Altitudinal changes in the incidence of crassulacean acid metabolism in vascular epiphytes and related life forms in Papua New Guinea

Abstract

The occurrence of Crassulacean acid metabolism (CAM), as judged from δC values, was investigated in epiphytes and some related plant species at a series of sites covering the approximate altitudinal range of epiphytes in Papua New Guinea. Comprehensive collections were made at each site and the occurrence of water storage tissue and blade thickness was also determined. Some 26% of epiphytic orchids from a lowland rainforest (2-300 m.a.s.l) showed δC values typical of obligate CAM and possessed leaves thicker than 1 mm. A second group of orchids, mostly with succulent leaves, possessed intermediate δC values between -23 and -26% and accounted for 25% of the total species number. Some species of this group may exhibit weak CAM or be facultative CAM plants. The remainder of the lowland rainforest species appeared to be C plants with δC values between -28 and -35%. and generally possessed thin leaves. Obligate CAM species of orchids from a lower montane rainforest (1175 m.a.s.l) comprised 26% of the species total and mostly possessed thick leaves. The remainder of the species were generally thin-leaved with δC values between -26 and -35%. largely indicative of C photosynthesis. Orchids with intermediate δC values were not found in the lower montane rainforest. Obligate CAM appeared to be lacking in highland epiphytes from an upper montane rainforest and subalpine rainforest (2600-3600 m.a.s.l). However the fern, Microsorium cromwellii had a δC value of -21.28%. suggesting some measure of CAM activity. Other highland ferns and orchids showed more negative °C values, up to-33%., typical of C photosynthesis. The highland epiphytic orchids possessed a greater mean leaf thickness than their lowland C counterparts due to the frequent occurrence of water storage tissue located on the adaxial side of the leaf. It is suggested that low daytime temperatures in the highland microhabitats is a major factor in explaining the absence of CAM. The increased frequency of water storage tissue in highland epiphytes may be an adaptation to periodic water stress events in the dry season and/or an adaptation to increased levels of UV light in the tropicalpine environment.

The occurrence of crassulacean acid metabolism among vascular epiphytes from Central Panama

“…In tropical lowland forests there is a steady increase of CAM species with a decrease in precipitation. Numbers increase from 26 °y and 24''[i in New Guinean and Australian rainforests, respectively (Winter et aL, 1983;Earnshaw et al, 1987), to 40°o in the wet forest of this study, to 62 °o in relatively dry, open forests in Australia (Winter et al, 1983) to 100% in a Mexican dry forest (Mooney f/a/., 1989). Analogous to this relative increase in CAM species from wetter to dryer forest types there is an increase of the proportion of CAM species from shaded to exposed habitats within the forest (Table 1, Fig.…”

“…The second problem is in part associated with the first and has been noted in % irtually all publications where 8^^C values have been used to study CAM in epiphytes (Griffiths & Smith, 1983;Winter et aL, 1983;Earnshaw et al, 1987). Additional field measurements of changes in titratable acidity and/or COj gas exchange are needed to determine how many of the 86 species (about 75 '\, of the total) may ha\e some capacity for nocturnal acidification.…”

“…Our number of 40 °o CAM orchids on Barro Colorado Island (Table 2) is rather close to the estimate given by Winter & Smith (1996) for the family as a whole. However, the comparatively small number of studies to date and the huge variability of the proportion of species with CAM depending on microsite (Griffiths & Smith, 1983), forest type (Winter et aL, 1983;Earnshaw et aL. 1987;Mooney et aL, 1989) or elevation (Earnshaw e( aL, 1987) still does not allow us to come up with more than a rough estimate of the real number of CAM species.…”

Evolution along the crassulacean acid metabolism continuum

“…Family nomenclature follows those provided by the Angiosperm Phylogeny website (Stevens 2008 sites between sea level and 500 m and no CAM species occurring above 2400 m (Silvera et al 2009). At two lowland forest sites in Panama, 36% and 42% of epiphytic orchid species displayed CAM isotopic values (Zotz and Ziegler 1997;Zotz 2004), and these percentages were 26% and over 60% among tropical epiphytic orchids collected in Papua New Guinea (Earnshaw et al 1987) and Australia (Winter et al 1983), respectively. Also, within a single site, the percentage of CAM epiphytes tended to increase from shaded understory sites to exposed canopy sites.…”

The effects of salinity, crassulacean acid metabolism and plant age on the carbon isotope composition of Mesembryanthemum crystallinum L., a halophytic C3-CAM species

“…Winter et al 1983;Earnshaw et al 1987;Crayn et al 2004;Holtum et al 2004). If C 4 photosynthesis is absent, d 13 C values permit the assignment of plants as either C 3 or as crassulacean acid metabolism (CAM), provided the latter species obtain roughly 30% or more of their carbon via phosphoenolpyruvate carboxylase (PEPC)-catalysed bcarboxylation.…”