1 Introduction

Timber harvests normally produce significant amount of slash including tops, limbs, and defective logs. After harvesting, a great deal of slash may be left on the harvested sites. Untreated or poorly treated slash left in the woods can restrict access of humans to the harvested sites, and may cause elevated fire hazards in the year immediately following the harvest. It is necessary to handle the slash after harvesting operations because of the potential fire hazards, limited access of humans and some requirements of operations following harvesting, such as site scarification and planting trees. In addition, slash disposal is quite helpful for the harvested sites to maintain productive soil if handling the slash in a reasonable way. Slash disposal is an important consideration and should be addressed in the timber harvest plan.

According to the Stipulations of Forest Harvesting and Regeneration issued by Forestry Ministry of China in 1987, there are three methods of slash disposal permitted in China: piling, burning and scattering. Piling is to pile the slash along the hill, mostly along the contour. Burning is to burn the slash collected and piled randomly at the harvested site.Scattering is to scatter the slash evenly over the site. All three methods were popular during the period from the 1970's to the 1980's, but the burning method was forbidden in the dry seasons due to risk of forest fire. The scattering method was less used hardly ever for last 10 years because of new regeneration requirements.

In western countries such as the United States, slash treatment or disposal may involve simply lopping and scattering the slash evenly over the area.The slash was cut into a specified maximum length to help speed its decomposition. For example, following pine harvests, lopped and scattered slash is normally cut until it lies within 20 ~ 50 cm of the ground. In some cases following clear cutting, slash is chopped, using dozer pulled roller-choppers, to break it up and put it in close contact with the soil to speed decomposition. Slash can also be burned in place, or piled and burned. If the whole tree skidding is used, the cut trees will be skidded with tops and all branches to a landing then processed into logs. In that case nearly all the slash will be concentrated around the landings. Slash piled at landings can, in some cases, be sold as firewood or biomass fuel, burned in place, or simply left for widlife use and decomposition.

Some scientists published reports or papers concerning the slash bur paper.Ining and its effects on soil (Boone et al., 1993; Covington et al., 1991; Debano, 1979; Xiang et al., 1990).The dynamic state of nutrient elements in the soil, relations between fuel properties and slash burning, and soil nutrient changes associated with slash burning were studied in these literatures. Results of these studies showed different conclusions. Some authors concluded that burning was a poor measure to handle the slash because it caused lose of the nutrient elements in soil. Other authors found, through their field surveys, that slash burning increased the nutrient elements content and availability in the soil. In addition, the impact of forest harvesting operations, including cutting methods, skidding system, and road construction on the properties of soil on the harvested sites were discussed in some other literatures (Sun, 1994; Wang et al., 1994; 1995; Wang, 1994).

2 Materials and methods 2.1 Experimental site selection

Field survey method was used to study the effects of slash disposal methods. The method particularly focused on the burning and piling methods. The study surveys on soil properties were carried out in Dailing Forest Experiment Bureau, Heilongjiang Province, China.The experimental sites are located in Dongfanghong Forestry Farm, 128°47′32″E and 47°13′10″N. The forest stands before harvesting were mixed stands of conifer and broadleaf trees, 70% to 30% in proportion. The conifer trees included Korean pine (Pinus koraiensis), korean spruce (Picea koraiensis) and khingan fir (Abies nephrolepis).The broadleaf trees were mainly consisted of Ribbed birch (Betulacostata), Mongolian oak (Quercus mongolica) and Vssuri poplar (Populus ussuriensis).Average Elevation of the experimental sites is 400 ~ 500 meters high. Annual precipitation is around 850 mm. The slope on experimental sites ranges from 12°~ 25°.Forest soil is dark brown soil. Clear cutting and selective cutting systems with ground crawler skidding and animal skidding were employed in the harvesting.

Four big blocks, one responding to slash pilelasted for 5, 10, 15 and 20 years respectively, were chosen as the experimental sites for piling method. Selective cutting method was used in the harvesting operations at these sites. At each block, nine square sites, 4m ×4m in area for each, were arranged at the positions of slash piles. Also nine soil samples, distributed evenly on each square site, were obtained. The layout of soil sample positions is shown as Fig. 1. The reference sites, also 4m ×4m in area for each, were followed the squae sites at the unpiling positions.

Three big blocks, which represented the site with slash burnt in 1986, 1990 and 1996 respectively, were selected as the experimental sites for the survey on burning method. Clear cutting was employed in the harvesting operations. Nine small square sites sitting on the positions where the slash was burnt, in the same space as that on piling site, were most evenly distributed at each block (as shown in Fig. 2).The reference sites on the unburnt area, also 4m ×4m in area for each, were put as close to the square sites as possible.Same as that from piling sited, nine soil samples were taken from each square site.

2.2 Experimental methods

The soil samples, used for analyzing the physical properties, were picked up by soil cylinders, and those used for chemical property analysis were obtained by special spades and contained in plastic containers. All the soil samples were from the topsoil with 0 ~ 20 cm of depth beneath the topsoil layer.

The analysis for soil physical properties was the conventional method. The bulk density, saturated water retention capacity and total porosity of soil were taken as the main indicators to present the soil physical properties. And those indicators to present the chemical properties of soil in this study were pH value, organic matter, total nitrogen, hydrolytic nitrogen, total phosphorus, available phosphorus, total potassium and available potassium. The methods used for obtaining those indicators were as follows. pH value: electric potential method, Organic matter:potassium bi-chromate oxidized method, Total N: Kjedahl Method (digested by sulfuric acid), Hydrolytic N: basic diffusion method, Total P: acid soluble-molybdenum colorimetric method, Available P: acid extracted-molybdenum method, Total K: acid soluble-flame photometric method, Available K: neutral ammonium acetate extracted-flame photometric method.

3 Results and discussions 3.1 Results

The main laboratory works were consisted of soil drying, physical property analysis and chemical property analysis. The results of laboratory analysis, in mean values, are shown in Tab. 1, Tab. 2, Tab. 3 and Tab. 4.

3.2 Discussions 3.2.1 Effects of piling method on soil physical properties

Bulk density, saturated water retention capacity and total porosity of soil were measured. The bulk density of soil on reference sites is slightly higher, only 3. 2% at average, than that on piling sites. The later two indexes of soil on reference sites are lower than those on piling sites, 10. 9% and 13. 5% at average respectively. The reason for this is that the touching of machines or labor workers from the operations after harvesting was exempted from the piling sites due to occupation of the slash like tree branches and tops, but the reference sites were impacted in some extent.

3.2.2 Effects of burning method on soil physical properties

The Tab. 2 shows, at average level, the bulk density on burning site is higher (5.2%)than that on reference sites, and the saturated water retention capacity and total porosity on burning sites are lower (14. 85 and 8. 6%) than those on reference site respectively. The possible reasons are as follows: (1) tons of tree leaves or needles and other organic matters on the top soil were burnt while burning the slash, and much of them were also washed away by rainfall year by year after burning. This made the top soil of burning site more soild, (2) the top soil on the reference sites was mixed with tree leaves, grass and other organic matters which makes the soil more soft.

3.2.3 Effects of piling method on soil chemical properties

Parameters of soil such as pH value, organic matter, total N, total P, total K, hydrolytic N, available P, and available K were measured. There is no significant difference between pH values of soil on piling sites and those on reference sites, but the soil in piling site seems a little more acidic than that on reference sites. Much more coniferous tree needles and tree branches were piled for the slash disposal and they made the soil more acidic than that on the reference sites because of the high acidic content in conifer tree needles and branches.

The organic matter content in the soil on piling site is significantly higher, 24. 6% at average, than that on reference site. And the contents of nutrient elements on piling site are also higher than those on reference site, 14.3%, 16. 6%., 8. 3%, 9. 2%, 8. 0% and 15. 1% higher for Total N, Hydrolytic N, Total P, Available P, Total K and Available K respectively. The reasons would be: (1)the nutrient elements lost less on the piling site than reference site because of the top soil protected well by slash on piling site; (2)the new nutriet elements from the decomposition of slash joined into the top soil on the piling site.

3.2.4 Effets of burning method on soil chemical properties

The pH value of soil on burning site is higher, 8.7% at average, than that on reference site.It is explicit that the burning of slash made the topsoil become more basic.

The organic content on burning site is lower, 13. 2% at average, than that on reference site.The topsoil on site after using burning method is fragile and easy to be taken away by running water from rainfall or snow melting water in the spring. And it would cost several years to recover the a new top layer mixed with the decomposed of grass and other plants.

The nutrient elements of soil on burning site are significantly lower than those on reference site, with the changed percentage from 12% to 38. 6%.Of them the soil compositions related to N and P, such as total N, hydrolytic N, total P and avilable P, decreased 34%, 39%, 13%and 30% respectively due to the slash burning. Part of compositions from slash, related to N, was gone with wind while slash burning or after burning, and part of them was gone with water after burning. Other research results (Boone et al., 1993; Covington et al., 1991) also showed that more than 50% of nitrogen in the slash was lost due to slash burning. Soil erosion after burning was one of the prevailing reasons that made soil compositions related to P losing. But the soil compositions related to K, such as total K and avaiable K, increased in some extent, 12% and 22% respectively, because of slash burning. Much more compositions, with K were produced during the slash burning and part of them was kept into soil, which changed the soil element proportion structure, while part of them was gone away with running water after burning.

4 Conclusions

The following conclusions can be derived from the laboratory results and analysis above: Both piling and burning methods do not affect the physical properties of soil on the harvested sites significantly, piling method is quite helpful for the harvested site to maintain the productive soil, burning method is useful for soil acidification improvement, but it is easy to lose nutrient elements.

It must point out that unless the slash is properly chipped or chopped the scattering method will be detrimental to site scarification and plantation operations following harvesting. This is the reason why this method has been rarely use at present.

Acknowledgement

Ms. Sun Molong, Ms. Liu Jingyu, Mr.Li jinbo, Mr.Yang Xuechun, Mr.Xu Maokun, Mr.Meng Qingguo, Mr. Yuan Shiming and Mr. You were also involved in the field works or laboratory works. Some unknown reviewers from Journal of Forest Engineering made a great deal of contribution to revise this paper.I am very gratedful to all of these people and founding institutions.