Michigan Forest Research Articles

Observational and modeling studies of the impacts of agriculture-related land use change on planetary boundary layer processes in the central U.S.

The impact of agricultural land use change on atmospheric boundary layer processes, the associated feedbacks and their regional scale impacts, are examined with particular emphasis on the central United States. Specifically, the role of contrasting forested and agricultural land covers in the initiation and subsequent evolution of summertime cloud patterns in the U.S. Midwest; and the impact of agricultural practices, including irrigation, on the surface climate of the U.S. High Plains are discussed in detail. Satellite-based observational results of previous work summarized in this paper indicate that the timing and intensity of cloud development appears to be influenced by both synoptic flow regimes and agricultural land use type. For example, under conditions characterized by high pressure with surface winds generally less than 5 m s −1, peak cloud development occurred almost two hours earlier over cropland than over the forest or boundary locations in Michigan. Cloud masses were also considerably taller over cropland in the mid-afternoon than over forest and land cover transition zones. The modeling results discussed here for a model domain centered over Nebraska indicate significant differences in the surface energy fluxes between the irrigated (control) and non-irrigated (dry) simulations. Surface latent heat flux was higher by 36% and dewpoint temperature higher by 2.3 °C in the control simulation. Also, surface sensible heat flux of the control simulation was 15% less and the near-ground (2 m) temperature was 1.2 °C less compared to dry run, indicating irrigation-induced surface cooling effect. Recent investigations on crop–climate interactions in which crop and ecological models were coupled to regional climate models show that incorporating important perturbations such as prolonged droughts and the resulting changes in soil and plant nutrient conditions remains one of the biggest challenges in developing effective and realistic ecological-climate integrated modeling systems.

Effects of nitrogen supply and wood species on Tsuga canadensis and Betula alleghaniensis seedling growth on decaying wood

Eastern hemlock (Tsuga canadensis (L.) Carrière) and yellow birch (Betula alleghaniensis Britt.) in primary Michigan forests depend on decaying wood for seedling-establishment sites, but seedling densities vary across wood species (hemlock, yellow birch, and sugar maple (Acer saccharum Marsh.)). We collected seedlings and wood from a natural field experiment and conducted a companion greenhouse experiment to determine whether seedling mass and nitrogen (N) content varied with wood species and whether they were related to wood inorganic N supply. Yellow birch seedlings were largest on hemlock wood in the field (P = 0.003) and greenhouse (but P > 0.05), while hemlock seedling mass did not vary across wood species. N concentration and N mineralization rate varied by species (N concentration: hemlock < yellow birch < maple; N mineralization rate: hemlock > yellow birch ≈ maple), but neither seedling mass nor N content was significantly correlated with wood inorganic N supply. In the greenhouse, yellow birch seedlings responded to fertilization with N when growing on hemlock and maple but not yellow birch wood and appear to be limited by phosphorus when growing on yellow birch wood. We conclude that yellow birch seedling growth varies with wood species, and is limited by both N and phosphorus, while hemlock seedlings are unresponsive to variation in wood species during the first two growing seasons.

The Effect of Increased N Deposition on Nitrous oxide, Methane and Carbon dioxide Fluxes from Unmanaged Forest and Grassland Communities in Michigan

Atmospheric nitrogen deposition is anticipated to increase over the next decades with possible implications for future forest-atmosphere interactions. Increased soil N2O emissions, depressed CH4 uptake and depressed soil respiration CO2 loss is considered a likely response to increased N deposition. This study examined fluxes of N2O, CH4 and CO2 over two growing seasons from soils in unmanaged forest and grassland communities on abandoned agricultural areas in Michigan. All sites were subject to simulated increased N-deposition in the range of 1–3 g N m−2 annually. Nitrous oxide fluxes and soil N concentrations in coniferous and grassland sites were on the whole unaffected by the increased N-inputs. It is noteworthy though that N2O emissions increased three-fold in the coniferous sites in the first growing season in response to the low N treatment, although the response was barely significant (p<0.06). In deciduous forests, we observed increased levels of soil mineral N during the second year of N fertilization, however N2O fluxes did not increase. Rates of methane oxidation were similar in all sites with no affect of field N application. Likewise, we did not observe any changes in soil CO2 efflux in response to N additions. The combination of tillage history and vegetation type was important for the trace gas fluxes, i.e. soil CO2 efflux was greater in successional grassland sites compared with the forested sites and CH4 uptake was reduced in post-tillage coniferous- and successional sites compared with the old-growth deciduous site. Our results indicate that short-term increased N availability influenced individual processes linked to trace gas turnover in the soil independently from the ecosystem N status. However, changes in whole system fluxes were not evident and were very likely mediated by competitive N uptake processes.

Quantifying the seasonal and interannual variability of North American isoprene emissions using satellite observations of the formaldehyde column

Quantifying isoprene emissions using satellite observations of the formaldehyde (HCHO) columns is subject to errors involving the column retrieval and the assumed relationship between HCHO columns and isoprene emissions, taken here from the GEOS‐CHEM chemical transport model. Here we use a 6‐year (1996–2001) HCHO column data set from the Global Ozone Monitoring Experiment (GOME) satellite instrument to (1) quantify these errors, (2) evaluate GOME‐derived isoprene emissions with in situ flux measurements and a process‐based emission inventory (Model of Emissions of Gases and Aerosols from Nature, MEGAN), and (3) investigate the factors driving the seasonal and interannual variability of North American isoprene emissions. The error in the GOME HCHO column retrieval is estimated to be 40%. We use the Master Chemical Mechanism (MCM) to quantify the time‐dependent HCHO production from isoprene, α‐ and β‐pinenes, and methylbutenol and show that only emissions of isoprene are detectable by GOME. The time‐dependent HCHO yield from isoprene oxidation calculated by MCM is 20–30% larger than in GEOS‐CHEM. GOME‐derived isoprene fluxes track the observed seasonal variation of in situ measurements at a Michigan forest site with a −30% bias. The seasonal variation of North American isoprene emissions during 2001 inferred from GOME is similar to MEGAN, with GOME emissions typically 25% higher (lower) at the beginning (end) of the growing season. GOME and MEGAN both show a maximum over the southeastern United States, but they differ in the precise location. The observed interannual variability of this maximum is 20–30%, depending on month. The MEGAN isoprene emission dependence on surface air temperature explains 75% of the month‐to‐month variability in GOME‐derived isoprene emissions over the southeastern United States during May–September 1996–2001.

Sap flux–upscaled canopy transpiration, stomatal conductance, and water use efficiency in an old growth forest in the Great Lakes region of the United States

Combining sap flux and eddy covariance measurements provides a means to study plant stomatal conductance and the relationship between transpiration and photosynthesis. We measured sap flux using Granier‐type sensors in a northern hardwood‐dominated old growth forest in Michigan, upscaled to canopy transpiration, and calculated canopy conductance. We also measured carbon and water fluxes with the eddy covariance method and derived daytime gross primary production (GPP). The diurnal patterns of sap flux and canopy transpiration were mainly controlled by vapor pressure deficit (D) and photosynthetically active radiation (PAR). Daily sums of sap flux and canopy transpiration had exponential relationships to D that saturated at higher D and had linear relationships to PAR. Sugar maple (Acer saccharum) and yellow birch (Betula alleghaniesis) had higher sap flux per unit of sapwood area than eastern hemlock (Tsuga canadensis), while sugar maple and hemlock had higher canopy transpiration per unit of leaf area than yellow birch. Sugar maple dominated canopy transpiration per ground area. Canopy transpiration averaged 1.57 mm d−1, accounting for 65% of total evapotranspiration in the growing season. Canopy conductance was controlled by both D and PAR, but the day‐to‐day variation in canopy conductance mainly followed a negatively logarithmic relationship with D. By removing the influences of PAR, half‐hourly canopy conductance was also negatively logarithmically correlated with D. Water use efficiency (WUE) had a strong exponential relationship with D on a daily basis and approached a minimum of 4.4 mg g−1. WUE provides an alternative to estimate GPP from measurements of sap flux.

Effects of an introduced pathogen on resistance to natural disturbance: beech bark disease and windthrow

In forests of eastern North America, introduced pathogens have caused widespread declines in a number of important tree species, including dominant species such as American beech (Fagus grandifolia Ehrh.). Most studies have focused on changes in forest composition and structure as a direct result of mortality caused by a pathogen. Our field studies of windthrow resistance in forests of northern New York and northern Michigan demonstrate that resistance of beech trees to windthrow is severely reduced by beech bark disease (BBD). This reduced resistance was primarily due to the increase in the probability of stem breaks of moderately and highly infected beech trees. The severity of BBD infection on individual trees has a significant negative effect on resistance to windthrow. We tested potential consequences of this for long-term composition and structure in these forests by using a simulation model, SORTIE. We found that species such as yellow birch (Betula alleghaniensis Britt.) and eastern hemlock (Tsuga canadensis (L.) Carr.) increased in basal area primarily because of the effect BBD had on the creation of new seedbed substrates. Our results highlight the indirect effects that host-specific pathogens can have on community dynamics and species coexistence in forests.

Base-cation Cycling by Individual Tree Species in Old-growth Forests of Upper Michigan, USA

The influence of individual tree species on base-cation (Ca, Mg, K, Na) distribution and cycling was examined in sugar maple (Acer saccharum Marsh.), basswood (Tilia americana L.), and hemlock (Tsuga canadensis L.) in old-growth northern hardwood – hemlock forests on a sandy, mixed, frigid, Typic Haplorthod over two growing seasons in northwestern Michigan. Base cations in biomass, forest floor, and mineral soil (0–15 cm and 15–40 cm) pools were estimated for five replicated trees of each species; measured fluxes included bulk precipitation, throughfall, stemflow, litterfall, forest-floor leachate, mineralization + weathering, shallow-soil leachate, and deep-soil leachate. The three species differed in where base cations had accumulated within the single-tree ecosystems. Within these three single-tree ecosystems, the greatest quantity of base cations in woody biomass was found in sugar maple, whereas hemlock and basswood displayed the greatest amount in the upper 40 cm of mineral soil. Base-cation pools were ranked: sugar maple > basswood, hemlock in woody biomass; sugar maple, basswood > hemlock in foliage; hemlock > sugar maple, basswood in the forest floor, and basswood > sugar maple, hemlock in the mineral soil. Base-cation fluxes in throughfall, stemflow, the forest-floor leachate, and the deep-soil leachate (2000 only) were ranked: basswood > sugar maple > hemlock. Our measurements suggest that species-related differences in nutrient cycling are sufficient to produce significant differences in base-cation contents of the soil over short time intervals (<65 years). Moreover, these species-mediated differences may be important controls over the spatial pattern and edaphic processes of northern hardwood-hemlock ecosystems in the upper Great Lakes region.

A Cross-Sectional Analysis of Michigan Nonindustrial Private Forest Landowners

Abstract Incentive and assistance programs for nonindustrial private landowners in Michigan were evaluated for their effectiveness in encouraging forest management activities. This article reports on selected results of a comparative analysis of program enrollment, landowner characteristics, and management accomplishments. The analysis was based on data from a survey of 2,230 nonindustrial private forestland (NIPF) owners who were members of the Michigan Forest Association (MFA) or were enrolled in the Commercial Forest Program (CF), the Forest Stewardship Program (FSP), or the Two-Hearted River Watershed (TRW) landowner program in 2000. With a 55% overall response rate, landowners reported on present and past management activities and program enrollment. To address the question of the effectiveness of incentive programs, this comparative analysis tested the hypotheses that forest management activity reported by Michigan NIPF landowners who were enrolled in several types of incentive programs did not differ significantly by program and that management activity was not significantly explained by landowner demographics and parcel characteristics. Landowner program enrollment was compared with respect to tree-planting, timber harvesting, timber stand improvement, wildlife habitat improvement, and soil and water protection. To examine the differences, if any, between landowners who practice forest management and those who do not, explanatory demographic and parcel characteristic variables also were tested for their effect on management activity levels.North. J. Appl. For. 22(2):132–138.

Respiratory carbon losses and the carbon‐use efficiency of a northern hardwood forest, 1999–2003

Quantitative assessment of carbon (C) storage by forests requires an understanding of climatic controls over respiratory C loss. Ecosystem respiration can be estimated biometrically as the sum (R Sigma) of soil (Rs), leaf (Rl) and wood (Rw) respiration, and meteorologically by measuring above-canopy nocturnal CO2 fluxes (Fcn). Here we estimated R Sigma over 5 yr in a forest in Michigan, USA, and compared R Sigma and Fcn on turbulent nights. We also evaluated forest carbon-use efficiency (Ec = P(NP)/P(GP)) using biometric estimates of net primary production (P(NP)) and R Sigma and Fcn-derived estimates of gross primary production (P(GP)). Interannual variation in R Sigma was modest (142 g C m(-2) yr(-1)). Mean annual R Sigma was 1425 g C m(-2) yr(-1); 71% from Rs, 18% from Rl, and 11% from Rw. Hourly R Sigma was well correlated with Fcn, but 11 to 58% greater depending on the time of year. Greater R Sigma compared with Fcn resulted in higher estimated annual P(GP) and lower annual Ec (0.42 vs 0.54) using biometric and meteorological data, respectively. Our results provide one of the first multiyear estimates of R Sigma in a forested ecosystem, and document the responses of component respiratory C losses to major climatic drivers. They also provide the first assessment of Ec in a deciduous forest using independent estimates of P(GP).

Long‐term isoprene flux measurements above a northern hardwood forest

We report continuous whole canopy isoprene emission fluxes from a northern hardwood forest in Michigan for the 1999–2002 growing seasons. The eddy covariance fluxes of isoprene, CO2, latent heat, and sensible heat are presented along with an analysis of the seasonal and year‐to‐year variations. Measurements were made in collaboration with the AmeriFlux site located at the University of Michigan Biological Station (UMBS) and the Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET). In general, isoprene emissions increased throughout the day with increasing temperature and light levels, peaked at midafternoon, and declined to zero by night. There were significant variations from one 30‐min period to the next, and from one day to the next. Average midday isoprene fluxes were 2.8, 3.2, and 2.9 mg C m−2 h−1 for 2000 through 2002, respectively. Insufficient data were available to include 1999. Last frost and full leaf out were significantly later in 2002 compared to the other years; however, total accumulated isoprene emissions for each year varied by less than 10%. Fully developed isoprene emissions occurred between 400 and 500 heating degree days, roughly half those required at other sites. Using long‐term net ecosystem exchange measurements from the UMBS∼Flux group, isoprene emissions represent between 1.7 to 3.1% of the net carbon uptake at this site. Observations for 2000–2002 were compared with the BEIS3 emission model. Estimates agree well with observations during the midsummer period, but BEIS3 overestimates observations during the spring onset of emissions and the fall decline in emissions. This work provides a unique long‐term data set useful for verifying canopy scale models and to help us better understand the dynamics of biosphere‐atmosphere exchange of isoprene.

Host Breadth and Ovipositional Behavior of AdultPolydrusus sericeusandPhyllobius oblongus(Coleoptera: Curculionidae), Nonindigenous Inhabitants of Northern Hardwood Forests

Polydrusus sericeus (Schaller) and Phyllobius oblongus (L.) are nonindigenous root-feeding weevils in northern hardwood forests of Wisconsin and Michigan. Detailed studies of adult host range, tree species preferences, and effects of food source on fecundity and longevity have not been conducted in North America. P. sericeus and P. oblongus adults fed on leaves of all 11 deciduous tree species offered in no-choice assays, but amount of consumption varied among species. P. sericeus consumed more yellow birch (Betula alleghaniensis Britton), basswood (Tilia americana L.), and ironwood [Ostrya virginiana (Miller) K. Koch] than maple (Acer spp.). Conversely, P. oblongus consumed more ironwood than poplar (Populus spp.) and yellow birch, with maple being intermediate. Females ate 2.5 times as much as males. Mean frass production by P. sericeus was strongly correlated with foliage consumption among host tree species. In feeding choice assays, P. sericeus preferred yellow birch over ironwood, basswood, and aspen (Populus tremuloides Michaux). P. sericeus produced 29.93 ± 1.43 eggs/d when feeding on yellow birch compared with 2.04 ± 0.36 eggs/d on sugar maple (Acer saccharum Marshall). P. oblongus produced 4.32 ± 1.45 eggs/d when feeding on sugar maple compared with just 0.2 ± 0.1 eggs/d on yellow birch. Overall, total egg production for P. sericeus and P. oblongus averaged 830.1 ± 154.8 and 23.8 ± 11.8 eggs, respectively, when feeding on their optimal host plants. P. sericeus survived approximately five times longer in assays than P. oblongus, and it oviposited a total of 25.8 ± 4.0 d, whereas P. oblongus oviposited 1.9 ± 0.9 d. Egg size among P. sericeus and P. oblongus was 0.53 ± 0.008 by 0.32 ± 0.003 and 0.56 ± 0.005 by 0.29 ± 0.004 mm, respectively. In laboratory rearing of P. sericeus on yellow birch seedlings, 18% of the initial 500 larvae completed development to adults.

Aspen Harvest Intensity Decreases Microbial Biomass, Extracellular Enzyme Activity, and Soil Nitrogen Cycling

Forest harvesting alters plant litter inputs to soil and modifies the soil environment, which could alter the composition and function of soil microbial communities. Harvest-induced reductions in microbial activity could eventually feed back to modify soil N availability and forest productivity. We reasoned that increasing harvest intensity should decrease microbial community biomass and function via reduced litter input to soil. We further expected microbial communities to differ in response to harvest intensity in aspen-dominated (Populus tremuloides Michx. and P. grandidentata Michx.) forests located in climatically and edaphically different conditions. To test these ideas, we quantified microbial community composition and function 8 to 10 yr following harvest in two climatically and edaphically distinct aspen-dominated forests in Michigan. Harvest treatments included control (no harvest), merchantable bole harvest (MBH), total tree harvest (TTH), and total tree harvest + forest floor removal (FFR). Microbial community composition was quantified using phospholipid fatty acid (PLFA) analysis, and microbial community function was assayed using extracellular enzyme activity and 15 N pool dilution. All harvest methods reduced microbial biomass (-24%), the activity of extracellular enzymes involved with litter decomposition (-10 to -30%), gross N mineralization (-36%), and microbial N immobilization (-38%), regardless of climatic and edaphic differences between stand locations. Microbial community composition was not affected by harvest treatment, nor did it differ between locations. Lower rates of extracellular enzyme activity and gross N transformation in harvested aspen stands corresponded with a reduction in microbial biomass, which in turn may be driven by reduced litter input and changes in soil microclimate following clear-cut harvest.

Environmental controls on sap flow in a northern hardwood forest

Our objective was to gain a detailed understanding of how photosynthetically active radiation (PAR), vapor pressure deficit (D) and soil water interact to control transpiration in the dominant canopy species of a mixed hardwood forest in northern Lower Michigan. An improved understanding of how these environmental factors affect whole-tree water use in unmanaged ecosystems is necessary in assessing the consequences of climate change on the terrestrial water cycle. We used continuously heated sap flow sensors to measure transpiration in mature trees of four species during two successive drought events. The measurements were scaled to the stand level for comparison with eddy covariance estimates of ecosystem water flux (Fw). Photosynthetically active radiation and D together explained 82% of the daytime hourly variation in plot-level transpiration, and low soil water content generally resulted in increased stomatal sensitivity to increasing D. There were also species-specific responses to drought. Quercus rubra L. showed low water use during both dry and wet conditions, and during periods of high D. Among the study species, Acer rubrum L. showed the greatest degree of stomatal closure in response to low soil water availability. Moderate increases in stomatal sensitivity to D during dry periods were observed in Populus grandidentata Michx. and Betula papyrifera Marsh. Sap flow scaled to the plot level and Fw demonstrated similar temporal patterns of water loss suggesting that the mechanisms controlling sap flow of an individual tree also control ecosystem evapotranspiration. However, the absolute magnitude of scaled sap flow estimates was consistently lower than Fw. We conclude that species-specific responses to PAR, D and soil water content are key elements to understanding current and future water fluxes in this ecosystem.

Photosynthetic and Water Use Efficiency Responses to Diffuse Radiation by an Aspen-Dominated Northern Hardwood Forest

Abstract Clouds can exert strong effects on ecosystem CO2 and water vapor fluxes and may be important determinants of terrestrial primary production. We used three years of eddy-covariance and meteorological data from an aspen-dominated northern hardwood forest in Michigan to investigate how canopy photosynthesis (P), evapotranspiration (E), and water use efficiency (WUE) responded to changes in cloud cover, or the proportion of diffuse (If) to total (It) photosynthetically active radiation (PAR). Canopy quantum efficiency (the initial slope of the P versus PAR response curve) increased with increasing cloud cover, but both midday canopy P and canopy photosynthetic potential (the asymptote of the P versus PAR response curve) were greatest under partly cloudy skies (If/It = ∼0.57). Midday canopy E decreased and midday canopy WUE increased with increasing If/It. The relationship between canopy P and cloud cover varied with soil moisture. Canopy P was insensitive to changes in soil moisture under overcast skies (If/It &amp;gt; 0.70), whereas under clearer skies, canopy P was lower during periods of low compared to high soil moisture. These results further our understanding of cloud cover effects on canopy physiological processes and will aid in more accurate assessments of forest responses to climatic change. FOR. SCI. 50(6):793–801.

Gypsy Moth Defoliation of Oak Trees and a Positive Response of Red Maple and Black Cherry: An Example of Indirect Interaction

We calculated relative growth rates of all species over 10 cm circumference at breast height from 1997–1999, 1999–2001 and 2001–2003 in a 70 by 100 m plot in an oak forest in southeastern Michigan. Red maple and black cherry represented most of the understory species while white and black oaks dominated the canopy. During the summer of 1999, outbreak levels of gypsy moths preferentially defoliated oaks on the plot, often resulting in 100% defoliation. The relative growth rates (RGR) of red maple and black cherry were significantly higher during the time period where gypsy moths were at outbreak levels. Oaks had lower RGR during the gypsy moth attack than in the subsequent time period. Consequently, the preference gypsy moths exhibited towards oak species indirectly resulted in a higher growth rate for understory tree species, possibly due to increased light availability and the decreased competitive capacity of the defoliated oaks. This indirect effect could accelerate the succession at which oak dominated communities become red maple forests.

The forests of Michigan

The forests of Michigan

Characterization of woody species distribution in riparian forests of Lower Michigan, USA using map-based models

The goal of this study was to identify and characterize the range of variation in riparian forests across Lower Michigan, USA as a basis for assessing the utility of map-based information in the characterization of riparian environments. We obtained a regional sample of riparian forests and valley-bottom physiography from 94 locations throughout Lower Michigan and distinguished seven major riparian forest types using hierarchical clustering and NMDS ordination. NMDS ordination distances accounted for more than 70% of the distances in species space and achieved excellent discrimination among riparian types. We then evaluated a set of map-based variables indicative of regional climate, catchment hydrology, and valley character relative to ordination axes and interpretations of the autecology of principal tree species from each forest group. Map-based predictors accounted for 83% of the variation in sample scores along NMDS Axis 1 and explained 42% of the variation in Axis 2. Species and riparian types varied along two principal gradients, one associated with climate and geology along a north-south gradient, the second associated with flood duration and power. Map-based interpretations of regional climate and hydroperiod dynamics agreed closely with species-based interpretations of riparian character, although in certain cases, a similar biotic response arose from apparently distinct hydrogeomorphic contexts. Such dynamic patterns underscore the need for better and more explicit linkages between the controls of riparian hydrology and more proximal physical cues on biotic communities in order to understand the drivers of spatial variation in riparian ecosystem structure and composition.

Simulated chronic NO3− deposition reduces soil respiration in northern hardwood forests

AbstractChronic N additions to forest ecosystems can enhance soil N availability, potentially leading to reduced C allocation to root systems. This in turn could decrease soil CO2 efflux. We measured soil respiration during the first, fifth, sixth and eighth years of simulated atmospheric NO3− deposition (3 g N m−2 yr−1) to four sugar maple‐dominated northern hardwood forests in Michigan to assess these possibilities. During the first year, soil respiration rates were slightly, but not significantly, higher in the NO3−‐amended plots. In all subsequent measurement years, soil respiration rates from NO3−‐amended soils were significantly depressed. Soil temperature and soil matric potential were measured concurrently with soil respiration and used to develop regression relationships for predicting soil respiration rates. Estimates of growing season and annual soil CO2 efflux made using these relationships indicate that these C fluxes were depressed by 15% in the eighth year of chronic NO3− additions. The decrease in soil respiration was not due to reduced C allocation to roots, as root respiration rates, root biomass, and root turnover were not significantly affected by N additions. Aboveground litter also was unchanged by the 8 years of treatment. Of the remaining potential causes for the decline in soil CO2 efflux, reduced microbial respiration appears to be the most likely possibility. Documented reductions in microbial biomass and the activities of extracellular enzymes used for litter degradation on the NO3−‐amended plots are consistent with this explanation.

Missing OH reactivity in a forest: evidence for unknown reactive biogenic VOCs.

Forest emissions of biogenic volatile organic compounds (BVOCs), such as isoprene and other terpenes, play a role in the production of tropospheric ozone and aerosols. In a northern Michigan forest, the direct measurement of total OH reactivity, which is the inverse of the OH lifetime, was significantly greater than expected. The difference between measured and expected OH reactivity, called the missing OH reactivity, increased with temperature, as did emission rates for terpenes and other BVOCs. These measurements are consistent with the hypothesis that unknown reactive BVOCs, perhaps terpenes, provide the missing OH reactivity.

Ecosystem‐atmosphere exchange of carbon dioxide over a mixed hardwood forest in northern lower Michigan

We report results from the first 3 years (1999–2001) of long‐term measurements of net ecosystem exchange (NEE) at an AmeriFlux site over a mixed hardwood forest in northern lower Michigan. The primary measurement methodology uses eddy covariance systems with closed‐path infrared gas analyzers at two heights (46 and 34 m) above the forest (canopy height is ∼22 m). One objective is to contribute to a more firmly established methodology of estimating annual net ecosystem production (NEP), by systematically examining the consequences of several variations of criteria to identify periods of unreliable measurements, and to fill gaps in the data. We compared two methods to fill data gaps (about 30% of time in 1999) due to missing observations or rejected data after quality control; one using short‐term ensemble averages of the daily course and the other by semiempirical parametric models based on relationships between ecosystem respiration and soil temperature and between gross ecosystem photosynthetic uptake and photosynthetically active radiation. The modeled estimates were also used to replace eddy covariance fluxes during periods of weak and/or poorly developed turbulence when eddy covariance measurements cannot be expected to represent the ecosystem exchange. Examination of the fractions of eddy covariance fluxes and storage change relative to the expected ecosystem respiration suggested a friction velocity (u*) of 0.35 m s−1 as the lower limit for the acceptance of micrometeorologically determined NEE for this site. The differences in estimated annual NEP due to different criteria of data acceptance, measurement height, or gap‐filling strategies turned out to be at least as large as the interannual variations over the 3 years. After discussing various analysis strategies we conclude that the best estimate of annual NEP at our site is achieved by replacing data gaps and measurements in low‐u* conditions at all times with site and period‐specific parametric models, using the upper measurement level (about 2.1 canopy heights). These “best estimates” of annual NEP for 1999–2001 amounted to 170 (1999), 160 (2000), and 80 (2001) g C m−2. We also discuss some problems of assigning quantitative estimates of uncertainty for annual NEP.