4 | DISCUSSION
Our study provided a new and simple method of independently estimating
GPP and compared it to estimates from PRELES, a model parameterised with
EC data. The two methods yielded similar estimates for both annual
totals and seasonal patterns. We then used the two methods to compare a
fertilised to an unfertilised plot. Both methods detected higher GPP on
the F plot, but only when using the more abundant daily estimates
(Figure 7).
Several previous studies have estimated GPP from Scots pine forests in
northern Europe. Such EC estimates include 1001 g C
m-2 y-1 (Magnani et al., 2007), 940
g C m-2 y-1 (Kolari, Pumpanen,
Rannik, Ilvesniemi, Hari, & Berninger, 2004), 1047 g C
m-2 y-1 (Lagergren et al., 2008),
and 1072 g C m-2 y-1 (Duursma et
al., 2009). There have been two estimates that were independent of EC.
The first was a chamber-based estimate of 982 g C m-2y-1 (Zha, Xing, Wang, Kellomäki, & Barr, 2007). The
second, based on earlier measurements of NPP at our site, was
~1000 g C m-2 y-1(Lim et al., 2015). We compared our GPPiso/SF estimate
minus our standard deviation for the reference plot (1350 - 43 = 1303 g
C m-2 y-1) to the mean of these
published values plus the standard deviation (1007 + 43 = 1050 g C
m-2 y-1) and found that the
published values were consistently lower than our
GPPiso/SF estimate. The differences between the studies
and our GPPiso/SF can be attributed to differences in
site conditions, stand age, and method-specific shortcomings, including
the filters and corrections embedded in each case. We discuss the
strengths and weaknesses of each method below without claiming that
either method is the “true” value of GPP.