- Top of page
- Materials and Methods
Epiphytic lichens comprise a notable, but declining part of the biodiversity in boreal forests. Concern for this spectacular biomass component has stimulated experimental studies to assess biomass accumulation in terms of lichen growth rates (e.g. Dahlman & Palmqvist, 2003; Gaio-Oliveira et al., 2004; Gauslaa et al., 2006, 2007; Coxson & Stevenson, 2007; Larsson et al., 2009). There is a long tradition of measuring radial growth in rosette-forming foliose and crustose lichens, as reviewed by Armstrong & Bradwell (2011). However, many epiphytic lichens do not form rosettes and have growth forms with substantial intercalary growth (e.g. Honegger, 1993) requiring other methods than radial growth measurements. From a functional perspective, thallus area and biomass are important parameters, and assessment of their dynamics can provide new knowledge.
So far, lichen growth has been reported on a range of temporal scales in various habitats. Yet, few studies have quantified seasonal variation in growth of thallus area and biomass simultaneously. A comparison of available seasonal growth studies shows that site- and/or stand-specific factors tend to shape recorded contrasts in growth rates (e.g. Benedict, 1990; Muir & Shirazi, 1997; Kytöviita & Crittenden, 2002; Caldiz, 2004). Thus, growth experiments under more standardized field conditions are needed to understand mechanisms behind lichen responses and variations in seasonal growth patterns. Such basic knowledge has practical importance for modeling studies and for lichen conservation.
Like most photosynthetic organisms in seasonal climates, lichens are known to acclimate temporarily by tuning various functions to seasonal climatic changes (McEvoy et al., 2007; Vráblikováet al., 2006; Lange & Green, 2005). They probably need a high resilience to maximize growth rate and/or survival under fluctuating conditions (Schofield et al., 2003). Lichen growth is often three-dimensional (Gauslaa et al., 2009). Mass growth responds to photosynthesis (Palmqvist, 2000), whereas the area expansion is poorly understood. Changes in specific thallus mass (STM) occur when area and mass do not increase in parallel. STM, a main driver of water-holding capacity in lichens (Gauslaa & Coxson, 2011), may respond to seasonally changing humidity availability and thereby impact lichen fitness. When laminar growth occurs in older thallus portions of Lobaria (Honegger, 1993), evidenced by enlargements of the characteristic reticulate patterns, simultaneous or delayed changes in STM may also occur as increase in thickness or as thinning during area expansion. STM is significantly higher in lichen populations growing in open forests than in those inhabiting shaded habitats (Gauslaa & Coxson, 2011), but it is not known if STM varies in response to seasonally changing evaporative demands. One aim is to quantify changes in STM in studied lichens during an annual cycle in a site with humid autumns and drier springs (Thue-Hansen & Grimenes, 2010).
This study focuses on two epiphytic old forest lichens, the cephalolichen Lobaria pulmonaria which has a green alga as its main photobiont and the cyanolichen Lobaria scrobiculata. These foliose species are used to indicate ecological continuity in boreal forest conservation management. Nevertheless, such lichens can grow fast in clear-cuts and forest edges because they benefit from the higher light intensity than that available in forest stands (Renhorn et al., 1997; Gauslaa et al., 2006, 2007). By running the growth experiment in a boreal clear-cut, we can separate the effects of seasonal climatic patterns from those derived from spatial (Gauslaa & Solhaug, 2001) and temporal (Gauslaa & Solhaug, 2000) variations in canopy cover, and thus reduce unexplained variation in growth. Lichen growth in open sites should, compared with growth inside forests, be more directly linked to physical parameters such as seasonally changing light, temperature and humidity. At high latitudes with low sun, the aspect of lichen-inhabited surfaces is probably important, and the aspect-dependent light exposure changes substantially during a year. Therefore, our two main aims were (1) to quantify seasonal changes in biomass and thallus area during a boreal year, and (2) to analyze how aspect in interaction with season influences the growth of the studied lichens.
- Top of page
- Materials and Methods
The growth data in Figs 2–4 are unique, as repeated simultaneous seasonal changes in biomass and area have not previously been recorded for a large set of lichen thalli. Area increment is rarely recorded in lichen studies, and lichen growth is often considered a synonym for mass growth or radial increment. Traditionally, environmental conditions experienced while thalli are moist have been considered to determine lichen growth (Lange, 2002, 2003b; Jonsson et al., 2008), and the irradiance during hydration explains a substantial part of the variation in lichen biomass increase (Dahlman & Palmqvist, 2003; Gaio-Oliveira et al., 2004).
The diverging annual courses of biomass (Fig. 2) and area growth rates (Fig. 3) provide a basis for better understanding of lichen growth processes. Comparisons of climatic variables (Fig. 1, Table 1) and growth patterns (Figs 2, 3) on a seasonal time-scale suggest that light is a driver of RGR, whereas humidity is required for area expansion (RTAGR). This was previously inferred in a spatial study of L. pulmonaria transplanted in three successional forest stands differing in canopy cover in each of three climatic zones along a macroclimatic humidity gradient from continental to oceanic sites (Gauslaa et al., 2009). Consistent responses along spatial (Gauslaa et al., 2009) and temporal (this study) gradients provide strong support for the view that lichen biomass growth and area growth are different processes. Area expansion is important as it is the only way in which a lichen thallus can increase its light-harvesting area and occupy new space. In view of the strong spatial (Gauslaa et al., 2007) and seasonal (this study) discrepancies in mass and area growth, future modeling studies should include both growth processes.
Our study documents strong seasonal changes in STM in lichens (Fig. 4). The photobiont drives mass growth through its CO2 assimilation (Palmqvist, 2000; Dahlman & Palmqvist, 2003), whereas the mycobiont contributes to area growth in a process that apparently requires hydration to form turgor pressure in expanding hyphae (Money, 2008). Cell expansion growth in plants is determined by cell wall properties and turgor pressure as described by Eqn 1:
- (Eqn 1)
(m, the wall extensibility; Ψp, the turgor pressure; Y, the yield threshold which Ψp must exceed to allow growth (e.g. Nobel, 1999).) Lichens already lose their turgor pressure at relative water contents of 43–60% (Beckett, 1995, 1996) which still may allow 50% of maximal net photosynthesis (Tuba et al., 1996). With no turgor pressure, area growth is hardly possible (Eqn 1), causing increased STM when photosynthesis is positive. STM stays fairly constant or decreases in seasons with short days (October–March) and long hydration periods without or with low light (Table 1; Fig. 4). Under such conditions, the carbon gain is low. In April–June when most rain comes during the day (Fig. 1), facilitating photosynthesis, STM increases strongly (Fig. 4). Climatic factors thus seem to drive seasonal changes in STM, consistent with an inducible response by external factors rather than a constitutive intraspecific regulation of thallus thickness. However, we do not know whether this is an active or passive response to prevailing climatic conditions.
According to the study of Gauslaa & Coxson (2011), the water-holding capacity is tightly coupled to STM, and within sites with similar rainfall regimes, STM significantly increases with canopy openness, and thus with increasing evaporative demands. Regardless of whether such responses are actively or passively regulated, an increase in STM may function as a compensatory mechanism to prolong hydration periods in sites with strong evaporative demands. The seasonal course in STM (Fig. 4) is consistent with the hypothesis that such a compensatory mechanism can be a part of seasonal acclimation in lichens. From a life cycle time perspective, STM increases with thallus size (e.g. Dahlman et al., 2002; Dahlman & Palmqvist, 2003; Gauslaa et al., 2009; Larsson & Gauslaa, 2011). In the cyanolichen Degelia plumbea, STM, total thallus thickness, and water-holding capacity are all strongly and positively correlated with thallus size, resulting in a > 10 times longer hydration period in the largest compared with the smallest thallus (Gauslaa & Solhaug, 1998). We can use the data set on juvenile thalli published in Larsson & Gauslaa (2011) to calculate that STM significantly increases with thallus size from 7.5 mg cm−2 in very small thalli (2–4 mm2) of both species to 9.8 and 13.5 mg cm−2 in larger thalli (10–20 cm2) of L. pulmonaria and L. scrobiculata, respectively. Thus, mature thalli may grow quickly in ecological niches where juvenile thalli are unable to establish because their water storage is too low. A size-dependent functional relationship between STM and duration of hydration cycles may explain the frequently reported discrepancy between fundamental and realized ecological niches in lichens (e.g. Antoine & McCune, 2004). Another implication of strongly seasonal STM is that studies of lichen performance need to consider the collection date of sampled thalli.
Cyanolichens, unlike cephalo- and chlorolichens, need liquid water to activate photosynthesis (Lange et al., 1986). They need much more water to become saturated than chlorolichens (Lange et al., 1993). At 90% relative humidity, the green alga Dictyochloropsis reticulate in L. pulmonaria efficiently assimilates carbon in the absence of liquid water, whereas the cyanolichen L. scrobiculata releases carbon, indicating active mycobiont respiration but inactive photosynthesis (Máguas et al., 1995). Thus, cyanolichen respiration in humid air represents a net carbon loss, although the loss is lower than after activation in water (Máguas et al., 1995). If hydration is mostly caused by humid air rather than rain or dew, L. scrobiculata will experience fewer active hours with photosynthesis than L. pulmonaria, but similar periods of fungal respiration. This may explain the lower growth rate in L. scrobiculata. To compensate for an inability to use humid air to photosynthesize, L. scrobiculata has a substantially higher water-holding capacity at a given STM than L. pulmonaria (Gauslaa & Coxson, 2011), presumably because of the gelatinous sheath surrounding their Nostoc cells. The highly significant regressions (r2 = 0.804 and 0.833; n = 120 in L. pulmonaria and L. scrobiculata, respectively) between water-holding capacity and STM (Gauslaa & Coxson, 2011) give computed start water-holding capacity as 24.6 and 14.7 mg H2O cm−2 for our thalli of L. scrobiculata and L. pulmonaria, respectively. Taking the highest annual increase in STM, 2.5 and 1.9 mg cm−2 for the July set for L. scrobiculata (top) and the January set for L. pulmonaria (north; see Fig. 4), respectively, these values would imply an increase in water storage of 8.7 mg H2O cm−2 for the cyanolichen vs 4.7 mg H2O cm−2 for the cephalolichen, equivalent to a 32–35% increase. The stronger increase in STM from the dark, humid October–December season to the sunny, dry April–June season for L. scrobiculata than for L. pulmonaria (Fig. 4) thus probably improves the fitness of the cyanolichen by prolonging hydration periods. The high rise of STM in L. scrobiculata at the extreme top aspect receiving abundant direct sun during summer (Fig. 5; July–September) can similarly be seen as beneficial at strong evaporative demands. Night-time dew providing liquid water is strongest at the top facing the open sky and being subjected to rapid nocturnal cooling (P. Larsson; unpublished field obs.; see also Stoutjesdijk & Barkman, 1987). In open sites, Lange (2003a) has shown that clear days with no rain contribute to a much higher proportion of the annual carbon gain than rainy days for the chlorolichen Lecanora muralis. Higher dew formation and a correspondingly high water-holding capacity probably explain the faster growth of L. scrobiculata at the top than at vertical aspects (Fig. 2). At optimal hydration, L. scrobiculata had much higher CO2 assimilation (2.3 ± 0.1 μmol CO2 m−2 s−1) than L. pulmonaria (0.9 ± 0.1 μmol CO2 m−2 s−1; mean ± SE; P. Larsson, unpublished data). Lobaria pulmonaria (e.g. Gauslaa & Solhaug, 1999), unlike L. scrobiculata (K. A. Solhaug, D. Coxson & Y. Gauslaa, unpublished data), is high-light susceptible. Therefore, it responds in an opposite way to aspect during the dry, sunny April–June season. Strong photoinhibition in L. pulmonaria at the top and south aspects (P. Larsson; unpublished data), also evidenced by localized bleached portions, presumably reduces the potential of this lichen to increase STM (Fig. 4).
In winter (October–December or January–March), newly transplanted thalli grew faster than older transplants that had been exposed for the first time to the clear-cut in summer (April–June or July–September; Figs 2, 3). This is interesting, as temperature and light during winter are low (Fig. 1). Solar radiation-screening compounds may explain the contrast between winter and summer sets. Lobaria pulmonaria and L. scrobiculata use cortical melanins and usnic acid, respectively, to reduce light at the underlying photobiont layer to nonharmful levels (Solhaug & Gauslaa, 2012; Asplund et al., 2010). As these pigments are rapidly induced by UV-B (Solhaug et al., 2003; McEvoy et al., 2006), they were absent or at low concentrations in the transplants when collected in forests, evidenced by a pale ash-gray color in the dry state. After exposure to the clear-cut in the sunny periods of April–June or July–September, these pigments were induced, as evidenced by brown pigmentation in L. pulmonaria and a yellowish color of L. scrobiculata. We believe that the higher growth in newly transplanted thalli in the dark winter is caused by high availability of light at the photobiont layer beneath pigment-deficient and transparent upper cortices in shade-adapted thalli (Gauslaa & Solhaug, 2001). Such an effect was not found during transplantation in the summer months, because the sudden exposure of shade-adapted forest thalli to a sun-exposed environment caused temporarily severe photoinhibition before pigments were formed (e.g. Gauslaa & Solhaug, 2000). We also believe that little or no pigment synthesis in UV-B-deficient winter seasons may have caused elevated growth also in parts of the next transplantation season with low light. In general, lichens function well at low temperatures that normally result in slow metabolic activity implying low respiratory losses and a lower light compensation point (Kershaw, 1985; Lange, 2002). Lichen photosynthesis can be active down to −10°C (Lange, 1965; Green et al., 2002), provided that there is a minimum hydration level. The corresponding lower maximal light-saturated CO2 uptake at low temperatures probably does not matter for winter photosynthesis because light at high latitudes is constantly below light saturation in winter (Fig. 1).
An additional explanation for reduced growth rates with time may be nutrient depletion in transplants taken out of the canopy drip zone which naturally adds leachates to lichens in forests (Goward & Arsenault, 2000). Recent studies have shown that lichen growth may be limited by low phosphorous and/or nitrogen availability (Benner et al., 2007; McCune & Caldwell, 2009; Johansson et al., 2011). However, such mechanisms hardly account for the strong seasonal trends in our study.
In conclusion, growth of L. pulmonaria and L. scrobiculata occurs in all seasons of the year in open boreal sites, although at slower rates in winter than in late summer. Mass and area growth in both species are separated along a temporal scale, resulting in seasonal changes in STM. Growth in mass may not result in area gain during dry and sunny periods, whereas area expansion can occur without detectable mass increase in dark and humid seasons.