Post-harvest respiration of Pinus radiata logs under different temperature and storage conditions

Background: Radiata pine (Pinus radiata D. Don) log exports are a growing and important trade for New Zealand. Oxygen (O-2) depletion and carbon dioxide (CO2) accumulation in ship holds caused by log respiration may affect the efficacy of fumigation treatments and safety to people working around the holds. Currently very little is known about changes in respiration rates of harvested logs. This study examined the respiration rate of harvested logs after temperature equalisation overnight to a range of temperatures and when stored at typical indoor or outdoor conditions to identify key factors affecting log respiration rate. Results: The results indicate that the respiration rate of logs increased as a function of temperature following Arrhenius' equation, with activation energy (Ea) value of 27708 J mol(-1). The respiration rate at a reference temperature of 20 degrees C decreased from 25.1 mg CO2 kg DW-1 h(-1) one week after harvest to 16.2 and 11.5 mg CO2 kg DW-1 h(-1) three and six weeks after harvest, respectively. The dominant respiration tissue shifted from cambial and parenchymatous tissues in 'fresh' logs towards the internal sapwood over a six-week period. This shift may be attributed to severe dehydration of the outer layers. Microbial activity also contributed to the respiration of logs when stored at humid conditions. Conclusion: Irrespective of storage conditions, the rates of respiratory CO2 release of log sections remained high during the first two weeks post harvest and declined rapidly within the next a few weeks as logs become dehydrated. Respiration rates at a given storage time increased nearly 50 % for every 10 degrees C increase in temperature from 10 to 40 degrees C. Drying logs for 4-6 weeks before fumigation treatment at moderate temperature may help to achieve consistent fumigation results, prevent life-threatening conditions and to reduce transportation cost.