David Bush

What is the relevance of smallholders’ agroforestry systems for conserving tropical tree species and genetic diversity in circa situm, in situ and ex situ settings? A review

Smallholders’ agroforests may be valuable for conserving tropical trees through three main mechanisms. First, trees planted and/or retained by farmers in agricultural landscapes where wild stands were once found may be circa situm reservoirs of biodiversity. Second, farmland trees may support conservation in situ by providing an alternative source of product to reduce extraction from forest, and by acting as ‘corridors’ or ‘stepping stones’ that connect fragmented wild stands. Third, the additional value that planting assigns to trees may result in greater interest in including them in seed collections, field trials and field ‘genebanks’ that support ex situ conservation. Here, we critically review the evidence for these mechanisms, and highlight areas for research and for intervention so that agroforestry practices can better support conservation in each setting, with an emphasis on often neglected genetic-level considerations. Based on current global challenges to diversity, conservation will need to rely increasingly on a smallholder-farm circa situm approach, but concerns on long-term effectiveness need to be properly quantified and addressed. Connectivity between widely dispersed, low density trees in agricultural landscapes is an important factor determining the success of the circa situm approach, while improving farmers’ access to a diversity of tree germplasm that they are interested in planting is required. The circumstances in which agroforestry plantings can support in situ conservation need to be better defined, and research on the stability of active tree seed collections (how long are species and populations retained in them?) as ex situ reservoirs of biodiversity is needed.

Eucalypts as a biofuel feedstock

Eucalypts are both a proven but largely unexplored source of woody biomass for biofuel production. Few of the some 900 species have been evaluated for cropping, yet among them are the most productive and versatile biomass species in the world, grown in over 90 countries, with species found to suit most tropical and temperate climates. The biology, science and technology underlying the breeding and growing of eucalypts and their potential for biofuel production are reviewed. How eucalypts meet sustainability and economic criteria for biofuel feedstocks, and the advantages of woody feedstocks broadly, are considered. Relevant aspects of eucalypt taxonomy, evolution, natural distribution, human dispersal, composition, domestication and biotechnology of the groups’ potential as a biofuel feedstock resource are reviewed. Two case studies are outlined, illustrating species identification, domestication and harvesting processes where eucalypts are prospective biofuel feedstocks. Eucalypts are strong contenders as a universal woody biomass feedstock for biofuel.

Genetic variation of natural durability traits in Eucalyptus cladocalyx (sugar gum)

Abstract• IntroductionWe present a study on genetic variation in natural durability traits of young-aged Eucalyptus cladocalyx, a species adapted to temperate, low rainfall regions. Our motivation was the production of naturally durable posts for applications such as vine trellises, a sector dominated by heavy metal preservative-treated wood in some of the world’s main wine-producing countries.• MethodsStem diameter at breast height over- and under-bark, heartwood proportion, wood density, methanol extractives and fungal decay were assessed in a progeny test on a set of 48 families from eight provenances nested within three regions of provenance (ROP) from the species’ natural range. Near-infrared reflectance (NIR) was examined as an efficient assessment method.• ResultsStrong variation among ROP was indicated for all traits, with low-moderate narrow-sense heritability for growth traits and extractives content and moderate-high heritability of basic density and decay resistance to two of the three fungi. Trait–trait correlations ranged from low to high, with basic density and extractives content being negatively correlated to decay mass loss.• DiscussionNIR was an effective predictor of methanol extractives, moderately effective for basic density, but unsuccessful for fungal decay. Generally, there were no practically adverse correlations between growth and durability traits.• ConclusionSubstantial genetic variation in natural durability traits is indicated, with wide scope for genetic improvement.

Marker-based adjustment of the additive relationship matrix for estimation of genetic parameters - an example using Eucalyptus cladocalyx

The effects of adjusting additive (numerator) relationship matrices (A) for inbreeding estimates taken from molecular markers were investigated using a small, model population of Eucalyptus cladocalyx. A number of individual-tree, mixed-models were compared, incorporating estimates of population- and family-level selfing and ancestral inbreeding applied either as average values to the entire population or as variable estimates for subpopulation and family groups. The consequences of ignoring inbreeding were inflated additive genetic variance estimates and underestimation of residual variance, with resulting inflation of heritability. We found models that correct for differential inbreeding at the subpopulation level give similar results to more complex ones including family-level estimates. Our analysis indicates that the commonly applied coefficient of relationship for first-generation eucalypt progeny of ρ = 1/2.5 appears to be quite suitable for correcting variance component and heritability estimates. However, if inbreeding is not specifically corrected for by adjustment of A, some minor rank changes of individual breeding values can occur, especially where levels of inbreeding vary among families, and some suboptimal selections and loss of genetic gain may ensue.

Variation in Growth Traits and Wood Density in Whitewood (Endospermum medullosum): a Major Timber Species in Vanuatu

SUMMARY A breeding program aimed at increasing economic value of whitewood plantations in Vanuatu and conserving declining genetic resources will take direction from the results of this study. Genetic parameters for stem diameter breast height over bark (DBHOB), straightness and branching were estimated in an 11.4-year-old whitewood breeding population on Espiritu Santo Island. Wood density variation in one subpopulation was also studied. Trees with the fastest growth and best form were from Espiritu Santo. Growth and form trait heritability estimates ( = 0.10–0.16) were low to moderate, with moderate phenotypic variation (CVP = 17–36%). DBHOB at 11.4-years and 4-years were very closely genetically correlated (rA = 1.00 ± 0.17). Mean unextracted wood basic density at breast height was 330 (SD±21) kg/m3, or similar to mature-tree densities. Economic gain in growth traits can be expected from a recurrent selection and breeding program given the heritable genetic variation indicated in this study. Conserving genetic diversity in this species is a high priority.

Genetic parameters of red mahogany breeding populations grown in the tropics.

Summary The expansion of red mahogany (Eucalyptus pellita) plantations in the wet tropics of northern Australia has increased interest in the domestication of the species. A review of an overseas breeding population was undertaken to enhance our current understanding of genetic parameters for E. pellita, which are essential for the development of advanced-generation breeding programs. The large genetic differences between and within provenances, as well as the moderate heritability estimates for growth and form traits, imply that tree breeding can create improved breeds and reduce variability in plantation forests. Our analysis suggests that breeding value predictions of parents tested in first-generation provenance—progeny trials are less indicative of an individuals genetic merit than breeding values generated from second-generation trials. The low heritability estimate in the first generation relative to the second generation and the reduced inter-generational correlations will influence the level of genetic gain that can be realised. The implications of these findings for the management of E. pellita breeding populations and the production of improved seed for plantation forestry in the tropics are discussed.

Genetic parameter estimates informed by a marker-based pedigree: A case study with Eucalyptus cladocalyx in southern Australia

Analysis of stem diameter, height and axis persistence (AP) in a first-generation Eucalyptus cladocalyx breeding population comprising 137 wild and land-race families planted at 11 sites in southern Australia revealed significant genetic variation among subpopulations and among families within subpopulations. Alternative analyses were carried out using individual-tree mixed models that (i) assumed the trees within families were half-siblings (HS) and (ii) used molecular marker-based information to account for highly heterogeneous relatedness and inbreeding depression among families resulting from mixed mating (MM). For certain site and trait combinations, the HS models would not converge, as estimates of additive variance exceeded the total phenotypic variance, demonstrating the fundamental unsuitability of the HS assumption for this breeding population. Where HS models converged, moderate to very high heritability estimates resulted for growth traits. The MM assumption resulted in re-ranking of individual-tree breeding values and markedly lower estimates of narrow-sense heritability for all trait-site combinations. In some cases, however, heritability remained moderate to high, probably reflecting unquantified dominance variance in some highly inbred subpopulations. Genotype-by-environment interaction was significant overall due to reactivity of genotypes on a few sites, with type-B correlations between pairs of sites ranging from 0.06 to 0.99. Generally, families from the Australian land race were found to perform particularly well for both growth and AP traits. Some wild families were found to be vigorous, despite significant inbreeding. The study has demonstrated that traditional models assuming non-relatedness and/or homogeneous inbreeding in first-generation eucalypt breeding populations can be significantly improved upon by flexible mixed models that integrate marker-based data.

Eucalypt Taxa for Low- to Medium-rainfall Farm Forestry in South-Eastern Australia

Summary Enhanced knowledge of on-farm forestry opportunities in the low to medium (500–750 mm) mean annual rainfall zone of south-eastern Australia is needed to maximise commercial and environmental benefits. Key research issues include species and provenance selection, site preparation and silviculture. As part of the ‘Heartlands Initiative’, CSIRO established several taxa evaluation trials in southern NSW and northern Victoria (within the Murray Darling Basin) in 2002. Results from four of these trials, comprising 16 taxa, are presented. Large differences in survival were evident amongst sites and species. Mean survival after 5 y was highest (89%) at Coomalong (near Violet Town, north-eastern Victoria), followed by Brooklyn West (near Wagga Wagga, NSW; 85%), Byawatha Hills (near Springhurst, north-eastern Victoria; 76%) and Koora (near Holbrook, NSW; 54%). Mean stem diameter and calculated stem volume at 5 y were greatest at Coomalong, but mean height was similar at the three sites. Survival of the commercial Eucalyptus camaldulensis × E. globulus hybrid clone, E. cladocalyx, E. argophloia and E. camaldulensis was consistently high. Eucalyptus camaldulensis × E. globulus hybrid clone had the best growth across all sites, followed by E. benthamii, E. botryoides and Corymbia maculata. Best tree form was achieved by E. camaldulensis × E. grandis, E. camaldulensis × E. globulus, E. benthamii, C. maculata and C. variegata. Growth of E. crebra was consistently poorest, with E. occidentalis also having slow growth and poor form. Selection of suitable taxa and best-practice establishment and silviculture are critical to establishing good plantations on these sites.

Basic Density, Diameter and Radial Variation of Vanuatu Whitewood (Endospermum medullosum): Potential for Breeding in a Low Density, Tropical Hardwood

SUMMARY Vanuatu whitewood (Endospermum medullosum L.S. Smith) is an economically important timber species for Vanuatu. Inter- and intraprovenance genetic parameters for stem diameter at breast height, basic density and radial variation in density, were estimated for two provenances selected from a 12.4-year-old open-pollinated seed orchard on Espiritu Santo Island, Vanuatu. Kole provenance exhibited the highest mean basic density, greatest mean diameter and greatest radial variation in density across the stem. Mean basic density at breast height was 345±2 kg/m3. Growth rings were not visually discernable and colour was homogenous across all samples. For radial variation determination, each pith-bark core was sectioned into four equi-length subcores (A–D). Subcore density increased consistently and significantly from pith to bark, with mean basic density of 308±3, 327±3, 343±3 and 359±3 kg/m3 for cores A–D respectively. The narrow-sense heritability estimate () was low (0.26±0.2) for diameter, with a moderate coefficient of additive genetic variation (CVA= 17.61%). Estimated for density was moderate (0.49±0.24), with a low coefficient of additive genetic variation (CVA= 5.39%). Though the precision of these estimates is modest, reflective of the small sample size, indications are that the heritable genetic variation in both growth and quality traits will result in economic gain from a recurrent selection and breeding program.

Pinus pinaster and Pinus radiata survival, growth and form on 500–800 mm rainfall sites in southern NSW

Summary Pinus radiata (GF27- and Guadalupe-based intraspecific hybrids) and Pinus pinaster were trialled to examine the potential for commercial deployment on three lower-rainfall sites within the 500–800 mm rainfall zone of New South Wales. All three ex-pasture sites were considered to be dry for P. radiata; two lower-elevation sites to the west of Gundagai being more challenging than a high-elevation site near Bombala. Cuttings and seedlings of genetically improved P. pinaster and P. radiata and a Guadalupe × routine cross seedlot of P. radiata were planted at three stockings (625, 1250 and 1850 stems per hectare (sph)) and were assessed for survival, growth (diameter at breast height, height, volume), form (straightness, malformations, branch size, branch angle) and number of commercially acceptable trees per hectare up to age 8 years. Survival in P. pinaster didn’t decline after minor post-establishment mortality and was not affected by stocking or plant type. In contrast, survival in P. radiata continued to decline over time at 1250 and 1850 sph stockings at the two low-rainfall, low-elevation sites. Guadalupe-based seedlings had better survival than GF27 cuttings, but both present a high risk under dryland or drought conditions. Site and stocking affected growth of the two species differently. P. radiata grew larger than P. pinaster at all sites. The combination of survival and growth resulted in site volume production that increased with stocking and site quality for P. pinaster. P. radiata exhibited a similar pattern but with reduced volume production at higher stockings on the high-quality Bombala site due to poor survival. The form and branching of P. pinaster was better than that of P. radiata on all sites except the high-rainfall Oak Range. Form of P. pinaster cuttings was better than of seedlings. Form and branch size were improved by higher stockings and were worse on higher quality sites for both species. However, branch angle was better for P. radiata than for P. pinaster. P. pinaster was reasonably productive on the best performing of the low-rainfall, lower-elevation sites. Given periods of very low rainfall since the establishment of these trials, the productivity of 6–7 m3 ha–1 y–1 at age 8 years at stocking of 1250–1875 suggests that 12–15 m–3 ha–1 y–1 may be possible when mean annual increment (MAI) peaks at a later age. Although P. radiata growth rate was high, the poor survival and form of GF27, and to a lesser extent the Guadalupe cross, suggest it presents a high risk on these sites. Continued monitoring of the sites is therefore suggested. Further work could look at more drought-resistant taxa of P. radiata.