# Photosynthetic Responses of Forests to Elevated CO2: A Cross-Scale Constraint Framework and a Roadmap for a Multi-Stressor World

## TL;DR

This paper explains how forests respond to higher CO2 levels and why their ability to absorb carbon is limited by nutrients and other factors.

## Contribution

The paper introduces a cross-scale framework to understand how multiple stressors limit forest growth under elevated CO2.

## Key findings

- Nutrient availability, especially nitrogen and phosphorus, is the main long-term constraint on forest productivity under elevated CO2.
- Photosynthetic acclimation reduces the initial boost in carbon uptake from higher CO2 levels.
- Forest responses to CO2 depend on interactions with water and other environmental factors.

## Abstract

Forests store carbon and so are often seen as a simple fix for climate change. In reality, their growth under higher carbon dioxide is limited by several everyday needs. This review follows carbon dioxide from the air into a leaf and then through the whole forest. The first step—the leaf’s sugar-making process—can rise for a while, but plants soon adjust and the boost fades. The next hurdle is food for plants: nutrients such as nitrogen and phosphorus. Without enough of these, extra carbon dioxide is like pressing the accelerator with an empty tank. Water and heat also matter, as do choices plants “make” about whether to build leaves, wood, or roots. Looking across many field trials and computer studies, we find that nutrient limits are the main brake on long-term gains. This matters for society because climate plans should not assume large, lasting growth everywhere. Targeting restoration on fertile soils and improving models to include nutrients and water can deliver more reliable climate benefits.

Rising atmospheric CO2 is expected to fertilize forest photosynthesis; yet, ecosystem-scale observations often reveal muted responses, creating a critical knowledge gap in global climate projections. In this review, we explore this paradox by moving beyond the traditional ‘CO2 fertilization’ paradigm. We propose an integrated framework that positions elevated CO2 as a complex modulator whose net effect is determined by a hierarchy of cross-scale constraints. At the plant level, photosynthetic acclimation acts as a universal first brake on the initial biochemical potential. At the ecosystem level, nutrient availability—primarily nitrogen in temperate/boreal systems and phosphorus in the tropics—emerges as the dominant bottleneck limiting long-term productivity gains. Furthermore, interactions with the water cycle, such as increased water-use efficiency, create state-dependent dynamic responses. By synthesizing evidence from pivotal Free-Air CO2 Enrichment (FACE) experiments, we systematically evaluate these constraining factors. We conclude that accurately predicting the future of the forest carbon sink necessitates a paradigm shift: from single-factor analysis to multi-stressor approaches, and from ecosystem-scale observations to an integrated understanding that links these phenomena to their underlying molecular and genetic mechanisms. This review provides a roadmap for future research and informs more realistic strategies for forest management and climate mitigation in a high-CO2 world.