# Variation in Tree Growth Increases With Global Warming

## TL;DR

Global warming is making tree growth more unstable, with growth variations increasing more than average growth rates.

## Contribution

First evidence that tree growth variability has increased 40% globally since the 20th century, faster than mean growth.

## Key findings

- Global tree growth variance increased 40% compared to 8.5% mean growth increase over the past century.
- Tree growth variance closely correlates with accelerated global warming since the 1970s (r = 0.93).
- Growth instability is higher in wetter habitats and less drought-resistant species under warming.

## Abstract

Global warming is raising both climate and weather variability. However, how this tendency may destabilise forest ecosystems is poorly understood. Using a set of global tree‐ring data, we calculated the 5‐year variance and mean of tree growth rate over 1401–2010, and modelled the variance–mean relationship. We found that the global averaged variance increased much faster than the mean in the past century (+40.0% vs. +8.5%), and closely covaried with the accelerated global warming since the 1970s (r = 0.93). The exponent of tree‐level variance–mean power law was higher in wetter habitats and less drought‐resistant species, and has increased significantly under global warming, indicating an environment‐ and trait‐dependent growth‐safety tradeoff and a decreasing resistance to a warmer climate. Our study shows that global warming may have strongly destabilised tree growth and made forest dynamics less predictable, adding to the growing concern that global warming is jeopardising the functioning of forest ecosystems.

In recent decades, considerable research has focused on the long‐term trend of tree growth rates under global warming, yet little attention has been paid to trends in high‐frequency tree growth variations—that is, year‐to‐year differences in growth rates. Our study reveals for the first time that, over the past century, the global average tree growth variation has increased by 40%, a far more pronounced shift than the 8.5% increase observed in long‐term mean tree growth rates. These findings indicate that global forest growth is becoming significantly more unstable, which may jeopardize the well‐functioning of forest ecosystems.