• Chitin;
  • chlorophyll;
  • ergosterol;
  • microclimate;
  • symbiosis


  1. Top of page
  2. Introduction
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References


  • 1
    To assess how internal and external factors contribute to lichen growth, light, water and nutrient supplies were manipulated during 3 months in the field for the lichens Nephroma arcticum (L.) Torss. and Peltigera aphthosa (L.) Willd. Concomitant measures of weight and area gain, microclimatic conditions and investments in photobiont vs mycobiont tissue were also conducted.
  • 2
    In both lichens ≈80% of the variation in weight gain was explained by a linear regression model including light received during wet active periods, chlorophyll a concentration and area gain. All three parameters had a positive effect on weight gain.
  • 3
    About 80% of the variation in area gain was explained by a model including variation in weight gain, initial thallus specific weight, ergosterol and chitin concentration. The model was identical for the two lichens, with a positive effect of weight gain and thallus specific weight and a negative effect of ergosterol and chitin.
  • 4
    Peltigera aphthosa grew faster than N. arcticum when exposed to the same environmental conditions. This could be explained by its higher chlorophyll a to ergosterol ratio, and a greater water-holding capacity prolonging the active time in light.


  1. Top of page
  2. Introduction
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References

Little is known about the mechanisms that regulate growth and development of lichen thalli (Honegger 1991; Richardson 1999), but the general view is that the heterotrophic fungus (mycobiont) controls both photobiont cell division and the extent of its carbohydrate and, in cyanobacterial lichens, nitrogen export (Richardson 1999). However, the lichen thallus must still be organized to favour a positive net carbon gain, which implies that resource investments must be balanced so that the mycobiont demands remain below the carbon-gain capacity of the photobiont (Palmqvist 2000). Growth of the partners must be further coordinated so that neither becomes more favoured than the other by the resource supply (Smith, Muscatine & Lewis 1969). Moreover, environmental conditions have a large impact on lichens because most are poikilohydric – unable to control their water status (Kappen 1988). Although respiration is detectable within 2–4 min after rehydration (Smith & Molesworth 1973), full activation of photosynthesis might require up to 1 h (Lange, Kilian & Ziegler 1986). Further, the two processes are differently sensitive to variations in thallus water content and temperature, and photosynthesis is also dependent on photon flux density (Kappen 1988; Palmqvist 2000).

Despite this environmental control of their metabolism, lichens can convert incident light energy into new biomass as efficiently as vascular plants (Palmqvist & Sundberg 2000). This implies that photosynthetic and respiratory processes in lichen thalli can be regulated in relation to each other and to prevailing light, a view that is also supported by the literature. Photosynthetic capacity and respiration are, for instance, related similarly to each other in a broad range of lichen associations (Palmqvist et al. 2002), and metabolic capacity can vary between seasons (MacKenzie et al. 2002) and populations (Sancho et al. 2000). Yet other studies have shown that environmental changes can reduce lichen vitality (Gauslaa & Solhaug 1999), and disturb their development by de-coupling weight gain from area expansion (Sundberg, Näsholm & Palmqvist 2001). Such disturbance might be caused by a differential response of the two growth processes to altered conditions, weight gain being related primarily to net C gain (Palmqvist & Sundberg 2000); and area gain to current N acquisition (Sundberg et al. 2001) and turgor pressure (Wessels 1993). However, such changes in thallus development have also been interpreted as a regulated response because increased thallus specific weight also increases their maximal water-holding capacity, which can be beneficial if the thallus becomes exposed to drier conditions (Green & Lange 1991; Hilmo 2002).

In this study two relatively fast-growing lichens, Nephroma arcticum and Peltigera aphthosa, were exposed to different light, water and nutrient regimes, and their growth was measured during 3 months in the field. Relations between area and weight gain, environmental conditions, and investments in photo- vs mycobiont tissue, were investigated by monitoring the microclimate, and by quantifying chlorophyll a as a marker for the photobiont's photosynthetic capacity, chitin for fungal biomass, and ergosterol for active fungal biomass and respiration (cf. Palmqvist et al. 2002; Sundberg et al. 1999).


  1. Top of page
  2. Introduction
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References


Nephroma arcticum (L.) Torss. and Peltigera aphthosa (L.) Willd. are tripartite lichen associations between green algal Coccomyxa sp. as the primary photobiont (Tschermak-Woess 1988), N2-fixing Nostoc sp. in cephalodia as the secondary photobiont (Paulsrud, Rikkinen & Lindblad 2000), and a fungal partner belonging to the Peltigerineae, Ascomycota (Eriksson & Winka 1998). Both are foliose with broad lobes, growing on bare soil or moss carpets, and are common in Swedish forests (Moberg & Holmåsen 1982). Nephroma arcticum was collected at Kulbäcksliden and P. aphthosa at Svartberget, outside Vindeln in the county of Västerbotten, Sweden in early June 1999. Sixty healthy-looking thalli of each species, without or with few maturing apothecia, collected from the same population, were chosen for the experiment. The thalli were rinsed to remove debris and their initial dry weights (DW1) determined to the nearest 0·1 mg after drying in darkness for 48 h (15 °C, 30–40% RH). Each thallus was then sprayed with water to determine its area (a1) when fully expanded (Palmqvist & Sundberg 2001). Dry weights varied between 0·1 and 0·8 g; area between 12 and 130 cm2; and thallus specific weights (TSW1) between 70 and 160 g m−2. Ten thalli of each species were mounted together on a nylon net frame (0·25 m2) fixed by four flexible aluminium rods (Palmqvist & Sundberg 2000). The six frames were used for different treatments in the field over 99 days, 23 June to 30 September 1999.

field site and manipulations

The study site was the same mixed Norway spruce (Picea abies) and pine (Pinus sylvestris) forest stand at Ulterviken, Umeå, Sweden (63°48′ N, 20°27′ E) as described by Palmqvist & Sundberg (2000); Sundberg et al. (2001); Dahlman, Näsholm & Palmqvist (2002). The transplant frames were placed on the forest floor connecting the thalli to the moss carpet. Each frame was assigned to one of the following six treatments: low light (LL); low light plus irrigation (LLw); medium light (ML); medium light plus irrigation (MLw); high light plus irrigation (HLw); or high light with irrigation and NH4NO3-phosphorus fertilization (HLNP). The LL and LLw frames were placed under a dense cover of north-facing branches of a Norway spruce tree, and the ML and MLw frames under a semi-closed canopy of spruce and pine. A patch without canopy cover was chosen for the HL and HLNP frames. The ML treatment was regarded as the control, being similar to the lichen's natural conditions. The ML frame was exposed to the natural precipitation at the site, amounting to 97 mm, while the LL frame was exposed to less precipitation due to shading of the dense spruce tree branches. The LLw, MLw and HLw frames were irrigated twice a week (32 times in total) with 2 l artificial rainwater (Dahlman et al. 2002), amounting to 256 mm precipitation during the transplantation period, and giving a total precipitation of up to 353 mm for the irrigated thalli. The artificial irrigation started by wetting the lichens so that all thalli were fully hydrated and expanded, and watering continued for another 5 min. The HLNP frame received NH4NO3 + P dissolved in the same amount of artificial rainwater to a total dosage of 500 mg N m−2 during the experiment (Dahlman et al. 2002). Three thalli of each species on the MLw frame were used as controls in a parallel experiment (Dahlman et al. 2002), while the other seven were used for this study. The frames in the same light regime were placed adjacent to each other, sharing the same temperature and light sensors.

climate monitoring

Precipitation (rain) was measured with a rain gauge (ARG100, Environmental Measurements Ltd, Wearfield, Sunderland, UK). Temperature was measured with thermocouples (chromel-alumel, type K, 0·05 mm diameter). Photon flux density (PFD) was measured with a quantum sensor (Skye SKP 215, Skye Instrument Ltd, Llandrindod Wells, UK) for the frames in the medium light, and with gallium-arsenide-phosphide photodiodes (5 mm diameter, G1125-02; Hammamatsu Photonics, Hammamatsu City, Japan) in the low and high light regimes. These have a flat sensitivity in the spectral range between 400 and 700 nm (Palmqvist & Sundberg 2000) and were calibrated twice against the Skye quantum sensor in the field. Thallus water contents (WC) were monitored with the impedance technique (Palmqvist & Sundberg 2000) for one separate thallus of each species and treatment. Data from all sensors were recorded and stored with a data logger (CR 10, Campbell Scientific Ltd, Logan, UT, USA) and a Relay Multiplexer (AM 416, Campbell Scientific). The sensors were read every minute and averaged over 15 min intervals during the 99 days of transplantation.

analysis of cellular components and data processing

After harvest, each thallus DW2 and a2 were determined, followed by freeze-drying and milling to a homogeneous powder. An elemental analyser (Model 2400 CHN, Perkin-Elmer, Wellesley, MA, USA) was used for quantification of total N. Chlorophyll was quantified after extraction in MgCO3-saturated dimethyl sulphoxide (DMSO; 60 °C for 40 min) (Palmqvist & Sundberg 2001). The fungal components ergosterol and chitin were measured by HPLC (Dahlman et al. 2001).

Microclimatic conditions for each species and treatment were extracted from the logger data as follows. Impedance values of fully hydrated thalli were set to 100% water content (WCmax) and dry thalli to 0% WC, and the impedance data from each species and treatment were then linearized over this interval to obtain their relative WC. The lichens were then assumed to be wet and metabolically active when their relative WC was above 5% (Palmqvist & Sundberg 2000). The accumulated irradiance intercepted by metabolically active thalli, denoted as Iwet (mol photons m−2), was obtained by summing all photons received by the lichens when their WC was above this limit (Palmqvist & Sundberg 2000; Sundberg et al. 2001). Average PFDwet and temperature in the light (Tlight) represent periods when PFDs were above 0 µmol m−2 s−1 and when the lichens were wet. Average Tdark represents periods below this light limit. Iwet was recalculated to its energy equivalent (J m−2) using the mean energy content of photons in the measured spectral range: 550 nm = 216·8 kJ mol−1. The energy content of lichen dry matter was set to 17·5 kJ g−1 (Palmqvist & Sundberg 2000). Lichen weight gain (ΔDWa; g m−2) could then be transformed to its energy equivalent (ΔDWe; J m−2) and divided by incident Iwet to obtain the apparent energy-use efficiency (e) for lichen growth.

light absorptance

Absorptance was determined in an Ulbricht integrating sphere (15 cm diameter) coated on the inside with magnesium oxide (Ögren 1988). A circular thallus disc (3 cm diameter) was punched from eight samples of each species, representing four marginal and four interior parts of thalli. Total chlorophyll in these samples varied between 50 and 240 mg m−2 in N. arcticum and 90–260 mg m−2 in P. aphthosa. Light from a projector lamp was focused on the upper photobiont-rich side of the sample, held at the centre of the sphere by a holder. A quantum sensor (Model Li-185 A; Li-Cor Inc., Lincoln, NE, USA) was shielded from the light beam and from light directly reflected from the sample. The PFD was read with the lichen sample inside (I) and outside (Io) the beam, and absorptance (Abs) calculated as: Abs = 1 − (I/Io) (Ögren 1988).

water-holding capacity

The water-holding capacity (WHC) – the maximum amount of water the thallus can hold when surface water films are removed – was determined for 16 thalli of each species, covering the same range of thallus sizes as the transplants. Dry thalli were weighed and thereafter soaked in water for 10 min, excess water was gently shaken off, and the thalli were allowed to equilibrate with the water for 30 min at 20 °C and 50% RH in the laboratory. The thallus was finally gently blotted with tissue paper immediately prior to weighing their fresh weight (FW). WHC was thereafter related either to the thallus DW as WHC = (FW − DW)/DW (g H2O g−1 DW), or to fully expanded ‘wet’ area (a) as WHC = (FW − DW)/a (g H2O m−2).

statistics and modelling

The effect of treatment and species on growth rates, energy-use efficiencies and subcellular component concentrations were analysed by a two-way general anova using a statistical package (statistix 7, Analytical Software, Tallahassee, FL, USA). An empirical model for area change (dependent variable) of each species was obtained by pooling all thalli from the six different treatments, then combining a best subset regression analysis and stepwise linear regression using the following independent variables: DW1, a1, TSW1, ΔDW, Iwet, average PFDwet, Chl a:b ratio, and area-based concentrations of Chl a, chitin, ergosterol, and N. A model for weight change (dependent variable) was obtained in a similar way using the same independent variables, but replacing DW change with area change, and using weight instead of area-based concentrations of the cellular components. Pooling all 57 thalli in the six treatments of the respective species was justified by comparing slopes and elevations for the ML (control) treatment vs the rest of the treatments, yielding similar empirical growth models between the 10 ML thalli and the rest.


  1. Top of page
  2. Introduction
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References

climatic conditions

The lichens were exposed to different environmental conditions in the six treatments (Table 1; Figs 1 and 2). PFD was below 75 µmol m−2 s−1 in the low light, up to 200 µmol m−2 s−1 in the medium light, and commonly exceeding 250 µmol m−2 s−1 in the high light treatments (Fig. 1). Daytime temperatures were cooler in the low light (12 °C) compared to the medium and high light (14–15 °C), while night-time temperatures were lowest in the more exposed high light (8–9 °C) (Table 1). The lichens became wet almost exclusively when it rained or when they were irrigated (see Methods; Fig. 1), so thalli in the irrigation treatments were wet for a longer total time (1200–1500 h) compared to the unwatered thalli (450–1200 h) (Table 1). Average PFD when the lichens were wet (PFDwet) increased with the light regime from 20 in the low light to 60 in the medium and 80–90 µmol m−2 s−1 in the high light (Table 1). This, together with the increased length of total wet time from LL to HLw and HLNP, subsequently increased the total light dose received by wet thalli (Iwet) from 16 to 30 in the low light, up to 200–300 mol m−2 in the MLw, HLw and HLNP treatments (Table 1). The percentage of active time occurring in the light increased slightly from the low light (40–50%) to the medium and high light regimes (50–60%) (Table 1). On average, a wet period lasted 15–30 h in N. arcticum and 20–40 h in P. aphthosa (Fig. 2), in accordance with the greater WHC of P. aphthosa compared to N. arcticum (Table 2; Fig. 3). As a result, both the percentage of the wet time occurring in the light, and average PFDwet for P. aphthosa, exceeded those of N. arcticum (Table 1).

Table 1.  Climatic conditions during the transplantation period (23 June−30 September 1999), corresponding to 2380 hours with climate monitoring. Total irradiance (Itot, mol m−2) amounted to 140 in LL, 440 in ML and 590 in HL treatments, respectively
Wet ‘active’ time (h)N. arcticum8901300110015001400
P. aphthosa4501200120015001400
Wet occasions (No)N. arcticum 50  70  40  70  70
P. aphthosa 20  40  40  40  40
Active in light (%)N. arcticum 40  50  60  60  50
P. aphthosa 50  50  60  60  60
Iwet (mol m−2)N. arcticum 29  56 144 175 225
P. aphthosa 16  49 153 208 290
Average PFDwet (µmol m−2 s−1)N. arcticum 20  25  60  60  80
P. aphthosa 20  20  60  65  90
Average Tlight (°C)N. arcticum 12·8  13·9  14·7  14·7  13·7
P. aphthosa 12·4  13·6  14·7  15·0  14·1
Average Tdark (°C)N. arcticum 10·3  10·6   9·9   9·2   8·3
P. aphthosa 11·4   9·8   9·4   9·3   8·6

Figure 1. Rain, light, temperature and thallus relative water contents (WC) of Nephroma arcticum (solid lines) and Peltigera aphthosa (dashed lines) in the different treatments, during an 11-day period in August 1999. For WC, the upper row shows LL and ML treatments; the second row LLw, MLw and HLw + HLNP treatments. Arrows indicate the four irrigation events during this period.

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Figure 2. Duration of wet active periods presented as relative frequency of occurrence in relation to total wet time (h, in italics). Lichens were assumed to be wet and metabolically active when their relative WC was >5% (see Methods).

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Table 2.  Mean (± 1 SE) absorptance of eight thalli of each lichen and their associated chlorophyll concentrations, and water-holding capacity (WHC), of 16 water-saturated thalli of each lichen. Measurements were made on additional thalli collected from the same area as the transplants. Means with a different letter are significantly different from each other for P < 0·05 (Tukey's HSD)
SpeciesAbsorptance (%)Chl a mg m−2Chl a + b mg m−2WHC
g H2O g−1 thallusg H2O m−2 thallus
N. arcticum67 ± 3A110 ± 20A130 ± 20A2·7 ± 0·1A2·1 ± 0·2A
P. aphthosa82 ± 2B140 ± 20A180 ± 20A5·2 ± 0·3B4·9 ± 0·2A

Figure 3. Water-holding capacity (WHC) of water-saturated lichen thalli in relation to thallus specific weight (TSW) for Nephroma arcticum (•) and Peltigera aphthosa (○). Linear regressions yielded the following equations where slopes were not significantly different between the two species (P = 0·59): WHC in N. arcticum = −117 + 4·2 × TSW (r2 = 0·89); WHC in P. aphthosa = 14 + 4·9 × TSW (r2 = 0·45).

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growth and energy-use efficiency

The lichens displayed significant differences in weight gain between both species and treatment (Table 3). Relative growth rate (RGR) increased from 0·4 to 0·5 in the LL treatment up to 2·5–4·4 mg g−1 day−1 in the MLw, HLw and HLNP treatments. Peltigera aphthosa grew more quickly than N. arcticum in the medium and high light treatments (Table 3). In N. arcticum, there was an average increase in area of 9% in the LL treatment, up to 26–28% in the ML and HLw treatments. Peltigera aphthosa displayed a similar response of thallus expansion, with a 14% increase in the LL treatment and ≈40% increase in the MLw, HLw and HLNP treatments. Thallus expansion was significantly greater in P. aphthosa compared to N. arcticum in the MLw treatment (Table 3). There was a tendency for decreased TSW in the low light, increasing with the light regime with the largest increase in TSW displayed by the fertilized N. arcticum thalli (Table 4). The apparent light-use efficiency (e) of lichen growth in P. aphthosa exceeded that in N. arcticum in the LL and LLw treatments, while e was similar between species in the other treatments (Table 3). The light absorptance of wet thalli was 82% for P. aphthosa and 67% for N. arcticum (Table 2). On average, this trait could not be related to differences in chlorophyll concentration between the two species (Table 2), but there was a correlation between total chlorophyll and absorptance when all samples were pooled (Fig. 4).

Table 3.  Weight and area gain, relative growth rate, and light-use efficiency of weight gain (e), 23 June−30 September 1999. Data are presented as mean ± 1 SE for each species and treatment with 10 samples in each group, except MLw where n = 7. Means with a different letter are significantly different from each other for P < 0·05 (Tukey's HSD) when a general anova was carried out using both treatment and species in the model
ΔDW (g g−1)N. arcticum0·04 ± 0·01a0·08 ± 0·02ab0·18 ± 0·01bc0·25 ± 0·03cd0·26 ± 0·02cd0·26 ± 0·03cd
P. aphthosa0·05 ± 0·01a0·18 ± 0·02bc0·29 ± 0·02de0·43 ± 0·03f0·39 ± 0·04ef0·37 ± 0·03ef
Δa (m2 m−2)N. arcticum0·09 ± 0·01a0·16 ± 0·03abc0·28 ± 0·02bcde0·23 ± 0·02abcd0·26 ± 0·03bcde0·16 ± 0·04abc
P. aphthosa0·14 ± 0·03ab0·30 ± 0·03cde0·34 ± 0·02de0·40 ± 0·04e0·38 ± 0·04de0·36 ± 0·05de
RGR (mg g−1 day−1)N. arcticum 0·4 ± 0·1a 0·8 ± 0·2ab 1·8 ± 0·1bc 2·5 ± 0·3cd 2·6 ± 0·2cd 2·7 ± 0·3cd
P. aphthosa 0·5 ± 0·1a 1·8 ± 0·2bc 3·0 ± 0·2de 4·4 ± 0·4f 4·0 ± 0·4ef 3·8 ± 0·3ef
e (%)N. arcticum 1·2 ± 0·2ab 1·2 ± 0·3ab 1·1 ± 0·1ab 1·2 ± 0·1ab 0·9 ± 0·1a 1·0 ± 0·1ab
P. aphthosa 2·6 ± 0·6cd 3·6 ± 0·4d 1·8 ± 0·1abc 2·1 ± 0·2bc 1·4 ± 0·1ab 1·3 ± 0·1ab
Table 4.  Initial thallus specific weight and its change during the experiment, and subcellular component concentrations in relation to thallus area at harvest. Data are presented as mean ± 1 SE for each lichen and treatment with 10 samples in each group, except MLw where n = 7 and chitin in P. aphthosa LL where n = 8. Means with a different letter are significantly different from each other for P < 0·05 (Tukey's HSD) when a general anova was carried out using both treatment and species in the model. Lower-case letters reflect the interaction term, capital letters the main effect of species
TSW1 (g m−2)N. arcticum 104 ± 7ab 111 ± 5abc 117 ± 4abc 110 ± 6abc 100 ± 2a 105 ± 4ab 108 ± 2A
P. aphthosa 118 ± 5abc 132 ± 8bc 122 ± 5abc 125 ± 7abc 140 ± 12c 126 ± 8abc 127 ± 3B
ΔTSW (%)N. arcticum      −5 ± 1a     −7 ± 2a  −10 ± 2a   2 ± 4ab   0 ± 2ab  11 ± 4b    −1 ± 1A
P. aphthosa      −9 ± 3a  −12 ± 3a     −4 ± 3ab   3 ± 4ab   3 ± 6ab   3 ± 5ab    −3 ± 2A
Nitrogen (g m−2)N. arcticum 2·4 ± 0·1ab 2·3 ± 0·1a 2·1 ± 0·1a 2·0 ± 0·2a 2·0 ± 0·1a 2·4 ± 0·2ab 2·2 ± 0·1A
P. aphthosa 3·4 ± 0·1cd 3·6 ± 0·1d 2·9 ± 0·1bc 3·1 ± 0·1cd 3·6 ± 0·2d 3·5 ± 0·1cd 3·4 ± 0·1B
Chl a (mg m−2)N. arcticum 110 ± 10a 110 ± 10a 100 ± 10a  90 ± 10a  70 ± 5a 100 ± 10a 100 ± 4A
P. aphthosa 190 ± 10bc 200 ± 10bc 180 ± 10bc 210 ± 10c 160 ± 10b 170 ± 10b 180 ± 5B
Chl a[N] (%)N. arcticum0·29 ± 0·03ab0·32 ± 0·03abc0·32 ± 0·03abc0·31 ± 0·03ab0·23 ± 0·01a0·28 ± 0·01ab0·29 ± 0·01A
P. aphthosa0·36 ± 0·03bc0·35 ± 0·01bc0·38 ± 0·02bc0·43 ± 0·02c0·28 ± 0·01ab0·30 ± 0·01ab0·35 ± 0·01B
Chl a:bN. arcticum 2·8 ± 0·1abc 2·9 ± 0·1abc 3·1 ± 0·1ab 3·2 ± 0·1a 3·1 ± 0·1ab 3·1 ± 0·1ab 3·0 ± 0·04A
P. aphthosa 2·7 ± 0·1c 2·7 ± 0·0c 2·8 ± 0·0bc 2·8 ± 0·0abc 2·6 ± 0·1c 2·8 ± 0·1bc 2·7 ± 0·02B
Ergosterol (mg m−2)N. arcticum 190 ± 10ab 160 ± 10abcd 170 ± 10abcd 190 ± 10abc 170 ± 10abcd 200 ± 10a 180 ± 5A
P. aphthosa 140 ± 10d 140 ± 10d 150 ± 5cd 170 ± 10abcd 160 ± 10abcd 150 ± 10bcd 150 ± 3B
Chitin (g m−2)N. arcticum 2·2 ± 0·2a 2·6 ± 0·2a 2·6 ± 0·2ab 2·8 ± 0·2ab 2·4 ± 0·1a 2·8 ± 0·2ab 2·6 ± 0·1A
P. aphthosa 3·5 ± 0·2bc 4·0 ± 0·1cde 3·8 ± 0·2cd 4·2 ± 0·3cde 4·6 ± 0·3e 4·3 ± 0·1de 4·1 ± 0·1B

Figure 4. Light absorptance (Abs) in Nephroma arcticum (•) and Peltigera aphthosa (○) in relation to total chlorophyll concentration. Both species were pooled in a linear regression analysis yielding Abs = 57 + 0·11 × Chl a + b (r2 = 0·42). When the species were separated the regression was significant only for N. arcticum (P < 0·05).

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component concentrations

Thallus N concentrations ranged from 2·0 to 2·4 g m−2 in N. arcticum and from 2·9 to 3·6 g m−2 in P. aphthosa (Table 4; Fig. 5a–f), and were similar between treatments within species. The photobiont component Chl a ranged from 70 to 110 mg m−2 in N. arcticum, and from 160 to 200 mg m−2 in P. aphthosa, without any treatment effects within species, but was consistently higher in P. aphthosa (Table 4; Fig. 5a,b). The Chl a:b ratio was higher in N. arcticum (3·0) compared to P. aphthosa (2·7) when all treatments were pooled, caused primarily by a smaller Chl a:b ratio of P. aphthosa in the HLw treatment (Table 4). Peltigera aphthosa contained more chitin (4·1 g m−2) than N. arcticum (2·6 g m−2) in all the treatments, but as for the N and Chl a concentrations there was no intraspecific variation between treatments (Table 4). The fungal plasma membrane component ergosterol was higher in N. arcticum (180 mg m−2) compared to P. aphthosa (150 mg m−2) when all samples were pooled, and in some treatments (Table 4). Within species, the Chl a concentration was positively correlated to thallus N concentration in N. arcticum (Fig. 5a), and chitin was positively correlated to N in P. aphthosa (Fig. 5d).


Figure 5. Relationships between thallus nitrogen (N) and Chl a (a,b); chitin (c,d); and ergosterol (e,f) at harvest. To avoid autocorrelation the N content of Chl a (6·27% of MW), and chitin (6·33% of MW) was subtracted from total N in their respective plots. Regression lines are given with 95% CI when the relation was significant for P < 0·05. Triangles, LL, LLw; circles, ML, MLw; squares, HLw, HLNP; open symbols, LL, ML; black symbols, LLw, MLw, HLw; grey symbols, HLNP.

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empirical growth modelling

Weight and area changes of individual thalli were coupled to each other, with 34% of the variation in either growth process being explained by the other (Fig. 6a,b). Variation in Iwet accounted for 66% of the variation in weight gain in N. arcticum and 63% in P. aphthosa (not shown). When also adding the variation in Chl a concentration, 70% of the variation in weight gain was accounted for in N. arcticum, and 74% in P. aphthosa (not shown). Weight gain was best explained by a model with Iwet, weight-based Chl a concentration and area change, explaining 78% of the variation in weight gain in N. arcticum and 79% in P. aphthosa (Table 5). The resulting equations were similar for the two species, although the models predicted greater productivity per Chl a in P. aphthosa than in N. arcticum (Table 5). The stepwise procedure included area change earlier than Chl a concentration in N. arcticum, while this pattern was reversed in P. aphthosa. The best model for area gain was similar for the two species, explaining 76% of the variation in N. arcticum and 82% in P. aphthosa (Table 5). In this model, weight change and TSW1 were positively correlated with area gain, while the fungal components ergosterol and chitin had a negative effect.


Figure 6. Area gain (Δa) as a function of weight gain (ΔDW) for Nephroma arcticum (a) and Peltigera aphthosa (b). All samples of the respective lichen (n = 57) were pooled in the regression analysis (solid line) yielding the following linear equations: ΔaNarc= 0·1 ± 0·02 + 0·6 ± 0·1 × ΔDW with adj r2 = 0·34 and P = 0·0000; ΔaPaph = 0·2 ± 0·03 + 0·5 ± 0·1 × ΔDW with adj r2 = 0·34 and P = 0·0000. Regression lines are given with ±95% CI. Triangles, LL, LLw; circles, ML, MLw; squares, HLw, HLNP; open symbols, LL, ML; black symbols, LLw, MLw, HLw; grey symbols, HLNP.

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Table 5.  Stepwise linear regression models of Δa (m2 m−2) and ΔDW (g1 g−1) during the experimental period (dependent variables) using the following independent variables: TSW1 (g m−2), ΔDW (g g−1), Δa (m2 m−2), DW1 (g), a1 (m2), Iwet (mol m−2), average PFDwet (µmol m−2 s−1), Chl a, Chl a:b ratio, chitin, ergosterol and nitrogen, and by pooling all thalli from the six treatments. Component concentrations were expressed on an area basis for area-change regressions and on a weight basis for weight-change regressions. Values have been rounded
Growth parameterSpeciesResulting model
VariableCoefficient ± SEStudent's tP variableadj r2FP regression
Weight change (g g−1)N. arcticumConstant−0·15 ± 0·04 −3·4  0·001   
Iwet  0·001 ± 0·0001  10·3<0·001   
Chl a (mg g−1)    0·1 ± 0·04   3·1  0·003   
Δa    0·3 ± 0·1   4·8<0·0010·7868·6<0·001
P. aphthosaConstant  −0·3 ± 0·06−4·7<0·001   
Iwet  0·001 ± 0·0001  10·9<0·001   
Chl a (mg g−1)    0·2 ± 0·03   5·5<0·001   
Δa    0·3 ± 0·1   3·7<0·0010·7970·8<0·001
Area change (m2 m−2)N. arcticumConstant   0·02 ± 0·06−0·29  0·77   
ΔDW    0·8 ± 0·1  11·0<0·001   
TSWstart  0·005 ± 0·001   8·1<0·001   
Ergosterol (g m−2)  −1·4 ± 0·3  −5·6<0·001   
Chitin (g m−2)  −0·1 ± 0·02−4·6<0·0010·7640·5<0·001
P. aphthosaConstant−0·02 ± 0·05   0·31  0·76   
ΔDW    0·8 ± 0·1  12·6<0·001   
TSWstart  0·005 ± 0·0005  12·5<0·001   
Ergosterol (g m−2)  −1·4 ± 0·5−3·0<0·001   
Chitin (g m−2) −0·1 ± 0·01−4·9<0·0010·8263·8<0·001

Initial area (a1) and initial dry weight (DW1) were rejected from the above models describing relative growth. However, there was a positive effect of DW1 and a1 on absolute weight and area gain (not shown). This is in agreement with the notion that, on an absolute scale, a large thallus can grow more quickly than a small thallus provided the larger thallus has not aged (Hill 2001).


  1. Top of page
  2. Introduction
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References

weight gain model

In agreement with previous studies of these and other tripartite lichens (Renhorn et al. 1997; Palmqvist & Sundberg 2000; Sundberg et al. 2001; Hilmo 2002), the major part of the variation in weight gain was attributed to water availability and irradiance (Tables 1 and 3), with the combined parameter Iwet having the largest effect in the empirical weight-gain model (Table 5). This parameter alone accounted for 66% of the variation in weight gain in N. arcticum and 63% in P. aphthosa (not shown), emphasizing, as discussed previously (Palmqvist 2000), that lichen weight gain is primarily limited by the environmental factors that limit their photosynthetic activity. Moreover, as a result of their cephalodial N2-fixation activity (Rai 1988) the two lichens also have a relatively high concentration of N compounds in their thalli (Palmqvist et al. 2002). As N2 fixation (similarly to lichen photosynthesis) is also limited by water availability and photosynthetically transduced energy (Rai 1988), Iwet may subsequently control the accumulation of weight in the form of combined N in addition to assimilated C compounds.

However, the relatively large variation in weight gain between thalli exposed to the same Iwet (Fig. 6) emphasizes that internal factors are also involved. Variation in Chl a concentration among the thalli (Fig. 5a,b) could explain some of this variation, with Iwet and Chl a together explaining 70–74% of the variation in weight gain. This suggests that photosynthetic activity, and possibly also N2 fixation, should increase with the Chl a concentration of the thallus. This is supported by recent findings showing a strong correlation between photosynthetic capacity and Chl a concentration across a broad range of lichens (Palmqvist et al. 2002). Light-absorption efficiency was also increased with increased Chl pigment concentrations, particularly in N. arcticum (Fig. 4).

In addition to Iwet and Chl a there was also a significant effect of area gain on weight gain (Table 5; Fig. 6). In part this is because light is absorbed on an area basis, and it might be more advantageous to invest new resources into area expansion rather than in thickening of the thallus – a thallus that continues to invest gained weight (resources) in new area will increase its capacity for further resource acquisition.

area gain model

Area gain was less affected than weight gain by environmental factors, overlapping between treatments (Table 3; Fig. 6). Area gain might then be more strongly regulated by internal factors, or be regulated by some other factors not measured here, such as current N acquisition (Dahlman et al. 2002). Nevertheless, as much as 80% of the variation in area gain between individual thalli was accounted for in a linear regression model including ΔDW, TSW1 and the area-based ergosterol and chitin concentrations of the thallus.

Area expansion of foliose lichens is the result of marginal hyphal growth in conjunction with photobiont cell division near the growing hyphal tips (Jahns 1988). The formation of this new tissue requires the input of new resources to the thallus or, alternatively, recycling of elements from ageing parts, as in mat-forming lichens (Crittenden 1991). However, as also proposed by Crittenden (1991), in the two foliose lichens studied here new thallus area is formed without significant dieback of the existing thallus, as evidenced by a similar composition of old and new tissue (Sundberg et al. 2001; Dahlman et al. 2002). This suggests that new thallus area is produced predominantly from recently assimilated resources rather than recycling within the thallus, which could explain why their area gain is dependent on their weight gain (Table 5; Fig. 6). However, in the low light treatments there was a tendency for decreased TSW during the transplantation period, while in the high light treatments TSW increased (Table 4), suggesting that weight and area gain are not always causally linked. The significant increase in weight in the absence of area expansion in the high light treatments (Fig. 6) supports previous findings that high light exposure might trigger a thickening of the thallus (Hilmo 2002).

Area gain was positively correlated with TSW1 (Table 5). There are several explanations for this. First, thalli with a large initial TSW may have relatively larger pools of previously assimilated resources compared to thalli with smaller TSW. Such putative pools can then be used for new thallus synthesis even if resource supply from the present environment is limiting, as in the low light treatments. Furthermore, the WHC of the two lichens increased significantly with TSW (Table 2; Fig. 3), in agreement with Hilmo (2002). Increased WHC would prolong metabolically active periods and, provided light is available, this will increase Iwet and subsequently also weight gain (Table 5). Hyphal extension would subsequently be faster in thalli with a high TSW because water is required to create the turgor pressure that drives the process (Wessels 1993).

Increased area-based concentrations of the two fungal components ergosterol and chitin was correlated negatively, and similarly to area gain in both lichens (Table 5; Figs 5c–f and 6 for individual thalli). This may be related to an increased respiratory load relative to photosynthetic capacity in thalli with higher area-based concentrations of fungal tissue, so reducing net accumulation of assimilates. In agreement with the larger ergosterol to Chl a ratio of N. arcticum in comparison to P. aphthosa (Table 4; Fig. 5a,b) the ratio between maintenance respiration and photosynthetic capacity is also significantly larger in N. arcticum (Palmqvist et al. 2002). Increased ergosterol and chitin concentrations might also be a sign of thallus maturation, further triggering a transition of vegetative expanding hyphae into reproductive and non-expanding hyphae (Jahns & Ott 1997). This is supported by a parallel experiment with 10 mature P. aphthosa thalli with many apothecia. These apothecia-rich thalli were treated similarly to the ML thalli, but had a significantly smaller area gain and greater ergosterol and chitin concentrations per unit area (K.P. and L.D., unpublished results).

The area gain models obtained were similar for N. arcticum and P. aphthosa (Table 5), emphasizing a similar regulation of this growth process in the two lichens. This might be expected as the two lichens investigated here are closely related and have similar developmental patterns (Jahns & Ott 1997). Both lichens can also regulate their N status through cephalodial N2 fixation, resulting in similar concentrations of N, ergosterol and chitin (Fig. 5; Palmqvist et al. 2002). Taken together, this can explain the similar coefficients in their respective area-gain model. On the other hand, even if the coefficients to the four variables – weight gain, TSW1, ergosterol and chitin – were identical for the two species, the constant's coefficient in the equation differed and the error terms were high (Table 5). Evidently, we need to know more about thallus expansion processes in lichens, and collect data from more foliose lichens in various climatic regimes, before we can say anything about the generality of the area gain model presented here.

concluding remarks

Our study emphasizes that lichen growth is primarily limited by external factors such as water and irradiance – factors that limit photosynthetic activity. There was a large variation in growth between thalli exposed to the same environmental conditions (Fig. 6); P. aphthosa generally grew faster than N. arcticum (Table 3). The differences in growth rates between the lichens could be explained in part by the larger water-holding capacity of P. aphthosa (Table 2), which enhances the time the lichen is wet in light (Table 1). Peltigera aphthosa also had a larger Chl a to ergosterol ratio (Table 4), increasing photosynthetic capacity in relation to respiration. Once wet and metabolically active, and exposed to light, a lichen's capacity for growth should then increase with the higher Chl a status of the thallus. This conclusion is in agreement with the overall increase in net carbon-gain capacity with increasing Chl a concentration in lichens (Tretiach & Carpanelli 1992; Palmqvist et al. 2002), and the generally high Chl a concentrations of competitive, fast-growing species (Rogers 1990).


  1. Top of page
  2. Introduction
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References

The Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS) has provided grants to K.P. (24·0795/97). The Center for Environmental Research (CMF, Umeå, Sweden) provided a grant to L.D. (993194). Margareta Zetherström (Department of Forest Genetics and Plant Physiology, SLU, Sweden) assisted with HPLC measurements and gave skilful technical support throughout. We would also like to thank P. Crittenden and an anonymous referee who gave valuable comments and helped to improve the final version of this manuscript. Roland Wass (UPSC, Umeå University) assisted with the climate sensors, and Bodil Sundberg (Örebro University) took active part in the initiation of this study. The owners of Stubbmyrberget, Ulterviken are acknowledged for allowing us to use their forest.


  1. Top of page
  2. Introduction
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References
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