Regeneration Burning Studies in High Elevation Mixed Species Forests in East Gippsland Gregory J. McCarthy and Glenn M. Dooley FOREST SCIENCE CENTRE Eastern Research Centre, Orbost Department of Sustainability and Environment Victoria Parks and Forests Report Series 04-3 August 2004 i © State of Victoria, Department of Sustainability and Environment, 2004 Published by the Department of Sustainability and Environment PO Box 500, East Melbourne, Victoria, 3002, Australia www.dse.vic.gov.au This publication is copyright. Apart from any fair dealing for private study, research, criticism or review as permitted under the Copyright Act 1968 , no part of this publication may be reproduced, store in a retrieval system or transmitted in any form or by any means, electronic, photocopying or otherwise, without prior permission of the copyright owner. ISSN 1449-2067 ISBN 1 74152 006 1 The Forest Science Centre was commissioned to undertake this project by Forestry Victoria and the Fire Management Branch, Department of Sustainability and Environment. General Disclaimer This publication may be of assistance to you, but the State of Victoria and its employees do not guarantee that the publication is without flaw of any kind, or is wholly appropriate for your particular purposes, and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication. Cover photographs (All taken by Greg McCarthy): 1. HEMS logging slash showing typical arrangement of different size classes of material. 2. Helicopter with an Aerial Drip Torch lighting a slash burn. 3. Strong convection column resulting from HEMS burn (Clarkeville 1) conducted under weather conditions in mid range of prescriptions. ii SUMMARY A suitable seedbed for the natural regeneration of High Elevation Mixed Species (HEMS) in south-eastern Australia can be achieved operationally by the application of high intensity fire to the logging debris (slash). However high elevations can often mean that it is difficult to achieve the required fire intensity to produce satisfactory seedbeds. This problem of reduced fire intensity, in addition to problems of achieving adequate regeneration generally in HEMS, prompted forest managers to investigate the prescriptions associated with high intensity slash burning on HEMS sites. While traditional burning methods had involved igniting logging slash by hand, the advent of a helicopter-mounted incendiary device - the Aerial Drip Torch (ADT) - had meant changed options for burn managers. A perception developed that the ADT could apply much more intensive ignition to a given area over a shorter time, and therefore may be able to achieve higher fire intensities under less favourable weather conditions. As there was only limited knowledge about what contributed to a successful HEMS slash burn, this study undertook a detailed appraisal of fuel, weather, topography and ignition method variables at a total of 16 operational logging coupes over two years. Fire intensity achieved was specifically examined by recording: observable fire behaviour; fuel consumption of various fuel size classes; and soil/subsoil temperatures reached. (The second phase of this project, which examined regeneration results, will be reported separately.) The results obtained indicated that the current HEMS Slash Burning Prescriptions were generally correct. There were also strong indications that soil moisture and Drought Index variables were particularly important to fire intensity achieved. This lead to two small supplementary studies. The first supplementary study looked at the application of a simple field test for surface soil moisture content, based on the ability of the soil to yield dust when impacted, with dust formation correlated to surface soil moisture content (and therefore fuel moisture content).The second supplementary study looked at Drought Index trends at HEMS elevations. The major findings were: 1) Fuel moisture and weather conditions, at the experimental regeneration burns studied, supported fuel moisture and weather condition prescription ranges given in the current HEMS Burning Prescriptions (NRE 1998). 2) The additional prescriptive factors of: • % direct sunlight on the coupe; • % cloud cover; • atmospheric stability; and • a field soil moisture test; could be usefully added to the HEMS Burning Prescriptions to increase the level of confidence in obtaining successful burns. iii 3) Burn escapes from spotting were uncommon in the HEMS regeneration burns studied. Additionally, the propagation of spot-fires in uncut forest adjacent to HEMS coupes was inhibited by the presence of a secondary or understorey canopy, which gave profile fuel moisture contents outside the coupe of greater than 20 %. 4) Drought Indices at high elevations varied significantly from those at lower elevations, and, given their important influence on soil and fuel moisture conditions (and hence burning opportunities), Drought Index trends at HEMS elevations could usefully be investigated in further research. 5) The ADT, although an extremely useful tool for burn managers (in terms of speed and safety), is no more successful under marginal weather conditions than hand lighting, in producing a level of fire intensity which gives acceptable fuel consumption and soil heating for silvicultural purposes (i.e. receptive seedbed production). iv CONTENTS SUMMARY ..........................................................................................................iii CONTENTS...........................................................................................................v List of Tables and Figures .........................................................................vii 1 INTRODUCTION......................................................................................... 1 2 METHODS................................................................................................... 2 2.1 Sample Coupe Details........................................................................ 2 2.2 Experimental Design and Analysis..................................................... 4 2.3 Fuels .................................................................................................. 5 2.3.1 Slash fuel distribution transects ...................................................6 2.3.2 Branch and stem fuel consumption - wire tie transects..................6 2.4 Weather............................................................................................. 7 2.5 Fuel Moisture and Soil Moisture ........................................................ 7 2.5.1 Moisture content of leaves and soil..............................................7 2.5.2 Field soil moisture test................................................................8 2.5.3 Seasonal soil moisture trends - Drought Index..............................9 2.5.4 Slash fuel curing time .................................................................9 2.6 Topography...................................................................................... 10 2.7 Lighting Method .............................................................................. 10 2.8 Fire Behaviour ................................................................................. 10 2.9 Fuel Consumption............................................................................ 11 2.10 Soil Heating ..................................................................................... 11 2.11 Seedbed Condition Following Burning............................................ 12 3 RESULTS .................................................................................................. 13 3.1 Fuel Structure on Slash Distribution Transects ............................... 13 3.2 Short Term Weather, Fuel Moisture and Soil Moisture.................... 16 3.2.1 Field soil moisture test - dust formation moisture contents .......... 17 3.3 Long-term Weather ......................................................................... 18 3.3.1 Drought Index ......................................................................... 18 3.3.2 Weather availability for HEMS slash burning ............................... 19 3.4 Topographic Conditions................................................................... 21 3.5 Ignition Method............................................................................... 21 3.6 Fire Behaviour Observations............................................................ 22 3.6.1 Convection column formation.................................................... 22 3.6.2 In-draught and tree sway ......................................................... 25 3.6.3 Spot fire formation ................................................................... 25 3.6.4 Fire behaviour effect on seedbed production .............................. 26 v 3.7 Fuel Consumption............................................................................ 27 3.7.1 Consumption of fuel by size classes ........................................... 27 3.7.2 Correlation of fuel consumption with other factors ...................... 29 3.7.3 Curing time.............................................................................. 30 3.7.4 Total slash volume and volume consumed.................................. 31 3.8 Soil Heating ..................................................................................... 32 3.8.1 Soil temperature correlation with other factors ........................... 33 3.8.1.1