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Forests occupy one-third of the land surface of the Earth, and account for almost one-half of the carbon (C) stored in the terrestrial biosphere (Schlesinger & Lichter, 2001). In a summary of studies conducted to investigate the effects of increased atmospheric CO2 on forest C cycles, Norby et al. (2005) calculated that an enrichment of 200 ppm CO2 above the current ambient CO2 level caused a 23% median increase in forest net primary productivity (NPP). However, interactions with other environmental factors may dampen such a response at larger temporal or spatial scales (Leuzinger et al., 2011). Nevertheless, increasing atmospheric CO2 concentrations may fundamentally alter forest ecosystem functioning by altering species growth, resource use and community interactions (Eamus & Jarvis, 1989). As forests are inextricably linked to the global C cycle, elevated CO2-driven environmental change may impact upon global C storage in phytomass, complex biogeochemical feedback mechanisms and, ultimately, long-term C sequestration in soils.
Empirical studies on woody plants exposed to elevated atmospheric CO2 have demonstrated that growth and aboveground biomass production increase, but that there is a considerable variation in response (Curtis & Wang, 1998). The observed variation in responses to elevated CO2 has been attributed to a large number of confounding factors, such as the length of study, interactions with other environmental stresses, plant functional group, species morphological physiology (Poorter, 1993), symbiotic associations (Godbold et al., 1997) and community dynamics (Kozovits et al., 2005). Recent research efforts have focused on the study of the whole ecosystem responses in near-natural conditions, chiefly achieved by employing free-air carbon dioxide (CO2) enrichment (FACE) facilities (Hattenschwiler et al., 2002; Karnosky et al., 2003; Körner et al., 2005; Hoosbeek et al., 2011). Körner (2006) has suggested that elevated CO2 studies should be divided into the following two types: high abundance of major resources other than C – ‘decoupled systems’ (type I); and near to steady-state nutrient cycling and full canopy development – ‘coupled systems’ (type II). Type I systems include the present study, aspen FACTS II FACE (Karnosky et al., 2003) and EuroFACE (Calfapietra et al., 2003) experiments. The remaining three (type II) experiments have used CO2 enrichment in stands with an already closed canopy. The Oak Ridge (Norby et al., 2002) and DukeFACE (Oren et al., 2001) experiments both started enrichment c. 10–20 yr after planting, whereas, at the Basel Web-FACE (Körner et al., 2005), enrichment was conducted in a mature deciduous forest comprising four species > 100 yr old. Using data from four of these studies (DukeFACE, FACTS II FACE, Oak Ridge and EuroFACE), Norby et al. (2005) demonstrated that an enrichment of 200 ppm CO2 above the current ambient CO2 level caused a 23% median increase in forest NPP. This conclusion was largely based on the initial response of forest ecosystems to elevated CO2. Subsequent investigations have shown that this response may not be maintained over a longer time horizon (Norby et al., 2010), as the response to elevated CO2 has been found to decline (Norby et al., 2010) or be maintained (Drake et al., 2011; Zak et al., 2011) after 10–11 yr of exposure. In both of these examples, the response to elevated CO2 was probably mediated by nitrogen (N) availability. The decline in response to elevated CO2 was attributed to N limitation (Norby et al., 2010), whereas no change in response was a result of greater N cycling (Zak et al., 2011). The comparison of these two studies clearly demonstrates that nutrient availability, in particular N, is a strong factor mediating the response of woody plants to elevated CO2.
Much of the research investigating species diversity, ecosystem functioning and productivity has been focused in grasslands (Hooper et al., 2005). Many experiments have shown a positive relationship between productivity and increased biodiversity (Tilman et al., 1996, 1997). Fornara & Tilman (2009) suggested that the increased productivity of N-limited, species-rich plant communities is dependent on the seasonal accumulation of root N pools by N-fixing plants. The importance of the incorporation of N-fixing plants in the facilitation of greater plant community productivity was also supported by Hooper & Dukes (2004), but it was argued that N fixation is not the only mechanism explaining the overyielding of species-rich communities. Elevated CO2 has been found to stimulate symbiotic N fixation in several studies (e.g. Hungate et al., 1999; Schortemeyer et al., 2002), and the incorporation of N-fixing plants to facilitate the N dynamics of co-occurring species with elevated CO2 was explored by Lee et al. (2003), who found that, in nine different grassland species, assemblages incorporating N-fixing Lupinus did not facilitate a larger community growth response to elevated CO2.
In forests, controversy surrounding the benefits of mixed species stand productivity dates back to the 18th century (Hartig, 1791), with the silvicultural practice of mixed species forests being subject to much conjecture. Only recently have rigorous scientific studies been initiated to elucidate the precise mechanisms mediating the productivity differences of trees grown in polyculture (Pretzsch, 2005). For example, in southern Germany, mixed stands of Fagus sylvatica and Picea abies produced up to 59% more aboveground biomass than adjacent pure stands (Pretzsch & Schütze, 2009). By contrast, Jacob et al. (2010) found decreases in aboveground biomass of F. sylvatica with increasing species richness in comparison with F. sylvatica in monoculture. Early on, most research on forest diversity focused on one or two tree species, but recent studies have included more species in an attempt to verify the applicability of grassland findings to forest stands (DeClerck et al., 2006; Vila et al., 2007; Paquette & Messier, 2010). In large-scale investigations, support has been found for the assertion that increased tree diversity leads to increased biomass production (Vila et al., 2007; Paquette & Messier, 2010). The studies of both Vila et al. (2007) and Paquette & Messier (2010) used databases originating from national forest inventories, whilst taking into account the effects of environment. Paquette & Messier (2010) used 12 000 permanent forest plots in boreal and temperate forest in Canada, and showed a strong positive and significant effect of tree biodiversity on aboveground productivity. The study of Vila et al. (2007) used over 8000 permanent forest plots in Mediterranean forests in Catalonia, and showed a mean 30% higher wood production in mixed forest compared with monospecific stands, and a production increase from 23% in two-species stands to 59% in five-species stands. In a meta-analysis of 54 forest studies investigating diversity–productivity relationships, Zhang et al. (2012) showed a 24% higher productivity in polycultures than in monocultures, with most of the variation accounted for by evenness, the heterogeneity of shade tolerance, species richness and stand age, in decreasing order of importance. Recently, high plant diversity has been shown to be required to maintain ecosystem function and services through time (Isbell & Wilsey, 2011); however, the role of tree diversity in ecosystem productivity, resistance and resilience is still poorly investigated (DeClerck et al., 2006). In the case of resistance to drought, DeClerck et al. (2006) found that the relative percentage of different species was more important than the species richness per se. Differing species resistance to drought can change the competitive relationship between the species, and may thus result in altered species composition. Reich et al. (2001) showed that the enhancement of biomass accumulation in response to elevated levels of CO2 was smaller in species-poor than in species-rich assemblages of herbaceous plants. However, although it has long been known that tree seedlings of co-occurring species show differing response to CO2 (Bazzaz & Miao, 1993), the influence of elevated CO2 on tree competition and the influence of tree biodiversity on community response to CO2 have not been investigated.
The objectives of this work were to investigate the effects of elevated CO2 (580 μmol mol−1) on the species and community response of monocultures and polycultures of tree mixtures under field conditions. Using a FACE system, we investigated the aboveground response of monocultures and a three-species polyculture of Alnus glutinosa, Betula pendula and F. sylvatica to elevated CO2 over 4 yr. We tested the hypothesis that interspecific competition modifies the response of tree species to elevated CO2.
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Allometric relationships have commonly been used to estimate the biomass of aboveground compartments. The allometric coefficients generated in this study were broadly similar to previously published coefficients (Hughes, 1971; Bartelink, 1997; Pajtik et al., 2011), with the exception of F. sylvatica. The dimorphic growth characteristics of juvenile F. sylvatica under different light regimes during canopy development may explain the difference observed (Delagrange et al., 2006). The application of species and site-specific allometric relationships is likely to be valid for A. glutinosa and B. pendula. However, the relationship for F. sylvatica appears to be slightly weaker and may benefit from closer examination of the differences in morphology when trees are shade suppressed and growing in full light.
In this study, aboveground woody biomass accumulation in A. glutinosa and B. pendula was greater in polyculture than in monocultures. In species-diverse communities, the complementary use of resources may lead to higher yields than in monocultures (Loreau & Hector, 2001). Differences in the tree species life history character traits, such as crown structure, rooting depth, shade tolerance, phenology and photosynthetic light response, may allow for differential access to resources (Kelty, 1992). If the chosen species occupying the same site differ substantially in these characteristics, they may capture site resources more completely or use resources more efficiently to produce biomass. Species with contrasting trait characteristics can be described as having complementary resource use (Haggar & Ewel, 1997) or good ecological combining ability (Harper, 1977). However, it should be noted that complementarity may not necessarily result in a positive effect on productivity; antagonistic interactions (negative complementarity) between species may also occur as a result of character trait interferences that may lower the productivity of species mixtures over those expected from monocultures (Wardle et al., 1998; Loreau & Hector, 2001; Eisenhauer, 2012). In their study, Paquette & Messier (2010), in an analysis of naturally occurring tree biodiversity, showed a strong positive effect of biodiversity on tree productivity. They further suggested that, in the more productive environment of temperate forest, competitive exclusion is the most probable outcome of species interactions, but, in the more stressful environment of boreal forest, beneficial interactions, such as niche partitioning and facilitation, may be more important.
In our temperate forest mixture, we used two pioneer species and a late successional species that strongly differ in their functional traits. Betula pendula is a light-demanding, early successional pioneer species which casts little shade and rapidly occupies open areas as a result of fast juvenile growth (Fischer et al., 2002). Alnus glutinosa is an N-fixing, water-demanding pioneer species, also with high juvenile growth rates (Braun, 1974). The root system of A. glutinosa is adapted to wet soils, with many vertically growing sinker roots that may reach a depth of 5 m (Claessens et al., 2010). In mixed forests, its limited height growth and shade intolerance prevent it from dominating in late successional forest. Lastly, Fagus sylvatica is shade tolerant and slow growing when juvenile (Ellenberg et al., 1991), can persist in the understorey and often dominates late successional forest. The higher polyculture productivity in our 4-yr-old plantation suggests that the dominant pioneer species A. glutinosa and B. pendula are partitioning canopy space made available by F. sylvatica. However, the flattening of the diameter class distribution in B. pendula, but not in A. glutinosa, suggests that some B. pendula are being excluded. In our study, we did not systematically determine crown architecture, but observed that, in polyculture, both B. pendula and shorter A. glutinosa had deeper crowns. Indeed, we saw higher leaf area index in A. glutinosa and B. pendula in polyculture relative to monocultures, but no difference in stem height, which suggests alteration of crown architecture between monoculture- and polyculture-grown trees. Claessens et al. (2010) suggested that A. glutinosa grown in monoculture produces a straight bole and round crown, whereas, when grown in an admixture with other species, forms a stratified canopy. In the meta-analysis of species richness productivity relationships by Zhang et al. (2012), heterogeneity of shade tolerance was the second most important factor explaining increased productivity in mixtures. In addition to an aboveground partitioning of canopy space, an increase in N availability via the N-fixing A. glutinosa could also be a factor in the higher productivity of the polyculture. In A. glutinosa under ambient CO2, the amount of N content in the leaves did not differ between monoculture or polyculture (Millett et al., 2012); however, in polyculture, leaves of F. sylvatica and B. pendula were less enriched in 15N compared with the leaves of these species growing in monoculture. This difference suggests an incorporation of N fixed by the symbionts of A. glutinosa. In other investigations, the contribution of transferred N to total N was 5–15% (Arnebrant et al., 1993) and 1–3% (Ekblad & HussDanell, 1995), on average, between A. glutinosa and P. contorta and A. incana and P. sylvestris, respectively. Furthermore, leaves of both F. sylvatica and B. pendula with greater numbers of A. glutinosa as direct neighbours were significantly depleted in 15N relative to the leaves of those with fewer A. glutinosa as direct neighbours (Millett et al., 2012), suggesting a competition for N as a possible mechanism for the exclusion of some B. pendula.
In response to elevated CO2, aboveground woody biomass for all three species combined was increased by 22% in monocultures. A response of this magnitude is consistent with a previously reported woody plant response of 28% calculated from meta-analyses of elevated CO2 experiments (Curtis & Wang, 1998; Ainsworth & Long, 2005) and 23% from four forest FACE experiments after 6 yr of enrichment (Norby et al., 2005). Utilizing observations spanning somewhat longer exposure to elevated CO2 (up to 11 yr), Norby et al. (2010) have shown that NPP responsiveness decreases in time. The limitation of NPP stimulation may largely be attributed to progressive N limitation (PNL); however, the observed reduction in NPP stimulation was almost entirely accounted for by changes in fine root production. Given the life history character traits of the species chosen in our experimental plantation, it is possible that the increased accrual of woody biomass observed in polyculture may not decrease as the forest stand develops. The presence of A. glutinosa in the mixture should compensate for increased N uptake, and thus negate or at least delay the onset of PNL. Several studies have shown that the rate of N fixation in the nodules of trees supporting this type of symbiosis increases under elevated CO2, presumably as a result of increased C availability (Hungate et al., 1999; Schortemeyer et al., 2002). Betula pendula and F. sylvatica growing in our plantation have been shown to utilize N fixed by A. glutinosa, suggesting that the presence of an N-fixing species might alleviate N limitation for all species grown in a polyculture.
There were considerable temporal differences in the response to elevated CO2 at our site. In the first growing season before canopy closure, all species responded to elevated CO2 enrichment by increasing total biomass by 27–29%. The stimulation of B. pendula began to decline during the second growing season, whereas the response of F. sylvatica declined during the last two growing seasons – an effect often attributed to acclimation to elevated CO2 (Ainsworth & Long, 2005) or nutrient limitation (Oren et al., 2001). In the present study, leaf N was unaffected by elevated CO2 during all stages of development, and thus it is unlikely that the decreasing overall elevated CO2 effect is caused by N limitation. As a result of the history of land use at the site, we did not expect a lack of N to limit plant growth within the first 4 yr. Indeed, foliar N increased, whereas leaf NUE decreased, with time in all treatments, indicating sufficient N uptake. In all species pooled together, leaf NUE was increased by elevated CO2 and also by growing trees in a mixture. However, we did not observe any differences in leaf NUE in individual species, suggesting that a different mechanism may explain the observed species-specific responses.
As we observed an expanding system with at least two canopy levels, the developmental phase of the stand and the strength of competition in our experiment must also be considered. Each species used in this study differs in their shade tolerance. Ellenberg et al. (1991) characterized F. sylvatica, A. glutinosa and B. pendula as shade tolerant (‘3’, out of ‘9’), intermediate (‘5’) and light-demanding (‘7’), respectively. Low leaf mass per leaf unit area and high rate of C assimilation per unit leaf area of light-demanding species allow rapid occupancy of available space and some canopy light penetration (Niinemets, 2006). Considering only monocultures in 2005, the saplings of each species were initially not influenced by intraspecific competition for light and space, allowing a greater response to elevated CO2. The subsequent decline in response of F. sylvatica to elevated CO2 may be explained by strong intraspecific competition through leaf morphology and crown architecture that minimizes canopy light penetration. By contrast, A. glutinosa sustained the stimulation by elevated CO2, ranging between 25% and 32% throughout the 4-yr experiment. Claessens et al. (2010) described A. glutinosa as fast growing when juvenile, but as a poor competitor that does not produce shade leaves. Respirational losses of crown-shaded leaves may result in a leaf C balance that approaches zero, which can lead to rapid leaf death (Reich et al., 2009). In our ecosystem, fast juvenile growth, coupled with rapid self-pruning, enabled A. glutinosa grown in monoculture to fully utilize elevated levels of atmospheric CO2 to accumulate aboveground woody biomass; however, the aboveground growth response to elevated CO2 was dramatically reduced when species were grown in polyculture. Initial increases in biomass of F. sylvatica were marginal, eventually becoming suppressed in the last growing season. The lack of stimulation of F. sylvatica is most probably a result of faster canopy occupation by A. glutinosa and B. pendula under elevated CO2. Changes in leaf area index may influence canopy light penetration and interspecific competition under elevated CO2. In our study, in monocultures, the leaf area index was unaffected by elevated CO2, but there was a consistently higher trend in B. pendula for the first 3 yr. During the summer of 2006, a severe drought resulted in partial canopy defoliation, which may explain the dramatic leaf area index increase in 2007. Both species possess indeterminate growth characteristics that enabled an additional leaf flush when environmental conditions improved later in the 2006 season. We propose two mechanisms to explain this phenomenon: differences in rooting depth between the two species; and the ability to recover from defoliation related to N storage. Alnus glutinosa has been characterized as possessing extensive root systems, with particularly deep tap roots that enable it to access water below the normal water table (Schmidt-Vogt, 1971; Claessens et al., 2010). This confers a considerable advantage in leaf production during, and following, drought conditions. The second explanation centres on the storage of N in tree perennial organs which can be re-mobilized and support leaf re-growth after defoliation. In combination with a flush of C and organic N compounds released for root uptake as the abscised litter decomposed mid-growing season, this mechanism may have facilitated the development of leaf primordia and a greater leaf area index during the following season (Tromp, 1983). Oksanen et al. (2001) found that elevated CO2 consistently increased leaf area index throughout the growing season in aspen, birch and maple stands, which was attributed to larger leaves. By contrast, Gielen et al. (2001) found that the leaf area index of P. nigra increased by 225% during the first growing season. However, a post-canopy closure analysis using a fish-eye canopy analyser revealed no increase in leaf area index, which is in agreement with data obtained at the Oak Ridge deciduous closed-canopy elevated CO2 experiment (Norby et al., 2003).
Our results clearly show that the aboveground response to elevated CO2 is species dependent, but is also affected by intra- and interspecific competition. Indeed, old growth F. sylvatica have been reported to show only a limited response to CO2 enrichment (Körner et al., 2005). In our study, a small, but statistically nonsignificant, positive effect of elevated CO2 on F. sylvatica in polyculture was shown in 2006, a year in which a severe summer drought in June and July resulted in strong leaf loss in A. glutinosa and B. pendula. During this period, only 44 mm of precipitation fell, compared with 101, 216 and 85 mm in the same period of 2005, 2007 and 2008, respectively. In July 2006, the maximum temperature was 34.5°C, 10°C warmer than in other years. The increase in light penetration to the understorey formed by F. sylvatica, in combination with improved water use efficiency, may have stimulated a response to elevated CO2, at least until A. glutinosa and B. pendula re-grew some of their foliage in late August. The literature suggests that much of the response of trees to elevated CO2 is linked to greater water availability, and that trees may be more drought tolerant under elevated CO2 (Eamus, 1991; Holtum & Winter, 2010; Leuzinger et al., 2011). If elevated CO2 had conferred a greater tolerance to drought in our experiment, we would have expected the highest response to elevated CO2 in 2006; this was clearly not the case for A. glutinosa and B. pendula; however, the severity of the drought, in combination with higher temperatures and photosynthetic oxidative stress, should also be considered.
To date, the majority of tree elevated CO2 experiments have used monospecific tree stands and have reported a mean stimulation of NPP for the duration of the observation (Norby et al., 2010). We have shown that, in a short-term empirical study of juvenile deciduous temperate trees grown in polyculture, the aboveground woody biomass response to elevated CO2 is strongly decreased. This result may have implications for the estimation of the global forest response to elevated CO2, as in natural mixed species forest the response to CO2 may be lower than previous estimates. However, caution must be exercised when extrapolating data from small-scale temperate plantations, particularly when there is the potential for experimental artefacts, arising from CO2 enrichment systems and edge effects influencing the response of saplings planted in complex arrangements at high planting densities. Although providing useful data, experimental plantations do not directly mimic the natural species-diverse, multi-aged and complex structures of the majority of the world's forests that grow in differing biomes, constrained by other physical and environmental drivers. Leuzinger et al. (2011) suggested that an increase in the number of driver variables, such as elevated CO2, drought and N addition, will dampen the ecosystem response to single factors through contrasting driver interactions. Similarly, Langley & Megonigal (2010) showed that, in a grassland system, the addition of N under high CO2 promoted a shift in community composition to C4 species that were less responsive to CO2, thus decreasing overall community response. Further, Langley & Megonigal (2010) suggested that, if the addition of N favours species that respond strongly to CO2, the community response to CO2 should increase. In our experimental mixture, complementary resource acquisition led to greater community productivity, which dampened the aboveground woody biomass response to elevated CO2, even though the most responsive species in the monoculture (A. glutinosa and B. pendula) were promoted within the mixed community. This is most probably a result of changes in source–sink relationships and C allocation to belowground organs. Indeed, tree root systems under elevated atmospheric CO2 have been shown to expand more deeply into the soil (Lukac et al., 2003; Iversen, 2010; Smith et al., 2013). Clearly, we are only beginning to understand how changes in elevated CO2-influenced above- and belowground processes may alter plant community dynamics.
In conclusion, atmospheric CO2 enrichment did not alter species-specific allometric relationships. The estimation of aboveground biomass stocks and productivity revealed a differential response to elevated atmospheric CO2. Aboveground biomass responses to CO2 enrichment were species specific and strongly reduced when species were grown in polyculture. In monoculture, A. glutinosa produced the largest and most consistent response, maintaining growth response until the experiment's conclusion. By contrast, the growth response of B. pendula and F. sylvatica diminished with time. In polyculture, the growth of F. sylvatica was not enhanced by elevated CO2. Our results suggest that the determination of how the aboveground biomass response of deciduous species grown in polyculture differs from that of single-species plantations is imperative to improve our understanding of how future CO2 will impact upon natural forest community dynamics.