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Summary: Increasing atmospheric CO2 is changing the dynamics of tropical savanna vegetation. C3 trees and grasses are known to experience CO2 fertilization, whereas responses to CO2 by C4 grasses are more ambiguous.Here, we sample stable carbon isotope trends in herbarium collections of South African C4 and C3 grasses to reconstruct 13C discrimination.We found that C3 grasses showed no trends in 13C discrimination over the past century but that C4 grasses increased their 13C discrimination through time, especially since 1950. These changes were most strongly linked to changes in atmospheric CO2 rather than to trends in rainfall climatology or temperature.Combined with previously published evidence that grass biomass has increased in C4‐dominated savannas, these trends suggest that increasing water‐use efficiency due to CO2 fertilization may be changing C4 plant–water relations. CO2 fertilization of C4 grasses may thus be a neglected pathway for anthropogenic global change in tropical savanna ecosystems. [ABSTRACT FROM AUTHOR]

Modern tropical and subtropical C4 grasslands and savannas were established during the late‐Miocene and Pliocene, over 20 Myr after evolutionary originations of the C4 photosynthetic pathway. This lag suggests environmental factors first limited and then favored C4 plants. Here, we examine the timing and drivers for the establishment of C4 grasslands on the Indian Subcontinent using carbon and hydrogen isotope signatures of plant‐wax n‐alkanes recovered from turbidites in the Bengal Fan. Like prior studies, we find C4 ecosystems in the Ganges‐Brahmaputra catchment first emerged at 7.4 Ma and subsequently expanded between 6.9 to ∼6.0 Ma. Hydrogen isotope values varied from 10.2 to 7.4 Ma and then increased after 7.4, which suggests intermittent drying began before the establishment of C4 grasslands with further drying at the onset of C4 expansion. Synthesis of published plant fossil data from the Siwalik Group of the Himalayan foreland basin documents an ecosystem trajectory from evergreen tropical forests to seasonally deciduous forests, and then expansive C4 grasslands. This trajectory coincided with a seasonally uneven drying trend due to both increased evaporation of plant leaf and soil waters and reduced rainfall, as identified in soil carbonate and tooth enamel data sets. Collectively the fossil, biomarker, and isotopic evidence reveal the development of modern C4 ecosystems on the Indian Subcontinent followed a series of ecosystem transformations driven by drying and fire feedbacks, and possibly declining atmospheric pCO2, beginning at 10.2 Ma and strengthening through the late Miocene. Key Points: Late Miocene expansion of C4 grasslands in the Bengal Fan catchment followed prior ecological change from evergreen to dry deciduous forestsSynthesis of isotopic and ecological records from Himalayan foreland basin reveals ecological change and multifaceted hydrologic change preceded C4 grassland expansionSeasonal reduction in rainfall and increased evaporation promoted ecosystem change, accelerated by ecological feedbacks [ABSTRACT FROM AUTHOR]

Fire dynamics potentially account for the asynchronous timing of the expansion of C4 grasslands throughout the Mio‐Pliocene world. Yet how fire, climate, and ecosystems interacted in different settings remain poorly constrained because it is difficult to quantify fires and fuel source over these timescales. Here, we apply molecular proxies for fire occurrence alongside records of vegetation change and paleohydrology in Bengal Fan sediments (ODP Leg 116) to examine fire feedbacks on the south Asian continent. We employ abundances of polycyclic aromatic hydrocarbons (PAHs) to reconstruct fire occurrence and δ13C measurements of pyrogenic PAHs to constrain fuel source and grassland burning. This combination allowed us to test whether: (1) a fire‐seasonality forcing facilitated the expansion of grassland ecosystems and (2) a fire‐C4 grass burning feedback maintained these systems. PAHs can be sourced from weathered fossil carbon (i.e., a petrogenic source) and from burned terrestrial biomass (i.e., a pyrogenic source). Alkylated and non‐alkylated structure abundance data distinguished pyrogenic from petrogenic sourced samples. A sharp increase in pyrogenic PAHs along with increases in δ2H and δ13C values of plant waxes at 7.4 Ma indicates increased fire coincided with the onset of C4 expansion and hydrologic change in South Asia. The correlated 13C enrichment in PAHs, 13C enrichment in plant waxes, and increased abundances of PAHs suggest burning of C4 grasslands likely maintained open ecosystems. Our results link fire to the initial opening of grassland ecosystems on a subcontinental‐scale and support disturbance as a critical mechanism of terrestrial biome transition. Key Points: Alkylated polycyclic aromatic hydrocarbon (PAH) distributions can constrain relative pyrogenic and petrogenic sources in settings with mixed recalcitrant inputsCarbon isotopes of PAHs indicate that C4 grasses preferentially burned relative to C3 vegetation on Indian SubcontinentFire both facilitated and maintained open C4 grasslands at local and subcontinental scales [ABSTRACT FROM AUTHOR]

C4 grasslands proliferated later in Australia than they did on other continents (∼3.5 Ma vs. 10–5 Ma). It remains unclear whether this delay reflects differences in climate conditions or ecological feedbacks, such as fire, that promote C4 ecosystems. Here, we evaluated these factors using terrestrial biomarkers from marine sediments off western Australia. Fire‐derived polycyclic aromatic hydrocarbons (PAH) indicate fire ecology did not substantially change during or following C4 expansion. The presence of fire‐adapted C3 woody vegetation likely diminished the role of fire and delayed C4 expansion until it was prompted by climate drying between 3.5 and 3.0 Ma. At the same time, mass accumulation rates of weathered PAHs increased 100‐fold, which indicates a significant loss of soil carbon accompanied this ecosystem shift. The tight couplings between hydroclimate and carbon storage altered boundary conditions for Australian ecosystems, and similar abrupt behavior may shape environmental responses to climate change. Plain Language Summary: Today, grassland ecosystems are characterized by frequent fires. This study tested the role of fire in the establishment of these ecosystems 3 Myr ago in Australia using molecular fossils. Results show that fire occurrence did not drastically change as C4 ecosystems were established. Instead, changes in rainfall caused massive amounts of soil carbon to be transported off of the continent, which may have favored C4 plants. These dynamics are different than what is seen on other continents, but have important implications for how Australian ecosystems may respond to future climate changes. Key Points: Molecular markers indicate that fire preexisted in the Australia system and did not further increase with C4 grassland expansionA 100‐fold increase in weathered Polycyclic aromatic hydrocarbons at ∼3.5 Ma indicates increased soil carbon remobilization off AustraliaIncreasingly variable hydrology drove both the expansion of C4 biomes and diminished carbon storage in the Australian terrestrial system [ABSTRACT FROM AUTHOR]

Increasing atmospheric CO 2 is changing the dynamics of tropical savanna vegetation. C 3 trees and grasses are known to experience CO 2 fertilization, whereas responses to CO 2 by C 4 grasses are more ambiguous. Here, we sample stable carbon isotope trends in herbarium collections of South African C 4 and C 3 grasses to reconstruct 13 C discrimination. We found that C 3 grasses showed no trends in 13 C discrimination over the past century but that C 4 grasses increased their 13 C discrimination through time, especially since 1950. These changes were most strongly linked to changes in atmospheric CO 2 rather than to trends in rainfall climatology or temperature. Combined with previously published evidence that grass biomass has increased in C 4 -dominated savannas, these trends suggest that increasing water-use efficiency due to CO 2 fertilization may be changing C 4 plant-water relations. CO 2 fertilization of C 4 grasses may thus be a neglected pathway for anthropogenic global change in tropical savanna ecosystems., (© 2024 The Author(s). New Phytologist © 2024 New Phytologist Foundation This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.)