Results
Interspecific variation in leaf
NSC
In this study, there were significant species variations in leaf soluble
sugar, starch, and NSC concentrations (p < 0.001 for
all, Table 2). In the CK treatment, the leaf soluble sugar concentration
of S. octophylla was generally higher than that of the other
three species, with concentrations of 124.54, 174.28, and 179.58 mg
g-1, in 2012, 2015, and 2017, respectively. In
addition, the starch concentration of S. octophylla in the CK
treatment was approximately twice as high as the other three species
(Fig. 1, e-h). The NSC concentrations of S. octophylla in the CK
plot were generally higher than those of the other three species (Fig. 1
i-l), with concentrations of 179.81, 219.00, and 199.44 mg
g-1, in 2012, 2015, and 2017, respectively.
Leaf NSC concentration in response to N and P
addition
Neither N addition nor N×P interactions had significant effects on the
total leaf NSC concentrations (p =0.746 and p =0.320,
respectively), while P addition significantly affected leaf NSC
concentrations (p <0.001). P addition significantly
influenced the NSC concentrations of U. macrophylla , S.
octophylla, and P. rubra in 2012, those of S. octophyllain 2015, and those of P. rubra in 2017. Overall, +P reduced the
NSC concentrations by 18.14% (except for U. macrophylla ),
15.13%, and 24.68% (except for U. macrophylla and S.
octophylla ), in 2012, 2015, and 2017, respectively (Fig. 2).
In this tropical forest, neither N addition nor N×P significantly
affected leaf soluble sugar concentrations (p =0.719 andp =0.866, respectively, Table 2). However, the leaf soluble sugar
concentrations were significantly changed by P addition
(p =0.016). Overall, +P decreased the leaf soluble sugar
concentrations by an average of 12.78% across the three sampled years
(except for S. bullockii in 2012 and U. macrophylla in
2017, Fig. 2). Neither N nor P addition had significant effects on leaf
soluble sugar concentrations in any of the species in 2012 (Table S1).
However, there was a significant reduction in the soluble sugar
concentrations of S. octophylla in 2015 and those of P.
rubra in 2017 under P addition (Table S1).
Nitrogen addition did not change the leaf starch concentrations of the
sample species (Table 2). However, both P addition and N×P interactions
significantly affected starch concentrations (p =0.001 andp =0.030, respectively). Overall, +P reduced leaf starch
concentrations by 38.10% (except for S. octophylla in 2017, Fig.
2), +NP also decreased the starch concentrations by 28.28% (except forU. macrophylla and P. rubra in 2015, Fig. 2).
Additionally, P addition also had significant effects on the leaf starch
concentrations of P. rubra in 2012, and those of S.
bullockii in 2015 (Table S1).
Although the ratios of soluble sugar to starch showed significant
interspecific variation (p <0.001), this ratio was
generally not affected by N, P, nor N×P interaction at the intra-species
level in this study (Table S1).
Relationships between leaf NSC and other
traits
Leaf soluble sugar and total NSC concentrations, and the ratios of
soluble sugar to starch were significantly negatively related to the LMA
(Table 3). Leaf starch concentrations was significantly positively
correlated with photosynthesis (p <0.05). In addition,
both the soluble sugar and NSC concentrations were significantly related
to the leaf N concentrations (p <0.05). Although there
was no clear relationship between NSC (soluble sugar and starch) and
leaf P concentrations, the structural P concentration had significantly
positive correlations, while residual P concentration had significantly
negative correlations, with soluble sugar and NSC concentrations, and
the ratios of soluble sugar to starch (p <0.05, Table
3).