Soils and vegetation of central Australian sandridges I. Introduction

This is the first of a series of papers describing catenary patterns in central Australian sand-ridge soils and vegetation, and observations and experiments designed to assess the interrelationships among them and the impact of fire. The aims and study area are described and previous literature reviewed briefly. Later papers will detail soil and vegetation patterns and discuss the relative importance of the dune-swale gradients in soil texture, mobility, fertility and moisture status as controls and constraints on the vegetation, with reference to rainfall fluctuations and to changes in plant cover and environment at various time scales. These embody experimental studies on plant regeneration to suggest an overall model for the maintenance of the present open-crested dune vegetation.     

Soils and vegetation of central Australian sandridges. IV. Soils.

Catenary patterns on central Australian sandridges are assessed for the following soil parameters: sand depth, texture, mobility and penetrability; moisture content and characteristic; organic C content, total N and extractable P, K and Ca. There are marked dune-swale gradients in sand depth, soil texture, mobility and penetrability, moisture content at — 15 kPa, and N and organic C contents. There are dune-swale gradients in soil Ca content at a few sites only. Soil P shows significant heterogeneity but no catenary patterns, whilst K levels are homogeneous throughout. S, Mg, Fe, Mn, Cu, Zn, B, Mo, Co and CI were also measured for a number of sandridge soils but showed no significant catenary patterns and were therefore not studied in detail.     

Qualitative effects of patchy stomatal conductance distribution features on gas-exchange calculations

The qualitative influence of patchy stomatal conductance distributions on the values of photosynthesis (A) and intercellular CO₂ concentration (c_i) as determined by gas-exchange measurements were investigated using computer modelling. Gas-exchange measurements were simulated for different conductance distributions by modelling photosynthesis explicitly for each patch, summing these rates, and inferring c_i from a diffusion equation. Qualitative relationships are presented between conductance distribution features and the difference between assimilation rates measured for patchy and homogeneous leaves at the same c_i (A_p and A_h, respectively). These data show that, although most conductance distributions have little effect on the value of A measured for a given c_i, some distribution features (which we have termed ‘bimodality’, ‘position’, ‘skewness’ and ‘range’) play a key role in controlling the magnitude of these effects. Distributions that are more nearly bimodal, span regions of lower conductance, are right-skewed, or have broader conductance ranges are associated with larger effects on the A(c_i) relationship. To clarify our mathematical analysis and illustrate some of the trends it predicts, we present conductance distributions and gas-exchange data from leaves of Malus dolgo var. Spring Snow Dial were treated with ABA. The results are discussed in the light of recent controversy over the effect of patchy stomatal conductance on gas-exchange data.     

A spatially explicit model of patchy stomatal responses to humidity

Stomata of leaves can exhibit either temporally stable, spatially homogeneous behaviour or complex spatial and temporal dynamics, depending on environmental and physiological conditions. To test the ability of accepted physiological mechanisms to describe these patterns, we developed a simple, spatially explicit model of stomatal responses to humidity that incorporated hydraulic interactions among stomata. Model results showed qualitative agreement with experimental evidence for a number of phenomena: (1) at high humidities, whole-leaf steady-state conductance is a monotonic function of humidity; (2) the initial stomatal response following a perturbation in humidity is in the direction opposite to the final response, and (3) spatial dynamics include patch formation and self-organization similar to that observed in actual leaves. These comparisons do not eliminate other explanations, but do suggest that novel mechanisms need not be invoked to explain the diversity of spatial and temporal patterns of stomatal behaviour in leaves.