Increased leaf area expansion of hybrid poplar in elevated CO2. From controlled environments to open-top chambers and to FACE
Introduction
When plants are exposed to elevated atmospheric carbon dioxide, photosynthesis and the accumulation of plant biomass are often increased in the short-term (Woodward, 1992, Pritchard et al., 1999). Whether this effect is maintained during long-term exposure of forest trees will depend, at least in part, on growing conditions (including nitrogen supply) and on the interactions and feedback between root and soil processes (Norby et al., 1999). In order to understand these complex feedbacks it is necessary to undertake large, field-scale exposures and for forest trees, these free-air CO2 enrichment (FACE) experiments are only now being used (http://oden.nrri.umn.edu/factssii/; Hendrey et al., 1999, Ferris et al., 2001, Norby et al., 2001). Generalisations are therefore difficult, but species with rapid, continuous growth may not become 'sink limited' and could maintain a positive response to CO2 (Arp, 1991, Farrar & Williams, 1991). Such responsive species are often characterised by increased leaf area development, from either lamina expansion or from the production of new leaves, or both (Taylor et al., 1994).
The genus Populus may be unusual for a woody plant in this respect, showing a large positive response to elevated CO2 with little evidence that this effect diminishes with time (Ceulemans et al., 1995a, Ceulemans et al., 1995b, Gardner, 1995, Taylor et al., 1995). This is in contrast to other woody species, such as Quercus for example, where long-term growth responses are thought to decline (Hättenschwiler et al., 1997), although past experiments on Populus have tended to be short-term using controlled environment (CE) and open top chamber (OTC) exposure systems. Leaf growth in poplar trees is an important determinant of total tree productivity and so an understanding of leaf growth physiology in elevated CO2 is warranted. Such studies usually occur in one type of growing environment, for example, in CE chambers (Taylor et al., 1995) or in a field system such as OTCs (Curtis et al., 1990), or FACE (Norby et al., 2001) with few studies aimed at comparing responses or validating each individual approach. In a recent review, Norby et al. (1999) claimed that OTC experiments have provided qualitatively similar and valid results, although quantitatively, they may be inadequate in explaining ecosystem response to elevated CO2, since they have only characterised the initial positive, although possibly transitory, response. However, it is possible that for long-lived trees, even this ‘qualitative’ consensus may be absent, but there are very few experiments on which to examine this contention.
Highly mechanistic studies of leaf growth in elevated CO2 have been undertaken and have shown that leaf growth in elevated CO2 is stimulated following enhanced cell expansion (Ranasinghe and Taylor, 1996) resulting from enhanced cell wall loosening and extensibility, associated with an increase in the activity of the wall loosening enzyme xyloglucan endo-transyglycosylase. The importance of such mechanistic explanations for increased leaf growth in elevated CO2 remains largely unknown but in a recent paper it was confirmed that enhanced leaf cell expansion and increased leaf size do contribute to rapid growth, even in a FACE (POPFACE) exposure (Ferris et al., 2001).
The overall objective of this study was to quantify the effect of exposure to elevated CO2 on leaf growth of hybrid poplar and to determine the effects of exposure technique, plant size and plant age on these responses. In particular, this paper aimed to (1) quantify the effect of elevated CO2 on leaf area development in three growing environments, and (2) determine whether short-term experiments provide useful indicators of long-term responses to elevated CO2, and whether mechanisms are similar in different environments.
Section snippets
Materials and methods
The contrasting experimental approaches are summarised in Table 1 illustrating the similarity between the experiments (clonal material, measurements performed) as well as their difference and complementarity (contrasting growth and exposure techniques, different duration of treatments, number of replicates and time periods for measurements).
Results
Fig. 1a–f illustrate the effects of elevated CO2 on leaf extension rate (LER) for P.×interamericana and P.×euramericana hybrids grown in either OTCs, CEs or POPFACE. For both hybrids and in all environments there was a stimulation of leaf extension rate (LER) in elevated CO2 although for the clone Robusta, LER was extremely small and no significant effects were detected. Leaf growth was most rapid in the CEs, an effect most pronounced for the P.×euramericana trees (Fig. 1c). Fig. 1 also
Discussion
This study has highlighted a number of important and consistent effects of elevated CO2 following exposure in either controlled environment glasshouse, open top chambers or FACE. In addition, some contrasting responses to elevated CO2 were also observed. A central finding was the consistent effect of elevated CO2 on LER, which for poplars is closely linked to stemwood productivity (Ridge et al., 1986, Barigah et al., 1994). LER was stimulated in elevated CO2 and this response occurred in all
Acknowledgements
This research was supported financially by the EC through its Environment R & D programme at the University of Antwerp, Belgium (Contract No. EV57-CT92-0127) and at the University of Southampton (Contract No. ENV4-CT97-0657, POPFACE) and by the BBSRC BAGEC Programme (PG085/0524). The support from the British–Flemish Academic Research Collaboration programme (British Council grant No. 26/94) is gratefully acknowledged. Further financial support was provided by the Fund for Scientific Research,
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