Physiological‐environmental Interactions in Lichens X. Light as an Ecological Factor

Abstract

Summary Little attention has been paid to the potential importance of light in the ecology of lichens. Data is presented which shows that Peltigera canina var. praetextata from a woodland floor can rapidly acclimate to low or high light levels and thus maintain a given level of photosynthesis irrespective of the leaf canopy status. This photosynthetic adjustment takes place naturally in the field during leaf fall in October. P. scabrosa shows a similar response to low light and furthermore, acclimation to low levels of illumination can readily take place in air‐dry material. In contrast populations of Peltigera aphthosa collected from the dense shade of spruce in northern Ontario are extremely sensitive to quite modest levels of ‘high’ light, with pronounced damage occurring under 350 μE m‐3 8‐1 illumination. Contrasting populations of P. aphthosa collected from open habitats are not affected by this level of illumination. The maintenance of the correct day length during air‐dry storage is also very important since a rapid alteration of daylength produces a marked although temporary photosynthetic decline. On the other hand continuous daylight induces a rapid and continuous decline of photosynthesis probably leading eventually to the death of the phycobiont.

Physiological‐environmental Interactions in Lichens

“…For example, the nitrogenase response matrix of P. praetextata (MacFarlane and Kershaw, 1977) shows that maximum rates of nitrogenase activity are generated at 25 to 30 °C in this species. In marked contrast, Collema shows optimum nitrogenase activity only at 25 °C, the rates at 30 °C declining markedly, indicating thermal deactivation of the nitrogenase enzymes (MacFarlane and Kershaw, 1977;Kershaw, MacFarlane and Tysiaczny, 1977). Rates of nitrogenase activity at 15 °C are only reduced to approximately 75% of the maximum levels at 25 °C whereas in P. praetextata the decline of activity at 15 °C is much more marked and achieves approximately 50% of the maximum values generated at 25 to 30 °C.…”

“…Nitrogenase activity was assayed using the acetylene reduction technique and a Hewlett Packard 5710A gas cbromatograph fitted with a Porapak R column and flame ionization detector. Experimental thalli were carefully removed from the bark and soaked overnight at 5 °C and 300 //E m -s"' PAR (MacFarlane and Kershaw, 1977). The thalli were incubated in gas exchange cells similar in design to those used by Larson and Kershaw (1975), with an atmosphere 10 "o by volume acetylene in air.…”

“…The marked summer decline of nitrogenase activity in Peltigera (MacFarlane and Kershaw, 1977) was shown to be due to summer temperature stress. Conversely, the elimination of nitrogenase activity under the winter snow-pack was shown to be due to the depletion of the carbohydrate pool supplying energy to the reaction in the dark, rather than the direct inactivation of the nitrogenase enzymes by freezing temperatures as had previously been suggested (Alexander and Kallio, 1976;Kallio, Kallio and Rasku, 1976;Crittenden and Kershaw, 1979).…”

Understory vegetation-environment relationships of lichen-rich forests in north-central British Columbia.

“…For dry, northern habitats, lichen growth rate actually increases with precipitation, because increased photosynthetic activity can be sustained for longer periods of time ). In addition, observations of microsite-specific adaptation to differing light environments within Cladonia populations Adams 1973, Legaz et al 1986), and of other lichen species showing considerable acclimation to both seasonal and experimentally induced changes in light (Kershaw and MacFarlane 1980), suggests that Cladonia species should be able to survive canopy closure in the absence of competition. Most vascular plants and feathermosses grow faster than lichens when moisture and temperature are not limiting, such that under dense forest canopies (which reduce the evaporation caused by direct solar radiation), or over thick organic layers (which both insulate the soil and increase water holding capacity), lichens are probably outcompeted for space and light (Yarranton 1975, Brown et al 2000, Crittenden 2000.…”

“…This may be explained, in part, by the existence of upper and lower hydration thresholds at which lichens can maintain positive net assimilation; if the lichen mats are in relatively wet locations, net assimilation may become negative as respiration outpaces photosynthesis (MacFarlane and Kershaw 1982, Kershaw 1985. Yet, while a lower threshold of irradiance (Kershaw and MacFarlane 1980), and greater threshold of moisture (MacFarlane and Kershaw 1982) at which these lichens can grow certainly exists, the requirement for exposed, dry conditions may also be the result of lichens being competitively inferior when conditions are otherwise (Oksanen 1986, Crittenden 2000. For dry, northern habitats, lichen growth rate actually increases with precipitation, because increased photosynthetic activity can be sustained for longer periods of time ).…”