The influence of Carboniferous palaeoatmospheres on plant function: an experimental and modelling assessment

Abstract

Geochemical models of atmospheric evolution predict that during the late Carboniferous, ca. 300 Ma, atmospheric O 2 and CO 2 concentrations were 35% and 0.03%, respectively. Both gases compete with each other for ribulose-1,5-bisphosphate carboxylase/oxygenaseöthe primary C-¢xing enzyme in C 3 land plantsöand the absolute concentrations and the ratio of the two in the atmosphere have the potential to strongly in£uence land-plant function. The Carboniferous therefore represents an era of potentially strong feedback between atmospheric composition and plant function. We assessed some implications of this ratio of atmospheric gases on plant function using experimental and modelling approaches. After six weeks growth at 35% O 2 and 0.03% CO 2 , no photosynthetic acclimation was observed in the woody species Betula pubescens and Hedera helix relative to those plants grown at 21% O 2 . Leaf photosynthetic rates were 29% lower in the high O 2 environment compared to the controls. A global-scale analysis of the impact of the late Carboniferous climate and atmospheric composition on vegetation function was determined by driving a process-based vegetation-biogeochemistry model with a Carboniferous global palaeoclimate simulated by the Universities Global Atmospheric Modelling Programme General Circulation Model. Global patterns of net primary productivity, leaf area index and soil carbon concentration for the equilibrium model solutions showed generally low values everywhere, compared with the present day, except for a central band in the northern land mass extension of Gondwana, where high values were predicted. The areas of high soil carbon accumulation closely match the known distribution of late Carboniferous coals. Sensitivity analysis with the model indicated that the increase in O 2 concentration from 21% to 35% reduced global net primary productivity by 18.7% or by 6.3 Gt C yr À1. Further work is required to collate and map at the global scale the distribution of vegetation types, and evidence for wild¢res, for the late Carboniferous to test our predictions.

Stomatal density and stomatal index as indicators of paleoatmospheric CO2 concentration

“…One pathway to mediate this decline is increasing SD and/or SI. Experimental work on Hedera helix and Betula pubescens show slightly higher stomatal indices in a 35% versus 21% O 2 atmosphere (Beerling et al, 1998b). If correct, this factor may be particularly important during the Carboniferous and early Permian when O 2 concentrations are modeled to exceed 30% (Berner and Can®eld, 1989;Berner et al, 2000).…”

Evolution of leaf-form in land plants linked to atmospheric CO2 decline in the Late Palaeozoic era

“…1). The reduced stomatal resistance of these planate leaves bene®ted photosynthetic productivity under the unusually low atmospheric CO 2 /O 2 ratio by increasing CO 2 diffusion into leaf mesophyll and thereby minimizing photorespiratory CO 2 evolution through the carbon oxidation pathway 15 . We tested our biophysical simulations of ancient land plants by ®rst computing their water-use ef®ciencies (carbon gain per unit of water lost) using modelled gas exchange characteristics.…”

Description and validation of the Simple, Efficient, Dynamic, Global, Ecological Simulator (SEDGES v.1.0)

“…These results suggest that max should be lowered/raised in SEDGES for higher/lower-thanpresent atmospheric O 2 levels. (In addition, although optimization theory by Prentice et al (2014) and experimental measurements show that the ratio of intercellular to atmosphere CO 2 c i c a increases/decreases with higher/lower atmospheric O 2 , the increase is found to be only ≈ 0.06 when going from the current (21 %) to the extreme high (35 %) O 2 level over the last 400 million years (Beerling et al, 1998). This c i c a difference is small in comparison with other uncertainties and inaccuracies in the model and can thus probably be neglected.…”

“…Higher/lower O 2 would lead to greater/lesser fertilization. Next, for a given CO 2 concentration, higher O 2 increases photorespiration and decreases gross photosynthesis, both experimentally and according to the aforementioned Farquhar model (Beerling et al, 1998) (although the effect was also found to be less pronounced for evolutionarily older taxa). These results suggest that max should be lowered/raised in SEDGES for higher/lower-thanpresent atmospheric O 2 levels.…”