Young Jack Pine Research Articles

A comparison of the mosaic and aggregated canopy frameworks for representing surface heterogeneity in the Canadian boreal forest using CLASS: a soil perspective

The Canadian Land Surface Scheme is employed in off-line tests to represent a 1200 km 2 patch of the Boreal Ecosystem-Atmosphere Study Northern Study Area, a heterogeneous region of boreal forest in north-central Manitoba. The soils in the region vary from rapidly draining coarse sand to poorly drained areas of peat overlying clay, including wetlands. Observed surface fluxes from four sites, a young jack pine stand, an old jack pine stand, an old black spruce stand and a FEN, are scaled by proportional weighting to represent the study area. The performance of the mosaic approach, in which the region has been represented by four distinct patches, is compared with that of the aggregated approach, in which mean surface parameters have been defined based on fractional coverage. Various soil columns are employed in the aggregated approach, in order to assess the effects of representing the heterogeneous soils of the region in a single column. While the conservative water use at all of the sites limits the effects of changes in the soil column on the turbulent fluxes, important effects are noted. Soil evaporation is overestimated when an organic or composite soil layer is represented at the surface, and underestimated when a sandy soil column is represented. Soil evaporation can be calibrated to match that of the mosaic method, however, the need for more robust methods of accomplishing this is acknowledged. The process of aggregating the soil column eliminates the dry sandy zones which experience periods of moisture stress. This results in periods of increased canopy conductance and an overestimation of the latent heat flux. A 2-patch mosaic, with one patch consisting of sandy soil and the other patch of organic soil over clay, allows moisture stress to occur and produces modelled half-hourly sensible and latent heat fluxes that agree more closely to those of the 4-patch mosaic than any of the aggregated runs, across the range of hydro-meteorological variables. Throughout the course of the model runs presented here, the 2-patch mosaic also exhibits cumulative drainage and evapotranspiration closer to that of the 4-patch mosaic than any of the aggregated soil columns.

KIRTLAND'S WARBLER DIET AS DETERMINED THROUGH FECAL ANALYSIS

The endangered Kirtland's Warbler (Dendroica kirtlandii) nests primarily in large (>32 ha) stands of young (5- to 25-yr-old) jack pine (Pinus banksiana) which grow on Grayling sand soil. These specific habitat requirements restrict the Kirtland's Warbler breeding range to only 13–16 counties in the northern lower peninsula of Michigan. Although the nature of the species' affinity for this habitat is poorly understood, one theory suggests that higher prey abundance in young jack pine may play a role. To explore further the hypothesis that Kirtland's Warblers choose nesting habitat due to prey abundance, a more thorough knowledge of the warblers' diet is needed. To better understand the diet, we identified arthropod and plant fragments found in 202 Kirtland's Warbler fecal samples, collected from June to September, 1995–1997. The major food items recorded were spittlebugs and aphids (Homoptera; found in 61% of all samples), ants and wasps (Hymenoptera; 45%), blueberry (Vaccinium augustifolium; 42%), beetles (Coleoptera; 25%), and moth larvae (Lepidoptera; 22%).

Near‐surface climate in the boreal forest

This paper addresses processes that affect near‐surface climate over the boreal forest, using data from the Boreal Ecosystem‐Atmosphere Study (BOREAS) northern study area just west of Thompson, Manitoba. The boreal forest is marked by a very large seasonal cycle with below‐freezing temperatures for half the year. The freezing and thawing of the soil plays an important role in the climate at high latitudes. It moderates winter temperatures (together with the insulating snow cover), because during the freeze process, the effective heat capacity of the soil is greatly increased, and it introduces a significant lag into the climate system. Perhaps the most important consequence is that water is unavailable for evaporation and photosynthesis until snow melts and the ground thaws, which occurs late in spring. As a result, in April and in early May, relative humidity (RH) is a minimum, the surface sensible heat flux is large, and the daytime boundary layer (BL) is very deep, because of this unavailability of water. The situation reverses in the fall, when the ground is warmer than the cooling atmosphere, and mean RH is high and BL depths low. This asymmetry between spring and fall can be seen in both seasonal and diurnal cycles. The forest is heterogeneous, and there is a marked difference in summer in daytime evaporative fraction between the conifers and the deciduous forest and fens. However, above the forest the daytime BL has a strong homogenizing effect, and it is the dominant coniferous forest that controls the mean BL depth. The impact of recent rainfall, stored on the canopy, in the surface moss layer, and in the top soil layer can be readily seen in summer. BL depths rise on succeeding days without rain. A comparison of the fen and young jack pine sites shows the important role of the stomatal control by conifers on transpiration. Since evaporation goes down at high net radiation and low RH for conifer sites, it is clear that the low RH and high BL depth over the forest are a direct consequence of stomatal control. At night, however, temperature, relative humidity, and CO2 are quite heterogeneous under the stable BL. We show that uncoupling of the stable BL at night inside the forest canopy occurs at low wind speeds and high outgoing net radiation and can lead to a 5K cooling within the canopy.

COMMUNITY-WIDE IMPACTS OF A GENERALIST BROOD PARASITE, THE BROWN-HEADED COWBIRD (MOLOTHRUS ATER)

Many ecologists have searched for species that contribute strongly to the structure and composition of communities of organisms. It has been speculated that the Brown-headed Cowbird (Molothrus ater), a generalist brood parasite, is capable of changing songbird communities. Cowbird parasitism may reduce numbers of suitable hosts, i.e., songbirds that accept cowbird eggs and raise cowbird young. In contrast, songbird species that eject cowbird eggs, nest in cavities, feed their nestlings mainly seeds or fruits, or are too big to parasitize, should escape the effects of cowbird parasitism. Thus, cowbirds may change the composition of songbird communities by selectively depressing numbers of suitable host individuals and species. We tested this hypothesis using an existing cowbird removal program in the state of Michigan, USA. This program was designed to protect the endangered Kirtland's Warbler (Dendroica kirtlandii) from cowbird parasitism throughout its 19 200-km2 breeding range. We compared songbird community composition in stands of young jack pine (Pinus banksiana) where cowbirds had been removed for 5–11 yr to communities in Control sites 5–10 km from cowbird traps and >10 km from cowbird traps. We predicted that cowbird Removal sites would support greater songbird richness and evenness and a greater proportion of suitable host vs. unsuitable host individuals relative to Control sites. Results from songbird point counts revealed that species richness and evenness were very similar at cowbird Removal and Control sites and that Removal sites contained only 4.0–8.7% more suitable host individuals than Control sites. Our results suggest that cowbirds do not strongly influence the composition of songbird communities in jack pine forests of Michigan. Several factors may explain a lack of community-wide response to long-term cowbird removal, including the population dynamics of songbirds on a broader scale. We conclude that there is little support for adding cowbirds to the short list of species that can regulate entire communities. Our results have implications for the increasingly widespread use of cowbird removal as a management tool.

Short-term Effects of Harvest Technique and Mechanical Site Preparation on Arthropod Communities in Jack Pine Plantations

Arthropods play a key role in the functioning of forest ecosystems and contribute to biological diversity. However, the influence of current silvicultural practices on arthropod communities is little known in jack pine (Pinus banksiana) forests, a forest type comprising a major portion of the Canadian boreal forest. In this study, the effects of silvicultural treatments on arthropod communities were compared to identify those treatments that minimize ecological impacts on arthropods. The influence of harvesting techniques and mechanical site preparations on insect family richness and abundance of arthropods (total, by orders and by trophic groups) was examined in young (three-year-old) jack pine plantations of northern Ontario. Each of the following treatments were conducted in three plots: (1) tree length harvest and trenching; (2) full tree harvest and trenching; (3) full tree harvest and blading; and (4) full tree harvest and no site preparation. Arthropods were collected using sweepnets and pitfall traps over two years. Blading significantly reduced insect family richness, the total abundance of arthropods, abundance of Orthoptera, Heteroptera, Hymenoptera, Diptera, insect larvae, and plant feeders when compared to the other treatments. The use of either full tree or tree length harvesting had similar short-term effects on family richness and the abundance of arthropods. Arthropod diversity declined with increasing post-harvest site disturbance. These results suggest that arthropod communities in the understory and on the ground are reduced most on sites mechanically prepared by blading, but are similar under conditions immediately following either full tree or tree length harvesting. The implications for regenerating jack pine in the boreal forest are discussed.

Foliar application of benzyladenine (BA) as a means to enhance shoot production for asexual propagation of young jack pine ( Pinus banksiana Lamb.) seedlings

Foliar applications of benzyladenine (BA) wereinvestigated as a potential means forenhancing development of proliferated dwarfshoot (pds), which may serve as stem cuttingmaterial for asexual propagation of young jackpine seedlings. BA application in combinationwith pruning promoted pds production. Potential production of 30–40 shoots per BAtreated plant were obtained within 6 monthsfrom sowing of seed, compared to 14–17 forpruned controls (no BA application) in thesame time period. The BA treatment thatresulted in the highest number of pds variedwith the frequency of BA application. Higherconcentrations of BA generally resulted ingreater numbers of pds. Increasing BAconcentration and frequency of applicationreduced rooting capacity of cuttings as wellas lengths of pds. The optimal BA treatmentsfor both total pds production and high rootingwas found to be BA applied singly at 62.5 or125 mg/l or applied twice at 31.2 or62.5 mg/l. The potential utilization of BAtreatments in operational asexual propagationprocedures of jack pine is discussed.

Landscape influence on the spatial and temporal distribution of the Kirtland's warbler at the Bald Hill burn, northern Lower Michigan, U.S.A.

The Kirtland's warbler (Dendroica kirtlandii Baird) is an endangered songbird that nests in northern Lower Michigan in ecosystems dominated by young jack pine (Pinus banksiana Lamb.). We used the landscape ecosystem approach to identify two adjacent landform-level ecosystems located at different elevations within the area burned by the 1975 Bald Hill fire in Crawford County, Michigan. Eighty-five percent of the warblers occupied the high-elevation landform from 1982 to 1987 compared with only 25% from 1992 to 1997, indicating a shift in warbler occupancy of the burn from the high- to the low-elevation landform by 1991. Although similar in soil, the landforms have significantly different microclimates as mediated by physiography. Because of a warmer microclimate, jack pines in the high-elevation landform exhibited faster growth and were colonized first by the warbler; the trees in the cooler low-elevation landform grew more slowly and were colonized later. We suggest that the presence of two adjacent landform-level ecosystems within a single burned area prolonged the warbler occupancy of the burn. Thus, a broad-scale ecosystem approach that considers physical site factors as well as vegetative characteristics is useful for understanding the complex interactions that affect spatial and temporal species distributions across the landscape. Such an approach may benefit the management of particular endangered species, especially when the size of management areas is considered to be a crucial element of species preservation.

Hierarchical habitat selection by woodland caribou: its relationship to limiting factors

Habitat selection is a hierarchical process that may yield various patterns depending on the scales of investigation. We employed satellite radio‐telemetry to examine patterns of habitat selection by female woodland caribou in central Saskatchewan at both coarse (seasonal range) and fine (daily area) scales. At each scale, we converted spatial data describing compositions of available and used habitat to standardised resource selection indices and examined them with multivariate analyses of variance. Seasonal ranges generally showed preferential inclusion of peatlands and black spruce dominated stands relative to recently disturbed stands and early seral stage forests. In all populations, caribou preferred peatlands and black spruce forests to all other habitat types at the daily area scale, in general, these patterns may reveal the effective avoidance of wolves, the primary factor limiting caribou throughout the boreal forest. In three populations where seasonal ranges showed the selective inclusion of either young jack pine stands or clearcuts along with peatlands and black spruce forests, we found a relative avoidance of the clearcuts and young jack pine stands at the daily area scale. As all caribou populations in the area are thought to be relics of a once more continuous distribution, the seasonal range selection by animals in disturbed areas may better describe historic rather than current habitat selection. We found inter‐annual variation in selection at the coarser spatial scale in one population, and inter‐seasonal variation in selection at the finer spatial scale in three populations, indicating that the relative grains of the spatial and temporal scales coincide. We were better able to explain the seasonal variations in finer scale selection by considering available forage, a factor less likely than predation to limit woodland caribou populations. The data agree with the theory that the spatial and temporal hierarchy of habitat selection reflects the hierarchy of factors potentially limiting individual fitness.

Performance of the Canadian land surface scheme at temperate aspen‐birch and mixed forests, and a boreal young jack pine forest: Tests involving canopy conductance parametrizations

The Canadian Land Surface Scheme (CLASS) was evaluated using off‐line tests, with data collected at temperate aspen‐birch and mixed forests located near Chalk River, Ontario, and at a young jack pine forest located near Thompson, Manitoba. Measurements of stomatal conductance and leaf area index were used to develop canopy conductance parametrizations for the aspen‐birch and the jack pine stands, and these were incorporated into CLASS in a modified run. The behaviour of the residuals from the unmodified run with respect to environmental variables at the mixed stand were used as a guide to producing a modified ad‐hoc canopy conductance parametrization for use at that site. It was found that CLASS exaggerates the diurnal range in soil heat flux. This leads to an underestimation of the diurnal range in available energy, and it is argued that this has implications for its partitioning into the turbulent fluxes of sensible and latent heat. Over time, systematic errors will accumulate which, in the case of the latent heat flux, can affect the long‐term water balance. Errors in the size of the turbulent fluxes over short time periods can limit the usefulness of CLASS for resolving mesoscale circulations in a regional model. These problems are believed to be related to CLASS's assumption of zero wind speed under the canopy, and the lack of an insulating litter layer at the soil surface. An unmodified CLASS produced estimates of sensible and latent heat fluxes that broadly agreed with measured values at the temperate stands but not at the jack pine stand, where the latent heat flux was significantly overestimated. The modified canopy conductance parametri‐zations improved the representation of the latent heat flux at all of the sites, particularly at the jack pine stand. Overestimation at the aspen‐birch stand may be related to site heterogeneity not being adequately represented in the measurement of stomatal conductance, or to difficulties in defining the vertical distribution of leaf area index for scaling stomatal conductance to the canopy level. As expected, a positive or negative bias in the modelled latent heat flux is associated with an opposite bias in the modelled sensible heat flux. The tendency towards positive intercepts and slopes less than unity in linear regressions relating modelled and measured fluxes is a symptom of the underestimation of the diurnal range in available energy. Recommendations are made to allow for vertical discontinuities in texture within a soil layer and a variable permeable depth, and to allow transpiration when the soil water suction exceeds –1.5 MPa. This is particularly important when soil moisture or texture is heterogeneous. Finally, it is suggested that volumetric soil moistures should be allowed to fall below the originally prescribed limiting value of 0.04. This is necessary for evaporation to continue in very sandy, dry soils.

Landscape influence on the spatial and temporal distribution of the Kirtland's warbler at the Bald Hill burn, northern Lower Michigan, U.S.A.

The Kirtland's warbler (Dendroica kirtlandii Baird) is an endangered songbird that nests in northern Lower Michigan in ecosystems dominated by young jack pine (Pinus banksiana Lamb.). We used the l...

Performance of young jack pine trees originating from two different branch angle traits under different intensities of competition

Le developpement de jeunes pins gris (Pinus banksiana Lamb.), issus d'arbres parents localises dans un verger a graines et differencies par deux caracteres d'angle des branches, a ete analyse sous differentes intensites de competition avec des mesures morphologiques de developpement des cimes et d'efficacite de croissance. Les semis ont ete plantes selon un dispositif en parcelles divises a cinq niveaux d'espacement (0,5 m, 0,75 m, 1,0 m, 1,5 m et 2,0 m), deux classes d'angle des branches, trois blocs et quatre repetitions. Le taux relatif de croissance en diametre a hauteur de poitrine (dhp) a presque double de l'espacement le plus serre a l'espacement le plus large. La largeur de la cime, le rapport cime-hauteur et les rapports de densite et de masse des aiguilles ont diminue de facon significative avec une diminution de l'espacement initial. Ces resultats indiquent que l'efficacite des couronnes du pin gris a occuper leur espace de croissance et la proportion de tissu assurant la photosynthese par rapport a la proportion de tissu qui respire a ete affectee negativement par la competition. La concentration en azote des aiguilles, qui a diminue avec une reduction de l'espacement, a ete reliee de facon significative au rapport de masse des aiguilles. La variation, en fonction de la taille des arbres, des rapports de croissance en diametre sur la biomasse foliaire et le contenu en azote des aiguilles indique que, en l'absence de competition severe, les petits arbres ont produit plus efficacement de la matiere ligneuse par unite de tissu photosynthetique et d'azote que les gros arbres et que cette tendance s'est inversee a mesure que l'espacement diminuait. L'angle de branchaison des arbres ne s'est pas revele presenter un avantage significatif pour le developpement des cimes et la croissance des tiges.

High temporal resolution NDVI phenology from micrometeorological radiation sensors

The boreal forest is a region characterized by wide swings in temperature and light levels over the course of a year. This seasonal variability strongly effects the vegetation of this biome. Normalized difference vegetation index (NDVI) values were observed at daily timescales for key land cover types of the boreal forest, developing a more detailed description of seasonal changes in NDVI than could be produced from satellite data. NDVI values were calculated from tower‐mounted photosynthetically active radiation (PAR) and global solar sensors measuring both incoming and reflected radiation above the canopies at four Boreal Ecosystem‐Atmosphere Study (BOREAS) sites. Comparisons were made between the tower‐based broadband hemispherical NDVI values and the narrowband nadir‐viewed NDVI values from helicopter modular multiband radiometer (MMR). The comparisons indicate that the tower NDVI values are close to the MMR NDVIs in value for the BOREAS sites, but the range in tower NDVIs is not so great as in the MMR NDVIs. In 1996, BOREAS towers operated from before thaw to freeze‐up, allowing a complete picture of growing season NDVI for fen, young jack pine, black spruce, and aspen sites. The tower‐based NDVI time series display different patterns for each vegetation type, showing the effects of snow cover and vegetation green‐up and senescence. Changes in solar zenith angles are shown to have little effect on the seasonal NDVI patterns.

Water and carbon dioxide exchange at a boreal young jack pine forest in the BOREAS northern study area

Climatological measurements, including carbon dioxide flux density, were made from May to September 1994 and from May to November 1996 at a young jack pine (Pinus banksiana Lamb.) forest near Thompson, Manitoba, Canada, as part of the Boreal Ecosystem‐Atmosphere Study (BOREAS). The study periods were warmer and drier than the 24 year climate normals, and in general, 1996 was warmer but wetter than 1994. Volumetric soil moisture above a depth of 8 cm in 1994 was often less than 5%. Albedos for solar radiation and photosynthetic photon flux density (PPFD) were 0.13 and 0.05, respectively, in both years. The Bowen ratio for days of year 144–262 was 1.7 in 1994 and 1.5 in 1996, with maximum daily Bowen ratios from 4 to 6. The range in CO2 flux densities was the same in both years, with maximum uptake values of −0.55 mg m−2 s−1 in midsummer. Average diurnal patterns of CO2 exchange for 1994 were similar to those for 1996, characterized by maximum uptake in the morning and a gradual decrease through the afternoon. Rectangular hyperbolic response curves showing CO2 uptake in relation to incident PPFD show that changes between the morning and the afternoon, and because of environmental controls, were as great or greater than changes between designated 3 and 4 week measurement periods. Over a period of 119 days the young jack pine forest accumulated approximately −200 g C m−2 in 1994 and −267 g C m−2 in 1996. Reproduction of CO2 exchange trends will be important for modeling efforts involving northern ecosystems.

Investigation of directional reflectance in boreal forests with an improved four-scale model and airborne POLDER data

Airborne Polarization and Directional Earth Radiation (POLDER) data acquired during the boreal ecosystem-atmosphere study (BOREAS) and the four-scale model of Chen and Leblanc (1997) are used to investigate radiative transfer in boreal forest. The four-scale model is based on forest canopy architecture at different scales. New aspects are incorporated into the model to improve the physical representation of each canopy, as follows: 1) Elaborate branch architecture is added. 2) Different crown shapes are used for conifer and deciduous forests. 3) Bilayer version of the model is introduced for forest canopies with an important understory. 4) Natural repulsion effect is considered in the tree distribution statistics. Ground measurements from BOREAS sites are used as input parameters by the model to simulate measurements of bidirectional reflectance distribution function (BRDF) from four forest canopies (old black spruce, old aspen, and old and young jack pine) acquired by the POLDER instrument from May-July 1994. The model is able to reproduce with great accuracy the BRDF of the four forests. The importance of the branch architecture and the self-shadowing of the foliage is emphasized.

Monitoring tree moisture using an estimation algorithm applied to SAR data from BOREAS

During several field campaigns in spring and summer of 1994, the NASA/JPL airborne synthetic aperture radar (AIRSAR) collected data over the southern and northern study sites of BOREAS. Among the areas over which radar data were collected was the young jack pine (YJP) tower site in the south, which is generally characterized as having short (2-4 m) but closely spaced trees with a dense crown layer. In this work, the AIRSAR data over this YJP stand from six different dates were used, and the dielectric constant and hence the moisture content of its branch layer components were estimated. The approach was to first derive a parametric scattering model from a numerical discrete-component forest model, which is possible if the predominant scattering mechanism can be identified. Here, a classification algorithm was used for this purpose, concentrating on areas where the volume scattering mechanism from the branch layer dominates. The unknown parameters mere taken to be the real and imaginary parts of the dielectric constant, from which the moisture content can be derived. Once the parametric model was derived, a nonlinear estimation algorithm was employed to retrieve the model parameters from SAR data. This algorithm is iterative, and takes the statistical properties of the data and unknown parameters into account. The inversion process was first verified using synthetic data. It was observed that the algorithm is robust with respect to the a priori estimate. The estimation algorithm was then applied to AIRSAR data of BOREAS. The results show how the environmental conditions affected the moisture state of this forest stand over a period of six months. It is observed that canopy moisture increased during the thaw season, was stable starting from the end of the thaw season throughout most of the growing season, after which a period of dry-down was observed at the end of the growing season.

The Impact of Simulated Acid Rain on Soil Leachate and Xylem Chemistry in a Jack Pine (Pinus Banksiana Lamb.) Stand in Northern Ontario, Canada

The impact of simulated acid rain on soil leachate and xylem chemistry in a young Jack pine (Pinus banksiana Lamb.) stand in the boreal forest of northern Ontario was evaluated. Permanent plots (5 × 2 m) were established in 1981 which were sprayed twice monthly with simulated acid rain, adjusted to pH 5.6, 4.0, 3.5, 3.0 or 2.5 with a 2:1 molar ratio of sulphuric (H2SO4) to nitric acid (HNO3) in addition to ambient rainfall. Sprays were applied between June and September for 5 yr. Unsprayed plots were also monitored. The pH of soil leachate collected between 1981–1985 was reduced significantly by the acid sprays and concentrations of sulphate (SO42-), nitrate (NO3-), calcium (Ca), magnesium (Mg), aluminium (Al), manganese (Mn), and zinc (Zn) increased in the A, B and C horizons. The soil recovered rapidly from the spray treatments, although residual effects were found in soil leachate samples collected during 1986–1987, particularly in the C horizon. Trees receiving spray acidified to pH 2.5 had higher concentrations of Ca, Mn, Cd and Rb in tree-rings formed between 1981–1985 compared to trees receiving spray acidified to pH 4.0 or to trees receiving ambient rainfall alone. Some of the changes in soil chemistry resulting from the application of acidic sprays are reflected in the chemistry of Jack pine tree rings and these chemical signals in tree rings may be used as indicators of soil acidification.

Population dynamics of the white pine weevil, Pissodes strobi, infesting jack pine, Pinus banksiana, in Ontario, Canada

1. Eight generations of white pine weevil, Pissodes strobi (Peck) , infesting the terminal shoots of young jack pine trees, were sampled as larvae, pupae, and adults between 1988 and 1995.2. The density of adult weevils increased rapidly for the first 3 years of the study, then declined in the next 4 years. Between‐generation rates of change in density of weevils were related to mean number of weevils produced per terminal shoot of the host tree. Net increases in population density followed years in which a relatively high number of weevils emerged per terminal shoot and vice versa.3. The mean number of weevils emerging per terminal shoot was determined by survival of weevils between the end of the larval stage and the successful emergence of adult weevils.4. There was no clear relationship between survival of the post‐feeding stages of the weevil and rates of parasitism or bird predation on these same stages. There was, however, a strong negative relationship between survival of weevils within the terminal shoot and the abundance of the facultative dipteran predator Lonchaea corticis Taylor.5. The relationship among generational rates of change in populations, survival, and predation was evident irrespective of the age of the trees infested, suggesting that population dynamics of the weevil may be influenced as much by local predation pressure as by physical and biotic changes associated with growth of the young host trees.

Parameterization and testing of a coupled photosynthesis-stomatal conductance model for boreal trees.

A coupled photosynthesis-stomatal conductance model was parameterized and tested with branches of black spruce (Picea mariana (Mill.) B.S.P.) and jack pine (Pinus banksiana Lamb.) trees growing in the Northern Study Area of the Boreal Ecosystem-Atmosphere Study (BOREAS) in Manitoba, Canada. Branch samples containing foliage of all age-classes were harvested from a lowland old black spruce (OBS) and an old jack pine (OJP) stand and the responses of photosynthesis (A(n)) and stomatal conductance (g(s)) to temperature, CO(2), light, and leaf-to-air vapor pressure difference (VPD) were determined under controlled laboratory conditions at the beginning, middle, and end of the growing season (Intensive Field Campaigns (IFC) 1, 2, and 3, respectively). The parameterized model was then tested against in situ field gas-exchange measurements in a young jack pine (YJP) and an upland black spruce (UBS) stand as well as in the OBS and OJP stands. Parameterization showed that Rubisco capacity (V(max)), apparent quantum yield (alpha') and Q(10) for sink limitation were the most crucial parameters for the photosynthesis sub-model and that V(max) varied among different measurement series in the laboratory. Verification of the model against the data used to parameterize it yielded correlation coefficients (r) of 0.97 and 0.93 for black spruce and jack pine, respectively, when IFC-specific parameters were used, and 0.77 and 0.87 when IFC-2 parameters were applied to all IFCs. For both measured and modeled g(s), the stomatal conductance sub-model, which linearly relates g(s) to (A(n)h(s))/c(s) (where h(s) and c(s) are relative humidity and CO(2) mole fraction at the leaf surface, respectively), had significantly steeper slopes and higher r values when only the VPD response data were used for parameterization than when all of the response data were used for parameterization. Testing the photosynthesis sub-model against upper canopy field data yielded poor results when laboratory estimates of V(max) were used. Use of the mean V(max) estimated for all upper canopy branches measured on a given day improved model performance for jack pine (from a nonsignificant correlation between measured and modeled A(n) to r = 0.45), but not for black spruce (r = 0.45 for both cases). However, when V(max) was estimated for each branch sample individually, the model accurately predicted the 23 to 137% diurnal variation in A(n) for all stands for both the upper and lower canopy. This was true both when all of the other parameters were IFC-specific (r = 0.93 and 0.92 for black spruce and jack pine, respectively) and when only mid-growing season (IFC-2) values were used (r = 0.92 for both species). Branch-specific V(max) estimates also permitted accurate prediction of field g(s) (r = 0.75 and 0.89 for black spruce and jack pine, respectively), although parameterization with all of the response data overestimated g(s) in the field, whereas parameterization with only the VPD response data provided unbiased predictions. Thus, after parameterization with the laboratory data, accurately modeling the range of A(n) and g(s) encountered in the field for both black spruce and jack pine was reduced to a single unknown parameter, V(max).

Dry Weight and N Partitioning in Relation to Substrate N Supply, Internal N Status and Developmental Stage in Jack Pine (Pinus banksianaLamb.) Seedlings: Implications for Modelling

Abstract Dry weight and nitrogen (N) partitioning of sand-cultured young jack pine ( Pinus banksiana Lamb.) seedlings under controlled environments were studied 3, 6, 9, 12 and/or 15 weeks after the initiation of six dynamic N supply treatments. The supply of other nutrients was maintained at optimal levels. Total dry weight varied widely among treatments and whole plant total N concentration ranged from 10 to 32 mg g -1 d. wt at most sampling intervals. Whole plant N concentration changed, with time, according to three distinct patterns: (1) stable; (2) rapidly increasing; or (3) gradually declining. Regardless of N treatment and sampling interval, whole plant N concentration was linearly and positively correlated with root, needle and stem N concentration. Dry weight and N weight ratios of needles declined, whereas those of roots increased linearly with decreasing whole plant N concentration ( r 2 =0.43 to 0.76) regardless of N regime. Dry matter partitioning to stems, however, was better explained by developmental stage than by whole plant N concentration. With the decline in internal N status, N was increasingly concentrated in roots at the expense of needles and stems. These results suggest: (1) dry weight and N partioning may be largely a function of the internal N status of plant rather than root and shoot activities; (2) both shoot and root specific activities may have a close, positive association with whole plant N concentration; (3) N-partitioning may be an active process itself and may warrant separate consideration from dry weight; and (4) developmental stage may be a significant determinant of partitioning, particularly to stems.

Magnitudes and seasonal patterns of energy, water, and carbon exchanges at a boreal young jack pine forest in the BOREAS northern study area

Seasonal patterns of the energy balance of a young jack pine site near Thompson, Manitoba, in the summer of 1994 are reported. The experiment was part of the Boreal Ecosystem‐Atmosphere Study (BOREAS), and it ran from May 24 to September 19. The average tree density was 4.4 trees m−2, but there was substantial spatial variation associated with three primary vegetation patterns: dense cover of short trees, sparse cover of tall trees, and a mixture of short and tall trees. The frequency distribution of tree heights was bimodal with peaks at 1 and 2 m and a range from 0.3 to 5.7 m. The average tree height was 2.3 m. The daily average photosynthetically active radiation albedo was conservative, varying from 0.054 under clear‐sky conditions to 0.051 under cloudy skies. The shortwave albedo was 0.136 under clear‐sky conditions and decreased by 1% following rain; the presence of smoke over the site increased it by 1%. Heat storage in the soil, trees, and air was an important component of the energy balance throughout the season with soil heat flux comprising the bulk of the total storage. On the average, for the whole experimental period, the sensible heat flux was approximately twice the latent heat flux (Bowen ratio ≈2). The average daily energy balance closure varied from 85% before day‐of‐year (DOY) 200 to 95% after DOY 200. As a result of problems with the measurement of the net CO2 flux above the canopy under stable nighttime conditions, all nighttime values were modeled. The typical diurnal pattern of net CO2 flux shows maximum uptake by the surface in the morning and a gradual decrease through the afternoon. For the whole field season of 118 days, the site fixed 224 g m−2 of carbon.