Drought response of European beech trees is not straight forward

Drought and rising temperatures can cause massive forest diebacks around the world. When a tree is experiencing drought, “shedding off” the leaves can enhance tree water status by reducing canopy transpiration. Whilst botanists can investigate the effect of drought relatively simply in a greenhouse of a single species, forests are complex ecosystems and require long-term research efforts. 

Dr Sylvie Oddou-Muratorio from INRAE and colleagues from Sorbonne University and local nature reserves investigated the relationship between tree growth, reproductive success (fecundity) and defoliation over a decade of European beech populations in Southern France. The scientists found that some trees reduce their growth and maintain their reproduction rate whilst others keep on growing but with reduced fecundity. 

Researchers have been monitoring European beech trees in the Massane Forest National Nature Reserve in Southern France since 2002. This location is prone to spring and summer drought. The team selected 439 trees in two plots and measured their growth between 2002-2012. 

Oddou-Muratorio and colleagues recorded branch mortality, competition with other trees and scored annual defoliation (how often a tree drops its leaves and whether it can regrow them). To understand the reproductive success of these trees, the researchers counted and sampled all the seedlings around the adult trees. 

The team established the origin of 365 seedlings with genotyping, and the reproductive success of male and female trees was analysed with Mixed Effect Mating Models (MEMMs). All this data was used to investigate the impact of environmental conditions on tree growth and reproduction.

“This study is among the rare ones to bring observational evidence that increasing crown defoliation decreases individual female fecundity”, Oddou-Muratorio wrote.

The scientists found that both female reproductive success and growth decreased with increasing crown defoliation and competition amongst trees. Some defoliated trees maintained most of the female fecundity at the expense of reduced growth, while others maintained high growth at reduced female fecundity. 

“The most vulnerable individuals (those that are the most impacted by stress) still contribute to regeneration, which could lead the population to evolve traits compromising its adaptation to stress.”

These observations highlight the need for long-term monitoring of forest trees experiencing drought. Drought response is not straightforward. In a greenhouse study, scientists would have been able to look at the tree growth and anatomical changes, but the competition between trees and seedling recruitment could not be examined on a large scale. This project has shown that trees have different growth and reproduction investment strategies.

“Our study overall stresses the need to investigate simultaneously the impact of climate change on reproduction, growth and survival, and how the inter-individual variations in these responses may affect the adaptation or maladaptation of forest trees population in face of climate change.”