Abstract Rising atmospheric concentrations of CO 2 ( C a ) can reduce stomatal conductance and transpiration rate in trees, but the magnitude of this effect varies considerably among experiments. The theory of optimal stomatal behaviour predicts that the ratio of photosynthesis to transpiration (instantaneous transpiration efficiency, ITE ) should increase in proportion to C a . We hypothesized that plants regulate stomatal conductance optimally in response to rising C a . We tested this hypothesis with data from young Eucalyptus saligna Sm. trees grown in 12 climate‐controlled whole‐tree chambers for 2 years at ambient and elevated C a . Elevated C a was ambient + 240 ppm, 60% higher than ambient C a . Leaf‐scale gas exchange was measured throughout the second year of the study and leaf‐scale ITE increased by 60% under elevated C a , as predicted. Values of leaf‐scale ITE depended strongly on vapour pressure deficit ( D ) in both CO 2 treatments. Whole‐canopy CO 2 and H 2 O fluxes were also monitored continuously for each chamber throughout the second year. There were small differences in D between C a treatments, which had important effects on values of canopy‐scale ITE . However, when C a treatments were compared at the same D , canopy‐scale ITE was consistently increased by 60%, again as predicted. Importantly, leaf and canopy‐scale ITE were not significantly different, indicating that ITE was not scale‐dependent. Observed changes in transpiration rate could be explained on the basis that ITE increased in proportion to C a . The effect of elevated C a on photosynthesis increased with rising D . At high D , C a had a large effect on photosynthesis and a small effect on transpiration rate. At low D , in contrast, there was a small effect of C a on photosynthesis, but a much larger effect on transpiration rate. If shown to be a general response, the proportionality of ITE with C a will allow us to predict the effects of C a on transpiration rate.
Summary A poplar short rotation coppice (SRC) grown for the production of bioenergy can combine carbon (C) storage with fossil fuel substitution. Here, we summarize the responses of a poplar ( Populus ) plantation to 6 yr of free air CO 2 enrichment (POP/EUROFACE consisting of two rotation cycles). We show that a poplar plantation growing in nonlimiting light, nutrient and water conditions will significantly increase its productivity in elevated CO 2 concentrations ([CO 2 ]). Increased biomass yield resulted from an early growth enhancement and photosynthesis did not acclimate to elevated [CO 2 ]. Sufficient nutrient availability, increased nitrogen use efficiency (NUE) and the large sink capacity of poplars contributed to the sustained increase in C uptake over 6 yr. Additional C taken up in high [CO 2 ] was mainly invested into woody biomass pools. Coppicing increased yield by 66% and partly shifted the extra C uptake in elevated [CO 2 ] to above‐ground pools, as fine root biomass declined and its [CO 2 ] stimulation disappeared. Mineral soil C increased equally in ambient and elevated [CO 2 ] during the 6 yr experiment. However, elevated [CO 2 ] increased the stabilization of C in the mineral soil. Increased productivity of a poplar SRC in elevated [CO 2 ] may allow shorter rotation cycles, enhancing the viability of SRC for biofuel production.
We investigated the individual and combined effects of elevated CO2 concentration and fertilization on aboveground growth of three poplar species (Populus alba L. Clone 2AS-11, P. nigra L. Clone Jean Pourtet and P. × euramericana Clone I-214) growing in a short-rotation coppice culture for two growing seasons after coppicing. Free-air carbon dioxide enrichment (FACE) stimulated the number of shoots per stool, leaf area index measured with a fish-eye-type plant canopy analyzer (LAIoptical), and annual leaf production, but did not affect dominant shoot height or canopy productivity index. Comparison of LAIoptical with LAI estimates from litter collections and from allometric relationships showed considerable differences. The increase in biomass in response to FACE was caused by an initial stimulation of absolute and relative growth rates, which disappeared after the first growing season following coppicing. An ontogenetic decline in growth in the FACE treatment, together with strong competition inside the dense plantation, may have caused this decrease. Fertilization did not influence aboveground growth, although some FACE responses were more pronounced in fertilized trees. A species effect was observed for most parameters.
Background If biofuels are to be a viable substitute for fossil fuels, it is essential that they retain their potential to mitigate climate change under future atmospheric conditions. Elevated atmospheric CO2 concentration [CO2] stimulates plant biomass production; however, the beneficial effects of increased production may be offset by higher energy costs in crop management. Methodology/Main Findings We maintained full size poplar short rotation coppice (SRC) systems under both current ambient and future elevated [CO2] (550 ppm) and estimated their net energy and greenhouse gas balance. We show that a poplar SRC system is energy efficient and produces more energy than required for coppice management. Even more, elevated [CO2] will increase the net energy production and greenhouse gas balance of a SRC system with 18%. Managing the trees in shorter rotation cycles (i.e., 2 year cycles instead of 3 year cycles) will further enhance the benefits from elevated [CO2] on both the net energy and greenhouse gas balance. Conclusions/Significance Adapting coppice management to the future atmospheric [CO2] is necessary to fully benefit from the climate mitigation potential of bio-energy systems. Further, a future increase in potential biomass production due to elevated [CO2] outweighs the increased production costs resulting in a northward extension of the area where SRC is greenhouse gas neutral. Currently, the main part of the European terrestrial carbon sink is found in forest biomass and attributed to harvesting less than the annual growth in wood. Because SRC is intensively managed, with a higher turnover in wood production than conventional forest, northward expansion of SRC is likely to erode the European terrestrial carbon sink.
Abstract To determine whether globally increasing atmospheric carbon dioxide (CO 2 ) concentrations can affect carbon partitioning between nonstructural and structural carbon pools in agroforestry plantations, Populus nigra was grown in ambient air (about 370 μmol mol −1 CO 2 ) and in air with elevated CO 2 concentrations (about 550 μmol mol −1 CO 2 ) using free‐air CO 2 enrichment (FACE) technology. FACE was maintained for 5 years. After three growing seasons, the plantation was coppiced and one half of each experimental plot was fertilized with nitrogen. Carbon concentrations and stocks were measured in secondary sprouts in seasons of active growth and dormancy during 2 years after coppicing. Although FACE, N fertilization and season had significant tissue‐specific effects on carbon partitioning to the fractions of structural carbon, soluble sugars and starch as well as to residual soluble carbon, the overall magnitude of these shifts was small. The major effect of FACE and N fertilization was on cell wall biomass production, resulting in about 30% increased above ground stocks of both mobile and immobile carbon pools compared with fertilized trees under ambient CO 2 . Relative C partitioning between mobile and immobile C pools was not significantly affected by FACE or N fertilization. These data demonstrate high metabolic flexibility of P. nigra to maintain C‐homeostasis under changing environmental conditions and illustrate that nonstructural carbon compounds can be utilized more rapidly for structural growth under elevated atmospheric [CO 2 ] in fertilized agroforestry systems. Thus, structural biomass production on abandoned agricultural land may contribute to achieving the goals of the Kyoto protocol.
Summary Plant light interception efficiency is a crucial determinant of carbon uptake by individual plants and by vegetation. Our aim was to identify whole‐plant variables that summarize complex crown architecture, which can be used to predict light interception efficiency. We gathered the largest database of digitized plants to date (1831 plants of 124 species), and estimated a measure of light interception efficiency with a detailed three‐dimensional model. Light interception efficiency was defined as the ratio of the hemispherically averaged displayed to total leaf area. A simple model was developed that uses only two variables, crown density (the ratio of leaf area to total crown surface area) and leaf dispersion (a measure of the degree of aggregation of leaves). The model explained 85% of variation in the observed light interception efficiency across the digitized plants. Both whole‐plant variables varied across species, with differences in leaf dispersion related to leaf size. Within species, light interception efficiency decreased with total leaf number. This was a result of changes in leaf dispersion, while crown density remained constant. These results provide the basis for a more general understanding of the role of plant architecture in determining the efficiency of light harvesting.
ABSTRACT Under elevated atmospheric CO 2 concentrations, soil carbon (C) inputs are typically enhanced, suggesting larger soil C sequestration potential. However, soil C losses also increase and progressive nitrogen (N) limitation to plant growth may reduce the CO 2 effect on soil C inputs with time. We compiled a data set from 131 manipulation experiments, and used meta‐analysis to test the hypotheses that: (1) elevated atmospheric CO 2 stimulates soil C inputs more than C losses, resulting in increasing soil C stocks; and (2) that these responses are modulated by N. Our results confirm that elevated CO 2 induces a C allocation shift towards below‐ground biomass compartments. However, the increased soil C inputs were offset by increased heterotrophic respiration (Rh), such that soil C content was not affected by elevated CO 2 . Soil N concentration strongly interacted with CO 2 fumigation: the effect of elevated CO 2 on fine root biomass and –production and on microbial activity increased with increasing soil N concentration, while the effect on soil C content decreased with increasing soil N concentration. These results suggest that both plant growth and microbial activity responses to elevated CO 2 are modulated by N availability, and that it is essential to account for soil N concentration in C cycling analyses.
Plants have been widely documented to respond to mechanical stimuli such as wind and touch. Well-known and long-studied examples of these are carnivorous plants (e.g. Darwin, 1893), but nonspecialized plants are also sensitive and responsive to mechanical perturbation. Studies on this phenomenon, called ‘thigmomorphogenesis’ (Jaffe, 1973), have been conducted for several decades, revealing complex signaling and response pathways (Braam, 2005). Common thigmomorphogenetic responses include altered shoot elongation vs radial expansion ratios, delayed flowering, changes in chlorophyll content, etc. (see Biddington, 1986 and Cahill et al., 2002 for a review and a concise overview, respectively). In nature, such changes usually occur in response to wind and as a result of contact with neighbouring plants. Humans can unwillingly mimic these effects when studying plants, as several studies have shown that the mere act of touching plants by hand can have significant effects (Braam & Davis, 1990; Cahill et al., 2001). Moreover, in a considerable number of plant studies, measurements are not limited to touching plant tissue but include destructive sampling of leaves, roots, etc. It is apparent that if such (repeated) plant measurements, whether destructive or nondestructive, affect plant functioning, this could have far-reaching implications. Nevertheless, the attention given to such ‘observer effects’ in plant science has been limited. If studying plants indeed implies involuntarily altering their morphology and/or physiology, then two main problems could arise. First, in studies on the state of nature (e.g. ozone damage in European forests, Ferretti et al., 2007), the presence of an observer effect could cause such assessments to deviate from reality, leading to erroneous conclusions. Second, in studies with an experimental treatment, a further problem arises if handling plants results in different effects in the different treatments (as already suggested by Cahill et al., 2001). Such a treatment × handling interaction would again distort the study's results and conclusions, as it implies inflation or understatement of the treatment effects. As treatment studies are often future oriented (e.g. investigating the effects of elevated CO2 concentrations or increased temperatures), this could subsequently lead to an incorrect impact assessment of several global changes. As an example from an actual experiment, we processed data from POP-EUROFACE in Central Italy (42°22¢N, 11°48¢E), a large-scale experiment for studying the long-term effects of elevated CO2 concentrations on carbon sequestration and bio-energy production in a short rotation coppice. An overview of the set-up can be found in Scarascia-Mugnozza et al. (2006). During six consecutive years (1999–2004), poplars were exposed to elevated CO2 concentrations (550 ppm) in three free air CO2 enrichment (FACE) areas, and three areas with ambient CO2 concentrations served as a control. Each area was divided into six sectors that were planted with three different poplar species (Populus alba L., P. nigra L. and P. × euramericana). Throughout these 6 yr, over 10 different research teams carried out measurements in this plantation, from the leaf level up to the canopy scale. Most of the wide array of common ecological measurements took place in ‘permanent growth plots’ (PGPs), and consisted of both destructive (e.g. leaf chlorophyll, nitrogen, rubisco, leaf area, soil coring) and nondestructive (e.g. tree diameter, height, canopy light transmission) measurements, with a similar intensity in each year. The PGPs consisted of a group of six adjacent trees within each sector, surrounded by at least one row of trees of the same genotype and treatment (Supplementary material Fig. S1). To assess the occurrence of observer effects, trees with very limited exposure to handling were randomly selected from the remaining poplars inside each sector (i.e. 18 trees during the first rotation (until 2001), and nine trees during the second rotation (until 2004)). In this study, we compared poplar biomass production (scaled up from stem diameter via allometric relations, cf. Calfapietra et al., 2003; Liberloo et al., 2005, 2006) inside and outside the permanent growth plots, to assess the following: whether an observer effect was detectable; whether there was an interaction of this effect with the CO2 treatment; whether the three poplar species responded differently to handling; and whether any of the observer effects changed over time. To this end, data sets from 2000, 2001, 2003 and 2004 were used. Stem diameter was the only measurement consistently made inside and outside PGPs during the course of the experiment, but is considered a suitable parameter for a general assessment of observer effects because of its nondestructive nature and its use as a proxy for tree vigour and health. Data were examined in sas (SAS 9.1; SAS Institute, Cary, NC, USA), first using an analysis of variance (ANOVA) with repeated measures in time on the biomass data (log transformed for normalisation) to test the significance (P < 0.05) of observer effects. The design was a randomized complete block, with CO2 treatment, species, year and plot identity (PGP or non-PGP) as fixed factors, block (i.e. the combination of one control and one treatment area) as a random factor, and plot as the unit of replication. Upon confirmation of the significance of observer effects, an identical-repeated measures ANOVA was then performed on the observer effect itself (i.e. the percental difference in above-ground biomass between PGPs and non-PGPs (these data had a normal distribution after removal of one outlier)), to test specifically for effects of treatment, species or year (fixed factors). An a posteriori comparison of means was performed with the Bonferroni correction for multiple comparisons. The analysis showed that trees inside and outside PGPs differed significantly (P < 0.001), with biomass reduced by up to 50% because of handling (Fig. 1). The observer effect differed between species (P < 0.01), with significantly lower adverse effects of handling in P. alba compared with the other two species (P < 0.05 after correction). Observer effects were furthermore strongly affected by the measurement year (P < 0.001) as they only reached significance in the last 2 yr of the study (2003–2004). Finally, a posteriori comparison revealed a trend towards differences in the size of observer effects in both treatments for P. × euramericana (P = 0.10 after correction), which was also visible in Fig. 1. Other factors and interactions were not significant. The percental difference in above-ground biomass (calculated from stem diameter via allometric relationships) between trees inside and outside permanent growth plots (PGPs), growing under ambient (open symbols, dashed line) or elevated (closed symbols, solid line) CO2 concentrations. Three Populus species are depicted: (a) P. alba; (b) P. nigra; and (c) P. × euramericana. Only averages and standard errors are shown. Symbols are slightly shifted, with respect to the x-axis, for clarity. The experiment that we scanned for observer effects yielded several interesting results. In general, handling was found to decrease productivity and did so by proportionally the same extent under treatment and control conditions, even though there were indications that the observer effect differed somewhat between treatments for one species. Our data thus provide further evidence to disprove the assumption of researchers as ‘benign observers’, as indeed the act of conducting an experiment can alter the experimental results (Cahill et al., 2001). The general absence of observer effects in 2000 and 2001, and the markedly steep decline in biomass inside vs outside PGPs in the last 2 yr of the study, furthermore suggest that adverse handling effects can build up (even across coppicing events), as the measurement intensity was similar in all years. Such an effect would be of particular importance in long-term experiments in which the same plots and plants are sampled continuously (similar to POP-EUROFACE). The recent trend towards such long-term studies (e.g. Wullschleger & Hanson, 2006; Mohan et al., 2007), invoked by their great scientific value, especially in determining impacts of global changes beyond single growing seasons, could therefore lead to a growing relevance of observer effects. It must be noted that, because there was no general treatment × handling interaction, conclusions of the POP-EUROFACE studies regarding effects of elevated CO2 concentrations on poplar growth were probably correct. Apart from direct effects of measuring, observers can also cause indirect effects that affect plant functioning. Among these are altered incidence of herbivory or plant diseases (Latimer & Oetting, 1999; Niesenbaum et al., 2006), soil compaction (Hik et al., 2003; Andres-Abellan et al., 2005) and changes in light conditions (Cahill et al., 2001). An indirect effect that probably caused a proportion of the observer effect in the POP-EUROFACE experiment was the combination of a windy site and the presence of scaffolding towers, causing mechanical damage to (predominantly) the PGP trees. We rule out that observer effects were solely attributable to the towers, as these were located at one side of the PGPs (Supplementary material Fig. S1) and therefore did not impact all PGP trees (affirmed by visual inspection of the damaged tree tops). As significant biomass reduction was found throughout the PGPs, negative direct impacts of measurements and sampling must have contributed to the observed growth reduction. The multitude of possible observer effects, both direct and indirect, renders it extremely difficult to predict their combined outcome. Moreover, sampling has been documented to affect different species in different ways (Hik et al., 2003), which was confirmed by the POP-EUROFACE data. This further increases the difficulties of quantifying observer effects, and hence makes it paramount to avoid or minimize such effects in the first place. In animal studies, and especially those concerned with behaviour, observer effects have long been known and acknowledged (Wade et al., 2005). In that field of research, avoidance is also deemed the best strategy for coping with observer effects rather than taking these into account somehow (e.g. Baker & McGuffin, 2007). In plant science, noninvasive techniques exist as an alternative to certain destructive measurements (such as leaf area determination). However, accuracy problems often make these alternatives less reliable (e.g. Broadhead et al., 2003). In many cases it is unavoidable that researchers do exert an influence, as, for example, cuvette measurements (which can damage leaves) provide data that are often essential but currently impossible to collect in less intrusive ways. Given the constraints imposed by the measurement technology currently available, the most appropriate solution to minimize observer effects seems to be to lower the measurement intensity. This can be achieved either by taking fewer samples per unit of time, or by spreading out the measurements over a larger number of study objects (plants, communities, etc.). Two main problems are associated with this. In the first case, reducing the number of samples would lower the statistical power, whereas the second proposed solution goes against the often-adhered researcher's philosophy to make full use of the money granted by maximizing the amount of data collected per unit of currency funded. Nevertheless, to avoid the risk of the experimental results becoming flawed, either of our two proposed solutions should be considered. Of course, only in hindsight can it be confidently stated whether the applied intensity affected plant functioning. It would nevertheless be prudent to design all sampling protocols for minimal disturbance while maintaining a statistically adequate number of data samples. This is especially relevant for treatment-type studies, in which the same limited number of experimental objects are sampled continuously and which therefore seem much more prone to oversampling than state-of-nature studies that usually have a lower sampling intensity. Field experiments regarding observer effects have almost uniquely been conducted to test the ‘herbivory uncertainty principle’, which states that researcher visitation and plant measurements may alter herbivore and pathogen damage (Cahill et al., 2001; Schnitzer et al., 2002). The manipulations in these type of experiments can be considered as mild, as they consist mainly of visual observations and height measurements. Even under these conditions were observer effects, although not consistently (e.g. Bradley et al., 2003; Cahill et al., 2004). We have demonstrated that in a long-term experiment with frequent and invasive measurements, observer effects are potentially larger (although the largest effects were observed only in the later stages). To elucidate the uncertainties associated with observer effects, research is needed: to unveil the generality of observer effects (i.e. whether they are more outspoken in certain ecosystems (e.g. tundra) and species or functional groups than in others); to clarify the relationship between measurement intensity and effect (i.e. is there a dose–response relationship (linear or otherwise) or are there thresholds?); to assess which types of measurements have the largest impact; and to uncover which plant process is the most sensitive to handling. To help resolve these important questions, we advise leaving part of any experiment unsampled, allowing for an a posteriori assessment of observer effects (such as for POP-EUROFACE). From contacts with international colleagues, we understand that a majority of scientists dealing with long-term experiments are aware of the existence of observer effects. However, by quantifying these effects, we have shown that the often underlying assumption that they are negligible is not necessarily true. Observer effects should therefore always be considered in setting up new experiments and drawing up sampling strategies, by focusing on minimizing disturbance. Such considerations are, in our eyes, vital to further plant research. Indeed, the issue of observer effects is a genuine concern, which will not be resolved by ignoring its existence. H. J. De Boeck holds a grant from the Institute for the Promotion of Innovation through Science and Technology in Flanders (IWT-Vlaanderen). M. Liberloo and B. Gielen are postdoctoral research associates of the Fund for Scientific Research – Flanders. POP-EUROFACE was supported by EU-POPFACE (ENV4-CT97-0657), EU-EUROFACE (EVR1-CT-2002-40027), the Center of Excellence ‘Forest and Climate’ (Italian Ministry of University and Research), and the Italy-USA Bilateral Project on Climate Change of the Italian Ministry of Environment. We thank V. Sluydts and S. Van Dongen for statistical advice. The following supplementary material is available for this article online: Fig. S1 Layout of the POP-EUROFACE plantation with two poplar fields, divided by a country road, and six experimental areas (black = free air CO2 enrichment (FACE)). This material is available as part of the online article from: http://www.blackwell-synergy.com/doi/abs/10.1111/j.1469-8137.2007.02329.x (This link will take you to the article abstract). Please note: Blackwell Publishing are not responsible for the content or functionality of any supplementary materials supplied by the authors. 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To determine whether long-term growth in elevated atmospheric CO2 concentration [CO2] and nitrogen fertilization affects woody tissue CO2 efflux, we measured stem CO2 efflux as a function of temperature in three different size classes of shoots of Populus nigra L. (clone Jean Pourtet) on two occasions in 2004. Trees were growing in a short rotation coppice in ambient (370 µmol mol-1) and elevated (550 µmol mol-1, realised by a Free Air Carbon dioxide Enrichment system) [CO2], and measurements were performed during the third growing season of the second rotation. Elevated CO2 did not affect Q10 or specific stem CO2 efflux (E10) of overall poplar shoots. The lack of any effect of N on stem CO2 efflux indicated that nutrients were sufficient. Specific stem CO2 efflux differed significantly between shoot sizes, emphasizing the importance of tree size when scaling-up respiration measurements to the stand level. Variation in stem CO2 efflux could not be satisfactorily explained by temperature as the only driving variable. We hypothesize that transport of CO2 with the sapflow might have confounded our results and could explain the high Q10 values reported here. Predicting the respiratory carbon loss in a future elevated [CO2] world must therefore move beyond the single-factor temperature dependent respiration model and involve multiple factors affecting stem CO2 efflux rate.
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