Temperature gradients can have pronounced effects on the abundance of epiphytic lichens and bryophytes (see Figure 8-3). For example, lowland tropical rainforests are rich in species, especially crustose lichens (Cornelissen and ter Steege 1989; Komposch and Hafellner 2000), but they support a much lower macrolichen and bryophyte biomass than cooler tropical montane rainforests (Pócs 1980;Nadkarni 1984a; Hofstede et al. 1993; Wolf 1993a; Büdel et al. 2000; Freiberg and Freiberg 2000) or lowland rainforests at higher latitudes (Nadkarni 1984b; Ellyson and Sillett 2003). Consistently high temperatures lead to high respiration rates in low-elevation tropical forests (Zotz et al. 1998; Lange et al. 2000). The situation is exacerbated by the fact that lichens and bryophytes have small quantities of chlorophyll per unit area or mass compared to vascular plants (Green and Lange 1994; Martin and Adamson 2001). As a result, many lichens and bryophytes are simply incapable of the high photosynthetic rates required to overcome respiratory energy losses during warm, wet periods (Frahm 1990b). High temperatures during the summer rainy season may also explain why the biomass of epiphytic lichens and bryophytes is so low in deciduous forests of the southeastern United States (Becker 1980). The increasing abundance of epiphytic lichens and bryophytes with altitude in tropical forests is, however, more a consequence of the positive correlation between moisture availability and elevation than it is a reflection of the underlying temperature gradient (Wolf 1993a).