Is resource overlap disadvantageous to three sympatric legumes?

Abstract The sympatric legumes Dillwynia hispida, Dillwynia uncinata and Pultenaea densifolia overlap in flowering phenology, floral characters and native bee pollinators. Tests for pollen-limitation were conducted to determine whether or not the overlap in pollinator use was detrimental to individuals of these species. Pollen limitation was only detected in D. hispida. Pollen loads on bees were examined and it was found that pollen loads from Dillwynia and P. densifolia were spatially segregated on the pollinator's body reducing the likelihood of pollen transfer between these species. The pollen of D. hispida and D. uncinata overlapped on the heads of bees suggesting that interspecific pollinations may be responsible for lowered fruit-set in D. hispida. However, hand pollinations involving heterospecific pollen and controlled floral visits by bees to the two Dillwynia species revealed that heterospecific pollen loads did not significantly depress fruit-set in either D. hispida or D. uncinata. Pollinator preferences were examined in detail and it was found that D. uncinata flowers received twice as many bees in a day compared with D. hispida and P. densifolia. Competition through pollinator preference is implicated here as a mechanism of competition. Intraspecific competition is also implicated as D. hispida was pollen limited in the absence of D. uncinata and P. densifolia.     

A dynamic model of photosynthesis in varying light taking account of stomatal conductance, C3-cycle intermediates, photorespiration and Rubisco activation

Abstract. A dynamic model of whole leaf C₃ photosynthesis is constructed using a modified version of the Farquliar-von Caemmerer approach. The model is designed to provide a physiological basis to understand observations of assimilation in environments with varying photon flux densities, including induction phenomena. The model couples the effect of light activation and dark deactivation of enzymes, stomatal conductance responses, and variations in the pools of carbon cycle intermediates. The dynamic components are viewed on three time scales, the slowest of which (min to h) involves changes in stomatal conductance and the activation stale of Rubisco. On a time scale of seconds to a few minutes, adjustments in pools of biochemical components of the photosynthetic pathway occurs. The most rapid time scale corresponds to the equilibration time of intercelluar CO₂ concentration through gaseous diffusion and is here assumed to occur instantaneously. The model form includes a single pool for reduced intermediates including RuBP, a single pool for components of the glycolate pathway, and a third component corresponding to the activation state of Rubisco. This is coupled to a previously described model for the dynamics of stomatal conductance, giving a final model form consisting of six non-linear ordinary differential equations, of which three control conductance dynamics and three control assimilation. The coupling between these occurs through the variable p_i, the intercellular partial pressure of CO_2. Only three of the parameters for the assimilation portion of the model require dynamic data to estimate. The remaining parameters are estimated from steady-state data. The model is calibrated using previously collected data on the tropical understory plant Alocasia macrorrhiza and is shown to have qualitatively similar behaviour to that of experimental measurements using simple changes in PFD, as well as a complex sequence of such changes.     

Subfossil Cladocera in relation to contemporary environmental variables in 54 Pan-European lakes

1. Changes in cladoceran subfossils in the surface sediments of 54 shallow lakes were studied along a European latitude gradient (36–68°N). Multivariate methods, such as regression trees and ordination, were applied to explore the relationships between cladoceran taxa distribution and contemporary environmental variables, with special focus on the impact of climate. 2. Multivariate regression tree analysis showed distinct differences in cladoceran community structure and lake characteristics along the latitude gradient, identifying three groups: (i) northern lakes characterised by low annual mean temperature, conductivity, nutrient concentrations and fish abundance, (ii) southern, macrophyte rich, warm water lakes with high conductivity and high fish abundance and (iii) Mid‐European lakes at intermediate latitudes with intermediate conductivities, trophic state and temperatures. 3. Large‐sized, pelagic species dominated a group of seven northern lakes with low conductivity, where acid‐tolerant species were also occasionally abundant. Small‐sized, benthic‐associated species dominated a group of five warm water lakes with high conductivity. Cladoceran communities generally showed low species‐specific preferences for habitat and environmental conditions in the Mid‐European group of lakes. Taxon richness was low in the southern‐most, high‐conductivity lakes as well as in the two northern‐most sub‐arctic lakes. 4. The proportion of cladoceran resting eggs relative to body shields was high in the northern lakes, and linearly (negatively) related to both temperature and Chl a, indicating that both cold climate (short growing season) and low food availability induce high ephippia production. 5. Latitude and, implicitly, temperature were strongly correlated with conductivity and nutrient concentrations, highlighting the difficulties of disentangling a direct climate signal from indirect effects of climate, such as changes in fish community structure and human‐related impacts, when a latitude gradient is used as a climate proxy. Future studies should focus on the interrelationships between latitude and gradients in nutrient concentration and conductivity.     

An improved dynamic model of photosynthesis for estimation of carbon gain in sunfleck light regimes

A dynamic model of leaf photosynthesis for C₃ plants has been developed for examination of the role of the dynamic properties of the photosynthetic apparatus in regulating CO₂ assimilation in variable light regimes. The model is modified from the Farquhar-von Caemmerer-Berry model by explicitly including metabolite pools and the effects of light activation and deactivation of Calvin cycle enzymes. It is coupled to a dynamic stomatal conductance model, with the assimilation rate at any time being determined by the joint effects of the dynamic biochemical model and the stomatal conductance model on the intercellular CO₂ pressure. When parametrized for each species, the model was shown to exhibit responses to step changes in photon flux density that agreed closely with the observed responses for both the understory plant Alocasia macrorrhiza and the crop plant Glycine max. Comparisons of measured and simulated photosynthesis under simulated light regimes having natural patterns of lightfleck frequencies and durations showed that the simulated total for Alocasia was within ±4% of the measured total assimilation, but that both were 12–50% less than the predictions from a steady–state solution of the model. Agreement was within ±10% for Glycine max, and only small differences were apparent between the dynamic and steady–state predictions. The model may therefore be parametrized for quite different species, and is shown to reflect more accurately the dynamics of photosynthesis than earlier dynamic models.