13.5.5  Growth, competition and ecosystem structure

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In tropical trees both total plant mass and leaf area per plant vary in their sensitivity to elevated CO2. Percentage changes range from –10% to + 300%. Despite some exceptions, plant growth is generally enhanced by CO2 enrichment. Such increases result from increased availability of fixed carbon due to increased assimilation rate and decreased respiration rates. In some cases, CO2 enrichment stimulates leaf initiation and branching (Section 6.2) and faster assimilation per unit leaf area becomes compounded by a larger assimilatory surface,

Species thus differ in both the nature and extent of biomass increase under elevated CO2, with correlated changes inmorphology and phenology. Accordingly, their competitive abilities in communities will also be affected by climate change. In one empirical study on tropical species where elevated CO2 was taken as a driving variable (Rekkie and Bazzaz 1989) five tropical tree species were grown separately and in competition with each other, under ambient or CO2-enriched conditions. CO2 enrichment significantly influenced the relative biomass of each species. In ambient conditions species ranking in terms of whole-plant biomass was Senna multijuga > Cecropia obtusifolia > Trichospermum mexicanum > Piper auritum > Myriocarpa longipes. In contrast, under CO2 enrichment, the ranking changed to Trichospermum mexicanum > Cecropia obtusifolia > Senna multijuga > Piper auritum > Myriocarpa longipes. There were clear differences in comparative response to CO2 enrichment between species. The importance of S. multijuga declined significantly whereas the importance of T. mexicanum and C. obtusifolia increased substantially.

C4 species and ecosystem structure

In view of their photosynthetic attributes, C4 plants are not expected to respond to CO2-enrichment because their natural CO2-concentrating mechanisms ensure that Rubisco is working closer to a saturating partial pressure of CO2 than in C3 plants. Paradoxically, a number of studies have shown increased leaf area and plant height. For example, Gifford et al. (1984) found 14%, 29% and 40% increase in leaf area per plant for amaranth, sorghum and maize respectively. Such increases may result from a decrease in respiration and/or a stimulation of leaf/branching initiation and emphasise that factors beyond leaf photosynthesis need to be recognised as components of plant response to elevated CO2.

That aside, large-scale ecosystem dynamics are nevertheless underpinned by biomass accumulation, and events that either add or substract from that biomass capital will impact on species composition. In Australian savannas at least 90% of grasses are C4 plants and all trees are C3 plants. On average, C3 species show a larger response to CO2 enrichment than C4 species. If climate change results in a longer wet season and warmer conditions, canopy cover by trees may increase and grass cover may therefore decline. Since grasses represent the major fuel load for bush fires and their persistence is related to burning frequency, global CO2 enrichment combined with less frequent fires will impact on vegetation structure with a possible shift in species composition towards C3 plants including trees.