Issue 3 -  2002/03

 ISSN 1311-8978

Effects of shrubs (Calicotome villosa (P.Link)

on surface soil properties in degradating ecosystems of west Lesvos Island

Ambatzis E., Alifragis D., Papaioannou A., Orfanoudakis M.

Aristotele University of Thessaloniki, Department of Forestry and Natural Environment, Laboratory of Forest Soils

Received: 25.10.03; Cited: 05.12.03

Abstract

Calicotome villosa forms a typical type of dry Mediterranean ecosystems in the central part of the Lesvos island. Soil properties were investigated at the periphery of randomly selected C. villosa plants, at the distance of 0, 0.5, and 1m from the shrub. The data were indicated a variation in the soil properties in proportion to the distance from the shrub, while soil degradation, wind direction, and the nature of topography of the site were also played role in the variation of the soil properties. There was a reduction in most of the elements N, C, P, K, Ca, Mg and Na determined, in proportion to the distance from the shrub. Those variations were greater and were intensified in extreme degredating environments. Plant species, which are tolerant to the dry conditions, they are important, factors in the soil protection, and modification of the soil properties.

Key words: forest floor, Mediterranean ecosystems, Western Lesvos, Calicotome villosa.

INTRODUCTION

Soils worldwide exhibit a large variation concerning their properties, depending both on biotic and abiotic factors. Several studies indicated that vegetation, production of organic matter, quality of forest floor and availability of nutrient elements, are strongly combined with the local precipitations, temperature, structure and texture of the soil as well as the history of land use (Burke et al. 1991, 1997, Epsetin et al. 1996, 1997a, 1997b, 1998). Recent studies showed that the total organic matter and the recycling of nutrient elements sustain also a strong effect by the vegetation site layout as well as its synthesis (Wedin and Tilman 1990, Wedin 1995, Vinton and Burke 1995, 1997).

The effect of vegetation on soil formation is mainly caused by the addition of organic residues (forest floor). This addition influences the natural and chemical properties of the soil directly in a significant way (Dickson and Crocker 1954, Crocker and Major 1955, Crocker 1960).

Organic matter that is added in soil derives from different parts of the plant (leaves, branches, branchlets etc.) and its effect is localized mostly on the surface horizon (Zinke and Crocker 1962, Kellman 1979). These residues are decomposed as time passes and after a series of modifications, the residues are reverted to their basic components i.e. CO₂, water and chemical elements so that the production of new organic matter is rendered possible. The procedure of organic matter decomposition is exclusively a matter of biochemical nature and is aided by soil microorganisms.

Trees and shrubs are functioning as natural fences, able to reduce wind strength (Milchels 1994, Kainkwa and Stigter 1994), particularly where tree crowns are large and strong. Wind strength possibly in certain cases removes part of the forest dry leaf matter as well as dust and soil particles from the surface layers. Besides the fact that vegetation occurring at the direction of the wind reduces its strength, it also retains part of the detached soil and leaf dry matter. In this way small humps are formed by the deposition of this material at the base of several shrubs. Wezel et al., (2000) at Nigeria observed the formation of such a small hump at the base of a shrub belonging to the species of Guiera senegalensis, the height of which reached to 11cm.

Most of the deposition occurs around shrubs when the soil is affected by the stormy winds (Sterk et al., 1998). This phenomenon tends to be more intense in the case that the soil lacks vegetation.

Finally, grazing causes a significant change to soils at the concentration of organic matter and the availability of nutrient elements (Reuss and  Mcnaughton 1987, Milchunas and  Lauenroth 1993, Manley et al., 1995, Biondini and  Manske 1996). Grazing creates some other problems as well caused by the effect of soil natural properties.

The aim of the present research is to point out the effect of shrub vegetation on the modifications of forest floor properties in degraded ecosystems of western Lesbos. In order to reach our aim, the properties of forest floor and especially the concentration of organic matter and nutrient elements Ν, P K, Ca, Mg and Νa, were studied.

MATERIALS AND METHODS

STUDY AREA

The area of research is located in the western part of Lesbos island and specifically at the location of Skafi that constitutes a brushwood ecosystem.

The reasons that leaded us to the selection of the particular study area are the following:

1. Brushwood ecosystems occupy the largest part of the land in western Lesbos.

2. The soil of the area exhibits high variability due to the large fluctuation of the geological conditions while the climate is more or less the same.

3. Brushwood ecosystems constitute a typical form of vegetation in degraded areas of eastern Mediterranean.

Research area is situated at the altitude of 320m. In the area there are slopes ranging mainly from 35% to 60%. It is considered a brushwood ecosystem and is used as grassland. The geologic background of the study area consists of lava and volcanic tuffs. The average amount of argil, slime and sand are: 15,3%, 17,7% and 67% respectively, fact that places the study area into the category of sand-clay soils. The bulk density of the soil is 1,15 g/cm3. The soil is characterized as slightly acid (pH=6,2). The average values of the soil chemical properties are given in the following table (depth 0-10cm):

Table 1. Average values of some soil properties of the study area (depth 0-10cm).

*        Numbers written in italics are the values of the standard deviation.

Meteorological data from the station of Mitilini, indicate the occurrence of a warm and dry period lasting from April till October with an average temperature of 26,1 °C and a moist and cold period from November till March with an average temperature of 10,4 °C. During summer, rainfall is rare with an average minimum height reaching to 2,7 mm in July and totally to 14,7mm from July till August. The average annual rainfall varies from 725mm in the eastern part of the island to 415mm in the western part. In general, western Lesbos belongs to the sub-dry zone, according to the climatic classification of FAO-UNESCO (1977). Finally, the effect of erosion and grazing is strong.

SAMPLING PROCEDURE

In order to estimate the quantity of forest floor and nutrient elements that are accumulated on it, in each sampling surface 12 samples of the forest floor were taken, peripherally of the stump of the Calicotome villosa shrub. Sampling was conducted in a cross-like shape and in a distance of 0m, 0,5m and 1m respectively.

Sampling was conducted by the aid of a metal frame (25x25cm), placed on the undisturbed surface of the forest floor and the frame was pressed till it reached the zone of mineral soil. Then the organic horizon, included in the metal frame, was collected with care till the depth of the mineral soil. The samples were placed in polyethylene bags, and were transferred to the laboratory for chemical analysis and weight measurement.

LABORATORY ANALYSIS

Forest floor samples were transferred to the laboratory and inorganic particles were removed by successive siftings. Small branches, fruits as well as bark and wood residues were left on the forest floor. Then the samples were oven-dried for 48 hrs at 84°C and their dry weight was determined. A representative sample was ground in a No 40 mesh laboratory mill and was stored in air-tight plastic vessels, until its chemical analysis.

The determination of the concentration in ash was conducted by the loss on ignition method by burning to ashes ground samples (2-3gr) in a high temperature oven for 4,5 hrs at 600°C. Total nitrogen was measured by the liquid oxidation macrokjedahl method with a Tecator apparatus. Distillation and volumetric analysis of ammonia was automatically determined by using 0,05HCl. For the determination of P,K,Mg,Ca and Na, the dry ashing method was used, by placing samples of 0,5gr in an oven for 3 hrs at 515°C. In order to disolve the ash 18% HCl solution was used with simultaneous heating in a waterbath for 15 min and subsequent filtration with Whatman No1 filters. A quantity of this solution was used for the determination of phosphorus by the molybdate blue method. Color development was conducted by using a mixed solution of: SnCl2, ascorbic acid, formic acid, Na-EDTA and dense HCl. Color was fixed after 30min, and it was determined by a visible light spectrophotometer (in the area of infrared spectrum 710mm). K,Ca,Mg and Na were determined by an atomic absorbtion spectrophotometer-Perkin Elmer AAnalyst 300.

The results of chemical analysis and the measurements of dry weight of forest floor samples were used for the reduction of these values in organic matter and nutrient element concentration per hectare. There were also estimated the mean numbers of the concentrations and quantities of nutrient elements.

STATISTICAL ANALYSIS

The statistical analysis of the results was conducted by using ONE WAY ANOVA analysis and the mean numbers were compared by using the Duncan test (Matis 1994). The significance level was at α=0,05. For the statistical analysis the SPSS ver. 10 statistical program was used.

RESULTS

Forest floor-distance properties

Modifications in forest floor properties according to distance are presented in table 2.

The results of the tab2 it is obvious that the concentration of organic matter (Kg/ha) and nitrogen present significant reduction in relation to distance from the stump (p<0,001). The modification was more intense between the extreme ends of the sampling points (0 and 1m from the stump). The extractable P follows the course of the two previously discussed elements (p<0,01). The reduction of the extractable P between the maximum and minimum distance is 75%. The in-between distance shows a reduction of ~ 40%.

K, Mg, Ca, Na, elements follow a similar modification in their concentrations (p<0,001). The above elements present their maximum value under the shrub and their minimum at the distance of 1m. The reduction percentages that were observed for these two sites for each element were the following: Κ 75%, Mg 70%, Ca 75%, Na 75%. The in-between distance presents also reduction for these elements as follows: for K, Mg, Ca, Na, the pecentages were 50%, 44%, 45%, 50%, respectively.

Table 2. Modification of forest floor properties according to distance from the stump.

  *    Values of standard deviation are written in italics

  **  Significance level α=0,05

   *** a, b, c,  Statistically significant differences are indicated by exponents a, b, c.

Forest floor-side properties

Modifications of forest floor properties according to side are given in table 3.

Results from  table3 it is obvious that the organic matter quantity presents a statistically significant variation depending on the side of the location (p<0,05). Comparing between upper and lower sides, the maximum value was observed at the lower side while in the case of right–left sides the maximum was observed at the left side

Na is the only element that does not present significant change in relation to the effect of side location (p>0,05). Comparing the values observed for the upper-lower and left-right sides, the maximum appeared at the lower and right side (slightly higher values).

Table 3. Modification of forest floor properties according to side.

*    Values of standard deviation are written in italics

  **  Significance level α=0,05

   *** a, b, c,  Statistically significant differences are indicated by exponents a, b, c.

The rest of the elements: N, P, K, Mg, Ca, present a significant statistical modification (p1<0,001, p2<0,001, p3<0,01, p4<0,01, p5<0,001) in relation to the side location. The comparison between the upper and lower sides showed the higher values at the lower side. Concerning the elements N, P, Mg and Ca, the difference observed, was 70%, 63%, 55%, 58% respectively. As for the left-right sides, Ν follows the course of modification observed for organic matter, i.e. it presents the higher concentrations at the left side. The fact that this was an expected case is due to the strong correlation of Ν with the organic matter. On the contrary the elements P, K, Mg and Ca show their higher concentrations at the right side (Mg and Ca did not show significant differences and P and K showed their higher values, at the right side, reaching to 25% and 20%, respectively). 

DISCUSSION

Distance

In all the comparisons it was confirmed the fact that the elements studied: organic matter, N, P, K, Mg, Ca, and Na are reduced as we are moved away from the base of the plant. Similar results are reported by Garcia-Moya and McKell 1970, Charley and West 1975. The interpretation of this phenomenon may be attributed to the reduction of organic remnants that are accumulated and decomposed in relation to the distance (Wezel et al., 2000). The closer to the plant are these elements, the more are protected from water erosion, as well as drifting away by the wind. Similarly in many cases favorable conditions are created for the decomposition of the materials under or close to the above standing plants (decrease of extreme temperatures, increased humidity). Moreover, through the crown flow the elements that have deposited on the shrub (solid deposition) and added to the forest or washed way from the above standing biomass.

These modifications are more intense in the highly degrading areas (Perez 1995). In the ecosystems that appear in semi-dry areas, in particular, these modifications are more intense.

In the study area, the effect of distance prevails on the modification of organic matter and Ν (p<0,001). P exhibits almost the same course of modification since this element is also altered in a statistically significant way in relation to the distance (p<0,01). In every 50 cm there is a decrease of 38% and 60%, respectively.

For the elements K, Mg, Ca, Na, the effect of distance is more intense. The more we move away from the base of the shrub their concentrations are decreased. Particularly in the study area, the decrease for every 50 cm remains constant (for the first meter) and ranges from 45 to 55%. The main reasons are: that already from the first 30-40cm there is no affect of shrub crown and the soil is exposed to the erosion caused by the water and the wind, the decrease in the addition of organic materials, and grazing that starts to make its appearance as well, as we move away from the particular shrub (thorns – difficulty in approaching).

Side

In the study area, the concentrations of organic matter are modified in a statistically significant way in relation to the side. The maximum values are presented on the left and lower side. These maximum values at the left side are justified as the N/NW exposure, receives the effect of N/NW winds that carry away organic materials (branches, leaves, etc.) and accumulates them on the left side of the shrub where it entraps them, while at the lower side the materials are accumulated under the effect of gravity and erosion. Similar action of the wind was observed in other works as well (Wezel et al., 2000).

Ν concentration usually follows the variation of organic matter (Perez 1995). This is an expected fact due to the strong correlation between organic matter and Ν that is observed in the Mediterranean ecosystems.

The variation of P as well as that of Κ, Mg, Ca except of Na (p>0,05) follows a different way of modification in relation to Ν, since the right side is richer than the left one. The higher concentration of P in the right side of the shrub, may be justified by the immobilization of this element due to the unfavorable conditions of decomposition. Concerning Κ, the reduced concentration that is observed at the left side, may be attributed to the faster wash off of the particular element, due to the favorable decomposition conditions. Finally the higher concentration of Mg and Ca nutrient elements on the right side of the shrub is possibly due to the mixing of the inorganic soil with the organic residues (Tantos et al., 2002). Concerning the side (upper, lower) we can easily claim that there is a statistically significant difference with higher values in the lower side. The exception is only the concentration of Na (p>0,05).

We can conclude that the micro-terrain and especially the micro-transfer of the organic residues due to the wind effect, the condensation or the retaining of micro-droplets seems to play the first role in this comparison. However, there is probably a connection to the appearance of the pair Κ – Ca with similar behavior at least as far as their appearance in the lower side is concerned.

CONCLUSIONS

The main conclusions of the present work are the following:

It was proved that the shrub Calicotome villosa has the ability to retain large quantities of organic materials, as its branching starts a little below the soil surface. The organic materials derive either from the very plant, but in most of the cases from the upper side as these materials move under the influence of gravity and erosion.

Calicotome villosa was found to function also as a protective barrier against the wind, enclosing the deposition materials that the wind detaches close to the branches.

The results for the nutrient elements were interesting. In all the cases, we observed a decrease in concentration, in proportion to the distance. The reasons are mainly the decomposition of organic matter as well as the crown outflow. The suspended materials that are deposited on the shrub are washed off by the rain and result in a close distance from the shrub. The side that the elements appear seems to be affected by the winds, the decomposition conditions, the erosion as well as the terrain topography.

From the above it seems that vegetation affects in a determinative way the modification of the properties and the productivity of the site by the addition of organic materials and their protection as well as the soil from erosion. The understanding of the mechanisms that rule the effect of vegetation and the variation of properties, will aid us in our interference with greater precision and efficiency in nature. In the same time this variation must be always taken into account during the procedure of soil sampling, in order to reflect the mean values of soil properties.

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