factors are altered by compaction and topsoil removal, but do not reveal clear effects of these changes on tree seedling growth. Although the most consistent ...
Fores;~;ology Management EL-SEWER
Forest EcologyandManagement82 ( 1996) 197-209
Soil compaction and topsoil removal effects on soil properties and seedling growth in Amazonian Ecuador C.L. Woodward Department
of Soil Science, 1525 Observatory
Drive,
University
of Wisconsin,
Madison,
WI 53703,
USA
Accepted2 October 1995
Abstract The effects of soil compaction and topsoil removal on soil physical and chemical properties, and growth of planted trees were investigated. The study was conducted during an oil extraction project in tropical moist forest on a Typic Paleudult in Amazonian Ecuador. The forest was being cleared by bulldozers that compacted the subsoil and scraped away the topsoil. Compaction and topsoil removal resulted in a 70% increase in bulk density, with a 23% increase caused by subsoil compaction alone. There were also significant decreases in organic matter, nitrogen and phosphorus content, total porosity and base saturation, and an increase in cation exchange capacity. Both compaction and topsoil removal caused a decrease in macroporosity of the subsoil and reduced water availability. Seedlings of each of three mature canopy tree species (Cedrelinga catenifonnis (Fabaceae), Caryodendron orinocense (Euphorbiaceae) and Virola elongata (Myristicaceae), were planted in undisturbed soil, compacted subsoil and uncompacted subsoil. Growth responses to soil compaction and topsoil removal, measured over 9 months, were not consistent among species. Height growth was reduced in all species but diameter growth decreased only in Cedrelinga cateniformis. Subsoil compaction reduced height growth and increased mortality of only one species, and topsoil addition to compacted subsoil did not generally increase growth. Fertilization had the most consistently positive effect on growth. The results of this study confirm that many soil physical and chemical factors are altered by compaction and topsoil removal, but do not reveal clear effects of these changes on tree seedling growth. Although the most consistent result was reduced height growth in compacted subsoil with topsoil removed compared with undisturbed soil, this decrease cannot be attributed consistently to either soil compaction or topsoil removal. Keywords:
Soil compaction;Topsoil; Cedreiinga
cateniformis;
Caryodendron
1. Introduction
Virola
elongata;
Fertilization
tration rates, decreased diffusive
Trafficking of forest soils by heavy machinery during extraction of forest resources alters soil physi-
cal and chemical properties that Negative effects on seedling growth are often attributed to with soil compaction, including
orinocense;
affect plant growth. establishment and changes associated reduced water infil-
and mass flow
of
nutrients and solutes through the soil, anoxia and increased resistance to root penetration (for a review see Greaten and Sands, 1980). Loss of organic matter when topsoil is scraped away by bulldozers may also negatively affect plant growth (La1 et al., 1975; Nussbaum et al., 1995). The effects may be especially important in tropical soils
0378-1127/96/$15.00 0 1996Elsevier ScienceB.V. All rights reserved SSDIO378-1127(95)03667-9
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dominated by low-activity clays where organic matter is important not only as a source of nutrients, but also to soil physical properties, phosphorus availability and cation retention (Greenland et al., 1992). Although degradation of soil physical conditions may cause more serious reductions in growth and yield than removal of organic matter (La1 and Greenland, 1979), improvement of soil physical conditions is difficult, costly, and varies in effectiveness. Alegre and Cassel (1983), for example, found that tilling and mulching of an Ultisol (Podzol, FAO) compacted by bulldozers had no effect and a negative effect, respectively, on grain yields. Improvement of soil chemical conditions following compaction with or without topsoil removal through fertilization can increase productivity on compacted soils. Nussbaum et al. (1995) found that fertilizing tree seedlings produced a significant increase in growth rates on compacted soils in Malaysia. The effect of fertilization was greater than all other treatments investigated, including mulching and cultivation aimed at improving soil physical characteristics. The objectives of this study were (1) to investigate the changes in soil physical and chemical characteristics resulting from compaction and topsoil removal, (2) to investigate the effects of soil compaction and topsoil removal on seedling survival and growth and (3) to determine the effect of topsoil and fertilizer addition on survival and growth of seedlings of three tropical tree species.
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82 ( 1996) 197-209
removed was variable, but ranged from IO to 30 cm in the areas under study. Larger amounts were moved in the making of road cuts and fills. Left behind were areas of compacted and scraped soil 3-25 m wide on either side of the 6 m wide road bed. Only areas with less than a 5” slope that were not located on deep road cuts or fills were included in the study. Forest restoration was being carried out in these areas under a contract between Fundacion Jatun Sacha (an Ecuadorian conservation organization) and Maxus Ecuador, Inc., as mandated by environmental guidelines of the United States and Ecuadorian governments. 2.2. Vegetation, climate and soil
2. Study site
Species-rich tropical moist forest (about 240 tree spp. per hectare; D. Neill, personal communication) vegetate terrain that undulates about 245 m a.s.1. Annual precipitation is about 3500 mm and mean annual temperature is 254°C (Instituto National .de Metereologia y Hidrologia, Quito). Soil in the study area is classified as a Typic Paleudult (Ultisd), derived from sedimentary Tertiary clay conglomerates over saprolitic sandstone. The heavily leached soil is clayey, dominated by variable charge clays (kaolinite) and oxides of iron and aluminum. The soil is moderately well-drained with a low pH of 4.2, low base saturation, and low fertility (Instituto Geografico Militar, 1986). Organic matter content decreases from 3 to 5% in the upper 10 cm of the soil profile to less than 0.6% between 20 and 30. cm (Woodward, 1995).
2.1. Location
2.3. Experimental plots
The study was conducted in Napo Province of eastern Ecuador, in northern Yasuni National Park :0”27”S, 76”38”W). Tracts of tropical wet forest that vegetate the hilly terrain were cleared by Maxus Ecuador, Inc. for oil exploration and construction of an oil pipeline, oil wells, production facilities, and a 120 km access road into the 2000 km2 petroleum Block No. 16 (Fig. 1). After tree felling, D8 and DlO bulldozers with 60and 90-cm wide tracks were used to clear stumps and slash, removing topsoil in the process and leaving compacted subsoil beneath. The depth of soil
Soil sampling was done in areas where bulldozers were presently clearing forest (September 1943) and in previously cleared areas (January 1993) that represented three soil conditions commonly found inor near bulldozed areas: (1) compacted subsoil with topsoil removed, (2) uncompacted subsoil with topsoil removed and (3) undisturbed forest soil (uncompacted subsoil with topsoil intact). Plots in these three soil conditions were utilized for seedling growth experiments. Plots in the first two conditions had a minimum size of 270 m2 and were established in exposed subsoil adjacent to the road bed. Two 90&m’
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1°S
76"W
77"W Fig. 1. Map showing study area that extends the lower center of the map.
south of the Pompeya
plots in undisturbed soil were cleared by hand at the start of the study 50 m into the forest from the edge of the mechanically cleared areas. All vegetation was cut and removed from the plot, minimizing foot traffic that could cause compaction. The local political climate generated by the incursion of oil developers into indigenous reserve lands made it not permissible to clear more than two 900-m* plots. The clearance of any forest over the amount initially agreed upon between oil developers and indigenous communities was opposed by community leaders. The width of undisturbed soil plots was approximately equal to that of the mechanically cleared areas where plots were placed (about 30 m). This plot size was selected to minimize the differences in light environment; however, five measurements per plot with a spherical densitometer revealed mean
community
into Maxus
Petroleum
Block No. 16, indicated
by the box in
canopy cover to be 24% among the roadside plots and 42% in the natural forest plots. 3. Materials
and methods
3.1. Efsects of compaction soil properties
and topsoil removal on
3.1 .I. Bulk density and soil chemical characteristics
To quantify the physical and chemical changes that result from topsoil removal and subsoil compaction with bulldozers, ten samples of the top 10 cm of soil were taken at random in each of three 15 X 30 m* plots prior to and immediately following clearing by bulldozers (25 August 1993 and 8 September 1993, respectively). About lo-15 cm of
200
C.L. Wmdward/Foresr
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topsoil was estimated to have been removed from these areas by random measurements of the amount of exposure of tree roots and boles along the edge of the bulldozed area. The samples were oven-dried to constant weight at 100°C to determine bulk density and total porosity. A subsample of these (three per plot) was analyzed for pH, organic matter content, total nitrogen, extractable phosphorus and potassium, cation exchange capacity, and base saturation (analytical methods are described below). Differences in each of these variables were analyzed with paired r-tests. To determine the change in bulk density with depth, five soil samples were taken at increments of 12 cm from 0 to 60 cm depth in three plots that were compacted with topsoil removed and in the two forest plots that had been hand cleared with minimal soil disturbance. Samples were oven-dried at 100°C and weighed to obtain a bulk density profile for the top 60 cm of soil. 3. I .2. Soil water characteristics
and porosity
To compare the effects of topsoil removal and soil compaction on water availability in the soil, 12 samples of the top 10 cm were taken in each of the three soil conditions as described above: (1) compacted subsoil with topsoil removed. (2) uncompacted subsoil with topsoil removed, and (3) uncompacted subsoil with topsoil intact (undisturbed soil). Soil samples from the three conditions were analyzed for water release characteristics using the pressure plate method (for description see Hanks, 1992). Soil water content was determined from samples taken over 2-week periods in August and October 1993, and January and June 1994. Pore size distribution was determined by calculation from water release data (Marshall and Holmes, 1988). 3.1.3. Analytical methods
Organic carbon was analyzed using the Walkley-BIack method (Nelson and Sommers, 1982). The method described by Gillman (1979) was used to determine extractable phosphorus in 0.5 M sodium bicarbonate at pH 8.2. Extractable potassium was measured using cold sulfuric acid extraction and flame photometry (Hunter and Pratt, 1957). Cation exchange capacity was measured at soil pH by the method described by Gillman (1979) for acid soils.
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Exchangeable bases (Ca2’, Mgzt, Na’, and K ’ 111 meq per 100 g soil) were determined by ammonium acetate extraction and atomic absorption spectrometry (see Page et al., 1982 for a description of all soil analysis methods used in this study). 3.2. The efect qf soil compaction and topsoii rc’moual on seedlinggrowth
3.2. I. Planting materials
The species studied are all economically important species of primary lowland forest surrounding the study area: Cedrelinga cateniformis (Ducke) Ducke (Fabaceae) and Virofa elongata (Benth.) Warb. (Myristicaceae), both valuable timber species, and Caryodendron orinocense Karsten (Euphorbiaceae), the nuts of which are marketed and consumed locally. (Species hereafter referred to by their generic epithets). Seedlings were raised at the Jatun Sacha nursery in Pompeya, where trees were being produced for reforestation of the road and pip&&e rights-of-way being constructed by Maxus Ecuador, Inc. Nursery seedlings were raised from seed (Viroln) OT as wildlings transplanted from the forest to the nursery at approximately 1 month of age. Healthy J-monmold seedlings growing in 15 X 25 cm2 cybndrical planting bags were transferred from the nursery to experimental plots 30 days prior to phuui.ng to acclimate. 3.2.2. Experimental treatments
In September 1993, approximately 260 seedlings of each of the three species were planted -in experimental plots to obtain the following five treatments. (1) Compacted subsoil-seedlings were planted directly in compacted soil where topsoil was removed. The planting hole was backfilled with the same compacted soil. (2) Compacted subsoil with topsoil added-seedlings were planted in holes excavated in compacted soil, but topsoil from the adjacent forest was used to backfill the planting hole. The upper 2 cm of the compacted surface soil were replaced with topsoil to a 50 cm radius around the seedling. Nutrient analysis of the topsoil revealed 5.23% organic matter, 0.27% total nitrogen, 2.67 ppm extractable phosphorus and
C.L. Woodward/Forest
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and Manqement
30 ppm extractable potassium 61 = 3, see analytical methods). (3) Compacted subsoil with fertilizer added-seedlings were planted directly in compacted soil with commercial 10-30-10 NPK granular fertilizer added in and around the planting hole (about 1 m’). The fertilizer application rate was 500 g rns2 as diammonium phosphate and potassium chloride, giving each plant approximately 50 g of elemental N, 150 g of available P,O, and 50 g of water-soluble K,O. The same amount of fertilizer was reapplied as a topdressing after 3.5 months. subsoil with topsoil (4) Uncompacted removed-seedlings were planted in areas where subsoil was moved by bulldozers but not compacted. (5) Undisturbed forest soil-seedlings planted in natural forest soil in plots that were cleared manually with minimal soil disturbance. All seedlings were planted in random order in rows 3 m apart. Planting holes were 20 X 20 X 25 cm3 in size and seedlings were removed from planting bags and planted with minimal disturbance to the root ball. Planting took place on rainy days and no supplemental watering was given. Natural regeneration was allowed to proceed in all plots without interference.
Table 1 Physical and chemical properties of the top 10 cm of the Typic standard deviations and results of paired r-tests
(1) Physical properties Bulk density Total porosity (o/o) pores (%I Air-filled (II) Chemical
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3.2.3. Data collection
Height and basal diameter were measured at the time of planting (September-October 1993), and 8.5 months following planting (June 1994). The few plants that died within 14 days were replaced, assuming they had suffered damage during planting. None of the replanted seedlings subsequently died within 14 days. Mortality was calculated after 1 month, 3.5 months and at the end of the study (8.5 months). Height and diameter relative growth rates for surviving trees without terminal bud damage was calculated at the end of the 8.5 month period using the equation d(lnX)/dt (adapted from Pearcy et al., 1991) where X is the height or diameter measurement and t is time in days. Results were analyzed using analysis of variance followed by multiple comparisons. 4. Results 4.1. The effect of soil compaction and topsoil removal on soil properties 4.1.I. Bulk density
Compaction and topsoil removal resulted in a 70% increase in bulk density of the top 10 cm of soil
Paleudult
before
Before
After
Mean (SD)
Mean (SD)
0.70 (0.12) 73.0 (0.05) 34.7 (0. IO)
1.19(0.12) 54.2 (0.05) 19.0 (0.06)
3.9 (0.2) 3.36 (0.88) 0.16 (0.04) 3.33 (I .26) 36.0 (5.53) 5.59 (0.66) 12.0 (2.95) 0.91 (0.40)
3.93 (0.19) 0.44 (0.30) 0.02 (0.0 I ) 0.07 (0.10) 38.6 (5.00) 7.05 (0.64) 5.5 (1.48) 0.66 (0.09)
and after soil compaction
and topsoil
removal.
T
Prob > 1T 1
13.41 13.38 7.55
o.coo1 * * ’ 0.0001 * * * o.oOOl**’
0.37 8.59 8.58 7.65 0.44 2.92 5.70 1.93
0.7200 ns 0.0001 ** * o.OOOl**’ 0.0001*** 0.6715 ns 0.0194 4 0.0005 * * * 0.0897 ns
Means,
properties
PH Organic matter (%o) Total nitrogen (%I Phosphorus (ppm) Potassium (ppm) CEC (meq per 100 g) Base saturation (%) Exchangeable Al ’ Significant
82 (1996)
(P < 0.05);
* * * highly
significant
(P < O.OOl>, and ns, not significant.
N = 9 for ah parameters
except
CEC, where
n = 6.
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undMunagement
to 1.19 g cmm3 (Table I), however a greater proportion of this increase was attributed to exposure of denser subsoil than to compaction. When removal of 12 cm of topsoil was taken into account. compaction resulted in only a 23% increase in bulk density (Fig. 2). Bulk density profiles of roadside and forest plots show that the increase in bulk density following compaction and topsoil removal extended to 36 cm depth with no significant differences beyond this. Samples taken immediately following compaction and topsoil removal were not significantly different from compacted plots that were cleared 9 months earlier (P = 0.79, data not shown). This result suggests that sites for reforestation following soil compaction and topsoil removal can be adequately described by immediate post-disturbance sampling as soil physical properties do not rapidly revert to predisturbance conditions. This finding is consistent with that of Alegre and Cassel (1993) who found no change in bulk density following compaction on the same soil type in Yurimaguas, Peru, after 7 years.
o-12
a iii It 7
12-24
5 24-36 I@ '0 E s E s f F n
+---ezyw-!
\ ..
397-209
Other studies confirm that soil physical properties may take years (Sands, 1983; Dias et al., 1985) or even decades to recover fUh1 et al., 1982). 4.1.2. Soil chemical properties
Chemical changes in the soil can be attributed to topsoil removal. TopsoiI removal resulted in reduced nitrogen and extractable phosphorus content of the top 10 cm of the soil, through loss of organic matter (Table 1). No changes in potassium content or pH occurred. The observed increase in cation exchange capacity is attributable to higher clay content of the remaining subsoil after topsoil removal, though the concomitant decrease in base saturation suggests higher proportions of exchangeable acids, It is apparent that nutrient availability to plants is reduced as a result of topsoil removal. 4. i .3. Soil water characteristics
Water release curves (Fig. 3) indicate that soil water availability is reduced following compaction
‘1,
1
82 (19%)
i Approximate depth , of topsoil removed 4
\
t / /
\ \
1 -j
AH
/
\ \
\ \
36-484 \
\ \
48-60
i
I
0 0
before
- 36-46
".-'
after
c-^----,-T 0.7
0"
0.8
-i-----f0.9
1.0
1.1
1.2
48-60 1.3
Bulk Oensity (g/cxn3) Fig. 2. Bulk density with increasing depth from the surface of the soil before and after soil compaction and topsoil removal. Shown are means and standard errors of five samples.
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70 l m
Undisturbed Compacted
soil subsoil/m,
topsoil
60
0.0010
0.0100 Tension
Fig. 3. Soil water characteristic (- 0.01 MPa) and ‘permanent
Table 2 Height and diameter relative growth rates and mortality letter are not significantly different (P < 0.05)
Cedrelinga
lines show water contents at ‘field water content in each condition.
tree species under five treatments.
Values
followed
Undisturbed soil
Treatment Compacted/ no topsoil
Compacted topsoil
0.0048 a 0.0033 a 3a
0.0021 b 0.0026 b 16b
0.0016 b 0.0030 ab 20b
0.0030 bc 0.0051 c 51 c
0.0028 c 0.0037 ac 14b
+
Compacted fertilizer
+
by the same
Uncompacted/ no topsoil
elongata
Height RGR Diameter RGR Mortality (%) Caryodendron
Height RGR Diameter RGR Mortality (o/o)
capacity’
about 40%. Available water (defined as the volumetric water content between - 0.1 and - 1.5 MPa) was also reduced in both uncompacted and compacted
careniformis
Height RGR Diameter RGR Mortality (o/o) Virola
of three tropical
1 .oooo
(MPa)
curves for three soil conditions in a Typic Paleudult. Horizontal wilting point’ (- 1.5 MPa). Arrows indicate range of available
and topsoil removal. In both compacted and uncompacted subsoil, matric potentials were lower than in undisturbed soil at volumetric water contents below
Species
0.1000
0.0034 a 0.0023 ac 3a
0.0017 b 0.0018 bc 8b
0.0019 b 0.0018 b 16b
0.0019 b O.OQlS b 55 c
0.0014 b 0.0015 b 5a
0.0023 a 0.0024 a 8a
0.0020 a 0.0031 a 7a
0.0023 a 0.0036 b 4a
0.0034 b 0.0044 c 28 b
0.0025 a 0.0035 abc 17 c
orinocense
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C.L. Woodwrd/
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subsoil compared with undisturbed soil (Fig. 3). The similarity in water release characteristics in the former two conditions may reflect the inherent quality of the subsoil and suggests an effect of topsoil removal. The effect of subsoil compaction is indicated by higher (less negative) matric potentials in uncompacted subsoil than in compacted subsoil at water contents below 45%. Matric potential in the compacted condition was reduced to - 0.10 MPa at the mean water content of 40%, from -0.03 MPa in the undisturbed condition where mean water content was 35%. These mean manic potentials support the assumption that undisturbed soils in the area remain near field capacity for most of the year. and suggest that plants growing in exposed subsoil may experience more severe and prolonged water stress than in undisturbed soil, especially when the subsoil is compacted.
and Management
82 (19961 197-209
60 T--
---------~-
4.1.4. Pore size distribution
Compaction and topsoil removal resulted in complete disappearance of large pores in the soil, a reduction in medium to large pores (1 .O pm < r < 50 pm) and a concomitant increase in the proportion of smaller pores ( < 1.O pm, Fig. 4). These results are similar to those from previous work on the effects of compaction (e.g. Chauvel et al., 1991). A comparson of pore size distribution in uncompacted subsoil and undisturbed soil (Fig. 3) suggests that part of the shift toward smaller size pores in the former condition can be explained by removal of soil organic matter in the topsoil. The importance of organic matter to soil porosity, particularly its contribution to the proportion of large pores in clay-dominated soil is well documented (e.g. Oades et al., 19891. The effect of subsoil compaction alone is also evident from differences in pore size distribution in uncompacted and compacted subsoil. The reduction in the proportion of medium size pores (10 km to 1 pm in radius) and the increase in small pores (1.0 Frn and 0.1 pm> in compacted versus uncompacted subsoil reflects the effect of compaction. Differences in pore size distribution explain the reduction in soil water availability (lower manic potentials) in compacted versus uncompacted subsoil even though the volume of available water per unit volume of soil was similar in these two conditions.
co 1
01 to10
iOlol0
IOto‘WO
%OO
Pore radius (pm) Fig. 4. Pore size distribution in undisturbed subsoil with topsoil removed and uncompacted soil removed iu a Typic Paleudult in Amazonian
4. I .S. Air-filled
soil, compacted subsoil with topEcuador.
porosity
Air-filled porosity (AFP) across the range of water contents is reduced as a result of compaction, reaching levels critical to plants at water contents at and above field capacity. In undisturbed soil and uncompacted subsoil, AFP was adequate at field capacity, based on mean bulk densities obtamed in the field (Table 1). Following compaction and topsoil removai, however, there was a 26% decrease in
C.L.
Woodward/Forest
Ecology
total porosity and a 45% decrease in AFT at the mean observed water content relative to the undisturbed condition. Despite this decrease, mean AFP was still 19% following compaction and topsoil removal at the observed mean water content of 35%. Only when compacted soil was at or above field capacity did AFP fall below 1I%, levels that are probably critical for plant growth (e.g. Ruark et al.,
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82 (1996)
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205
1982; Kramer and Kozlowski, 1991). Therefore, at least transient anoxic conditions are possible on compacted soil between field capacity and saturation, but firm conclusions regarding the importance of anoxia cannot be made without measurements of oxygen diffusion rates and plant respiration rates, combined with measurements of soil water content over time.
0.0060
0.0040
0.0030
0.0040
0.0020 Y ?o
‘E g
0.0020 0.0010
E!
0.0000
0.0000
i 6
0.0060
i g d
g 6
00040
9 g
0.0020
nt
CS
CT
US
CF
UN
CS
CT
US
CF
UN
Treatment Fig. 5. Mean height (a) and diameter (b) relative growth rates for seedlings of Caryodendron orinocense (Co), Cedrelinga cateniformis and Virola elongata (Ve) growing in compacted subsoil (CS), compacted subsoil with topsoil added (CT), uncompacted subsoil compacted subsoil with fertilizer added (CF), and undisturbed soil (UN) (standard deviations shown).
(Cc)
(US),
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Woodwurd/
4.2. The effect of soil compaction moval on seedling growth rates
Forest
Ecology
and topsoil re-
4.2.1. Height and diameter relative growth
rates
There was a highly significant treatment effect on height and diameter growth rates (Table 2 and Fig. 51, but few responses were consistent among all species. Cedrelinga and Virola exhibited the highest growth rates in undisturbed soil. Height and diameter growth rate of Curyodendron was greatest in fertilized plants growing in compacted subsoil. Growth rates were consistently lowest for all species growing in compacted subsoil with topsoil removed, though the difference compared with undisturbed soil was not significant in Caryodendron. Comparing growth in uncompacted versus compacted subsoil revealed that subsoil compaction negatively affected height and diameter growth of Cedrelinga but did not significantly decrease height nor diameter growth of the other two species. Topsoil addition to compacted subsoil had no affect on any species, except for an increase in diameter growth of Curyodendron. Fertilization of plants growing in compacted soil had the most consistent positive effect on plant growth, improving height and diameter growth of Cedrelinga and Caryodendron, but not Virolu. Growth rates for fertilized Caryodendron growing on compacted soil were higher than in undisturbed soil. All three species exhibited visible improvement in nutrient status with fertilization on compacted subsoil, manifested by reduced chlorosis and larger and/or more numerous leaves (Woodward, 1995). 4.2.2. Seedling survival
Seedling mortality in undisturbed soil was less than 10% for all species, within generally accepted limits for reforestation. Compaction and topsoil removal increased mortality of Cedrelinga and Virolu, but not Caryodendron. Subsoil compaction produced equivocal results, increasing mortality in Virola, decreasing it in Caryodendron and having no affect on mortality of Cedrelinga. Topsoil addition to compacted soil had no affect on mortality. Fertilization consistently increased seedling mortality, but most fertilized trees died within 2 months of the first fertilization, with little additional mortality after subsequent fertilization. This pattern reduces the likeli-
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hood that mortality following fertilization was caused by alteration of soil pH or other soil chemical changes initiated by fertilizer addition, and may be explained by early application of fertilizer before seedlings had recovered from transplanting. 5. Discussion The results of this study clearly demonstrate that soil compaction and topsoil removal change the physical and chemical characteristics of the Typic Paleudult studied, consistent with findings from others conducted on clay-dominated tropical soils (e.g. Dias et al., 1985: Chauvel et al., t991). Topsoil removal may be equally or more important than compaction in altering soil properties important to plant growth. A large proportion of the change m soil bulk density, macropore space, and water and nutrient availability is attributable to loss of topsoil. Compaction further reduces water availability and porosity of the subsoil. These soil changes control multiple factors that may interact to have deleterious effects on plant growth. The relative importance of compaction or topsoil removal in eliciting plant response, however, is difficult to ascertain as the response of tree seedlings to soil compaction and topsoil removal appears to depend heavily on species. Soil compaction and topsoil removal .decreased the growth of two out of three species, indicated by higher growth rates in undisturbed soil than in compacted subsoil. Reduced plant growth on compacted soil has been found in other studies conducted insthe tropics (e.g. Alegre and Cassel, 1993; Nussbaum et al., 1995) and temperate zones (e.g. Alban et ai.. 1994). The lack of a significant difference in growth of Curyodendron between undisturbed soil and compacted subsoil may be due to the fact that many seedlings of Curyodendron in the undisturbed plots were noted as having up to 75% of their leaves severely damaged by insect herbivores (Woodward, 1995). Loss of biomass associated with herbivore attack may have offset the beneficial effects of more favorable soil characteristics in undisturbed soil. The lack of a generally negative effect of subsoil compaction on plant growth is in disagreement with other studies conducted in the field (e.g. Sands. 1983: Nambiar and Sands, 1992; Nussbaum et al.. 19951, and indicates that reductions in plant growth
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following compaction and topsoil removal may be more affected by soil changes brought about by topsoil removal than by subsoil compaction. The uncompacted subsoil condition used in this study also was not equivalent to undisturbed subsoil since the soil had been moved. This may have resulted in a condition that more closely resembled tilled or dug soil. Positive (Greaten and Sands, 1983; Nussbaum et al., 1995) and equivocal (Cassel and Lal, 1993) plant growth responses to tillage and digging of compacted subsoil have been reported. Water stress may have contributed to reduced growth in uncompacted and compacted subsoil relative to undisturbed soil. Available water was reduced and observed mean water content was well below field capacity in both subsoil treatments. Reduced height growth of Cedrelinga and increased mortality of Virola as a result of subsoil compaction may have been caused by reduced water availability exacerbated by a reduction in root growth. Although mean bulk density following compaction to 1.19 g cmV3 in the soil studied may not have been sufficiently great to cause substantial reduction in plant growth, impedance to root elongation has been found in fine textured soils at a bulk density as low as 1.20 g cme3 (Ruark et al., 1982). Casual observation of roots from a random sample of trees, revealed shorter thicker roots in compacted versus uncompacted subsoil. Caryodendron, a tap-rooted species, had shorter more branched roots, but did not suffer reduced growth as a result of compaction. Topsoil removal and the associated loss of nutrients may have a greater affect on growth than soil compaction, evidenced by the overall decrease in growth from undisturbed soil to uncompacted subsoil and the general lack of a difference between uncompacted and compacted subsoil, as mentioned above. Although less favorable physical properties of subsoil may be partly responsible for reduced growth in uncompacted subsoil compared with undisturbed soil, nutrient limitation is a likely explanation given the favorable growth response of two out of three species to fertilization. Nussbaum et al. (1995) also reported positive responses to fertilization of plants in compacted soil. The general lack of a response to topsoil addition to compacted subsoil may be due to compaction effects on the mobility or uptake of nutrients. Com-
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paction effects on nutrient availability would also explain the failure of fertilization to completely overcome decreases in height growth rates relative to undisturbed soil. Some artifact of the topsoil addition treatment, such as puddling of rainwater observed in planting holes filled with topsoil may have led to transient anaerobic conditions with negative consequences on growth, although no data were taken to test this. Other studies, have also reported no beneficial effect of organic matter addition to compacted soil (e.g. Alegre and Cassel, 1983; Nussbaum et al., 1995), while others confirm that topsoil removal decreases growth (e.g. La1 et al.,’ 1975; La1 and Greenland, 1979; Cassel and Lal, 1992). The lack of consistent responses among species reflects their ecological and physiological differences. For example, the positive growth response of Caryodendron to both fertilization and topsoil addition on compacted soil, coupled with the lack of a difference in growth between compacted and uncompacted subsoil, suggests that nutrient availability may be more limiting to growth of this species than water availability or mechanical impedance. This is consistent with the natural distribution of Caryodendron on more fertile and seasonally inundated microsites (Gentry, 1993). Soil conditions may be secondary to other factors on disturbed sites in eliciting responses of some species. For example, there was no treatment effect on growth of Virolu but a reduction in growth in all high-light roadside plots, regardless of treatment, compared with the low-light forest plot. High light levels thus correlate well with reduced height growth, observed chlorosis, and reductions in leaf area (Woodward, 1995) in all disturbed plots. Higher light could account for lower growth rates, reduced leaf area and observed chlorosis if light levels caused photoinhibition. Lovelock et al. (1994) found rapid and irreversible photoinhibition of shade-tolerant tropical tree seedlings placed in high light environments. Damage caused by photoinhibition is exacerbated when plants are water stressed (BjGrkman and Powles, 1984; Araus and Hogan, 1994). This phenomena may explain the higher mortality of Virolu as a result of subsoil compaction. Seedlings of Virola were smaller and less deeply rooted than the other two species studied, and thus may have suffered more prolonged or severe water stress.
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The outcomes of this study suggest that soil compaction and topsoil removal negatively affects plant growth, but that responses are species-specific. Negative plant responses discussed herein may be surmounted by minimizing soil compaction and avoiding topsoil removal whenever possible. Soil amendments, such as fertilization and organic matter addition, may be useful to improve plant growth on compacted soil. Further research is needed to identify the most important mechanisms operating to reduce plant growth following disturbance. It is also evident from this study that research on ecological characteristics of tropical trees, such as shade-tolerance, flood-tolerance and rooting patterns, is essential before predictions and recommendations can be made regarding species best suited to reforestation of mechanically cleared areas. Acknowledgements
Many thanks to F.E. Putz, K. Williams and N. Comer-ford of the University of Florida for valuable input, D. Neill, of Fundacion Jatun Sacha for logistical support, Jody Stallings, USAID, ClFOR and the New York Zoological Society for financial support, and the many other people in Ecuador and the University of FLorida that contributed to the realization of this research. References Alban, D.H., Host, G.E., Elioff, I.D. and Shadis, D.A., 1994. Soil and vegetation response to soil compaction and forest floor removal after aspen harvesting. USDA Forest Service Research Paper, NC-3 15. Alegre, J. and Cassel, D.K., 1983. Reclamation of compacted bulldozed areas. In: J.J. Nicholaides, W. Couto and M.K. Wade (Editors), Agron. Econ. Res. Soils Trap. Tech. Rep. 1980- 198 1. North Carolina University Press, Raleigh, NC. Araus, J.L. and Hogan, K.P., 1994. Leaf structure and patterns of photoinhibition in two neotropical palms in clearings and forest understory during the dry season. Am. J. Bot., 81: 726-738. BjBrkman, 0. and Powles. S.B., 1984. Inhibition of photosynthetic reactions under water stress: interaction with light level. Planta, 161: 490-504. Cassel, D.K. and Lal, R., 1992. Soil physical properties of the tropics: common beliefs and management restraints. In: R. La1 and P. Sanchez (Editors), Myths and Science of Soils in the Tropics. Soil Sci. Sot. Am. Spec. PubI. No. 29, Madison, WI.
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Chauvel, A., Grimaldi, M. and Tessier, D., 1991. Changes in pore-space distribution following deforestation and revegetation: an example from the Central Amazon Basin. Brazil. For. Ecol. Manage., 38: 259-271. Dias. A.C. and Nortcliff. S., 1985. Effects of two land clearing methods on the physical properties of an oxisol in the Brazilian Amazon. Trop. Agric. (Trinidad), 62: 207-212. Gentry, A., 1993. A Field Guide to the Families and Genera 01 Woody Plants of Northwest South America (Colombia, Ecuador, Peru). Conservation International. Washington. DC. Gillman, G.P., 1979. A proposed method for the measurement of exchange properties of highly weathered soils. Aust. J. Soil Res., 17: 129- 139. Greaten, E.L. and R. Sands, 19X0. Compaction of forest soils. A review. Aust. J. Soil Res., 18: 163- 189. Greenland, D.J.. Wild. A. and Adams, D., 1992. Organic matter dynamics in soils of the tropics: From myth to complex reality. In: R. La1 and P.A. Sanchez (Editorsl, Myths and Science of Soils of the Tropics. Soil Sci. Sot Am. Spec. Pub1 No. 29, Madison, WI, pp. 17-33. Hanks, R.J., 1992. Applied Soil Physics: Sail Water and Temperature Applications. 2nd edition. Springer-Verlag. New York, NY. Hunter, A.H. and Pratt, P.F.. 1957. Extraction of potassium from soils by sulfuric acid. Soil Sri. Sot. Am. Proc., 21: 595.-598. Institute Geogrrifico Militar. 1986. Mapa General de Suelos de1 Ecuador. Sociedad Ecuatoriana de la Ciencia de1 Suelo, Quito. Kramer, P.J. and Kozlowski, T.T.. 1091. The Physiological Ecology of Woody Plants. Academic Press, San Diego, CA. Lal, R. and Greenland, D.J. (Editors). 1979. Soil Physical Properties and Crop Production in the Tropics. John Wiley. New York, NY. Lal, R., Kang, B.T., Mootman, F.R.. Juo, A.S.R. and Moomaw, J.C.. 1975. Soil management problems and possible solutions in western Nigeria. In: E. Bomemisza and A. Alvarado (‘Editors), SoiI Management in Tropical America, North Carolina State University Press, Raleigh, NC, pp. 372-408. Lovelock, C.E., Jebb, M. and Osmond, C.B., 1394. Photoinhibition and recovery in tropical plant species: response to distur. bance. Oecologia, 97: 297-307. Marshall. T.J. and Holmes. J.W.. 1988. Soil Physics. 2nd edition. Cambridge University Press, Cambridge. Nambiar, E.K.S. and Sands, R., 1992. Effects ol compaction and simulated root channels in the subsoil on root development, water uptake and growth of mdiata pine. Tree Physiol., IO 297-306. Nelson, D.W. and Sommers. L.E., 1982. Total carbon, organic carbon and organic matter. in: A.L. Page. R.H. Miller and D.R. Keeney (Editors). Methods of Soil Analysis, Part 2. Chemical and Microbiological Properties. American Society of Agronomy, and Soil Science Society of America, Madison WI. Nusshaum, R.. Anderson, J. and Spencer, T., 1995. Factors limit, ing the growth of dipterocarp and pioneer tree seedlings. planted on disturbed soils following rainforest logging ir: Sabah. Malaysia. For. Ecol. Manage., 00: tX&OOO. Oades, J.M.. Gillman, G.P. and Uehara, G.. 1989. Interactions of soil organic matter and variable-charge clays. In. D.C. Coie-
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man, J.M. Oades and G. Uehara (Editor), Dynamics of Soil Organic Matter in Tropical Ecosystems. Niftal Project. University of Hawaii, pp. 69-95. Page, A.L., Miller, R.H. and Keeney, D.R. (Editors), 1982. Methods of Soil Analysis, Part 2. Chemical and Microbiological Properties. American Society of Agronomy, and Soil Science Society of America, Madison, WI. Pearcy, R.W., Ehrlinger, J., Mooney, H.A. and Rundel, P.W., 1989. Plant Physiological Ecology. Chapman and Hall, London.
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Sands, R., 1983. Physical changes to sandy soils planted to radiata pine. IUFRO Symposium on Forest Site and Continuous Productivity. USDA Forest Service Report, Portland, OR. Uhl, C., Jordan, C., Clark, K., Clark, H. and Herrera, R., 1982. Ekosystem recovery in Amazon caatinga forest after cutting, cutting and burning and bulldozer clearing treatments. Oikos, 38: 313-320.
Woodward, C., 1995. Soil compaction and topsoil removal during mechanized clearing of a tropical forest: effects on soil properties and seedling growth. Master’s thesis. University of Florida, Gainesville.
