Foliage deficit: a tool to describe tree vitality

Foliage deficit refers to an estimation of tree foliage loss (graded from 0 to 100%). This indicator reflects all of the potential hazards to which the tree may be exposed.

It is in the tree's leaves that photosynthesis takes place and where water, mineral elements taken up from the soil and carbon dioxide (CO2) absorbed from the air are combined to create the compounds the tree needs to live. If the tree's foliage is reduced due to climatic events (droughts, hail, storms...), damage by pests (insects...), parasitic infestations (mistletoe...), fungi, nutrient or water deficiency in the soil or human disturbance (soil compaction, pollution...), the tree's growth, reproductive capacity or even survival can be affected.

Monitoring foliage deficit is therefore an interesting way to evaluate tree health and determine how trees respond to environmental stresses which may lead to dieback or decline.

An increase in foliage deficit indicates that the tree's health is deteriorating; inversely, a decreasing foliage deficit indicates an improvement in overall tree health.

From 1999 to 2009 (no foliage observations were made in 2003), a trend toward a limited increase in foliage deficit was often observed. However, more locally, trends differed from site to site, and in certain cases, foliage deficit decreased. These differences depended in particular on tree species.

For silver fir and Scots pine, a stable foliage deficit rate of around 10% was observed. For European spruce, sessile oak and beech, the foliage deficit increased after 2004 in the aftermath of the drought/heat wave in 2003, then stabilised during the following years (at 20% for European spruce and beech and at 25% for sessile oak). Pedunculate oak showed a different trend: after an increase in foliage deficit in the years following 2003, quite rapidly the deficit returned to levels similar, or even inferior (20%) to those before 2003.

Many different factors can cause leaf loss but the main parameters explaining the variations in foliage deficit from one year to the next are linked to the amount of water available to the trees. Typically, a foliage deficit is observed in the years following a drought. However, in some cases, the reverse is true, with a foliage deficit appearing after a rainy year.

The amount of water available to trees does not depend solely on precipitation. The ability of the soil to store water must also be taken into account. Indeed, a drought will not have the same impact on a stand growing in soil with a limited water storage capacity as it will if the soil has a high storage capacity. Furthermore, some soils can become temporarily waterlogged, thus preventing the tree roots from breathing.

On average, trees which are subjected to a higher foliage deficit show less diameter growth. Compared to perfectly healthy trees, this reduction is visible from the first stages of foliage deficit, then intensifies as foliage is lost (1% loss in foliage ≈ 1% decrease in growth rate).