Report - ICP Forests

46 3. Intensive Monitoring
in beech stands because stemflow was not taken into account in the present study as it had not
been measured continuously from 1998 to 2007 on most plots.
The observed annual mean throughfall deposition is interpreted always together with the
respective bulk deposition in order to allow for an estimation of effective enriching and reducing
canopy effects. The plot specific annual sums of bulk and throughfall deposition of nitrate
(NO 3 ), ammonium (NH 4 + ), sulphate (SO 4 2- ), calcium (Ca 2+ ), sodium (Na + ), and chlorine (Cl - )
were basis for the evaluations. Bulk and throughfall depositions expressed in kg ha -1 yr -1 in the
text and in the figures refer to the chemical element considered, e.g. to sulphur (S-SO 4 2- ) instead
of sulphate (SO 4 2- ).
Data selection criteria and calculations follow the approach already described by LORENZ et al.
(2005) for the calculation of deposition data from 1996 to 2001. The numbers of plots with
available data fulfilling the selection criteria for mean annual deposition calculations from the
year 2005 to 2007 are presented in table 3.2.1-1. In addition to mean annual deposition rates, the
development of throughfall and bulk deposition over time was object of the present study. The
slope of plot specific linear regression over the years of observation was used for mapping and
quantifying the general temporal devolpments. Whereas in previous reports temporal trends
were presented for 6 consecutive years, this evaluation prolongs the period to a time span of 10
years, i.e. to the years from 1998 – 2007. Due to the continuously ongoing monitoring activities
this prolongation still resulted in a satisfying high number and spatial resolution of plots
fulfilling the selection criteria (Table 3.2.1-1).
Table 3.2.1-1: Number of plots which fulfilled the selection criteria.
No. of observations Na + Cl - Ca 2+ +
N-NH 4
-
N- NO 3
2-
S- SO 4
Trend
Bulk 155 156 155 155 156 151
1998 – 2007 Throughfall 163 164 163 163 164 157
Mean
Bulk 288 288 288 288 288 288
2005 – 2007 Throughfall 215 214 215 215 215 215
Throughfall
> Bulk
169
of 205
179
of 204
187
of 205
131
of 205
162
of 205
168
of 205
The slopes of the linear equations were statistically tested and depicted in maps according to the
following classification:
Decrease: negative slope, error probability lower or equal 5% (green)
No change: negative slope with error probability greater than 5%, or same deposition in
each year, or positive slope with error probability greater than 5% (yellow)
Increase: positive slope, error probability lower or equal 5% (red)
Even with an enlarged time span of ten years, results must be understood as a mere description
of the changes over time rather than a trend analysis which would require an even longer period
of observation and respective statistical models for time series analyses.
Sulphate is an important constituent of sea salt, and in many coastal areas (e.g. western Norway)
most sulphate in deposition may originate from sea salt rather than anthropogenic sources. As
the relationship between chloride and sulphate in sea water is almost constant and assuming that
chloride is almost entirely derived from sea salt and hardly affected by biogeochemical
processes (which may not always be correct), measured sulphate concentrations can be easily
corrected for the sea salt contribution using the formula
non-marine S-SO 4 2- = total S-SO 4 2- - (0.054 * Cl - ) ; where all values are in mg/l.