Photosynthesis, carboxylation and leaf nitrogen responses of 16 species to elevated pCO 2 across four free-air CO 2 enrichment experiments in forest, grassland and desert

The magnitude of changes in carboxylation capacity in dominant plant species under long-term elevated CO 2 exposure (elevated pC a ) directly impacts ecosystem CO 2 assimilation from the atmosphere. We analyzed field CO 2 response curves of 16 C 3 species of different plant growth forms in favorable growth conditions in four free-air CO 2 enrichment (FACE) experiments in a pine and deciduous forest, a grassland and a desert. Among species and across herb, tree and shrub growth forms there were significant enhancements in CO 2 assimilation ( A ) by +40±5% in elevated pC a (49.5–57.1 Pa), although there were also significant reductions in photosynthetic capacity in elevated pC a in some species. Photosynthesis at a common pC a ( A a ) was significantly reduced in five species growing under elevated pC a , while leaf carboxylation capacity ( V cmax ) was significantly reduced by elevated pC a in seven species (change of −19±3% among these species) across different growth forms and FACE sites. Adjustments in V cmax with elevated pC a were associated with changes in leaf N among species, and occurred in species with the highest leaf N. Elevated pC a treatment did not affect the mass-based relationships between A or V cmax and N, which differed among herbs, trees and shrubs. Thus, effects of elevated pC a on leaf C assimilation and carboxylation capacity occurred largely through changes in leaf N, rather than through elevated pC a effects on the relationships themselves. Maintenance of leaf carboxylation capacity among species in elevated pC a at these sites depends on maintenance of canopy N stocks, with leaf N depletion associated with photosynthetic capacity adjustments. Since CO 2 responses can only be measured experimentally on a small number of species, understanding elevated CO 2 effects on canopy N m and N a will greatly contribute to an ability to model responses of leaf photosynthesis to atmospheric CO 2 in different species and plant growth forms.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/72832/1/j.1365-2486.2004.00867.x.pd

Maintenance of leaf N controls the photosynthetic CO 2 response of grassland species exposed to 9 years of free-air CO 2 enrichment

Determining underlying physiological patterns governing plant productivity and diversity in grasslands are critical to evaluate species responses to future environmental conditions of elevated CO 2 and nitrogen (N) deposition. In a 9-year experiment, N was added to monocultures of seven C 3 grassland species exposed to elevated atmospheric CO 2 (560 μmol CO 2  mol −1 ) to evaluate how N addition affects CO 2 responsiveness in species of contrasting functional groups. Functional groups differed in their responses to elevated CO 2 and N treatments. Forb species exhibited strong down-regulation of leaf N mass concentrations (−26%) and photosynthetic capacity (−28%) in response to elevated CO 2 , especially at high N supply, whereas C 3 grasses did not. Hence, achieved photosynthetic performance was markedly enhanced for C 3 grasses (+68%) in elevated CO 2 , but not significantly for forbs. Differences in access to soil resources between forbs and grasses may distinguish their responses to elevated CO 2 and N addition. Forbs had lesser root biomass, a lower distribution of biomass to roots, and lower specific root length than grasses. Maintenance of leaf N, possibly through increased root foraging in this nutrient-poor grassland, was necessary to sustain stimulation of photosynthesis under long-term elevated CO 2 . Dilution of leaf N and associated photosynthetic down-regulation in forbs under elevated [CO 2 ], relative to the C 3 grasses, illustrates the potential for shifts in species composition and diversity in grassland ecosystems that have significant forb and grass components.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/78679/1/j.1365-2486.2009.02058.x.pd

Sources of increased N uptake in forest trees growing under elevated CO 2 : results of a large‐scale 15 N study

Nitrogen availability in terrestrial ecosystems strongly influences plant productivity and nutrient cycling in response to increasing atmospheric carbon dioxide ( CO 2 ). Elevated CO 2 has consistently stimulated forest productivity at the Duke Forest free‐air CO 2 enrichment experiment throughout the decade‐long experiment. It remains unclear how the N cycle has changed with elevated CO 2 to support this increased productivity. Using natural‐abundance measures of N isotopes together with an ecosystem‐scale 15 N tracer experiment, we quantified the cycling of 15 N in plant and soil pools under ambient and elevated CO 2 over three growing seasons to determine how elevated CO 2 changed N cycling between plants, soil, and microorganisms. After measuring natural‐abundance 15 N differences in ambient and CO 2 ‐fumigated plots, we applied inorganic 15 N tracers and quantified the redistribution of 15 N for three subsequent growing seasons. The natural abundance of leaf litter was enriched under elevated compared to ambient CO 2 , consistent with deeper rooting and enhanced N mineralization. After tracer application, 15 N was initially retained in the organic and mineral soil horizons. Recovery of 15 N in plant biomass was 3.5 ± 0.5% in the canopy, 1.7 ± 0.2% in roots and 1.7 ± 0.2% in branches. After two growing seasons, 15 N recoveries in biomass and soil pools were not significantly different between CO 2 treatments, despite greater total N uptake under elevated CO 2 . After the third growing season, 15 N recovery in trees was significantly higher in elevated compared to ambient CO 2 . Natural‐abundance 15 N and tracer results, taken together, suggest that trees growing under elevated CO 2 acquired additional soil N resources to support increased plant growth. Our study provides an integrated understanding of elevated CO 2 effects on N cycling in the Duke Forest and provides a basis for inferring how C and N cycling in this forest may respond to elevated CO 2 beyond the decadal time scale.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87073/1/gcb2465.pd

Complexity in water and carbon dioxide fluxes following rain pulses in an African savanna

The idea that many processes in arid and semi-arid ecosystems are dormant until activated by a pulse of rainfall, and then decay from a maximum rate as the soil dries, is widely used as a conceptual and mathematical model, but has rarely been evaluated with data. This paper examines soil water, evapotranspiration (ET), and net ecosystem CO2 exchange measured for 5 years at an eddy covariance tower sited in an Acacia–Combretum savanna near Skukuza in the Kruger National Park, South Africa. The analysis characterizes ecosystem flux responses to discrete rain events and evaluates the skill of increasingly complex “pulse models”. Rainfall pulses exert strong control over ecosystem-scale water and CO2 fluxes at this site, but the simplest pulse models do a poor job of characterizing the dynamics of the response. Successful models need to include the time lag between the wetting event and the process peak, which differ for evaporation, photosynthesis and respiration. Adding further complexity, the time lag depends on the prior duration and degree of water stress. ET response is well characterized by a linear function of potential ET and a logistic function of profile-total soil water content, with remaining seasonal variation correlating with vegetation phenological dynamics (leaf area). A 1- to 3-day lag to maximal ET following wetting is a source of hysteresis in the ET response to soil water. Respiration responds to wetting within days, while photosynthesis takes a week or longer to reach its peak if the rainfall was preceded by a long dry spell. Both processes exhibit nonlinear functional responses that vary seasonally. We conclude that a more mechanistic approach than simple pulse modeling is needed to represent daily ecosystem C processes in semiarid savannas

Nitrate deposition in northern hardwood forests and the nitrogen metabolism of Acer saccharum marsh

It is generally assumed that plant assimilation constitutes the major sink for anthropogenic Nitrate NO 3 − deposited in temperate forests because plant growth is usually limited by nitrogen (N) availability. Nevertheless, plants are known to vary widely in their capacity for NO 3 − uptake and assimilation, and few studies have directly measured these parameters for overstory trees. Using a combination of field and greenhouse experiments, we studied the N nutrition of Acer saccharum Marsh. in four northern hardwood forests receiving experimental NO 3 − additions equivalent to 30 kg N ha −1 year −1 . We measured leaf and fine-root nitrate reductase activity (NRA) of overstory trees using an in vivo assay and used 15 N to determine the kinetic parameters of NO 3 − uptake by excised fine roots. In two greenhouse experiments, we measured leaf and root NRA in A. saccharum seedlings fertilized with 0–3.5 g NO 3 − −N m −2 and determined the kinetic parameters of NO 3 − and NH 4 + uptake in excised roots of seedlings. In both overstory trees and seedlings, rates of leaf and fine root NRA were substantially lower than previously reported rates for most woody plants and showed no response to NO 3 − fertilization (range = non-detectable to 33 nmol NO 2 − g −1 h −1 ). Maximal rates of NO 3 − uptake in overstory trees also were low, ranging from 0.2 to 1.0 μmol g −1 h −1 . In seedlings, the mean V max for NO 3 − uptake in fine roots (1 μmol g −1 h −1 ) was approximately 30 times lower than the V max for NH 4 + uptake (33 μmol g −1 h −1 ). Our results suggest that A. saccharum satisfies its N demand through rapid NH 4 + uptake and may have a limited capacity to serve as a direct sink for atmospheric additions of NO 3 − .Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47695/1/442_2004_Article_BF00334659.pd

Food, Nutrition and Agrobiodiversity Under Global Climate Change

Available evidence and predictions suggest overall negative effects on agricultural production as a result of climate change, especially when more food is required by a growing population. Information on the effects of global warming on pests and pathogens affecting agricultural crops is limited, though crop–pest models could offer means to predict changes in pest dynamics, and help design sound plant health management practices. Host-plant resistance should continue to receive high priority as global warming may favor emergence of new pest epidemics. There is increased risk, due to climate change, to food and feed contaminated by mycotoxin-producing fungi. Mycotoxin biosynthesis gene-specific microarray is being used to identify food-born fungi and associated mycotoxins, and investigate the influence of environmental parameters and their interactions for control of mycotoxin in food crops. Some crop wild relatives are threatened plant species and efforts should be made for their in situ conservation to ensure evolution of new variants, which may contribute to addressing new challenges to agricultural production. There should be more emphasis on germplasm enhancement to develop intermediate products with specific characteristics to support plant breeding. Abiotic stress response is routinely dissected to component physiological traits. Use of transgene(s) has led to the development of transgenic events, which could provide enhanced adaptation to abiotic stresses that are exacerbated by climate change. Global warming is also associated with declining nutritional quality of food crops. Micronutrient-dense cultivars have been released in selected areas of the developing world, while various nutritionally enhanced lines are in the release pipeline. The high-throughput phenomic platforms are allowing researchers to accurately measure plant growth and development, analyze nutritional traits, and assess response to stresses on large sets of individuals. Analogs for tomorrow’s agriculture offer a virtual natural laboratory to innovate and test technological options to develop climate resilience production systems. Increased use of agrobiodiversity is crucial to coping with adverse impacts of global warming on food and feed production and quality. No one solution will suffice to adapt to climate change and its variability. Suits of technological innovations, including climate-resilient crop cultivars, will be needed to feed 9 billion people who will be living in the Earth by the middle of the twenty-first century