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Deer and Moose Browsing in Hemlock Removal Experiment at Harvard Forest 2008

Hemlock decline in New England is caused by direct and indirect effects of invasion of the hemlock woolly adelgid. Direct damage from the insect is causing gradual mortality of hemlock, and widespread harvesting of hemlock in advance of mortality, in contrast, causes immediate mortality and removal of biomass from the site. Although both processes affect thousands of acres of forest annually we have only a limited understanding of their effects on forest ecosystem function and productivity and the nature of the subsequent forest community. We anticipate that harvesting will yield different consequences than gradual mortality from the insect. Therefore we designed an experiment to simulate these contrasting impacts, by logging or girdling hemlock stands. Results from the experimental treatments will be compared to the changes observed in forests that are being infested by the adelgid, and can also be included in integrated analyses of a suite of large experiments that form a core component of the Harvard Forest LTER program. Deer and moose foraging can play a key role in shaping forest regeneration after disturbance in temperate forest. In 2008, we initiated a browsing survey of woody stems in the Simes hemlock removal experiment plots. There are regular moose sightings in the study area, and moose pellets are commonly found within the plots. Also, extensive browsing of tree regeneration in the logged plots was noted starting in 2007. Sampling of the 8 study plots was completed in Summer 2008. The next sampling is tentatively scheduled for Summer 2013.

Coarse Woody Debris in Hemlock Removal Experiment at Harvard Forest since 2005

The woody detritus survey is designed to measure coarse woody detritus that includes snags, logs, and stumps, and to estimate fine woody detritus which includes smaller pieces of downed wood. To capture both standing and downed wood, we based our survey around two main types of methods, the line intercept method and the fixed radius plot method. Surveys have been completed for 2005, 2007, 2009, 2011, 2013, 2015, 2017, and 2021.

Overstory Vegetation in Hemlock Removal Experiment at Harvard Forest since 2003

Linking Xylem Diameter Variations with Sap Flow Measurements at Harvard Forest 2003-2006

Measurements of variation in the diameter of tree stems provide a rapid response, high resolution tool for detecting changes in water tension inside the xylem. Water movement inside the xylem is caused by changes in the water tension and theoretically, the sap flow rate should be directly proportional to the water tension gradient and, therefore, also linearly linked to the xylem diameter variations. The coefficient of proportionality describes the water conductivity and elasticity of the conducting tissue. Xylem diameter variation measurements could thus provide an alternative approach for estimating sap flow rates, but currently we lack means for calibration. On the other hand, xylem diameter variation measurements could also be used as a tool for studying xylem structure and function. If we knew both the water tension in the xylem and the sap flow rate, xylem conductivity and/or elasticity could be calculated from the slope of their relationship. In this study we measured diurnal xylem diameter variation simultaneously with sap flow rates (Granier-type thermal method) in six deciduous species (Acer rubrum L., Alnus glutinosa Miller, Betula lenta L., Fagus Sylvatica L. Quercus rubra L., and Tilia vulgaris L.) for 7-91 day periods during summers 2003, 2005 and 2006 and analyzed the relationship between these two measurements. We found that in all species xylem diameter variations and sap flow rate were linearly related in daily scale (daily average R 2 = 0.61-0.87) but there was a significant variation in the daily slopes of the linear regressions. The largest variance in the slopes, however, was found between species, which is encouraging for finding a species specific calibration method for measuring sap flow rates using xylem diameter variations. At a daily timescale, xylem diameter variation and sap flow rate were related to each other via a hysteresis loop. The slopes during the morning and afternoon did not differ statistically significantly from each other,

Litterfall, Biomass and Productivity in CRUI Land Use Project at Harvard Forest 1996-1997

Prior agricultural land use often plays an important role in the present condition of forests. This research was conducted to find any productivity differences among previously plowed, pastured, and woodlot sites. There are six sites, two from each of the land use types. With this research we wanted to estimate the productivity for each of the six stands and test the hypothesis that rates of annual aboveground biomass and annual litter fall in recovering forests would decline in the order woodlot - pastured - plowed. Twenty-five trees, of various diameters and species, were cored at each site. The cores were analyzed and the radial growth was calculated for the most recent 5 and 10 year intervals. Biomass increments were also calculated for the same time span using published allometric equations. Aboveground woody productivity was calculated for diameter classes within species and semi-log relationships of productivity per diameter were used to calculate the productivity of all trees in the stand that were not cored. Site-specific and species-specific equations were used. Litter fall was estimated by collecting litter from ten traps each in the six stands. Litter was sorted by species and reported on a dry matter basis. Red oaks and red maples were the major contributors of biomass and litter fall in all six of the stands, accounting for about seventy percent of the total aboveground biomass. Although woodlots had higher total biomass there was not a significant difference in productivity across the six sites. The range of productivity including both litter fall and aboveground woody production was from 7524-9014 Kg/ha. This fell into the range for temperate forests according to results published by the International Biological Program. Production per unit biomass was greater in the plowed stand and least in the woodlots.

Tree Canopy Leaf Area Index in CRUI Land Use Project at Harvard Forest 1997

Numerous variables related to land use disturbance and recovery processes can influence forest composition and structure. We’ve measured differences in forest communities in six sites that were formerly plowed, pastured, or continuously forested woodlots in Prospect Hill. None of the sites had noticeable canopy gap disturbance at the time of the measurements. Leaf area index (LAI) was measured with an LAI-2000 plant canopy analyzer (Li-Cor, Inc., Lincoln, NE) at all 77 edge and interior intersection points in the 30 m x 50 m permanent plot (7 columns x 11 rows) in 5 of our 6 land use sites. Under-canopy measurements were made in each site over 25-30 minutes during midday hours (11:00-2:30 EST) on overcast days near solstice (June 13, 18). The under-canopy readings were contrasted with an open-sky measurement taken in an open field near the Harvard Forest headquarters just before beginning data collection in each site. LAI averaged 3.98 and ranged from 2.28 to 5.93 across all sites. W1 had the highest site-level mean (4.62) and maximum (5.93) LAI while S2 had the lowest values (mean = 3.41, max = 4.55). The woodlot also showed the greatest spatial variation as measured by C.V., while plow #1 showed the least variation.

Canopy Sky Factors in CRUI Land Use Project at Harvard Forest 1998

Hemispherical photos were taken at 35 interior grid corners in each land use site’s 30 m x 50 m permanent plot on uniformly overcast days during summer 1998. A Canon A-1 35 mm camera with a 7.5 mm lens was mounted on a tripod, positioned so the film plane was 0.5 m above ground, oriented N-S with a compass, and leveled. Ektachrome 100 slide film was used. Two exposures were made for each location. The slides were processed and digitized in jpeg format by Kodak, Inc. at a resolution of 1536 x 1024 dpi. The jpeg images were screened for nearground obstructions. Fifteen of the 35 photo locations per site were used in subsequent analysis. Hemiview software (v. 2.0, Delta-T Devices Ltd., Cambridge, UK) was used to process the images and calculate indirect, direct, and global (= indirect + direct) sky factors for each sample location, plus the proportion of global sky factor represented by direct beam radiation. Several images spanning a wide range of canopy density were used to develop protocols for setting optimal thresholds in converting from color to black and white images. Each B/W image was then analyzed at three threshold levels, the optimum level plus 5 threshold units (~ 2.5%) above and below the optimum. The results were averaged across the three threshold levels to provide one set of sky factor data per sample location. Above-canopy incident radiation was calculated daily for the June-September growing season based on (1) latitude and longitude, (2) average atmospheric transmission, and (3) monthly averages for fractions of the day in diffuse radiation due to cloud cover for the Harvard Forest area. Data for 2 and 3 above were provided by researchers at the State University of New York at Albany. Below-canopy radiation was estimated daily across half-hour intervals as the sum of diffuse and direct beam components. Sky factors were then calculated from the above- and below-canopy radiation values and averaged both monthly and across the growing season.

Herbaceous Species in CRUI Land Use Project at Harvard Forest 1996

Forests recovering from agricultural legacies differ in many ways that influence the landscape distribution, local abundance, and dispersion of herbaceous populations, as well as patterns of species associations within the herbaceous stratum. We recorded the presence/absence of 10 herbaceous species on a 1 x 1 m resolution in a 30 x 50 m permanent plot grid in six land use legacy sites (2 plowed, 2 pastured, 2 permanent woodlot) in summer 1996 to test predictions about herbaceous species abundance, richness, dispersion patterns, patterns of association, and their relationships to forest structure, microclimates, and soil resources. The 10 species chosen differed in several ways (evergreen vs. deciduous, stature, clonal architecture, affinity for disturbed vs. undisturbed forests) that allowed us to probe the impacts of prior disturbance and current resource patterns (light, water, nutrients) on the spatial patterns of individual species and species associations.

Mapped Trees in CRUI Land Use Project at Harvard Forest 1996-2006

Numerous variables related to land use disturbance and recovery processes influence forest composition, structure, and growth. We measured forest communities in six sites that were formerly plowed, pastured, or continuously forested woodlots in Prospect Hill to test predictions about species composition, stand structure, and productivity in response to agricultural land use legacies. A permanent 30 x 50 m plot gridded in 5 x 5 m sub-plots was established in each of the 6 sites. All trees at least 2.5 cm DBH (diameter at breast height, 1.3 m) were mapped visually in the field, marked with an aluminum tag, and their DBH recorded in summer 1996. Individual boles of multi-stemmed trees were measured separately and a composite single DBH was calculated. Standing dead trees were also mapped and their DBH recorded as well. The 1996 data were used to determine above ground woody biomass using allometric equations for individual trees. The permanent plots were re-surveyed ten years later in fall 2006. The status of each tree mapped in 1996 was recorded (alive, dead standing, dead fallen, forked) and DBH’s were re-measured. Additional trees that grew across the 2.5 cm DBH threshold during the ten-year period were mapped and their DBH’s measured. Composite diameters and above ground woody biomass were determined for forked trees as in 1996. The 2006 re-measurements were used to analyze changes in species composition, stand structure (density, diameter distribution), and mortality patterns among species and size classes, and to calculate net changes in above ground woody biomass across the ten-year period.

Microclimate in CRUI Land Use Project at Harvard Forest 1997

Understory microenvironmental patterns are strongly influenced by both the forest community and substrate, which are in turn affected by site factors, disturbance history and recovery processes. We measured ground-level microclimates across the growing season as part of a multi-faceted study of soil-plant-atmosphere interactions in sites with different agricultural land-use legacies (formerly plowed, formerly pastured, permanent woodlots) in Prospect Hill. A permanent 30 x 50 m plot was established in each of six sites (2 per land use category) and gridded into 5 x 5 m sub-plots. Fifteen microclimate stations were used to measure photosynthetic photon flux (PPF, umol m-2 s-1, LI-190 quantum sensor, Li-Cor, Inc), soil temperature (107B thermistor, Campbell Scientific, Logan, Utah), and air temperature (107 thermistor, Campbell Scientific, Logan, Utah) at 5 locations each in three sites simultaneously. PPF and air temperature were measured at 50 cm, while soil temperature was measured 5 cm. The central sample station was located in the center (sub-plot D6) of the 30 x 50 m grid and four additional stations were positioned 10 m from the central station in the cardinal directions. In addition to the variables described above, relative humidity (%, 207 air temperature/relative humidity probe, Campbell Scientific, Logan, Utah) was measured at 50 cm at the central station. All sensors were connected to a Campbell Scientific 23X datalogger and sampled continuously every 10 sec and averaged every 10 minutes for 5 days at a time. The stations were switched between two sets of sites (P1-S2-W2 followed by P2-S1-W1) at the end of every week, so the data record for any one site includes alternate weeks. Measurements were made during April through September, 1997. The data archived here are daily means per variable per station. Missing records are due to inclement weather, while blank fields represent occasional sensor or datalogger connection problems.

Photosynthetic Light Response Curves in CRUI Land Use Project at Harvard Forest 1998

Ambient CO2 concentrations in terrestrial ecosystems vary substantially on several spatial and temporal scales as numerous soil, plant, and atmospheric processes respond to irradiance, temperature, moisture and wind. There is one widespread microhabitat in terrestrial ecosystems, the nearground zone, in which CO2 is naturally enhanced above average background levels. CO2 produced by soil respiration diffuses through the litter and boundary layers and dissipates fairly rapidly into the overlying bulk air. However, a marked vertical profile of nearground enriched CO2 (hereafter NEC) is usually present in the first 0-50 cm above ground. The degree of enrichment varies primarily with soil respiration rate and turbulent mixing, secondarily with photosynthesis by plants in the herbaceous stratum, and usually shows marked diel and seasonal variation. References to this CO2 "subsidy" and its effects on plants have occurred occasionally in the literature since 1939, but there have been few detailed studies of either the nearground profile or plant responses in the field, particularly for species that consistently occupy the nearground stratum. Considerable research over the last twenty years in both controlled and field environments has shown that co-occurring plant species may respond differently to artificially elevated CO2. But in contrast to light, temperature, water, and nutrients, plant community ecologists have generally not considered CO2 among the factors that regulate species’ distribution and abundance, except indirectly as it may affect water balance. We have documented differences in forest composition (woody and herbaceous), soil characteristics, microclimates, and nearground CO2 levels among six sites that were formerly plowed, pastured, or continuously forested woodlots in Prospect Hill. We selected three perennial herbaceous species (Aralia nudicaulis, wild sarsaparilla; Clintonia borealis, blue-bead lily; Medeola virginiana, Indian cucumber root) and two do

Herbaceous Stratum Sunfleck Regimes in CRUI Land Use Project at Harvard Forest 1998

Numerous variables related to land use disturbance and recovery processes can influence forest understory irradiance regimes. In addition to season-long measurements of herbaceous stratum microclimates (data archived separately), we explored the degree to which differences in forest community composition and structure generate differences in near-ground sunfleck regimes in our six land use sites (2 plowed, 2 pastured, 2 permanent woodlot). Considerable research has shown that photosynthetic responses to fluctuating irradiance cannot be predicted accurately from steady-state gas-exchange measurements, and that sunflecks may account for both a large fraction of total daily photon fluence and net carbon assimilation by plants in the herbaceous stratum of moist forests. We measured clear-day irradiance patterns to test predictions about how sunfleck frequency, duration, and brightness should vary across the three land-use legacies. We also wished to document sunfleck regimes as part of a companion study on photosynthetic responses by herb and tree species to sunflecks. Twenty-three quantum sensors (Li-190, Li-Cor, Inc., Lincoln, NE) were placed at regularly-spaced interior intersection points across the 30 m x 50 m permanent plot grid in each land use site. The sensors were mounted on stakes 50 cm above ground, leveled, and attached to a Campbell Scientific CR23X datalogger. Photosynthetic photon flux (PPF) values were recorded at all 23 microsites every 6 seconds for 3.5-4.5 hours in the morning (8:00-11:30+ a.m. EST) on one clear day per site in July and August 1998. Using custom software and a sunfleck definition threshold of 100 umol m-2 s-1, sunfleck events were extracted for each microsite across the sampling period and the following were calculated for each fleck event: number of fleck events per hour, sunfleck duration (sec), duration of the shade period prior to the sunfleck (sec), mean PPF across the sunfleck event, and peak PPF during the sunfleck. Descrtipti

Tree Seedlings in CRUI Land Use Project at Harvard Forest 1996

Forests recovering from agricultural legacies differ in many ways that influence tree seed dispersal and seedling establishment patterns. We recorded the number of seedlings (less than 0.5 m tall) of all tree species on a 1 x 1 m resolution across six land use legacy sites (2 plowed, 2 pastured, 2 permanent woodlot) in summer 1996 to test several predictions about seedling abundance, diversity, and dispersion patterns, and their relationships to forest structure, microclimates, and soil resources.

Herbaceous Community Composition in CRUI Land Use Project at Harvard Forest 1996

Patterns of vascular plant species richness were investigated in six land use legacy sites (2 formerly plowed, 2 formerly pastured, and 2 permanent woodlot) in Prospect Hill to test predictions about the effects of disturbance, light and soil resources, and forest floor environmental heterogeneity on community composition. The occurrence of vascular taxa was recorded in each of the 60 5 m x 5 m contiguous plots within the 30 m x 50 m permanently gridded study plot in each land use legacy site in June 1996. Identification was made to species in most cases. Woodlots showed higher average species richness at the site level (53) than either pastured (52 species) or plowed (49) sites. However woodlots also show greater spatial variation in richness at the 5 m x 5 m resolution than either the plowed or pastured sites (in that order) as represented by the range, standard deviation, and coefficient of variation. Seasonally-averaged light levels at 50 cm are approximately twice as high in the plowed and pastured sites as in the woodlots, but the woodlots show significantly greater soil organic matter, carbon, nitrogen and water-holding capacity than the post-agricultural sites. These results suggest that soil resources may be more important than light in fostering higher herbaceous stratum richness. However, woodlot richness is also affected by the presence of taxa that are slow to re-colonize heavily disturbed sites (e.g., Epigaea repens) and to distinctive microsites that are less common in the plowed or pastured sites in the Harvard Forest system (e.g., exposed boulders). The greater spatial variation in woodlot richness is strongly influenced by both substrate diversity and by scattered hemlock trees, which substantially depress the herb stratum in localized patches.

Hydrocarbon Concentrations at Harvard Forest EMS Tower 1992-2001

Hydrocarbons are products of incomplete combustion and also emitted by vegetation. Hydrocarbons are important precursors for photochemical ozone formation. A system to quantify concentrations of several low-molecular weight hydrocarbons was installed at the EMS tower in the summer of 1992. The measurements were intended to help resolve questions about the potential reactivity in rural New England atmosphere and identify and quantify biogenic hydrocarbon emissions.

Organic and Inorganic Nitrogen Uptake by Sarracenia Purpurea at Harvard Forest and Fort Albany ON 2007

The nitrogen-limited carnivorous pitcher plant Sarracenia purpurea and its associated detritus-based food web is a model system for studying plant nutrient dynamics. We tested if S. purpurea can directly take up intact amino acids and compared uptake of organic and inorganic forms of nitrogen (N) across a gradient of N deposition. At sites in Canada and the United States, individual pitchers with complete or incomplete food webs were fed U-13C-15N-glycine, U-13C-15N-phenylalanine and 15NH415NO3 individually and in mixture. Plants took up intact amino acids. Acquisition of each N form provided in isolation exceeded uptake of the same form in mixture. At the high deposition site, uptake of 15N from amino acids was higher than uptake of 15N from inorganic nitrogen. At the low deposition site, uptake of 15N from all three forms of N were similar. Completeness of the associated food web had no effect on 15N uptake. By taking up intact amino acids, Sarracenia purpurea can short-circuit the inorganic N cycle, thus minimizing potential bottlenecks in N availability that result from the plant’s reliance for nitrogen mineralization on a seasonally reconstructed food web operating on infrequent and irregular prey capture.

Tree Growth and Above-Ground Biomass at Harvard Forest HEM and LPH Towers since 2001

Tree diameter (“dbh”) at 1.25 m above ground were recorded and stainless steel dendrometer bands for trees above 10 cm dbh were attached. Increases in tree diameter were calculated from increases in the distance between holes punched in the dendrometer bands. Tree diameters and diameter increases were used to estimate aboveground biomass and aboveground carbon storage in order to characterize the forest at the flux tower sites, and to quantify the amount of carbon being stored aboveground annually.

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Land Use and Forest Dynamics at Harvard Forest 1937-2013

Modern vegetation patterns have developed as a result of species response to environmental gradients, disturbance history, and biotic interactions. However, the relative importance of environmental versus historical factors in determining modern species distributions may differ from site to site and may change with increasing time since disturbance. In addition, life-forms may differ in their response to environmental versus historical disturbance gradients. In the current study, we relate vegetation patterns across the Prospect Hill Tract of Harvard Forest to variability in edaphic factors and disturbance histories. Modern species distribution patterns are related to both edaphic and disturbance gradients. Although species respond individualistically to edaphic and disturbance gradients, land-use history, soil drainage, and C:N ratio are good predictors of the distribution of many species. In contrast, 1938 hurricane damage is less important. Vascular species richness is related to historical land-use, with fewer species in historical woodlots than on former agricultural sites. This is contrary to what has been reported from several other studies, and may reflect the dominance of historical woodlots in the study area by hemlock, a species that creates environmental (i.e. light and soil) conditions that are unsuitable for many species. Historical land-use does not appear to influence bryophyte species richness, although the distributions of several species do follow patterns of land-use. Species richness also varies according to soil drainage, with higher bryophyte, herb, and shrub richness and lower overstory and understory tree richness on wetter sites.

Leaf Area Index at Harvard Forest HEM and LPH Towers since 1998

Leaf area index (LAI) measurements are made to detect changes in forest leaf area that are important in understanding forests’ carbon dioxide (CO2) uptake and water use. Measurements of LAI in different forest types are useful understanding differences between forests in CO2 uptake and water use, while variation in LAI in the same forest type over time helps to explain interannual variation and long-term trends in carbon storage and water use. Within a single year, especially for deciduous forests, seasonal changes in LAI are very important in determining the forest’s cycle of CO2 uptake and water use. LAI measurements at Harvard Forest were begun in the old-growth hemlock stand in 1998 in order to understand and develop a predictive model for its carbon exchange, and were begun at the Little Prospect Hill site in 2002 to better understand CO2 uptake and carbon storage at this site, which were measured by the eddy-covariance method beginning in 2002. Due to a lack of personnel, multiple annual measurements of LAI to examine seasonal change in leaf area did not begin until 2007.

Litterfall at Harvard Forest HEM and LPH Towers since 2002

We measure the dry mass and estimated carbon content of leaves and fine twigs that abscise each year. These data are used to quantify one component of annual net primary productivity (ANPP), an important measure of ecosystem productivity and carbon cycling. Leaf area index (LAI) can also be estimated from these data, if area-to-mass ratios are calculated for leaves of each species. This was done for 2002 and 2003 leaf litterfall at the Little Prospect Hill (LPH) site. The fraction of total leaf mass or estimated total leaf area that is formed by leaves of each species also shows the species composition of the forest canopy. If repeated over many years, these data can indicate whether the forest canopy species composition is changing over time.