Forest Condition in Europe - ICP Forests

80 Forest Condition in Europe 2013
Spruce: The percentage of damage classes 2 to 4 is 27 %, which means no change compared
to the previous year. 35 % (2011: 33 %) of the trees were in the warning stage. 38 % (2011:
40 %) showed no defoliation. Mean crown defoliation increased from 19.1 to 19.3 %.
Scots pine: The share of damage classes 2 to 4 decreased from 13 % to 11 %. 39 % (2011:
42 %) are in the warning stage. 50 % (2011: 45 %) showed no defoliation; this is the best result
since the beginning of the surveys. Accordingly the mean crown defoliation of Pine decreased
to 14.5 % which is the lowest score since the beginning of the surveys in 1984.
Beech: The share of damage classes 2 to 4 decreased by 19 percentage points from 57 to
38 %. 40 % (2011: 31 %) were classified in the warning stage. The share of trees without defoliation
increased from 12 to 22 %. Mean crown defoliation decreased from 30.4 to 24.3 %.
The high defoliation rates in 2011 were mainly due to the intense fruiting of the beech trees.
In 2012 almost no fruiting was recorded and the trees were able to recover. However the defoliation
rates of beech are still higher than they were in the period before 2004. In 2004, the
crown condition of beech trees worsened due to the extreme drought and heat in summer 2003
and did not yet completely regenerate.
Oaks: The share of damaged trees increased to 50 % (2011: 41 %). The share of the warning
stage was 33 % (2011: 38 %). Only 17 % (2011: 21 %) of the oaks showed no defoliation.
Mean crown defoliation was 29.4 % (2011: 26.3 %). High defoliation rates in oaks have already
being recorded during the past ten years. This is partly due to defoliation by a number
of insects belonging to different species. The “oak defoliator community” includes caterpillars
of different moth species, namely the green oak leaf roller (Tortrix viridana), the mottled umber
moth (Erannis defoliaria) and the winter moth (Operophthera brumata) and, regionally,
also the gypsy moth (Lymantria dispar). Recently published research regarding the susceptibility
of oaks for herbivory by the green oak leaf roller showed biochemical and genetic differences
between oak types within the same species (Quercus robur). It has been found that
oaks tolerant against herbivory use the strategy to emit deterring volatiles to avoid the moths
laying their eggs, whereas susceptible oaks (meaning most defoliated) emit volatiles to attract
predators of the moth’s larvae. Furthermore, a different food quality for the moth’s larvae
between tolerant and susceptible oak leaves has been found: larvae prospered better on susceptible
than on tolerant oaks. Most conspicuous is the result that T. viridana uses the predators-attracting
volatiles of the susceptible oaks to find most suitable hosts for its offspring.
Thus, it appears that the plants’ volatiles thought to function as defensive compounds actually
act as a boomerang signal that attracts this pest species.
Some laender report significant damage caused by the oak processionary moth (Thaumetopoea
processionea). Until recently, this species was considered to be mainly a human health
problem. From the third larval stage on the caterpillars are covered in hairs containing toxins
that cause skin irritations and allergic reactions. In recent years, this species has extended its
range to more northern and eastern regions of Germany. Shoots appearing after insect damage
events are often affected by mildew.
Figure 1 shows the input of acid deposition for selected Level-II plots in Germany over a minimum
of 15 years. The acid deposition is composed of the input of H + , NH 4 -N and canopy
uptake (CU); while NH 4 -N acidifies the soil due to its complete transformation to NO 3 -N, CU
describes the canopy uptake of H and NH 4 -N. This term is calculated using the total leaching
of Ca 2+ , Mg 2+ , K + and Mn 2+ minus the canopy leaching of those elements associated with foliar
excretion of weak acids (Draaijers and Erisman 1995).
Generally the acid deposition decreases in most of the plots, whereas the single components
develop differently. The reduction concerns especially H and CU, however, the input of NH 4 -
N does not change noticeably. The decrease of H is particularly obvious for plot 1402, which