# Minimal impact of recent decline in C4 vegetation abundance on atmospheric carbon isotopic composition

## TL;DR

The decline in C4 plants since 1982 has had minimal impact on atmospheric carbon isotope trends, suggesting other factors are at play.

## Contribution

A model-based analysis shows that shifts in C3/C4 plant abundance explain only part of the observed carbon isotope changes.

## Key findings

- The proportion of C4 plants in terrestrial biomass declined from 16% to 12% between 1982 and 2016.
- Global gross primary production increased by 16.5 ± 1.8 petagrams of carbon during the same period.
- Changes in C3/C4 abundance partially explain but do not fully account for observed atmospheric carbon isotope trends.

## Abstract

Changes in atmospheric carbon dioxide concentrations, climate, and land management influence the abundance and distribution of C3 and C4 plants, yet their impact on the global carbon cycle remains uncertain. Here, we use a parsimonious model of C3 and C4 plant distribution, based on optimality principles, combined with a simplified representation of the global carbon cycle, to assess how shifts in plant abundances driven by carbon dioxide and climate affect global gross primary production, land carbon isotope discrimination, and the isotopic composition of atmospheric carbon dioxide. We estimate that the proportion of C4 plants in total biomass declined from about 16% to 12% between 1982 and 2016, despite an increase in the abundance of C4 crops. This decline reflects the reduced competitive advantage of C4 photosynthesis in a carbon dioxide-enriched atmosphere. As a result, global gross primary production rose by approximately 16.5 ± 1.8 petagrams of carbon, and land carbon isotope discrimination increased by 0.017 ± 0.001‰ per year. Accounting for changes in C3 and C4 abundances reduces the difference between observed and modeled trends in atmospheric carbon isotope composition, but does not fully explain the observed decrease, pointing to additional, unaccounted drivers.

The fraction of C4 plants in terrestrial biomass decreased from 16% to 12% between 1982 and 2016, but the change is too small to explain the observed change in the isotopic composition of atmospheric carbon dioxide, according to estimates based on a C3/C4 distribution model and a carbon cycle box model.