Comparative performance of Eucalyptus bicostata, E. globulus, E. maidenii and E. pseudoglobulus on three northern New Zealand sites

The blue gum group comprises four closely related taxa that are variously treated as separate species Eucalyptus bicostata Maiden, Blakely & Simmons, E. globulus Labill., E. maidenii F. Muell., and E. pseudoglobulus Naudin ex Maiden (Brooker, 2000) or as subspecies of E. globulus (Jordan et al., 1993; Kirkpatrick, 1975). For this paper, they are treated as separate species. The species complex appears to have radiated from an origin in eastern Victoria (Jones, 2002), with well-defined core populations (Figure 1), and extensive intergrades between species at the boundaries. Eucalyptus globulus has the most southerly distribution of the four species, occurring in Tasmania, the Bass Strait Islands and the southern extremities of eastern Victoria (Table 1). The core distributions of the other blue gums are more northerly than that of E. globulus (Figure 1). Of these, E. pseudoglobulus intergrades with E. globulus, E. bicostata and E. maidenii(Jones, 2002), from its east Victorian core. Eucalyptus bicostata occurs on the inland watershed of Victoria and southeast New South Wales, is geographically separated from other species, occupies a substantial latitudinal range, and has extreme outlying populations such as Mount Bryan (South Australia) (Vaillancourt et al., 2001), and Wollemi (New South Wales). Over this range, two rainfall distribution zones are recognised: (1) predominantly winter rainfall in the south; and (2) uniform annual rainfall at Wollemi^a. Also, frosts are more common and severe in the south. The range of E. maidenii occurs on the coastal catchments of south-eastern New South Wales, which experiences a more uniform rainfall distribution (Bureau of Meteorology 2012).

Eucalyptus globulus, along with its intergrades with E. pseudoglobulus, has proved suited to both kraft pulp production (Kibblewhite et al., 2000), and solid wood products (Boland et al., 1992); and together have formed the genetic base of large-scale plantations across xeric, temperate, winter-maximum rainfall landscapes in the Iberian Peninsula, Australia and South America (Lanfranco & Dungey, 2001; Potts et al., 2004; Díez 2005). Species × provenance trials in Tasmania (Volker & Orme, 1988) and Portugal (Almeida et al., 1995) confirm the superiority of E. globulus over the other blue gum species at sites with a dry summer climate but with good winter rainfall. However, different specific rankings have been reported from tests in warmer climates with more uniform seasonal distribution of rainfall. These include: anecdotal reports from central Africa and southern China (Eldridge et al., 1993); and formal comparisons among the species in northern New Zealand (Low & Shelbourne, 1999) and South Africa (Komakech et al., 2009). When compared in ex-situ species comparison tests, the relative performance of these species is expected to be related to the fit between the environmental parameters of the collection and test sites, and such relationships have been considered in planning introduction trials (e.g. Booth, 2005).

Variations in disease resistance (Hood et al., 2002), wood chemical composition (Rencoret et al., 2007), and pulp yield (Kibblewhite et al., 2000) have been demonstrated among species. Genetic variation within and among subraces, which has been demonstrated in many E. globulus traits, is likely to be similarly exploitable within the other three blue gum species (Dutkowski & Potts, 1999; Jordan et al., 1993; Nesbitt et al., 1995; Potts et al., 2004). This has already been reported for disease resistance in young E. bicostata (Hood et al., 2002).

The wood of the blue gum complex is recognised for its suitability for producing chemical pulps (Cotterill & Brolin, 1997; Kibblewhite et al., 2000; Miranda & Pereira, 2001), and has higher density, modulus of elasticity, modulus of rupture, and lower shrinkage than that of E. nitens (Deane & Maiden) Maiden, a common plantation alternative (McKinley et al., 2002). In New Zealand, research has shown that E. maidenii matched E. globulus in growth and foliage health (Low & Shelbourne 1999). Although the pulp yield of E. maidenii, was lower (Kibblewhite et al., 2000), its density was higher by some 50 kg m^-3 (Jamieson, 1997).

In 1999, a series of species / provenance trials were established in New Zealand, designed to assess the potential of E. bicostata, E. globulus, E. maidenii and E. pseudoglobulus, and to gain insight into the effects of different provenances of these species across three sites in New Zealand that were located within the equivalent range of the Australian populations. This paper presents and discusses the results of growth, form and health assessments of these trials at age seven years.

The stocking at each trial was 1114 stems per hectare, equivalent to 3.5 m between rows and 3 m within rows. McKinnon was an ex-pasture site, with a 2-year-old crop of pine which was pre-sprayed with glyphosphate (10 L ha^-1) by helicopter. The area was also fenced off at the time of planting due to concern about damage from wild goats. The trial area was spot-mounded. Otaua was an ex-pasture site that was spot-cultivated using a Savanah Cultivator head (spot size: 1.20 m) to a depth of 60 cm. A mixture of glyphosphate 6 L ha^-1 + terbuthylazine 20 L ha^-1 was applied to each spot at a rate of 30 mL. Kawerau was on an ex-pasture site prepared by ripping and mounding and a pre-plant application of terbuthylazine 6 L ha^-1 + glyphosphate 4 L ha^-1 + organo-silicone penetrant 500 mL ha^-1, water rate 150 L ha^-1.

Traits measured were:

At age 7 years, the inner 25 trees of the original 49 trees planted per block were measured. The inter-planted E. globulus seedlots rapidly suppressed the two E. pseudoglobulus entries, which were excluded from the analysis. The block containing the E. nitens and E. globulus control seedlots was not included in the analysis. Data were analysed using ANOVA to test for species, site, and provenance effects, and the interactions species × site, and provenance (species) × site. Owing to a combination of the layout, varying numbers of replicates at each site, and uneven number of individuals in each provenance, F-ratios (Satterthwaite-type approximate tests) were obtained using the TEST option within the RANDOM statement in SAS.

The models were simplified where trait classes had some missing values, in order to enable estimation of least-squares means.

These analyses were performed using the Proc GLM procedure in SAS 9.1. Tukey’s multiple range test was used to indicate significance of difference among sites, species or provenance within species means for all traits.

The suppression of Eucalypus pseudoglobulus by E. globulus meant that results from only three species (E. globulus, E. maidenii and E. bicostata) are reported here.

Site effects

Using the full data set, site significantly affected all traits (p < 0.002, Table 4). The main site effects were DBH and HT, which were highest at McKinnon for all species (Figure 2a,b). The McKinnon site was by far (p < 0.001) the most productive, with DBH of 16.9 cm compared with Otaua 15.4 cm and Kawerau 13.2 cm; and HT at McKinnon of 15.6 m compared with Otaua 12.2 m and Kawerau 10.4 m at age 7 years (Figure 2a,b). Further, McKinnon was ranked first overall for DBH, HT, FH, GCH and STR (Table 7). Trait means for Otaua were usually intermediate, except for SUR (highest) and FH (lowest) (Table 7). Kawerau was severely frosted in the first growing season, and this appears to have contributed to its lowest rank for all traits except for FH.

Effect	Levels	Trait
		DBH	HT	GCH	STR	FH	SUR
Species	E. maidenii	15.2 a	12.7 a	6.5 a	5.7 a	5.4 a	0.81 a
	E. globulus	10.4 b	9.9 b	4.9 b	5.1 b	2.6 b	0.57 b
	E. bicostata	10.2 b	9.0 b	4.0 b	4.3 c	3.0 b	0.71 a
	E. nitens1	15.8	13.9	8.3	3.3	7.1	0.81
Site	Otaua	11.7 b	9.9 b	5.1 b	5.2 a	3.2 b	0.87 a
	Kawerau	10.1 c	8.2 b	3.3 c	4.4 b	3.8 a	0.47 c
	McKinnon	13.9 a	13.6 a	7.0 a	5.4 a	4.0 a	0.75 b

Demonstration plots of E. nitena included as a comparison.

¹ Differences for E. nitens were not tested against blue gum species means. Values for species means (within each trait group) when followed by the same letter are not significantly different (p > 0.05).

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.

Eucalyptus maidenii appears to be the best-suited blue gum species to sites in the northern half of New Zealand’s North Island. This may be due to similarities in climatic characteristics among the trial sites and natural E. maidenii populations. The relatively close grouping of natural provenances and variation in E. maidenii are unlikely to affect which provenances should be used for the selection of genetic material for further development in New Zealand, except that as wide a collection as possible would help ensure a broad genetic base.

Adaptation to uniform rainfall climate is also evident in the superior performance of the northerly Wollemi provenance of E. bicostata in the present trial series.

^aThe range of Eucalyptus bicostata extends to sites in northern New South Wales (e.g. outlier populations in the Walcha area) that experience a third type of rainfall pattern, summer maximum rainfall distribution. This type of site was not represented in the New Zealand trials.

^bwhere 0 = dead or dying, 1 = severe damage, 2 = bad damage and tree obviously suffering, 3 = moderate-to-bad damage, 4 = moderate damage, but above average, 5 = moderate damage, but below average, 6 = low-moderate damage, 7 = damage clear but low, 8 = some damage evident, 9 = healthy.