Savannahs dominated by grasses with scattered C3 trees expanded between 24 and 9 million years ago in low latitudes at the expense of forests. Fire, herbivory, drought and the susceptibility of trees to declining atmospheric CO2 concentrations ([CO2]a) are proposed as key drivers of this transition. The role of disturbance is well studied, but physiological arguments are mostly derived from models and palaeorecords, without direct experimental evidence. In replicated comparative experimental trials, we examined the physiological effects of [CO2]a and prolonged drought in a broadleaf forest tree, a savannah tree and a savannah C4 grass. We show that the forest tree was more disadvantaged than either the savannah tree or the C4 grass by the low [CO2]a and increasing aridity. Our experiments provide insights into the role of the intrinsic physiological susceptibility of trees in priming the disturbance-driven transition from forest to savannah in the conditions of the early Miocene.
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We acknowledge funding through an ERC advanced grant (CDREG, no. 322998) awarded to D.J.B. C.B. and N.U. received funding from the European Union’s Horizon 2020 research and innovation programme through an MSCA individual fellowship (grant agreement ID no. 702755) awarded to C.B.
The authors declare no competing interests.
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A) Seedlings of Celtis africana and Vachellia karroo during the growth phase before measurements began. B) A seedling of Vachellia karoo being weighed to gravimetrically determine watering amount. C) Seedlings inside the double-door growth chambers during the night phase of the photoperiod. D) Eragrostis curvula growing in the growth chambers; this picture was also taken during the dark phase of the photoperiod. The tobacco plants in C and D were grown alongside experimental plants throughout the experiment and were regularly monitored for growth and morphological traits to ensure they were reproducing treatment expected differences. E) Example of in-situ leaf area measurement prior to operational gas exchange data collection. The leaf is clamped between a piece of white Perspex mounted on a white card background and it is placed at a fixed distance from the camera lens. Subsequently, images are processed in Image J to determine area as illustrated in F, wich displays monochrome images of Vachellia karroo leaflets.
Substrates were loosely filled into pots identical to those used in the experiments, watered to field capacity and left to dry under normal growth cabinet conditions. Soil water potential was measured every 2–3 days using a psychrometer (Psypro with L-51 hygrometers, Wescor Inc., Logan, UT, US) calibrated with five standard NaCl solutions according to the manufacturer’s instructions. Soil samples were extracted from the centre of the pot at a depth of ~10 cm using a weighing spatula. Soil and John Innes No. 3 (green up-triangles) was selected as the experimental substrate because it had the steadiest decrease in soil matric potential as it dried. Ψm = Ψe∙(θ/θs)-b, where Ψm is soil matric potential, Ψe = 0.0101 and b = 3.01 are the fitted parameters.
Extended Data Fig. 3 Response of leaf-level CO2 assimilation (A) to increasing photosynthetic photon flux density (PPFD), A - PPFD curves.
Values are means ± 1 SE (n = 4) for Celtis africana (top), Vachellia karroo (middle), and C4 Eragrostis curvula (bottom) plants, meassured at five watering levels (80, 60, 50, 40 and 30% of pot capacity) and grown at either 200 ppm (left), 400 ppm (centre) or 800 ppm (right) [CO2]a. Blue symbols with solid blue lines represent measurements after re-watering (recovery phase).
Extended Data Fig. 4 Response of CO2 assimilation (A) to increasing [CO2] in the sub-stomatal cavity (Ci), A - Ci curves.
Values are means ± 1 SE (n = 4) for Celtis africana (top), Vachellia karroo (middle) and C4 Eragrostis curvula (bottom) plants, meassured at five watering levels (80, 60, 50, 40 and 30% of pot capacity) and grown at either 200 ppm (left), 400 ppm (centre) or 800 ppm (right) [CO2]a. Blue symbols with solid blue lines represent measurements after re-watering (recovery phase).
Extended Data Fig. 5 [CO2] in the sub-stomatal cavity (Ci) and its ratio to [CO2] in the measuring cuvette (Ci/Ca) under operational growing conditions.
Values are means ± 1 S.E (n = 4) for the forest broad-leaf tree Celtis africana (left), the savanna tree Vachellia karroo (centre), and the C4 savanna grass Eragrostis curvula (right), meassured at five watering levels (80, 60, 50, 40 and 30% of pot capacity followed by a recovery back to 80%) and grown at either 200 ppm (Low), 400 ppm (Amb) or 800 ppm (Ele) [CO2]a. Gas exchange was measured in-cabinet with gas-analyser set points for temperature, humidity, [CO2]a, and light intensity set at cabinet levels.
Values are means (n = 4) ± 1 SE for the forest broad-leaf tree Celtis africana (left), the savanna tree Vachellia karroo (middle), and the C4 savanna grass Eragrostis curvula (right), meassured at five watering levels (80, 60, 50, 40 and 30% of pot capacity) followed by a recovery back to 80% and grown at either 200 ppm (Low), 400 ppm (Amb) or 800 ppm (Ele) [CO2]a. FV/FM was measured from pulse-amplitude modulated (PAM) chlorophyll fluorometry within the cabinets in the dark.
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Bellasio, C., Quirk, J., Ubierna, N. et al. Physiological responses to low CO2 over prolonged drought as primers for forest–grassland transitions. Nat. Plants 8, 1014–1023 (2022). https://doi.org/10.1038/s41477-022-01217-8