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a Museum National d'Histoire Naturelle, Laboratoire d'Écologie Générale, 4 Ave. du Petit-Château, 91800 Brunoy, France
b Universidad de Navarra, Facultad de Ciencias, Departamento de Zoología y Ecología, 31080 Pamplona, Spain
Corresponding author(jean-francois.ponge{at}wanadoo.fr)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIAL AND METHODSRESULTSDISCUSSIONREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIAL AND METHODSRESULTSDISCUSSIONREFERENCES |
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Previous studies (Bernier and Ponge, 1994; Ponge and Delhaye, 1995; Bernier, 1996; Ponge et al., 1997; Bernier, 1998; Ponge, 1999) have shown that the interplay between plant and soil animal communities was reflected in the composition of humus profiles. In particular, the analysis of humus profiles by laboratory optical methods can inform us about the dynamic state of the ecosystem (Ponge et al., 1998) and the level of soil biodiversity (Ponge et al., 1997). In this study we applied these methods to an assessment of the effects of aleppo pine on soil biological activity in Mediterranean landscapes where pine occurs either as a component of unmanaged vegetation or is planted in monoculture for site reclamation.
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MATERIAL AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIAL AND METHODSRESULTSDISCUSSIONREFERENCES |
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The first site was located in the southern part of Navarre, Spain, near the village of Funes, close to the Bardenas area. This strongly eroded agricultural land had been overgrazed for centuries. Reafforestation programs were started 30 to 40 yr ago in order to create islands of soil restoration. Aleppo pine was planted at a high density and these stands had not been thinned at the time of our study. Average tree height was 7.5 m. Some young isolated pine trees established themselves in the scrubland surrounding the plantations, thus forming a transition zone (outer edge) between the scattered, still grazed, scrubland and the plantation. Within the boundary of the plantation we distinguished another transition zone (inner edge), where ground vegetation was more abundant. The geological substrate was gypsum. Soils were Haploxerepts, with little accumulated litter, except under pine where a continuous layer of needles was visible on the ground. Climate was of the Mediterranean type, fairly warm and dry (mean annual temperature 14°C, mean annual rainfall 419 mm), with moderate winter. The study site was located on a slope (13%) at 470 m above sea level, with a north-northeast aspect. The scrubland was dominated by rosemary and thyme (Thymus vulgaris L.). In the plantation false brome [Brachypodium ramosum (L.) R. & S.] dominated the understory. In the inner edge scrubland species were also present in mixture.
The second site was located in southeastern Sicily, in the Ippari River Valley, near Vittoria Veneta. Aleppo pine was dominant in patches of undisturbed scrubland, with rosemary and pistachio as accompanying species, which are remnants of the climax vegetation that once covered Sicily. The surrounding landscape is mainly composed of orange (Citrus reticulata L.) groves. The time interval between recurrent fires affecting the scrubland is 
70 yr, and this was the age of most dominant individuals of aleppo pine. Aleppo pine, rosemary, and pistachio were growing isolated, except in the periphery of pine crowns where branches of pistachio were growing in the understory. It appears that after fire aleppo pine is the first vegetation to become established and gaps are filled thereafter by other scrubland species. We considered that ecoclines were present under the crown of each individual pine (mean height 9 m), from the trunk base to the crown border, where there was an admixture of other scrubland species. Small areas not covered with vegetation were present (1% of total surface), but in these areas the ground was still covered with pine litter. The herb layer was practically absent. Pine pollen was especially abundant in the litter, because of intense flowering. The geological substrate was gypsum. Soils were Haploxerepts with more accumulated litter than in the Navarre site, especially under the pine and the pistachio. The slope was 16%, with a northwest aspect. Climate was of the warm Mediterranean type, with a short rainy period in winter and a long warm dry period from May to September.
Sampling Design
In the Navarre site, five transect lines about 90 m long and 2 m wide were established that crossed the southeastern side of the plantation at 10-m intervals. These transects ran from the overgrazed scrubland (outer edge) to the interior of the plantation (farthest point from all edges of the plantation). Only three of the five transect lines were used for sampling humus profiles. Along each transect line the topsoil was sampled under seven vegetation types, including both the overgrazed scrubland and the pine plantation as follows:
Pine plantation (inside of the plantation, without any admixtion of scrubland vegetation)Inner edge 1 (most internal part of the inner edge, showing first signs of admixtion)
Inner edge 2 (midpart of the inner edge)
Inner edge 3 (most external part of the inner edge, under the crown of most external trees)
Isolated pine (outer edge, scrubland)
Bare ground (outer edge, areas not covered with scrubland vegetation)
Rosemary (outer edge, scrubland)
Care was taken to collect samples under these well-defined cover types rather than at fixed intervals.
In the Sicily site, three transect lines about 7 m long were established under three adjoining pine trees, converging towards a central area occupied by other scrubland species or bare ground. The sampling was replicated three times in the central area. The seven vegetation types sampled (three samples each) were
Pine 1 (near the trunk base)Pine 2 (at mid distance between the trunk base and the crown border)
Pine 3 (at mid distance between Pine 2 and the crown border)
Pine 4 (just at the crown border)
Pistachio (central area)
Bare ground (central area)
Rosemary (central area)
Sampling was completed in November 1994 at the Navarre site and in December 1994 at the Sicily site. A total of 42 soil samples (21 in Navarre, 21 in Sicily) were collected.
Sampling Procedure
Soil cores were collected using a specially built steel and aluminum corer (Fig. 1)
. It included an external jacket and an internal sleeve, thus resembling the corer devised by Macfadyen (1962). However, it had a 45 by 45 mm square section and the cutting edge was not the jacket, but rather a discardable section of the internal sleeve, which had been slanted at 45° and sharpened. The jacket and the cutting portion of the sleeve were made from standard square steel profile. The jacket had a foot pad of steel soldered to the bottom. The sleeve consisted of a range of several seamless square, open aluminum boxes of equal size, made from standard square aluminum profile, prolonged distally by the steel cutting edge and proximally by a piston made of reinforced steel. The sleeve glided freely inside the jacket on soldered steel rails. The train made of the cutter and of the desired number of boxes (each 25 mm deep) was inserted at the bottom of the corer, then forced vertically into the soil with the piston, the reinforced top of which was hammered when necessary. The sampling depth was variable (according to stoniness of the soil), ranging from 40 mm (two boxes) to 100 mm (five boxes).
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For transportation to the French laboratory, where analysis of the soil matrix took place, alcohol was allowed to seep out of the boxes, but drying of the samples was avoided by putting each box in a sealed bag with an alcohol-impregnated cotton swab. Sets of sealed samples were then placed in air-tight containers for transportation and storage until analysis.
Morphological Analysis
The boxes filled with litter and soil material were gently immersed into 95% ethyl alcohol. Then the upper surface was observed under a dissecting microscope. Soil and litter components were sorted into categories, taking into account their origin (plant, animal, microbial, mineral, and mixed) and their stage of decomposition or weathering. Following the technique by Ponge (1984) modified by Bernier and Ponge (1994), a piece of counting glass etched with a regular 200-point grid was placed at the top of each box, and the number of points that covered each category was counted. This procedure allowed determination of the percentage in volume of each category at a given depth level. Afterwards, the material contained in the boxes was progressively excavated with tweezers down to a new horizon, if any. Then a new set of observations was completed. Successive observations were used to follow changes in the composition of the soil matrix along a given profile. When necessary for calculations, the composition prevailing at a given depth level was estimated by interpolation between observations immediately above and below the level.
Statistical Analysis
Data (percentages of occurrence of different categories at different depth levels in different humus profiles) were analyzed by simple correspondence analysis, a multivariate method using the chi-square distance (Greenacre, 1984). This graphical data exploratory technique has been previously applied to the analysis of sets of humus profiles by Ponge (1999). It allows classification of horizons as well as successions of horizons (humus profiles) on the basis of the composition of the soil matrix. Active variables or rows (variables from which characteristic roots of the covariance matrix were derived) were the percentages of occurrence of the categories. They were standardized according to Ponge and Delhaye (1995), by arbitrarily fixing the mean to 20 and the variance to one. In this way, factorial coordinates of the variables along a given axis were interpreted in terms of their contribution to this axis; that is, the farther a point is from the origin (barycenter) along an axis, the more it has contributed to the formation of the axis and, thus, the more weight it should be given when interpreting the axis. Observations (columns) were the different counts, which were done over the whole set of humus profiles. Thus, each count was assigned both to a given depth level and to a given humus profile. As this is possible in correspondence analysis, rows (variables) and columns (observations) were projected together on planes made of combinations of the first factorial axes (those corresponding to the highest eigenvalues), thus describing by a geometrical model global patterns emerging from the data matrix. Departure of the first factorial axes from random partitioning of the total variance was tested using the procedure of Lebart et al. (1979).
Passive (additional) variables were used to facilitate interpretation of the factorial axes and to make charts easier to evaluate. In order to assess the influence of depth on the composition of humus profiles, depth levels (cm) were added to the data matrix, without contributing to the formation of factorial axes. They were projected on the factorial axes, taking into account their distance to the counts. Each depth level (each centimeter) was considered as a distinct variable. Fuzzy coding (0.x) was used when a given count was made between two successive centimeters, otherwise depth levels were coded as 1 or 0. Vegetational types and horizons were also used as passive variables, following the same procedure. In correspondence analysis all points corresponding with rows (active and passive variables) and columns (counts) can be projected as points on the same factorial axes. This property facilitates interpretation. If we want to know whether a humus component is (on average) associated with a given depth level or with a given vegetation type, we need only to search for those depth level indicators (centimeters) and those vegetation types that are projected in its vicinity. In our analyses, only passive and active variables were projected on factorial axes, since we did not need information about individual counts. Rather, we desired a synthetic view of changes with depth and changes among vegetation types.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIAL AND METHODSRESULTSDISCUSSIONREFERENCES |
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In Navarre (Fig. 2) , pine components comprised more than one-half of the litter only under pine, whether planted or disseminated in the scrubland. Within the interior of the plantation they comprised the bulk of surface litter, while in the inner edge and under isolated pines they comprised only from 60 to 80% of total litter and 15 to 30% only in the rest of the scrubland, significantly less than in the plantation. Under rosemary (the most typical vegetation of the overgrazed scrubland) the litter comprised 10% of non-plant components (animal feces, roots, mineral soil). In bare areas non-plant litter components comprised 25% of total counts (significantly more than in other vegetational types, except rosemary), while the remaining counts consisted of a mix of pine, moss, lichen, rosemary, and thyme litter, in decreasing order. The importance of lichens covering bare areas should be emphasized, as this vegetation was often invisible to the naked eye. The composition of litter in the inner edge was characterized by greater amounts of false-brome litter than other vegetation types. An increase in moss litter was perceptible from the inside to the edge of the pine plantation and then to isolated pines, but strong variations in moss cover from one sampling point to another made the response of this vegetation to environmental influences rather erratic.
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In Sicily, Axes 1 and 2 of correspondence analysis extracted 6.3 and 5.1% of the total variance, which indicated a significant departure from random according to the number of rows and columns of the data matrix (126 categories and 155 counts, respectively). The projection of humus components in the plane of Axes 1 and 2 of correspondence analysis (Fig. 5) indicated three different kinds of composition, corresponding with Oi, Oe, and A horizons, whatever the vegetation type. Oi horizons were characterized by orange entire pine needles (3), brown entire pine needles (4), entire pine short shoots (8), pine seed wings (88), brown entire pistachio leaflets (62), rosemary stems <3 mm (28), entire orange rosemary leaves (26), and undetermined grasses (96). Oe horizons were characterized by fragmented and nibbled pine needles (60), fragmented brown pine needles (15), nibbled and cut off pine male cones (53), fragmented brown pistachio leaflets (70), holorganic millipede fecal pellets made of pollen grains (77), holorganic millipede fecal pellets (68), food remains made of wood and scales (50), holorganic mite fecal pellets (78), and faunal-modified old fecal pellets (72). A horizons were characterized by mineral particles <0.5 mm (20), fragmented snail shells (66*), organo-mineral fecal pellets without definite shape (86), and coarse-grained organo-mineral fecal pellets (75).
Depth trajectories in Sicily (Fig. 6 and Fig. 7) indicated some shifts in the passage from Oi to Oe then to A horizons from one vegetation type to another. Under a pine crown, the appearance of a distinct Oe horizon was less pronounced as distance from the trunk base increased (i.e., along the sequence Pine 1, Pine 2, Pine 3, Pine 4). Trajectories under rosemary and pistachio were not very different from what could be observed under the peripheral half of pine crowns (Pine 2 to Pine 4). Bare ground exhibited the more direct passage from the Oi to A horizon, despite the abundance of pine litter at the ground surface (Fig. 2).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIAL AND METHODSRESULTSDISCUSSIONREFERENCES |
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All humus profiles studied are characterized by a crumby A horizon mainly made of organo-mineral fecal pellets and sand particles. In the hemorganic part of the humus profile, differences between Navarre and Sicily are related to the animal origin of hemorganic feces, which are seemingly produced by earthworms in Navarre and by insects in Sicily. Mull is the dominant humus form in both sites and under every vegetation type. Under aleppo pine, either planted or naturally established, there is an accumulation of fragmented pine litter, mixed with holorganic feces of epigeic fauna (mainly mites and millipedes) at the top of the mull profile (Oe horizon). In Sicily, the Oe horizon is visible under pistachio and rosemary as well (Fig. 6). All studied humus profiles exhibit a crumby hemorganic A horizon; thus the humus form is mull. However strong variations have been observed in the development of the Oe horizon. Brêthes et al. (1995) described a variety of humus forms belonging to the mull group, from eumull (mull with a thin Oi horizon and absence of Oe horizon) to amphimull (mull with Oi, Oe, and Oa horizons). Mesomull (Oi and thin Oe) and dysmull (Oi and thick Oe) are intermediary. In our sample the humus form varies from eumull, under rosemary and bare ground in Navarre, to dysmull, under pine in Navarre and under all vegetation types in Sicily. Thus, in our study sites, the influence of pine is minor compared with what is commonly reported in the literature, where moder or even mor humus forms are commonly observed under pine trees (Kendrick and Burges, 1962; Bal, 1970; Mettivier Meyer et al., 1986). Changes from mull to moder often follow establishment of softwood species in forests previously occupied by hardwoods (Bonneau, 1978; Arpin et al., 1986a). Reasons for such discrepancies are probably due to the milder climate and more base-rich substrate common in the south of Europe relative to northern countries (Elmi and Babin, 1996). Mull has been associated with an increase in biodiversity under warmer climatic and richer trophic conditions (Ponge and Bernier, 1995; Bernier, 1996; Ponge et al., 1997). It cannot be excluded that an excessive development of some soil organisms typical of soils with a strong accumulation of organic matter, such as the enchytraeid Cognettia sphagnetorum (Vejdovsky) and the ascomycete Cenococcum geophilum Fr., could be responsible for the commonly reported detrimental effects of conifer crops on the biological activity of the soil, by reinforcing the effects of harsh climate and poor substrate quality in the absence of competitors (Meyer, 1964; Toutain, 1987; Ponge, 1990).
Despite minor changes in humus forms, aleppo pine exerts a marked influence on the biological functioning of the studied soils. This was ascertained by comparing plots increasingly dominated by pine. The gradient of development of the Oe horizon observed under the crown of adult pine trees in Sicily (Fig. 6) is probably at least partly due to the progressive development of the crown over the course of time, and thus to a longer-lasting influence of pine, from the time of its establishment, as the trunk is approached. In Navarre, edge effects have been observed near the border of the aleppo pine plantation, pointing to some improvement in litter decomposition in the understory due to changes in litter composition and microclimate (Collins and Pickett, 1987). The favorable influence of understory plant species on pine litter decomposition and humus form has been reported (Tappeiner and Alm, 1975; Ponge and Prat, 1982; Emmer, 1994). This is probably due to a more diverse plant and decomposer community (Törne, 1978), but mechanisms by which pine litter is consumed at a higher rate in the presence of less recalcitrant litter are unknown.
The high level of soil biological activity observed under rosemary in the derelict scrubland of Navarre merits discussion. In Navarre, abundance of earthworm casts in the A horizon and rapid disappearance of litter (absence of an Oe horizon) are typical of humus profiles under rosemary bushes. The presence of the big soil-dwelling earthworm S. campoii, which has been observed to consume aerial as well as subterranean litter and mineral matter, is probably the reason for the rapid disappearance of rosemary litter. Under pine, the same earthworm species has been observed to consume pine needles and roots. It probably adapted its regime to changes in food resources following afforestation with pine, but the amount of pure pine litter reaching the ground at the inside of the plantation is probably higher than the amount populations of this endogeic species can consume each year. Before pine needles can be consumed by S. campoii they temporarily accumulate, forming the Oe horizon, a habitat favorable to fungal and arthropod activity (Ponge, 1991a; Torgersen et al., 1995). This adaptive behavior of soil animal and microbial communities is not unexpected since aleppo pine is a common inhabitant of present as well as past Mediterranean landscapes (Gindel, 1964; Naveh, 1971; Poli Marchese et al., 1988). Needle-consuming earthworms have been observed in places where conifer species are in their natural range (Bernier and Ponge, 1994; Bernier, 1998). In contrast, most inhibitory changes in soil biological activity have been observed in places where afforestation utilized exotic species (Hamilton, 1965). Nevertheless we can postulate that in Navarre, pine litter needs to be conditioned by microflora more than rosemary litter before being consumed or buried by native earthworms (Satchell and Lowe, 1967; Ponge, 1991b). This delay offers a permanent yet constantly renewed habitat for fungi and fungal-consuming fauna, whose contribution to the total biomass is known to increase under pine (Arpin et al., 1986a, 1986b; Arpin and Ponge, 1986).
In Sicily, we observed more uniformity among humus profiles. This could be due to the fact that pine litter dominates the forest floor everywhere, except under pistachio (Fig. 7). Personal observation suggests that pistachio leaves are more recalcitrant to comminution due to strong resistance to mechanical disruption; thus they share some mechanical features with pine needles. This can explain why, in contrast to Navarre, the Oe horizon was always present in Sicily. We observed a gradient in the development of the Oe horizon under pine crowns, pointing to the influence of pine on litter decomposition. The acidification of the soil in the vicinity of the trunk has been often attributed to stemflow (Mina, 1967; Bollen et al., 1968; Neite and Wittig, 1985), but this phenomenon occurs also in cases where stemflow is negligible (Beniamino et al., 1991). Bark deposition also contributes to soil acidification near the stem of pine (Zinke, 1962). We can postulate that a cumulative process of local soil impoverishment partly explains this phenomenon. As the tree grows, its crown enlarges. Thus, under an individual tree places far from the tree trunk have been influenced by pine much more recently than places located in the vicinity of the stem, which has undergone soil impoverishment from the time of pine establishment (Hamilton, 1965; Mettivier Meyer et al., 1986). Each time a wild fire destroys the forest and burns the accumulated litter, these effects disappear to appear again elsewhere during revegetation, which is always initiated with establishment of a new pine cohort.
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ACKNOWLEDGMENTS |
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Received for publication January 20, 2000.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIAL AND METHODSRESULTSDISCUSSIONREFERENCES |
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This article has been cited by other articles:
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  J.-F. Ponge, R. Chevalier, and P. Loussot Humus Index: An Integrated Tool for the Assessment of Forest Floor and Topsoil Properties Soil Sci. Soc. Am. J., November 1, 2002; 66(6): 1996 - 2001. [Abstract] [Full Text] [PDF]
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