*3.1. Variations of P. crassifolia Leaf* δ*13C and* δ*15N Values with Leaf and Tree Ages*

Changes of leaf δ13C and δ15N in relation to leaf age and tree age are shown in Figures 2 and 3. All leaf <sup>δ</sup>13C values varied from <sup>−</sup>28.22‰ to <sup>−</sup>24.09‰, with a mean of <sup>−</sup>26.76‰ and a variance of

3.39%. The average carbon isotopic values from current year to 3-year-old leaves were −25.54‰, <sup>−</sup>27.15‰, <sup>−</sup>27.24‰ and <sup>−</sup>27.07‰, respectively (Figure 2A). The <sup>δ</sup>13C was significantly more enriched in current year leaves than others at each tree age level (*p* < 0.01, Figure 3A), while no differences were observed among other older leaves. Meanwhile, at each leaf age level, the δ13C value of mature trees (>150 years old) was lower than that of other aged trees (Figure 2A), but the difference was not significant (Figure 3A).

The mean leaf <sup>δ</sup>15N of all samples was <sup>−</sup>5.91‰, with a range of <sup>−</sup>8.7‰ to <sup>−</sup>2.89‰ and a variance of 16.80%. The average nitrogen isotopic values from current year to 3-year-old leaves were −4.86‰, <sup>−</sup>6.00‰, <sup>−</sup>6.15‰ and <sup>−</sup>6.55‰, respectively (Figure 2B). Leaf <sup>δ</sup>15N showed substantial variability among leaf ages and tree ages. Older leaves were more depleted in 15N than current year leaves at each tree age level (Figure 3B). Moreover, there was a significant difference in δ15N between middle-aged trees (50–100) and juvenile trees (<50) except for 1-year-old leaves (*p* < 0.05).

**Figure 2.** Box and scatter plots showing that the spatial patterns and variations of the δ13C (**A**) and δ15N (**B**) values changed with differently aged leaves and trees of *P. crassifolia.* Leaf ages including current year to 3-year-old leaves were represented by cy, 1y, 2y, and 3y, respectively.

**Figure 3.** Differences in leaf δ13C (**A**) and δ15N (**B**) between 4 years' leaves collected along different tree age levels. The value was Mean± SE (standard error). Different uppercase letters represent significant differences among leaf ages at each tree age level, while different lowercase letters represent significant differences among tree ages at each leaf age level.
