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- Materials and Methods
Attempts to predict forest ecosystem functioning in response to rising atmospheric CO2 concentration typically emphasize tree growth responses through a photosynthetic enhancement. Mean stimulations of 66% and 73% (median = 55%) for photosynthesis and above-ground woody biomass, respectively, were reported for hardwood trees grown in the field under elevated CO2 concentrations (Norby et al., 1999). However, evidence that growth stimulation may be limited to the initial phase of CO2 enrichment is emerging (Ward & Strain, 1999). To understand this limited effect, and adequately predict future forest functioning and productivity, responses other than tree growth per se need careful attention.
Dry-matter production of forests is related to the amount of intercepted light, which is influenced by leaf area, display and distribution (Halléet al., 1978). These canopy characteristics are in part determined by woody crown architecture. Despite its importance, experimental studies on canopy architecture responses to elevated CO2 are scarce. A better understanding of crown architecture may help scaling from the leaf level to the stand level, and is particularly crucial for predictions of elevated CO2 responses of mixed stands (Diaz, 1995; Norby et al., 1999; Pritchard et al., 1999). Species composition of temperate forests, for example, might change in response to rising atmospheric CO2 concentrations, but shifts wil depend on light patterns, hence on canopy architecture (Hättenschwiler & Körner, 2000).
Related to biomass production and stem wood quality with respect to knots (Brazier, 1977), studies of crown architecture are of particular interest in managed forests and short-rotation culture systems (Mäkinen, 1996). Ideotype breeding includes crown architecture traits as selection criteria to improve forest productivity (Dickmann, 1985; Ceulemans et al., 1990; Hinckley et al., 1992; St. Clair, 1994). Studies of crown architecture of poplar (Populus spp.) have tried to identify criteria to further increase biomass yield of short-rotation poplar plantations (Nelson et al., 1981; Isebrands & Nelson, 1982; Burk et al., 1983; Ceulemans et al., 1990). Because symmetric branching patterns result from the phyllotactic control of bud arrangement, few studies have investigated crown architecture in a CO2-enriched atmosphere. Nevertheless, phenotypic plasticity induced by the environment has already been documented, and branching patterns can be altered by branch mortality and abscission beyond current-year stem growth (Fisher, 1986). Moreover, within-stand competition modifies the growth of individual trees and the stand development in several ways (Kellomäki, 1986).
The present study was performed in a high-density poplar plantation with a free-air CO2 enrichment (FACE) system. With this technique stand-level responses can be assessed under natural conditions, and therefore largely eliminating the shortcomings of past CO2 enrichment techniques (Hendrey et al., 1993 and 1999). The questions addressed in the present study are: does CO2 enrichment affect woody crown architecture of Populus?; If so, to which extent is this caused by a growth stimulation, or by increased competition in the FACE treatment? To answer these questions, we measured crown depth, number of branches, branch dimension and inclination. Additionally, apical dominance was studied by determining branch insertion height. Emphasis was given to first-order branches due to their size and disposition dominance, and because they determine the arrangement of higher-order branches (Nelson et al., 1981; Burk et al., 1983). Finally, we examined species differences that may help explain variable responses to elevated CO2 concentration.
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- Materials and Methods
For all three species the canopy was deeper in the FACE stand at the end of the second growing season as a result of the first-year stimulation of sylleptic branching. The FACE treatment significantly increased stem internodal length because sylleptic branching was unaffected in the second growing season. This agrees with most studies that were reviewed recently; these showed increased stem or branch elongation without elevated CO2 effects on the number of nodes, and suggested a greater growth stimulation as compared to the formation of organ primordia at the shoot tip (Pritchard et al., 1999). Nevertheless, many studies have reported stimulated branch initiation even without increased plant height (Pritchard et al., 1999), or showed no response, as also recently reported for Betula pendula (Centritto, 2000). Earlier CO2 experiments with Populus in controlled- and open-top-chambers showed higher absolute number of branches, and increased stem elongation (Radoglou & Jarvis, 1990; Ceulemans et al., 1995). However, Kubiske et al. (1997) showed that internodal distance of P. tremuloides was not affected by elevated CO2.
Our observations of deeper crowns, fewer proleptic branches, and longer internodes in the FACE treatment confirm our analysis of LAI (Gielen et al., 2001). Increased crown depth confirms our previous suggestion that the loss of a significant CO2-stimulation of LAI after canopy closure was caused by increased shading. Although we observed deeper crowns in response to FACE, the possible positive effect on growth may be restricted to the initial period of CO2 enrichment, and may disappear once leaf-fall starts in the lower part of the canopy.
In general, our results showed evidence for increased branch dimensions under elevated CO2 concentrations, in agreement with previous results on Pinus sylvestris and other tree species (Jach & Ceulemans, 1999; Pritchard et al., 1999), but in contradiction to the results of a model ecosystem with Picea abies (Hättenschwiler & Körner, 1998). Based on our observations, branch growth and stem growth were similarly affected in the first growing season, with branches having longer internodes. Nevertheless, this was not true for sylleptic branches produced in the second growing season, indicating a decline in the response to the FACE treatment.
Large increases of higher-order branch biomass in response to elevated CO2 have been observed previously (Sionit et al., 1985; Idso et al., 1991; Wilkins et al., 1994; Jach & Ceulemans, 1999), but also a lack of response in second-order branching (Barton & Jarvis, 1999). Our study showed a reduced number of second-order branches, but future characterization of higher-order branching is needed to verify these preliminary observations. Based on these findings and evidences, we conclude that woody crown architecture is affected by FACE through CO2-induced stimulation of growth, and of initial branching.
Because CO2 effects on branch insertion height and branch dimension were observed, apical dominance and apical control were examined in response to the FACE treatment. In the first case, the position of the top branch was relatively higher in the FACE than in control treatments at the end of the first growing season. This suggests a partial loss of apical dominance, since this dominance determines whether the meristem initially forms a sylleptic branch, or a bud (Brown et al., 1967; Ford, 1985; Wilson, 2000). In the case of apical control (the process that regulates the amount of elongation, and diameter growth of proleptic branches) (Brown et al., 1967; Ford, 1985; Wilson, 2000) we found that proleptic branch length appeared to be stimulated by the FACE treatment for P. nigra (+22%) and P.×euramericana (+6%). Nevertheless, these observations were not statistically significant and may be solely caused by an initial stimulation of branching and growth under elevated CO2. This prevents us from concluding that either apical dominance or apical control was reduced in an elevated CO2 atmosphere. Reduced apical dominance under elevated CO2 has been suggested in earlier studies (Pritchard et al., 1999), but also contrary results have been observed, for example, under a combination of elevated temperatures and increased CO2 (Martin et al., 1995).
Apart from the FACE effects, our observations confirm the importance of crown architecture to discriminate among poplar species (Ceulemans et al., 1990). For example P. alba, was characterized by longer, more horizontally orientated branches as compared to both other species. Such differences could influence future responses to CO2 enrichment through light interception, and through other canopy-atmosphere exchange processes. Moreover, within-stand competition could also be influenced by these canopy architectural differences, which, in turn, could alter growth performance in response to FACE. The simplified schematic architecture representations of the species reflect this competition aspect when the planting distance between trees (represented by the horizontal 1 m line), is compared with tree crown width. Contacting and interacting branches are more abundant within the stands of P. alba (Fig. 3). This may further affect crown architecture in the forthcoming years as it is well known that competitive processes modify the structure of a crown (Kellomäki, 1986). Competition also reduced branch dimension in the middle and lower canopy of P. sylvestris stands, although this was likely caused by reduced overall tree growth (Mäkinen, 1996). The competition issue is related to the effect of plant density on branch inclination angles, for example, widely spaced poplars develop wider crowns (Nelson et al., 1981). Optimal performance under closed spacing is, of course, partly genetically determined. We speculate that competition could play a more important role in our P. alba stands, as compared to P. nigra because P. alba has much wider crowns.
The importance of an elevated CO2 effect on canopy architecture was also demonstrated by Reekie & Bazzaz (1989) in a simplified tropical community. Contrary to elevated CO2 effects on individually grown tree seedlings, biomass was significantly affected by CO2 when the trees were grown under competitive conditions. The authors concluded that CO2 affected competitive outcome through its effect on canopy architecture (Reekie & Bazzaz, 1989). Similarly, differences in crown architecture of the three Populus species in our study, and their different relative response to CO2 enrichment, might influence their competitive ability when grown in multispecies stands.
Following two growing seasons of FACE, we found no evidence for altered crown architecture due to CO2 effects on within-stand competition. FACE caused slightly sharper branch termination angles in the second growing season, but this was correlated with increased branch length, that is, long branches grew vertically to the light source. Such a negative correlation between branch length and termination angle was found previously (Bozzuto & Wilson, 1988), although also no correlation has been reported (Cluzeau et al., 1994). Our observations of wider branch angles in the lower part of the canopy has been attributed to self-weight, and to a way to avoid self-shading (Bozzuto & Wilson, 1988; Deleuze et al., 1996; Goulet et al., 2000).
We examined branch abscission of both sylleptic and proleptic branches (data not shown), but only species differences were observed, showing that FACE did not indirectly affect branch abscission through competition, or light (deeper canopy).
In conclusion, crown architecture of two Populus species and one hybrid was affected by the FACE treatment mainly through stimulated growth, that is, increased canopy depth, internodal length, and number and dimension of branches. Nevertheless important species differences were observed, which may further influence responses to elevated CO2 in the long term. Additionally, canopy closure was reached only in the summer of the second growing season, hence competition may become more important in the coming years and could further influence crown architecture.