Pine Canopy Research Articles

In situ measurements of C2‐C10 volatile organic compounds above a Sierra Nevada ponderosa pine plantation

A fully automated gas chromatograph‐flame ionization detector system was designed and built to measure ambient concentrations of C2‐C10 volatile organic compounds, including many oxygenated compounds, without using liquid cryogen. It was deployed at Blodgett Forest Research Station in Georgetown, California, 38°53′42.9″N, 120°37′57.9″W, 1315 m elevation. More than 900 in situ measurements were made above a ponderosa pine canopy at 40‐min intervals, continuously from July 2 through August 1, 1997. Factor analysis and observed temporal patterns were used to categorize sources for measured compounds as biogenic or anthropogenic or both. Compounds that were clearly biogenic included methylbutenol, isoprene and its oxidation products (methacrolein and methyl vinyl ketone), and terpenes (α‐pinene, 3‐carene, d‐limonene). Other compounds were partially biogenic, including acetone, ethene, propene, hexanal, acetaldehyde, and methanol. Hydroxyl radical (OH) loss rates were dominated by the clearly biogenic compounds, accounting for 70% of the loss under mean midday conditions. The most important single compounds were isoprene (33%) and methylbutenol (21%). These two compounds were dominant under all conditions, including the coldest and most polluted days. Under the most polluted conditions, acetaldehyde became very important, accounting for 13% of the total. Total OH loss rates were highly correlated with temperature because emissions of biogenic compounds, which dominate OH loss, are strongly temperature dependent. Much of the research on biogenic volatile organic compounds has focused on isoprene and terpenes. Our results suggest that quantifying and understanding factors controlling biogenic emissions of other compounds such as methylbutenol, acetone, hexanal, methanol, and acetaldehyde are critical for improving our understanding of regional photochemistry.

Interactions between precipitation and Scots pine canopies along a heavy-metal pollution gradient

Bulk precipitation and stand throughfall were collected during 1992–96 at distances of 0.5, 4 and 8 km from the Harjavalta Cu–Ni smelter, southwestern Finland. The amounts of heavy metals (Cu, Ni, Zn, Fe) and mineral nutrients in bulk precipitation and throughfall were highest at 0.5 km. Although the canopy coverage was low at 0.5 km, the amounts of heavy metals intercepted by the canopy were extremely high. The proportion of foliar leaching relative to the wash-off of dry deposition from the needle surfaces decreased on moving towards the smelter for all elements, except for K. The high rate of K leaching from the needle tissues close to the smelter demonstrated that the K throughfall flux has been greatly altered by the heavy pollution load.

Below-canopy and soil CO 2 fluxes in a ponderosa pine forest

Below-canopy eddy covariance measurements of CO 2 flux ( F cb) and soil surface CO 2 flux measurements ( F s) were made seasonally in a ponderosa pine forest in central Oregon in 1996 and 1997. The forest ecosystem has a very open canopy, and it is subject to drought and high vapor pressure deficits in summer. Below-canopy flux measurements in March, May, and August 1997 showed increasing effluxes from the forest floor as soils warmed. In July 1996, daytime F cb measurements appeared to have been influenced by photosynthetic uptake of CO 2 by ground vegetation. We did not see a similar diurnal trend in F cb data in August 1997, probably because photosynthesis may have decreased with senescence of ∼1/3 of the pine canopy and the herbaceous species. On 4 days in August 1997, the mean nocturnal F s (2.6 ± 0.08 μmol m −2 s −1) was lower than nocturnal F cb (3.5 ± 0.28 μmol m −2 s −1) by 26%, and daytime F s was lower than nocturnal F cb by 18%, possibly because F cb includes respiration by understory and the lower portions of trees. The mean nocturnal NEE calculated from above-canopy flux and storage in the canopy airspace ( F ca + F stor) at this time was 2.8 ± 0.40 μmol m −2 s −1, 23% lower than ecosystem respiration calculated from chamber measurements on soils, wood, and foliage. The largest difference was observed on a more a turbulent night ( u * = 0.30 m s −1) when F ca + F stor was even significantly less than F cb and F s. Our hypothesis is that under calm conditions (e.g. u * < 0.15 m s −1 as observed on three of the nights), F ca is negligible and has no impact on the CO 2 budget. Under weak wind conditions (e.g. u * = 0.30 m s −1), F ca begins to become significant and fluxes missed by the above-canopy eddy correlation system degrade the CO 2 budget. Under windy conditions, the above-canopy eddy correlation measurement is a good approximation and the CO 2 budget improves again. Below-canopy flux measurements provided useful temporal information for understanding seasonal differences in diel patterns, while the chambers allowed us to characterize spatial variation in CO 2 fluxes. It is important to measure below-canopy fluxes along with above-canopy fluxes throughout the year to understand CO 2 exchange components and annual contributions to the carbon budget of open canopy forest systems.

Moose, Trees, and Ground-Living Invertebrates: Indirect Interactions in Swedish Pine Forests

The role of moose in structuring the boreal forest ecosystem has been studied extensively in recent years. This research has focused mainly on the effects of moose on vegetation and soil dynamics. However, the extent to which these effects influence animal communities has received little attention. We studied how invertebrate assemblages on the forest floor of two Swedish pine forests were affected by the presence of moose. Our approach was to trap invertebrates using pitfall traps in unbrowsed exclosures and browsed reference plots, estimate the percentage cover of herb-layer plants, measure the height and density of tree species, and measure the accumulation of leaf litter. The effect of moose on tree strata differed between the two areas. In Sunnas, where the stand was a mixture of deciduous trees and pines, the main effect of moose was to eliminate most of the deciduous trees from the canopy. Here the amount of deciduous leaf litter was ten times higher in unbrowsed exclosures compared with browsed plots. The stands on the dry sites in the other area, Furudal, were almost pure pine. Here, moose browsing reduced the density of taller pines, but increased the number of pine saplings. At Sunnas, the herb-layer plant cover did not differ between browsed and unbrowsed plots. However, there was a difference between exclosures and browsed plots in terms of the ground-dwelling invertebrate assemblage. Number of individuals of most invertebrate groups that differed between exclosures and browsed plots were higher in unbrowsed plots and the total number of invertebrates was also higher in unbrowsed plots, but diversity of invertebrate fauna was higher in browsed plots. At Furudal both the vegetation and invertebrate assemblages differed between browsed and unbrowsed plots. Browsed plots had a higher cover of ground lichens, while unbrowsed plots had more mosses. Among the invertebrate taxa that differed between treatments, the number favored by browsing and the number disfavored by browsing were roughly equal, and diversity was not affected by moose. We suggest that by affecting the density and composition of tree species, moose indirectly influenced the structure of invertebrate communities. At Sunnas, the reduction in the amount of deciduous leaf litter was probably the key effect. Vegetation was not affected much, but the differences in litter layer composition had affected the microclimate on the forest floor while also providing an important additional trophic resource and habitat for ground-dwelling invertebrates. At Furudal the main effect of moose was to reduce the density of the pine canopy, thus increasing the amount of light penetrating to the forest floor. As a consequence, the floral composition and microclimate was altered which, in turn, affected the invertebrate fauna.

Seed predation and dispersal in relict Scots pine forests in southern Spain

For two years, the seed rain and magnitude of seed losses due to predation were evaluated in Scots pine forests in southern Spain. The Crossbill was the most important pre-dispersal predator, consuming more than 80% of ripening seeds. In addition, other birds, mainly Tits and Siskin, also consumed seeds just before seed dispersal, reaching values of 16 and 51% losses in 1996 and 1997, respectively. Seed rain was monitored in different microhabitats (under pine canopies, under shrubs and in open areas), and was most intense under the canopy of mother plants both years. Post-dispersal seed predators (rodents and birds) consumed up to 96% of seeds reaching the ground. Both pre- and post-dispersal seed predators preferentially harvested filled seeds. Post-dispersal predation was similarly intense in all microhabitats, so predators did not change the spatial distribution of the seed rain. These high predation rates were constant between years, localities and habitats (woodland and treeline). We hypothesize that this high rate of seed predation is a major factor limiting the regeneration of these relict populations of Scots pine in its southernmost limit.

Quantifying spatial variability of photosynthetically active radiation within canopies using a multipoint measuring system

A multipoint measuring system of photosynthetically active radiation (PAR) was used to investigate the effect of the number and position of measuring points in determining the dependence of the photosynthetic rate on PAR within a Scots pine canopy. A regular grid of 400 measuring points was incorporated into a cuvette to observe the distribution of PAR falling on the needles of a twig along with simultaneous measurements of the CO 2 exchange. Measurements were made in the middle layer of the crown, at a level characterised by a high spatial and temporal variability in PAR. The results show that measuring points covering the entire needle region in an even manner are needed for accurately quantifying PAR and reliable estimation of the photosynthetic rate. The positioning of the measuring points is of primary importance: even a large number of measuring points can lead to high errors if the positioning is not representative in relation to the twig.

Gap-phase regeneration in longleaf pine wiregrass ecosystems

Naturally regenerated seedlings of longleaf pine are typically observed to cluster in the center of tree fall canopy gaps and be encircled by a wide zone from which they are generally excluded. Twelve representative canopy gaps distributed across 600 ha of a naturally regenerated uneven-aged longleaf pine forest in the sandhills of north central Florida were examined to determine which aboveground and belowground factors are responsible for development of this seedling exclusionary zone. Within 12 m of adult trees growing along the gap edge, significantly fewer longleaf pine seedlings were present. The canopy of overstory trees, however, extended only 4–5 m into the gap. The relatively open structure of the longleaf pine canopy (57% cover) allowed photosynthetically active radiation (PAR) to be evenly distributed upon the forest floor across each canopy gap. Thus, light availability was not related to pine seedling clustering near gap centers. Significantly greater forest litter mass beneath adult trees (5 Mg ha −1) could result in fires more intense than would be supported by the litter mass near gap centers (2.5 Mg ha −1). However, litter mass was significantly elevated only within 4 m of the gap edge. The fine root biomass within 12 m of the gap edge (3–4.5 Mg ha −1) was two to six times that measured near the gap center and most closely coincided with the width of the seedling exclusionary zone along the margin of each gap. Thus, while the canopy of adjacent adult trees may indirectly influence seedling mortality through deposition of needle litter and greater fire intensity within 4 m of the gap edge, the root systems of these adults also appear to directly compete with seedlings within 12–16 m of the gap edge for limited site resources. To effectively regenerate and sustain longleaf pine wiregrass ecosystems, caution should be used in prescribing single-tree selection harvest methods so as not to create gap openings so small (<30 m diameter) that intraspecific competition from adult trees totally excludes seedlings. Group selection methods may prove more effective in creating canopy gaps of sizes suitable (≥40 m diameter) for ensuring that intraspecific competition from adults is maintained at a level which allows for survival and growth of seedlings. Shelterwood and deferment methods may also prove useful where more open canopy conditions are desired. However, the pine seedlings' need for light cannot be used as an appropriate rationale for application of clearcutting methods.

Development of oaks ( Quercuspetraea (Matt.) Liebl.) emerged from bird-dispersed seeds under old-growth pine ( Pinussilvestris L.) stands

In East Germany, there are a lot of areas covered by old pine stands. They are growing on soils, on which under natural conditions without anthropogenic impact mixed stands of oak ( Quercus petraea (Matt.) Liebl.) and pine ( Pinus silvestris L.), would form a natural forest. An important objective of ecological silviculture in these areas is to convert the pure pine stands into mixed oak–pine stands by using natural regeneration methods. A highly appreciated assistant in this connection is the European jay ( Garrulus glandarius L.). The remarkable results of its work have been found by analyzing the natural regeneration in a few old pine stands in the forest district of Weißwasser in Saxony. Although mother trees of oak are very scarce there, natural regeneration of oak was found everywhere in the research area. The oak regeneration, undoubtedly created by the jay, amounted to at least 2000 oaks/ha. They were mainly distributed at random, while pine regeneration was aggregated in places where gaps in the pine canopy occurred. Oak regeneration was established much earlier than pine regeneration. Thus, oaks exceeded pines in age, height and diameter. The oaks were also superior to pines concerning height increment for the last three years. Thus, there is a good chance for oak to defend its prevailing role in the regeneration in the future. Probably, the next forest generation will be composed of oak and pine trees. The analysis of the quality of the oak regeneration shows that there is no substantial difference to artificially sown oak stands. This indicates that the European jay creates oak stands sufficiently both in number and quality.

Using Caribbean pine to establish a mixed plantation: testing effects of pine canopy removal on plantings of rain forest tree species

Our study tested the potential for establishing shade-tolerant tree species within different canopy removal treatments of an 18-year-old Pinus caribaea (Caribbean pine) plantation. We investigated whether planting within a Pinus plantation can be a solution to the dispersal, weed competition, and pathogen/insect problems rain forest tree species have during their initial establishment on sites previously cleared of forest. The plantation was originally established on abandoned swidden adjacent to the Sinharaja Man and the Biosphere rain forest reserve in southwestern Sri Lanka. The five species selected for the study were Caryota urens, Dipterocarpus zeylanicus, Pericopsis mooniana, Shorea stipularis, and Swietenia macrophylla. Seedlings were monitored over two years for survival, root collar diameter and height growth along transects that were in seven different environmental treatments within the Pinus plantation. Environmental treatments comprised: (i) under plantings beneath a closed canopy of Pinus that were unaffected by any row removal; (ii) the centres of clearcut strips of Pinus that were 6 m width; (iii–vii) and five treatments across clearcut strips within closed canopy Pinus that were 12 m wide. The five treatments within the 12 m wide clearcut strip included inside edges beneath the Pinus canopy on the eastern (iii) and western (iv) sides of the strip; outside edges on the eastern (v) and western (vi) sides of the strip; and the centre (vii) of the strip opening. At the end of two years, a sample of the seedlings growing within each environmental treatment were taken and measured for dry masses and leaf areas. Results demonstrate that all species grew poorly and had higher mortality in the Pinus understorey than the other environmental treatments. The dipterocarp species ( D. zeylanicus, S. stipularis) were slower growing and more site-sensitive to changes in environmental treatment than the non-dipterocarp species. S. macrophylla grew tallest in all environmental treatments as compared to the other species. Overall, the best environments for seedling establishment and growth for all species were in the centres of the canopy strips (6–12 m). This study contributes to investigations testing the feasibility of using Pinus as a nurse for establishing more shade-tolerant species; and as a technique for forest restoration in south and southeast Asia.

Effects of acidic deposition on soil ecosystems under forest in the Chongqing region of China

The effects of acidic deposition on soil ecosystems under temperate coniferous forest in the Chongqing region of China were investigated from 1993 to 1994. Precipitation, throughfall, stemflow, soil solutions, and soil samples were collected to estimate the acidification of soil ecosystems through the changes of their chemical components. The concentrations of ion species in the throughfall and stemflow under masson pine forest in Mt. Zhenwu were higher than those under mixed coniferous forest in Mt. Jinyun and under camphor tree forest in Laojundong, suggesting that Mt. Zhenwu is located in the vicinity of the Chongqing center and that it allowed the canopy of masson pine to intercept air pollutants. However, the level of aluminum dissolution into soil solutions was relatively low under masson pine forest in Mt. Zhenwu in spite of the low pH in the stemflow.

Palatability of Coloradia pandora (Lepidoptera: Saturniidae) Eggs to a Rodent Predator: Contributions of Physical and Chemical Characteristics

Field observations of remnants from predation on pandora moths, Coloradia pandora Blake, suggested that eggs from gravid females were being rejected. Captive feeding trials with golden-mantled ground squirrels, Spermophilus lateralis Say, verified that this rodent is a natural enemy of C. pandora that rejects eggs while feeding on moths. Nutritional analyses indicated C. pandora moths (both females and males) and eggs are of comparable food quality, so additional feeding trials were conducted to determine why eggs were unpalatable to the ground squirrels. Chemical fractions of eggs did not inhibit consumption consistently enough to explain the rejection behavior; neither did the extracted residue of egg tissue. Whole eggs, whole shells, and ground shells did inhibit consumption by the same ground squirrels. A follow-up trial with a naive group of S. lateralis confirmed that whole eggs and whole shells inhibit consumption. Consumption of egg contents was equivalent to controls for both groups of S. lateralis. We conclude that C. pandora eggs are protected physically, but not chemically, from consumption by S. lateralis. The foraging range of S. lateralis, a generalist predator in central Oregon, includes the lower canopy of pine ( Pinus spp.) forests where C. pandora eggs are deposited. Therefore, the tough eggshell probably inhibits S. lateralis predation of C. pandora eggs under natural conditions.

VEGETATION-ENVIRONMENT RELATIONSHIPS IN THE ULTRAMAFIC AREA OF MONTE FERRATO, ITALY

Relationships between some environmental features and species composition and abundance of grassland and dwarf shrub vegetation were investigated on Monte Ferrato, one of the best known ultramafic (serpentine) sites of Italy. The main aim was to test the importance of the available fraction of soil metals in causing the typical infertility of ultramafic soils. The physical and chemical features of soil were determined for each plot in which species composition and cover were recorded. The plots were classified by cluster analysis and ANOVA was applied to compare the environmental variables of groups of plots. Canonical Correspondence Analysis (CCA) was used to detect the principle factors related to the gradient of species composition within the plant communities. It was found that the grassland and dwarf vegetation of Monte Ferrato is not negatively influenced by soil content of nickel and other metals. Pine canopy cover, which provides additional nutrient input and protects against erosion, was found to be important for evolution of the garigues into grasslands. The evolution of grassland turf induced the retention of higher levels of exchangeable cations, including potentially toxic metals, in the evolved soil.

A novel multipoint measuring system of photosynthetically active radiation

A novel multipoint measuring system of photosynthetically active radiation (PAR) has been designed. It is used as a component of a field measurement system of photosynthesis. The system consists of a multichannel fiberoptic sensor, Intel 486-based microcomputer (PC) with software for control and analysis and interface electronics. The fiberoptic sensor comprises 800 measuring points which are arranged in a regular grid on a plane. This grid is attached to a cuvette to observe the spatial and temporal distribution of PAR falling on the needles along with simultaneous measurements of CO 2 exchange. PAR is registered through a fiberoptic bundle using a charge coupled device (CCD) sensor. The system has been in operation between July and October 1996 within a Scots pine canopy. The results demonstrate that the obtained regression between the photosynthetic rate estimated with the multipoint PAR measuring system and the measured CO 2 exchange rate is as tight within a canopy as in unshaded conditions.

Utilizing a multipoint measuring system of photosynthetically active radiation in photosynthetic studies within canopies

A novel multipoint measuring system of photosynthetically active radiation (PAR) has been constructed and operated within a Scots pine canopy. A regular grid of 800 measuring points has been incorporated into a cuvette to observe the spatial and temporal distribution of PAR incident on the needles of a twig along with simultaneous measurements of the CO2 exchange in order to determine the dependence of photosynthesis on PAR. It was shown that large errors can result if the photosynthetic rate is estimated using the mean value of PAR instead of the instantaneous values of PAR detected at given points in the region of the needles. The results demonstrate that the obtained regression between the CO2 exchange rate estimated using the multipoint PAR measuring system and the measured CO2 exchange rate is as good within a canopy as in unshaded conditions.

Bidirectional reflectance of selected BOREAS sites from multiangle airborne data

Hyperspectral, multiangle reflected radiances were obtained using the airborne advanced solid‐state array spectroradiometer (ASAS)over Boreal Ecosystem‐Atmosphere Study (BOREAS) sites in Canada during four field campaigns in 1994. Atmospherically corrected bidirectional reflectance factors and estimates of spectral hemispherical reflectance for three canopies in the BOREAS southern study area (old aspen, old black spruce, and old jack pine) are presented. The multiangle spectral reflectance factors derived from data acquired July 21, 1994 (for a limited solar zenith angle range of 34°–37°) for the forested sites showed distributions of high backscatter and consistently low forward scatter due to shadowing. Position and shape of the retrosolar maximums for the three sites varied. Spectral distinction among the sites was evident in the red, where reflectance factors for the jack pine exceeded those for spruce and aspen, and in the near infrared (NIR), where the aspen reflectance factors were twice those for the conifers. Data presented here suggest that spectral reflectances acquired at 26° backscatter in the principal plane would provide better discrimination among the major cover types than those obtained from a nadir view. Red reflectance was strongly dependent on view geometry for the spruce and jack pine sites due to the varying amounts of deep shadow and red‐reflecting materials observed as a function of view azimuth and zenith. At the aspen site the red reflectance displayed much less variation with changing view zenith and azimuth. Accordingly, angular effects on the normalized difference vegetation index were large for the spruce and jack pine canopies but small for the aspen site. Estimated spectral hemispherical reflectances for photosynthetically active radiation (PAR) (0.4–0.7 μm), red (0.63–0.69 μm), and NIR (0.83–0.87 μm) calculated using various combinations of azimuthal data sets as input to the Walthall et al. [1985] model showed the following trends: maximum estimates were generated using data from the solar principal plane (spp) only; minimum values were derived from perpendicular plane data and amounted to 50–83% (relative) of the corresponding spp value; and inclusion of data from three view azimuths together (spp + perpendicular + oblique) produced intermediate values totaling 73–91% (relative) of the spp result. A preliminary review of ASAS‐derived and independent ground‐based measures of PAR hemispherical reflectance revealed a sizable range in this parameter.

Structure and abundance of arachnid communities in Scots and lodgepole pine plantations

Between April and September 1994, six plots within Aberfoyle Forest in Scotland, three of Scots ( Pinus sylvestris) and three of lodgepole pine ( Pinus contorta), were surveyed for spiders and harvestmen, as these are predators of several forest invertebrate pests. A total of 64 species of spider were trapped in Scots and lodgepole pine; 56 in Scots pine and 32 in Lodgepole pine. The most abundant spiders, both in Scots and Lodgepole pine, were Linyphiidae. Scots pine plots were floristically more diverse and structurally more complex than lodgepole pine plots. More species of spiders and harvestmen (midsummer only) were found at ground level in Scots pine than lodgepole pine plots. More species and a greater numbers of spiders were found in the canopy of Scots pine than in that of lodgepole pine. However, no differences in the abundance of harvestmen were found between the canopies of Scots and lodgepole pine. Spider species diversity was greatest in Scots pine.

Remote sensing the biochemical composition of a slash pine canopy

Airborne imaging spectrometers can record spatially-explicit information on the absorption features associated with foliar biochemicals in a forest canopy. The spectra of a single species pine canopy were recorded by the National Aeronautics and Space Administration's (NASA) Airborne Visible/Infrared Imaging Spectrometer (AVIRIS). Up to three wavebands were correlated to the concentration of chlorophyll, nitrogen, lignin, and cellulose (R/sup 2/=0.96,0.94,0.93, and 0.61, respectively) and the content of these four biochemicals (R/sup 2/=0.98,0.91,0.88, and 0.92, respectively). The AVIRIS data were used, for the first time, to map the content of these biochemicals within the forest canopy and the accuracy was between 3-7% of the mean.

Measurement and modelling of the photosynthetically active radiation transmitted in a canopy of maritime pine

Pour la modelisation de la photosynthese d'un couvert vegetal, il est important de connaitre la quantite de rayonnement utile a la photosynthese (PAR) absorbe par les couronnes et le sous-bois. Dans cet article, un modele semi-empirique, exploitant la loi de Beer, ainsi que les variations saisonnieres du PAR sont presentes. L'objectif de l'etude est de confirmer que le rayonnement utile a la photosynthese et le rayonnement solaire suivent les memes lois d'interception pour le direct et pour le diffus en integrant les valeurs mesurees de reflectance et de transmitance. Le modele etabli prend en compte le rayonnement direct et le rayonnement diffus. Le rayonnement rediffuse par le houppier est estime a partir des equations de Kubelka-Munk. Lorsque ce terme est neglige, on montre que l'erreur induite sur le bilan radiatif est faible. Les entrees du modele sont deduites des mesures effectuees sur une foret de pin maritime durant l'ete et l'automne 1995. La comparaison entre les resultats du modele et les mesures est satisfaisante aussi bien pour le rayonnement direct que pour le rayonnement diffus. En conclusion, bien que les ordres de grandeurs et les domaines spectraux des mesures soient differents, la penetration du rayonnement utile a la photosynthese peut etre estime par un simple modele semi-empirique deja etabli pour le rayonnement solaire.

A β-sitosterol assay for fine-root mass in soils

The structural complexity and spatial heterogeneity of soils make the direct measurement of biomass a tedious task. These difficulties have led to the development of indirect assays based on biochemical indicators. We have investigated the potential of β-sitosterol, a component of plant membranes, as an indicator of phytomass in soils. β-sitosterol was extracted from soil, root and litter samples with hot methanol, following the procedure used for fungal ergosterol, and quantified by HPLC. Concentrations in fine roots (<2 mm dia) of 16 herbaceous and woody plants ranged from 0.2 to 2.0 μg mg −1 organic mass (mean = 1.0, SD = 0.5). Concentrations in 2 cm dia × 15 cm deep soil cores collected under old field, deciduous hardwood and pine canopies ranged from 2.4 to 26.5 μg g −1 soil dry mass, equivalent to fine-root standing crops of 5–60 Mg ha −1. Microcosm studies indicated that in the early stages of decomposition β-sitosterol degrades at about the same rate as bulk litter. The β-sitosterol assay may facilitate the analysis of mycorrhizal associations and the quantification of total fine-root mass.

Seasonal variation of carbon dioxide exchange rates above and below a boreal jack pine forest

Fluxes of carbon dioxide were measured above and below a boreal jack pine stand with the eddy covariance method. Our goal was to examine the seasonal variation of the net carbon dioxide exchange between the ecosystem and the atmosphere and to understand the relative contributions of the vegetation and rhizosphere to these fluxes. Over the course of the growing season the jack pine stand was a small, net sink of carbon dioxide—47 g C m −2 were assimilated during a 115 day period between spring (day 144) and autumn (day 259). On a daily basis, the magnitude of the net uptake of CO 2 by the forest was relatively small, less than 4 g C m −1 day −1. We also observed many days when carbon dioxide was lost from the forest stand (up to 2 g C m −2 day −1). Day to day sums of net carbon dioxide exchange exhibited little seasonality, despite the seasonality experienced by controlling environmental and biological variables. The aseasonality of net canopy CO 2 exchange occurred because temporal variations associated with canopy photosynthesis and plant and microbial respiration offset one another over the growing season. Canopy photosynthesis rates of the jack pine stand were evaluated using CO 2 flux measurements made above and below the canopy. Daily totals of canopy photosynthesis were a linear function of available solar energy and exhibited a quadratic dependence on temperature, with optimum photosynthesis occurring between 15 and 25°C. We also observed that the net daily uptake of carbon dioxide was independent of soil moisture in the upper 0.15 m of the soil profile, despite the wide range of soil moisture that was experienced. Finally, the net gain of carbon by a jack pine canopy came with a relatively large respiratory expense. On average, 68% of the carbon gained by photosynthesis was consumed by plant respiration, over the course of a day.