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Getting more out of your radiata pine woodlot Mid-rotation fertiliser use

Graham Coker and Harriet Palmer, New Zealand Tree Grower November 2019.

Owners of small forests who grow radiata pine stand to benefit from levy-funded research into tree nutrient balances and the cost-effectiveness of applying fertiliser at different times throughout a rotation. The research is producing promising results and will increase options for the precision management of radiata pine.

Radiata pine is regarded as a very undemanding species. This prompts growers of small forests generally to close the gate once the trees are established and after any pruning or thinning is completed.  Traditionally, no further intervention has been considered necessary. Barring some catastrophe, growers can justifiably expect trees to keep growing steadily with productivity determined by genetics and the growing environment.

Few small-scale forest growers would consider applying fertiliser to a radiata pine crop and even fewer would probably think about matching nutrient supply with changing nutrient demands as the trees grow.  However, new research shows that being more proactive with radiata management throughout a rotation could well be worthwhile.

Nutrient needs throughout a rotation

Scion’s six-year Growing Confidence in Forestry’s Future research programme has usually been aimed at increasing the productivity of radiata pine. Much of the research has focused on improving our understanding of the fundamental biology of tree growth within forest ecosystems, and then translating this knowledge to more precise methods of growing timber crops.

Research to determine the nutrient requirements of radiata pine trees throughout their life is an example of this. Significant effort has gone into a radiata pine nutrient balance model NuBalM, which predicts how
nutrient supply and demand varies over a rotation.  The diagram below illustrates how nitrogen supply and demand change through the life of a hypothetical second rotation forest stand.

Nitrogen supply initially exceeds demand, but as the trees develop their demand increases while the supply of nutrients from harvest residues decreases, creating a nitrogen shortfall. Nitrogen supply and demand does not balance until the trees themselves start cycling their own nutrients after the first thinning and beyond. Simulations such as this have enabled researchers to identify times when the supply of critical nutrients on ome sites may be limiting radiata pine growth. This enables intervention, such as adding fertiliser, to be made with greater precision.

Applying fertiliser at mid-rotation

Nutrient supply to trees inevitably varies with site type. To reflect this in nutrition management, Scion researchers led by Graham Coker, have set out to develop new site-specific fertiliser application guidelines for forest growers.

Nine trial sites were established in commercial midrotation radiata pine forests in different parts of New Zealand. Soil samples were taken at each of the sites from which growth-limiting nutrients were identified.
A site-specific fertiliser treatment, determined by the results of the soil sampling and a standard urea treatment, were applied at each site. Tree growth responses have now been measured for three years at five of the sites and for two years at the other four sites.

Modelling cost-effectiveness

As well as learning about the magnitude of growth responses on the different sites, forest growers need to know whether applying fertiliser at mid-rotation is money well spent. To assess this, the growth responses from the trials have been evaluated using the Atlas Forecaster model. The predicted growth, log outputs and discounted harvest returns were all modelled. This enabled an economic comparison of what happened with and without fertiliser as well as site specific versus standard fertiliser treatments on the different site types, allowing the cost-effectiveness of the intervention to be determined.

Forecaster results indicated that, in 80 per cent of cases, the site-specific fertiliser treatment gave the greatest gains in nett harvest returns, log grades and nett present value. On one site, the nett present value increased by just over $9,000 a hectare. The average increase, of $1,400 over conventional practices, indicates that site-specific fertiliser application is cost-effective. In addition, due to increased growth rates, modelling results suggest that growers can expect to harvest earlier or take advantage of higher grade logs at the original harvest time. The graphs above show the modelled results of fertiliser application versus a control with no fertiliser on the out-turn of different log grades. The graphs show the mean predicted harvest volumes for large pruned export peelers P1 and unpruned medium export A grade S2 at age 26 years with three different treatments.

 

Main findings

Overall, some main findings of the mid-rotation fertiliser application research have been that −
• Fertiliser means logs go into a higher log grade
• Fertiliser increases growth efficiency as small changes in water and photosynthetic efficiencies over long timeframes accrue to bring profitable gains
• Fertiliser applications are flexible in that they can be applied almost anywhere using conventional practices
• Fertiliser may improve the health of stands and therefore also reduce management risk
• Site-specific fertiliser is increasing responses from the dominant trees, suggesting that sites may be raised to new productive capacities.

The research has also included comparisons of standard aerial granular fertiliser application which lands on the ground and is taken up through the tree roots compared with aerial liquid fertiliser, which is applied directly to tree foliage. Drones have been used for the foliar applications – technology likely to be suited for applications to small woodlots with boundary or neighbouring area challenges.

Monitoring stand nutrition and growth

This research offers owners of small forests and woodlot an opportunity to increase the productivity of their trees. The adage of ‘the more you put in, the more you will get out’ generally applies. However, to maximise returns, growers will need to know more about their woodlot’s soil and accurately identify areas with differential growth.

Advances in technology, especially drone technology, mean gathering geo-located woodlot growth data is likely to become much cheaper. People offering drone services to the forest industry, including to owners of small forests, should also become more readily available.

Permanent sample plots can be installed, ideally in areas identified with contrasting growth. This will enable foliage and soil samples to be collected from the same locations throughout the rotation, along with associated growth responses.

Sampling foliage early in the rotation at around age four years is sufficient to identify some main nutrient limitations of the site, and a foliage sampling regime at years four, seven and 12 is likely to be optimal. With older woodlots over 15 years of age, growers can start building knowledge about the site nutrition by sampling the soil, because any fertiliser application at this stage can also help to supply the nutrient needs of the next rotation of trees.

The illustration above shows how growers might plan to take foliage and soil samples, and apply fertiliser if results of the sampling show this as likely to be cost-effective. Taking foliage and soil samples is not difficult but needs to be done correctly. Scion has produced some videos demonstrating recommended techniques which can be found on the website gcff.nz/publications/videos.

Samples can be sent to Scion’s Veritec lab where analysis will identify limiting nutrients. Scion can further help by interpreting the results and producing fertiliser recommendations. Growers can then contact local contractors and agree on a site-specific fertiliser blend to be applied according to the requirements of different areas throughout the woodlot or forest.

Conventionally, granular fertiliser would be applied, but as the research on precision application of liquid fertilisers continues, there is likely to be a move towards this technique. For small woodlots of under 10 hectares, fertiliser application by drone could become the norm. Ultimately, growers will be able to develop a precision site-specific and intra-site-specific, cost-effective fertiliser regime.

Guidelines for growers

• Collect foliar information at three critical periods during a rotation at around ages four, seven and 12 years
• Collect soil information at least once during a rotation at around age 20 years
• Match Permanent Sample Plot locations with soil and foliage sample locations.
• Never apply fertiliser without first establishing Permanent Sample Plots to measure response
• A fertiliser response builds over time so do not expect miracles
• Applying greater quantities of fertiliser does not always equal more growth response
• A fertiliser blend including nitrogen is usually the best practice
• Ensure environmental safeguards are maintained during fertiliser application, including that correct buffer zones are applied − 10 metres for foliar fertiliser and 30 metres for granular fertilisers.

Testing water quality

Water quality is an increasingly critical national concern for all managed land uses. As researchers, we are aware that innovations to improve forest productivity need to occur within acceptable environmental limits. As
part of the research programme, Scion’s Dr Brenda Baillie sampled water quality in forest streams in paired catchments near three of the mid-rotation trial sites with contrasting soil types. Samples were taken before and after standard granular and experimental liquid foliar fertiliser applications. The samples were taken hourly on the day of application, and then with decreasing frequency.

The results showed that any effects on water quality were short-term, and mainly confined to the day of fertiliser application. As anticipated, foliar fertiliser applications showed minimal effects on water quality.
Concentrations of all nutrients were lower from liquid foliar fertiliser applications than standard fertiliser treatments.

The results will help the forest industry to improve environmental risk management practices, and ensure that there are good operational guidelines around midrotation fertiliser applications.

Graham Coker is Project Leader, Soil Plant Ecology at Scion.
Harriet Palmer is an independent forestry communications specialist.

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