Rainforest plantations - restoring productivityand biodiversity to degraded lands

Top: Young mixed species plantationBottom: Older monoculture plantation withdeveloping understorey of rainforest species(photographs: Rod Keenan)

May 1999

Rainforests around the world continueto be cleared or degraded at rapid rates.Some of these lands are now being usedproductively, but many are not. Planta-tions of rainforest species may hold thekey to returning productivity,biodiversity and community benefitsthrough the ecological restoration ofthese lands. The cessation of mostrainforest logging in north Queenslandin 1988 renewed interest in rainforestplantations and their potential. Whilemuch is known about traditionalmonoculture plantations based onexotic pines, silviculture knowledge fornew forms of rainforest plantations is inits early days. Could the higher marketvalue of these timbers compensate fortheir slower growth, now timber fromnative forests is no longer available? Ifso, how should these plantations beestablished?

Seeing the trees for the forestResearchers are tackling some of thesebig questions about rainforest planta-tions in a collaborative program withlandholders in north Queensland,particularly with Errol Wiles ofBabinda. Program Leader David Lambsays that the rainforest plantationresearch is asking four main questions:

1. Which species to use?2. Which are the best sites?3. What plantation design to use?4. What are the best plantation management practices?Practical research to address thesequestions requires long term trials andplantations. A number of plantationswere established in north Queenslandin the past by the Queensland ForestService, some over 60 years old.These provide baseline data on growth

rates of commercially attractive treesbut it is clear that better growth ratesmight be possible if more up-to-dateestablishment and managementmethods are used.In more recent years there have beenextensive rainforest plantings in northQueensland by the Community Rainfor-est Reafforestation Program (CRRP) andthese plantings are providing valuabledata. Current research is being under-taken by the Rainforest CooperativeResearch Centre working withlandholders to establish trial plantations

on their lands. Establishing a plantationis a considerable long term investmentfor landholders, and many are unwill-ing to invest while they cannot beguaranteed a market for the timbers.Paradoxically, no market exists untilplantations are created.

Which species to use?The attributes of commercially success-ful plantation trees include rapidgrowth under plantation conditions,straight stems with limited branching,and appropriate wood quality forparticular end-uses. Plantation speciesshould be tolerant of a wide variety ofsoils and site conditions and be resistantto pests and diseases. The CRRP hasexplored a large list of up to 80 speciesin their plantings.Researchers are now focussing fieldtrials on a smaller simpler list ofpromising species as follows:

Eucalyptus cloeziana Gympie Messmate

Eucalyptus microcorys Tallow Wood

Eucalyptus grandis Blue Gum

Toona ciliata Red Cedar

Melia azerderach White Cedar

Agathis robusta Kauri Pine

Flindersia brayleyana Queensland Maple

Araucaria cunninghamii Hoop Pine

Eucalyptus pellita Red Mahogany

Elaeocarpus angustifolia Blue Quandong

Acacia aulacocarpa Hickory Wattle

Which are the best sites?

CRRP planting sites are being exam-ined to draw from their experience. Alarge number of species have beenplanted at a variety of sites. In someplaces they have grown well but inothers their growth has been lessvigorous. What are the reasons forthese differences? Is is due to climaticfactors such as rainfall or temperatureor because of differences in otherfactors? The objective is to end up witha set of predictors describing the bestsites for particular species. Note thatthe alterations following clearing (eg.pasture establishment, fertilising) meanthat these optimum sites may not beexactly the same sites these speciesoriginally occupied.

What plantation design to use?While traditional plantations are grownas monocultures (single-speciesplantations), mixed plantings ofrainforest species may hold the key tohigher productivity and increasedbiodiversity. Mixtures have manypotential benefits. Using nitrogen fixingspecies in deficient soils can boost thegrowth of the other species. Mixingtrees with different root or canopyarchitectures can reduce between-treecompetition. Mixed species plantationsmight also be less susceptible to fungal,insect or animal damage than monocul-ture plantations.Trials suggest that a mixture of rainfor-est and eucalypts associated withrainforest margins may provide the bestproductivity. Trial plots of differentmixtures have been planted at anumber of sites to explore growthpatterns and relationships and thebeneficial effects of individual plantsupon each other. Early results so farsuggest mixtures can improve thegrowth of some species over that theywould have when planted in a mono-culture. On the other hand, the growth

of some other species is worse inmixtures than in monocultures. Re-searchers are examining these differ-ences, and recommendations aboutwhich species to combine or not shouldbe drafted after the trials have beenoperative for 5 years.

What are the best plantationmanagement practices?Pruning ensures trees grow straight andtall and early pruning has been found tobe very important from the first year on.There is a high market value for straightunknotted timber, but there is a tradeoff- pruning side branches can slowphotosynthesis and growth. Researchresults show that half the crown can bepruned without slowing growth.Competition between trees is a bigissue. The questions of when, how oftenand how intensely to thin out trees arebeing investigated. There are alsotradeoffs concerning planting densityand space. Traditional plantings are in3m x 3m grids (densities of 1100 treesper hectare) which shade out grassesand competing weeds. Trees growquickly at first and then slow down, thecrowns merge and start to interfere witheach other. Thinning of slower growingtrees and those with poor form is thenneeded to reduce trees to around 300trees per hectare. This begs the question- why not just start with 300 trees perhectare? The answer is that with lowdensity plantings weeds become amajor management problem and theircontrol may be costly.

Biodiversity and commercial valueA mixed plantation can also be a morebiodiverse one. Researchers are explor-ing how to build biodiversity withoutimpeding commercial value. Thisresearch is challenging the traditionalnotion that plantations are biologicaldeserts. In north Queensland it ispossible to have your cake(biodiversity) and eat it too (economic

return). Here the distances to intactforests are not so far, soils are richer,and rainforest plantations need regularburning regimes like southern planta-tions, so northern plantations cansupport a variety of native plant andanimal life.Understanding the tradeoffs is crucial.By encouraging biodiversity in planta-tion design, at what point doesbiodiversity impact on commercialproductivity? In some situations thebiodiversity benefits will come at noadditional cost or may even improveproduction, in others increasingbiodiversity may slow production. Thisraises many other questions about howjudgements are made on non marketvalues? What is the price of clean airand water or of increasing wildlife, andhow are these factored into the commer-cial value of the plantation? Thesequestions are acutely important and arebeing investigated by researchers SteveHarrison (UQ) and John Herbohn(JCU).Plantation researchers include DavidLamb, David Doley, David Yates andRobin Thwaites from the University ofQueensland; Bob Congdon of JamesCook University and Rod Keenan of theDepartment of Primary IndustriesForest Research Institute.

For further information:

Dr David Lamb, Botany Department ,University of Queensland,St Lucia, QLD 4072.

Phone: (07)3365 2045,Fax (07)3365 2045Email: d.lamb@botany.uq.edu.au