Leaf Demography, Sclerophylly and Ecophysiology of Two Banksias With Contrasting Leaf Life Spans

Abstract

Leaf demography, sclerophylly and ecophysiology were assessed on Banksia petiolaris (ground creeper, large vertical leaves) and B. baueri (shrub, smaller horizontal leaves) in the dry mediterranean scrub-heath of Western Australia. Leaves survived for up to 13 and 5 years respectively (50% survival was 5.9 and 2.0 years). Leaf specific mass increased with age for both species, due to increase in leaf density. Shading was negligible for 1-year-old B. baueri leaves but increased sharply with age, whereas shading was moderate and similar for leaves of all ages in B. petiolaris. Transpiration rates and stomatal conductance during cloudy autumn weather were higher in B. petiolaris, whereas photosynthesis was lower. Stomatal conductance, transpiration and photosynthesis declined with leaf age in both species, but more steeply in B. baueri. Dawn and midday xylem pressure potentials and hydraulic conductance decreased with leaf age and were higher and declined more gradually in B. Petiolaris. Water-use efficiency declined slightly with age. The water potential at zero turgor, osmotic potential at 100% relative water content and bulk elastic modulus values were closer to zero in B. Petiolaris and changed little with age, while the relative water content at zero turgor and bound water were higher in older leaves of B. petiolaris. No trends in chlorophyll concentrations or fluorescence kinetics were observed with aging except that the oldest leaves of B. baueri had lowest activity. Area based leaf N decreased in B. baueri with leaf age, but was unchanged in B. petiolaris up to 10 years. K and Mg did not vary with leaf age, but P decreased and Ca increased in both species. P levels were extremely low, yet 70% resorption prior to leaf death was apparent. Photosynthetic-N and -P use efficiencies were high in the 1-year-old relative to the 2-year-old leaves, and declined gradually with further aging, but more steeply in B. baueri. We conclude that the exceptionally long-lived leaves of B. Petiolaris are consistent with the substantial investment they represent (large surface area, mass and total nutrient store) and low levels of physiological activity associated with low soil nutrient and moisture availability and moderate illumination at all leaf ages.