Carbon and nitrogen isotope discrimination and nitrogen nutrition of trees along a rainfall gradient in northern Australia

Transcription

1 Aust. J. Plant Physiol., 1998, 25, CSIRO Australia 1998 Carbon and nitrogen isotope discrimination and nitrogen nutrition of trees along a rainfall gradient in northern Australia E.-D. Schulze A, R.J. Williams B, G.D. Farquhar C, W. Schulze D, J. Langridge E, J.M. Miller C and B.H. Walker E A Max-Planck-Institute of Biogeochemistry, Sophienstr. 10, Jena, Germany. Corresponding author; Detlef.Schulze@BGC-Jena.MPG.de B Division of Wildlife and Ecology, CSIRO, PMB Box 44 Winnellie, NT 0822, Australia. C RSBS-Environmental Biology. Group, Australian National University, GPO Box 475, Canberra, ACT 2601, Australia. D Botanisches Institut, Universität Tübingen, Auf der Morgenstelle 1, Tübingen, Germany. E Division of Wildlife and Ecology, CSIRO, PO Box 84, Lyneham, ACT 2601, Australia. Abstract. Carbon isotope discrimination ( ) and nitrogen isotope ratios, N-concentrations and specific leaf area of 50 tree species were investigated along a continental-scale transect through northern Australia over which annual rainfall varied from 1800 mm to 216 mm rainfall. Average specific leaf area (SLA, m 2 kg 1 ) of leaves ranged from 10.7 ± 1.7 (av. ± s.d.) in N 2 fixing deciduous trees to 0.8 ± 0.4 in spinescent sclerophylls shrubs. SLA generally decreased with increasing aridity. N 2 fixing species had higher leaf N concentration (average N-concentration 20.1 ± 3.7 mgn g 1 ) than non- N 2 fixing (10.8 ± 3.3) or spinescent species (7.05 ± 1.8). Community-averaged was approximately constant at rainfalls above 475 mm (average = 19.4 ± 1.2 ). Where rainfall was less than 475 mm, decreased from 19 to 17 at 220 mm. was positively correlated with SLA. of deciduous N 2 fixing species and spinescent species were 1 and 2.4 lower than in evergreen sclerophyllous species. in the N 2 fixing Allocasuarina was 1.2 lower than in non- N 2 fixing sclerophyllous species. The δ 15 N-values indicated N 2 fixation only at high rainfall. Burning of the field layer in a Eucalyptus forest had no effect on all measured tree parameters including δ 15 N, but δ 15 N increased under grazing conditions to >5. The constant value of community averaged between 1800 and 450 mm may be the result of replacement of functional types and species. The decline in in the more arid section may be a function of both low species diversity, and a highly aseasonal and unpredictable rainfall regime. Keywords: carbon and nitrogen isotope ratios, nitrogen concentration, specific leaf area, drought, burning, grazing, plant functional types, Eucalyptus. Introduction Northern Australia exhibits strong aridity gradients, generally within a wet-dry monsoonal climate. Mean annual rainfall ranges over an order of magnitude, from about 2000 mm on Melville Island off the coast of Darwin to 200 mm in the center of the continent. Over most of northern Australia, the native savanna vegetation has remained relatively intact, because land-use has concentrated primarily on cattle grazing, mining and nature conservation, while activities such as forestry and agriculture have been negligible. The woodland vegetation of northern Australia is burnt regularly with effects on tree species composition (Lonsdale and Braithwaite 1991). Such features make this region globally unique for the study of the effects of conitental-scale variation in aridity on the responses of trees (Williams et al. 1996). Patterns of carbon isotope discrimination ( ), which is negatively related to water use efficiency, nitrogen isotope ratios (δ 15 N) as an indicator of N 2 fixation and ecosystem N- cycling, N-concentrations as measure of N-nutrition and specific leaf area (SLA) were measured for species of several plant functional types. Along continental-scale gradients elsewhere in the world, it has been shown that, for herbaceous species and shrubs, increasing aridity can result in increasing stomatal closure and associated decrease in carbon isotope discrimination of leaves (Ehleringer and Cooper 1988; Ehleringer, 1995; Comstock and Ehleringer 1992; Lin et al. 1996). In contrast to these studies on herbs and shrubs, the responses of trees have not been investigated as thoroughly. Stewart et al. (1995) showed a linear decrease in community averaged carbon isotope discrimination with decreasing annual rainfall of south-eastern Queensland. However, in that case, woody and non-woody species were apparently averaged and the response of trees remains unclear. In contrast, Read and Farquhar (1991) observed in a common garden (i.e. in a constant test environment) a greater carbon isotope discrimination in species originating from drier sites. This raises the possibility that species-specific responses may /PP /98/040413

2 414 E. D. Schulze et al. dampen the effect of drought on community averaged carbon isotope discrimination ( ) in trees as was observed for the tree flora of Namibia (Schulze et al. 1976, 1991a), and for the vegetation of trees and shrubs along an aridity gradient in Patagonia (Schulze et al. 1996b). The aim of this paper is to quantify the relationship between isotope discrimination and increasing aridity along a continental-scale aridity gradient. We investigated carbon and nitrogen isotope ratios and associated leaf parameters (nitrogen concentration, specific leaf area) in relation to rainfall. We studied these parameters along the strong gradient in mean annual rainfall that exists in the Northern Territory, from the mesic northern section to the arid southern section. Given that species replacement patterns along the gradient may buffer any effect of drought on isotope discrimination at the community level, we examined the dominant Eucalyptus species in detail. Most of these species have relatively wide latitudinal ranges, and show some overlap in their distribution. They thus provide a framework for testing hypotheses concerning species replacement and its potential effect on isotope discrimination averaged over the community. Materials and methods The North Australian Tropical Transect The North Australian Tropical Transect (NATT) is one of the continental transects of the IGBP core project GCTE (Walker and Steffen 1997). It is approximately 900 km long, and reaches from Darwin (about 12 S) in the mesic north of the Northern Territory to the edge of the Tanamai Desert (near Kalkaringi, about 17 S). The NATT thus encompasses the majority of the Northern Territory landscapes where the occurence of the summer rains is predictable, in that a wet season occurs each year. However, in order to encompass a larger range of rainfall the present study extended this transect to Melville Island in the north (about 11 S) offshore from Darwin, to Giles and the Olgas in the center of the continent (about 26 S, Fig. 1). Along this geographic range, there is a strong gradient in annual rainfall which is closely related to latitude. Rainfall decreases from about 1800 mm on >100 raindays in the north to 200 mm on about 30 raindays in the south, while potential evapotranspiration increases from about 1800 mm to about 3500 mm (Table 1; Fig. 2). In the north the rainfall is consistently seasonal, with only 3% of the annual total falling in the 5 months between May and September in Darwin. In the south the seasonality is less pronounced. In the arid interior, where rainfall is less than 350 mm, the annual rainfall is very Fig. 1. Geographic location of the North Australian Tropical Transect and of the sample sites. Mel: Melville Island, Kap: Kapalga, Kath: Katherine, Vic: Victoria River, Kid: Kidman Springs, San: Mt Sanford, TC1: North Tennant Creek, TC2: Tennant Creek, TC3: South Tennant Creek, Kin: Kintore, LH: Lake Hopkins, TP: Tylers Pass, SB: Sandy Blight Junction, Gil: Giles, GJ: Giles Junction, MM: Mt. Miller, Olg: Olgas, AR: Ayers Rock. Rainfall map after Climate Atlas of Australia (1977).

3 Table 1. Locations of plant collection and collected species Material was collected between 8 and 26 October 1993 Deciduous N 2 fixing species Evergreen N 2 fixing species Evergreen sclerophylls Erythrophleum chlorostachys Er.ch. Acacia aneura Ac.an. Atalaya hemiglauca At.he. Lysiphyllum cunningamii Ly.cu. Acacia coriacea Ac.co. Carissa lanceolata Ca.la. Melaleuca sp. Me.sp. Deciduous non-n 2 fixing species Acacia dimidiata Ac.di. Eucalyptus bleeseri Eu.bl. Persoonia falcata Pe.fa. Brachychiton diversifolius Br.di. Acacia holosericea Ac.ho. Eucalyptus brevifolia Eu.br. Petalostigma quadriloculare Pe.qu. Brachychiton megaphyllus Br.me. Acacia mimula Ac.mi. Eucalyptus confertiflora Eu.co. Terminalia arostrata Te.ar. Buchanania obovata Bu.ob. Acacia olgana Ac.ol. Eucalyptus gummifera Eu.gu. Terminalia canescens Te.ca. Cochlospermum fraseri Co.fr. Allocasuarina decaisneana Al.di. Eucalyptus miniata Eu.mi. Xanthostemon paradoxus Xa.pa Eucalyptus clavigera Eu.cl. Cycas armstrongii Cy.ar. Eucalyptus nesophila Eu.ne. Eucalyptus porrecta Eu.po. Ventilago viminalis Ve.vi. Eucalyptus papuana Eu.pa. Eucalyptus tectifica Eu.tec. Eucalyptus pruinosa Eu.pr. Eucalyptus setosa Eu.se. Planchonia careya Pl.ca. Eucalyptus tectifica Eu.tec. Evergreen spinescent species Eucalyptus terminalis Eu.ter. Grevillea pyramidalis Gr.py. Eucalyptus tetrodonta Eu.tet. Cultivated fruit trees Grevillea juncifolia Gr.ju. Grevillea dimidiata Gr.di. Anacardium occidentale An.oc. Deciduous baobab Hakea leucoptera Ha.le. Hakea arborescens Ha.ar. Garcinia mangostana Ga.ma. Adansonia gregorii Ad.gr. Hakea divaricata Ha.di. Livistona humilis Li.hu. Mangifera indica Ma.in. Sites of sampling Coordinates Weather Median Mean Community Canopy Forest Collected species Location South East station ann. rain- rain height (m) density fall (mm) days av. (s.d) (trees ha 1 ) Melville I Pickertaramoor tall open forest 20.5 (4.6) 760 Co.fr.,Cy.ar.,Er.ch.,Eu.mi., Eu.ne.,Eu.tet.,Li.hu.,Pe.fa. Melville I Pickertaramoor plantation An.oc.,Ma.in,,Ga.ma. Darwin Airport plantation An.oc.,Ma.in.,Ga.ma. Kapalga Gunbalunya open forest 14.8 (2.6) 390 Ac.mi.,Bu.ob.,Er.ch.,Eu.cl., Eu.mi.,Eu.po.,Eu.tec.,Eu.tet., Li.hu.,Pl.ca.,Xa.pa. Katherine K. Council Depot open forest 11.5 (3.4) 156 Ac.di.,Br.di.,Br.me.,Bu.ob.,Co.fr., Er.ch.,Eu.co.,Eu.bl.,Eu.mi., Eu.tet.,Gr.py.,Ly.cu.,Pl.ca.,Pe.qu. Victoria R Timber Creek woodland 11.6 (4.2) 136 Ad.gr.,Er.ch.,Eu.mi.,Eu.tec. Kidman Springs Victoria R. Downs open woodland 8.5 (2.1) 122 Ac.ho.,Ca.la.,Eu.br.,Eu.pr.,Eu.tec., Eu.ter.,Ha.ar.,Ly.cu.,Me.sp. Mt.Sanford Kalkaringi spinifex woodland 6.7 ( 0.9) 66 Eu.br.,Eu.ter.,Gr.di.,Ha.le., Te.ar.,Ve.vi Tennant Creek Elliott spinifex woodland 7.8 (1.9) <10 Eu.pa.,Eu.se. Tennant Creek TC Post Ooffice spinifex woodland 7.5 (0.6) <10 Ac.an. Tennant Creek TC Post Office spinifex woodland 7.4 (0.6) <5 Eu.pa.,Eu.ter. Sandy Blight Jct Newhaven spinifex woodland <10 Ac.co.,Eu.ter.,Ha.di. Kintore Newhaven spinifex woodland 6.7 (1.8) Ac.co.,Eu.ter.,Ha.di. Tyler Pass Alice Springs spinifex woodland 6.0 (1.0) <5 Eu.ter. Lake Hopkins Giles spinifex woodland Al.di.,Gr.ju. Giles Giles spinifex woodland 9.3 (2.6) Ac.an.,Eu.ter. Giles Jct Giles spinifex woodland Al.di.,Eu.ter.,Gr.ju. Mt Miller Ayers Rock spinifex woodland Ac.an.,Eu.gu.,Eu.ter. Olgas Ayers Rock spinifex woodland Al.di.,Ac.an.,Eu.ter. Ayers Rock Ayers Rock spinifex woodland Ac.an.,Ac.ol.

4 416 E. D. Schulze et al. Fig. 2. Median annual rainfall (R, mm), potential evapo-transpiration (E, mm), average canopy height of trees (H, m), and forest density (D, trees ha 1 ) along the transect. unpredictable. Here, there is no guarantee of summer rains as there is in the north of the Northern Territory, and rain-free periods may be 2 or more years in length. Associated with this climatic gradient are distinct changes in the composition and structure of the vegetation (Wilson et al. 1990, Williams et al. 1996). Over most of the range the vegetation is savanna with a sparse tree cover over grass. In the north the savannas are dominated by >20 m tall forest (maximum height 27 m). In the south the vegetation is dominated by Spinifex grasslands on sand dunes with or without trees of about 7 m height growing in open savannas. Woodlands with tussock grasslands dominate the intermediate, semi-arid section of the NATT. Tree basal area decreases with decreasing mean annual rainfall. Most vegetation types are dominated by Eucalyptus. Woodland formations in the arid zone are dominated by Acacia aneura, but under some geological conditions Allocasuarina decaisneana may reach groundwater and form tall forest-like vegetations on sand dunes in an otherwise desert climate. Tree density decreased with rainfall from 760 to 10 tree ha 1. Plant material and sample locations Sun exposed leaves were collected at the end of the dry season in October 1993 at 20 locations (Table 1). At each site the main species dominating the tree canopy were sampled. The number of species sampled per site varied from 1 to 14. Between 3 and 5 individual trees were sampled per species in an area of about 1 hectare. Tree height and diameter at breast height were recorded for each sample tree. The sampling strategy was designed á priori to allow the collection of Eucalyptus species which were widespread in their distribution, and which showed overlap in their distribution with other, potentially co-ocurring species. All plots were flat habitats on light to medium textured soils (sands and loams), except for one sample site on a stony slope at Victoria River. Eucalyptus species which showed this pattern of overlapping distribution were E. miniata, E. papuana, E. tectifica, E. terminalis, and E. tetrodonta (Table 1). Erythrophleum chlorostachys, a tree legume, was sampled as a potentially N 2 fixing species of broad geographic distribution at high rainfall. Acacia aneura was the potentially N 2 -fixing species at low rainfall. Thus, species were collected under comparable edaphic conditions along their full range of distribution. In addition to the collection of Eucalyptus that show species replacement, accompanying sub-dominant tree canopy species and the taller woody species of the mid-stratum were collected in order to investigate tree-community-averaged carbon isotope discrimination. The Australian savanna tree flora consists of mixtures of deciduous and evergreen species in various proportions. For data analysis plant species were grouped into various functional types based on life form, leaf form and leaf phenology. The abundance of deciduous tree species declines with decreasing annual rainfall along the NATT (Williams et al. 1996). Hence, deciduous and evergreen trees were grouped separately, as were potentially N 2 fixing and non- N 2 fixing species. The bottle tree Adansonia gregorii was accorded its own because of its large storage stem. The spinescent species of Grevillea and Hakea were also treated as a separate group because of their distinct SLA. Allocasuarina was separated because of its characteristic assimilation organs (cladodes), and because of its potential for N 2 fixation (Boland et al. 1984). Leaves were also sampled from evergreen fruit tree plantations at Darwin and Melville Island as reference points for well watered and fertilised evergreen species. It would have been possible to define additional plant functional types (Egan and Williams 1996; Smith et al. 1997), but since plant traits do not always vary independently, for example rooting depth and leaf longevity (Schulze, 1982), we decided á priori to use this very simple scheme of classification which also represents the most obvious tree characteristics in the vegetation

5 Isotope discrimination and nutrition 417 of the Northern Territory. At the time of sampling the deciduous trees had begun to grow new leaves in the northern part of the transect. At the Kapalga site the collection included plots which were protected from burning (plot C, Q, M and S) and plots which had been burnt annually by June fires between 1990 and 1993 (plots B, K and P) (Anderson et al. 1998). Tree height and diameter at breast height were recorded for each sample tree. The potential effect of grazing was studied in a subset of sites where grazing intensity could be qualitatively assessed. Melville, Kapalga and Tyler Pass were apparently ungrazed by domestic stock, although Kapalga had been grazed by domestic water buffalo in the 1980s. In contrast, Kidman springs and Mt. Sanford were heavily grazed as indicated by the grass and weed cover and the damage to trees, while Kintore and Giles (Aboriginal land) were intermediate in grazing pressure. Sample analyses Leaves were sun-dried in the field, and oven dried again in the laboratory (80 C). Leaf area was determined by a leaf area meter (LiCor, Nebraska), and specific leaf area (SLA: m 2 kg 1 ) of leaves, and of spinescent assimilation organs such as in Hakea was calculated from oven dried leaf weights and projected leaf areas. It is possible that drying may underestimate leaf area by 5 10% in the deciduous species. Carbon isotope ratios of leaf samples of 250 tree samples were measured by an on-line mass spectrometer (Isomass, VG; precision 0.1 ) after combustion in an elemental analyser (Carlo Erba, Milano). Carbon isotope ratios were expressed as carbon isotope discrimination ( : ) by taking into account an assumed value for the isotope composition of the air ( 8 ) relative to the PeeDee Belemnite standard (Farquhar and Richards 1984). Nitrogen isotope ratios and nitrogen contents of samples were measured using a system combining an elemental analyzer (Heraeus CHN-O Rapid) for Dumas combustion of the samples, a Finnigan MAT Trapping box HT for automatic cryo-purification of the combustion products, and a Finnigan MAT mass spectrometer (delta D) with a dual inlet at a precision of 0.1 (Gebauer and Schulze 1991). Nitrogen contents were expressed as nitrogen concentrations on a leaf dry weight basis (mgn g 1 ). All differences between averages were tested by t-test at the P<0.05 level. The significance of trends was tested by ANOVA, where plant functional types were used as a factor, and by linear regression analysis, where the various traits were regressed against independent variables such as latitude, mean annual rainfall, and for some isotope parameters, against specific leaf area. Climate data The study sites were only rarely represented by climate stations. The climate data for each site were obtained from long-term observations made at meteorological stations in the vicinity. All data are accessible under Results Leaf phenology and ontogeny are both important when making comparisons on a large geographic scale, and many of the comparisons are based on newly expanded or old leaves. Therefore we investigated the change of leaf parameters with leaf age (Table 2). SLA increased from young to fully expanded leaves and decreased again in old leaves, but this change was only paralleled by N concentration in old leaves. Carbon isotope discrimination was about 2 lower in young leaves than in expanded leaves, but this value did not change any further between freshly expanded and old leaves, which had experienced a full dry season. Thus, the -values we present in Fig. 3 represent the climatic conditions during leaf expansion. There was an overall significant effect of leaf age on δ 15 N- values (ANOVA, P<0.01). Young leaves were more depleted in 15 N than old leaves. The following analysis will be based mainly on observations of expanded and old leaves. Community averaged carbon isotope discrimination ( ) of expanded and old leaves along the transect showed no trend with decreasing rainfall over the range from 1800 mm to 450 mm (Fig. 3). It was roughly constant (19.4 ± 1.2, (av. ± s.d.), n=146) over this wide range in annual rainfall. This is in clear contrast to the community-averaged values of reported by Stewart et al. (1995) who showed a linear decrease of with decreasing annual rainfall in southern Queensland (Fig. 3). In the north Australian area, decreased with decreasing rainfall only in that part of the transect where annual rainfall was between about 450 and 200 mm, but in this arid section of the transect decreased more steeply with decreasing rainfall than was the case in Queensland. This suggests a more complex interaction of plant composition and responses to climate in the north than in eastern Australia. Clearly, further studies are needed to confirm or clarify the response of in the 450 to 200 mm rainfall range. Most of the functional types used in this study co-existed in the high rainfall section of the transect, although the forest canopy was dominated by evergreens (Fig. 4). The number of functional types decreased with increasing latitude (increasing aridity) mainly because of a decline in the Table 2. Changes in specific leaf area (SLA: m 2 kg 1 ), nitrogen concentration (N: mgn g 1 ), carbon isotope discrimination ( : ), and the δ 15 N-values ( ) of different age leaves Young leaves were not fully expanded and often red in color, expanded leaves had reached full leaf size and were normally green, while old leaves were grown in the past rain season. The comparison is made for evergreen non-n 2 fixing trees at the sites Melville, Kapalga and Katherine, because only at these sites were all stages of leaf growth present. Small letters indicate significant differences within each column (Student t-test, P<0.05) Leaf type SLA N δ 15 N n av. s.d. av. s.d. av. s.d. av. s.d. young 3.70 a a a a expanded 6.01 b a b a old 4.66 c b b b

6 418 E. D. Schulze et al. Fig. 3. Change of carbon isotope ratios with median rainfall along an aridity gradient in northern Australia (av.± s.d.), as compared to observations made in eastern Australia (Stewart et al. 1995). Fig. 4. Latitudinal changes of specific leaf area (SLA, m 2 kg 1 ), leaf nitrogen concentration (mg g 1 ), 13 C-isotope discrimination ( ), and of the δ 15 N-isotope ratio ( ) in the plant functional types: potentially N 2 fixing deciduous and evergreen trees (d + N, ev + N) and non-n 2 fixing deciduous and evergreen trees (d N, ev N), spinescent species (spin), Adansonia (Ad), Allocasuarina (Allocas), and evergreen cultivated fruit tree plantations (ev.plant).

7 Isotope discrimination and nutrition 419 abundance of the deciduous component. Large differences existed in SLA between functional types and the response of SLA to increasing aridity along the moisture gradient (Fig. 4A). SLA decreased with decreasing rainfall in all functional types, except for the spinescent species and the N 2 fixing deciduous species. Non-N 2 fixing deciduous trees had higher SLA (8.4 ± 2.7, n = 34) than evergreen trees (4.3 ± 1.9, n = 179), while potentially N 2 fixing deciduous species had consistently higher SLA (10.7 ± 1.7, n = 15) than non- N 2 fixing species. In contrast to deciduous N 2 and non-n 2 fixing trees, evergreen N 2 fixing species reached lower SLA than non-n 2 fixing species especially in arid habitats. Cultivated fruit trees had the same SLA as naturally grown evergreens. The SLA of spinescent trees (0.8 ± 0.4, n = 6) was much lower than that of all other functional types, especially at high rainfall. The differences in SLA were mirrored in part by the leaf N concentrations (Fig. 4B). Highest values were observed in potentially N 2 fixing deciduous and evergreen species (20.1 ± 3.7 mg N g 1 n = 15), while leaf N concentrations of evergreen non-n 2 fixing species were, on average, only half of this value (10.8 ± 3.3, n =154, minimum 5.3). Evergreen fruit trees from plantations had higher N concentrations (14.8 ± 4.9, n =20) than the evergreen natural vegetation. However, the N concentration of cultivated evergreens (none of which were N 2 fixing) was similar to that of non- N 2 fixing deciduous trees in the same rainfall regime. The N concentration of spinescent plants was lower (7.1 ± 1.8, n =6) than in associated evergreen trees. Nitrogen per unit leaf area was constant in each functional type with decreasing rainfall. Differences between functional types may relate to different requirements for nitrogen. There was a non-significant decrease of leaf N concentration (Fig. 4B) with increasing latitude (increasing aridity), except for spinescent species, which reached higher N concentrations under arid conditions than at high rainfall. The increase of N concentrations in deciduous N 2 fixing species with increasing latitude resulted most likely from an effect of leaf age on N concentration in newly sprouted Erythrophleum trees. The large differences in SLA and N concentrations between different functional types did not appear as equivalent variation in 13 C discrimination (Fig. 4C). Potentially N 2 fixing species had a lower than in non- N 2 fixing species (ANOVA P < 0.01) and spinescent species had a lower discrimination than evergreens. The cultivated evergreen species had the same as other evergreen trees from natural vegetation. There was a general trend of decreasing at higher latitudes (lower rainfall) across the range of functional types, as reflected in the decrease in 13 C discrimination values across the community (Fig. 4C). This was most consistently expressed by the evergreen non-n 2 fixing species although the of this group was statistically not different between Melville Island and North Tennant Creek, and the decreased only between 20 S and 25 S. The water-storing bottle tree Adansonia exhibited a no different from other deciduous trees. The δ 15 N-values (Fig. 4D) exhibited a markedly different trend with respect to latitude compared with all other parameters. There was a maximum in the middle of the transect, which is the region of highest grazing intensity (as qualitatively assessed from the grass and weed cover, data not presented). Tyler Pass was á priori sampled because it was the only habitat which was relatively ungrazed in the arid part of the transect. This site exhibited low δ 15 N-values that appear almost as outliers when compared to other sites. In contrast to our expectations, the potentially N 2 fixing species did not show consistently lower δ 15 N-values than non- N 2 fixing species. Only in the high rainfall range did δ 15 N-values reach values which were typical for N 2 fixation (Shearer et al. 1983), with evergreen N 2 fixing species within this region exhibiting the lowest values. However, the pattern was complicated by the fact that non- N 2 fixing evergreen species reached the same δ 15 N-level as the one N 2 fixing deciduous species, most likely due to differences in rooting depth (Shearer and Kohl 1989; Handley and Scrimgeour 1997). The relationships between leaf N concentration, SLA and 13 C isotope discrimination are shown in Fig. 5. Since different numbers of species and functional types were measured at each site, we will make these comparisons based on average values per site but separate for each functional type. The most significant relation existed between leaf N concentration and SLA (Fig. 5A). Leaf N at a given level of SLA was consistently higher in the N 2 - fixing species than the non- N 2 fixing species. The relation between 13 C discrimination and SLA (Fig. 5B) or leaf N concentration (Fig. 5C) was less clear. The generally also increased with SLA, with deciduous N 2 fixing species having the highest values of SLA. 13 C discrimination also increased with leaf N concentration. In both cases, however, the relationship between dependent and independent variables was weak. Consistent with the observed differences in the relationship between N concentration and SLA, N 2 fixing species had the highest N concentration, but the range of values was similar to those of non N 2 fixing species. Based on the whole set of data, there was no significant difference between N 2 fixing and non- N 2 fixing species with respect to 13 C discrimination (ttest for expanded leaves from Melville and Kapalga P > 0.05), but for each site, values were lower for N 2 fixing than for non- N 2 fixing species (see Fig. 4). The high variation in the data indicates that additional site factors affected this trend. Clearly, species have different interactions between C, N, and SLA, and each functional type has its own combinations of relations.

8 420 E. D. Schulze et al. Burning at the Kapalga site had no significant effect on SLA, leaf N-concentration or (Table 3). The small change in the δ 15 N value was not significant. In contrast to burning, grazing intensity resulted in a major increase of the δ 15 N value. While N-concentration decreased at high grazing intensity there was no significant effect of grazing on the value. SLA changed with species composition from high to low values even at low grazing intensity, but within a given climatic regime, there was no significant effect of grazing intensity on SLA. Averaging the measured parameters for each site we observed a significant increase of SLA and N concentration with rainfall in evergreen sclerophylls (Fig. 6B). This contrasted with the response of carbon isotope discrimination, where the site averages remained constant between 1800 and 450 mm rainfall (Fig. 6A), and a decrease was observed only under drier conditions. The maximum values of responded more sensitively than the minimum values of to rainfall. It is interesting to note that ranges over 6 at high rainfall and almost 4 at low rainfall. In the following, we address the initial question of whether the observed geographic trends in SLA, N, and resulted from effects of climate on leaf properties within a species or if they were caused by a sequence of species replacements. We investigated this problem in nine of the Eucalyptus species which were sampled with replicates (Fig. 7). At the high rainfall range (Melville Island to Victoria River), the forests consisted of a range of Eucalyptus species which had inherent differences in SLA ranging from 6.3 (E. nesophila) to 4.5 (E. tetrodonta) (Fig. 7A,D). At higher latitudes and decreasing annual rainfall these species were replaced by others which in turn had an inherently lower SLA of 3 to 4. E. tectifica encompassed the largest range of SLA, between 6 and 4. Both, E. tectifica and E. miniata showed a SLA decrease over a latitudinal range of about 5, and a rainfall range of about 1800 to 800 mm. At higher latitudes, both species were replaced by E. terminalis, which had inherently lower SLA. E. terminalis also exhibited a significant decrease in SLA over its latitudinal range. In the present study E. brevifolia, E. setosa, E. papuana and E. Fig. 5. Relationships between, (A) leaf nitrogen concentration and specific leaf area (N 2 fixing: N=14.61 ± 1.15 SLA, r = 0.82, non- N 2 fixing: N= 5.69 ± 1.28 SLA, r =0.89), (B) 13 C discrimination and SLA (non-n 2 fixing: =18.03 ± 0.36 SLA, r =0.61), and (C) 13 C discrimination and N-concentration (N 2 fixing: =17.38 ± 0.19 N, r =0.44; non- N 2 fixing: =15.90 ± 0.16 N, r =0.62). The data points represent average values (av.± s.d.) at each sample site for different functional types as observed along the transect.

9 Isotope discrimination and nutrition 421 gummifera were collected only in a narrow range of distribution. In these habitats these species had generally a lower SLA than the co-dominant E. terminalis. The general decrease in SLA with increasing aridity was paralleled by decreasing N-concentrations with latitude in most Eucalyptus species, the exception being E. terminalis, which increased N concentration with increasing aridity (Fig. 7B, E). Eucalyptus species with narrow distributions had lower N concentrations in the mid latitudes, but showed a stronger increase with drought than E. terminalis. Thus, the parallel change of N and SLA could be regarded as a response to changing nutrient availability, while decreasing SLA at increasing N concentration might reflect a response to drought. The geographic pattern of SLA and N concentration resulted from a combined effect of species replacement and environmental conditions. The carbon isotope discrimination (Fig. 7C,F) showed distinct species related differences at each location Table. 3. Effects of burning and grazing intensity on specific leaf area (SLA: m 2 kg 1 ), N-concentration (N: mgn g 1 ), carbon isotope discrimination ( : ), and the δ 15 N-values ( ) of expanded leaves of evergreen non- N 2 fixing species Small letters indicate significant differences within each column (Student t-test, P<0.05) Treatment Location SLA N δ 15 N Burning unburnt Kapalga C,Q, M,S 5.51 a a a 0.89 a burnt Kapalga K,M,P,S 5.50 a a a 1.44 a Grazing low Melville-Kapalga 6.17 a a a 1.54 a Tyler Pass 3.87 b a b 1.27 a medium Kintore/Giles 3.44 b a b 4.96 b high Kidman/Mt.Sanford 3.24 b 8.16 b ba 7.09 c Fig. 6. Change of 13 C discrimination ( ) (A), and changes of specific leaf area (m 2 kg 1 ) and nitrogen concentration (mg g 1 ) (B) in evergreen sclerophyllous species with median rainfall. The figure shows average values ± standard deviation for each site. (A) shows also the maximum and minimum value which was observed at each site.

10 422 Eterm Fig. 7. Changes of average values at each sample site (av. ± s.d.) of (A, D) specific leaf area (SLA = 2.18 ± R, r = 0.93) and of (B, E) nitrogen concentration (N =10.91 ± R, r = 0.53) with latitude (A, B) and (D, E) with rainfall (R in mm). Changes of average values as well as of maximum and minimum values of 13 C discrimination ( ) with latitude (C, F) with rainfall (R in mm). E. D. Schulze et al.

11 Isotope discrimination and nutrition 423 (see Anderson et al. 1996). In the high rainfall range there was a trend (all species) of decreasing discrimination with increasing latitude (or decreasing discrimination with decreasing rainfall), but at certain critical environmental conditions these species were replaced by others with an initially higher discrimination, but which decreased with decresing rainfall or increasing latitude. E. tectifica which showed the highest plasticity in its SLA had lower and higher leaf N concentrations than E. miniata or E. tetrodonta. E. tectifica reached higher latitudes (more arid environment) than the other two forest species, but it was replaced by E. terminalis and E. brevifolia at Kidman Springs with decreasing rainfall. At Kidman Springs E. terminalis and E. brevifolia had higher than E. tectifica. In a similar way E. papuana and E. setosa also had higher than E. terminalis in the central part of the transect, where leaf N concentrations were lower. E. terminalis maintained higher under severe drought conditions than other species (Fig. 4). Discussion There are relatively few studies that have investigated the carbon isotope discrimination of trees over large geographic areas and along climatic transects. Little change in was seen in earlier transect studies for C 4 grasses of Namibia (Schulze et al. 1996a), or for the diverse tree and shrub flora of N 2 and non-n 2 fixing woody C 3 species of Namibia (summer rainfall) between 30 and 400 mm annual rainfall (Schulze et al. 1976, 1991a), as well as for a range of plant life forms along an aridity gradient between 125 and 770 mm annual rainfall in Patagonia, a winter-rainfall area (Schulze et al. 1996b). As expected from the theory of isotope discrimination (Farquhar 1983), the variation in is smaller among C 4 than C 3 species. The change in species composition and plant life forms and their specific changes in rooting depth and biomass allocation were discussed as the major causes for the observed maintenance of community-averaged carbon isotope discrimination in Namibia and Patagonia. It was not possible to examine root parameters in the present study. In contrast to earlier studies, though, the present study specifically examined the effect of changing species composition, using a single tree genus (Eucalyptus) as an example, and by measuring additional leaf parameters, such as SLA and leaf N concentration. The geographic pattern of a very minor response of community-averaged 13 C discrimination values resulted from a sequence of species replacements, in which increasing aridity favors species that have inherently lower SLA, lower N-concentration and higher 13 C discrimination. This is consistent with Read and Farquhar (1991). On the other hand the observation is difficult to reconcile with Anderson et al. (1996) where was positively correlated with seasonality and precipitation. In the present study species replacement acted towards maintaining assimilation at low nutrient and water availability in the arid region. Following the reviews by Canadell et al. (1996) and Jackson et al. (1996), changing rooting depth may be an additional important variable in supporting assimilation. Low nutrient availability is likely to correlate with the availability of water, but leaf nitrogen concentrations remained constant within each plant functional type at decreasing rainfall. This could support the maintenance of isotope discrimination with increasing aridity. If we generalise for the north Australian transect that changes in species composition act to maintain a relatively constant carbon isotope discrimination along continental transects of aridity, at least between 450 mm and 1800 mm annual rainfall, then we still need to explain the contrasting result of Stewart et al. (1995) who observed a decrease in with decreasing rainfall. Both linear and constant responses of 13 C to rainfall have been described in the literature (Stewart et al. 1995, Schulze et al. 1996a, b). A comparison of the pattern from north Australia with SE Queensland is confounded by one being a study of trees, the other being a study of trees and herbs, and it is not possible to separate trees from herbs in the latter study since the species were not listed. Additionally, there are climatic differences between the two regions. The northern regions are characterized by a higher seasonal rainfall (seasonality index >3; Climate Atlas of Australia 1977) compared with Queensland (seasonality index close to 1). The comparison of young and old leaves indicates that represents the conditions during the growing season rather than the conditions over the whole year, despite the fact that leaves start sprouting at the end of the dry season with deciduous species being earlier than evergreens (Williams et al. 1997). Thus, following monsoon rain, all leaves expand under more or less favorable conditions, at similar levels of isotope discrimination. The decrease in mainly occurred in that part of the transect below the zone of influence of the monsoonal rains, and where conditions were inherently drier, and the variation in annual rainfall is more variable. The effect of changing Eucalyptus species on community averaged carbon isotope discrimination was enhanced by additional changes in the contribution of other functional types (deciduous, evergreen, spinescent) to the plant community which increased N per unit leaf area. Apparently, N 2 fixing species do not use the high N concentration to support high photosynthesis rates relative to conductance which would result in decreased carbon isotope discrimination. We observed an increased with increasing SLA especially in evergreen non- N 2 fixing species. Thus functional types with high N concentration conserve rather than spend water. Although the functional type of potentially N 2 fixing species is distinct in its leaf anatomy and N concentration, it

12 424 E. D. Schulze et al. remains difficult to quantify N 2 fixation on the basis of 15 N discrimination following the approach of Shearer and Kohl (1989). Species-specific rooting depth and grazing overlaid the pattern to an extent that N 2 fixation could not be quantified in relation to accompanying reference plants. N 2 fixation was most likely limited to the high rainfall region (Melville Island) and to species with access to ground water (Allocasuarina). Apparently N 2 fixation contributes very little to the N relations of Acacia aneura based on δ 15 N measurements. An interpretation of the δ 15 N data with respect to N 2 fixation is difficult because we observed a steep increase in δ 15 N at the arid sites which were also heavily grazed sites. In an earlier study in Namibia, an increase in δ 15 N was observed in the region of main grazing activity (Schulze et al. 1991a). δ 15 N decreased again in the less grazed but more arid part. A separation of effects of burning or grazing was not possible in the case of Namibia. In the present study burning had no effect on carbon and nitrogen isotope discrimination and N concentration. The high δ 15 N ratios in the arid part of the NATT might be related to grazing. We are aware that the effect of grazing is difficult to quantify, because the δ 15 N ratio would represent a longterm rather than a short term effect, and we cannot rule out an interaction between aridity and grazing. Tyler Pass was a location which was sampled because of its apparently ungrazed condition. The δ 15 N ratios were low at this site despite its arid climate confirming the Namibia observation. Obviously the latitudinal change in δ 15 N ratios and the influence of grazing needs further experimental work. We intend to explore the hypothesis that grazing results in a loss of the light 14 N which over time leads to an accumulation of the heavy 15 N in the ecosystem. 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