Report - ICP Forests

3. Intensive Monitoring 83
3.4.5 Discussion
In contrast to the previous analysis of De Vries at el. (2002), where the effect of N-deposition
was only just significant, there are now clearly significant (sometimes even highly significant)
effects of N-deposition. These effects are found both when using EMEP model output and when
using measured throughfall (and sometimes bulk deposition) as estimates for deposition.
Significant effects of the 'calculated' deposition were not found. Although the effect of
deposition on the individual species cannot be clearly defined, the effect on the vegetation as a
whole is a shift towards nitrophytic species, which is found irrespective of the estimator for N
deposition (modelled or measured). In most cases the effect is due to the deposition of NO 3 ;
effects of NH 4 deposition were not found. This agrees with the analysis of De Vries et al. (2002)
who also reported an effect of NO 3 . On the basis of the present analysis it is not possible to
determine whether bulk or throughfall measurements or the EMEP model yields the 'best'
estimates for the 'true' deposition. On average, EMEP estimates seem to be a slightly better
predictor for the vegetation than measured deposition, however the difference appears to be
small and a real comparison is hampered by the far larger number of plots that have EMEP
deposition estimates compared to bulk and throughfall measurements. Also, being a better
predictor for the vegetation is not a guarantee for being a better estimator for the true value of
the deposition. The present analysis is solely based on a comparison of the spatial patterns of
deposition and vegetation, and the absolute values are irrelevant in this type of statistical
evaluation.
The conclusion that the composition of the ground vegetation mainly depends upon the
traditional factors soil, climate and dominant tree species is not different from the conclusion of
De Vries et al. (2002). However, the present study yields clear indications for a small but
significant effect of NO 3 deposition. This effect of N deposition is even larger when the change
in vegetation is considered instead of the vegetation at a single point in time. It is not possible to
determine whether the change in the vegetation coincides with a change in the N deposition
itself because the period over which the change was considered is different per plot both in
starting point and in length. This is true for both the vegetation data and the measured deposition
data, and their periods do not necessarily coincide. Moreover, EMEP simulations were used at
only two points in time (1995 and 2000), and the deposition significantly decreased (P < 0.001)
between these points in time for both N-total, NH 4 , NO 3 and SO 4 . However, there is no
significant trend in measured deposition. The vegetation in the last relevé per plot is best
explained by the EMEP simulation for 2000, while the change is better explained by the
simulation for 1995 (but note that a better fit for a certain date is caused by differences in the
spatial pattern at the two dates and not by the absolute amount of deposition).
It is difficult to indicate the exact nature of the vegetation change induced by N deposition.
There were no large changes in single species, but rather small changes occurring over a wide
range of species. Therefore the change is only apparent for generalised measures viz. those
derived from multivariate statistics, or indicator values. The change is in agreement with the
expected change at increasing N availability (increasing Ellenberg-N, increase in some
individual nitrophytic species). National studies from France and Switzerland indicate that a
shift towards more nitrophilic species is partly due to changes in the forest canopy, induced by
storms. Less dense canopies support mineralization processes in the forest soils and thus can
increase nitrogen availability.
There is no apparent explanation for the strong increase of the number of species per plot.
Possible explanations are (1) N deposition, (2) climate change, and (3) methodological causes. It