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Keywords:

  • life history;
  • nitrogen uptake;
  • nitrogen-use efficiency;
  • spring ephemeral;
  • understorey herbs

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  • 1
    We compared nitrogen (N) uptake and whole-plant N dynamics in three deciduous-forest herbs of contrasting life histories: the spring ephemeral Alliumtricoccum, the summergreen Violapubescens and the semievergreen Tiarellacordifolia. We predicted that differences in above-ground physiology would translate into differences in N acquisition and partitioning, such that nitrogen-use efficiency (NUE) would increase from Allium to Viola to Tiarella.
  • 2
    Patterns of N uptake were generally the opposite of our predictions. Allium had the lowest N uptake capacity in both laboratory and field experiments whereas roots of Tiarella had the highest specific N uptake capacity.
  • 3
    Viola was the only species in which the specific uptake capacity of roots was related to photosynthetic activity of leaves, both decreasing by a factor of two from spring to summer. In contrast, Tiarella consistently had the lowest photosynthetic capacity and the highest specific uptake capacity whereas Allium maintained substantial root uptake capacity throughout the summer when it had no photosynthetic activity.
  • 4
    There were no significant differences between species in overall NUE. However, there were differences in the components of NUE: nitrogen productivity (A) and mean residence time of N in the plant (MRT). Nitrogen productivity increased, and MRT decreased, from Allium to Viola to Tiarella.
  • 5
    In all three species, there was a balance between acquisition of N and building of biomass over the annual growth cycle, despite dramatic disjunctions between the tissue-specific rates of carbon and N acquisition in Allium and Tiarella. The variation in A and MRT we observed among co-occurring species of a single N-rich habitat was comparable with that observed by other researchers studying plants adapted to habitats of widely varying N availability.

Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Dramatic, but predictable, changes in light availability occur beneath the canopy of temperate deciduous forests through the seasons (Hicks & Chabot 1985). Herbaceous plants exhibit two distinct strategies for coping with the deep shade cast by the overstorey canopy in summer. Spring ephemeral herbs avoid shade stress by exploiting the narrow window of full sun and adequate temperatures prior to canopy closure whereas summergreen and evergreen species have adapted to tolerate the long summer shade period (Hicks & Chabot 1985). Spring ephemeral herb abundance tends to be associated with high soil fertility (Al Mufti et al. 1977; Rogers 1982; Host & Pregitzer 1991), possibly because the rapid photosynthetic rates characteristic of this life history require an abundance of water and mineral nutrients (Grime 1979, 1994). In contrast, deciduous forests on infertile sites are expected to be dominated by summergreen and evergreen herbs (Grime 1994).

Spring ephemerals might therefore be expected to possess a suite of characteristics associated with plants from high-resource habitats (i.e. rapid growth rate, high physiological capacity for resource capture and low nutrient-use efficiency) whereas shade-tolerant herbs would show the opposite traits, typically associated with low-resource habitats (Grime 1979, 1994; Vitousek 1982; Schulze & Chapin 1987; Chapin 1991). Above-ground resource acquisition, reflected by photosynthetic capacity, is indeed higher in spring ephemerals (Sparling 1967; Taylor & Pearcy 1976; Rothstein 1999). Patterns of mineral nutrient capture should parallel those of photosynthetic capacity as these two processes are believed to be interdependent (Bloom et al. 1985; Campbell et al. 1991; Chapin 1991; Grime 1994). However, to the best of our knowledge, relationships between life history and the physiology of mineral nutrient capture by deciduous forest herbs have never been investigated.

The sugar maple-basswood–Osmorhiza ecosystem type of north-western lower Michigan, USA, is characterized by both the highest rates of N mineralization and nitrification in the region (Zak & Pregitzer 1990), and an abundant and diverse herbaceous plant community (Host & Pregitzer 1991). We investigated patterns of N use in three members of this community: Alliumtricoccum Ait., a spring ephemeral; Violapubescens Ait., a summergreen species; and Tiarellacordifolia L., a semievergreen (sensuMahall & Bormann 1978). We therefore compared N-use among plant species adapted to temporal niches of variable light availability, while holding soil fertility constant, rather than among plant species adapted to habitats differing in soil fertility (as in Ingestad 1976; Aerts & Caluwe 1994; Vázquez de Aldana & Berendse 1997). We hypothesized that N uptake capacity would parallel differences in photosynthetic capacity among species (Tiarella < Viola < Allium), as well as the seasonal changes in photosynthetic capacity observed in Viola and Tiarella (Rothstein 1999). We also hypothesized that nitrogen-use efficiency (NUE; or the efficiency of biomass production with respect to nitrogen) would be inversely related to leaf N concentration and positively related to leaf longevity, and species would thus be ranked in the reverse order to N uptake. In particular, we expected that the components of NUE (sensuBerendse & Aerts 1987) would vary between species, with nitrogen productivity decreasing, and mean residence time increasing, from Allium to Viola to Tiarella.

Materials and methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Study site, species and sampling protocol

Our study was conducted in a single stand of northern hardwood forest located in Wexford County, northern lower Michigan, USA (44°20′ N, 86°00′ W) classified by Host & Pregitzer (1991) as belonging to the sugar maple-basswood–Osmorhiza ecosystem type. Mahall & Bormann’s (1978) phenological classification was used to assign herbaceous species as (i) spring ephemerals: leaf development occurs at or before snowmelt and leaf mortality occurs during expansion of the forest canopy; (ii) summer greens: leaf development occurs before expansion of the forest canopy and leaf mortality occurs prior to canopy leaf drop in the autumn; and (iii) semievergreens, as for (ii), but leaf mortality occurs over winter or early the following spring. We chose one spring ephemeral (Alliumtricoccum), one summergreen (Violapubescens) and one semievergreen (Tiarellacordifolia); nomenclature for all species follows Gleason & Cronquist 1991. In Fig. 1, we illustrate leaf phenology of the three study species in relation to canopy development and photosynthetically active radiation (PAR) reaching the forest floor.

image

Figure 1. Leaf phenology of Allium tricoccum, Viola pubescens and Tiarella cordifolia in relation to the average photosynthetic photon flux density (PPFD) reaching the forest floor each day of the snow-free year in 1997 (data from Rothstein 1999). Horizontal lines represent the time period during which each species has green leaves above ground. Numbered arrows at the top represent the timing of the following events: (1) snowmelt, (2) beginning of canopy bud-break, (3) canopy at full leaf expansion, (4) beginning of canopy leaf senescence, and (5) first snow accumulation.

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Mitelladiphylla, a closely related species that is difficult to distinguish from Tiarella when not flowering (Voss 1985), also occurs in this stand. It is therefore likely that some non-flowering individuals of M. diphylla were mistaken for Tiarella, but these two species also appear indistinguishable in terms of phenology, life history and selected parameters of leaf physiology and chemistry (Rothstein 1999).

Patterns of carbon (C) and N metabolism were determined through a series of physiological measurements conducted in both spring and summer, with a third sample for Allium and Viola in the late summer, and Tiarella in the autumn. Two sets of five and 10 ramets of each species were collected for measurement of CO2 exchange and tissue N, and of specific root uptake of 15NH4+ and 15NO3, respectively. For each set, a transect (random start point) across the stand was sampled at 20-m intervals, and the nearest plant (or for Allium and Tiarella, which reproduce clonally, the nearest solitary ramet) selected. Intact cylinders of soil, approximately 30 cm deep and 25 cm in diameter, containing each plant were placed in c. 10-L buckets for transport to the field laboratory (approximately 5 min).

Photosynthesis and plant n

Light response curves were determined for the three species as part of a parallel study of the their photosynthetic adaptations (Rothstein 1999), and used to estimate the instantaneous photosynthetic N-use efficiency (PNUE). Upon completion of CO2-exchange measurements, each plant was divided into component tissues (i.e. roots, stems, leaves), oven-dried to a constant weight at 70 °C, and then ground to a fine powder using a mortar and pestle. The ratio of root mass to total plant mass (i.e. root mass ratio; RMR) was calculated for each plant. The N content of each tissue sample was determined using an NC 2500 Elemental Analyser (CE Elantech Inc., Lakewood, NJ, USA). Because a substantial amount of leaf N can exist as NO3 (Chapin et al. 1988), we determined the NO3 content of each component tissue by extracting a c. 50-mg subsample in 5 mL of 100 °C deionized water, then analysed the extractant colourometrically on a Rapid Flow Analyser (Alpkem Inc., Clackamas, OR, USA).

Because spring was the only time in which all three species displayed leaves, we only compared PNUE for this season. Springtime photosynthetic rate was estimated by entering a representative midday photon flux density (700 µmol m−2 s−1; 10-day average) into the light response function for each individual. Leaf-level PNUE was calculated for each plant, as the rate of CO2 assimilation per unit of leaf N (µmol CO2 g N−1 s−1).

15n uptake

Ten plants of each species were washed free of soil particles in tap water, and then their root systems suspended in glass jars containing 750 mL of either 300 µmol L−1 K15NO3 (n = 5) or 300 µmol L−115NH4Cl (n = 5) in 0.5 mmol L−1 CaCl (to preserve membrane integrity; Epstein et al. 1963). These N concentrations were chosen to be high enough that uptake rates would not be limited by substrate concentration (Lewis 1986), but low enough to be within the range of soil solution concentrations observed in the field (Rothstein 1999). The plants were incubated at room temperature (18–21 °C) in the field laboratory for 1 h, after which the root system of each plant was washed in rapidly flowing tap water for c. 5 min. After drying, separation into component tissues and grinding to a fine powder, the 15N content of each tissue was determined by mass spectrometry (Stable Isotope Facility, Department of Agronomy and Range Science, University of California, Davis, CA, UISA). The amount of 15N label within each plant tissue was calculated as the atom percentage 15N in excess of the background atom percentage 15N (determined on unlabelled plant samples). The amount of isotope within each tissue was summed to calculate whole-plant 15N. We then calculated specific uptake rate (µmol N g root−1 h−1) by dividing whole-plant 15N by the dry biomass of roots.

N resorption

In order to compare patterns of N resorption, we collected shed leaves from five ramets each of Allium (June 1997) and Tiarella (April 1998). Nitrogen concentrations were analysed as described earlier and compared with green-leaf values (spring and summer, respectively, for the two species). Allium appeared to undergo active leaf senescence, as evidenced by a clear pattern of basipetal yellowing whereas leaf loss in Tiarella appeared to be due to general necrosis, and/or mechanical damage. Tiarella leaves initiated in the spring of 1997 suffered 30% mortality by November of that year, 80% by snowmelt in the spring of 1998, and 100% within 1 month of snowmelt (Rothstein 1999). We could find no intact, shed leaves of Viola suggesting that this species loses its leaves primarily through mechanical damage and perhaps herbivory.

Whole-plant n dynamics

In order to characterize patterns of in situ plant growth, N uptake, NUE and N partitioning, approximately 75 ramets of each species with intact roots were collected and labelled with 15N before return to the field for 1 full year of growth. Plants were collected when their leaves began to emerge from the soil in the spring of 1997 (6–10 April for Allium, 23–26 April for Viola and 10–13 May for Tiarella), assigned an identifying number, washed free of soil particles with tap water, blotted dry and then weighed. The root system of each plant was then placed for c. 2 h in a beaker containing a labelling solution of 500 µmol L−1 each of 15NH4Cl, K15NO3, in 0.5 mmol L−1 CaCl2. Roots were then washed in rapidly flowing tap water for c. 5 min. A randomly selected subsample of 15 plants from each species (hereafter referred to as ‘initial’ plants) was harvested immediately. The remaining plants were placed in 10-cm diameter by 15-cm deep PVC sleeves, sealed at the bottom with nylon screen (c. 1 mm openings), which were then filled with homogenized native soil and placed back in the ground in the original stand. Harvests of each species (10 randomly selected plants per harvest) were made throughout the next year to coincide with import-ant phenological events (e.g. top senescence for Allium, canopy leaf senescence for Tiarella). Tiarella experienced significant mortality (c. 33%) in June and July, and the three subsequent harvests of this species were therefore of only five plants each. There was very little mortality of Viola (c. 3%) or Allium (c. 10%) throughout the course of the experiment, and no other apparent effects of transplanting; in particular, none of the containers appeared root bound, even at final harvest.

At each harvest, dry weight, total N and 15N were determined as described previously. The initial harvest of 15 plants for each species was used to determine the relationships between fresh and dry weight, and between fresh weight and total N and 15N contents. These relationships were used to estimate initial values for the remaining (transplanted) individuals.

Allium

DW = (0.1891 × FW) − 0.1641, r2 = 0.968; N = (0.006 × FW) − 0.0001, r2 = 0.992; 15N = (10.589 × FW) − 2.654, r2 = 0.843.

Viola

DW = (0.1878 × FW) + 0.0032, r2 = 0.922; N = (0.006927 × FW) − (0.00049 × DAY) + 0.001402, r2 = 0.890;15N = (32.333 × FW) − (3.991 × DAY) + 3.426, r2 = 0.879.

Tiarella

DW = (0.2068 × FW) − 0.118, r2 = 0.846;N = (0.004954 × FW) + (0.000631 × DAY) − 0.00245; r2 = 0.805;15N = (36.675 × FW) − 3.986; r2 = 0.847.

Where DW is the initial plant dry weight (g), FW, the initial plant fresh weight (g), N, the initial N content of the plant (g), 15N, the initial 15N content of the plant (µg), and DAY the day on which the plant was initially collected (1–4). Collection day was included as a coefficient in regression equations for Viola and Tiarella, where it improved the coefficient of determination for each. This effect was likely due to changing root:shoot ratios, because rapid leaf expansion was occurring over the 4 days it took to process all the plants.

Relative biomass growth (g g−1) for each subsequently harvested plant was calculated by subtracting the estimate of its initial dry weight from its final dry weight and dividing by the estimate of initial dry weight, and relative N uptake (mg N g−1) as final N minus initial N divided by the initial dry weight.

Patterns of N partitioning were followed by determining the amount of 15N label within each plant tissue at each harvest. Atom percentage 15N in excess of the background atom percentage 15N (see 15N uptake assay) was multiplied by the total N in that tissue to give a value in µg of 15N in that tissue per plant, and then adjusted for differences in plant size by dividing by initial dry weight.

We used these same plants to determine NUE as defined by Berendse & Aerts (1987), i.e. the product of nitrogen productivity (A; or dry matter production per unit of plant nitrogen) and mean residence time (MRT) of nitrogen in the plant. The equations of Vázquez de Aldana & Berendse (1997) were used to estimate N productivity for the entire year of growth (A) and for the initial spring light phase (Ai):

  • image(eqn 1)

and MRT:

  • image(eqn 2)

where B is plant biomass, N is total plant nitrogen content and 15N is the total amount of labelled nitrogen in the plant.

Statistical analysis

Leaf N concentration, leaf NO3 concentration, 15N uptake and RMR were analysed using one-way analysis of variance (anova) with species–season combination as a single factor (either seven or nine levels); differences in mean values were evaluated using Tukey’s HSD test. For whole-plant relative growth and relative N uptake, we compared only the final (i.e. after 1 year) values for each species using Bonferroni’s multiple comparison procedure, with probabilities scaled for three pairwise comparisons. We used this same procedure to test for differences in PNUE, A, MRT and total NUE. Student’s t-tests were used to compare N concentrations between green and shed leaves of Allium and Tiarella. Tissue NO3 concentrations, NO3 and NH4+ uptake rates and leaf NRA were log-transformed prior to statistical analyses to meet the assumption of normality. All statistical analyses were performed using systat for personal computers (Wilkinson 1990), and significance was accepted at α = 0.05.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

There were significant differences among species and seasons in both total leaf N and leaf NO3 concentration (Fig. 2). Spring-time leaf N concentrations in Allium and Viola were approximately 70% greater than those of Tiarella (Fig. 2a), but while leaf N dropped dramatically in Viola from spring to summer, there were no seasonal changes in Tiarella. A substantial portion of the N in leaves of Viola and Tiarella was in the form of NO3 (1–16%), but NO3 was barely detectable in the leaves of Allium (Fig. 2b). Differences in leaf N concentration paralleled differences in photosynthetic rates (Rothstein 1999), such that there were no differences among species in springtime PNUE (Table 1; P = 0.653).

image

Figure 2. Seasonal and interspecific patterns of (a) leaf N and (b) leaf NO3 concentration in Allium tricoccum (●), Viola pubescens (▪) and Tiarella cordifolia (▴). Allium data are confined to the spring because of its limited leaf display. Values represent means (±1 SE) of five ramets of each species. Means within a panel with the same letter are not significantly different according to Tukey’s HSD test (P > 0.05).

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Table 1.  Components of nitrogen-use efficiency in Allium, Viola and Tiarella: springtime photosynthetic nitrogen-use efficiency (PNUESPR), springtime nitrogen productivity (ASPR), annual nitrogen productivity (A), mean residence time of nitrogen (MRT), and nitrogen-use efficiency (NUE). Values are means ± 1SE. Means in the same column followed by the same letter are not significantly different according to Bonferroni’s multiple comparison test
SpeciesPNUESPR (µmol CO2 g N−1 s−1)ASPR (g biomass g−1 N day−1)A (g biomass g−1 N day−1)MRT (years)NUE (g biomass g N−1)
Allium8.6 ± 0.2a0.98 ± 0.16a12.4 ± 2.1a2.84 ± 0.36a37.6 ± 8.8a
Viola7.8 ± 0.5a0.42 = 0.05b19.4 ± 2.5a2.05 ± 0.21a,b42.3 ± 8.3a
Tiarella8.8 ± 1.2a1.09 = 0.11a31.2 ± 3.7b1.69 ± 0.21b55.9 ± 9.2a

The roots of all three species took up less 15NO3 (Fig. 3a) than 15NH4+ (Fig. 3b), although their relative preferences for NH4+ (i.e. the difference between rates of NH4+ and NO3 uptake) diminished from Allium to Viola to Tiarella. Nitrate and NH4+ uptake both followed the same pattern among species, decreasing from Tiarella to Viola to Allium. Specific uptake capacity of both ions tended to decrease over time for all species, although these changes were, for the most part, not statistically significant.

image

Figure 3. Seasonal and interspecific patterns of (a) 15NO3 and (b) 15NH4+ uptake. Values represent means (±1 SE) of five ramets of each species. Means within a panel with the same letter are not significantly different according to Tukey’s HSD test (P > 0.05). Symbols are as follows: Alliumtricoccum (●), Viola pubescens (▪) and Tiarellacordifolia (▴).

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In situ N uptake by transplants of Viola and Tiarella (Fig. 4a) closely mirrored biomass accumulation (Fig. 4b). Allium, however, accumulated little N in the spring despite a very rapid relative growth rate, and even maintained net N uptake in the summer, despite a decline in biomass during this period. The negative relative N uptake in June was probably the result of N loss during leaf senescence.

image

Figure 4. (a) Relative biomass growth and (b) relative N uptake of Allium, Viola and Tiarella through 1 full year. Values represent means (±1 SE) of 10 plants, except for the final three samples of T.cordifolia, which were of five plants each. Final growth and N uptake values with the same letter are not significantly different according to Bonferroni pairwise comparisons. Symbols are as follows: Alliumtricoccum (●), Viola pubescens (▪) and Tiarellacordifolia (▴).

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Despite the fact that Tiarella roots had the greatest specific uptake capacity for NH4+ and NO3 (Fig. 3), Viola exhibited the most rapid rate of relative N uptake in situ (Fig. 4). This is probably due to the higher proportion of Viola’s total biomass in roots (averaged across all seasons, root mass ratio was significantly greater in Viola[0.46 ± 0.04] than in Allium[0.24 ± 0.02] or Tiarella[0.21 ± 0.04]). After 1 full year of growth, net relative biomass gain was significantly greater in Tiarella compared with Allium (P = 0.032), while Viola was intermediate, and not significantly different from either. Relative N uptake by Allium was significantly lower than that of Viola (P = 0.002), and marginally lower than that of Tiarella (P = 0.072). Values for the latter two species were equivalent.

These seasonal patterns of growth and N uptake gave rise to differences among the three species in the components of NUE (Table 1). Viola had a significantly lower springtime N productivity than the other two species (P = 0.001), although PNUE did not vary at this time. Over an entire year of growth, however, N productivity increased significantly from Allium to Viola to Tiarella (P = 0.001). Mean residence time of N followed the opposite pattern decreasing from Allium to Viola to Tiarella (P = 0.023). Therefore total NUE, the product of A and MRT, did not differ between species (P = 0.388).

Both Allium and Viola showed seasonal shifts in N allocation associated with leaf mortality, with N moving from shoot to storage pools (Fig. 5). In contrast, shoots remain the dominant pool of N in Tiarella throughout the year. In the spring, when all three species had green leaves and there was direct light reaching the forest floor, Allium (0.69 ± 0.01) and Tiarella (0.62 ± 0.02) allocated proportionately more N to their shoots than did Viola (0.50 ± 0.03; P < 0.001).

image

Figure 5. Seasonal patterns of N allocation to shoots (leaves, stems and flowers), roots and storage (bulbs or rhizomes) in (a) Alliumtricoccum, (b) Violapubescens, and (c) Tiarellacordifolia. Values represent means (+1 SE) of 10 ramets of each species, except for Tiarella in the autumn, in which means are of five ramets. All N values are expressed relative to the initial biomass of each plant.

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Patterns of 15N partitioning in the same plants (Table 2) are consistent with patterns of total N allocation. In all three species, 15N that was in roots immediately following labelling had moved to leaves by the second harvest. After this, 15N in Allium remained as a discrete pool (significantly the largest; Table 2) whereas the isotope dispersed rapidly throughout the entire plant in the other species so that no tissue pool could be distinguished statistically from any of the others after September 1997. After leaf senescence in Allium, the bulk of the label was translocated back to the bulb, which is consistent with the apparent resorption of 70% of green-leaf N in free-growing Allium plants (Table 3), and the increase in total N in the storage pool over this same period (Fig. 5). The relatively low resorption observed in Tiarella (Table 3) is consistent with leaves remaining the largest pool of isotope, even if not significantly so (Table 2). Viola roots always contained a relatively large 15N pool, although not always the largest and not significantly so after the initial harvest.

Table 2.  Changes in the distribution of 15N label in Allium, Viola and Tiarella through 1 year of growth. Values are means (±1 SE of the 15N label in each tissue, adjusted for initial plant size (µg 15N g−1 initial dry weight). Shoot includes leaves, stems and reproductive tissues. Storage includes bulbs and rhizomes. Values in bold type are significantly greater than all non-bold values within that sample date, using Tukey’s HSD multiple comparison procedure (P ≤ 0.05)
 Distribution of 15N label (µg 15N g−1 initial dry weight)
 April 1997May 1997June 1997September 1997October 1997November 1997April 1998
  1. *Label in senesced leaves (6.0 ± 1.04 µg g−1); assumed to be lost from the plant.

Allium       
Shoot  0.5 ± 0.1183.9 ± 12.59 6.38 ± 1.14* 8.5 ± 1.18
Storage  7.1 ± 0.77 13.8 ± 1.6368.5 ± 7.2171.0 ± 19.5541.3 ± 3.26
Roots57.2 ± 8.05 13.9 ± 1.63 12.7 ± 1.4719.1 ± 6.9012.2 ± 1.42
Viola       
Shoot108.7 ± 8.6567.4 ± 3.9210.0 ± 1.14
Storage 32.5 ± 2.84 11.0 ± 1.2114.0 ± 1.4531.0 ± 4.1019.8 ± 1.76
Roots109.0 ± 7.00 22.5 ± 2.0322.7 ± 2.0035.4 ± 4.632.3 ± 2.81
Tiarella       
1997 shoot 32.4 ± 7.71131.1 ± 15.6092.8 ± 13.5064.4 ± 11.2541.8 ± 15.44
1998 shoot14.0 ± 1.96
Rhizome 47.6 ± 6.16 16.7 ± 2.1232.8 ± 13.5612.5 ± 2.8416.0 ± 2.05
Roots121.1 ± 12.43 35.3 ± 5.6325.6 ± 4.9110.1 ± 1.9014.0 ± 2.86
Table 3.  Apparent N resorption from leaves of Allium and Tiarella. The differences in N content between green and shed leaves were significant for both species using a Student’s t-test (P < 0.001; n = 5). Values for leaf N are means ±1 SE
 Green leaf N (mg g−1)Shed leaf N (mg g−1)Resorption efficiency (%)
Allium49.8 ± 2.2414.8 ± 1.3469.8
Tiarella29.1 ± 0.5723.1 ± 0.8420.6

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

We initially proposed that interdependence of above- and below-ground resource capture would lead to N uptake capacity and photosynthetic activity varying in parallel, both within and among species. However, we found that the species showed opposite rankings for rates of N uptake and photosynthesis, and patterns within species were inconsistent. We also proposed that NUE should increase with decreasing leaf N concentrations and increasing leaf longevity, but overall NUE did not differ among species. Moreover, its components (A and MRT) differed in the opposite direction to our predictions. Above- and below-ground physiology can therefore be uncoupled, and a variety of combinations of assimilation rates, allocation patterns and tissue phenologies used to balance above- and below-ground resource capture. Patterns of N uptake and NUE also varied widely in response to ecological and life-history factors unrelated to N availability.

Species rank for photosynthetic capacity (Tiarella < Viola < Allium;Rothstein 1999) was the reverse of their rank for specific N uptake capacity (Fig. 3). Within species, Viola alone exhibited the predicted parallel response, with both photosynthetic capacity (Rothstein 1999) and N uptake capacity (Fig. 3) declining by a factor of two from spring to summer. Specific N uptake in Tiarella did not change from one season to another whereas its photosynthetic capacity decreased from spring to summer and then increased in autumn (Rothstein 1999). Perhaps the most unusual relationship between N uptake and photosynthesis was in Allium, which, despite the fact that it was leafless, sustained its NH4+ uptake capacity throughout the summer. Above- and below-ground resource capture, in terms of instantaneous physiological activity, are clearly not interdependent in Allium and Tiarella.

These disjunctions disappear for Tiarella when N uptake is measured in situ, with N uptake closely tracking biomass growth throughout the year (Fig. 4). The disjunction was, however, reinforced by in situ measurements on Allium, where net uptake of N continued over the summer months despite a loss of biomass. Clearly, the rapid photosynthetic rates necessary to sustain the spring ephemeral life history (Sparling 1967; Taylor & Pearcy 1976; Rothstein 1999) require a large investment of N. However, Allium meets this demand from rapid remobilization of N from its bulb, accumulated as a result of uptake throughout the year, rather than by rapid springtime uptake. Indeed, even though Allium shows efficient resorption of N, spring uptake of N is outweighed by losses of N due to leaf senescence. However, in other spring ephemeral species, root senescence coincides with leaf senescence (e.g. Erythroniumamericanum; D. Rothstein personal observation), and sufficient N uptake must therefore occur in the spring.

Predictions of interdependence are based on the premise that, to optimize growth, plants must maintain a balance between acquisition of above- and below-ground resources (Bloom et al. 1985). Although in Tiarella and Allium physiological processes measured over short time scales appear to violate this assumption, the whole plant over its entire annual growth cycle does not. The very low physiological activity of Tiarella leaves relative to its roots is compensated for by a greater allocation to leaves relative to roots. In contrast, Allium maintains its balance via differences in above- and below-ground phenology, and an ability to store and remobilize N (Tables 2 & 3).

While both high resorption efficiency and extended leaf lifespan are viewed as mechanisms of increasing MRT (Eckstein et al. 1999), differences here were determined primarily by differences in resorption efficiency, and MRT was actually inversely related to leaf lifespan. Despite having the shortest leaf lifespan, Allium’s efficient resorption and storage of leaf N resulted in very low annual N losses. In contrast, Tiarella’s poor resorption efficiency resulted in large losses of leaf N, even though it holds its leaves for the entire growing season. Although we are limited by a lack of data regarding leaf shedding in Viola, it appears from the changes in 15N pools (Table 2), and the autumnal increase in storage pools of N (Fig. 5), that Viola falls between Allium and Tiarella in its efficiency of leaf N resorption, contributing to its intermediate MRT. The unexpected inverse relationship between leaf lifespan and MRT may be explained by the fact none of these species has leaves that persist through more than one growing season, thus constraining any positive effects of leaf longevity on MRT. Furthermore, longer leaf display may actually increase the opportunity for N losses from the plant via leaching, particularly given the high leaf concentrations of soluble NO3 in Viola and Tiarella.

Patterns of tissue phenology and N allocation clearly exert more influence over A than does leaf N concentration. High springtime A in Allium and Tiarella is associated with a higher proportional allocation of N to shoots, relative to Viola (Fig. 5), rather than higher leaf N concentration (similar in Viola and Allium, lower in Tiarella). Differences in springtime A should approximate differences in maximal, instantaneous A, given that this period has light, temperature and moisture conditions that are the most conducive to growth, but in this rapidly changing environment, annual and springtime N productivity followed very different patterns among species. Not surprisingly, extended, slow C assimilation (Tiarella) resulted in greater annual N productivity than rapid C assimilation confined to a brief window of time (Allium).

Berendse & Aerts (1987) postulated that there were evolutionary trade-offs between traits leading to high A and those leading to high MRT, such that low-nutrient habitats should be dominated by plants with high MRT and low A whereas high-nutrient habitats should be dominated by plants with high A and low MRT. Even though our three species co-occur in a single high-nutrient habitat, their differences in life history still produced the expected negative relationship between A and MRT (Table 1). In fact, we found nearly as much variation in A and MRT among co-occurring species of differing phonologies as other researchers have found among species associated with habitats of widely varying N availability. Compare the 2.5-fold difference in A, and 1.7-fold difference in MRT, between Tiarella and Allium to the 2.3-fold difference in A, and a 1.9-fold difference in MRT, between a perennial grass species associated with acidic, nutrient-poor meadows and one associated with heavily fertilized pastures (Vázquez de Aldana & Berendse 1997).

Overall, the below-ground resource acquisition by deciduous-forest herbs does not relate to above-ground resource acquisition in nearly as clear-cut a manner as predicted, i.e. physiological rates of N uptake do not simply mirror rates of photosynthesis. However, patterns of allocation to, and phenology of, above- vs. below-ground structures appeared to compensate for disjunctions in physiology, resulting in relatively balanced C and N acquisition in all three species. Finally, life-history adaptations that appear to be related primarily to light availability also have important consequences for plant nitrogen economy.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

We thank the USDA Forest Service, North Central Research Station for providing access to the field site and laboratory space. We thank D. Harris of the UC Davis Stable Isotope Facility for help with 15N determinations. M. Holley, M. Hyde, and M. Tomlinson all provided assistance in the field and laboratory. We gratefully acknowledge B. Barnes, L. Nooden, K. Pregitzer, J. Teeri, L. Haddon and two anonymous referees for providing helpful comments on earlier drafts of this manuscript. This project was supported through a graduate research fellowship from the Matthai Botanical Gardens, University of Michigan, and grants from Sigma XI and the School of Natural Resources and Environment, University of Michigan.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
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Received 23 November 1999 revision accepted 26 September 2000