Tree canopy defoliation can reveal growth decline in mid-latitude temperate forests

Climate and weather fluctuations and changes are the most important environmental driver of tree canopy defoliation, an indicator of forest health. We examined the relationship between tree defoliation and Basal Area Increment (BAI), a dimension of tree growth related to wood biomass increment, carbon sequestration and therefore to the climate change mitigation potential of forests. We analysed data from mostly even-aged, single-species permanent monitoring plots in France over two growing periods (1995-2004: 47 plots, 2008 trees; 2000-2009: 63 plots, 3116 trees) and for which precipitation deficit was identified as the main environmental driver of defoliation. Trees from ten different species were assessed annually for defoliation and measured periodically for growth, from which we derived periodical (10-year) BAI (BAIperiod). We investigated (i) direction and significance of defoliation-BAIperiod relationship and (ii) occurrence, size and significance of BAI deviation of progressively defoliated trees in proportion to the BAI of undefoliated trees (BAIrel). Analyses were first carried out at the level of individual plots, with results subsequently evaluated using meta-analysis, and further aggregated at different levels (all species, functional groups, individual species). BAIperiod resulted negatively and significantly related to defoliation, with a significant reduction detected already at slight (15%) defoliation level. A generalized statistically significant reduction of BAIrel was obvious, leading to an estimated reduction of 0.7 - 0.8% per unit increase of defoliation for conifers, and 0.9% for broadleaves. Considering the observed distribution of trees along the defoliation range, our results indicate an overall growth reduction of ca. 42% in comparison to a theoretical population of undefoliated trees. Shifts in such a distribution can result into loss or gains of growth, which in turn may have cascading effects on carbon sequestration and therefore on land-climate interactions. In the context of the significant increase in defoliation observed in Europe in recent decades, our results suggest that even slight and moderate variations in defoliation may have had a significant impact on tree and forest growth.