4.5 | Role of understorey species
A key difference between the GPP methods is that
GPPiso/SF quantified GPP of the trees only whereas
GPPPRELES quantified GPP of the whole ecosystem, which
included understorey GPP. Understorey GPP was 41 g C
m-2 in a 120-year-old Scots pine boreal forest
(Kulmala et al., 2011) and 5% of the ecosystem GPP in mixed spruce-pine
forest (Palmroth et al., 2019). PRELES estimated understorey GPP at our
site to be 7 and 9% of the ecosystem GPP, i.e., 90 g C
m-2 y-1 and 123 g C
m-2 y-1, on the reference and the
fertilised plots in 2013, respectively (Tian et al., under
review.). In other words, this preliminary estimate of fertilisation
treatment would induce 2% increase of understorey GPP in 2013. A direct
comparison of tree GPP between the sap flux/isotopic and PRELES
(GPPPRELES-7% and 9%) method would lead to 1369 vs
1194 g C m-2 y-1 in the R plot and
1483 vs 1248 g C m-2 y-1 in the F
plot. However this estimate needs more replicates to confirm the
understorey contribution to global GPP. As these methods continue to
improve, it may become possible to solve for understorey GPP by
difference. Note that if a next study shows that the fertilisation
significantly increased understorey GPP, then GPPiso/SFwould not detect it, but GPPPRELES would. Future work
should explore this possibility.