J Plant Res DOI 10.1007/s10265-016-0826-z
REGULAR PAPER
Nutrient allocation among plant organs across 13 tree species in three Bornean rain forests with contrasting nutrient availabilities Ryota Aoyagi1 · Kanehiro Kitayama1
Received: 17 January 2015 / Accepted: 16 February 2016 © The Botanical Society of Japan and Springer Japan 2016
Abstract Allocation of nitrogen (N) and phosphorus (P) among plant organs is an important factor regulating growth rate, which is a key ecological process associated with plant life-history strategies. However, few studies have explored how N and P investment in photosynthetic (leaves) and non-photosynthetic (stems and roots) organs changes in relation to depletion of each element. We investigated nutrient concentrations of plant organs in relation to whole-plant nutrient concentration (total nutrient weight per total biomass) as an index of nutrient status of each individual using the saplings of the 13 species in three tropical rain forests with contrasting N and P availabilities (tropical evergreen forests and tropical heath forests). We found a steeper decrease in foliar N concentration than foliar P concentration with decreasing whole-plant nutrient concentration. Moreover, the steeper decrease in foliar N concentration was associated with relatively stable N concentration in stems, and vice versa for P. We suggest that the depletion of N is associated with a rapid dilution of foliar N because the cell walls in non-photosynthetic organs function as an N sink. On the other hand, these species can maintain foliar P concentration by decreasing stem P concentrations despites the depletion of P. Our results emphasize the significance of non-photosynthetic organs as an N sink for understanding the variation of foliar nutrient Electronic supplementary material The online version of this article (doi:10.1007/s10265-016-0826-z) contains supplementary material, which is available to authorized users. * Ryota Aoyagi
[email protected] 1
Graduate School of Agriculture, Kyoto University, Kitashirakawa Oiwake‑cho, Sakyo‑ku, Kyoto 606‑8502, Japan
concentrations for the tree species in the three Bornean rain forests with different N and P availabilities. Keywords Cell walls · Functional traits · Mixed dipterocarp forests · Nutrient productivity · Photosynthetic and non-photosynthetic organs · Tropical heath forests
Introduction Nitrogen (N) and phosphorus (P) are essential limiting resources for plant growth (Elser et al. 2007), and the balance of available N and P considerably varies across the landscape of terrestrial ecosystems (Coomes 1997; Crews et al. 1995; Laliberté et al. 2012; Moran et al. 2000; Richardson et al. 2008). Many studies have reported that plants growing on either N- or P-poor soils have convergent traits, such as a larger leaf mass per area, a longer leaf life span and a lower maximum photosynthetic rate (Chapin 1980; Craine 2009; Wright et al. 2002), associated with a lower growth rate. On the other hand, several studies demonstrate that plants in N-poor and P-poor conditions have different traits, e.g. a slower leaf turnover and a higher leaf mass per area in N-poor conditions (Aoyagi and Kitayama 2015; Cordell et al. 2001; Güsewell 2004, 2005; Hayes et al. 2014). These results suggest that the depletion of nutrients causes different consequences of plant adaptation and growth rate between N and P (Güsewell 2004). If so, the mechanisms why plants demonstrate different responses to N vs. P depletion need to be studied. Resource allocation among organs (leaves, stems and roots) is an important factor associated with plant growth and life-history strategies (Grime 1979; Kerkhoff et al. 2006), and plants may allocate nutrients among organs differently in N- vs. P-depleted environments. However,
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previous studies have focused on biomass allocation (Canham et al. 1996; Dent and Burslem 2009; Palmiotto et al. 2004; Poorter et al. 2012) or nutrient investment only in leaves (Reed et al. 2012; Richardson et al. 2004; Vitousek et al. 1995) in relation to soil properties, and allocations of N and P in non-photosynthetic organs have been rarely demonstrated. In stems and roots, most cells from vascular cambiums eventually die and lose cytoplasm, leaving cell walls behind. Thus, cell walls account for the bulk of dry biomass of non-photosynthetic organs. Cell walls contain 5 mg g−1 N or more as proteins (Lamport 1965; Onoda et al. 2004). N is also contained in living cells (vascular cambiums, companion cells, etc.) as proteins, amino acids, alkaloid, nucleic acids and nitrates (Chapin et al. 1990). On the other hand, P is contained mainly in living cells as phosphate, polyphosphate, phospholipids and nucleic acids, but is considered absent in cell walls (Sterner and Elser 2002, as will be shown in Table 3). Onoda et al. (2004) found that N concentration in cell walls of leaves did not differ among individuals with different foliar N concentration for a perennial herb, Polygonum cuspidatum Sieb. et Zucc., suggesting that N investment per unit mass of cell walls was constant against the changes in foliar N concentration (see also Takashima et al. 2004). By contrast, Hidaka and Kitayama (2011) demonstrated that the concentration of all cellular P fractions in leaves (both structural and other fractions) monotonously decreased with decreasing foliar P concentration across tropical tree species growing on soils with different P availabilities (see also Veneklaas et al. 2012). If the same patterns are observed for non-photosynthetic organs, N concentrations of non-photosynthetic organs, which are rich in cell walls, are maintained against decreasing N availability, while P concentrations in all plant organs decrease with decreasing P availability. Moreover, if biomass allocation to leaves does not change with nutrient availability, then N allocation to leaves decreases with decreasing N availability, while P allocation to leaves is maintained against decreasing P availability (Hypothesis 1). Hence, the depletion of N is expected to be a stronger factor associated with a lower foliar nutrient concentration than the depletion of P. The relationship between photosynthetic capacity and foliar nutrient concentration (i.e. photosynthetic nutrientuse efficiency) may be another important factor that differentiates plant growth rate in N- vs. P-depleted environments. It is well known that maximum photosynthetic rate positively correlates with both foliar N and P concentrations (Field and Mooney 1986; Reich et al. 2009; Wright et al. 2004) because these nutrients are required in the process of photosynthesis and other metabolisms in leaves (Evans 1989;Geiger and Servaites 1994; Sterner and Elser 2002). Hidaka and Kitayama (2009) investigated the relationship between photosynthetic nutrient-use efficiency and foliar
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nutrient concentrations using data from 340 plant species, and found that photosynthetic N-use efficiency decreased with decreasing foliar N concentration, whereas photosynthetic P-use efficiency increased with decreasing foliar P concentration. This suggests that plants disproportionately decrease their productivity rate with decreasing foliar N concentrations than with decreasing foliar P concentrations. By extending the leaf-level patterns demonstrated in Hidaka and Kitayama (2009) to the whole-plant level, we expect decreasing N productivity (biomass growth per unit N content in plant biomass, see “Materials and methods” for definition) with decreasing whole-plant or foliar N concentration, and increasing P productivity with decreasing whole-plant or foliar P concentration (Hypothesis 2). If this is true, the difference in trends of nutrient productivity can be a mechanism, which underlies the different plant growth rates under N-poor vs. P-poor conditions. In this study, we aimed to test the above two hypotheses using the saplings of 13 species growing in three Bornean rain forests with contrasting N and P availabilities (Fertile, P-poor, N-poor). Whole-plant nutrient concentration, defined as total nutrient weight per total biomass weight, was considered as an index of nutrient status of each sapling. Specifically, we asked the following research questions. (1) Does N allocation to leaves decrease with decreasing whole-plant N concentration, while P allocation to leaves is maintained with decreasing whole-plant P concentration (Hypothesis 1)? (2) Does N productivity decrease with decreasing whole-plant or foliar N concentration, while P productivity increases with decreasing whole-plant or foliar P concentration (Hypothesis 2)?
Materials and methods Study site and species Study sites were located in the forests of the Tawau Hills Park (4°27′N, 117°56′E, ca. 300 m asl), the Deramakot Forest Reserve (5°22′N, 117°25′E, ca. 250 m asl) and Nabawan (5°05′N, 116°29′E, ca. 500 m asl) in Sabah, the north part of Borneo. Soil characteristics of the forests were described by previous studies (Table 1). Based on the relative concentrations of inorganic N and soluble P in topsoils (Table 1), the forest of the Tawau Hills Park, the Deramakot Forest Reserves and Nabawan was considered as a Fertile, P-poor and N-poor forest, respectively. Note that relatively lower concentrations of soluble P in soils of the P-poor site (and lower concentrations of inorganic N in soils of the N-poor site) do not necessarily mean P-limitation (or N-limitation). Vegetation of Tawau (Seino et al. 2007) and Deramakot (Imai et al. 2010) is a mixed dipterocarp forest,
J Plant Res Table 1 Characteristics of the topsoils under litter layers in each study site Tawau (Fertile) Deramakot (P-poor) Nabawan (N-poor) 4.7 pH (H2O) Nutrients (mg kg−1) 69.8 NH4 + − 73.6 NO3 HCO3 −–Pi 5.3 −OH–Pi 41.7 Total P
512
3.4
3.7
16.8 19.9 3.2 5.6
9.9 1.1 6.3 8.7
246
NA
Soil pH (H2O), and concentrations of inorganic nitrogen (NH4+ and NO3−) and sequentially extracted phosphorus (Tiessen and Moir 1993). Inorganic nitrogen in soil samples was extracted by 1.5 N KCl. Inorganic phosphorus (Pi) was sequentially extracted by 0.5 M NaHCO3 adjusted to pH 8.5 (HCO3 −–Pi) followed by 0.1 M NaOH (−OH–Pi). Data are cited from Aoyagi et al. (2016) for the Tawau Hills Park, from Imai (unpublished data) and Imai et al. (2010) for the Deramakot Forest Reserve, and from Takahashi (unpublished data) for Nabawan. Details for soil sampling method are described in Online Resource 1
of each sapling, we took hemispherical photographs on the top of saplings at the first and second censuses. The photographs were taken under an overcast sky with a Coolpix 990 digital camera equipped with a FC-E8 fish-eye converter (Nikon). To estimate the allometric relationship between DGH and plant biomass, more than 20 saplings of each species (range 20–31, see Online Resource 2) were harvested at the second or third census after DGH and light conditions were measured. The replication number of growth measurements and light conditions of censused saplings for each species are summarized in Online Resource 3. Saplings to be harvested were selected from those used for the growth measurement or those nearby the censused individuals. Harvested saplings were divided into three organs (leaves, stems and roots) and oven-dried at 60 °C for 4 days. Dry weight of three organs was measured separately. Subsequently, three organs of each of four saplings under high light conditions [15–20 % global site factor (GSF), see data analysis for the GSF] for each species were finely grounded separately to determined elemental concentrations. Chemical analysis
whereas that of Nabawan is a tropical heath forest (Takahashi, unpublished data). We selected the same species as those used in Aoyagi and Kitayama (2015), which show high abundance in each forest; Shorea johorensis Foxw., Shorea macrophylla (de Vriese) Ashton, Shorea parvifolia Dyer, and Parashorea tomentella (Sym.) Meijer, in the Tawau Hills Park; Shorea multiflora (Burck) Sym., S. parvifolia, Shorea atrinervosa Sym., Parashorea malaanonan (Blanio) Merr., and Dryobalanops lanceolata Burck in the Deramakot Forest Reserve; and Dacrydium pectinatum de Lanb., Hopea pentanervia Sym. ex Wood, Cotylelobium melanoxylon (Hook) Pierre ex Heim, and Shorea venulosa Wood ex Meijer in Nabawan. All species except for D. pectinatum (Podocarpaceae) belong to Dipterocarpaceae. Field survey This study was conducted from September 2011 to February 2013. In the three forests, we tagged 30–40 saplings, defined as individuals <1 cm of diameter at ground height (DGH), across understory to forest gaps for each species. A permanent plot was established in each forest, and soil characteristics and vegetation structure were investigated in each plot by Aoyagi and Kitayama (2015). Census plots were placed around each permanent plot in our study and bamboos, lianas and herbaceous plants were cut off under canopy gaps in each census plot so as not to affect light conditions of censused saplings. DGH of each sapling was measured three times at 8-month intervals. To determine the light condition
C, N and P concentrations of each powdered plant sample (leaves, stems and roots) were determined. C and N concentrations were determined by the dry combustion method with an NC analyzer (JM1000CN). To determine P concentration, each sample was digested on a block digester with concentrated H2SO4 and H2O2. P concentrations in the solutions were determined using an inductively coupled plasma atomic emission spectrometer (ICPS-7510, Shimadzu Co., Kyoto, Japan). Furthermore, we extracted cell wall materials from stem and root samples following the method of Lamport (1965) with some modifications, and C, N and P concentrations in cell walls were determined. Each powdered plant sample was washed with citric acid buffer containing 3 % (w/v) sodium dodecyl sulfate (SDS). The homogenate was centrifuged at 1,500g for 5 min after heating at 90 °C for 1 h. The supernatant was discarded, and the residue was washed with 0.2 M NaOH to remove cytoplasmic proteins. The pellet was then washed twice with deionized water and twice with ethanol. After these treatments the pellet (cell wall material) was dried in an oven for 2 days. C and N concentrations of cell wall materials were determined by the dry combustion method with an NC analyzer (JM1000CN). To determine P concentration, cell wall samples were ashed at 550 °C on a muffle furnace and dissolved in 0.02 M potassium peroxydisulfate. The solution was then autoclaved at 120 °C for 1 h; total P in the solutions was determined using the Murphy–Riley method (Murphy and Riley 1962).
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Data analysis Hemispherical fish-eye photographs were processed using the HemiView image analysis software (ver. 2.1, Delta-T Devices), which allowed determination of indirect site factor (ISF), direct site factor (DSF) and GSF. ISF and DSF are the fractions of indirect and direct light relative to the amount of radiation above the forest canopy, respectively. GSF is calculated as a weighted average of ISF and DSF. GSF at the initial census or at the time of harvesting was used as an index of the light condition for each sapling. Whole-plant nutrient concentration was calculated as total nutrient weight per total biomass for each harvested sapling. Nutrient allocation to leaves was calculated as nutrient weight in leaves per nutrient weight in whole-plant biomass. To describe growth rate of each species, relative growth rate of biomass (RGR, biomass growth per unit wholeplant biomass, g g−1 month−1) and leaf productivity (biomass growth per unit leaf biomass, g g−1 month−1) were calculated based on the concept of growth analysis of Hunt (1978). Mean RGR and leaf productivity between the censuses were calculated following the equations;
RGR =
ln(Wt+1 ) − ln(Wt ) T
Leaf productivity =
(Wt+1 − Wt ) ln(Lt+1 − Lt ) × (Lt+1 − Lt ) T
(1)
(2)
where Wt and Wt+1 are weight of total plant biomass at the initial and the terminal census; Lt and Lt+1 are foliar weight at the initial and terminal census; and, T is the period (months) between the censuses. To estimate the total and foliar biomass of each sapling, we investigated the allometric relationship between DGH and biomass for each species. Plant biomass (g) was assumed to follow the following equation;
Total or foliar biomass = a × eb×DGH
(3)
where a and b are constant numbers. The values of a, b and R2 for each species are described in Online Resource 2. Because plant growth rate varies with light conditions (Poorter 1999), we fitted RGR and leaf productivity of saplings by light conditions (GSF) for each species using the linear regression analysis to compare RGR and leaf productivity among the 13 species. Linear regressions were implemented with the lm function in the R software. Because census periods (1st and 2nd) did not have significant effects on plant growth rate for all species (ANOVA, F = 0.20, P = 0.65), we considered only light conditions as the explanatory variable. Number of replicates, estimates, adjusted R2 and P values of the models for each species are described in Online Resource 3. Using the models,
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the mean RGR and leaf productivity at 15 % GSF were estimated for each species. Nutrient productivity (biomass growth per unit nutrient content in plant biomass, g mg−1 month−1) was defined by two equations as follows (cf. Ingestad 1979 for Eq. 4, and Harrington et al. 2001 for Eq. 5): Nutrient productivity =
dW W dW × = NW W NW =
RGR Whole − plant nutrient concentration
(4)
Nutrient productivity =
L dW dW × = NL L NL (5) Leaf productivity = Foliar nutrient concentration
where dW and W are biomass growth in the census periods and weight of total plant biomass; NW and NL are total nutrient weight in whole-plant biomass and foliar nutrient weight; L is foliar weight, respectively. Thus, nutrient productivity is calculated as RGR divided by whole-plant nutrient concentration or leaf productivity divided by foliar nutrient concentration. We compared nutrient concentrations of whole-plant biomass, leaves, stems, roots and cell wall materials among three sites by an ANOVA. We performed standardized major axis (SMA) slope-fitting techniques using the sma function in the package smart of the R software for the following analyses; the relationship between nutrient concentrations of whole-plant biomass and each organ, the relationship between whole-plant nutrient concentration and biomass/ nutrient allocation to leaves, and the relationship of elemental concentrations of plant tissues (leaves, stems, roots and whole-plant biomass) with growth rate and nutrient productivity. In addition, we tested if factors other than soil nutrients (e.g. water availability and species characteristics) influenced nutrient concentrations in plant organs and nutrient allocation by a multiple regression analysis (Online Resource 5). The regression analysis showed that including the effects of site differences did not change our results and discussion. All statistical analyses were performed by the R software, version 3.1.0 (R Core Team 2014).
Results Nutrient concentration and allocation Foliar N and P concentrations significantly differed among the three sites (ANOVA, P < 0.001 in all cases, Table 2). The rank of mean foliar nutrient concentrations was Fertile = P-poor > N-poor for N, and
Nutrient concentration in whole-plant biomass was calculated as total nutrient weight per total biomass. The difference among the sites was analyzed by an ANOVA. F and P values are also shown (degree of significance, * P < 0.05, ** P < 0.005, *** P < 0.001, N = 52). Groups sharing the same letters do not significantly different from each other at P = 0.05 (Tukey’s HSD)
**
6.4 1.47
0.24 0.88
0.1 14.3
*** **
6.4 1.7 32.7
***
27.8 37.2 28.6
***
5.9
**
6.3 F
P
**
459c ± 18 475b ± 6 493a ± 11 0.58a ± 0.18 0.43b ± 0.07 0.43b ± 0.14 Fertile P-poor N-poor
12.1a ± 1.5 12.4a ± 1.8 10.5b ± 1.6
***
***
0.20
451b ± 26 453b ± 11 474a ± 22 0.30 ± 0.12 0.24 ± 0.07 0.28 ± 0.14 8.1 ± 1.4 7.8 ± 1.3 7.9 ± 1.5 461c ± 9 472b ± 6 482a ± 15 0.44a ± 0.19 0.26b ± 0.07 0.44a ± 0.22 479b ± 12 487b ± 11 512a ± 13 0.94a ± 0.22 0.69b ± 0.10 0.51c ± 0.16 20.5a ± 1.8 18.7a ± 2.7 13.6b ± 1.9
8.7 ± 1.0 7.9 ± 1.6 7.9 ± 1.8
P N P N C P N
C P N
Stems (mg g−1) Leaves (mg g−1) Whole-plant (mg g−1)
Table 2 Mean values (± SD) of nutrient concentrations in whole-plant biomass and plant organs in each study site
C
Roots (mg g−1)
C
J Plant Res
Fertile > P-poor = N-poor for P (Table 2). Despite the considerable difference in N availability (Table 1) and wholeplant N concentration among the three sites (Table 2), stem and root N concentrations did not significantly differ among the three sites (ANOVA, P = 0.20 and 0.88, respectively, Table 2). Stem P concentration significantly differed among the three sites, and the rank of mean stem P concentration was Fertile = N-poor > P-poor (ANOVA, P < 0.005, Table 2). Root P concentration did not significantly differ among the three sites (ANOVA, P = 0.24, Table 2). Cell walls in non-photosynthetic organs contained N in the range of 4.3–5.2 mg N g−1, and the concentration of N in cell walls of non-photosynthetic organs did not differ among the three sites (Table 3). On the other hand, cell walls contained an extremely low amount of P (0.03– 0.05 mg P g−1, Table 3) in line with the earlier account (Sterner and Elser 2002). With decreasing whole-plant N and P concentrations, N and P concentrations of leaves, stems and roots significantly decreased (P < 0.001 in all cases, Fig. 1). The results of SMA regression analysis were summarized in Table 4. The slopes of the relationships of whole-plant N concentration on the x-axis with N concentrations of leaves, stems and roots on the y-axis were 2.45 (95 % CI 1.74–3.44), 0.80 (95 % CI 0.50–1.26) and0.60 (95 % CI 0.36–0.99), respectively (Table 4). The slopes of the relationships of whole-plant P concentration on the x axis with P concentrations of leaves, stems and roots on the y-axis were 1.89 (95 % CI 1.35–2.65), 1.21 (95 % CI 0.77–1.90) and 0.70 (95 % CI 0.47–1.04), respectively (Table 4). N allocation to leaves did not significantly vary in relation to whole-plant N concentration (P > 0.05, Fig. 2). On the other hand, P allocation to leaves significantly increased with decreasing whole-plant P concentration (P < 0.005, Fig. 2). Nutrient concentration and growth Leaf productivity correlated with foliar nutrient concentrations among the 13 species (P < 0.05 for N, P < 0.1 for P, Table 5). RGR nor leaf productivity did not correlate with N and P concentrations of whole-plant biomass (P > 0.05, Table 5), and of stems and roots (R2 < 0.1 in all cases, data not shown). Nutrient productivity did not correlate with either foliar (P > 0.05 for both N and P, Fig. 3) or whole-plant nutrient concentrations (P > 0.05 for both N and P, data not shown).
Discussion Nutrient allocation (Hypothesis 1) Our results showed contrasting trends of nutrient allocation between N vs. P, which was largely consistent with
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Table 3 Mean values (± SD) of nutrient concentrations of cell walls in non-photosynthetic organs in each study site
J Plant Res Stems (mg g−1)
Roots (mg g−1)
N
P
C
N
P
C
0.03 ± 0.03 0.03 ± 0.03 0.05 ± 0.03 1.2
482b ± 11 490ab ± 11 519a ± 66
5.2 ± 2.5 4.8 ± 0.8 4.8 ± 0.8
0.04 ± 0.04 0.04 ± 0.02 0.03 ± 0.02
475 ± 10 466 ± 16 478 ± 33
F
5.2 ± 1.3 4.4 ± 0.9 4.3 ± 1.6 2.8
4.6
0.3
0.3
1.3
P
0.07
0.29
*
0.72
0.75
0.29
Fertile P-poor N-poor
The difference among the sites was analyzed by an ANOVA. F and P values are also shown (degree of significance, * P < 0.05, N = 52). Groups sharing the same letters do not significantly different from each other at P = 0.05 (Tukey’s HSD)
Fig. 1 Relationships between whole-plant nutrient concentration and nutrient concentrations in plant organs for the 13 species. Wholeplant nutrient concentration was calculated as total nutrient weight per total biomass. Error bars indicate SD. The dotted line shows the
1:1 line (slope 1, intercept 0). Different symbols indicate saplings in different sites (fertile, P-poor and N-poor). Regression statistics is summarized in Table 3
Hypothesis 1; N allocation to leaves did not change with whole-plant N concentration, while P allocation to leaves increased with decreasing whole-plant P concentration (Fig. 2). Nutrient allocation is affected by both/ either nutrient concentration of plant organs and/or biomass allocation (Garnier et al. 1995). As to the former (nutrient concentration), the slopes of the SMA regression lines in Table 4 showed a steeper decrease in foliar N concentration than foliar P concentration with decreasing
whole-plant nutrient concentration. As to the later, biomass allocation to leaves was greater in the N-poor site (ANOVA, P < 0.05, data not shown). As the sum of reduced N concentration and increased biomass allocation, N allocation to leaves did not change when wholeplant N concentration decreased (Fig. 2). On the other hand, the relatively rapid decrease in stem P concentration (the slope of the SMA regression line >1; Table 4; Fig. 1) and greater biomass allocation to leaves (Fig. 2) caused
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J Plant Res Table 4 The results of standardized major axis analysis for the relationships between whole-plant nutrient concentrations and nutrient concentrations of plant organs for the 13 species
X variable (mg g−1)
Y variable (mg g−1)
Slope
Intercept
R2
Whole-plant N
Foliar N Stem N Root N
2.45 (1.74 to 3.44) 0.80 (0.50 to 1.26) 0.60 (0.36 to 0.99)
−10.7 (−20.7 to −0.71) −1.18 (−5.68 to 3.31) 1.02 (−2.68 to 4.73)
0.73 0.48 0.36
Whole-plant P
Foliar P Stem P
1.89 (1.35 to 2.65) 1.21 (0.77 to 1.90)
0.73 0.51
Root P
0.70 (0.47 to 1.04)
−0.18 (−0.50 to 0.14) −0.21 (−0.48 to 0.07)
−0.06 (−0.20 to 0.08)
0.62
Whole-plant nutrient concentration was calculated as total nutrient weight per total biomass. Numbers in parentheses indicate 95 % confidence intervals Fig. 2 Relationships between whole-plant nutrient concentration and nutrient/biomass allocation to leaves for the 13 species. Error bars indicate SD. Whole-plant nutrient concentration and nutrient allocation to leaves were calculated as total nutrient weight per total biomass and nutrient weight in leaves per nutrient weight in whole-plant biomass, respectively. Different symbols indicate saplings in different sites (fertile, P-poor and N-poor)
the increase of nutrient allocation to leaves with decreasing whole-plant P concentration. We suggest that the difference of N vs. P allocation among organs is intimately related to the difference in demand for N vs. P to build structural biomolecules. In line with this idea, N concentrations in cell walls of non-photosynthetic organs did not differ among the sites and cell walls contained an extremely low amount of P (Table 3, see also Sterner and Elser 2002). Thus, tree species must maintain N allocation/concentration in non-photosynthetic organs to meet a strong demand for N in cell walls. On the other hand, tree species can buffer dilution of foliar P by decreasing stem P concentration in response to P depletion. The function of stem P is less demonstrated, but it may act as storage (Sardans and Peñuelas 2015). Carbon
Table 5 The R2 of standardized major axis analysis for the relationships between growth rates and nutrient concentrations in leaves and whole-plant biomass for the 13 species (degree of significance; * P < 0.1, ** P < 0.05) X variables
RGR (g g−1 month−1)
Leaf productivity (g g−1 month−1)
0.14 0.18
0.31** 0.25*
Leaves (mg g−1) N P Whole-plant (mg g−1) N P
0.16
0.14
0.03
0.12
Nutrient concentration in whole-plant biomass was calculated as total nutrient weight per total biomass. Growth rates were described by relative growth rate (RGR) and leaf productivity at 15 % of global site factor
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Fig. 3 Relationships between foliar nutrient concentrations and nutrient productivity for the 13 species. Nutrient productivity was calculated as leaf productivity (biomass growth per unit leaf biomass) at 15 % global site factor per foliar nutrient concentrations. Different symbols indicate saplings in different sites (fertile, P-poor and N-poor)
and nutrient storages in long-lived organs (i.e. stems and coarse roots) are considered to be important to compensate the losses of biomass due to the fallen branches or natural enemies (Chapin et al. 1990; Poorter and Kitajima 2007). The species in the P-poor site had lower P concentrations in stems (Table 2), which implied that these species allocate P into the growth function at the expense of storage. Based on these results, we conclude that non-photosynthetic organs are strong sinks that compete with leaves for N among the 13 species in the three Bornean rain forests with contrasting nutrient availabilities. Nutrient productivity (Hypothesis 2) Based on the leaf-level study of Hidaka and Kitayama (2009), Hypothesis 2 expects that N productivity (RGR per whole-plant N concentration or leaf productivity per foliar N concentration) decreases with decreasing whole-plant or foliar N concentration, and that P productivity increases with decreasing whole-plant or foliar P concentration. However, this hypothesis was not supported in our study. Our results showed that both N and P productivities did not correlate with foliar nutrient concentrations (Fig. 3). This suggests that there is no difference in responses of N and P productivities to depletion of each element, and that the trend of nutrient productivity does not largely contribute to the difference in growth rates among plants in the N-poor and P-poor forests. Probably, loss of C to respiration, root symbionts and root exudates are intricately involved in the whole-plant C budget (i.e. growth) in relation to nutrient availability especially for P (cf. Lambers and Poorter 1992; Vicca et al. 2012). Previous studies demonstrate that nutrient productivity decreases with decreasing nutrient availability for N (Ingestad and Lund 1979) and P (Ericsson and Ingestad 1988). On the other hand, Ryser et al. (1997) demonstrated that P productivity did not change with nutrient
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availability for two grass species, which is consistent with our results. These studies suggest that more studies are needed to generalize the relationships between nutrient productivity and plant nutrient status. Implications for foliar nutrients in tropical heath forests and tropical evergreen forests The considerable variation in balance of available N and P is widely observed elsewhere in tropical regions (Coomes 1997; Crews et al. 1995; Moran et al. 2000). Previous studies have demonstrated that foliar nutrient concentration of plants in tropical heath forests (N-poor) tended to be lower than that of plants in tropical forests on P-poor soils (Aoyagi and Kitayama 2015; Chua et al. 1995; Reich et al. 1994; Turner et al. 2000). N depletion is more tightly associated with reductions in foliar nutrient concentration and growth rates than P depletion; this may be one of the key factors that differentiate forest structures among the three forests. To our knowledge, our study is the first to investigate nutrient concentrations of non-photosynthetic organs and their ecological significance among woody plants adapted to contrasting N and P availabilities. We suggest the significance of N and P allocations to non-photosynthetic organs for understanding the variation of foliar nutrient and growth rates for the three Bornean rain forests with contrasting N and P availabilities. Acknowledgments We thank L. Ajon, P. Lagan, Y. Onoda, A. Hidaka, N. Imai, T. Seino, and K. Miyamoto for assisting our fieldwork and providing valuable suggestions. Permission to conduct our research was granted by the Sabah Forestry Department and the Sabah Parks. This study was supported by the Grant-in-Aid from the Japanese MESSC (22255002) to K.K. and by the Global Environment Research Fund D-1006 and 1-1403 of the Ministry of the Environment, Japan, to K.K.
J Plant Res
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