University of Michigan Biological Station

The even-aged northern hardwood forests of the Upper Great Lakes Region are undergoing an ecological transition during which structural and biotic complexity is increasing. Early-successional aspen (Populus spp.) and birch (Betula papyrifera) are senescing at an accelerating rate and are being replaced by middle-successional species including northern red oak (Quercus rubra), red maple (Acer rubrum), and white pine (Pinus strobus). Canopy structural complexity may increase due to forest age, canopy disturbances, and changing species diversity. More structurally complex canopies may enhance carbon (C) sequestration in old forests. We hypothesize that these biotic and structural alterations will result in increased structural complexity of the maturing canopy with implications for forest C uptake. At the University of Michigan Biological Station (UMBS), we combined a decade of observations of net primary productivity (NPP), leaf area index (LAI), site index, canopy tree-species diversity, and stand age with canopy structure measurements made with portable canopy lidar (PCL) in 30 forested plots. We then evaluated the relative impact of stand characteristics on productivity through succession using data collected over a nine-year period. We found that effects of canopy structural complexity on wood NPP (NPPw) were similar in magnitude to the effects of total leaf area and site quality. Furthermore, our results suggest that the effect of stand age on NPPw is mediated primarily through its effect on canopy structural complexity. Stand-level diversity of canopy-tree species was not significantly related to either canopy structure or NPPw. We conclude that increasing canopy structural complexity provides a mechanism for the potential maintenance of productivity in aging forests.

A long-term stream fertilization experiment was performed to evaluate the potential eutrophication of an arctic stream ecosystem. During 16 years of summer phosphorus (H3PO4) fertilization, we observed a dramatic change in the community structure of the Kuparuk River on the North Slope of Alaska. A positive response to fertilization was observed at all trophic levels with increases in epilithic algal stocks, some insect densities, and fish growth rates. After approximately eight years of P fertilization, bryophytes (mosses) replaced epilithic diatoms as the dominant primary producers in the Kuparuk River. The moss impacted NH4+ uptake rates, benthic gross primary production, habitat structure, and insect abundance and species composition. This study documents the long-term changes in an arctic tundra stream in response to nutrient enrichment. Predicting stream ecosystem responses to chronic perturbation requires long-term observation and experiments.

Temperate forest soils store globally significant amounts of carbon (C) and nitrogen (N). Understanding how soil pools of these two elements change in response to disturbance and management is critical to maintaining ecosystem services such as forest productivity, greenhouse gas mitigation, and water resource protection. Fire is one of the principal disturbances acting on forest soil C and N storage and is also the subject of enormous management efforts. In the present article, we use meta-analysis to quantify fire effects on temperate forest soil C and N storage. Across a combined total of 468 soil C and N response ratios from 57 publications (concentrations and pool sizes), fire had significant overall effects on soil C (-26%) and soil N (-22%). The impacts of fire on forest floors were significantly different from its effects on mineral soils. Fires reduced forest floor C and N storage (pool sizes only) by an average of 59% and 50%, respectively, but the concentrations of these two elements did not change. Prescribed fires caused smaller reductions in forest floor C and N storage (-46% and -35%) than wildfires (-67% and -69%), and the presence of hardwoods also mitigated fire impacts. Burned forest floors recovered their C and N pools in an average of 128 and 103 years, respectively. Among mineral soils, there were no significant changes in C or N storage, but C and N concentrations declined significantly (-11% and -12%, respectively). Mineral soil C and N concentrations were significantly affected by fire type, with no change following prescribed burns, but significant reductions in response to wildfires. Geographic variation in fire effects on mineral soil C and N storage underscores the need for region-specific fire management plans, and the role of fire type in mediating C and N shifts (especially in the forest floor) indicates that averting wildfires through prescribed burning is desirable from a soils perspective.

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.

Agonistic behavior is a fundamental aspect of ecological theories on resource acquisition and sexual selection. Crustaceans are exemplary models for agonistic behavior within the laboratory, but agonistic behavior in natural habitats is often neglected. Laboratory studies do not achieve the same ecological realism as field studies. In an attempt to connect laboratory results to field data and investigate how habitat structure affects agonistic interactions, the nocturnal behavior of two crayfish species was observed by scuba diving and snorkeling in two northern Michigan lakes. Intraspecific agonistic interactions were analyzed in three habitats: two food resources-macrophytes and detritus-and one sheltered habitat. The overall observations reinforce the concept that resources influence agonistic bouts. Fights in the presence of shelters were longer and more intense, suggesting that shelters have a higher perceived value than food resources. Fights in the presence of detritus patches had higher average intensities and ended with more tailflips away from an opponent, suggesting that detritus was a more valuable food resource than macrophytes. In addition, observations of aggressive behavior within a natural setting can add validity to laboratory studies. When fights in nature are compared with laboratory fights, those in nature are shorter, less intense, and less likely to end with a tailflip, but do show the fundamental fight dynamics associated with laboratory studies. Extrinsic and intrinsic factors affect intraspecific aggression in many ways, and both should always be recognized as having the potential to alter agonistic behavior.

Summary 1 Primary succession on coastal sand dunes has generally been presumed to be driven by autogenic environmental change associated with dune stabilization and gradual soil development. 2 An extensive chronosequence of dune‐capped beach ridges is found adjacent to northern Lake Michigan and the youngest 13 ridges (aged 30–440 years old) show a clear pattern of primary succession and development of a forest ecosystem. 3 Seed‐addition and seedling‐transplant experiments indicated that colonization of young sand dunes by late‐successional Pinus and Quercus species is constrained by limited seed dispersal, seed and seedling desiccation, and seed predation rather than seedling success being constrained by low soil nitrogen availability. Their establishment may therefore depend on coincidence of chance seed dispersal, favourable weather conditions and low rodent densities. 4 In addition, episodic burial by sand prevents most species from colonizing young dune ridges, while burial of seedlings by litter limits recruitment on older dune ridges with developing forest. The intensity of competition increases during succession. 5 Dune succession is better described as the transient dynamics of colonization and competitive displacement rather than the result of gradual soil development and competitive displacement.

Vegetation and soil properties were described across a well-dated sand-dune chronosequence bordering northern Lake Michigan to document patterns and rates of primary succession and forest ecosystem development, and to determine environmental constraints that potentially drive succession and regulate species diversity. The site experienced frequent and continuing formation of 72 shore-parallel dune ridges over the past 2375 yr. Across the chronosequence represented by the youngest 13 dune ridges aged 25–440 yr, there were clear patterns of species turnover and community convergence as well as successional changes in species diversity, aboveground biomass, aboveground litter production, net ecosystem production, nutrient pools, and nutrient cycling. Dune-building species were replaced by evergreen shrubs and bunchgrass within 100 yr, which in turn, were replaced by mixed pine forest within 345 yr. Plant-species richness increased to a peak in developing forest at 285 yr but thereafter decreased as early-successional species disappeared from the communities. Rates of species addition peaked between 95 and 145 yr as forest species invaded, whereas rates of species loss peaked between 345 and 440 yr as early-successional species were lost from the developing forest. Development of the forest ecosystem required ∼300 yr (i.e., 145–440 years). Total ecosystem carbon increased in a logistic manner to 128 Mg C/ha, with net ecosystem production peaking at 30 g C·m−2·yr−1 in developing forest. Aboveground biomass and O horizon mass increased to ∼137 Mg/ha and ∼79 Mg/ha, respectively, whereas aboveground litter production increased to 3.5 Mg·ha−1·yr−1 at 440 yr, but thereafter varied between 175 and 350 Mg·ha−1·yr−1. Total carbon and total nitrogen in the upper 15 cm of mineral soil and O horizon accumulated to ∼42 Mg/ha and ∼1.36 Mg/ha, respectively. Estimated average rates of carbon and nitrogen accumulation over 440 yr of ecosystem aggradation were 23.2 g·m−2·yr−1 for carbon and 0.38 g·m−2·yr−1 for nitrogen. Because nitrogen-fixing plants are rare on the upland dune ridges, ecosystem aggradation depends largely on atmospheric nitrogen inputs. Following colonization by conifers, soil acidification resulted in rapid leaching losses of calcium and magnesium, whereas phosphorus and potassium were cycled more tightly. The dune chronosequence represents a complex gradient of changing environmental constraints that differentially reduce the survival, growth, and reproduction of plant species. Young dune ridges near the lake shore are characterized by strong winds, sand burial and erosion, high insolation, high rates of evaporation, and low availability of nitrogen and phosphorus. These conditions ameliorate with increasing dune age as wind velocities and sand movement diminish with distance from the lake, as accumulating organic matter improves the moisture-holding capacity and nitrogen availability of the soil, and as mineral weathering mobilizes soil phosphorus. However, in developing forest, light and cationic nutrients may become limiting, and decreased light availability, cool soil temperatures, and accumulation of a thick forest floor may limit recruitment from seed for many species. These numerous potential environmental constraints suggest a considerable complexity in this ostensibly simple ecosystem.

Abstract Forest harvesting and wildfire were widespread in the upper Great Lakes region of North America during the early 20th century. We examined how long this legacy of disturbance constrains forest carbon (C) storage rates by quantifying C pools and fluxes after harvest and fire in a mixed deciduous forest chronosequence in northern lower Michigan, USA. Study plots ranged in age from 6 to 68 years and were created following experimental clear‐cut harvesting and fire disturbance. Annual C storage was estimated biometrically from measurements of wood, leaf, fine root, and woody debris mass, mass losses to herbivory, soil C content, and soil respiration. Maximum annual C storage in stands that were disturbed by harvest and fire twice was 26% less than a reference stand receiving the same disturbance only once. The mechanism for this reduction in annual C storage was a long‐lasting decrease in site quality that endured over the 62‐year timeframe examined. However, during regrowth the harvested and burned forest rapidly became a net C sink, storing 0.53 Mg C ha −1 yr −1 after 6 years. Maximum net ecosystem production (1.35 Mg C ha −1 yr −1 ) and annual C increment (0.95 Mg C ha −1 yr −1 ) were recorded in the 24‐ and 50‐year‐old stands, respectively. Net primary production averaged 5.19 Mg C ha −1 yr −1 in experimental stands, increasing by < 10% from 6 to 50 years. Soil heterotrophic respiration was more variable across stand ages, ranging from 3.85 Mg C ha −1 yr −1 in the 6‐year‐old stand to 4.56 Mg C ha −1 yr −1 in the 68‐year‐old stand. These results suggest that harvesting and fire disturbances broadly distributed across the region decades ago caused changes in site quality and successional status that continue to limit forest C storage rates.

Temporal and spatial patterns of specific leaf weight (SLW, g/m 2 ) were determined for deciduous hardwood tree species in natural habitats in northern lower Michigan to evaluate the utility of SLW as an index of leaf photosynthetic capacity. No significant diurnal changes in SLW were found. Specific leaf weight decreased and then increased during leaf expansion in the spring. Most species, especially those located in the understory, then had relatively constant SLW for most of the growing season, followed by a decline in SLW during autumn. Specific leaf weight decreased exponentially down through the canopy with increasing cumulative leaf area index. Red oak ( Quercus rubra ), paper birch ( Betula papyrifera ), bigtooth aspen ( Populus grandidentata ), red maple ( Acer rubrum ), sugar maple ( A. saccharum ), and beech ( Fagus grandifolia ) generally had successively lower SLW, for leaves at any one level in the canopy. On a given site, comparisons between years and comparisons of leaves growing within 35 cm of each other showed that differences in SLW among species were not due solely to microenvironmental effects on SLW. Bigtooth aspen, red oak, and red maple on lower‐fertility sites had lower SLW than the same species on higher‐fertility sites. Maximum CO 2 exchange rate, measured at light‐saturation in ambient CO 2 and leaf temperatures of 20 to 25 C, increased with SLW. Photosynthetic capacities of species ranked by SLW in a shaded habitat suggest that red oak, red maple, sugar maple, and beech are successively better adapted to shady conditions.

Abstract Biophysical controls on plant water status exist at the leaf, stem, and root levels. Therefore, we pose that hydraulic strategy is a combination of traits governing water use at each of these three levels. We studied sap flux, stem water storage, stomatal conductance, photosynthesis, and growth of red oaks ( Quercus rubra ) and red maples ( Acer rubrum ). These species differ in stomatal hydraulic strategy and xylem architecture and may root at different depths. Stable isotope analysis of xylem water was used to identify root water uptake depth. Oaks were shown to access a deeper water source than maples. During non‐limiting soil moisture conditions, transpiration was greater in maples than in oaks. However, during a soil dry down, transpiration and stem water storage decreased by more than 80% and 28% in maples but only by 31% and 1% in oaks. We suggest that the preferential use of deep water by red oaks allows the species to continue transpiration and growth during soil water limitations. In this case, deeper roots may provide a buffer against drought‐induced mortality. Using 14 years of growth data, we show that maple growth correlates with mean annual soil moisture at 30 cm but oak growth does not. The observed responses of oak and maple to drought were not able to be explained by leaf and xylem physiology alone. We employed the Finite‐difference Ecosystem‐scale Tree Crown Hydrodynamics model version 2 plant hydrodynamics model to demonstrate the influence of root, stem, and leaf controls on tree‐level transpiration. We conclude that all three levels of hydraulic traits are required to define hydraulic strategy.

Sediments from Cheboygan Marsh, a coastal freshwater wetland on Lake Huron that has been invaded by an emergent exotic plant, Typhaxglauca, were examined to assess the effects of invasion on wetland nutrient levels and sediment microbial communities. Comparison of invaded and uninvaded zones of the marsh indicated that the invaded zone showed significantly lower plant diversity, as well as significantly higher aboveground plant biomass and soil organic matter. The sediments in the invaded zone also showed dramatically higher concentrations of soluble nutrients, including greater than 10-fold higher soluble ammonium, nitrate, and phosphate, which suggests that Typhaxglauca invasion may be impacting the wetland's ability to remove nutrients. Terminal restriction fragment length polymorphism analyses revealed significant differences in the composition of total bacterial communities (based on 16S-rRNA genes) and denitrifier communities (based on nirS genes) between invaded and uninvaded zones. This shift in denitrifiers in the sediments may be ecologically significant due to the critical role that denitrifying bacteria play in removal of nitrogen by wetlands.

Peer Reviewed

Afforestation (tree establishment on nonforested land) is a management option for increasing terrestrial C sequestration and mitigating rising atmospheric carbon dioxide because, compared to nonforested land uses, afforestation increases C storage in aboveground pools. However, because terrestrial ecosystems typically store most of their C in soils, afforestation impacts on soil organic carbon (SOC) storage are critical components of ecosystem C budgets. We applied synthesis methods to identify the magnitude and drivers of afforestation impacts on SOC, and the temporal and vertical distributions of SOC change during afforestation in the United States. Meta‐analysis of 39 papers from 1957 to 2010 indicated that previous land use drives afforestation impacts on SOC in mineral soils (overall average = +21%), but mined and other industrial lands (+173%) and wildlands (+31%) were the only groups that specifically showed categorically significant increases. Temporal patterns of SOC increase were statistically significant on former industrial and agricultural lands (assessed by continuous meta‐analysis), and suggested that meaningful SOC increases require ≥15 and 30 yr of afforestation, respectively. Meta‐analysis of 13 C data demonstrated the greatest SOC changes occur at the surface soil of the profile, although partial replacement of C stocks derived from previous land uses was frequently detectable below 1 m. A geospatial analysis of 409 profiles from the National Soil Carbon Network database supported 13 C meta‐analysis results, indicating that transition from cultivation to forest increased A horizon SOC by 32%. In sum, our findings demonstrate that afforestation has significant, positive effects on SOC sequestration in the United States, although these effects require decades to manifest and primarily occur in the uppermost (and perhaps most vulnerable) portion of the mineral soil profile.

The prevalence of nonrandom fertilization due to postpollination events has rarely been studied in natural populations, despite important implications for outcrossing rates, mate choice, and plant fitness. Nonrandom paternity within fruits can be caused by both unequal fertilization and unequal embryo abortion. Using self‐compatible Hibiscus moscheutos , we studied the potential for nonrandom fertilization by comparing growth rates of pollen‐tubes from different donors. The branched style of Hibiscus allowed within‐flower comparisons between pollen donors. Relative pollen‐tube growth rates were determined by applying pollen from pairs of donors to different stigmas on adjacent stylar branches. We then measured the number of callose plugs per tube in cross‐sectional transects across the style after 3 hr. We demonstrate that rates of callose plug formation can be used as a sensitive indicator of relative pollen‐tube growth rate. Differences between pollen donors were common and repeatable. Self‐pollen‐tubes grew slower than outcross pollen‐tubes in some crosses and faster in others. Allozyme variation in glucose phosphate isomerase was used to show that individuals with fast‐growing pollen‐tubes sired a disproportionate number of seeds following mixed pollinations (up to 72%). Since seed abortion was negligible, we conclude that variation in pollen‐tube growth rates leads to nonrandom paternity within fruits.

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).

Hydraulic capacitance and water storage form a critical buffer against cavitation and loss of conductivity within the xylem system. Withdrawal from water storage in leaves, branches, stems, and roots significantly impacts sap flow, stomatal conductance, and transpiration. Storage quantities differ based on soil water availability, tree size, wood anatomy and density, drought tolerance, and hydraulic strategy (anisohydric or isohydric). However, the majority of studies focus on the measurement of storage in conifers or tropical tree species. We demonstrate a novel methodology using frequency domain reflectometry (FDR) to make continuous, direct measurements of wood water content in two hardwood species in a forest in Michigan. We present results of a two month study comparing the water storage dynamics between a mature red oak and red maple, two species with differing wood densities, hydraulic architecture, and hydraulic strategy. We also include results pertaining to the use of different probe lengths to sample water content only within the active sapwood and over the entire conductive sapwood and the outer portion of heartwood in red oak. Both species studied exhibited diurnal cycles of storage that aligned well with the dynamics of sap flux. Red maple, a diffuse porous, relatively isohydric species showed a strong dependence on stored water during both wet and dry periods. Red oak, a ring porous relatively anisohydric species, was less reliant on storage, and did not demonstrate a dependence on soil water potential. Comparison between long and short FDR probes in the oak revealed that oaks may utilize water stored in the innermost layers of the xylem when soil moisture conditions are limiting. We found the FDR probes to be a reliable, functional means for continuous automated measurement of wood water content in hardwoods at a fast time scale. Application of FDR technology for the measurement of tree water storage will benefit forest ecologists as well as the modeling community as we improve our understanding and simulations of plant hydrodynamic processes on a large scale.

Up to 99% of the carbon fuelling the food webs of temperate woodland streams is derived from inputs of terrestrial leaf litter. Aquatic bacteria, fungi, and detritivore invertebrates directly utilize these inputs, transferring this energy to other components of the food web. Increases in atmospheric CO 2 could indirectly impact woodland stream food webs by chemically altering leaf litter. This study evaluated CO 2 ‐induced chemical changes in aspen ( Populus tremuloides ) leaf litter, and the corresponding effects on stream bacteria, fungi and leaf‐shredding cranefly larvae ( Tipula abdominalis : Diptera). Leaf litter from plants grown under elevated CO 2 had decreased nutritional value to aquatic decomposers and detritivores because of higher levels of structural compounds and lower nitrogen content. Consequently, elevated CO 2 ‐grown leaf litter supported 59% lower bacterial production in a stream than litter grown at ambient CO 2 levels, while not affecting fungal biomass. Larval craneflies fed elevated CO 2 ‐grown microbially colonized leaves consumed less, assimilated less, and grew 12 times slower than their ambient fed counterparts.

We used the Rassoulzadegan-DeMott bead bioassay to evaluate the ability of various pelagic microcrustaceans to discriminate between particles on the basis of taste and size. The test examined zooplankton reactions to fresh algal exudates adsorbed onto polystyrene microspheres. The investigation shows that there is a marked dichotomy between certain large-bodied freshwater microcrustaceans in how they respond to small particles; it confirms that large daphnid cladocerans exhibit no taste discrimination for small beads, whereas calanoid copepods continually appraise resource quality of both small and large particles with a fine degree of discrimination. In contrast, smaller bodied daphnids and other cladocerans show some degree of taste and acute size discrimination, the former presumably related to processing large particles one at a time and to the individual peculiarities of the filtering mechanisms. Based on the findings from taste discrimination tests, we suggest that the degree of taste discrimination is often related to particle size. Moreover, we assert that many cladocerans are categorized more appropriately as detritivores than as herbivores, although they exhibit modest, size-related taste discrimination.

Predicting forest responses to rising atmospheric CO2 will require an understanding of key feedbacks in the cycling of carbon and nitrogen between plants and soil microorganisms. We conducted a study for 2.5 growing seasons with Populus tremuloides grown under experimental atmospheric CO2 and soil-N-availability treatments. Our objective was to integrate the combined influence of atmospheric CO2 and soil-N availability on the flow of C and N in the plant–soil system and to relate these processes to the performance of this widespread and economically important tree species. Here we consider treatment effects on photosynthesis and canopy development and the efficiency with which this productive capacity is translated into aboveground, harvestable yield. We grew six P. tremuloides genotypes at ambient (35 Pa) or elevated (70 Pa) CO2 and in soil of low or high N mineralization rate at the University of Michigan Biological Station, Pellston, Michigan, USA (45°35′ N, 84°42′ W). In the second year of growth, net CO2 assimilation rate was significantly higher in elevated-CO2 compared to ambient-CO2 plants in both soil-N treatments, and we found little evidence for photosynthetic acclimation to high CO2. In the third year, however, elevated-CO2 plants in low-N soil had reduced photosynthetic capacity compared to ambient-CO2, low-N plants. Plants in high-N soil showed the opposite response, with elevated-CO2 plants having higher photosynthetic capacity than ambient-CO2 plants. Net CO2 assimilation rate was linearly related to leaf N concentration (log:log scale), with identical slopes but different intercepts in the two CO2 treatments, indicating differences in photosynthetic N-use efficiency. Elevated CO2 increased tissue dark respiration in high-N soil (+22%) but had no significant effect in low-N soil (+9%). There were no CO2 effects on stomatal conductance. At the final harvest, stem biomass and total leaf area increased significantly due to CO2 enrichment in high-N but not in low-N soil. Treatment effects on wood production were largely attributable to changes in leaf area, with no significant effects on growth efficiency. We conclude that harvest intervals for P. tremuloides on fertile sites will shorten with rising atmospheric CO2, but that tree size at canopy closure may be unaffected.

Abstract Intermediate disturbances shape forest structure and composition, which may in turn alter carbon, nitrogen, and water cycling. We used a large‐scale experiment in a forest in northern lower Michigan where we prescribed an intermediate disturbance by stem girdling all canopy‐dominant early successional trees to simulate an accelerated age‐related senescence associated with natural succession. Using 3 years of eddy covariance and sap flux measurements in the disturbed area and an adjacent control plot, we analyzed disturbance‐induced changes to plot level and species‐specific transpiration and stomatal conductance. We found transpiration to be ~15% lower in disturbed plots than in unmanipulated control plots. However, species‐specific responses to changes in microclimate varied. While red oak and white pine showed increases in stomatal conductance during postdisturbance (62.5 and 132.2%, respectively), red maple reduced stomatal conductance by 36.8%. We used the hysteresis between sap flux and vapor pressure deficit to quantify diurnal hydraulic stress incurred by each species in both plots. Red oak, a ring porous anisohydric species, demonstrated the largest mean relative hysteresis, while red maple, bigtooth aspen, and paper birch, all diffuse porous species, had the lowest relative hysteresis. We employed the Penman‐Monteith model for LE to demonstrate that these species‐specific responses to disturbance are not well captured using current modeling strategies and that accounting for changes to leaf area index and plot microclimate are insufficient to fully describe the effects of disturbance on transpiration.