Leaf structural and photosynthetic characteristics, and biomass allocation to foliage in relation to foliar nitrogen content and tree size in three Betula species

Abstract

Young trees 0.03-1.7 m high of three coexisting Betula species were investigated in four sites of varying soil fertility, but all in full daylight, to separate nutrient and plant size controls on leaf dry mass per unit area (MA), light-saturated foliar photosynthetic electron transport rate (J) and the fraction of plant biomass in foliage (F(L)). Because the site effect was generally non-significant in the analyses of variance with foliar nitrogen content per unit dry mass (N(M)) as a covariate, N(M) was used as an explaining variable of leaf structural and physiological characteristics. Average leaf area (S) and dry mass per leaf scaled positively with N(M) and total tree height (H) in all species. Leaf dry mass per unit area also increased with increasing H, but decreased with increasing N(M), whereas the effects were species-specific. Increases in plant size led to a lower and increases in N(M) to a greater FL and total plant foliar area per unit plant biomass (LAR). Thus, the self-shading probably increased with increasing N(M) and decreased with increasing H. Nevertheless, the whole-plant average M(A), as well as M(A) values of topmost fully exposed leaves, correlated with N(M) and H in a similar manner, indicating that scaling of MA with N(M) and H did not necessarily result from the modified degree of within-plant shading. The rate of photosynthetic electron transport per unit dry mass (J(M)) scaled positively with N(M), but decreased with increasing H and M(A). Thus, increases in M(A) with tree height and decreasing nitrogen content not only resulted in a lower plant foliar area (LAR = F(L)/M(A)), but also led to lower physiological activity of unit foliar biomass. The leaf parameters (J(M), N(M) and M(A)) varied threefold, but the whole-plant characteristic FL varied 20-fold and LAR 30-fold, indicating that the biomass allocation was more plastically adjusted to different plant internal nitrogen contents and to tree height than the foliar variables. Our results demonstrate that: (1) tree height and N(M) may independently control foliar structure and physiology, and have an even greater impact on biomass allocation; and (2) the modified within-plant light availabilities alone do not explain the observed patterns. Although there were interspecific differences with respect to the statistical significance of the relationships, all species generally fit common regressions. However, these differences were consistent, and suggested that more competitive species with inherently larger growth rates also more plastically respond to N and H.

Fig. 1. Scatter plot of foliar nitrogen content per unit dry mass vs. total tree height. The data were sampled from the bog (poor site) and from the clear fell sites in Viimsi (B. pubescens) and forest edges in Voore (B. pendula, Table 1). Average foliar nutrient contents (Table 1) provided the primary criterion to distinguish the sites on the basis of nutrient availability (fertile vs. infertile). The same symbols are used for the Viimsi data because (1) the primary difference between the Viimsi sites (Viimsi‐1, and Viimsi‐2, Table 1) was only the age of the clear fells, and (2) the sites did not differ in measured soil characteristics and foliar nutrient contents. The linear correlation is non‐significant (r = 0·21, P > 0·06 for all data pooled).

Fig. 2. Dependence of leaf size on foliar nitrogen content and on total tree height in Betula nana, B. pendula and B. pubescens. All data were fitted by a single multiple linear regression: logS = –0·190 + 0·358logH + 0·447N_M (r² = 0·60, P < 0·001 for both logH and N_M, P > 0·1 for the intercept). Symbols as in Fig. 1.

Fig. 3. Correlations between leaf dry mass per unit area (M_A) and leaf nitrogen content per unit dry mass (A) and N per unit area (B). In A, all data were fitted by a single linear regression (r² = 0·33, P < 0·001). In B, separate regressions were fitted for all sample points from the infertile site (upper regression line, r² = 0·31, P<0·001) and for B. pubescens from the fertile site (lower regression line, r² = 0·85, P<0·001). Symbols as in Fig. 1.

Fig. 4. Effects of leaf nitrogen content and total tree height on M_A (A) and on leaf area ratio (B). The regression surfaces were fitted through all data: M_A = 123·4 + 23·2logH – 18·8N_M (r² = 0·64) for A, and log(LAR) = 0·622 – 0·340LogH + 0·277N_M for B (r² = 0·49). All regression coefficients were significant at P < 0·001. Symbols as in Fig. 1.

Fig. 5. Leaf area ratio in relation to leaf dry mass per unit area. The regression was fitted through all data. Symbols as in Fig. 1.

Fig. 6. Relationships between the rate of light‐saturated photosynthetic electron transport (J) per unit leaf dry mass in dependence on leaf nitrogen content per unit dry mass (A) and M_A (B); and J per unit leaf area in relation to nitrogen content per unit area (C) and M_A (D). The rate of photosynthetic electron transport was determined from chlorophyll fluorescence measurements (eqn. 2). The data were fitted by linear regressions. Symbols as in Fig. 1.