold for possible toxicity in Pinus radiata (0.7 mg gÿ1, Marcos de Lanuza, 1966). Other authors (Tiarks and Haywood, 1996) have also reported increased Mn foliage content in the second rotation of pine plantations after intense site preparation. In these ®ne textured soils, the higher uptake of Mn may be associated with decreased porosity after site preparation, which can enhance local anaerobic conditions and, therefore, the availability of this element. Because organic matter combines with Mn, the lower organic matter content in such soils can also contribute to higher solubility of this metal. Aluminium levels in needles were unchanged after site preparation. Independently of the type of management, foliage Ca/Al molar ratios were in most cases higher than one (Table 1) which, according to Cronan and Grigal (1995), would indicate conditions of no Al stress to affect tree growth. Depletions of exchangeable Ca in the most-disturbed soils did not affect the content of this element in needles. According to the reference values given by Will (1985), Ca contents of the pine foliage indicated a good supply. K and Mg were normally found at concentrations >5 and 1 mg gÿ1 which should not be limiting for plant growth. No signi®cant correlations were found between the foliage contents of Ca, Mg or K and their respective contents in the soil.

In spite of the detrimental effects to tree nutrition following intensive site preparation, tree growth was observed to be affected in only some stands. The lack of consistent responses of the young pines' growth to increased bulk density or lower nutrient status was probably due to the heterogeneity of the plantations studied. The different pre-harvesting managements of the sites and their different expositions probably in¯uenced the tree growth and masked the effect of harvesting management on productivity. It may be possible, however, that decreased nutrient availability may alter tree productivity during the plantation development or in later rotations. 5. Conclusions Under the conditions of the study, intensive site preparation altered the physical properties and nutrient status of the soil in the following four years. The low rates of recovery of bulk density, organic matter, N and S found in severely disturbed soils were due to the removal of logging residues and soil humus layer and to the poor development of understorey vegetation, by which soil nutrient status and physical properties can be improved. The lower foliage levels of P, S and, in some cases, N, observed, along with the symptoms of chlorosis after highly mechanized management practices, indicate that the detrimental effects in soil properties brought about by intensive site preparation affect the tree nutrient uptake. Although the needle concentrations of Ca and Mg indicate a good supply of these elements, their low content in these soils may lead to nutrient de®ciencies in subsequent rotations. Acknowledgements We thank Dr. Socorro Seoane, Susana FernaÂndez, GuzmaÂn Ouro and Jose Antonio LarrioÂn for their assistance during the course of the study. We are grateful to Dr. Michael Bredemeier from the Forest Ecosystems Research Center at the University of GoÈttingen (Germany), for his valuable discussion and critical comments on the manuscript. This work was funded by the Environmental Department of the Basque Government.

A. Merino, J.M. Edeso / Forest Ecology and Management 116 (1999) 83±91

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