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
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
Percentage mineral soil exposed using various
timber harvest methods and logging systems in
Oregon and Washington (Rice, 1972)
22
Skyline, cable crane
23
(No Transcript)
24
(No Transcript)
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

27
(No Transcript)
28
(No Transcript)
29
(No Transcript)
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
32
(No Transcript)
33
Harvester in operation
34
(No Transcript)
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

36
(No Transcript)