CARBON AND NITROGEN ISOTOPE DISCRIMINATION AND NITROGEN NUTRITION OF TREES ALONG A RAINFALL GRADIENT IN NORTHERN AUSTRALIA

Carbon isotope discrimination (Delta) and nitrogen isotope ratios, N-c oncentrations and specific leaf area of 50 tree species were investiga ted 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 spi nescent sclerophylls shrubs. SLA generally decreased with increasing a ridity, 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 Delta was approximately constant at rainfalls above 475 mm (average Delta = 19. 4 +/- 1.2 parts per thousand). Where rainfall was less than 475 mm, De lta decreased from 19 parts per thousand to 17 parts per thousand at 2 20 mm, Delta was positively correlated with SLA. Delta of deciduous N- 2 fixing species and spinescent species were 1 parts per thousand and 2.4 parts per thousand lower than in evergreen sclerophyllous species. Delta in the N-2 fixing Allocasuarina was 1.2 parts per thousand lowe r than in non- N-2 fixing sclerophyllous species. The delta(15)N-value s indicated N-2 fixation only at high rainfall. Burning of the field l ayer in a Eucalyptus forest had no effect on all measured tree paramet ers including Delta(15)N, but Delta(15)N increased under grazing condi tions to >5 parts per thousand. The constant value of community averag ed Delta between 1800 and 450 mm may be the result of replacement of f unctional types and species. The decline in Delta in the more arid sec tion may be a function of both low species diversity, and a highly ase asonal and unpredictable rainfall regime.