Based on previous studies in various parts of the world, the relative performance of the four blue gum species, Eucalyptus bicostata, E. globulus, E. maidenii and E. pseudoglobulus, is expected to be related to the fit between the environmental parameters of both the collection (provenence) and test sites (e.g. Dutkowski and Potts 1999, Jordan et al., 1993). Given the equivalent latitude of New Zealand’s North Island to the core distributions of E. bicostata and E. maidenii then those species might be expected to perform better than E. globulus, the core of which comes from much higher latitudes (Jordan et al., 1993) (Figure 1).
The lack of a species × site effect for both growth variables (DBH and height), and the consistent high performance of E. maidenii, suggest that all three test locations are well suited to that species but not to the other three blue gum species. However, the three test sites varied greatly in early productivity, for reasons that could include: varying rainfall, nutrient availability or evapotranspiration among the sites, out-of-season frosts, and/or establishment practice. The three trial sites all had different pre-plant sprays, for example; broadcast at McKinnon, spot mounding and spraying at Otaua, and ripping and mounding at Kawerau followed by pre-plant spray. Regardless of the variation in environmental conditions among the three test sites, the ranking of the species was the same at each site. This result suggests that the climate of all three New Zealand test sites more closely matches the natural climate range of E. maidenii than those of the other species.
Warm, wet summer weather experienced at the three test locations is conductive to fungal Mycosphaerella leaf disease outbreaks (Park, 1988; Carnegie, 1994) that cause leaf necrosis and defoliation resulting in reduced growth (Park and Keane, 1982). Of the eucalypt species assessed at the three test locations only E. maidenii (with the exception of the Wollemi provenance of E. bicostata which grew well compared to the other provenances of this species tested) originate from regions that experience uniform rainfall throughout the year (Boland et al., 1992). Having evolved in regions where Mycosphaerella is more severe, E. maidenii is likely to have undergone more intense natural selection to such pathogens making them more resistant. This theory would explain the better foliage health and growth rate experienced by E. maidenii assessed in this study. Other studies have noted similar associations between high rainfall and warm temperatures and lower susceptibility to Mycosphaerella spp. in provenances of E. globulus (Dungey et al., 1997; Carnegie, 1994).
In addition to Mycosphaerella resistance, the three species (Eucalyptus maidenii, E. globulus and E. bicostata) may differ in their physiological adaptation to the test site conditions. For example, Anekonda et al. (1999) reported differences in respiratory metabolism between two Eucalyptus subgenera. It is presumed that physiological differences may be detected among species within subgenera, particularly where the species are related and are distributed along a climatic cline. Such variation is suggested by the latitudinal cline in lignin content among blue gum species (Rencoret et al., 2007), and the higher tolerance of inter-tree competition exhibited by E. maidenii versus E. globulus and other eucalypt species in even-spaced monoculture (Low & Shelbourne, 1999). The functional adaptation of genotypes to latitude is suggested within E. globulus, whereby density, and pulp yield show particular patterns of variation with latitude (Stackpole et al., 2010b). The broad latitudinal range of E. bicostata encompassed both the winter and uniform rainfall zones. The most northern E. bicostata provenance (Wollemi) in the present study occurred in a uniform rainfall zone, and displayed the best growth rate and foliage health of the seven E. bicostata provenences tested in the New Zealand test sites. This result is similar to the findings of (Komakech et al. 2009), although the causality of this association remains to be demonstrated.
The superior growth of E. maidenii found in this study compared with the other three blue gum species reflects the findings of (Shelbourne et al. 2002) from longer-term large-plot comparisons with E. globulus. This trait, plus the reasonable pulping qualities (Kibblewhite et al., 2000), high density (McKinley et al., 2002), and the favourable silvicultural characteristics of E. maidenii, seem to indicate the suitability of this species for projects where biomass, carbon or nutrient accumulation are contributing objectives. Similarly, the density of blue gum species varies; with E. maidenii reported at 550 kg m-3 (McKinley et al., 2002), well in excess of published density of E. globulus (470 kg m-3) from that and other studies (Miranda et al., 2001; Stackpole et al., 2010a).
While the results here have shown clear preference to E. maidenii, it is known that genotypes grown in mixtures do not always reflect the performance of genotypes in large plots (Fasoula and Fasoula 1997; Retief et al., 2001; Stanger et al. 2011). However, (Shelbourne et al. 2002) also found E. maidenii to be among the best performers in large-plot trials. Therefore consider that results are sufficiently robust to: (a) conclude this species outperforms the other blue gums tested; and (b) to recommend this species as suitable for further domestication for New Zealand conditions. In order to undertake this recommendation, further E. maidenii introductions from natural origins would be required. The lack of a significant provenance effect in the E. maidenii in these trials suggests that a collection based on range-wide open-pollinated families from well-defined populations is possible. A sampling strategy that obtains 200–400 family seedlots from across the provenances, and selection for key pulp production traits including density and pulp yield is recommended. Such a venture could be established in partnership with Australian co-operators with a view to characterising genotype x environment interaction.
The site at Kawerau was severely frosted in the first growing season with around three-quarters of the trees losing their foliage. The E. nitens plots suffered only minor damage, and E. maidenii appeared to be slightly less affected than E. bicostata, E. globulus, or E. pseudoglobulus. The majority of the plants re-sprouted and grew to crop trees, but this incident serves as a warning to planting these species in non-coastal regions of New Zealand. Relatively few frosts are predicted per year at Kawerau (~15, Table 2) compared to the conditions experienced in the species’ natural ranges (Table 1). However, it is likely that New Zealand is more prone to out-of-season frosts compared to Australia due to its maritime climate, and these frost events, rather than total number of frosts, are more damaging to young plants. Traditionally, small seedlings have been planted out in winter (these trials included) but planting in late spring is likely to improve survival. Spring-planted seedlings are likely to grow sufficiently in the first season to survive any frosts that occur the following year.