First preliminary results about the effects of the drought summer 2018 on forest trees

Tree water deficit (TWD)

The extremely dry summer 2018 left clear effects in forest trees. Trees at about 25 of the 30 measured TreeNet-sites showed record-high values of TWD exceeding any measurements of the previous five years before. TWD is known to be a largely tree-species independent indicator for missing water at a site explicable with the physical conditions in air (vapor pressure deficit) and soil (soil water potential).

Record-high tree water deficits (TWD) and strongly varying radial stem growth in 2018.

TWD is an indicator for missing water in a tree. The larger TWD is the higher is TWD and the greater is the drought stress of the trees. The box plots show the average range of TWD during the stem growth period at the TreeNet sites over five years since 2014. The red dots indicate the mean value of 2018. At most sites TWD was exceeding the previous measurements, whereas the growth response (GRO) of the trees varied from below average to above average.

Radial stem growth (GRO)

Radial stem growth responded much more heterogeneous than TWD to the dry and hot summer 2018. Trees at some sites had record-high radial stem increments in 2018 whereas other sites showed very low GRO. There is clear species-specific GRO responses to the year 2018. Spruce responded with low GRO in general whereas GRO of beech varied strongly.

Tree water deficit (TWD) and radial stem growth (GRO) of beech

Beech responded site-specifically within a wide range of GRO and TWD. At sites with enough soil water, beech was able to benefit from the high temperatures (high GRO), whereas at sites with dry soil the response was negative (high TWD and low GRO). The colored box plots show average ranges of TWD and GRO over five years. The red dots indicate the response in 2018.