We are pleased to share with you a new paper involving the lead PI of the SPECIAL group, Sandy Harrison. The paper outlines a new framework for modelling leaf area index (LAI) globally using the simplicity of eco-evolutionary optimality (EEO) theory and performs as well as, or better than, more complex dynamic global vegetation models.
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The paper “A Unifying Principle for Global Greenness Patterns and Trends.” was led by Wenjia Cai at Imperial College London and was produced as a contribution to the LEMONTREE project. The study introduces a minimalist framework for modelling LAI providing new insights into the mechanisms driving vegetation greenness and its response to environmental changes.
Vegetation plays a pivotal role in regulating the exchange of carbon, water, and energy between terrestrial ecosystems and the atmosphere, often quantified by the leaf area index (LAI). This vegetation characteristic determines how plants absorb photosynthetically active radiation (PAR) and governs processes like transpiration, a major contributor to global evaporation.
This study introduces an approach to modelling LAI based on eco-evolutionary optimality (EEO) principles, using an equation with only two globally fitted parameters to predict annual maximum fractional absorbed PAR (fAPARmax). The framework hypothesises that fAPARmax is constrained by water-limited and energy-limited conditions
Water Limited: The fraction of precipitation accessible to plants. This reflects the plant’s ability to capture water for growth, with higher values indicating greater water use efficiency.
fAPARmax = f0 P ca (1 – χ) / (1.6 D A0)
Energy- Limited: The cost of maintaining and replenishing the plant canopy, measured in terms of carbon allocation. This parameter reflects the balance plants must strike between above-ground (canopy) and below-ground (root) investments.
fAPARmax = 1 – z / (k A0)
When tested globally, the framework effectively reproduces vegetation greenness patterns and temporal trends seen in remote-sensing data, attributing observed greening to CO₂-driven efficiencies and environmental changes, while browning is linked to drying.
In performance comparisons with 15 dynamic global vegetation models (DGVMs) from the TRENDY project, the LAI framework matched or outperformed the best models.
Figure 1. Comparison of our model with that of 15 DGVMs participating in the TRENDY project24 version 9
This approach demonstrates how minimalist modelling can capture key ecological processes with precision, advancing the understanding of vegetation responses to environmental changes and informing next-generation ecosystem models.
For a full breakdown of the papers key findings you can read the extended blog post on the LEMONTREE project website or you can read the full paper here:
Cai, W., Zhu, Z., Harrison, S.P., Ryu, Y., Wang, H., Zhou, B. & Prentice, I.C. 2025. A unifying principle for global greenness patterns and trends. Communications Earth and Environment, https://doi.org/10.1038/s43247-025-01992-0