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The Log - Issue five- Research activity: Wood recovery from different harvest systems in blue gum plantations

image of chipping blue gumBy John Wiedemann

CRC for Forestry studies have commenced to determine the impact of different harvesting systems on yield in blue gum (Eucalyptus globulus) plantations destined for export woodchips. With a single product, obtaining maximum within-specification yield is an important factor in determining the overall net value of an operation.

The studies have considered two common harvesting systems in blue gum plantations:

  • in-field chipping systems (feller buncher, skidder and flail delimber-debarker-chipper), where woodchips are delivered to the customer
  • cut-to-length systems (harvester and forwarder), where logs are delivered to the customer to be chipped at a mill site.

Results

Results from two initial studies show considerable differences in available volume recovered through the harvest operation. In both cases, yield (delivered green tonnes of raw product to initial point of sale) from in-field chipping was greater than cut-to-length yield (18.4% and 1.5%).

The lower difference in yield between the harvest systems was in a stand of very small trees (less than 100t/ha) while the greater difference between the systems was in a low-yielding stand but more in line with normal tree sizes (~120 t/ha).

While the reason for the difference in yield between the two harvesting systems has not been directly investigated in these studies, observations and discussions with operators indicate the difference is related to the lower stump height that can be achieved with feller bunchers and the ability to recover all the volume in the stem with in-field chipping since it is not restricted by minimum log lengths. These factors favour the in-field chipping operation for higher volume recovery.

Additional studies needed

The study results for these individual trials will be made available in bulletins shortly, but additional studies are required before we can conclusively understand the differences in yield between these two commonly used harvest systems for different plantation productivity classes.

The key criteria required to obtain meaningful comparisons are:

  • a relatively uniform study site with knowledge of tree, stand and site parameters
  • both harvesting systems operating in the same stand to ensure that variations in stand characteristics are equally captured by each harvest system
  • operations for each system mimicking normal operating practices as far as practicable for each of the harvesting, extraction, processing and delivery phases
  • accurate measurement of the area harvested by each system.

If you are interested in participating in future trials to address this question, please get in touch with the CRC for Forestry ‘Harvesting and operations’ team, John Wiedemann (john.wiedemann@wapres.com.au;  (08) 9771-7400)