Light
Light availability varies along exposure gradients in forest canopies (Parker 1995), but photosynthesis in lichens and bryophytes may be limited less by light than by the interrelated effects of temperature and moisture. In upper forest canopies, poikilohydric epiphytes rapidly desiccate and become dormant under the lower humidities and higher temperatures accompanying bright, cloudless conditions. Many lichens and bryophytes become light-saturated at relatively low levels (Demmig-Adams et al. 1990; Green et al. 1997; Zotz et al. 1997), and some species can attain positive net photosynthesis in deep shade (Green et al. 1991). Furthermore, epiphytic bryophytes and lichens growing in exposed habitats often use pigmentation to shield themselves from excessive irradiation (Rikkinen 1995; Solhaug and Gauslaa 1996;Proctor 2000; Gauslaa and Solhaug 2001). The importance of light on the distribution of lichens and bryophytes cannot be ignored, however, because upper-canopy species tend to have higher light-saturation points than lower-canopy species (Hosokawa et al. 1964; Proctor 2000), and growth rates of some species increase with increasing light availability (Rincon and Grime 1989; Hilmo 2002). Light may only be limiting in the shaded lower canopies of dense forests, a hypothesis that is supported by observations of positive effects of hardwood gaps, riparian edges, and silvicultural thinning on epiphyte diversity (Rose 1992; Neitlich and McCune 1997; Peterson and McCune 2003). In order to test this hypothesis, however, the light environment of epiphytes must be experimentally manipulated in ways that do not also affect moisture availability or temperature.
แสงLight availability varies along exposure gradients in forest canopies (Parker 1995), but photosynthesis in lichens and bryophytes may be limited less by light than by the interrelated effects of temperature and moisture. In upper forest canopies, poikilohydric epiphytes rapidly desiccate and become dormant under the lower humidities and higher temperatures accompanying bright, cloudless conditions. Many lichens and bryophytes become light-saturated at relatively low levels (Demmig-Adams et al. 1990; Green et al. 1997; Zotz et al. 1997), and some species can attain positive net photosynthesis in deep shade (Green et al. 1991). Furthermore, epiphytic bryophytes and lichens growing in exposed habitats often use pigmentation to shield themselves from excessive irradiation (Rikkinen 1995; Solhaug and Gauslaa 1996;Proctor 2000; Gauslaa and Solhaug 2001). The importance of light on the distribution of lichens and bryophytes cannot be ignored, however, because upper-canopy species tend to have higher light-saturation points than lower-canopy species (Hosokawa et al. 1964; Proctor 2000), and growth rates of some species increase with increasing light availability (Rincon and Grime 1989; Hilmo 2002). Light may only be limiting in the shaded lower canopies of dense forests, a hypothesis that is supported by observations of positive effects of hardwood gaps, riparian edges, and silvicultural thinning on epiphyte diversity (Rose 1992; Neitlich and McCune 1997; Peterson and McCune 2003). In order to test this hypothesis, however, the light environment of epiphytes must be experimentally manipulated in ways that do not also affect moisture availability or temperature.
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