Leaf Area Determation

Annals of Botany 88: 915±927, 2001 doi:10.1006/anbo.2001.1536, available online at http://www.idealibrary.com on

Direct and Indirect Relationships Between Speci®c Leaf Area, Leaf Nitrogen and Leaf Gas Exchange. E€ects of Irradiance and Nutrient Supply

D R I SS M EZ I A N E{ and B I L L S H I P L E Y * { {DeÂpartement de Biologie, Universite Sidi Mohamed Ben Abdellah, FeÁs, Morocco and {DeÂpartement de Biologie, Universite de Sherbrooke, Sherbrooke, (Qc), Canada J1K 2R1

Received: 17 April 2001 Returned for revision: 17 April 2001 Accepted: 1 August 2001

We present a series of competing path models relating interspeci®c patterns between speci®c leaf area, leaf nitrogen content, net photosynthesis and stomatal conductance and test these against data from 22 species of herbaceous plants grown under controlled conditions with contrasting irradiance and nutrient supply rates. We then compare these results with two previous data sets, one based on ®eld measures and one based on glasshouse measures, to determine the robustness of the results. Only one model was able to account for the patterns of direct and indirect e€ects between the four variables to all data sets. In this model speci®c leaf area is the forcing variable that directly a€ects both leaf nitrogen levels and net photosynthetic rates. Leaf nitrogen then directly a€ects net photosynthetic # 2001 Annals of Botany Company rates which in turn then a€ect stomatal conductance to water. Key words: Comparative ecology, modelling, path analysis, photosynthesis, plant strategies, SLA, speci®c leaf area, stomatal conductance.

I N T RO D U C T I O N In this study we evaluate the robustness to changes in irradiance and nutrient supply of general interspeci®c patterns involving speci®c leaf area (SLA, cm2 g À1), leaf nitrogen concentration ([N], mg g À1), net leaf photosynthetic rate (A, nmol g À1 s À1), and stomatal conductance to water (G, mmol g À1 s À1). There is now a large comparative literature relating...