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One of the best documented direct effects of elevated CO2 concentrations [CO2] on plants is the stimulation of photosynthesis. In a recent meta-analysis on CO2 responses of more than 40 species from 12 free-air CO2 enrichment (FACE) sites, Ainsworth & Long (2005) found that exposure to elevated CO2 resulted in a 31% increase of leaf light saturated photosynthetic carbon uptake. The magnitude of this response, however, may be changed by abiotic factors such as irradiance, soil moisture and soil nutrient availability (Curtis & Wang, 1998). In addition, the light environment inside a forest canopy is very heterogeneous, and the differential effects of elevated [CO2] on the dynamic photosynthesis in sunflecks and shade might strongly affect the response of a forest to elevated [CO2] (Leakey et al., 2002).
Forests account for 70% of the terrestrial atmospheric carbon fixation (Waring & Schlesinger, 1985), and atmospheric CO2 rise could possibly be mediated by an increased carbon uptake by trees from the atmosphere. However, it is still uncertain if the increased photosynthetic rates will be sustained in the long term. A study of Bernacchi et al. (2003) on photosynthesis of the EUROFACE poplar leaves indicated that photosynthetic capacity of P. nigra was down-regulated in elevated [CO2] during the first year after coppice. Nevertheless, Davey et al. (2006) stated that poplar trees were able to ‘escape’ from long-term, acclimatory down-regulation of photosynthesis through a high capacity for starch synthesis and carbon export. The authors explained the downward acclimation observed by Bernacchi et al. (2003) during the first year after coppice as a short-term feedback response occurring at the level of enzyme activity in these poplar trees.
Down-regulation of the photosynthetic capacity has been found important in several long-term experiments conducted on field-grown trees (Tissue et al., 1999; Jach & Ceulemans, 2000; Rogers & Ellsworth, 2002). This reduction in photosynthetic capacity is often accompanied by a decrease in the leaf carboxylation capacity (Vcmax), related to lower leaf nitrogen (N) contents (Medlyn et al., 1999). Recent findings (Griffin et al., 2000; Rogers & Ellsworth, 2002) confirm that down-regulation of photosynthesis in elevated [CO2] is strongly linked to an increased C : N ratio of the photosynthesizing leaves, when the increased uptake of CO2 cannot be matched by a sufficient nutrient supply.
To assess the future role of forests in a globally changing atmosphere, scaling up individual leaf measurements to the canopy level is of major importance (Leuning et al., 1995). The vertical profile of a canopy is characterized by a decline of the irradiance from the top of the canopy downwards. Several studies pointed out that leaf morphology, physiology and photosynthetic capacity acclimate to the light environment in which the leaves are growing (Gratani, 1997; Niinemets et al., 1998; Casella & Ceulemans, 2002; Meir et al., 2002; Iio et al., 2005). The observed decline of leaf nitrogen with depth in a dense, closed canopy (Field & Mooney, 1986; Hirose & Werger, 1987) has been explained as a way to maximize the daily canopy photosynthesis, although in practice full acclimation to irradiance does not occur (Dewar et al., 1998; Kull, 2002; Meir et al., 2002). In addition, the partitioning of nitrogen between light harvesting (expressed as leaf chlorophyll content), electron transport (Jmax) and carboxylation components of photosynthesis (Vcmax) may also change with depth as a result of a relative increase in the light harvesting complexes compared with the rest of the photosynthetic apparatus (Kull & Niinemets, 1998). The altered leaf properties with depth can, in turn, change the light environment.
Several studies on deciduous and conifer trees have frequently reported that, in a future high-CO2 world, the leaf area index (LAI) of trees might increase (Wullschleger et al., 1997; Norby et al., 1999; Janssens et al., 2000), even after canopy closure (Liberloo et al., 2005). Likewise, certain leaf properties, such as leaf thickness, nitrogen and chlorophyll content, could change under CO2 enrichment (Epron et al., 1996; Gielen et al., 2003). Such CO2-induced changes in individual leaf characteristics may alter both light quantity and quality, affecting in turn leaf characteristics (Gielen et al., 2003).
In addition, responses to elevated CO2 are reported to depend on light (Tissue et al., 2001; Kubiske et al., 2002), leaf morphology and physiology (Liozon et al., 2000; Herrick & Thomas, 2001; Takeuchi et al., 2001; Kubiske et al., 2002), which also vary across the canopy. In a recent study on Pinus taeda (Crous & Ellsworth, 2004) growing for 6 yr under FACE, it was shown that canopy position interacted with CO2 effects on photosynthetic capacity: Rubisco and electron transport capacity were down-regulated by c. 17–20% in elevated [CO2], but only in the upper sun leaves.
Here, we investigate the combined effects of nitrogen fertilization and long-term elevated [CO2] on canopy profiles of photosynthetic parameters and related leaf characteristics. Measurements were taken during the third year of the second rotation cycle across the canopy of a short-rotation coppice plantation (POP/EUROFACE) of three poplar species (Populus alba, P. nigra and P. × euramericana). Trees had been growing for 6 yr (consisting of two rotations, 3 + 3 yr) in elevated [CO2] realized by a FACE system. After the first cycle, trees were cut back, and many shoots resprouted from each stool during the second cycle. Half of each plot was fertilized to study interactions between CO2 and nitrogen availability. This research was carried out to answer the following questions:
Does elevated [CO2] affect the light environment inside the closed canopy of a poplar coppice?
Do leaf characteristics change differently across height in elevated [CO2]?
Is there evidence of down-regulation of photosynthesis of Populus trees after 6 yr of growth in elevated [CO2], and does this down-regulation vary among different crown positions?
Is the response of photosynthetic profiles to elevated [CO2] modified by nitrogen fertilization?