Physiological Ecology of Tropical Bryophytes

Abstract

Bryophytes in the tropics occur from cool alpine grasslands to warm lowland sites and from cloud forests to dry forests, varying markedly in abundance and diversity in these habitats. This chapter deals with the current knowledge of the ecophysiology of tropical bryophytes attempting to explain some of these abundance patterns, in particular the marked increase in bryophyte biomass with altitude in rain and cloud forests. As data are scarce, we include data on, physiologically rather similar, lichens in our account where appropriate. We focus mostly on carbon relations, and water, nutrients, light, CO 2 and temperature are discussed as co-determinants of the carbon balance. In particular, we address the hypothesis that the surprisingly low bryophyte abundance in lowland rainforests is due to the limitation of net carbon gain by fast drying and low light levels during the day combined with moist and warm conditions at night, which promote high respiration rates. The timing of hydration is crucial in determining this diel balance between photosynthesis and respiration. Temperature is important in determining moisture loss rates and nocturnal carbon loss through respiration – if respiration does not acclimatize to higher temperatures. Since carbon balance precariously depends on daily hydration patterns, future climate change may pose a serious problem to tropical lowland bryophytes. I. Introduction Tropical forests harbor a particularly large diversity of bryophytes (mainly mosses and liverworts). The Neotropics alone are home to some 4,000 species of bryophytes, which represents about a quarter of the global species number (Gradstein et al. 2001 ). The majority of species grows epiphytically on live stems and branches of trees and shrubs, although a variety of other substrates are also used, -e.g. leaves (these epiphytes are referred to as epiphylls, Ruinen 1953 ), rocks, decaying wood, or termite mounds (Pócs 1982 ). Lowland rainforest fl oors are virtually devoid of bryo- phytes, probably due to high litter input, but subalpine forests can boast fully moss-covered soils (Wolf 1993 ) and terrestrial mosses can also be found in alpine bogs (Bosman et al. 1993 ) and in drier regions as a component of biological soil crusts (Belnap and Lange 2001 ; Godínez-Alvarez et al. 2012 ). Bryophytes (and lichens) show a dramatic increase in abundance (e.g. Frahm 1990a ; Wolf 1993 ) and diversity (e.g. Seifriz 1924 ; Pócs 1982 ; Gradstein and Pócs 1989 ) as one moves uphill away from the tropical lowlands. This increase is particularly noticeable above 1,000 m elevation and often reaches a maximum in upper montane forests around 3,000 m, while decreasing again above the treeline (Grau et al. 2007 ). For example, the “bryomass” in the Uluguru mountains in Tanzania increased from 1 kg ha −1 at 20 m elevation to 440 kg ha −1 in the ‘mossy’ forest at 2,100 m. (Frahm 1990a ). Not surprisingly then, the impact of bryo- phytes on ecosystem function, particularly on hydrology (Pócs 1980 ; Hölscher et al. 2004 ) but also on nutrient cycling, is highest in montane systems (Hofstede et al. 1993 ; Nadkarni et al. 2004 ; Clark et al. 2005 ). For example, it has been extrapolated by Pócs ( 1980 ), that epiphytic bryophytes in an elfi n forest may intercept more than 40,000 l of precipitation ha −1 year −1 . Leaching and decomposition of bryophyte organic mate- rial result in a pulsed release of nutrients after rehydration of dry mosses. Coxson ( 1991 ) estimated an effl ux of 80 kg K ha −1 year −1 in a tropical montane rainforest on Guadeloupe. This ecological importance contrasts strongly with the availability of information on the ecophysiology of this plant group in the tropics. Just a handful of studies has addressed the ecophysiology of tropical low- land bryophytes, focusing on desiccation tolerance (Biebl 1964 ; Johnson and Kokila 1970 ), temperature tolerance (Biebl 1967 ), or photosynthetic light response and related functional traits (Waite and Sack 2010 ) and photosynthetic light, water, and temperature responses (Wagner et al. 2013 ). For bryo- phytes from the tropical montane zone the situation is somewhat better, with a number of published studies on CO 2 exchange (Frahm 1987a , b ; Lösch et al. 1994 ; Zotz et al. 1997 ; Wagner et al. 2013 ), water relations (Biebl 1964 ; Proctor 2002 ) and temperature responses (Lösch and Mülders 2000 ). For upper montane cloud forest the information is very scarce again (Lösch et al. 1994 ; Lösch and Mülders 2000 ; Romero et al. 2006 ), though an interesting recent contribu- tion addresses altitudinal patterns, including