- Top of page
- Materials and methods
1. Plants growing in deep shade and high temperature, such as in the understorey of humid tropical forests, have been predicted to be particularly sensitive to rising atmospheric CO2. We tested this hypothesis in five species whose microhabitat quantum flux density (QFD) was documented as a covariable. After 7 (tree seedlings of Tachigalia versicolor and Beilschmiedia pendula) and 18 months (shrubs Piper cordulatum and Psychotria limonensis, and grass Pharus latifolius) of elevated CO2 treatment (c. 700 μl litre–1) under mean QFD of less than 11 μmol m–2 s–1, all species produced more biomass (25–76%) under elevated CO2.
2. Total plant biomass tended to increase with microhabitat QFD (daytime means varying from 5 to 11μmol m–2 s–1) but the relative stimulation by elevated CO2 was higher at low QFD except in Pharus.
3. Non-structural carbohydrate concentrations in leaves increased significantly in Pharus (+ 27%) and Tachigalia (+ 40%).
4. The data support the hypothesis that tropical plants growing near the photosynthetic light compensation point are responsive to elevated CO2. An improved plant carbon balance in deep shade is likely to influence understorey plant recruitment and competition as atmospheric CO2 continues to rise.
- Top of page
- Materials and methods
Tropical forests, like all other vegetation on earth, existed at atmospheric CO2 concentrations as low as 180–220 μl litre–1 only 20 000 years ago (Neftel et al. 1988). Now plants experience almost twice that concentration. Research is devoted worldwide to study the potential quadrupling-effects of atmospheric CO2 expected to occur by the end of the next century. Because almost half of the carbon in global biomass is found in the tropics and subtropics (Brown & Lugo 1982), understanding the CO2 response of these biomes deserves high priority, in particular because predictions of plant responses to elevated CO2 commonly assume that responses are more pronounced the warmer the climate (Drake & Leadley 1991; Mooney et al. 1991; Gifford 1992; Rawson 1992; Koch & Mooney 1996).
Biomass responses in tropical plants growing in pots or model ecosystems (to date seven projects) were found to vary with experimental conditions (see review by Arnone 1996). Reekie & Bazzaz (1989) and Arnone & Körner (1993, 1995) found no significant overall biomass response in experimental communities exposed to elevated CO2. Körner & Arnone (1992) working with model communities in a greenhouse on more fertile substrate, found a moderate stimulation of plant community biomass (significant below ground, little above ground). In contrast Ziska et al. (1991, shade-house in Panama), Lovelock, Kyllo & Winter (1996, open top chambers in Panama) and Oberbauer, Strain & Fetcher (1985, growth chambers) observed significant CO2 stimulation of biomass in potted tropical plants. It appears that responses are species specific, related to plant morphology, and are dependent on soil fertility, plant competition and experimental duration. A problem with these previous experimental systems using tropical plants is that they were artificial to variable degrees, lacking one most fundamental prerequisite for assessing realistic CO2 responses, namely a natural plant–soil association, a feature that was present in the current study with plants growing in the understorey of an undisturbed tropical forest.
Seedlings below a tree canopy and understorey plants in general are constrained by a lack of solar radiation. Is this low level of quantum supply likely to preclude growth stimulation by CO2? It is important here to distinguish between relative and absolute effects of elevated CO2. Simply because most plants grow faster in light compared to shade, absolute gains in biomass owing to CO2 fertilization over similar time intervals can be expected to be higher in full sunlight. However, relative effects are likely to be more pronounced in deep shade because higher concentrations of CO2 reduce photorespiration, increase quantum yield and, thus, decrease the light compensation point of photosynthesis (e.g. Valle et al. 1985; Wong & Dunin 1987; Long & Drake 1991). Hence, the leaf carbon balance should be improved in very low light under elevated CO2 and the largest relative effects should occur in plants whose leaves are operating close to the light compensation point under current ambient CO2 concentrations. Although small in absolute terms, such large relative effects could induce far-ranging changes in community dynamics in a forest (Bazzaz & Miao 1993; Hättenschwiler & Körner 1996), the motive for the current investigation.
In situ CO2 enrichment in the understorey of a natural forest is challenging. The first problem is the extremely patchy light climate. Without carefully accounting for this, it obviates any meaningful interpretation of growth responses to CO2. We met this challenge by installing 64 light sensors next to our experimental plants in the experimental tents, so that we could treat microhabitat QFD as a measured covariable. The second potential problem relates to the frequent assumption that CO2 concentrations are naturally high in the forest’s understorey, and that any further increase in CO2 concentrations would have minimal effects. However, there is no substance to this belief. A substantial body of evidence, particularly in the less known older forest meteorology literature, shows that natural CO2 enrichment during the day in forests really is confined to the immediate soil boundary layer (e.g. Medina et al. 1986) and to calm night-time hours. Daytime CO2 concentrations at heights between 0·1 m and the top of the canopy rarely deviate by more than 30 μl litre–1 from concentrations measured in the free atmosphere (Schimper & Von Faber 1935; Mitscherlich, Kern & Künstle 1963; Baumgartner 1967; Elias et al. 1989; Bazzaz & Williams 1991; Buchmann, Kao & Ehleringer 1996; for tropical forests see also Allen & Lemon 1976; Kira & Yoda 1989). We will present data for our experimental site which support these earlier findings and justify experimental CO2 enrichment in the understorey.
Thus, there are good reasons to assume that growth of plants in the understorey of a tropical forest should be stimulated by elevated CO2. We tested this hypothesis by exposing populations of understorey plants in situ to CO2-enriched air while leaving all other environmental conditions largely unchanged. A great advantage to working in the understorey is that CO2 enrichment can be achieved in transparent enclosures without risking any of the well-known greenhouse effects. Plant responses were assessed as biomass accumulation, leaf number, leaf area, specific leaf area and non-structural leaf carbohydrate pools.