Science - USA (2022-05-06)

eCO2 may not necessarily result in higher
woody cover. Possible explanations for the
small CO 2 effect on woody cover are that it
can be counteracted by impacts from climate-
driven changes ( 51 ) or by water or nutrient
limitations on vegetation productivity ( 17 ), or
fire could limit woody cover expansion in tropi-
cal grasslands and savannas ( 22 )[butsee( 28 )].
Generating comparable, high-resolution, multi-
proxy records to test our findings in different
biomes across the tropics is now a priority.
Our study confirms that an improved under-
standing of vegetation community change (C 4
versus C 3 plant response) under eCO 2 is needed
for dynamic global vegetation and Earth sys-
tem modeling of the future. The small effects
of CO 2 on past woody cover challenges the
long-term capacity of tropical savannas and
forests to slow anthropogenically eCO 2 through
increased greening as woody taxa recruitment
and growth are enhanced through CO 2 fertil-
ization effects, as predicted by these models
( 14 – 16 ). Therefore, our findings suggest that
the long-term effectiveness of proposed trop-

ical vegetation-based strategies to mitigate

anthropogenic eCO 2 through enhanced car-

bon sequestration rates ( 52 , 53 ) is contingent

on sufficient moisture availability.

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ACKNOWLEDGMENTS

We thank four anonymous reviewers and the editor for their extensive

comments, which have no doubt helped to improve the quality

of this manuscript.Funding:The past environmental change data

from Lake Bosumtwi used in this study were funded by a NERC/The

Open University Charter studentship (NE/H525054/1 to C.S.M.)

and a NERC New Investigator Award (NE/G000824/1 to W.D.G.).

We thank the Lake Bosumtwi Drilling Project team, the International

Continental Drilling Program, the US National Science Foundation,

and National lacustrine core repository (LacCore) USA for

supporting the acquisition and curation of Bosumtwi sediment

cores and age model data.Author contributions:The study

was conceived and developed by W.D.G. and F.v.L. The past

environmental change data from Bosumtwi was generated by C.S.M.

and W.D.G. The statistical analysis was conducted by F.v.L. Expertise

on past climate change and the Bosumwti sedimentary record was

provided by T.M.S. and J.T.O. Expertise on Earth system modeling

was provided by P.B.H. The manuscript was written by W.D.G. and

F.v.L. with contributions from all authors.Competing interests:The

authors declare no competing interests.Data and materials

availability:The past environmental change data reported in this

manuscript are archived in Figshare ( 54 ).

SUPPLEMENTARY MATERIALS

science.org/doi/10.1126/science.abg4618

Materials and Methods

Figs. S1 to S8

Table S1

References ( 55 – 93 )

Submitted 12 March 2021; accepted 29 March 2022

10.1126/science.abg4618

656 6 MAY 2022¥VOL 376 ISSUE 6593 science.orgSCIENCE

Fig. 3. Distributions of the standardized coefficients for the predictor variables in the SEMs for the
~500,000-year interval for Bosumtwi (Fig. 2C).The SEMs were run against 1000 possible alternative versions
of the chronology to explore for possible bias caused by chronological uncertainty. The predictor variables are
shown per response variable (fire and woody cover). The most important drivers of woody cover are moisture
availability and fire because they do not include zero in their distributions. The statistics are reported in ( 33 ).

RESEARCH | REPORTS