Shedding of older needle age classes does not necessarily reduce photosynthetic primary production of Norway spruce

The hypothesis that the frequently observed shedding of older needle age classes in stands of Norway spruce suffering from atmospheric pollutants has minimal effect on photosynthetic primary production was assessed. Using structural parameters of young Norway spruce [Picea abies (L.) Karst.] trees, an existing 3-dimensional canopy photosynthesis model was adapted and validated for conifer canopies. This model was employed to quantitatively demonstrate that, depending on the stand density, the loss in photosynthetic active foliage area may be compensated by higher photosynthetic rates of the remaining younger foliage age classes due to higher levels of photosynthetically active radiation (PAR) within the thinned tree crowns. Under certain circumstances, beneficial effects on the tree's water use efficiency may be expected. Extrapolating the model results to mature tree stands is addressed. The potential effects of higher PAR levels on abundance and species diversity of the epiphytic and understory vegetation, as well as the general importance of the older foliage age classes for spruce trees on a long term scale, are discussed.     

Daylight spectra (400–740 nm) beneath sunny,blue skies in Tasmania,and the effect of a forest canopy

Abstract The photosynthetically active radiation (PAR) incident on a horizontal surface at an open mountain site is positively correlated with solar altitude for sunny, blue sky conditions. The proportion of red light in PAR decreases with increasing solar altitude, while that of blue increases. These results are consistent with the wavelength dependency of Rayleigh and Mie scattering. The ratio of near infrared radiation to PAR decreases with increasing solar altitude towards solar noon and with decreasing solar altitude towards sunset. Thus surface reflection seems to be an important part of the light climate. The relative transmission of daylight through a forest canopy to a horizontal surface is not correlated with solar altitude for sunny, blue sky conditions at a mountain site. The amount of diffuse daylight is negatively correlated with per cent canopy interception, and the amount of direct sunlight is negatively correlated with per cent solar track interception. Daylength is negatively correlated with both canopy and solar track interceptions. The proportion of red light in PAR increases with increasing solar altitude, while that of blue decreases. These results are opposite those for the open site and are due to the spatial patterns of canopy obstruction of the sky vault, and of the spectral quality of daylight across the sky. The ratio of near infrared radiation to PAR in shadelight increases with increasing canopy interception due to the selective scattering properties of the canopy. The ratio for shadelight is positively correlated with the ratio for sunflecks.     

Simulating stand climate, phenology, and photosynthesis of a forest stand with a process-based growth model

In the face of climate change and accompanying risks, forest management in Europe is becoming increasingly important. Model simulations can help to understand the reactions and feedbacks of a changing environment on tree growth. In order to simulate forest growth based on future climate change scenarios, we tested the basic processes underlying the growth model BALANCE, simulating stand climate (air temperature, photosynthetically active radiation (PAR) and precipitation), tree phenology, and photosynthesis. A mixed stand of 53- to 60-year-old Norway spruce (Picea abies) and European beech (Fagus sylvatica) in Southern Germany was used as a reference. The results show that BALANCE is able to realistically simulate air temperature gradients in a forest stand using air temperature measurements above the canopy and PAR regimes at different heights for single trees inside the canopy. Interception as a central variable for water balance of a forest stand was also estimated. Tree phenology, i.e. bud burst and leaf coloring, could be reproduced convincingly. Simulated photosynthesis rates were in accordance with measured values for beech both in the sun and the shade crown. For spruce, however, some discrepancies in the rates were obvious, probably due to changed environmental conditions after bud break. Overall, BALANCE has shown to respond to scenario simulations of a changing environment (e.g., climate change, change of forest stand structure).     

利用多光谱扫描仪测定空心莲子草冠层光谱的影响因素

【背景】太阳光的入射辐射是影响多光谱扫描仪测定植物冠层光谱反射率的主要因素。本研究旨在探讨不同测定条件对空心莲子草冠层光谱特征值的影响,确定应用多光谱扫描仪测定空心莲子草冠层光谱特征的适宜环境条件。【方法】采用多光谱扫描仪,在不同天气条件、仪器探头高度、太阳高度角、叶片湿度等外界因素条件下,测定空心莲子草冠层光谱反射率的变化。【结果】天气条件、仪器探头高度、太阳高度角、叶片湿度4个因素对空心莲子草冠层在绿光区560nm、黄光区660nm和近红外光区810nm3个特征波段处的光谱反射率均有显著影响。【结论与意义】利用多光谱扫描仪测定空心莲子草冠层光谱数据,应选择晴朗无云的天气,仪器探头距离冠层1.3m,适宜的数据采集时间为10：00～14：00,以保障适度的太阳高度角和干燥的叶面,从而保持稳定的光谱测量结果。本研究可为完善多光谱扫描评价技术提供理论依据。     

Light environment, growth and morphogenesis in a peach tree canopy

Morphogenic and growth processes were studied in relation to photosynthetically active radiation (PAR), and red, far red and blue spectral bands monitored at three different heights of a peach canopy during the vegetative season. The PAR intercepted by the bottom of the tree was significantly lower than that at the top, and blue fluence rate changed with height and season in a manner similar to PAR. Phytochrome photoequilibria indicated spatial and temporal differences in the three layers of the canopy: significantly lower values were detected at the bottom in correspondence with the maximum leaf area index. In this layer, a stimulation of internode elongation and a decrease of flower density were detected. Higher shoot growth rates and about double number of lateral shoots were found at the top of the canopy, where a greater number of sun leaves was present. Possible explanations in terms of different growth strategies induced by shade, depletion of blue, and low phytochrome levels at the bottom of the canopy are given.     

The effect of two growth forms of Norway spruce on stand development and radiation interception: a model analysis

Summary Development of tree and canopy structure, and interception of photosynthetically active radiation (PAR) were studied in two model stands of Norway spruce consisting of trees with rapid versus slow site capture. The tree models were derived using Burger's (1953) sample tree material, from which two subpopulations of dominant trees were selected using the rate of horizontal site capture of the tree crowns as the criterion of division. The development of stand structure and interception of PAR were simulated in the two model canopies. The simulation period covered the period from tree age 15–80 years. The average development of the trees in the two subpopulations proved to be very different. The rapidly expanding trees were characterized by low mean within-crown needle area density and a long crown. The slowly expanding trees were smaller but had a higher mean within-crown needle area density. Up to approximately 40 years of age the stand of rapidly expanding trees contained more leaf area and intercepted more radiation than the stand of slowly expanding trees, when canopy cover was held constant. After 40 years of age this relationship was reversed due to the subsequent decline of leaf area in the stand of rapidly expanding trees and the increase in leaf area in the stand of slowly expanding trees. The biological relevancy and silvicultural implications of the simulated patterns of tree and stand development are discussed.     

The ratio of transmitted near-infrared radiation to photosynthetically active radiation (PAR) increases in proportion to the adsorbed PAR in the canopy

The daily total photosynthetically active radiation (400??00?nm, PAR) and near-infrared radiation (700??000?nm, NIR) were measured in the understory beneath the canopy (PARt and NIRt) and above the canopy (PARi and NIRi) of a Japanese cool-temperate deciduous broad-leaved forest during the snow-free period (May to November). The integration of spectral radiation for NIR and that for PAR, and the daily integrations of instantaneous NIR and PAR, reduced the noises from the optical difference in spectrum and from canopy structure heterogeneity, sky condition and solar elevation. PARi/PARt was linearly related to NIRt/PARt (R2?=?0.96). The effect of cloudiness was negligible, because the fluctuation of NIRi/PARi was quite small regardless of season and weather conditions compared with the range of NIRt/PARt in the forest. The ratio of NIRt/PARt beneath the canopy was log-linearly related to the in situ leaf area index (LAI) with a wide range from 0 to 5.25 (R2?=?0.97). We conclude that seasonal changes in fAPAR (=?1???PARt/PARi) and LAI of a canopy can be estimated with high accuracy by transmitted NIRt and PARt beneath the canopy.     

Ecophysiological controls over the net ecosystem exchange of mountain spruce stand. Comparison of the response in direct vs. diffuse solar radiation

Cloud cover increases the proportion of diffuse radiation reaching the Earth's surface and affects many microclimatic factors such as temperature, vapour pressure deficit and precipitation. We compared the relative efficiencies of canopy photosynthesis to diffuse and direct photosynthetic photon flux density (PPFD) for a Norway spruce forest (25‐year‐old, leaf area index 11 m² m⁻²) during two successive 7‐day periods in August. The comparison was based on the response of net ecosystem exchange (NEE) of CO₂ to PPFD. NEE and stomatal conductance at the canopy level (G_canopy) was estimated from half‐hourly eddy‐covariance measurements of CO₂ and H₂O fluxes. In addition, daily courses of CO₂ assimilation rate (A_N) and stomatal conductance (G_s) at shoot level were measured using a gas‐exchange technique applied to branches of trees. The extent of spectral changes in incident solar radiation was assessed using a spectroradiometer. We found significantly higher NEE (up to 150%) during the cloudy periods compared with the sunny periods at corresponding PPFDs. Prevailing diffuse radiation under the cloudy days resulted in a significantly lower compensation irradiance (by ca. 50% and 70%), while apparent quantum yield was slightly higher (by ca. 7%) at canopy level and significantly higher (by ca. 530%) in sun‐acclimated shoots. The main reasons for these differences appear to be (1) more favourable microclimatic conditions during cloudy periods, (2) stimulation of photochemical reactions and stomatal opening via an increase of blue/red light ratio, and (3) increased penetration of light into the canopy and thus a more equitable distribution of light between leaves. Our analyses identified the most important reason of enhanced NEE under cloudy sky conditions to be the effective penetration of diffuse radiation to lower depths of the canopy. This subsequently led to the significantly higher solar equivalent leaf area compared with the direct radiation. Most of the leaves in such dense canopy are in deep shade, with marginal or negative carbon balances during sunny days. These findings show that the energy of diffuse, compared with direct, solar radiation is used more efficiently in assimilation processes at both leaf and canopy levels.     

Spectral multichannel monitoring of radiation within a mature mixed forest

A multi-sensor system is described based on fiber optic technology and a diode array spectrometer for near-simultaneous measurement of spectral photon fluence rates (PFR) in the range of 360 nm to 1020 nm with a resolution of 0.8 nm, within a mature Norway spruce ( Picea abies [L.] Karst.) - European beech ( Fagus sylvatica L.) stand. 126 space-integrating spherical sensors, deployed in a regular grid above and within the canopy and on the forest floor, are sequentially connected to the spectrometer by means of fiber optics. About 1 s per sensor is needed to collect spectral data, store them on hard disk and move the channel multiplexer to the next fiber optic position. Data thus obtained serve to determine vertical profiles of wavelength-dependent photon extinction, especially for spectral ratios and wavebands, characterization of phenological stages, analyses of time series, and meteorological influences such as solar altitude and cloud cover. First measurements during leaf fall 2004 show a non-linear relation of the red/far-red ratio (R/FR) with relative photosynthetic PFR (PPFR (rel)). An analysis of relative PFR (PFR (rel)) quantifies the frequency of penumbral sunfleck occurrence and the fraction of incoming radiation on the forest floor. In-canopy measurements of daily means of PPFR (rel) and R/FR indicate that leaf unfolding and leaf fall can be described by a single sensor, independent of its vertical location within the canopy.     

Low clouds and cloud immersion enhance photosynthesis in understory species of a southern Appalachian spruce-fir forest (USA)

High-altitude forests of the southern Appalachian Mountains (USA) are frequently immersed in clouds, as are many mountain forests. They may be particularly sensitive to predicted increases in cloud base altitude with global warming. However, few studies have addressed the impacts of immersion on incident sunlight and photosynthesis. Understory sunlight (photosynthetically active radiation, PAR) was measured during clear, low cloud, and cloud-immersed conditions at Mount Mitchell and Roan Mountain, NC (USA) along with accompanying photosynthesis in four representative understory species. Understory PAR was substantially less variable on immersed vs. clear days. Photosynthesis became light-saturated between ～100 and 400 μmol · m(-2) · s(-1) PAR for all species measured, corresponding closely to the sunlight environment measured during immersion. Estimated daily carbon gain was 26% greater on clear days at a more open canopy site but was 22% greater on immersed/cloudy days at a more closed canopy site. F(v)/F(m) (maximum photosystem II efficiency) in Abies fraseri seedlings exposed to 2.5 min full sunlight was significantly reduced (10%), indicating potential reductions in photosynthesis on clear days. In addition, photosynthesis in microsites with canopy cover was nearly 3-fold greater under immersed (2.6 mmol · m(-2) · h(-1)) vs. clear conditions (0.9 mmol · m(-2) · h(-1)). Thus, cloud immersion provided more constant PAR regimes that enhanced photosynthesis, especially in shaded microsites. Future studies are needed to predict the survival of these refugial forests under potential changes in cloud regimes.     

Tree species (Picea abies and Fagus sylvatica) effects on soil water acidification and aluminium chemistry at sites subjected to long-term acidification in the Ore Mts., Czech Republic

The effect of European beech (Fagus sylvatica) and Norway spruce (Picea abies) on acid deposition and soil water chemistry was studied at a site in the Ore Mts., Czech Republic, that has been subjected to decades of elevated acidic deposition. Dry deposition onto the spruce canopy significantly increased acid input to the soil in comparison to the beech canopy. As a result soil waters were more acidic; Al, SO4(2-), and NO3- concentrations were significantly higher; and Ca and K concentrations were lower in the spruce stand than in the beech stand. The concentrations of potentially toxic inorganic aluminium (Al(in)) were, on average, three times higher in the spruce stand than in the beech stand. Thus, Al played a major role in neutralizing acid inputs to mineral soils in the spruce stand. Despite the higher dissolved organic carbon (DOC) concentrations in spruce organic soil solutions, organic Al (Al(org)) accounted for only 30% of total Al (Al(tot)), whereas in beech organic soil solutions Al(org) was 60% of Al(tot). Soil waters in the beech stand exhibited Al(in) concentrations close to solubility with jurbanite (Al(SO4)OH.5H2O). The more acidic soil waters in the spruce stand were oversaturated with respect to jurbanite. The Bc/Al(in) ratio (Bc = Ca + Mg + K) in O horizon leachate was 4.6 and 70 in spruce and beech stands, respectively. In beech mineral soil solutions, the Bc/Al(in) ratio declined significantly to about 2. In the spruce stand, mineral soil solutions had Bc/Al(in) values below the critical value of 1. The observed Bc/Al(in) value of 0.4 at 30 cm depth in the spruce stand suggests significant stress for spruce rooting systems. A more favourable value of 31 was observed for the same depth in the beech stand. The efficiency of the spruce canopy in capturing acidic aerosols, particulates, and cloud water has resulted in the long-term degradation of underlying soils as a medium for sustainable forest growth.     

Cooling streams with riparian trees: Thermal regime depends on total solar radiation penetrating the canopy

Riparian planting is often recommended for stream restoration, notably to mitigate solar heating of stream waters. However, plant leaves shade photosynthetically active radiation (PAR, 400–700 nm) more efficiently than the near-infrared (NIR, 700–3000 nm) wavelengths that comprise about half of incident solar radiation and so contribute equally to water heating. Total solar radiation (NIR + PAR) exposure at the reach scale is needed to avoid bias when predicting the thermal response of streams. In this article, we alert stream ecologists and riparian restorers to the water heating contribution of NIR and provide a means to account for both NIR and PAR in total solar radiation penetrating riparian canopies. We used spectral pyranometers to simultaneously measure total solar radiation and its NIR component under different tree canopies over a wide range of shade conditions as indexed by PAR exposure. Measurements were made during full overcast (diffuse lighting) conditions so as to ‘sample’ transmission of radiation through the complete canopy. The NIR proportion of sub-canopy solar radiation increased steadily with increasing shade and was appreciably greater under willow than pine canopies, while NZ native broadleaf rainforest had intermediate (rather variable) NIR content. Our trend lines for different tree canopies permit total sub-canopy solar radiation to be estimated from PAR exposure for unbiased modelling of stream thermal regimes.     

Hoverflies (DIPT.: Syrphidae) of spruce forest in different health condition

Our investigation was aimed at the composition of syrphid fauna in spruce stands damaged by different intensity of air pollution which caused differences in canopy density, stand opening and herbaceous stratum. The syrphids were collected by yellow dish method. Stands damaged by the highest degree of pollution showed the richest spectrum of Syrphidae representatives.     

Stand characteristics and biodiversity indicators along the productivity gradient in boreal forests: Defining a critical set of indicators for the monitoring of habitat nature quality

Abstract

We quantified the effects of anthropogenic disturbances on the structure and biodiversity of boreal forests on acidic soils and created a statistically supported rational set of indicators to monitor the stand “naturalness”. For that, we surveyed various traits of tree layer, understory, herb layer, forest floor and several widely accepted biodiversity epiphytic indicators in 252 old‐aged boreal stands in Estonia, mostly dominated by Scots pine or Norway spruce. Multifactorial general linear model analyses showed that many forest characteristics and potential indicators were confounded by the gradient of soil productivity (reflected by the forest site type), local biogeographic gradients and also by stand age. Considering confounding effects, boreal forests in a near‐natural state have more large‐diameter trees (diameter at breast height >40 cm) and larger variety of diameter classes, higher proportion of spruce or deciduous trees, a larger amount of coarse woody debris in various stages, a more closed tree canopy and denser understory than managed mature forests. By increasing light availability above the field layer, forest management indirectly increases the coverage of herbs and lichens on the forest floor but reduces the alpha‐ and beta‐diversity of herbs and the proportion of graminoids. Human disturbances reduce the relative incidence of many commonly accepted biodiversity indicators such as indicator lichens, woodpeckers, wood‐dwelling insects or fungi on trees. The test for the predictive power of characteristics reacting on disturbance revealed that only a fraction of them appeared to be included in a diagnostic easy‐to‐apply set of indicators to assess the nature quality of boreal forest: the amount of dead wood, the proportion of deciduous trees, the presence of specially shaped trees and woodpeckers and, as an indicator of disturbances, the forest herb Melampyrum pratensis. Many of these indicators have already been implemented in practice.     

Estimating photosynthetic radiation use efficiency using incident light and photosynthesis of individual leaves

It has been theorized that photosynthetic radiation use efficiency (PhRUE) over the course of a day is constant for leaves throughout a canopy if leaf nitrogen content and photosynthetic properties are adapted to local light so that canopy photosynthesis over a day is optimized. To test this hypothesis, 'daily' photosynthesis of individual leaves of Solanum melongena plants was calculated from instantaneous rates of photosynthesis integrated over the daylight hours. Instantaneous photosynthesis was estimated from the photosynthetic responses to photosynthetically active radiation (PAR) and from the incident PAR measured on individual leaves during clear and overcast days. Plants were grown with either abundant or scarce N fertilization. Both net and gross daily photosynthesis of leaves were linearly related to daily incident PAR exposure of individual leaves, which implies constant PhRUE over a day throughout the canopy. The slope of these relationships (i.e. PhRUE) increased with N fertilization. When the relationship was calculated for hourly instead of daily periods, the regressions were curvilinear, implying that PhRUE changed with time of the day and incident radiation. Thus, linearity (i.e. constant PhRUE) was achieved only when data were integrated over the entire day. Using average PAR in place of instantaneous incident PAR increased the slope of the relationship between daily photosynthesis and incident PAR of individual leaves, and the regression became curvilinear. The slope of the relationship between daily gross photosynthesis and incident PAR of individual leaves increased for an overcast compared with a clear day, but the slope remained constant for net photosynthesis. This suggests that net PhRUE of all leaves (and thus of the whole canopy) may be constant when integrated over a day, not only when the incident PAR changes with depth in the canopy, but also when it varies on the same leaf owing to changes in daily incident PAR above the canopy. The slope of the relationship between daily net photosynthesis and incident PAR was also estimated from the photosynthetic light response curve of a leaf at the top of the canopy and from the incident PAR above the canopy, in place of that measured on individual leaves. The slope (i.e. net PhRUE) calculated in this simple way did not differ statistically from that calculated using data from individual leaves.     

Predicting the composition of Canadian southern boreal forest in different fire cycles

Abstract. Post-fire succession was reconstructed for a sector located in the southern part of the Québec boreal forest. Forest composition for different periods since fire was evaluated using a stand initiation map together with ecological maps representing both site conditions and stand types. Nine fires covering at least 100 ha and representing a chronosequence of more than 230 yr were used. Although a relatively clear successional pattern from deciduous to coniferous composition relating to time-since-fire was observed, Pinus banksiana stands showed an erratic distribution not related to succession but possibly to the pre-fire stand composition. A comparison with forest cover maps produced after a recent spruce budworm outbreak, showed that succession toward coniferous dominance appeared to be interrupted by spruce budworm (Choristoneura fumiferana) outbreaks which, by killing Abies balsamea, lead to a mixed deciduous forest composition. A simple empirical model based on a negative exponential distribution of age classes was developed to evaluate how changes in the fire cycle would affect the composition of the forest mosaic. The transition between deciduous dominance and coniferous dominance occurs in a fire cycle > 200 yr. Although pure deciduous stands tend to disappear during long fire cycles, the proportion of mixed stands remains relatively constant. Prediction of the forest composition for longer fire cycles is complicated by the interaction between post-fire composition and stand vulnerability to spruce budworm outbreaks.     

Morphological plasticity of regeneration subject to different levels of canopy cover in mixed-species, multiaged forests of the Romanian Carpathians

Morphological plasticity was studied for advanced regeneration trees in different light environments of the mountainous, mixed-species forests in the Carpathian Mountains of Romania. The primary species in these mixtures were very shade tolerant silver fir (Abies alba Mill.) and European beech (Fagus sylvatica L.), and midtolerant Norway spruce (Picea abies (L.) Karst). Seedlings/saplings of these species were selected for measurements in different stands from two different geographical locations. Various morphological traits (specific leaf area, live crown ratio, crown width to length ratio, terminal to lateral ratio, number of internodal shoots, number of shoots in terminal whorl, stem symmetry, stem orientation, stem forking) for each regenerating tree were measured during summers of 2001 and 2002. Percentage of above canopy light and stand basal area measures were used to assess the available growing space for each seedling/sapling. Regression relationships were developed for the different morphological indicators as a function of these two variables. All species adapted their morphology along the gradient in light and basal area. Spruce seemed to be less adapted to low light conditions than both fir and beech. However, no significant differences in terms of shade tolerance were detected using the above indicators. In really dense stand conditions (less than 20% above canopy light and stand basal area above 36 m² ha⁻¹), probability for stem forking in beech increased. In open, all three species adapted their morphology for vigorous growth. Under such conditions, spruce was better adapted than fir.     

Effects of changes in tree species composition on water flow dynamics – Model applications and their limitations

Water-plant relations play a key role in the water cycling in terrestrial ecosystems. Consequently, changes in tree species composition may have distinct effects on the water retention capacity as well as on the pattern of streamflow generation. Such changes may result from modified interception properties and transpiration related to differences in canopy properties and root distribution. In order to evaluate the potential hydrological effects of the current silvicultural conversion from monocultural conifer stands into mixed or pure deciduous stands the hydrological model BROOK90 was applied to two forested upland catchments in Germany. The Rotherdbach catchment (9.4 ha, 93 yr-old Norway spruce) is situated in the Eastern Ore Mountains. The Schluchsee catchment (11 ha, 55-yr-old Norway spruce) is located in the higher altitudes of the Black Forest. The calibrated model is capable to describe rather well the temporal variation of streamflow but also the portions of the individual flow components. Data for a beech scenario were adapted for each site using a standard parameter set for deciduous trees provided by BROOK90. The annual discharge in the fictional beech stand at Rotherdbach is 30 to 50% higher compared to spruce with an increase of soil moisture and especially the slow streamflow components. This mainly results from low interception rates during winter time. In contrast, the spruce stand has a permanently higher interception rate. Effects of tree species conversion are moderate at Schluchsee. The annual discharge of a fictional beech stand at Schluchsee is 7 to 14% higher compared to spruce. There in contrast to Rotherdbach, effects of tree species conversion on soil moisture dynamics are small since vertical percolation in the highly permeable soil dominates and precipitation is abundant. Practical forestry will favorably establish mixed beech–spruce rather than pure beech stands. However, it is critical to simulate mixed stands with BROOK90. Therefore, a simple summation of model results from spruce and beech according to their respective area in a fictional mixed stand can only be a first approximation. Advanced hydrological simulation of mixed stand conditions should regard interactions of tree species and spatial parameter distribution. However, this is not yet feasible due to a distinct lack of information. As a consequence, there is a strong need to collect relevant hydrological and ecophysiological data in mixed stands in the future.     

Modelling Variable Fire Severity in Boreal Forests: Effects of Fire Intensity and Stand Structure

It is becoming clear that fires in boreal forests are not uniformly stand-replacing. On the contrary, marked variation in fire severity, measured as tree mortality, has been found both within and among individual fires. It is important to understand the conditions under which this variation can arise. We integrated forest sample plot data, tree allometries and historical forest fire records within a diameter class-structured model of 1.0 ha patches of mono-specific black spruce and jack pine stands in northern Québec, Canada. The model accounts for crown fire initiation and vertical spread into the canopy. It uses empirical relations between fire intensity, scorch height, the percent of crown scorched and tree mortality to simulate fire severity, specifically the percent reduction in patch basal area due to fire-caused mortality. A random forest and a regression tree analysis of a large random sample of simulated fires were used to test for an effect of fireline intensity, stand structure, species composition and pyrogeographic regions on resultant severity. Severity increased with intensity and was lower for jack pine stands. The proportion of simulated fires that burned at high severity (e.g. >75% reduction in patch basal area) was 0.80 for black spruce and 0.11 for jack pine. We identified thresholds in intensity below which there was a marked sensitivity of simulated fire severity to stand structure, and to interactions between intensity and structure. We found no evidence for a residual effect of pyrogeographic region on simulated severity, after the effects of stand structure and species composition were accounted for. The model presented here was able to produce variation in fire severity under a range of fire intensity conditions. This suggests that variation in stand structure is one of the factors causing the observed variation in boreal fire severity.     

Branch transpiration of pine and spruce scaled to tree and canopy using needle biomass distributions

Branch water exchange and total tree water uptake were measured in a mixed Norway spruce and Scots pine stand in central Sweden during the 1995 and 1996 growing seasons. Branch transpiration was scaled to canopy level on the basis of a branch conductance model, using vertical needle-area distributions obtained by destructive sampling. Comparison with total tree water uptake scaled to canopy level showed agreement within 10%, for periods when the canopy was not affected by climatically induced stress. Comparison of scaled fluxes on individual trees showed that measurements of transpiration at branch level provide information on the direct response of transpiration to variations in weather, and furthermore that the time-lag between transpiration and tree water uptake was as much as 3 h. The vertical needle-area distribution of Scots pine was similar to that found by other authors. Needle-area distribution on Norway spruce, which has not been described before, showed that it has its largest needle area at the top of the crown. Specific needle area varied considerably both within trees and between trees. For spruce, mean specific needle area (±SD) varied from 2.4±0.5 mm² mg^–1 at the top of the crown to 7.1±1.9 mm² mg^–1 at the base. Corresponding figures for Scots pine were 3.4±2.0 and 9.1±2.1. Received: 5 March 1999 / Accepted: 17 March 2000