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.