Forest water use and water use efficiency at elevated CO2: a model‐data intercomparison at two contrasting temperate forest FACE sites

Predicted responses of transpiration to elevated atmospheric CO₂ concentration (eCO₂) are highly variable amongst process‐based models. To better understand and constrain this variability amongst models, we conducted an intercomparison of 11 ecosystem models applied to data from two forest free‐air CO₂ enrichment (FACE) experiments at Duke University and Oak Ridge National Laboratory. We analysed model structures to identify the key underlying assumptions causing differences in model predictions of transpiration and canopy water use efficiency. We then compared the models against data to identify model assumptions that are incorrect or are large sources of uncertainty. We found that model‐to‐model and model‐to‐observations differences resulted from four key sets of assumptions, namely (i) the nature of the stomatal response to elevated CO₂ (coupling between photosynthesis and stomata was supported by the data); (ii) the roles of the leaf and atmospheric boundary layer (models which assumed multiple conductance terms in series predicted more decoupled fluxes than observed at the broadleaf site); (iii) the treatment of canopy interception (large intermodel variability, 2–15%); and (iv) the impact of soil moisture stress (process uncertainty in how models limit carbon and water fluxes during moisture stress). Overall, model predictions of the CO₂ effect on WUE were reasonable (intermodel μ = approximately 28% ± 10%) compared to the observations (μ = approximately 30% ± 13%) at the well‐coupled coniferous site (Duke), but poor (intermodel μ = approximately 24% ± 6%; observations μ = approximately 38% ± 7%) at the broadleaf site (Oak Ridge). The study yields a framework for analysing and interpreting model predictions of transpiration responses to eCO₂, and highlights key improvements to these types of models.     

Soil respiration is stimulated by elevated CO2 and reduced by summer drought: three years of measurements in a multifactor ecosystem manipulation experiment in a temperate heathland (CLIMAITE)

This study investigated the impact of predicted future climatic and atmospheric conditions on soil respiration (R_S) in a Danish Calluna‐Deschampsia‐heathland. A fully factorial in situ experiment with treatments of elevated atmospheric CO₂ (+130 ppm), raised soil temperature (+0.4 °C) and extended summer drought (5–8% precipitation exclusion) was established in 2005. The average R_S, observed in the control over 3 years of measurements (1.7 μmol CO₂ m^?2 sec^?1), increased 38% under elevated CO₂, irrespective of combination with the drought or temperature treatments. In contrast, extended summer drought decreased R_S by 14%, while elevated soil temperature did not affect R_S overall. A significant interaction between elevated temperature and drought resulted in further reduction of R_S when these treatments were combined. A detailed analysis of short‐term R_S dynamics associated with drought periods showed that R_S was reduced by ~50% and was strongly correlated with soil moisture during these events. Recovery of R_S to pre‐drought levels occurred within 2 weeks of rewetting; however, unexpected drought effects were observed several months after summer drought treatment in 2 of the 3 years, possibly due to reduced plant growth or changes in soil water holding capacity. An empirical model that predicts R_S from soil temperature, soil moisture and plant biomass was developed and accounted for 55% of the observed variability in R_S. The model predicted annual sums of R_S in 2006 and 2007, in the control, were 672 and 719 g C m^?2 y^?1, respectively. For the full treatment combination, i.e. the future climate scenario, the model predicted that soil respiratory C losses would increase by ~21% (140–150 g C m^?2 y^?1). Therefore, in the future climate, stimulation of C storage in plant biomass and litter must be in excess of 21% for this ecosystem to not suffer a reduction in net ecosystem exchange.     

Long‐term response of a Mojave Desert winter annual plant community to a whole‐ecosystem atmospheric CO2 manipulation (FACE)

Desert annuals are a critically important component of desert communities and may be particularly responsive to increasing atmospheric (CO₂) because of their high potential growth rates and flexible phenology. During the 10‐year life of the Nevada Desert FACE (free‐air CO₂ enrichment) Facility, we evaluated the productivity, reproductive allocation, and community structure of annuals in response to long‐term elevated (CO₂) exposure. The dominant forb and grass species exhibited accelerated phenology, increased size, and higher reproduction at elevated (CO₂) in a wet El Niño year near the beginning of the experiment. However, a multiyear dry cycle resulted in no increases in productivity or reproductive allocation for the remainder of the experiment. At the community level, early indications of increased dominance of the invasive Bromus rubens at elevated (CO₂) gave way to an absence of Bromus in the community during a drought cycle, with a resurgence late in the experiment in response to higher rainfall and a corresponding high density of Bromus in a final soil seed bank analysis, particularly at elevated (CO₂). This long‐term experiment resulted in two primary conclusions: (i) elevated (CO₂) does not increase productivity of annuals in most years; and (ii) relative stimulation of invasive grasses will likely depend on future precipitation, with a wetter climate favoring invasive grasses but currently predicted greater aridity favoring native dicots.     

Sustained effects of atmospheric [CO2] and nitrogen availability on forest soil CO2 efflux

Soil CO₂ efflux (F_soil) is the largest source of carbon from forests and reflects primary productivity as well as how carbon is allocated within forest ecosystems. Through early stages of stand development, both elevated [CO₂] and availability of soil nitrogen (N; sum of mineralization, deposition, and fixation) have been shown to increase gross primary productivity, but the long‐term effects of these factors on F_soil are less clear. Expanding on previous studies at the Duke Free‐Air CO₂ Enrichment (FACE) site, we quantified the effects of elevated [CO₂] and N fertilization on F_soil using daily measurements from automated chambers over 10 years. Consistent with previous results, compared to ambient unfertilized plots, annual F_soil increased under elevated [CO₂] (ca. 17%) and decreased with N (ca. 21%). N fertilization under elevated [CO₂] reduced F_soil to values similar to untreated plots. Over the study period, base respiration rates increased with leaf productivity, but declined after productivity saturated. Despite treatment‐induced differences in aboveground biomass, soil temperature and water content were similar among treatments. Interannually, low soil water content decreased annual F_soil from potential values – estimated based on temperature alone assuming nonlimiting soil water content – by ca. 0.7% per 1.0% reduction in relative extractable water. This effect was only slightly ameliorated by elevated [CO₂]. Variability in soil N availability among plots accounted for the spatial variability in F_soil, showing a decrease of ca. 114 g C m^?2 yr^?1 per 1 g m^?2 increase in soil N availability, with consistently higher F_soil in elevated [CO₂] plots ca. 127 g C per 100 ppm [CO₂] over the +200 ppm enrichment. Altogether, reflecting increased belowground carbon partitioning in response to greater plant nutritional needs, the effects of elevated [CO₂] and N fertilization on F_soil in this stand are sustained beyond the early stages of stand development and through stabilization of annual foliage production.     

Divergent terrestrial responses of soil N2O emissions to different levels of elevated CO2 and temperature

Understanding the responses of soil nitrous oxide (N₂O) emissions from terrestrial ecosystems to future CO₂ enrichment and warming is critical for the development of mitigation and adaptation policies. The effects of continuous increase in elevated CO₂ (EC) and elevated temperature (ET) on N₂O emissions are not fully known. We synthesized 209 measurements from 70 published studies and carried out a meta-analysis to examine individual and interactive effects of EC and ET on N₂O emissions from grasslands, croplands and forests. On average, a significant increase of 23% in N₂O emissions was observed under EC across all case studies. EC did not affect N₂O emissions from grasslands or forests, but significantly increased N₂O emissions in croplands by 38%. The extent of ET effects on N₂O emissions was nonsignificant and there was no significant difference in N₂O emission responses among these three terrestrial systems. ET only promoted N₂O emissions in forest by about 32% when ET was less than 2°C. The interactive effect of EC and ET on N₂O emissions was significantly synergistic, showing a greater increase than the sum of the effects caused by EC and ET alone. Our findings indicated that the combination of EC and ET substantially promoted soil N₂O and highlighted the urgent need to explore its mechanisms to better understand N₂O responses under future climate change.     

Flowering phenology in a species-rich temperate grassland is sensitive to warming but not elevated CO2

* Flowering is a critical stage in plant life cycles, and changes might alter processes at the species, community and ecosystem levels. Therefore, likely flowering-time responses to global change drivers are needed for predictions of global change impacts on natural and managed ecosystems. * Here, the impact of elevated atmospheric CO2 concentration ([CO2]) (550 micromol mol(-1)) and warming (+2 masculineC) is reported on flowering times in a native, species-rich, temperate grassland in Tasmania, Australia in both 2004 and 2005. * Elevated [CO2] did not affect average time of first flowering in either year, only affecting three out of 23 species. Warming reduced time to first flowering by an average of 19.1 d in 2004, acting on most species, but did not significantly alter flowering time in 2005, which might be related to the timing of rainfall. Elevated [CO2] and warming treatments did not interact on flowering time. * These results show elevated [CO2] did not alter average flowering time or duration in this grassland; neither did it alter the response to warming. Therefore, flowering phenology appears insensitive to increasing [CO2] in this ecosystem, although the response to warming varies between years but can be strong.     

No growth stimulation by CO2 enrichment in alpine glacier forefield plants

Since 1850, glaciers in the European Alps have lost around 40% of their original area, releasing bare forefields, which are colonized by alpine pioneer species, setting the scene for later successional stages. These expanding pioneer communities are likely less restricted by resources and competition than late‐successional systems. We thus hypothesized that rising atmospheric CO₂ concentration will enhance plant growth in these high‐elevation communities. Nine characteristic, perennial glacier forefield species were assembled in microcosms and grown at a nearby experimental site in the Swiss Alps (2440 m a.s.l.). The communities were exposed to an elevated CO₂ concentration of 580 ppm by free‐air CO₂ enrichment for three seasons. Four study species were additionally grown in isolation in containers, half of which received a low dose of mineral fertilizer (25 kg N ha^‐1 a^‐1) in order to explore a potential nutrient limitation of the CO₂ response. Responses of growth dynamics and peak season biomass of the two graminoid species, four forbs and three cushion forming species were analysed by repeated nondestructive assessments and a final biomass harvest. After three seasons, none of the species were stimulated by elevated CO₂, irrespective of mineral nutrient addition, which by itself enhanced growth in the fertilized plants by +34% on average. Increased CO₂ concentration did not affect total (above‐ plus belowground) biomass but reduced aboveground biomass by ?35% across all species, even in the fast growing ones. This reduced aboveground biomass was associated with higher biomass partitioning to roots. Foliar nonstructural carbohydrate concentration increased and nitrogen concentration in leaves decreased under elevated CO₂. We observed downward adjustment of photosynthetic capacity by on average ?26% under long‐term exposure to 580 ppm CO₂ (assessed in graminoids only). Our results indicate that glacier forefield pioneers, growing under harsh climatic conditions are not carbon limited at current atmospheric CO₂ concentration.     

Interacting effects of atmospheric CO2 enrichment and solar radiation on growth of the aquatic fern Azolla filiculoides

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The influence of elevated atmospheric CO2 on fine root dynamics in an intact temperate forest

Root dynamics are important for plant, ecosystem and global carbon cycling. Changes in root dynamics caused by rising atmospheric CO₂ not only have the potential to moderate further CO₂ increases, but will likely affect forest function. We used FACE (Free‐Air CO₂ Enrichment) to expose three 30‐m diameter plots in a 13‐year‐old loblolly pine (Pinus taeda) forest to elevated (ambient + 200 µL L^?1) atmospheric CO₂. Three identical fully instrumented plots were implemented as controls (ambient air only). We quantified root dynamics from October 1998 to October 1999 using minirhizotrons. In spite of 16% greater root lengths and 24% more roots per minirhizotron tube, the effects of elevated atmospheric CO₂ on root lengths and numbers were not statistically significant. Similarly, production and mortality were also unaffected by the CO₂ treatment, even though annual root production and mortality were 26% and 46% greater in elevated compared to ambient CO₂ plots. Average diameters of live roots present at the shallowest soil depth were, however, significantly enhanced in CO₂‐enriched plots. Mortality decreased with increasing soil depth and the slopes of linear regression lines (mortality vs. depth) differed between elevated and ambient CO₂ treatments, reflecting the significant CO₂ by depth interaction. Relative root turnover (root flux/live root pool) was unchanged by exposure to elevated atmospheric CO₂. Results from this study suggest modest, if any, increases in ecosystem‐level root productivity in CO₂‐enriched environments.     

Responses to atmospheric CO2 concentrations in crop simulation models: a review of current simple and semicomplex representations and options for model development

Elevated atmospheric CO₂ concentrations ([CO₂]) cause direct changes in crop physiological processes (e.g. photosynthesis and stomatal conductance). To represent these CO₂ responses, commonly used crop simulation models have been amended, using simple and semicomplex representations of the processes involved. Yet, there is no standard approach to and often poor documentation of these developments. This study used a bottom‐up approach (starting with the APSIM framework as case study) to evaluate modelled responses in a consortium of commonly used crop models and illuminate whether variation in responses reflects true uncertainty in our understanding compared to arbitrary choices of model developers. Diversity in simulated CO₂ responses and limited validation were common among models, both within the APSIM framework and more generally. Whereas production responses show some consistency up to moderately high [CO₂] (around 700 ppm), transpiration and stomatal responses vary more widely in nature and magnitude (e.g. a decrease in stomatal conductance varying between 35% and 90% among models was found for [CO₂] doubling to 700 ppm). Most notably, nitrogen responses were found to be included in few crop models despite being commonly observed and critical for the simulation of photosynthetic acclimation, crop nutritional quality and carbon allocation. We suggest harmonization and consideration of more mechanistic concepts in particular subroutines, for example, for the simulation of N dynamics, as a way to improve our predictive understanding of CO₂ responses and capture secondary processes. Intercomparison studies could assist in this aim, provided that they go beyond simple output comparison and explicitly identify the representations and assumptions that are causal for intermodel differences. Additionally, validation and proper documentation of the representation of CO₂ responses within models should be prioritized.     

Trait variation in juvenile plants from the soil seed bank of temperate forests in relation to macro- and microclimate

Aim

The soil seed bank is a key component of the biodiversity of plant communities, but various aspects of its functioning in temperate forest ecosystems are still unknown. We here adopted a trait-based approach to investigate the effects of macro- and microclimatic gradients on the juvenile plant communities from the realized seed bank of two types of European temperate forest.

Location

Oak-dominated forests in Italy and Belgium.

Methods

We analysed the variation of key functional traits (plant height, leaf area, leaf dry weight, specific leaf area and leaf number) of juvenile plants from the realised soil seed bank in relation to elevation (from 0 to 800 m a.s.l.), forest type (thinned and unthinned forest) and distance to the forest edge. We translocated soil samples from the forest core to the edge (and vice versa) and from high- to low-elevation forests to test the effects of edge and warming respectively.

Results

Taller communities developed at the forest edge due to higher light availability and warmer temperatures. The translocation from the core to the edge did not significantly modify mean trait values. Instead, the shadier and cooler microclimate of the forest core reduced the mean leaf area, mean dry weight, height and leaf number in the communities realised from the edge soil. The translocation from high- to lowland forests led to increased values for all traits (except specific leaf area). Edge vs core trait variation was more driven by intraspecific variability, whereas the translocation from high- to low-elevation forests caused trait changes mostly due to species turnover.

Conclusions

Global warming might result in a functional shift of the understorey due to both an early filtering effect on the seedlings from soil seed banks and their adaptive trait adjustments to temperature increase. Furthermore, our study underpins the importance of edge vs core microclimate in driving the functional composition of the realised soil seed bank.     

Short‐term carbon cycling responses of a mature eucalypt woodland to gradual stepwise enrichment of atmospheric CO2 concentration

Projections of future climate are highly sensitive to uncertainties regarding carbon (C) uptake and storage by terrestrial ecosystems. The Eucalyptus Free‐Air CO₂ Enrichment (EucFACE) experiment was established to study the effects of elevated atmospheric CO₂ concentrations (eCO₂) on a native mature eucalypt woodland with low fertility soils in southeast Australia. In contrast to other FACE experiments, the concentration of CO₂ at EucFACE was increased gradually in steps above ambient (+0, 30, 60, 90, 120, and 150 ppm CO₂ above ambient of ~400 ppm), with each step lasting approximately 5 weeks. This provided a unique opportunity to study the short‐term (weeks to months) response of C cycle flux components to eCO₂ across a range of CO₂ concentrations in an intact ecosystem. Soil CO₂ efflux (i.e., soil respiration or R_soil) increased in response to initial enrichment (e.g., +30 and +60 ppm CO₂) but did not continue to increase as the CO₂ enrichment was stepped up to higher concentrations. Light‐saturated photosynthesis of canopy leaves (A_sat) also showed similar stimulation by elevated CO₂ at +60 ppm as at +150 ppm CO₂. The lack of significant effects of eCO₂ on soil moisture, microbial biomass, or activity suggests that the increase in R_soil likely reflected increased root and rhizosphere respiration rather than increased microbial decomposition of soil organic matter. This rapid increase in R_soil suggests that under eCO_2, additional photosynthate was produced, transported belowground, and respired. The consequences of this increased belowground activity and whether it is sustained through time in mature ecosystems under eCO₂ are a priority for future research.     

Using an ecosystem model linked to GCM-derived local weather scenarios to analyse effects of climate change and elevated CO2 on dry matter production and partitioning, and water use in temperate managed grasslands

Local effects of climate change (CC) and elevated CO₂ (2 × CO₂, 660 μmol mol^–1) on managed temperate grasslands were assessed by forcing a dynamic ecosystem model with weather scenarios. The aims of the study were to compare the relative importance of individual and combined effects of CC, 2 × CO₂, and photosynthetic acclimation, and to assess the importance of local site conditions. The model was driven by hourly means for temperature (T), precipitation (P), global radiation (G), vapour pressure (VP), and wind speed (U). Local climate scenarios were derived by statistical downscaling techniques from a 2 × CO₂ simulation with the General Circulation Model of the Canadian Climate Centre (CCC-GCMII). Simulations over 14 growing seasons to account for year-to-year variability of climate were carried out for a low, relatively dry site, and a high, more humid site. At both sites, shoot dry matter responded positively to 2 × CO₂ with the site at low elevation being more sensitive than the higher site. The effect of assumed changes in climate was negative at the lower, but positive at the higher site. Shoot dry matter was more sensitive to the effects of 2 × CO₂ than to CC. Both effects combined increased shoot dry matter by up to 20%. This was attributed to direct effects of 2 × CO₂ and increased T, and indirect stimulation via increased soil N availability. Biomass partitioning to roots increased with 2 × CO₂ but decreased with CC, while an intermediate response resulted from the combination. Leaf area index (LAI) increased under 2 × CO₂, but not enough to compensate fully for a decrease in leaf conductance. Under the 2 × CO₂ scenario evapotranspiration (ET) decreased, but increased under CC. Photosynthetic acclimation reduced the effect of 2 × CO₂ on shoot growth, but had little effect on ET. The seasonal water use efficiency (WUE) was improved under 2 × CO₂, and reduced under CC. With the combination of both factors, the change was small but still positive, especially at the high elevation site with more favourable soil water conditions. This reflects the stronger positive yield response in combination with a smaller increase in ET under cooler, more humid conditions. The results for the combination of factors suggest that except for shoot growth, effects of 2 × CO₂ and CC tend to offset each other. While CC determines the sign of the ET response, the sign of the biomass response is determined by 2 × CO₂. The results highlight the importance of a site-specific analysis of ecosystem responses by using a flexible approach based on a combination of state-of-the-art downscaling, spatially resolved data sets, and a mechanistic model to obtain quantitative and reproducible assessments of climate change impacts at the ecosystem level.     

Fate of the soil seed bank of giant ragweed and its significance in preventing and controlling its invasion in grasslands

Giant ragweed (Ambrosia trifida, L. henceforth referred to as GR), an annual non‐native invasive weed, may cause health problems and can reduce agricultural productivity. Chemical control of GR in grasslands may have irreversible side effects on herbs and livestock. In an attempt to propose a solution to the harmful effects of GR on grasslands, this study explores the fate of its soil seed bank (SSB) and considers the physical control of its SSB reduction. By studying GR distributed in grasslands of the Yili Valley, Xinjiang, China, we measured the spatial and temporal changes in seed density, seed germination, dormancy, and death. We analyzed seed germination, dormancy, and death following different storage periods. The study analyzed population characteristics over time, including seed fate, and examined physical control methods for reducing the SSB density. The SSB of GR occurs in the upper 0–15 cm of soil in grasslands. Seed density in the SSB decreased by 68.1% to 82.01% from the reproductive growth period to the senescence period. More than 98.7% of the seeds were rotten, eaten, germinated, dispersed, or died within one year after being produced. The seed germination rate of the SSB decreased with the number of years after invasion. When stored for 0.5 or 3.5 years, seed germination rates fell by 40%, during which time seed death rate increased by almost 40%. When GR was completely eradicated for two consecutive years, the SSB and population densities decreased by >99%. The vast majority of GR seeds germinated or died within one year; the germination rate decreased significantly if the seeds were stored dry at room temperature for a long time. Newly produced seeds are the main source of seeds in the SSB. Therefore, thoroughly eradicating GR plants for several years before the seeds can mature provides an effective control method in grasslands.     

Atmospheric CO2 enrichment and drought stress modify root exudation of barley

Rising CO₂ concentrations associated with drought stress is likely to influence not only aboveground growth, but also belowground plant processes. Little is known about root exudation being influenced by elements of climate change. Therefore, this study wanted to clarify whether barley root exudation responds to drought and CO₂ enrichment and whether this reaction differs between an old and a recently released malting barley cultivar. Barley plants were grown in pots filled with sand in controlled climate chambers at ambient (380 ppm) or elevated (550 ppm) atmospheric [CO₂] and a normal or reduced water supply. Root exudation patterns were examined at the stem elongation growth stage and when the inflorescences emerged. At both dates, root exudates were analyzed for different compounds such as total free amino acids, proline, potassium, and some phytohormones. Elevated [CO₂] decreased the concentrations in root exudates of some compounds such as total free amino acids, proline, and abscisic acid. Moreover, reduced water supply increased proline, potassium, electric conductivity, and hormone concentrations. In general, the modern cultivar showed higher concentrations of proline and abscisic acid than the old one, but the cultivars responded differentially under elevated CO₂. Plant developmental stage had also an impact on the root exudation patterns of barley. Generally, we observed significant effects of CO₂ enrichment, watering levels, and, to a lesser extent, cultivar on root exudation. However, we did not find any mitigation of the adverse effects of drought by elevated CO₂. Understanding the multitude of relationships within the rhizosphere is an important aspect that has to be taken into consideration in the context of crop performance and carbon balance under conditions of climate change.