Savanna woody encroachment is widespread across three continents

Tropical savannas are a globally extensive biome prone to rapid vegetation change in response to changing environmental conditions. Via a meta‐analysis, we quantified savanna woody vegetation change spanning the last century. We found a global trend of woody encroachment that was established prior the 1980s. However, there is critical regional variation in the magnitude of encroachment. Woody cover is increasing most rapidly in the remaining uncleared savannas of South America, most likely due to fire suppression and land fragmentation. In contrast, Australia has experienced low rates of encroachment. When accounting for land use, African savannas have a mean rate annual woody cover increase two and a half times that of Australian savannas. In Africa, encroachment occurs across multiple land uses and is accelerating over time. In Africa and Australia, rising atmospheric CO₂, changing land management and rainfall are likely causes. We argue that the functional traits of each woody flora, specifically the N‐fixing ability and architecture of woody plants, are critical to predicting encroachment over the next century and that African savannas are at high risk of widespread vegetation change.     

Increased tree densities in South African savannas: >50 years of data suggests CO2 as a driver

For the past century, woody plants have increased in grasslands and savannas worldwide. Woody encroachment may significantly alter ecosystem functioning including fire regimes, herbivore carrying capacity, biodiversity and carbon storage capacity. Traditionally, increases in woody cover and density have been ascribed to changes in the disturbance regime (fire and herbivores) or rainfall. Increased atmospheric CO₂ concentrations may also contribute, by increasing growth rates of trees relative to grasses. This hypothesis is still heavily debated because usually potential CO₂ effects are confounded by changes in land use (disturbance regime). Here we analyse changes in woody density in fire experiments at three sites in South African savannas where the disturbance regime (fire and herbivores) was kept constant for 30 and 50 years. If global drivers had significant effects on woody plants, we would expect significant increases in tree densities and biomass over time under the constant disturbance regime. Woody density remained constant in a semiarid savanna but tripled in a mesic savanna between the 1970s and 1990s. At the third site, a semiarid savanna near the southern limits of the biome, tree density doubled from the mid 1990s to 2010. Interpretation of the causes is confounded by population recovery after clearing, but aerial photograph analysis on adjacent non‐cleared areas showed an accompanying 48% increase in woody cover. Increased CO₂ concentrations are consistent with increased woody density while other global drivers (rainfall) remained constant over the duration of the experiments. The absence of a response in one semiarid savanna could be explained by a smaller carbon sink capacity of the dominant species, which would therefore benefit less from increased CO₂. Understanding how savannas and grasslands respond to increased CO₂ and identifying the causes of woody encroachment are essential for the successful management of these systems.     

Sixteen hundred years of increasing tree cover prior to modern deforestation in Southern Amazon and Central Brazilian savannas

Tropical ecosystems are under increasing pressure from land‐use change and deforestation. Changes in tropical forest cover are expected to affect carbon and water cycling with important implications for climatic stability at global scales. A major roadblock for predicting how tropical deforestation affects climate is the lack of baseline conditions (i.e., prior to human disturbance) of forest–savanna dynamics. To address this limitation, we developed a long‐term analysis of forest and savanna distribution across the Amazon–Cerrado transition of central Brazil. We used soil organic carbon isotope ratios as a proxy for changes in woody vegetation cover over time in response to fluctuations in precipitation inferred from speleothem oxygen and strontium stable isotope records. Based on stable isotope signatures and radiocarbon activity of organic matter in soil profiles, we quantified the magnitude and direction of changes in forest and savanna ecosystem cover. Using changes in tree cover measured in 83 different locations for forests and savannas, we developed interpolation maps to assess the coherence of regional changes in vegetation. Our analysis reveals a broad pattern of woody vegetation expansion into savannas and densification within forests and savannas for at least the past ~1,600 years. The rates of vegetation change varied significantly among sampling locations possibly due to variation in local environmental factors that constrain primary productivity. The few instances in which tree cover declined (7.7% of all sampled profiles) were associated with savannas under dry conditions. Our results suggest a regional increase in moisture and expansion of woody vegetation prior to modern deforestation, which could help inform conservation and management efforts for climate change mitigation. We discuss the possible mechanisms driving forest expansion and densification of savannas directly (i.e., increasing precipitation) and indirectly (e.g., decreasing disturbance) and suggest future research directions that have the potential to improve climate and ecosystem models.     

Rates of woody encroachment in African savannas reflect water constraints and fire disturbance

Aim

The aims of this study were to (1) estimate current rates of woody encroachment across African savannas; (2) identify relationships between change in woody cover and potential drivers, including water constraints, fire frequency and livestock density. The found relationships led us to pursue a third goal: (3) use temporal dynamics in woody cover to estimate potential woody cover.

Location

Sub‐Saharan African savannas.

Methods

The study used very high spatial resolution satellite imagery at sites with overlapping older (2002–2006) and newer (2011–2016) imagery to estimate change in woody cover. We sampled 596 sites in 38 separate areas across African savannas. Areas with high anthropogenic impact were avoided in order to more clearly identify the influence of environmental factors. Relationships between woody cover change and potential drivers were identified using linear regression and simultaneous autoregression, where the latter accounts for spatial autocorrelation.

Results

The mean annual change in woody cover across our study areas was 0.25% per year. Although we cannot explain the general trend of encroachment based on our data, we found that change rates were positively correlated with the difference between potential woody cover and actual woody cover (a proxy for water availability; p < .001), and negatively correlated with fire frequency (p < .01). Using the relationship between rates of encroachment and initial cover, we estimated potential woody cover at different rainfall levels.

Main conclusions

The results indicate that woody encroachment is ongoing and widespread across African savannas. The fact that the difference between potential and actual cover was the most significant predictor highlights the central role of water availability and tree–tree competition in controlling change in woody populations, both in water‐limited and mesic savannas. Our approach to derive potential woody cover from the woody cover change trajectories demonstrates that temporal dynamics in woody populations can be used to infer resource limitations.     

Origin, fate and significance of CO2 in tree stems

Although some CO₂ released by respiring cells in tree stems diffuses directly to the atmosphere, on a daily basis 15–55% can remain within the tree. High concentrations of CO₂ build up in stems because of barriers to diffusion in the inner bark and xylem. In contrast with atmospheric [CO₂] of c.  0.04%, the [CO₂] in tree stems is often between 3 and 10%, and sometimes exceeds 20%. The [CO₂] in stems varies diurnally and seasonally. Some respired CO₂ remaining in the stem dissolves in xylem sap and is transported toward the leaves. A portion can be fixed by photosynthetic cells in woody tissues, and a portion diffuses out of the stem into the atmosphere remote from the site of origin. It is now evident that measurements of CO₂ efflux to the atmosphere, which have been commonly used to estimate the rate of woody tissue respiration, do not adequately account for the internal fluxes of CO₂. New approaches to quantify both internal and external fluxes of CO₂ have been developed to estimate the rate of woody tissue respiration. A more complete assessment of internal fluxes of CO₂ in stems will improve our understanding of the carbon balance of trees.     

Non-stomatal limitation of CO2 assimilation in three tree species during natural drought conditions

The effect of drought on CO₂ assimilation and leaf conductance was studied in three northern hardwood species: Quercus rubra L., Acer rubrum L. and Populus grandidentata Michx. Leaf gas exchange characteristics at two CO₂ levels (320 and 620 μl I⁻¹) and temperatures from 20 to 35°C were measured at the end of a dry period and shortly after 10 cm of rainfall. The effects of drought varied with species, temperature and CO₂ level. Calculated values of internal CO₂ concentration showed little or no decline during drought. Differences in assimilation, before vs after the rains, were most apparent at the higher CO₂ level. These latter two observations indicate nonstomatal disruption of CO₂ assimilation during the dry period. In P. grandidentata there was a substantial interaction between drought and temperature, with a resultant shift in the temperature for maximum assimilation to lower temperatures during drought. During drought, internal CO₂ concentrations increased sharply in all three species under the combined conditions of high temperatures and the higher CO₂ level.     

Cascading biodiversity and functional consequences of a global change–induced biome switch

Aim At a regional scale, across southern Africa, woody thickening of savannas is becoming increasingly widespread. Using coupled vegetation and faunal responses (ants), we explore whether major changes in woody cover in savannas represent an increase in the density of savanna trees (C₄ grass layer remains intact) or a ‘regime shift’ in system state from savanna to thicket (=dry forest) where broad‐leaved, forest‐associated trees shade out C₄ grasses. Location Hluhluwe Game Reserve, South Africa. Methods We sampled paired open (low woody cover) and closed (high cover that have undergone an increase in tree density) sites. Vegetation was sampled using belt transects, and a combination of pitfall trapping and Winkler sampling was used for ants. Results Closed habitats did not simply contain a higher density of woody savanna species, but differed significantly in structure, functional composition (high prevalence of broad‐leaved trees, discontinuous C₄ grasses) and system properties (e.g. low flammability). Ant assemblage composition reflected this difference in habitat. The trophic structure of ant assemblages in the two habitats revealed a functional shift with much higher abundances of predatory species in the closed habitat. Main conclusions The predominance of species with forest‐associated traits and concomitant reduction of C₄ grasses in closed sites indicate that vegetation has undergone a shift in fundamental system state (to thicket), rather than simply savanna thickening. This biome shift has cascading functional consequences and implications for biodiversity conservation. The potential loss of many specialist savanna plant species is especially concerning, given the spatial extent and speed of this vegetation switch. Although it is not clear how easily the habitat switch can be reversed and how stable the thicket habitats are, it is likely in the not‐too‐distant future that conservation managers will be forced to make decisions on whether to actively maintain savannas.     

Woody cover in African savannas: the role of resources, fire and herbivory

Aim To determine the functional relationships between, and the relative importance of, different driver variables (mean annual precipitation, soil properties, fire and herbivory) in regulating woody plant cover across broad environmental gradients in African savannas. Location Savanna grasslands of East, West and Southern Africa. Methods The dependence of woody cover on mean annual precipitation (MAP), soil properties (texture, nitrogen mineralization potential and total phosphorus), fire regimes, and herbivory (grazer, browser + mixed feeder, and elephant biomass) was determined for 161 savanna sites across Africa using stochastic gradient boosting, a refinement of the regression tree analysis technique. Results All variables were significant predictors of woody cover, collectively explaining 71% of the variance in our data set. However, their relative importance as regulators of woody cover varied. MAP was the most important predictor, followed by fire return periods, soil characteristics and herbivory regimes. Woody cover showed a strong positive dependence on MAP between 200 and 700 mm, but no dependence on MAP above this threshold when the effects of other predictors were accounted for. Fires served to reduce woody cover below rainfall‐determined levels. Woody cover showed a complex, non‐linear relationship with total soil phosphorus, and was negatively correlated with clay content. There was a strong negative dependence of woody cover on soil nitrogen (N) availability, suggesting that increased N‐deposition may cause shifts in savannas towards more grassy states. Elephants, mixed feeders and browsers had negative effects on woody cover. Grazers, on the other hand, depressed woody cover at low biomass, but favoured woody vegetation when their biomass exceeded a certain threshold. Main conclusions Our results indicate complex and contrasting relationships between woody cover, rainfall, soil properties and disturbance regimes in savannas, and suggest that future environmental changes such as altered precipitation regimes, N‐enrichment and elevated levels of CO₂ are likely to have opposing, and potentially interacting, influences on the tree–grass balance in savannas.     

A continental-scale analysis of tree cover in African savannas

Aim We present a continental‐scale analysis that explores the processes controlling woody community structure in tropical savannas. We analyse how biotic and abiotic factors interact to promote and modify tree cover, examine alternative ecological hypotheses and quantify disturbance effects using satellite estimates of tree cover. Location African savannas. Methods Tree cover is represented as a resource‐driven potential cover related to rainfall and soil characteristics perturbed by natural and human factors such as fire, cattle grazing, human population and cultivation. Within this framework our approach combines semi‐empirical modelling and information theory to identify the best models. Results Woody community structure across African savannas is best represented by a sigmoidal response of tree cover to mean annual precipitation (MAP), with a dependency on soil texture, which is modified by the separate effects of fire, domestic livestock, human population density and cultivation intensity. This model explains c. 66% of the variance in tree cover and appears consistent across the savanna regions of Africa. Main conclusions The analysis provides a new understanding of the importance and interaction of environmental and disturbance factors that create the broad spatial patterns of tree cover observed in African savannas. Woody cover increases with rainfall, but is modified by disturbances. These ‘perturbation’ effects depend on MAP regimes: in arid savannas (MAP < 400 mm) they are generally small (< 1% decrease in cover), while in semi‐arid and mesic savannas (400–1600 mm), perturbations result in an average 2% (400 mm) to 23% (1600 mm) decrease in cover; fire frequency and human population have more influence than cattle, and cultivation appears, on average, to lead to small increases in woody cover. Wet savannas (1600–2200 mm) are controlled by perturbations that inhibit canopy closure and reduce tree cover by, on average, 24–34%. Full understanding of the processes determining savanna structure requires consideration of resource limitation and disturbance dynamics.     

Effects of elevated CO2 concentrations on photosynthesis, growth and reproduction of branches of the tropical canopy tree species, Luehea seemannii Tr. & Planch.

Mature trees have already experienced substantial increases in CO₂ concentrations during their lifetimes, and will experience continuing increases in the future. Small open-top chambers were used to enclose branchlets that were at a height of between 20 and 25 m in the canopy of the tree species Luehea seemannii Tr. & Planch. in a tropical forest in Panamá. Elevated concentrations of CO₂ increased the rate of photosynthetic carbon fixation and decreased stomatal conductance of leaves, but did not influence the growth of leaf area per chamber, the production of flower buds and fruit nor the concentration of nonstructural carbohydrates within leaves. The production of flower buds was highly correlated with the leaf area produced in the second flush of leaves, indicating that the branchlets of mature trees of Luehea seemannii are autonomous to a considerable extent. Elevated levels of CO₂ did increase the concentration of nonstructural carbohydrates in woody stem tissue. Elevated CO₂ concentration also they increased the ratio of leaf area to total biomass of branchlets, and tended to reduce individual fruit weight. These data suggest that the biomass allocation patterns of mature trees may change under future elevated levels of CO₂. Although there were no effects on growth during the experiment, the possibility of increased growth in the season following CO₂ enrichment due to increased carbohydrate concentrations in woody tissue cannot be excluded.     

Trade‐offs between savanna woody plant diversity and carbon storage in the Brazilian Cerrado

Incentivizing carbon storage can be a win‐win pathway to conserving biodiversity and mitigating climate change. In savannas, however, the situation is more complex. Promoting carbon storage through woody encroachment may reduce plant diversity of savanna endemics, even as the diversity of encroaching forest species increases. This trade‐off has important implications for the management of biodiversity and carbon in savanna habitats, but has rarely been evaluated empirically. We quantified the nature of carbon‐diversity relationships in the Brazilian Cerrado by analyzing how woody plant species richness changed with carbon storage in 206 sites across the 2.2 million km² region at two spatial scales. We show that total woody plant species diversity increases with carbon storage, as expected, but that the richness of endemic savanna woody plant species declines with carbon storage both at the local scale, as woody biomass accumulates within plots, and at the landscape scale, as forest replaces savanna. The sharpest trade‐offs between carbon storage and savanna diversity occurred at the early stages of carbon accumulation at the local scale but the final stages of forest encroachment at the landscape scale. Furthermore, the loss of savanna species quickens in the final stages of forest encroachment, and beyond a point, savanna species losses outpace forest species gains with increasing carbon accumulation. Our results suggest that although woody encroachment in savanna ecosystems may provide substantial carbon benefits, it comes at the rapidly accruing cost of woody plant species adapted to the open savanna environment. Moreover, the dependence of carbon‐diversity trade‐offs on the amount of savanna area remaining requires land managers to carefully consider local conditions. Widespread woody encroachment in both Australian and African savannas and grasslands may present similar threats to biodiversity.     

Seventeen years of carbon dioxide enrichment of sour orange trees: final results

The long-term responses of trees to elevated CO₂ are especially crucial (1) to mitigating the rate of atmospheric CO₂ increase, (2) to determining the character of future forested natural ecosystems and their spread across the landscape, and (3) to determining the productivity of future agricultural tree crops. Therefore, a long-term CO₂-enrichment experiment on sour orange trees was started in 1987, and the final results after 17 years are reported herein. Four sour orange trees ( Citrus aurantium L.) were grown from seedling stage at 300 μmol mol⁻¹ CO₂ above ambient in open-top, clear-plastic-wall chambers at Phoenix, AZ. Four control trees were similarly grown at ambient CO₂. All trees were supplied ample water and nutrients comparable with a commercial orchard. After a peak 2–4 years into the experiment, there was a productivity plateau at about a 70% enhancement of annual fruit and incremental wood production over the last several years of the experiment. When summed over the duration of the experiment, there was an overall enhancement of 70% of total biomass production. Much of the enhancement came from greater numbers of fruits produced, with no change in fruit size. Thicker trunks and branches and more branches and roots were produced, but the root/shoot ratio was unaffected. Also, there was almost no change in the elemental composition of the biomass produced, perhaps in part due to the minimal responsiveness of root-symbiotic arbuscular mycorrhizal fungi to the treatment.     

Stocks and dynamics of carbon in trees across a rainfall gradient in a tropical savanna

In this study, systematic variation in tree morphology across a rainfall gradient in Australia's tropical savanna biome and its implications for carbon stocks and dynamics were quantified. The aim was to support efforts to manage fire regimes to increase vegetative carbon stocks as a greenhouse gas mitigation strategy. The height of trees for a given trunk diameter declines with decreasing rainfall from 2000 to 300 mm and increasing dry season length across the Australian savanna biome. It is likely that increasing dry season length is the main driver of this decline rather declining rainfall per se. By taking account of the response of total basal area to rainfall and soil type, stand structure, and tree height and diameter relationships, the carbon stocks in live trees were estimated to decline from about 34 t ha^?1 in the wetter savannas to 6 t ha^?1 in the drier savannas. These values are broadly consistent with field‐based estimates. Because of the declining ratio of height to trunk diameter, trees of a given diameter in drier regions will be more likely to be killed by fires of a given intensity than trees in wetter regions. Thus single fires of given intensity are likely to have a greater proportionate impact on live tree carbon stock in drier savannas, but a much greater absolute impact in wetter savannas due to the greater total carbon stock. Projected decreases in early wet season rainfall under climate change scenarios, despite projections of little change in total precipitation in northern Australia, may lead to decreased carbon stock in live trees through two mechanisms: a reduction in total basal area and decreases in tree height for given trunk diameters.     

Respiration in a future, higher-CO2 world

Abstract. Apart from its impact on global warming, the annually increasing atmospheric [CO₂] is of interest to plant scientists primarily because of its direct influence on photosynthesis and photorespiration in C₃ species. But in addition, 'dark' respiration, another major component of the carbon budget of higher plants, may be affected by a change in [CO₂] independent of an increase in temperature. Literature pertaining to an impact of [CO₂] on respiration rate is reviewed. With an increase in [CO₂], respiration rate is increased in some cases, but decreased in others. The effects of [CO₂] on respiration rate may be direct or indirect. Mechanisms responsible for various observations are proposed. These proposed mechanisms relate to changes in: (1) levels of nonstructural carbohydrates, (2) growth rate and structural phytomass accumulation, (3) composition of phytomass, (4) direct chemical interactions between CO₂ and respiratory enzymes, (5) direct chemical interactions between CO₂ and other cellular components, (6) dark CO₂ fixation rate, and (7) ethylene biosynthesis rate. Because a range-of (possibly interactive) effects exist, and present knowledge is limited, the impact of future [CO₂] on respiration rate cannot be predicted. Theoretical considerations and types of experiments that can lead to an increase in the understanding of this issue are outlined.     

Contrasting effects of climate and CO2 on Amazonian ecosystems since the last glacial maximum

The nature of Amazonian ecosystem responses to the large-scale environmental changes characterizing glacial–interglacial cycles is poorly understood. We investigated this issue with a series of transient, continuous 21 000-year simulations using a dynamic process-based ecosystem model. Our results indicate that the Amazon Basin has been dominated by evergreen rain forests since the last glacial maximum (LGM), demonstrating the resilience of this ecosystem to glacial–interglacial environmental change. We find that biome shifts in ecotonal areas since the LGM were driven predominantly by climate change, while coincident, increased ecosystem carbon storage throughout the Amazon Basin was driven largely by CO₂. Our findings imply that recent observed biomass increases in contemporary rain forest plots might be part of a long-term trend driven by the anthropogenic rise in CO₂ over recent centuries.     

Stomata of trees growing in CO2-enriched air show reduced sensitivity to vapour pressure deficit and drought

Stomatal conductance ( g _s) and photosynthetic rate ( A ) were measured in young beech ( Fagus sylvatica ), chestnut ( Castanea sativa ) and oak ( Quercus robur ) growing in ambient or CO₂-enriched air. In oak, g _s was consistently reduced in elevated CO₂. However, in beech and chestnut, the stomata of trees growing in elevated CO₂ failed to close normally in response to increased leaf-to-air vapour pressure deficit (LAVPD). Consequently, while g _s was reduced in elevated CO₂ on days with low LAVPD, on warm sunny days (with correspondingly high LAVPD) g _s was unchanged or even slightly higher in elevated CO₂. Furthermore, during drought, g _s of beech and chestnut was unresponsive to [CO₂], over a wide range of ambient LAVPD, whereas in oak g _s was reduced by an average of 50% in elevated CO₂. Stimulation of A by elevated CO₂ in beech and chestnut was restricted to days with high irradiance, and was greatest in beech during drought. Hence, most of the additional carbon gain in elevated CO₂ was made at the expense of water economy, at precisely those times (drought, high evaporative demand) when water conservation was most important. Such effects could have serious consequences for drought tolerance, growth and, ultimately, survival as atmospheric [CO₂] increases.     

A patch-dynamics approach to savanna dynamics and woody plant encroachment – Insights from an arid savanna

The coexistence of woody and grassy plants in savannas has often been attributed to a rooting-niche separation (two-layer hypothesis). Water was assumed to be the limiting resource for both growth forms and grasses were assumed to extract water from the upper soil layer and trees and bushes from the lower layers. Woody plant encroachment (i.e. an increase in density of woody plants often unpalatable to domestic livestock) is a serious problem in many savannas and is believed to be the result of overgrazing in ‘two-layer systems’. Recent research has questioned the universality of both the two-layer hypothesis and the hypothesis that overgrazing is the cause of woody plant encroachment.

We present an alternative hypothesis explaining both tree–grass coexistence and woody plant encroachment in arid savannas. We propose that woody plant encroachment is part of a cyclical succession between open savanna and woody dominance and is driven by two factors: rainfall that is highly variable in space and time, and inter-tree competition. In this case, savanna landscapes are composed of many patches (a few hectares in size) in different states of transition between grassy and woody dominance, i.e. we hypothesize that arid savannas are patch-dynamic systems. We summarize patterns of tree distribution observed in an arid savanna in Namibia and show that these patterns are in agreement with the patch-dynamic savanna hypothesis. We discuss the applicability of this hypothesis to fire-dominated savannas, in which rainfall variability is low and fire drives spatial heterogeneity.

We conclude that field studies are more likely to contribute to a general understanding of tree–grass coexistence and woody plant encroachment if they consider both primary (rain and nutrients) and secondary (fire and grazing) determinants of patch properties across different savannas.     

Drought-induced shifts in daily CO2 uptake patterns for leafy cacti

For cacti with persistent, relatively large leaves, most shoot CO₂ uptake under well-watered conditions occurs by the leaves using the C₃ pathway. For three species in the primitive subfamily Pereskioideae, droughts of 7 or 14 days decreased leaf daytime net CO₂ uptake by an average of 49 and 88%, respectively; these species always had a net CO₂ release at night by the leaves and both at night and during the day by the stems. For three leafy species in subfamily Opuntioideae, 7 and 14 days of drought reduced leaf daytime net CO₂ uptake by 90 and 100%, respectively. Although drought reduced the total CO₂ uptake over 24 h, the average percentage occurring at night by the leaves of these species increased from 5% under wet conditions to 71% after 7 days of drought to 99% after 14 days of drought. For two of the three species of Opuntioideae, 7 days of drought caused the small net CO₂ uptake by the sterns to shift from the daytime to the nighttime, while for the third species drought caused a reduction of its stem nocturnal net CO₂ uptake. Thus, shifts from predominantly daytime to predominantly nighttime net CO₂ uptake can be induced by drought for the leaves and the stems of leafy cacti in subfamily Opuntioideae, indicating a high degree of biochemical versatility.     

Coppicing shifts CO2 stimulation of poplar productivity to above-ground pools: a synthesis of leaf to stand level results from the POP/EUROFACE experiment

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 CO₂ 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 CO₂ concentrations ([CO₂]). Increased biomass yield resulted from an early growth enhancement and photosynthesis did not acclimate to elevated [CO₂]. 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 [CO₂] was mainly invested into woody biomass pools. Coppicing increased yield by 66% and partly shifted the extra C uptake in elevated [CO₂] to above-ground pools, as fine root biomass declined and its [CO₂] stimulation disappeared. Mineral soil C increased equally in ambient and elevated [CO₂] during the 6 yr experiment. However, elevated [CO₂] increased the stabilization of C in the mineral soil. Increased productivity of a poplar SRC in elevated [CO₂] may allow shorter rotation cycles, enhancing the viability of SRC for biofuel production.     

Rice responses to drought under carbon dioxide enrichment. 1. Growth and yield

Projections of future climate change include a strong likelihood of a doubling of current atmospheric carbon dioxide concentration ([ CO ₂]) and possible shifts in precipitation patterns. Drought stress is a major environmental limitation for crop growth and yield and is common in rainfed rice production systems. This study was conducted to determine the growth and grain yield responses of rice to drought stress under [CO₂] enrichment. Rice (cv. IR-72) was grown to maturity in eight naturally sunlit, plant growth chambers in atmospheric carbon dioxide concentrations [CO₂] of 350 and 700 μmol CO₂ mol^–1 air. In both [CO₂], water management treatments included continuously flooded (CF) controls, flood water removed and drought stress imposed at panicle initiation (PI), anthesis (ANT), and both panicle initiation and anthesis (PI & ANT). The [CO₂] enrichment increased growth, panicles plant^–1 and grain yield. Drought accelerated leaf senescence, reduced leaf area and above-ground biomass and delayed crop ontogeny. The [CO₂] enrichment allowed 1–2 days more growth during drought stress cycles. Grain yields of the PI and PI & ANT droughts were similar to the CF control treatments while the ANT drought treatment sharply reduced growth, grain yield and individual grain mass. We conclude that in the absence of air temperature increases, future global increases in [CO₂] should promote rice growth and yield while providing a modest reduction of near 10% in water use and so increase drought avoidance.