Neil J. Davidson

Photosynthetic responses of Eucalyptus nitens (Deane and Maiden) Maiden to green pruning

Abstract Three-year-old Eucalyptus nitens (Deane and Maiden) Maiden trees and 1-year-old ramets of a single clone of E. nitens were pruned to remove 0, 50% or 70% of the green crown length. This was equivalent to removal of 0, 55% or 88% of foliage area of trees, and 0, 77% or 94% of foliage area of ramets. CO2 assimilation (A) and stomatal conductance (gs) were measured at constant illumination in five height zones and three foliage-age classes of trees over a 16-month period following pruning. Foliar nitrogen (N) and phosphorus (P) concentrations were determined for each measurement time during the first 12 months of the experiment. In ramets A and gs were measured in four height zones and two foliage-age classes over a six-week period, and N and P concentrations were measured only once, at the end of the experiment. Rates of A increased by up to 175% following pruning. This response occurred throughout the canopy irrespective of position in the crown or foliage age. The magnitude of the response was generally greater in ramets than in trees, and increased with increasing severity of pruning. The initiation of the response was later, and the duration of the response was longer, in trees than ramets. In the lower crown of trees there was evidence of delayed senescence following pruning. Photosynthetic enhancement was not related to changes in foliar N concentrations. The ratio of A/N increased in many zones following pruning, especially after more severe defoliation. There was no evidence that changes in P concentrations were responsible for the result. The increases in A may have been related to changes in gs, as maximum values of gs were greater, and the ratio of A/gs was generally lower, in pruned than unpruned plants.

Cold hardening reduces photoinhibition of Eucalypts nitens and E. pauciflora at frost temperatures

Abstract Photoinhibition of photosynthesis at low temperatures was investigated in two species of subalpine eucalypt, Eucalypts nitens (Deane and Maiden) Maiden and E. pauciflora Sieb. ex Spreng. Imposition of an artificial cold-hardening treatment increased the frost tolerance of leaf tissue and increased tolerance to excess light. Cold-hardened seedlings of both species had a higher photosynthetic capacity than non-hardened seedlings at 6 and 16°C and lower levels of non-photochemical quenching (NPQ) at 20 and 5°C. Furthermore, hardened seedlings had faster rates of NPQ development at 5 and −3.5°C. An increase in minimal fluorescence, which indicates slowly reversible photoinhibition, was evident in all seedlings at −1.5 and −3.5°C but was less pronounced in hardened seedlings, with a threefold faster rate of development of NPQ, at −3.5°C than non-hardened seedlings. Hardened seedlings also recovered faster from photoinhibition at −3.5°C. Thus cold hardening increased tolerance to high light in these species. Differences between E. nitens and E. pauciflora in their response to excess light were small and significant only at −3.5°C. Faster recovery from photoinhibition of E. pauciflora was consistent with its occurrence in colder habitats than E. nitens.

Does low-intensity surface fire increase water- and nutrient-availability to overstorey Eucalyptus gomphocephala?

The objective of this study was to investigate how the management practices of prescribed fire and understorey vegetation removal affect water and nutrient relations of old, yet prematurely declining Eucalyptus gomphocephala. Long unburnt sites were established in Yalgorup National Park, Western Australia, adjacent to frequently burnt state forest sites. Trees were allocated to vegetation clearing, prescribed fire or no prescribed fire treatments. Prescribed fire was achieved in only one long unburnt national park site so that the results were pseudoreplicated but analysed accordingly. Soil chemistry, plant nutrient availability and tree foliar carbon and nitrogen isotope ratio and nutrient concentration were investigated. No effects of vegetation clearing were found. Prescribed fire sites were associated with sky exposure and bare ground whereas no prescribed fire sites were associated with shrub and litter cover and litter depth. Foliar carbon isotope ratios were significantly more negative in prescribed fire, relative to no prescribed fire, treatments on long unburnt sites. Soil exchangeable Zn and Mn and plant available (estimated by charged resin beads) Mg were higher on prescribed fire, relative to no prescribed fire, long unburnt sites. Seedling bioassays indicated elevated P and Cu availability on prescribed fire, relative to no prescribed fire, treatments. In overstorey E. gomphocephala, foliar N levels were elevated (but not to excessive levels), and there was a trend toward elevated foliar Mn, in prescribed fire relative to no prescribed fire treatments on long unburnt sites. In the context of our large-scale pseudoreplicated case study, prescribed fire provided a pulse of water and N, (with some indications towards provision of elevated Mn, Cu and Mg) availability to E. gomphocephala in decline on sites with a history of a long absence of fire that may in part underpin observations of elevated tree health on sites that have a history of relatively frequent fire.

Pigment dynamics during cold-induced photoinhibition of Acacia melanoxylon

Long-term acclimation of photo- and pigment-chemistry was investigated in a naturally-regenerating stand of Acacia melanoxylon R.Br. ex Ait. A pronounced decrease in photochemical efficiency of A. melanoxylon saplings was observed between autumn and winter in both thinned and unthinned treatments, but the decrease was more severe in the thinned treatment. Associated pigment changes in the unthinned treatment included a decrease in total chlorophyll content and a rise in chlorophyll a : b. Similar acclimation occurred in the thinned treatment with additional increase in zeaxanthin per unit chlorophyll observed. Saplings in the thinned treatment were exposed to lower minimum temperatures, more hours of frost and higher light intensities in the mid- to lower-crown. Growth chamber studies of the short-term acclimation of photo- and pigment-chemistry were conducted in a low / high light and cold / warm temperature factorial experiment. Photochemical efficiency and quantum yield adjusted within one day and then remained constant for 10 d in response to the imposed treatments. Chlorophyll concentration had decreased in all treatments by day 2 in the growth chambers, and subsequently increased in warm, but not in cold, treatments, irrespective of light level by day 10 in the growth chambers. The concentration of lutein-5,6-epoxide decreased in response to the cold-high light treatment and increased in response to other treatments by day 10 in the growth chambers, consistent with a function in sustained photoprotection in leaves of shade-adapted species. Our experiments indicated that A. melanoxylon is susceptible to cold-induced photoinhibition under cool temperatures (2-8°C) and moderate light intensities (450 μmol m-2 s-1).

Reflectance and phenolics of green and glaucous leaves of Eucalyptus urnigera

On MountWellington, Tasmania, Australia, there is an altitudinal cline in the juvenile leaf form of Eucalyptus urnigera from a green phenotype at lower altitudes (from 610 m) to a glaucous phenotype at higher altitudes (up to 1050 m). The objectives of this study were to (1) quantify reflectance of ultraviolet (UV) and photosynthetically active radiation (PAR) for the adaxial and abaxial surfaces of leaves of the glaucous and green phenotypes and (2) to investigate seasonal adjustment in photochemical efficiency (Fv/Fm) and tannin and flavonol compounds that absorb in UV spectrum. The following two research sites were identified: one at low altitude (at 910 m, solely green phenotype) and one at high altitude (1004 m, solely glaucous phenotype) on the E. urnigera cline. During the experimental period, average summer temperatureswere similar at the two sites but in winter, average temperatureswere 5–6◦C lower and minimum temperatures 0.4–0.7◦C lower at the high-altitude site occupied by the glaucous phenotype. Incident PAR was similar at the two sites. Reflectance was higher for glaucous leaves than for green leaves across the light spectrum of UVC (190–280 nm), UVB (280–320 nm), UVA (320–400 nm) and PAR (400–700 nm). Reflectance was significantly higher from the abaxial than from the adaxial surface of leaves. Fv/Fm was similar for glaucous and green leaves in summer but significantly lower in green than glaucous leaves in winter. Fv/Fm was significantly lower in winter than in summer for both leaf types. Tannins were significantly higher in green than glaucous leaves but there was no effect of season. Flavonol concentrations were similar for leaves of both types and did not vary with season. We conclude that high reflectance of UV radiation and PAR may confer benefit at the cooler, high-altitude portion of the cline of E. urnigera and that low reflectance of PAR may confer benefit at the relatively warm and densely vegetated, low-altitude portion of the cline of E. urnigera where interand intra-specific competition for light is likely to be intense. Introduction The

Early seedling establishment on aged Tasmanian tin mine tailings constrained by nutrient deficiency and soil structure, not toxicity

Revegetation of exposed wastes at historically abandoned mines is frequently constrained by inherent characteristics of poorly developed and contaminated soils. We tested whether the establishment of seedlings on 85-year-old arsenic rich tailings deposits at an abandoned tin mine in north-east Tasmania was limited by toxicity, nutrient limitation or structural factors. We conducted soil analyses, and tested growth of six native species in pot studies utilising both fertiliser treatments and a replacement series involving tailings and potting medium. An in situ three-year field trial was also conducted to assess the effects of adding sand, compost and biochar on plant growth and water infiltration. Analyses of the tailings identified a finely laminated structure and potential manganese, magnesium and potassium deficiencies. There was no evidence of microbial inhibition or adverse metals toxicity, pH or salinity effects. The pot trial indicated nutrient limitation acting on each of the species tested. Physical amendment within the field trial resulted in a highly significant and sustained improvement in infiltration rate, as well as improved growth in three of the six species tested. We conclude that the correction of nutritional and physical deficiencies in surface soils should overcome limitations to the early establishment of native seedlings at this site. This study shows that systematic site assessment and targeted trials is a valuable first step in the revegetation of previously recalcitrant sites.