Reviewing “sustaining plantation production”

A workshop in Perth on 5 December, “sustaining plantation production: a review” was a great example of how CRCs can deliver science that is both highly relevant to industry needs and scientifically challenging.

Dr Daniel Mendham (Project 1.1 “monitoring and measuring”) presented data showing changes in nutrient pools through the first rotation in blue gum plantations and showed how nutrient pools into the second rotation are affected by slash management in the inter-rotation period.  His work shows that although soil carbon and total nitrogen pools increase throughout the first rotation, the change in organic matter quality means that nutrient availability may decline by up to 50 per cent over a 10 year rotation.  The effect is likely to increase in the second rotation as the legacy of nitrogen rich organic matter from previous leguminous cropping systems diminishes.

Dr Paul Adams introduced process-based modelling work to investigate the long term sustainability of different management systems in Tasmania for sawlog production.  Using the CABALA model he showed that on low fertility sites inter-rotation management was important if production was to be sustained through many rotations.  Although the modelling was not without problems it clearly indicated how the new tools being developed can help us to anticipate and perhaps avoid the sustainability issues that initially confronted the Green Triangle pine grower’s in the 1960s.

Drs Don White and Richard Benyon reviewed our knowledge of soil water recharge following the depletion of soil water stores in the first rotation (see also Subproject 1.2.1 "sustaining site resources" update, The Monitor 4). While much has been published about changes in water yields from catchments following afforestation, it seems that little is known about recharge in the inter-rotation period and what this might mean for sustainability of plantation production.  Modelling work in the CRC for Sustainable Production Forestry alerted partners to the potential of second rotation declines associated with only partial recharge on the profile but had suggested that much was unknown about how trees would respond.  Two outstanding data sets were present in this talk.  Don showed how early evidence indicates that there will be recharge on higher rainfall sites but that on lower rainfall sites (such as the areas east of Albany) only a fraction of the soil water stores have been recharged in the first 18 months following harvesting.  Richard showed that in most cases recharge appears to happen rapidly under second rotation pine, and that evapotranspiration of pine plantations in the first two years may be lower than previously assumed.

Professor Bernie Dell discussed mirco-nutrients in plantations and suggested that we could learn from horticultural and agricultural sciences in ways to predict the frequency with which micro-nutrients should be applied.  Industry partners were also alerted to increased evidence of micro-nutrient deficiency symptoms and other forest health problems in coppice in the second rotation.

Dr Paul Drake examined physiological and growth differences between coppice and seedlings and speculated how this knowledge could be included in process-based models.  Coppice roots allow the plantation to extract water from at least the first five metres of soil within 12 months of the prior stand being harvested compared to seedlings which at this stage are only extracting water from the first metre of soil.  This may have important implications for the risk of drought mortality of stands growing in dry areas and particularly where the soil has been substantially dried during the first rotation.  The gas exchange properties of seedlings and coppice also appear different: photosynthetic rates of coppice are lower than seedlings experiencing similar conditions.  However, coppice grows more rapidly initially because stores of carbohydrate in the stump and roots lead to rapid foliage development.  Paul speculated that there appeared to be a strategy implicit in these findings that coppice rapidly foliates, apparently at a rate at which the development of structural material and perhaps new carbon acquisition outstrips nitrogen uptake so that coppice typically has lower leaf area nitrogen concentrations (and very much lower when expressed on a unit leaf mass) and that this perhaps leads to lower photosynthetic potential.

On the topic of coppice, Dr Tom Baker presented some comparisons of first rotation seedlings and second rotation coppice and seedlings.  At this stage the drought has made it difficult to interpret these results and although data generally suggest that the second rotation is growing more slowly this may be a reflection of weather rather than a change in soil properties or a result of the use of coppice.  Tom showed a decision tool he had built with Jim Knot of Midway to guide the choice to use coppice or seedlings to reafforest the second rotation.  The tools allowed the inclusion of uncertainty of outcomes in such matters as first rotation compared with second rotation growth to be included in the decision process.  At this stage the tools suggests that if growing the second rotation with coppice is likely to lead to reduced or even equivalent growth when compared with the first rotation then generally poorer sites should be replanted and only higher production sites coppiced if net present value is to be maximised.

In other presentations PhD student Nathalie Long (Project 1.2.1) explored the history of breeding for resource-use efficiency and speculated where this might lead us on the search for increased water-use efficiency in plantations.

Sadanandan gave a presentation on the CiFOR network of 16 international trials that explore the effects of inter-rotation management on second rotation yield.

Dr Michael Battaglia (right) discussed the use of the process-based models to provide a framework to explore second rotation sustainability and reminded the audience that by the time a pine plantation planted today is into its second rotation, climate change may have made the world a different place and that sustaining production in the second rotation may be about sustaining production under markedly different conditions than today.

Results from the workshop will be published on the CRC for Forestry website soon.