Simulated Atmospheric No3− Deposition Increases Soil Organic Matter By Slowing Decomposition

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/117243/1/eap20081882016.pd

Nitrogen turnover in the leaf litter and fine roots of sugar maple

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/117199/1/ecy201091123456.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 ( 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

Nitrogen and phosphorus constrain the CO2 fertilization of global plant biomass

Unidad de excelencia María de Maeztu MdM-2015-0552Elevated CO2 (eCO2) experiments provide critical information to quantify the effects of rising CO2 on vegetation. Many eCO2 experiments suggest that nutrient limitations modulate the local magnitude of the eCO2 effect on plant biomass but the global extent of these limitations has not been empirically quantified, complicating projections of the capacity of plants to take up CO2. Here, we present a data-driven global quantification of the eCO2 effect on biomass based on 138 eCO2 experiments. The strength of CO2 fertilization is primarily driven by nitrogen (N) in ~65% of global vegetation and by phosphorus (P) in ~25% of global vegetation, with N- or P-limitation modulated by mycorrhizal association. Our approach suggests that CO2 levels expected by 2100 can potentially enhance plant biomass by 12 ± 3% above current values, equivalent to 59 ± 13 PgC. The global-scale response to eCO2 we derive from experiments is similar to past changes in greenness and biomass10 with rising CO2, suggesting that CO2 will continue to stimulate plant biomass in the future despite the constraining effect of soil nutrients. Our research reconciles conflicting evidence on CO2 fertilization across scales and provides an empirical estimate of the biomass sensitivity to eCO2 that may help to constrain climate projections

A dynamic leaf gas-exchange strategy is conserved in woody plants under changing ambient CO2: evidence from carbon isotope discrimination in paleo and CO2 enrichment studies

Rising atmospheric [CO2 ], ca , is expected to affect stomatal regulation of leaf gas-exchange of woody plants, thus influencing energy fluxes as well as carbon (C), water and nutrient cycling of forests. Researchers have proposed various strategies for stomatal regulation of leaf gas-exchange that include maintaining a constant leaf internal [CO2 ], ci , a constant drawdown in CO2 (ca - ci ), and a constant ci /ca . These strategies can result in drastically different consequences for leaf gas-exchange. The accuracy of Earth systems models depends in part on assumptions about generalizable patterns in leaf gas-exchange responses to varying ca . The concept of optimal stomatal behavior, exemplified by woody plants shifting along a continuum of these strategies, provides a unifying framework for understanding leaf gas-exchange responses to ca . To assess leaf gas-exchange regulation strategies, we analyzed patterns in ci inferred from studies reporting C stable isotope ratios (δ(13) C) or photosynthetic discrimination (∆) in woody angiosperms and gymnosperms that grew across a range of ca spanning at least 100 ppm. Our results suggest that much of the ca -induced changes in ci /ca occurred across ca spanning 200 to 400 ppm. These patterns imply that ca - ci will eventually approach a constant level at high ca because assimilation rates will reach a maximum and stomatal conductance of each species should be constrained to some minimum level. These analyses are not consistent with canalization towards any single strategy, particularly maintaining a constant ci . Rather, the results are consistent with the existence of a broadly conserved pattern of stomatal optimization in woody angiosperms and gymnosperms. This results in trees being profligate water users at low ca , when additional water loss is small for each unit of C gain, and increasingly water-conservative at high ca , when photosystems are saturated and water loss is large for each unit C gain. This article is protected by copyright. All rights reserved.Rising atmospheric [CO2], c(a), is expected to affect stomatal regulation of leaf gas-exchange of woody plants, thus influencing energy fluxes as well as carbon (C), water, and nutrient cycling of forests. Researchers have proposed various strategies for stomatal regulation of leaf gas-exchange that include maintaining a constant leaf internal [CO2], c(i), a constant drawdown in CO2 (c(a)-c(i)), and a constant c(i)/c(a). These strategies can result in drastically different consequences for leaf gas-exchange. The accuracy of Earth systems models depends in part on assumptions about generalizable patterns in leaf gas-exchange responses to varying c(a). The concept of optimal stomatal behavior, exemplified by woody plants shifting along a continuum of these strategies, provides a unifying framework for understanding leaf gas-exchange responses to c(a). To assess leaf gas-exchange regulation strategies, we analyzed patterns in c(i) inferred from studies reporting C stable isotope ratios (C-13) or photosynthetic discrimination () in woody angiosperms and gymnosperms that grew across a range of c(a) spanning at least 100ppm. Our results suggest that much of the c(a)-induced changes in c(i)/c(a) occurred across c(a) spanning 200 to 400ppm. These patterns imply that c(a)-c(i) will eventually approach a constant level at high c(a) because assimilation rates will reach a maximum and stomatal conductance of each species should be constrained to some minimum level. These analyses are not consistent with canalization toward any single strategy, particularly maintaining a constant c(i). Rather, the results are consistent with the existence of a broadly conserved pattern of stomatal optimization in woody angiosperms and gymnosperms. This results in trees being profligate water users at low c(a), when additional water loss is small for each unit of C gain, and increasingly water-conservative at high c(a), when photosystems are saturated and water loss is large for each unit C gain

Long-term global change effects on forest biogeochemistry in the north-central United States

Human activities have substantially altered the composition of the atmosphere and many of these changes directly affect the biogeochemistry of forest ecosystems. Because of the geography of industrialization, these impacts are particularly acute in northern temperate forests. Unfortunately, most studies examining the effects of altered atmospheric composition on forest ecosystems may not be accurate predictors of the long-term impacts on mature forests because these studies used immature trees and were short in duration. Here, I use measurements from two large long-term collaborative experiments to examine the impacts of altered atmospheric composition on forest biogeochemistry in the north-central United States. At the Rhinelander free-air carbon dioxide (CO2) enrichment experiment in Wisconsin, I examined the independent and interactive effects of increased concentrations of atmospheric CO2 and tropospheric ozone (O3) on leaf production and soil carbon (C) storage in three forest communities. To estimate leaf production, litter traps were used to collect fallen leaves from 2002 to 2008 (years five through eleven of the experiment). In addition to leaf production (g m-2), these collections were used to assess leaf area (m2 m2 ), leaf litter nitrogen (N) concentration (mg g-1), and the leaf N content (g N m-2). On average, the factorial elevated CO2 effect (+CO2 and +CO2+O3) stimulated leaf production by 36% and the factorial elevated O3 effect (+O3 and +CO2+O3) decreased leaf production by 18%. Similar effects were observed for leaf area. However, the relative effects of the individual treatments were highly dynamic. From 2002 to 2008, the positive effect of the elevated CO2 treatment (+CO2) on leaf production relative to the ii ambient treatment decreased from +52% to +25%, while the negative effect of the elevated O3 treatment (+O3) relative to ambient changed from -5% to -18%. The CO2 and O3 treatments did not have significant overall effects on litter N concentrations. Consequently, the leaf litter N content (g m-2) was increased 30% by the elevated CO2 treatments and decreased 16% by the elevated O3 treatments. To estimate changes in soil C pools, the top 20 cm of the mineral soil was sampled seven times between 1998 and 2008. Despite an increase in the input of leaf and root litter by elevated CO2 and a decrease in litter inputs by elevated O3, there were no significant effects of CO2 and O3 on soil C storage for the overall experiment. However, within the forest community containing only aspen (Populus tremuloides), there was significantly less soil C (-17.4 Mg ha-1) beneath forests receiving the elevated CO2 treatments (+CO2 and +CO2+O3) in the 2008 samples. In addition, I was able to use the unique 13C signature of fumigation CO2 to trace the input of new C into the soil in the elevated CO2 treatments (+CO2 and +CO2+O3). Initially, soils from the +CO2+O3 treatment had less new C than soils from the +CO2 treatment, but this difference gradually disappeared. This gradual disappearance matched trends in fine root production. Combining the leaf production study with the soil C study, these results suggest that the rate of soil C cycling accelerated under elevated CO2 and declined under elevated O3 because changes in soil C accumulation did not match changes in litter production. The other long-term experiment tests the influence of atmospheric deposition on four mature northern hardwood forests spread across 500 km in northern Michigan. These four forests sit along a north to south gradient, with warmer temperatures and higher iii inputs of both acid deposition and N deposition at the southern end of the gradient. These sites were established in 1987 to examine the impacts of atmospheric deposition along this gradient, but a parallel experiment was established at the same four sites to simulate potential increases in N deposition. I utilized both aspects of this experimental design, using the existing deposition gradient to examine the ongoing effects of atmospheric deposition and using the N addition experiment to test the long-term influence of added N on leaf-level photosynthesis. Since these sites were established in 1987, there have been major changes in federal emissions regulations. These new regulations greatly restricted emissions of acid deposition precursors, but did not attempt to control overall N deposition. In the time since this policy was enacted, there have been remarkable changes in the impacts of acid deposition and N deposition on the biogeochemistry of these four forests. Using data only from the plots receiving ambient deposition, I found that there have been decreases in leaf sulfur, calcium, and aluminum concentrations over the past two decades. Acid deposition usually increases concentrations of these elements in soil solution, so the observed changes in leaf chemistry signal a waning influence of this pollutant. In comparison, leaf δ 15N and soil lysimeter data show that persistent ambient N deposition has caused widespread increases in both the availability of inorganic nitrogen and soil nitrate leaching. The declining influence of acid deposition shows that environmental policy can quickly and broadly influence forest biogeochemistry. Although there are large amounts of nitrate being leached from these forests as a result of ambient N deposition, the parallel N addition experiment at these same sites resulted in increased aboveground growth. Because of the key role of N in photosynthesis, conceptual models often attribute growth increases from increased N availability to iv higher photosynthesis. However, increases in leaf-level photosynthesis have not often been observed in long-term N addition experiments. We tested the effects of 14 years of N additions on photosynthesis in two ways: by making instantaneous measurement from both canopy towers and excised branches, and by analyzing leaf tissue for δ 13C and δ 18O, isotopes integrate changes in photosynthesis through time. Trees receiving N additions had higher foliar N concentrations, but there were no differences in instantaneous measurements of photosynthesis from canopy towers or excised branches. Further, there were no significant changes in δ 13C and δ 18O in either current foliage or leaf litter collected annually throughout the N addition experiment (1994-2007). Together, these data suggest that increases in photosynthesis are not responsible for the higher rates of aboveground growth. Together, these experiments show that changes in atmospheric composition expected to occur in the next century will alter the functioning of forest ecosystems in the northcentral United States. However, predictions from short-term experiments did not often match the results observed in these long-term projects. Alternately, the recovery of forests in the north-central United States from acid deposition suggests that forest biogeochemistry can respond positively if pollution reductions are prioritized by policy makers

Primary succession in sand dunes along the northeastern shore of Lake Michigan.

A study was conducted detailing the trends of succession at Sturgeon Bay, located in Wilderness State Park, Emmet County, Michigan. Data were gathered from 21 different plots located on 3 different transects on the first 7 dunes from the shore. It was found that in the fore-dunes, the most limiting factors to colonization were desiccation and sand movement due to wind. Light intensity was found to be higher in the fore-dunes (R2=.9344). Wind speed was faster in the fore-dunes compared to those farther back from the lake (R2= .9168). Sapling composition showed significant differences between the fore-dunes and the older dunes (Dunes 3-7: X2=80.66, d.f. 12, p< 0.05). Mature trees showed no significant difference in species composition, however, the calculated chi-squared value was very similar to the critical value suggesting a possible type I error (Dunes 4-7: X2=16.02, d.f = 9, p<O.05). In the later dunes, the most limiting factors were competition (mostly for light) and regeneration mutations on the species in the fore-dunes. This was shown by the decrease in vegetation density as the dunes matured. (R2 =.6605) Contrary to previous beliefs on succession, soil development is an effect rather than a driving factor. The soil horizons, pH, and moisture levels were correlated with dune age, showing the effects of succession. (Soil horizon: R2 = .8062, pH : R2= .939, and % Moisture: R2=.8661). The data suggest little correlation between soil nitrogen and phosphorous content and dune age. (Nitrogen: R2 = .3888, , Phosphorous: R2 = .125) This suggests that while soil nutrients increase growth rates, their presence does not facilitate succession.http://deepblue.lib.umich.edu/bitstream/2027.42/54973/1/3414.pdfDescription of 3414.pdf : Access restricted to on-site users at the U-M Biological Station