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Growth of trees in elevated carbon dioxide concentrations usually lead to increased in net photosynthetic rate (Huang et al. 2007, Ambebe et al. 2009). Doubling growth carbon dioxide concentration from 360 to 720 µmol mol-1 resulted in higher leaf-level net photosynthetic rate (Pn) in both black spruce and white spruce seedlings in the mono and mixed-species experiments. Johnsen (1993) reported that black spruce seedlings grown under elevated [CO2] had significantly greater Pn than seedlings at ambient [CO2]. Similarly, white spruce seedlings grown at elevated [CO2] had greater photosynthetic enhancement than at ambient [CO2] (Dang et al. 2008). The higher Pn at elevated [CO2] is presumably due to increased partition of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) to carboxylation and greater suppression of oxygenation (Dang et al. 2008, Lambers et al. 2008). In both experiments, the greater Pn of seedlings at elevated [CO2] was independent of nutrient supply. This was unexpected as the effect of increased [CO2] is usually magnified under high nutrient conditions (Murray et al. 2000, Springer et al. 2005). Nevertheless, our finding is similar to the observations of Norby and O'Neill (1991) who observed independent but none interactive significant effects of CO2 and nitrogen supply on yellow poplar seedlings. The hypotheses that elevated [CO2] would stimulate greater photosynthetic rate with increased nutrient supply and partially compensate for low nutrient supply are therefore rejected.
Photosynthetic down-regulation, a reduction in the photosynthetic capacity of plants grown in elevated [CO2] may occur when plants are grown in pots or under limited nutrient supply (Gunderson and Wullschleger 1994, Lambers et al. 2008). Measuring Pn at a common [CO2], usually ambient [CO2] is a common way of determining the existence or otherwise of photosynthetic down-regulation. Biochemically, a reduction in Rubisco's carboxylation activity (Vcmax), electronic transport capacity (Jmax) or reduced inorganic phosphate regeneration (reduced sink strength) signify photosynthetic down-regulation (Thomas et al. 1994, Lambers et al. 2008, Ambebe et al. 2009). Reduction in foliar nitrogen concentration (N) may also cause or accelerate down-regulation (Tissue et al. 1999). Notwithstanding the lower foliar [N] observed in black spruce (mono experiment), that did not result in significant decline in the species Pn in relation to white spruce (no nutrient-species interaction). In this study, we found no evidence to support photosynthetic down-regulation of seedlings grown in elevated [CO2]. Compared at a common [CO2] (Pn360), seedlings grown at ambient and elevated [CO2] had similar photosynthetic rate, indicating no down-regulation. The absence of down-regulation is further supported by the Vcmax, Jmax and triose-phosphate utilisation (TPU) data. In both experiments, elevated [CO2] did not cause a significant decline in Vcmax, Jmax or TPU as generally reported (Thomas et al. 1994). In other studies involving the species, photosynthetic down-regulation has been reported; black spruce (Johnsen 1993) and white spruce (Dang et al. 2008). The contradictory findings of this study and previous ones (Johnsen 1993, Dang et al. 2008) might be due to differences in experimental protocol. Secondly, it is known that CO2 enrichment have little or no effect on black spruce's sugar, starch or total non-structural carbohydrates content (Lamhamedi and Bernier 1994). As such, the species is able to maintain greater sink strength under conditions of increased [CO2] (Lamhamedi and Bernier 1994), thereby minimising photosynthetic down-regulation. The lack of photosynthetic down-regulation indicates that pot size did not limit sink development and that a balanced sink-source was maintained. In fact, in a recent review, Way and Oren (2010) found that growth rate in potted and field grown plants were similar. We however believe that, photosynthetic down-regulation would have occurred, especially in the mono-experiment if the study had been prolonged beyond the current duration. This is based on the fact that the Pn enhancement due to elevated [CO2] in the mixed-species experiment (larger pot size) was higher than in the mono experiment.
Growth of seedlings at elevated [CO2] resulted in greater photosynthetic water use efficiency (WUE) despite no significant reduction in either stomatal conductance (gs) or transpiration rate (E). This contradicts many reports which document either reduction in gs or E leading to increased WUE under elevated [CO2] (Thomas et al. 1994, Medlyn et al 2001, Körner 2006, Lambers et al. 2008). Our findings are however supported by the work of Teskey (1995) and Dang et al. (2008). In a loblolly pine (Pinus taeda) plantation, Teskey (1995) observed no significant CO2 effect on gs or E but a greater WUE under elevated [CO2]. Dang et al (2008) also reported of increased WUE in white spruce seedlings despite no significant reduction in gs. These findings suggest that needle stomata were insensitivity to increased [CO2], similar to Johnsen (1993) findings for black spruce seedlings exposed to short-term CO2 changes. In fact Ci/Ca ratio was about 0.75 and not significantly influenced by [CO2] (data not shown). The greater WUE at elevated [CO2] was therefore solely due to significantly greater Pn without a significant change in E. If black spruce and white spruce seedlings grown at elevated [CO2] and on fertile sites can maintain low evapo-transpiration rate but higher water use efficiency, it may be advantageous during drought. At the whole plant or ecosystem level however, this may not be achieved as increased CO2 elevation and fertilization may result in higher leaf area and subsequent greater transpiring surface.
The optimum soil temperature for maximum carbon assimilation in black spruce and white spruce may not shift upwards in response to changes in atmospheric carbon dioxide concentration. The hypothesis that current optimum soil temperature (20 oC) for the two species would shift upwards (25 oC) in response to increased [CO2] and rising temperatures is not supported by our data. Similarly, Tjoelker et al. (1998) working with black spruce and other tree species reported that a warmer environment failed to enhance the response of net photosynthesis to increased concentrations of CO2. In both the mono and mixed-species, there was no significant CO2-temperature interactive effect. The absence of significant CO2-temperature effect might be due to our experimental protocol. Soil temperature was lowered at each night to reflect naturally lower temperatures at night. This could have resulted in changes in the carbon gain-carbon loss dynamics of the seedlings (Wan et al. 2009). The lack of significant interaction between CO2 and soil temperature on Pn indicates that both species have wider temperature acclimation to optimise photosynthesis and that any change in their optimum temperature is independent of [CO2]. We can conclude that upward adjustment of soil temperature to maximise black spruce and white spruce photosynthetic rate may depend more on the species inherent genetic differences and soil nutrient status than to increased [CO2].
Increasing soil temperature from 20 to 25 oC may not have any effect on black spruce Pn but can increase white spruce Pn under favourable nutrient conditions, especially when the two species are grown in mixture. While black spruce maintained a constant Pn irrespective of the temperature, white spruce had declined Pn when grown in the 20 oC temperature. At the 25 oC -high nutrient however, Pn of white spruce was increased suggesting gains from the warmer temperature. In a recent study, Bronson and Gower (2010) observed that at the ecosystem level, a warmer climate would not change Pn of black spruce. Our latest findings and their work support the work of Way and Sage (2008) on black spruce greater temperature acclimation.
Low nutrient supply limits plant's photosynthetic rate (Aerts and Chapin 1999, Ainsworth and Long 2005) and the effects may be more severe in nutrient demanding species (Jackson et al. 1990). In this study, the low nutrient treatment significantly reduced seedlings Pn. In the mono experiment, the level of suppression was similar in both species despite the low level of tissue N in black spruce needles. In the mixed-species experiment however, the low nutrient treatment reduced on the photosynthetic rate of white spruce, partially supporting the hypothesis that white spruce, the higher nutrient demanding conifer species would have reduced Pn when nutrient supply is limited. Plants with high foliar nitrogen concentration generally have high Pn rate. (Aerts and Chapin 1999, Murray et al. 2000, Pallardy 2007, Lambers et al. 2008). One would therefore expect black spruce in the mono experiment to have lower Pn than white spruce since it had significantly reduced foliar [N]. Contrary, both species had similar Pn rate irrespective of the tissue [N]. While the exact mechanism that resulted in both species having similar Pn rate in the mono grown experiment is not clear, two underlying mechanisms can be assigned. First, the efficiency of nitrogen allocation between Rubisco carboxylation and electron transport apparatus was more effective in black spruce than in white spruce. It is estimated that up to 75% of nitrogen is allocated to the photosynthetic machinery (Lambers et al. 2008). In conifers however, the amount of total nitrogen allocated to Rubisco varies greatly from 4-30% (Warren and Adams 2004). It may therefore be the efficiency with which N is allocated to and utilised by the photosynthetic apparatus that determines Pn rate. Compared to broadleaves, conifers generally are inefficient in the allocation of N to other components of the photosynthetic apparatus (Field and Mooney 1986, Warren and Adams 2004). And in this study, it appears that white spruce was relatively inefficient in utilising N, compared to black spruce. Thus, black spruce had higher nitrogen use efficiency at both low and high nutrient treatments and maintained almost equal Jmax-Vcmax ratio as white spruce. Secondly, photosynthesis in conifers usually correlates positively with foliar [P] more than with [N] (Reich and Schoettle 1988, Conroy et al. 1990, Aerts and Chapin 1999). As such, N:P ratio rather than individual nutrient concentrations might have determined the seedlings Pn as reported earlier by Reich and Schoettle (1988) . In spruce seedlings, target N:P ratio of 10 is considered acceptable (Linder 1995). In the mono experiment, N:P ratio for both species was less than 10, and even lower in white spruce. The implication is that both species might have been nitrogen stressed. Because white spruce has higher nutrient requirement (Viereck and Johnston 1990, Nienstaedt and Zasada 1990), any nutrient stress would have been magnified in the species more than in black spruce. The low [P] in black spruce supports earlier reports that plants from nutrient-poor habitats have lower capacities for phosphate absorption than plants from fertile habitats (Chapin et al. 1982, Chapin 1983). Differential response of the two species in nutrient use efficiencies may be the key determinant of relative competitive ability of species, especially under limited nutrient conditions (Poorter et al. 1990).
The responses of isolated or monoculturally-grown plants in response to [CO2] and nutrient supply differs from that of species mixture (Navas et al. 1999). Between the mono and mixed species experiments, there were some distinct species differences to treatment. Photosynthetic rate, transpiration rate, maximum rate of carboxylation, total electron transport rate and triose-phosphate utilisation in the mixed-species experiment were different between the two species but similar when the seedlings were grown in isolation. For example, both species had statistically similar Pn rate in the mono experiment but in the mixed-species experiment, white spruce had reduced Pn rate under limited nutrient supply. Overall, black spruce appeared superior to white spruce in terms of carboxylation rate, efficiency of electron transport mechanism, sink strength nutrient use efficiencies, which cumulatively translated into higher photosynthetic rate in the mixed-species experiment. The expectation that white spruce would benefit more from increased nutrient supply and [CO2] is not supported by our data. Generally, both nitrogen and phosphorus use efficiencies were also higher in black spruce, especially at either elevated [CO2] or low nutrient treatment. Plant response to CO2 or nutrient supply when grown in mixture depends on the competitive ability the successfully species to acquire more or efficiently utilize acquired limited nutrient, relative to the co-occurring species (Reynolds 1996). Therefore, we can postulate that black spruce is a better competitor than white spruce, especially on poor sites.
Aside the species difference outlined, overall enhancement of the seedlings Pn rate in the mixed-species experiment was higher than in the mono experiment. The hypothesis that photosynthetic rate in the mixed-species experiment would be higher than in the mono experiments is supported by our data. The fact that elevated [CO2]-induced photosynthetic enhancement was greater in the mixed species experiment than in the mono experiment suggests that both nutrient (below ground resource) and carbon supply (above ground resource were better utilised when the two species were grown in mixture than in isolation
This study outlined some of the physiological responses of boreal forest's two most important conifer species, black spruce and white spruce, to soil temperature and nutrient supply under current and future [CO2] in mono and mixed growth conditions. The effects of soil temperature appeared marginal in relation to nutrient and inherent species differences suggesting that both species have wide optimum temperature range rather than a fixed single value. Increasing soil temperature from 20 to 25 oC may not have much effect on black spruce as compared to white spruce, especially when the two species are grown in mixture. Black spruce appeared superior to white spruce in terms of photosynthetic gain but only when the species were grown together. The effects of nutrient and CO2 on tree species are more realistic when trees are in mixture than grown in isolation (Navas et al.1999). If that is the case, then leaf-based photosynthetic gain from increased [CO2] would be higher when the two species are grown together than in isolation. A note of caution is however necessary as this study was short-termed and under controlled conditions. Field application therefore needs to cautiously done.