The influence of Tree harvesting methods on soil erosion

1. The influence of Tree harvesting methods on soil erosion By Simon Hart

2. Introduction • Forests cover almost one third of earths land surface. • Provide timber & non timber forest products. • Services • Stabilising soil against wind and water erosion • Moderate stream flow absorbing excessive rainfall • Recycling moisture completing hydrological cycle

3. The current situation • Many forests have already been lost • Estimated at 17 million ha/ year (CIDA, 2001) • Reasons are poverty and population growth. • Harvesting timber can have and adverse effect on the environment if not done correctly.

4. Soil erosion • Varies, dependent on: • Soil type • Climate • Vegetation • Aspect • Gradient

5. Tropics is considered more at risk • High amounts of rainfall • Fragile soils • Temperate forestry • Uplands • Steep slopes

6. What is erosion? • ‘Erosion results from accelerated removal of soil materials by running water, wind or gravitational creep’ (Pritchett, 1987) • Natural process existed through geological time • Ganges and Mississippi deltas • Before the arrival of Europeans

7. Forest harvesting Natural sequence of event for any crop Renewable resource Increased erosion following deforestation has been reported for centuries Increased sedimentation

8. Harvesting systems • Combination of methods to fell trees and extract to roadside • Felling and extraction methods vary • Mechanical • Manual • Combination

9. Clear felling/ clear cut • Both temperate & tropical • Economic • Easy to implement • Concern in recent years • Removes all merchantable timber • Often trees of same species and age

10. Effects • By removing trees you are removing water losses, evapotranspiration • Less interception • Roots holding soil together • Source of organic matter • Increase in surface runoff and soil erosion

11. Tropics more destructive • Brash is burnt • No protective layer • Soil left bare • Temperate brash often remains onsite • Brash mats, help protect soil

12. Surface erosion, mass wasting and channel scour can all result from forest harvesting • Mineral soils are exposed to raindrop impact • Detached particles are transported as surface erosion • Care taken with location, construction and maintenance of extraction routes (skidder tracks) reduces stream turbidity as a result of soil erosion.

13. Stream turbidity (JTU) Treatment Base flow mean Stormflow maximum Commercial clear-cut During logging 490 56000 First year after cut 38 5000 Second year after cut 2 170 Silvicultural clear cut During logging 6 90 First year after cut 5 35 Second year after cut 2 23 Undisturbed control 2 25 Effects of timber harvesting on stream turbidity in a humid region (Kochenderfer and Aubertin, 1975)

14. Whole tree harvesting • Method of clear felling • All material including brash is removed • More destructive • More traffic over site • No protective layer

15. Partial cut harvesting • Selection Method- harvesting selected trees in an uneven-aged stand either individually or in small groups at periodic intervals throughout a harvesting rotation. Harvested trees may be the most valuable trees, the poorest quality trees, the oldest trees or trees of a certain species. • Shelterwood Method-harvesting mature trees in two or more cuttings to allow establishment and early growth of seedlings under partial shade and shelter of older trees. • Seed tree Method- leaving individual trees or groups of trees uncut to provide seed to regenerate the cutover area.

16. Permanent cover = less erosion • Allows cuts to be made in sensitive areas • Slopes • Riverbanks Risk of wind throw Result being worse

17. Reduced impact logging (RIL) • Developed in Brazil • Array of best harvesting techniques to reduce damage to residual forest • 57 % less sediment yield compared to conventional logging (Malaysia) • Lower amount of bare soil

18. pre-harvest inventory and mapping of trees • pre-harvest planning of roads and skidtrails • pre-harvest vine cutting • directional felling • cutting stumps low to the ground • efficient utilization of felled trunks • constructing roads and skid trails of optimum width • winching of logs to planned skid trails • constructing landings of optimal size • minimizing ground disturbance and slash management.

19. Mechanical harvesting • Heavy trafficking on soft ground • Efficient drainage system • Construction of roads • Construction of skidder trails

20. An experiment in harvesting , piedmont forest showed that overall export was 90% due to poor roading and channel damage

21. Harvest method/ logging system Percentage bare soil Clear-cut/ tractor 26-29 Highlead 16-19 Skyline 6-12 Balloon 6 Selection/ tractor 16 Cable 21 Percentage mineral soil exposed using various timber harvest methods and logging systems in Oregon and Washington (Rice, 1972)

22. Skyline, cable crane

25. Benefits • Soil disturbance kept to a minimum • Absence of ground traffic • Caution, trails do not become warn by repeated dragging

26. Skidding • Serious soil disturbance • Dragging logs along behind a tractor unit • 11 times sediment yields have been reported as a result • Malaysia, 30% of ground was bare and damaged after mechanical extraction • Particularly damaging in wet conditions

30. Other mechanical harvesting methods • Forwarders • Harvesters • Fully laden forwarders can have a big impact on soil even with good brash mats. • Should be confined to dry periods to reduce compaction and erosion of soil.

31. Forwarder

33. Harvester in operation

35. Conclusion • Harvesting operations in the tropics are often more at risk of causing soil erosion • Planning and management of a harvesting operation is an important factor in reducing soil erosion • Cost of preventing soil erosion is often cheaper than repairing the damage