Elevated carbon dioxide and ozone alter productivity and ecosystem carbon content in northern temperate forests

Three young northern temperate forest communities in the north‐central United States were exposed to factorial combinations of elevated carbon dioxide ( CO 2 ) and tropospheric ozone (O 3 ) for 11 years. Here, we report results from an extensive sampling of plant biomass and soil conducted at the conclusion of the experiment that enabled us to estimate ecosystem carbon (C) content and cumulative net primary productivity ( NPP ). Elevated CO 2 enhanced ecosystem C content by 11%, whereas elevated O 3 decreased ecosystem C content by 9%. There was little variation in treatment effects on C content across communities and no meaningful interactions between CO 2 and O 3 . Treatment effects on ecosystem C content resulted primarily from changes in the near‐surface mineral soil and tree C, particularly differences in woody tissues. Excluding the mineral soil, cumulative NPP was a strong predictor of ecosystem C content ( r 2  = 0.96). Elevated CO 2 enhanced cumulative NPP by 39%, a consequence of a 28% increase in canopy nitrogen (N) content (g N m −2 ) and a 28% increase in N productivity ( NPP /canopy N). In contrast, elevated O 3 lowered NPP by 10% because of a 21% decrease in canopy N, but did not impact N productivity. Consequently, as the marginal impact of canopy N on NPP (∆ NPP /∆N) decreased through time with further canopy development, the O 3 effect on NPP dissipated. Within the mineral soil, there was less C in the top 0.1 m of soil under elevated O 3 and less soil C from 0.1 to 0.2 m in depth under elevated CO 2 . Overall, these results suggest that elevated CO 2 may create a sustained increase in NPP , whereas the long‐term effect of elevated O 3 on NPP will be smaller than expected. However, changes in soil C are not well‐understood and limit our ability to predict changes in ecosystem C content.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/108065/1/gcb12564.pd

Elevated carbon dioxide and ozone alter productivity and ecosystem carbon content in northern temperate forests

Three young northern temperate forest communities in the north-central United States were exposed to factorial combinations of elevated carbon dioxide (CO2) and tropospheric ozone (O3) for 11 years. Here, we report results from an extensive sampling of plant biomass and soil conducted at the conclusion of the experiment that enabled us to estimate ecosystem carbon (C) content and cumulative net primary productivity (NPP). Elevated CO2 enhanced ecosystem C content by 11%, whereas elevated O3 decreased ecosystem C content by 9%. There was little variation in treatment effects on C content across communities and no meaningful interactions between CO2 and O3. Treatment effects on ecosystem C content resulted primarily from changes in the near-surface mineral soil and tree C, particularly differences in woody tissues. Excluding the mineral soil, cumulative NPP was a strong predictor of ecosystem C content (r2 = 0.96). Elevated CO2 enhanced cumulative NPP by 39%, a consequence of a 28% increase in canopy nitrogen (N) content (g N m−2) and a 28% increase in N productivity (NPP/canopy N). In contrast, elevated O3 lowered NPP by 10% because of a 21% decrease in canopy N, but did not impact N productivity. Consequently, as the marginal impact of canopy N on NPP (ΔNPP/ΔN) decreased through time with further canopy development, the O3 effect on NPP dissipated. Within the mineral soil, there was less C in the top 0.1 m of soil under elevated O3 and less soil C from 0.1 to 0.2 m in depth under elevated CO2. Overall, these results suggest that elevated CO2 may create a sustained increase in NPP, whereas the long-term effect of elevated O3 on NPP will be smaller than expected. However, changes in soil C are not well-understood and limit our ability to predict changes in ecosystem C content

Phytoplankton productivity and growth rate kinetics in the Cedar River lakes

Lakes Findley, Chester Morse and Sammamish, Washington, are characterized by one major outburst of phytoplankton productivity and biomass (mainly diatoms) with usually no or low fall activity. Vernal outbursts were often delayed in the monomictic lakes by inadequate light because of unfavorable climate and/or a lack of thermal stratification. Strong inhibition by light (probably u.v.) was observed in Findley such that average maximum productivity occurred at 10% of surface intensity while maximum was customarily at 60% in the other lakes. Annual productivity was 369C/m2 in Findley, 479C/m2 in Chester Morse and 1989C/m2 in Sammamish. The range in mean chlorophyll a content was 0.8 to 10 ug/,for the same lakes respectively. Although more than three fourths of the productivity in the four lakes was contributed by nanoplankton (5-50u), a tendency for increased contribution from netplankton was observed with increasing trophic state. In vitro experiments during all parts of the growing season show that nitrogen (N) and phosphorus (P) were simultaneously limiting productivity increase in the three lakes. Growth rate kinetics experiments showed increasing half-saturation constants for P (0.17 to 2.8pgP/A) for the natural phytoplankton progressing from oligotrophy to eutrophy. Growth rate models using these parameters were evaluated in Findley Lake subsequent to iceout in 1973. The best agreement was obtained with a model using light (with a function that included inhibition) N and P in contrast to several other combinations of those variables. Light was the most important factor and adaptation problems to low experimental light necessitated increasing the maximum growth rate by a factor of 10 in order to obtain the best agreement with in situ growth rate

Phytoplankton productivity and response to altered nutrient content in lakes of contrasting trophic state

Lakes Findley, Chester Morse and Sammamish, Washington, are characterized by one major outburst of phytoplankton productivity and biomass (mainly diatoms) with usually no or low fall activity. Vernal outbursts were often delayed in some lakes and years probably by unfavorable climate (snow cover and cloudy rainy conditions). Mean spring-summer productivity ranged from 270 mgC/m² day in the most oligotrophic lake, Findley, to nearly 1000 mgC/m² day in mesotrophic-eutrophic Lake Sammamish. The range in mean Chlorophyll a content was 0.8 to 10 ug/l for the same lakes respectively. Mean biomass within and between the lakes was related to winter phosphorus content but not to nitrogen. However, nitrogen (N) and phosphorus (P) were simultaneously limiting productivity increase in the three lakes in summer. Carbon assimilation in response to added P showed increasing half-saturation constants for the natural phytoplankton progressing from oligotrophy to eutrophy. While diversion of over 1/2 the phosphorus from nearby Lake Washington during 1963-1967 was followed by reduction in winter mean P content and a rapid shift from eutrophy to mesotrophy (Edmondson 1970), mean winter P content and measured characteristics of plankton response have not changed significantly in Lake Sammamish following a diversion of similar magnitude. P availability in the water column (winter mean content) appears to be controlled by precipitation with Fe to a greater extent than in Lake Washington

Phytoplankton productivity and growth rate kinetics in the Cedar River Lakes

Sample collection to define the phytoplankton biomass and productivity in Lakes Findley, Chester Morse, and Sammamish was conducted according to the schedule In Figure 1. Methods and procedures are the same as those used in 1972 (Welch et al. 1972). The purpose of such sampling is to provide validation data of productivity, biomass and growth rate (P/B) of phytoplankton to evaluate the fit of models constructed from experimentally determined parameters. Also, productivity and biomass on a seasonal mean basis serve as a measure of lake response to the incoming nutrient supply rate

Tropospheric O 3 moderates responses of temperate hardwood forests to elevated CO 2 : a synthesis of molecular to ecosystem results from the Aspen FACE project

1.   The impacts of elevated atmospheric CO 2 and/or O 3 have been examined over 4 years using an open-air exposure system in an aggrading northern temperate forest containing two different functional groups (the indeterminate, pioneer, O 3 -sensitive species Trembling Aspen, Populus tremuloides and Paper Birch, Betula papyrifera , and the determinate, late successional, O 3 -tolerant species Sugar Maple, Acer saccharum ). 2.   The responses to these interacting greenhouse gases have been remarkably consistent in pure Aspen stands and in mixed Aspen/Birch and Aspen/Maple stands, from leaf to ecosystem level, for O 3 -tolerant as well as O 3 -sensitive genotypes and across various trophic levels. These two gases act in opposing ways, and even at low concentrations (1·5 × ambient, with ambient averaging 34–36 nL L −1 during the summer daylight hours), O 3 offsets or moderates the responses induced by elevated CO 2 . 3.   After 3 years of exposure to 560 µmol mol −1 CO 2 , the above-ground volume of Aspen stands was 40% above those grown at ambient CO 2 , and there was no indication of a diminishing growth trend. In contrast, O 3 at 1·5 × ambient completely offset the growth enhancement by CO 2 , both for O 3 -sensitive and O 3 -tolerant clones. Implications of this finding for carbon sequestration, plantations to reduce excess CO 2 , and global models of forest productivity and climate change are presented.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/72125/1/j.1365-2435.2003.00733.x.pd

Temporal perspective on acid deposition research

This statement presented to the Subcommittee on Natural Resources of the US House of Representatives gives a definition of acid rain, presents new data on the regional and temporal nature of the problem, and discusses research needs. (ACR

Biotic, abiotic and performance aspects of the Nevada Desert Free-Air CO2 Enrichment (FACE) Facility

Arid and semiarid climates comprise roughly 40% of the earth\u27s terrestrial surface. Deserts are predicted to be extremely responsive to global change because they are stressful environments where small absolute changes in water availability or use represent large proportional changes. Water and carbon dioxide fluxes are inherently coupled in plant growth. No documented global change has been more substantial or more rapid than the increase in atmospheric CO2. Free Air CO2 Enrichment (FACE) technology permits manipulation of CO2 in intact communities without altering factors such as light intensity or quality, humidity or wind. The Nevada Desert FACE Facility (NDFF) consists of three 491 m2 plots in the Mojave Desert receiving 550 μL L–1 CO2, and six ambient plots to assess both CO2 and fan effects. The shrub community was characterized as a Larrea–Ambrosia–Lycium species complex. Data are reported through 12 months of operation