Forest Ecology and Management 195 (2004) 85–95
Soil erosion after Eucalyptus globulus clearcutting: differences between logging slash disposal treatments Cristina Ferna´ndeza, Jose´ A. Vegaa,*, Jose´ M. Grasa, Teresa Fonturbela, Pedro Cuin˜asa, Etienne Dambrineb, Margarita Alonsoa a
Centro de Investigaciones Forestales y Ambientales de Louriza´n, Consellerı´a de Medio Ambiente, Xunta de Galicia, P.O. Box 127, 36080 Pontevedra, Spain b Equipe Cycles Bioge´ochimiques, INRA, 54280 Champenoux, France
Received 31 December 2002; received in revised form 22 October 2003; accepted 20 February 2004
Abstract The effect of different logging slash disposal techniques on soil erosion for 3 years after harvesting was evaluated in a clearfelled Eucalyptus globulus Labill. stand on a representative coastal site in Galicia (NW Spain). The treatments compared were: slash scattering; slash scattering þ fertilization; windrowing; scattering þ burning (broadcast burning) and windrowing þ strip burning (windrow burning). Accumulated soil losses were relatively small and particularly in slash scattering treatments. Both burning treatments resulted in significantly higher losses (between 7 and 110 times) than the other disposal methods. Broadcast burning generated less erosion than windrow burning in the first year after treatment but not in the second. The severe burning conditions in windrow burns reduced drastically the protective soil organic layer. Slash scattered (alone or combined with fertilization) on the ground was the most efficient treatment and gave negligible soil losses. Cover by slash or litter and duff significantly controlled soil losses. In burn treatments, remaining slash and litter þ duff reduced soil losses. The duration of soil heating significantly affected the remaining soil organic cover on burned soils and this, in turn, was significantly influenced by surface soil moisture content immediately before burning. Soil moisture content just before burns was the key to constrain soil losses after slash burning. # 2004 Elsevier B.V. All rights reserved. Keywords: E. globulus; Harvesting; Clearcutting; Logging slash disposal; Soil erosion; Burning
1. Introduction Eucalyptus globulus Labill. is one of the main forestry species in Galicia (NW Spain), covering more than 200,000 ha in pure stands (Tercer Inventario Forestal Nacional, 2001). Most of these plantations *
Corresponding author. Tel.: þ34-986-805011; fax: þ34-986-856420. E-mail address:
[email protected] (J.A. Vega).
are on steep slopes and shallow and acidic soils developed on granitic bedrock, in an ocean-influenced coastal area with a mild, humid climate. These eucalypt stands, characterized by their high growth rate (the highest in western Europe forests), are managed on short rotations (10–15 years) using mechanized skidding and intensive logging slash manipulation after clearcutting. Harvesting practices can increase the potential for sediment production through soil alteration and forest
0378-1127/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2004.02.052
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floor disturbance; this in turn, may cause an increase in runoff and in soil losses (e.g. Blackburn et al., 1986; Beasley and Granillo, 1988; Rab, 1994; Castillo et al., 1997; Edeso et al., 1999). These adverse effects can be magnified with the use of fire to eliminate logging slash (e.g. Megahan and Molitor, 1975; Shahlaee et al., 1991; Soler and Sala, 1992; Megahan et al., 1995; Rab, 1996; Leal da Silva et al., 1998). Although the erosion risk after forest fires has been evaluated in different situations in Galicia (e.g. Vega et al., 1982; Dı´az-Fierros et al., 1982, 1987, 1990; Vega and Dı´az-Fierros, 1987; Benito et al., 1991; Soto et al., 1994) there is no information about the effect on soil erosion of either tree clearcutting or the impact of burning and other slash disposal treatments after clearfelling. In Galician E. globulus plantations, two slash disposal techniques after harvesting, windrowing alone or windrowing þ burning, are frequently used to reduce the risk of insect infestation and specially fire hazard which is very high in this area. Intensive forest management as practised in Galician E. globulus stands raises concerns for sustainability (Dambrine et al., 2000; Ferna´ ndez, 2002; Vega and Ferna´ ndez, 2002). Furthermore, the extensive use of fire for logging slash disposal contributes to nutrient losses (Raison et al., 1985; Neary et al., 1999). Post-
harvesting erosion may exacerbate nutrient losses with consequences on long-term soil productivity. The aim of this work was to compare soil erosion losses of different logging slash management techniques after clearcutting in Galician E. globulus stands and to explore the influence of some factors related to treatments on soil losses in 3 years following harvesting and slash disposal.
2. Materials and methods 2.1. Study site This study was carried out on the SW slope of the Castrove Mountains (428250 4900 N; 88440 3000 W; 225 m a.s.l.) near the ocean inlet of Pontevedra, Galicia, NW Spain (Fig. 1), a representative site of the Galician coastal E. globulus stands. It is part of an integrated investigation on the effects of harvesting and logging slash disposal on soil properties, N mineralization and nutrient budget. A recently harvested area in a second-growth E. globulus stand on a relatively homogeneous slope of 50% was selected in the spring of 1994. Stand age was 15 years with a density of 1100 trees ha1, basal area was 28 m2 ha1 and dominant height 25 m. Mean
Fig. 1. Localization of study site.
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Table 1 Castrove soils main physico-chemical characteristics Depth (cm)
0–20 20–50 50–100
Texture
pH
Sand (%) Silt (%)
Lime (%)
73 70 72
8 10 10
19 20 18
4.5 4.9 4.8
C (%)
10.8 5.4 3.1
C/N
17.4 18.1 19.4
merchantable wood yield was 23 m3 ha1 per year. The understory is dominated by some Ericaceae (Erica cinerea L. and Calluna vulgaris (L.) Hull) and gorse (Ulex europaeus L.) plants. The climate is warm-humid oceanic. Average site rainfall is about 2000 mm per year with a marked dry period in summer (1 or 2 months). Mean annual temperature is 14 8C, January being the coldest month minimum temperature 2 8C and July the hottest with 30 8C (maximum temperature). The susbtrate is granite. Soils are Humic Cambisols (Macı´as and Calvo, 2001) with a sandy-loam texture and relatively stony. They are very acidic soils with a very low nutrient content (Table 1). 2.2. Experimental design Twenty-five experimental plots (30 m 19 m each) were installed, just after clearcut and logging, with their longest dimension along the maximum slope. A randomized block design was used with five different slash disposal treatments. The treatments were: slash scattering (S), slash scattering þ fertilization (F) (100 kg ha1 of 15N15P15K), windrowing (W), slash scattering þ burning (broadcast burning, BB) and windrowing þ burning (windrow burning, WB) and five replicates for each treatment. In an adjacent area within the same stand, five undisturbed plots (uncut trees) were also set up acting as control (T). The total number of plots was 30. A fuel inventory was carried out immediately after harvesting and slash disposal. Five 2 m 2 m subplots were randomly selected on each plot. In windrowing plots, they were located inside the windrow. Due to the abundance of unshed leaves on slash and shrub, this destructive sampling was preferred to linear sampling. All the slash within each subplot was cut and collected
Exchangeable cations (cmol(þ) kg1) Ca
Mg
Na
K
0.01 300 8C on the mineral soil surface >60 8C on the mineral soil surface >100 8C 2 cm below mineral soil surface *
P < 0:05. P < 0:01.
**
Windrowing þ burning (WB) (n ¼ 5)
487 204 47 33 27
(108) (69) (2) (3) (10)
720 387 68 48 32
(24)* (133)* (25) (13) (9)
258 60 1266 0
(66) (68) (295) (0)
1544 975 2898 24
(209)** (628)* (380)* (24)
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Table 6 Accumulated mean annual soil losses for different levels of treatment (g m1) (S.E. in parenthesis)
Control Slash scattered Slash scattered þ fertilization Windrowing Broadcast burning Windrowing þ burning Rainfall (mm) Mean rainfall kinetic energy (J m2) Mean rainfall intensity in 30 min, I30 (mm h1) Rainfall erosivity (105 W m1)
First year
Second year
Third year
0 35.2 136.3 337.6 2704.2 4116.3 2209 178 8 148
0 16.0 200.0 547.0 2328.8 2080.7 2210 152 6 126
0 7.4 42.0 69.0 263.6 268.4 1751 127 5 68
(29.6) a (89.5) ab (156.3) ab (716.9) bc (1858.7) c
(8.3) a (238.3) (507.6) (699.2) (656.7)
a a b b
(7.4) a (35.7) a (58.6) a (82.7) b (81.6) b
Also shown are annual rainfall, mean rainfall kinetic energy and intensity in 30 min, and rainfall erosivity during each period considered. Soil loss values followed by the same letter in the same column, means did not differ statistically (P < 0:05). Values of control included only for comparative purposes.
greater than in no-burned plots. There were no differences between no-burning treatment levels. For the whole study period, soil losses in windrow burnings (the treatment with the highest erosion rate) were 110 times greater than in slash scattering, the slash disposal method in which less soil erosion was measured after harvesting, whereas between broadcast burnings and slash scattering that value was 90. The ratios between accumulated soil losses for the 3 years after treatment measured in windrow burnings, broadcast burnings and windrowing were 7:1 and 6:1, respectively. Assuming the sediment trapped came from the whole length of the slope (e.g. Morgan, 1980, 1996; De Byle, 1981; Soler and Sala, 1992; Soler et al., 1994; Edeso et al., 1999), at the end of the 3 years after treatment, the mean total amounts of soil losses in windrow burnings and broadcast burnings were equivalent to 2.04 and 1.76 Mg ha1, respectively, whereas mean values for windrowing, scattering and scattering þ fertilization were 0.32, 0.02 and 0.13 Mg ha1, respectively. There were no differences in soil shear strength between treated plots immediately after treatments (Table 7). Bulk density, measured 3 years after treatments, increased significantly (Table 7) in burned plots compared with other treatments. 3.3. Variables influencing soil losses There was a significant and positive linear relationship between the mean accumulated soil erosion by burning level (n ¼ 2, windrow burnings and broadcast
burnings) in each measurement period throughout the first year (n ¼ 10), and the accumulated maximum rainfall intensity in 30 min (I30) in the same period (r ¼ 0:75; n ¼ 20; S:E: ¼ 144; P < 0:001). Likewise, soil erosion losses in the burned plots were significantly related with the accumulated rainfall kinetic energy (r ¼ 0:73; n ¼ 20; S:E: ¼ 148; P < 0:001) and precipitation during each measurement period (r ¼ 0:69; n ¼ 20; S:E: ¼ 157; P < 0:05) throughout the first year. For the 25 experimental plots, accumulated soil losses at the end of the study were significantly and positively related with soil bulk density (r ¼ 0:52; n ¼ 25; S:E: ¼ 3643; P < 0:01). There was a positive effect of soil organic layer cover measured immediately after treatments on the soil erosion control. There were significant, negative logarithmic relationships between soil organic cover Table 7 Soil shear strength (kg cm2) in E. globulus harvested plots immediately after slash disposal or burning and soil bulk density at the end of the study period (g cm3) (S.E. in parenthesis) Treatment
Shear strength
Bulk density
Control Slash scattered Slash scattered þ fertilization Windrowing Broadcast burning Windrowing þ burning
3.60 3.65 3.45 3.58 3.51 4.19
0.62 0.62 0.63 0.63 0.71 0.73
(0.30) (0.38) (0.46) (0.30) (0.14) (0.37)
a a a a a a
(0.01) (0.01) (0.01) (0.02) (0.03) (0.03)
a a a a b b
Values followed by the same letter in the same column, means did not differ statistically (P < 0:05).
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Fig. 2. Statistical relationships between: (a) accumulated soil erosion throughout 3 years in 25 experimental plots after E. globulus clearcutting and different slash disposal treatments vs. soil organic cover immediately after treatments; (b) soil organic cover immediately after treatments for nine experimental plots and duration of temperatures >60 8C at mineral soil surface during burnings; (c) duration of temperatures >60 8C at mineral soil surface in nine experimental plots during burnings and soil moisture content (0–5 cm) immediately before burns. S: slash scattering; F: slash scattering þ fertilization; W: windrowing; BB: broadcast burning; WB: windrowing þ burning.
and accumulated soil erosion losses measured in each plot during the first (r 2 ¼ 0:67; n ¼ 25; S:E: ¼ 1466; P < 0:001), second year (r 2 ¼ 0:62; n ¼ 25; S:E: ¼ 885; P < 0:001) and the whole study period (r 2 ¼ 0:72; n ¼ 25; S:E: ¼ 2081; P < 0:001, Fig. 2a). This relationship also held for the burned plots in the first year (r 2 ¼ 0:48; n ¼ 10; S:E: ¼ 2357; P < 0:05) and for the whole study period (r 2 ¼ 0:48; n ¼ 10; S:E: ¼ 3301; P < 0:05). Nevertheless, there were no direct relationships between remaining litter þ duff thickness, maximum temperature at soil surface and total fuel loading reduction versus soil erosion in burned plots. However, there was a significant logarithmic
relationship between remaining soil organic cover after burning and the duration of temperatures >60 8C on the mineral soil surface (r 2 ¼ 0:71; n ¼ 9; S:E: ¼ 8; P < 0:005, Fig. 2b). In turn, the duration of temperatures >60 8C on the mineral soil surface was negatively and exponentially related with soil moisture content (r 2 ¼ 0:69; n ¼ 9; S:E: ¼ 951; P < 0:005, Fig. 2c).
4. Discussion The absence of soil losses in undisturbed E. globulus plantations (Table 6) is in agreement with the
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low levels of soil erosion found in similar stands by Terry (1994) and Shakesby et al. (1994) in Portugal. E. globulus clearcut followed by mechanical harvesting and slash scattering or windrowing also resulted in negligible soil losses (Table 6). This supports the results observed by Shakesby et al. (1994) in Eucalyptus clearfelling stands in Portugal and in other experiments involving other tree species in which soil losses after clearfelling were more related with slash management than with tree removal (e.g. Douglass and Goodwin, 1980; Farrish et al., 1993; Nykvist, 1994; Edeso et al., 1999; Wilson, 1999). Slash scattering over the soil may be a slightly more efficient technique than windrow disposal (Table 6). This relatively lower soil protection in windrowing could be partially caused by an uneven logging debris distribution but especially because those post-harvesting residues were located along the maximum slope (as usually made by local loggers) and, that distribution could partially channel runoff increasing erosion. The advantage of maintaining the clearcutting slash compared to burn, detected in this study, was also found by Leal da Silva et al. (1998) who measured an increment of soil erosion losses with burning of 99.6% when compared with slash disposal on the ground. Oyarzun and Pen˜ a (1995) observed four times more erosion in plots burned after harvesting compared with soil losses in non-burned treatments. The significant relationship found between the rainfall kinetic energy in burned soils and first year soil losses, and its absence in scattered, windrowing or scattering þ fertilization treatments, could be related with a lower protective cover in burned plots. Although both types of burning drastically reduced soil organic cover, the negative relationship found between soil organic layer cover and soil erosion suggests the positive influence of that remaining layer. The absence of significant relationships between rainfall kinetic energy and soil erosion in burned plots in the following years may be partially due to the quick E. globulus Labill. regrowth in this area which could have dramatically reduced the rainfall energy reaching the soil (Terry and Shakesby, 1993; Shakesby et al., 1994). In this study, an increase in soil bulk density in the burned soils and a relationship between soil erosion and soil bulk density were detected. This is in line with
93
other investigations establishing that severe fires can alter some soil physical parameters, favoring an increment in bulk density or a decrease in porosity (Packer and Williams, 1976; De Byle, 1981; Giovannini et al., 1990; Imeson et al., 1992) especially when temperatures between 170 and 220 8C are reached in coarsetexture soils (Giovannini et al., 1990; Giovannini, 1994). Although soil hydrophobicity was not measured in this study, the temperatures reached in superficial mineral soil during burns could promote water repellency, contributing to soil losses. The absence of differences in soil shear strength between treatments tallies with the low maximum temperature measured at 2 cm below mineral soil surface during burns (Table 5) and could partially explain the relatively low soil losses observed in burned soils. In our case, even after a burning, soil erosion losses were not high and there was no evidence of rills or gullies formation. Our figures were similar to those found by Beschta (1978), after harvesting and burning or Clayton (1981), Megahan et al. (1995) and Elliot et al. (1996) in Idaho (USA), or after a wildfire and pine clearcut in Portugal (Terry, 1994). The relationship between remaining soil cover after burning and the duration of temperatures >60 8C at the soil surface during burning and between soil cover and erosion found in this study, suggests that duration of glowing combustion finally exerts an indirect but significant influence on soil losses. Furthermore, the apparent and close dependence of temperature duration from soil moisture just before burns indicates that this variable can effectively reduce soil erosion after slash burning through the control of remaining soil organic cover.
5. Conclusion No erosion was detected throughout the study period in undisturbed E. globulus Labill. stands, suggesting that an effective erosion control takes place in these intensively treated plantations. Soil erosion after harvesting and slash disposal treatments was relatively small for all the techniques employed. Nevertheless, slash scattering and windrowing brought out comparatively less soil losses than burning techniques.
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Soil cover by slash and litter þ duff efficiently controlled soil erosion losses even in burning treatments. The duration of temperatures greater than 60 8C at the soil surface during burning significantly affected the remaining soil organic cover in burning treatments. On its turn, surface soil moisture content had a significantly influence over soil heating. Consequently, if E. globulus slash burning needs to be prescribed to reduce fire hazard, it must be conducted with high soil moisture content just before burning and avoiding high slash accumulation to effectively control soil losses.
Acknowledgements This study has been funded by the National Institute of Agricultural Research of Spain (INIA) through the project SC-93-096. We are grateful to all those who have helped with field work and laboratory analyses, particularly A. Arellano, J.R. Gonza´ lez, E. Pe´ rez, A. Lo´ pez, J. Crespo and J.M. Mendan˜ a. We also like to thank the Forestry Service from the Department of Environment (Consellerı´a de Medio Ambiente) and in particular to J. Gea and Comunidad de Montes de Meis. We also acknowledge the critical reviews of two anonymous referees who improved the initial paper. References Beasley, R.S., Granillo, A.B., 1988. Sediment and water yields from managed forests on flat coastal plain sites. Water Resour. Bull. 24 (2), 361–366. Benito, E., Soto, B., Dı´az-Fierros, F.,1991. Soil erosion studies in NW Spain. In: Sala, M., Rubio, J.L., Garcı´a-Ruiz, J.M. (Eds.), Soil Erosion Studies in Spain. Geoderma, Logron˜ o, Spain. Beschta, R.L., 1978. Long-term patterns of sediment production following road construction and logging in the Oregon coast range. Water Resour. Res. 14 (6), 1011–1016. Blackburn, W.H., Wood, J.C., Dehaven, M.G., 1986. Sediment losses from forest management: mechanical versus chemical site preparation after clearcutting. J. Environ. Qual. 15, 413–416. BMDP, 1990. BMDP Statistical Software Inc., Los Angeles, CA, USA. Brown, J.K., 1974. Handbook for Inventorying Downed Woody Material. USDA. For. Serv. Int. For. Range Exp. Sta. General Technical Report INT-16, 24 pp. Byram, G.M., 1959. In: Davis, K.P. (Ed.), Combustion of Forests Fuels: Forest Fire Control and Use. McGraw-Hill, New York.
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