Australian Journal of Botany Australian Journal of Botany Society
Southern hemisphere botanical ecosystems
RESEARCH ARTICLE

The ratio of foliar nitrogen to foliar phosphorus: a determinant of leaf attributes and height in life-forms of subtropical and tropical plant communities

Ray L. Specht A C and Alison Specht B

A Emeritus Professor of Botany, The University of Queensland, 107 Central Avenue, St Lucia, Qld 4067, Australia.

B Program Manager, Terrestrial Ecosystem Research Network, The University of Queensland, Queensland 4072, Australia.

C Corresponding author. Email: r.specht@uqconnect.net

Australian Journal of Botany 58(7) 527-538 https://doi.org/10.1071/BT07110
Submitted: 12 June 2007  Accepted: 21 July 2010   Published: 27 October 2010

Abstract

In the species-rich overstorey of tropical and subtropical closed-forests (rainforests), a series of life-forms (emergent trees, canopy trees, subcanopy trees, mid-stratum trees and shrubs, interlaced with lianes) of increasing basal area, height and foliage attributes (leaf area, leaf specific weight and internode length) develop in equilibrium with aerodynamic fluxes (frictional, thermal, evaporative ± atmospheric salinity) in the atmosphere as it flows turbulently over and through a plant community. In both closed-forest and open-forest communities in eastern Australia, the translocation of high-energy nitrogen and phosphorus compounds into developing leaves – during the driest season of the year – increases as soil water becomes more available in the climatic gradient from the subhumid to the per-humid zone. Foliage attributes (leaf area and leaf specific weight) of vertical shoots are determined by the rate of input of high-energy compounds into developing shoot apices. Increasing nutrient input in the transpiration stream results in a greater number of leaves (with similar leaf specific weights) on vertical foliage shoots. The leaf area index of the tree is thus enhanced and leads to increased biomass, basal area and height at maturity. In each life-form within a closed-forest, the size of the root system is allometrically related to aboveground attributes. The ability of the root system to explore available nitrogen and phosphorus stored in the surface soil thus determines the attributes of developing foliage shoots in each of these life-forms. Both leaf areas and leaf specific weights decrease from maxima in canopy trees to leaves of subcanopy and mid-stratum trees in the milder climate under the dense structure of per-humid rainforests. In contrast, in the open-structured, subhumid rainforests, although leaf areas decrease in the gradient from canopy to mid-stratum trees – all exposed to direct solar radiation – leaf specific weights increase as temperatures in the boundary layer around growth-apices increase. The production of nitrate ions in soil, exposed to solar radiation in gaps, increases the uptake of nitrogen into leaves of pioneer trees. Larger and thinner leaves, with higher foliar N : P ratios and nitrate reductase activity, result in and enable rapid regeneration of the rainforest.


Acknowledgements

Many scientists have investigated the dynamic processes in tropical and subtropical rainforests in Australia. The subtropical rainforest vegetation of south-eastern Queensland has been studied by the following scientists: Len Webb, Geoff Tracey, Bill Williams, Paul Forster and Peter Bostock (floristics), and Elwyn Hegarty (structure and phenology). Both rainforest and eucalypt open-forest–woodland vegetation in south-eastern Queensland has been examined by the following scientists: Ray Specht, Margarita Arianoutsou of the University of Athens and Santi Sabaté of the University of Barcelona (foliage attributes), Carles Gracia of the University of Barcelona (Chlorophyll a and b ratios), Ray Specht, Phil Rundel of University of California, LA, and Roger Reeves of Massey University, NZ (foliar nutrients), Phil Rundel and George Stewart (carbon isotope ratios), David Lamb (soil nitrogen dynamics), and George Stewart of the University College, London (nitrate reductase activity). The subtropical rainforests of north-eastern New South Wales have been studied by the following scientists: Alec Floyd, Len Webb, Geoff Tracey, Bill Williams and Alison Specht (floristics), Alison Specht (structure and phenology), Alison Specht (foliage attributes), Alison Specht, John Turner, M. J. Lambert and Len Webb (foliar nutrients), and Alison Specht (soil nitrogen dynamics and nitrate reductase activity). The ‘dry rainforests’ in wet-watered sites in the serpentinite outcrop of central Queensland have been studied by the following scientists: George Batianoff and Ray Specht (floristics and structure), Ray Specht (foliage attributes), Alison Specht and Roger Reeves (foliar nutrients), and George Stewart (carbon isotope ratios). The tropical rainforests of north-eastern Queensland have been studied by the following scientists: Len Webb, Geoff Tracey and Bill Williams (floristics), Geoff Stocker and Greg Unwin (structure), Heather Brasell (foliar nutrients), and David Doley, David Yates and Greg Unwin (photosynthesis). David Doley provided invaluable comments on the manuscript.


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