3.2. Determination of the seasonal time course of leaf nitrogen
content
Since leaf nitrogen content (NL) is affected not only by nitrogen
supply but also by temperature and radiation, the accumulated
photo-thermal index (PTIsum) was used to describe the seasonal
time course of NL. The photo-thermal index (PTI)was defined as the
daily average normalized thermal timemultiplied by the daily total
PAR intercepted by a canopy (Xu et al., 2010), and was calculated
as follows:
PTI(j) =
1
24
24
h=1
TT(h, j)
× PARint(j) (1)
where PTI(j) (MJm−2 d−1) is the photo-thermal index on day j;
PARint(j) (MJm−2 d−1) is the daily total PAR intercepted by the
canopy on day j; TT(h,j) is the hourly based normalized thermal
time on day j. PARint(j) and TT(h,j) were calculated according to Xu
et al. (2010) using Eqs. (1a)–(1c) in Appendix A.
.
3.2 3.2. Determination of the seasonal time course of leaf nitrogen
content
Since leaf nitrogen content (NL) is affected not only by nitrogen
supply but also by temperature and radiation, the accumulated
photo-thermal index (PTIsum) was used to describe the seasonal
time course of NL. The photo-thermal index (PTI)was defined as the
daily average normalized thermal timemultiplied by the daily total
PAR intercepted by a canopy (Xu et al., 2010), and was calculated
as follows:
PTI(j) =
1
24
24
h=1
TT(h, j)
× PARint(j) (1)
where PTI(j) (MJm−2 d−1) is the photo-thermal index on day j;
PARint(j) (MJm−2 d−1) is the daily total PAR intercepted by the
canopy on day j; TT(h,j) is the hourly based normalized thermal
time on day j. PARint(j) and TT(h,j) were calculated according to Xu
et al. (2010) using Eqs. (1a)–(1c) in Appendix A.
.
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