CSIRO PUBLISHING
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Australian Journal of Botany, 2008, 56, 44–50
Patterns of growth and nutrient accumulation in expanding leaves of Eucalyptus regnans (Myrtaceae) Jacqueline R. EnglandA,B,C and Peter M. AttiwillA A
School of Botany, The University of Melbourne, Vic. 3010, Australia. Present address: CSIRO Forest Biosciences, Private Bag 10, Clayton South, Vic. 3169, Australia. C Corresponding author. Email:
[email protected] B
Abstract. Patterns of leaf growth and nutrient accumulation were investigated in relation to leaf ontogeny in the tree species Eucalyptus regnans F.Muell. Newly emergent leaves were tagged in the field and collected every 14 days for measurement of leaf dimensions and nutrient concentrations over a 113-day period. Patterns of growth in area, length, width and mass of leaves followed sigmoid curves. An exponential rate of growth for all measures was observed up to 56 days after leaf emergence, after which there was little increase. Conversely, specific leaf area (leaf area/leaf mass) decreased from emergence to about Day 56 and then remained relatively constant. Contents of all nutrients (measured on a leaf basis) increased during leaf expansion. Concentrations of N, P and K decreased and Ca concentration increased, but there was no clear trend for Mg concentration with leaf development. In general, the results of the present study verify previously developed ‘idealised curves’ of changes in dry mass and nutrient concentrations with leaf age for eucalypts. Patterns of leaf growth and nutrient accumulation (particularly N) show that leaves had reached full expansion and physiological maturity by ∼80–90 days after emergence. Introduction As a leaf grows, the concentrations of nutrients within the leaf change. In coniferous and deciduous trees, where it is relatively easy to determine the age of needles/leaves because all needles/leaves in the same cohort are initiated at the same time, several studies have measured changes in nutrients during needle/leaf expansion (e.g. Fife and Nambiar 1982; Oleksyn et al. 2000). However, in fast-growing, broad-leaved evergreens such as eucalypts, leaf age is more difficult to determine because shoot growth is continuous rather than determinate, and therefore we only have broad indications of changes in nutrient concentrations during expansion. Because the leaves of most species in the genus Eucalyptus are heteroblastic—that is, they show distinct stages of juvenile, intermediate and adult leaf forms (Blakely 1955)—studies of leaf development in eucalypts have often been related to the ontogenesis from juvenile to intermediate to adult leaf forms (e.g. Ashton 1956; Cameron 1970). Heteroblasty may have functional significance; for example, juvenile foliage may be shade-adapted (Ashton 1956; Cameron 1970), and glaucous juvenile foliage in some species may confer resistance to low temperatures (Thomas and Barber 1974) and insect attack (Edwards 1982). More recently, developmental changes in the morphology and anatomy of fully expanded, adult leaves of E. regnans have been reported in relation to tree age and height (England and Attiwill 2006). However, to our knowledge, there are no published studies that have followed leaf development from emergence to full-expansion in a given leaf form in eucalypts. While there is some information on changes in leaf size (Ashton 1975) and photosynthesis (Choinski et al. 2003) during © CSIRO 2008
leaf expansion in eucalypts, there is little information on changes in leaf nutrients. Attiwill and Leeper (1987) presented idealised curves of changes in dry mass and nutrient concentrations with leaf age for eucalypts in the absence of any published data. They predicted a decrease in concentrations of P, N, K and Mg, and an increase in the concentration of Ca. The aim of the present study was to investigate patterns of leaf growth and nutrient accumulation in expanding leaves of E. regnans. Materials and methods Site description The study region was in the Toolangi State Forest, ∼70 km north-east of Melbourne, Australia (37◦ 28 S–37◦ 32 S, 145◦ 27 E–145◦ 31 E). The climate is cool-temperate, characterised by cool, wet winters and warm, dry summers. Mean annual rainfall is ∼1400 mm. Mean daily maximum/ minimum temperatures of the hottest (February) and coldest (July) months are 22.9/12.0◦ C and 8.4/3.8◦ C, respectively. The soils of much of the region are Krasnozems (Stace et al. 1972) or Brown Earths, and have been described previously (Polglase and Attiwill 1992). Eucalyptus regnans (mountain ash) is the world’s tallest hardwood, growing to a height of at least 98 m (Hickey et al. 2000). E. regnans grows in cool-temperate areas of south-eastern Australia, in Victoria and Tasmania (Boland et al. 1992). The leaves of E. regnans are juvenile only at the seedling stage, and there are distinct morphological, anatomical and functional differences between adult and juvenile leaf forms (see Ashton and Turner 1979). Leaf development was studied in a young stand of E. regnans located within the Toolangi State Forest, at an elevation of 590 m 10.1071/BT07053
0067-1924/08/010044
Growth and nutrient accumulation in expanding E. regnans leaves
above sea level, with a slope of 4◦ in a SSW aspect. The stand was regenerated in 1992 following clear-felling and burning operations and was aged ∼3.5 years at the commencement of sampling. The young, even-aged stand of E. regnans formed ‘wet sclerophyll forest’ (Beadle and Costin 1952). At the beginning of the study, the E. regnans saplings were just emergent above a layer of Acacia dealbata Link (silver wattle). Beneath this, there was a layer of shrubs including Pomaderris aspera Sieber ex DC. (hazel pomaderris), Cassinia aculeata (Labill.) R. Br. (common cassinia), Coprosma quadrifida (Labill.) B.L.Rob. (prickly coprosma) and Olearia phlogopappa (Labill.) DC. (dusty daisy-bush). A ground layer of ferns, herbs, grasses and sedges included Pteridium esculentum (G.Forst.) Cockayne (austral bracken), Polystichum proliferum (R.Br.) C.Presl (mother shield-fern), Tetrarrhena juncea R.Br. (forest wiregrass), Lepidosperma elatius Labill. (tall saw-sedge) and Viola hederacea Labill. (ivy-leaf violet). Daily rainfall and temperature data were obtained from the nearest Bureau of Meteorology station, ∼8 km from the study site. Total rainfall for the study period (December 1995 to March 1996) was 596 mm, which was considerably higher than the long-term average of 377 mm (Table 1). February, normally the month with the lowest rainfall, was unusually wet during the study period, with rainfall more than 2.5 times that of the long-term average (Table 1). Monthly means of daily maximum temperatures were lower in December 1995 and February 1996 (by 4.2 and 1.8◦ C, respectively) than for the long-term means, whereas temperatures in January and March of 1996 were very similar to the long-term means. Tagging methods Tagging studies have been widely and successfully used for conifers, where it is possible to tag branches rather than individual needles, because all needles in the same cohort are initiated at the same time. In contrast, tagging studies in eucalypts are often problematic because the pattern of shoot growth is continuous rather than determinate, and therefore it is difficult to identify groups of leaves of the same age. Shoot growth of E. regnans follows the general pattern described for eucalypts by Jacobs (1955). In general, one bud produces two axillary branches with associated leaves, and a new growing apex. Leaves form in clearly defined pairs and are opposite in their early stages. Intranodal growth occurring between individuals of pairs of leaves results in opposite leaves Table 1. Comparison of total monthly rainfall and mean daily maximum temperature for the months December 1995–March 1996, with long-term data for the period 1953–1996 (Source: Bureau of Meteorology, Victorian Regional Office, Australia) Month
Total rainfall (mm) 1995–1996 Long-term
Mean daily maximum temperature (◦ C) 1995–1996 Long-term
December January February March
135.0 149.2 212.0 99.8
125.5 86.0 77.8 87.7
16.3 22.0 20.1 20.5
20.5 22.4 22.9 20.3
Total
596.0
377.0
–
–
Australian Journal of Botany
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of the same age being separated by several centimetres. Most eucalypt leaves live less than 18 months (Jacobs 1955), and this is also the case for E. regnans, where most leaves fall in their second year, although a large proportion do so in their first (Ashton 1975). The method of leaf tagging used in the present study was a modification of the technique used by Polglase (1989). Numbered plastic tags were attached to shoots in the upper crown with thin, plastic-coated wire. Newly emergent leaves are delicate and easily damaged by tagging. Therefore, tags were placed several intranodes/internodes from the leaf, lower down on the branch. A description of the position of the leaf relative to its tag was recorded. Sampling Ten trees were selected within the stand on the basis that they had a sufficient number of newly emergent leaves in the upper crown for tagging, and were at least 10 m from the boundary road. As a result, sample trees were quite widely separated within the stand. Mean diameter at breast height ± standard error of the mean of the sample trees was 3.6 ± 0.5 cm and mean height ± standard error of the mean was 5.7 ± 0.6 m. Newly emergent pairs of leaves (where the intranode had not begun to elongate) were tagged in the upper third of the tree crown on 30 November and 1 December 1995. About 90 leaves per tree were tagged to allow for leaf losses resulting from herbivory and senescence during the study period. Tagged leaves were small (length
