Dependence of winter water relations of mature high-elevation Picea engelmannii and Abies lasiocarpa on summer climate

Water relations of Engelmann spruce (Picea engelmannii Parry) and subalpine fir (Abies lasiocarpa (Hook.) Nutt.) trees growing at an elevation of 3230 m on Mt. Evans, Colorado, USA, were studied during the winters of 1995-1996 and 1996-1997. During both winters, current-year and 1-year-old shoots were collected weekly and their relative water contents (RWC) determined. Measured meteorological parameters were used in a conifer winter water relations model, WINWAT, to simulate changes in shoot RWC of P. engelmannii and A. lasiocarpa during the winter. The model failed to predict shoot RWCs in 1996-1997 when calibrated with 1995-1996 data. The cold early summer of 1995 inhibited xylem formation, which appears to have caused lower rates of water recharge to the needles during the 1995-1996 winter than during the 1996-1997 winter. We conclude that summer climate strongly affects winter water relations in these subalpine species, and that changes in both summer and winter climate must be considered when predicting future ranges of these species.     

Midwinter needle temperature and winter injury of montane red spruce

To assess the role of solar warming and associated temperature fluctuations in the winter injury of sun-exposed red spruce foliage, we used fine wire thermocouples to monitor midwinter needle temperature in the upper canopy of mature red spruce trees over two winters. In 1989-1990, 15-min mean temperatures were recorded for six needles in a single tree. In 1990-1991, 10-min mean temperatures of six needles in one tree, and 1-min mean temperatures of seven needles in a second tree were recorded during rapid temperature changes. Warming was more frequent and greatest on terminal shoots of branches with a south to southwest aspect. The maximum rise above ambient air temperature exceeded 20 degrees C, and the maximum one minute decrease in temperature was 9 degrees C, with maximum rates of 0.8 and 0.6 degrees C min(-1) sustained over 10- and 15-min intervals, respectively. These data demonstrate that red spruce is subject to rapid temperature fluctuations similar to those known to produce visible injury in American aborvitae, a much hardier species. We concluded that solar warming to temperatures above the freezing point was unlikely to result in dehardening and subsequent freezing injury, because warming was infrequent, of short duration, and did not always raise needle temperature above the freezing point. Parts of branches and some individual shoots were frequently covered by snow or rime that may have prevented injury by reducing the frequency or intensity of needle temperature fluctuations. Radiation load on exposed shoots may have been increased by reflection of short wave radiation from snow and rime deposits on surrounding surfaces, which would exacerbate temperature fluctuations.     

Effects of radiational heating at low air temperature on water balance, cold tolerance, and visible injury of red spruce foliage

Recent studies have shown that winter needle mortality in red spruce (Picea rubens Sarg.) is increased by exposure to direct solar radiation, possibly as a result of photo-oxidative damage, accelerated winter desiccation, or reduced cold tolerance due to heating of sun-exposed needles. In an experiment at controlled subfreezing air temperatures of -10 to -20 degrees C, visible radiation was less effective than infrared radiation in producing needle desiccation and visible injury during freeze-thaw cycles. However, visible radiation produced a red-brown color in injured needles, similar to natural winter injury, whereas injured needles exposed to infrared radiation were yellow and injured needles kept in darkness were dark brown. Thus, visible radiation was necessary to produce the red-brown color of damaged needles, but not the injury itself. Needle desiccation was not strongly correlated with visible injury, but the pattern of variation in visible injury among trees and the positive correlation between electrolyte leakage and visible injury suggested that freezing damage following freeze-thaw cycles might cause the visible injury. This was confirmed by a second experiment that showed loss of cold hardiness in needles thawed by radiational heating for six consecutive days. Even with a constant nighttime temperature of -10 degrees C, six days of radiational heating of needles to above freezing caused a small (2.8 degrees C) mean decrease in needle cold tolerance, as measured by electrolyte leakage. Continuous darkness at -10 degrees C for six days resulted in an estimated 5.6 degrees C mean increase in needle cold tolerance. Freezing injury stimulated desiccation: cooling at 4 degrees C h(-1) to -43 or -48 degrees C increased the dehydration rate of isolated shoots by a factor of two to three during the first day after thawing. Within three days at 15 to 22 degrees C and 50% relative humidity, the mean water content of these shoots fell to 60% or lower, compared to 90% or greater for unfrozen controls or shoots subject to less severe freezing stress. In some but not all severely freeze-stressed shoots, accelerated needle desiccation and abscission were accompanied by a red-brown color typical of red spruce winter needle injury. We conclude that severe winter desiccation in red spruce may often be due to prior freezing injury, increased as a result of exposure to direct solar radiation. Furthermore, freezing injury in red spruce may sometimes cause desiccation and abscission of green needles.     

Winter desiccation and injury of subalpine red spruce

Montane red spruce (Picea rubens Sarg.) in the northeastern United States has undergone a decline during the past two decades. One symptom associated with the decline syndrome is the episodic browning of first-year foliage in early spring. To examine the potential role of winter desiccation in this browning, the water relations of red spruce foliage in a subalpine forest on Mt. Moosilauke, New Hampshire, USA, were monitored from January to May, 1989. All sampled trees lost water during the winter and the first-year foliage on some trees turned brown in early spring. The relative water content of first-year shoots during the winter was a significant predictor of spring browning; red spruce trees that showed browning had desiccated faster and reached lower relative water contents. Damaged trees also had more closely packed needles and lower cuticular resistances to water loss. The first-year shoots had a significantly lower average relative water content than older shoots before and after browning. Cuticular resistance to water loss decreased with elevation. Sun-exposed shoots lost more water than shaded shoots because of solar heating of needles. Winter desiccation can occur before the decline-related spring browning of red spruce foliage.     

Winter water relations of New England conifers and factors influencing their upper elevational limits. I. Measurements

The upper elevational limits of tree species are thought to be controlled by abiotic factors such as temperature and the soil and atmospheric conditions affecting plant water status. We measured relative water contents (RWC), water potentials (Psi) and cuticular conductances (g(c)) of shoots of four conifer species-eastern hemlock (Tsuga canadensis (L.) Carr.), eastern white pine (Pinus strobus L.), red pine (P. resinosa Ait.) and red spruce (Picea rubens Sarg.)-during two winters on Mt. Ascutney, Vermont, USA. Some micrometeorological measurements are also reported. Eastern hemlock and white pine were studied near their upper elevational limits at a 640-m site, and red pine was studied near its upper elevational limit at 715 m. Red spruce was also studied at the 715-m site, which is in the middle of its elevational range on this mountain. There was no evidence of winter desiccation stress in any species. The observed distribution of seedlings suggested that the upper elevational limits on shade-intolerant eastern white pine and red pine are set by the absence of suitable seed beds after 100 years without fire. Eastern hemlock is able to reproduce in deep shade on organic substrates, but germination at high elevations may be restricted by low temperatures.     

Water relations of red spruce seedlings treated with acid mist

Pressure-volume curves, day and night transpiration rates, needle drying curves, and shoot water potentials were determined for 2-year-old red spruce trees that had been exposed for three months to a range of acid mists (pH 2.5 to pH 5.0) containing equimolar (NH(4))(2)SO(4) and HNO(3). No effect of acid mist was observed on cuticular resistance or on the rates of day and night transpiration, although trees exposed to acid mist exhibited symptoms of mild water stress. Significant decreases in maximum turgor, the relative water content (RWC) associated with zero turgor, and bulk volumetric elastic modulus occurred as the pH of the mist decreased from 5.0 to 2.5. At all RWC values, there was an increase in solute potential as mist pH decreased. Shoot water potential declined with a decrease in pH of the mist.     

Shoot and root sensitivity of containerized black spruce,white spruce and jack pine seedlings to late fall freezing

Damage to containerized forest seedlings due to freezing can occur in the fall or early winter in Canadian forest nurseries. The following spring, damage to shoots and impairment of growth is observed. The objectives of this experiment were to measure the impact of late fall low temperatures (0° to --30°C) on whole seedlings of the three most common species used for reforestation in Quebec: black spruce (BS), white spruce (WS) and jack pine (JP). Impacts of freezing temperatures on (i) whole seedling and apical bud mortality, (ii) shoot growth and root mortality, (iii) stem electrical resistance, (iv) shoot and root water relations, (v) concentrations of N, P, K, Ca, Mg, and total sugars in shoots were assessed. JP showed the highest rate of whole seedling mortality while WS showed the highest rate of apical bud mortality. JP was the most severely affected: destruction of the root system at low temperatures as well as a reduction of shoot growth and stem diameter and a decrease (more negative) in shoot and root water potential. WS showed a reduction of shoot growth despite no apparent damage to the root system at low temperatures. BS was not affected by temperatures as low as --30°C. Nutrient and sugar concentrations were not affected by low temperature treatments.     

Influence of nutrient supply on shade-sun acclimation of Picea abies seedlings: effects on foliar morphology,photosynthetic performance and growth

Norway spruce seedlings (Picea abies Karst.) were grown in low light for one year, under conditions of adequate and limiting nutrition, then transferred to high light. Three months after transfer we measured photosynthesis, leaf nitrogen concentration, leaf chlorophyll concentration and leaf mass per area (LMA) of current-year and 1-year-old shoots; silhouette area ratio (SAR, the ratio of shoot silhouette area to projected needle area) was also measured in current-year shoots. At the foliage level, the effects of light and nutrient treatments differed markedly. Light availability during foliage expansion primarily affected LMA and SAR (morphological acclimation at the needle and shoot level, respectively). By contrast, nutrient supply in high light affected photosynthetic capacity per unit of leaf tissue (physiological acclimation at the cellular level) but did not affect LMA and SAR. The capacity for shade-sun acclimation in foliage formed before transfer to high light differed greatly from that of foliage formed following the transfer. The morphological inflexibility of mature needles (measured by LMA) limited their shade-sun acclimation potential. In contrast, at high nutrient supply, shoots that developed just after the change in photosynthetic photon flux density largely acclimated, both morphologically and physiologically, to the new light environment. The acclimation response of both current- and 1-year-old shoots was prevented by nutrient limitation. Analysis of growth at the whole-plant level largely confirmed the conclusions drawn at the shoot level. We conclude that nutrient shortage subsequent to the opening of a canopy gap may strongly limit the acclimation response of Norway spruce seedlings. Successful acclimation was largely related to the plant's ability to produce sun foliage and adjust whole-plant biomass allocation rapidly.     

A comparison of red spruce and balsam fir shoot structures

I compared the shoot structures of high-elevation red spruce (Picea rubens Sarg.) and balsam fir (Abies balsamea (L.) Mill.). Needle widths, thicknesses and perimeters were measured to estimate total leaf areas from measured projected leaf areas. Measured needle perimeter/needle width ratios differed significantly from estimated ratios that assumed needles were either rhomboidal or elliptical in cross section. The vertical and horizontal silhouette shoot area to total leaf area ratios (STAR(v) and STAR(h)) of the two species were negatively correlated with needle packing and canopy height. Red spruce had higher values of STAR(v) than balsam fir at each canopy height, but STAR(v) declined with canopy height at a similar rate in the two species. The STAR(h) values of the two species did not differ significantly at a given canopy height. Needle packing increased with canopy height at the same rate in the two species. Needle weight increased in red spruce and decreased in balsam fir with increased needle packing, but showed no significant dependence on canopy height. Red spruce had higher values of STAR(h) than balsam fir at low values of needle packing, but STAR(h) values converged at high values of needle packing. The generally comparable values of STAR, along with similar needle diameters, may imply that red spruce and balsam fir have similar collection efficiencies of wet and dry particles. Measurements of STAR may be used to estimate leaf area indices (LAI) more accurately when using indirect techniques.     

Changes in the canopies of Pinus sylvestris and Picea abies under alkaline dust impact in the industrial region of Northeast Estonia

Studies were carried out in 1999, 2005 and 2007 in the area of Kunda cement plant in Northeast Estonia on sample plots 3 km W and 2.5 and 5 km E of Kunda. As control stands, two plots for pine and spruce were established in Lahemaa National Park (34-38 km W of Kunda). The selected pine and spruce stands were 75-85-year-old Myrtillus site type, of 0.7-0.8 density and II quality class, with moderately dense or sparse understorey. The values concerning needle density and number of needle scars were higher for shoots formed in the period of higher pollution than for the shoots grown under a considerably lower pollution load. Although the cement dust pollution has notably decreased from year to year, the number of needle pairs per 1 cm of the shoot was 1.8-2.1 times greater in the shoots formed in 1998 than in those formed in 2003, whereas the changes were statistically reliable. Possibly the low temperatures at the time of shoot and needle formation affected the density of needles on all sample plots, and thus the number of needles on shoots formed in 2003 was many times smaller. After the significant fall in the pollution load since 1996 the length growth of needles intensified around Kunda cement plant, at the same time no changes occurred in the length growth of needles in the control area. As compared to the data from 1998, the length growth of pine needles had improved, especially 2.5 and 5 km E from the cement plant, needles being respectively 1.5 and 1.1 cm longer than 6 years ago. The stimulation of the growth of pine and spruce needles 2.5 and 5 km E of the cement plant may be a sign of a positive effect of reduced doses of cement dust in soil. The greater length of pine and spruce needles is the reason for the larger biomass of the needles.     

In situ experimental freezing produces symptoms of winter injury in red spruce foliage

Two mechanisms have been proposed to explain winter injury to needles of red spruce (Picea rubens Sarg.): (1) desiccation, which is characterized by net loss of foliar water from the needle to the environment, with cell injury resulting from dehydration; and (2) freezing, which is characterized by direct injury to cells resulting from intracellular or extracellular ice formation during exposure to low temperature. To compare the separate and combined effects of freezing and desiccation, branches of a mature red spruce at 1160 m were (a) experimentally frozen in situ to -50 degrees C; (b) cut and tied in their original orientation and allowed to desiccate passively; or (c) both frozen in situ and cut and tied in their original orientation. Needle water content, electrolyte leakage (an index of cell injury), and needle color were monitored for 60 days after treatment. Freezing resulted in immediate increases in electrolyte leakage, rapid water loss, and reddening necrosis of needles similar to that of naturally injured needles. Cutting resulted in more gradual water loss, no significant changes in electrolyte loss until severe desiccation had occurred, and a change in the color of the needles to a dull green. Because freezing produced reddening necrosis, a key symptom of winter injury, whereas desiccation did not, we conclude that freezing is probably the primary cause of winter injury in red spruce, and that desiccation is a secondary effect.     

Tree recovery during the aftermath of an outbreak episode of the Hungarian spruce scale in southern Sweden

In 2010, the first, and so far only, infestation of the Hungarian spruce scale (Physokermes inopinatus) and accompanying sooty mould occurred in Scania, southernmost Sweden. About 1000?ha of Norway spruce (Picea abies) were affected, and the trees suffered from the sucking of the insects as well as from the dense sooty mould that covered the needles. Salvage cuttings were carried out in many of the massively attacked forest stands, both in response to the fear that the trees otherwise would die, e.g. from secondary bark beetle attacks and to prevent spreading of the infestation. The aim of this study was to provide basic, quantitative knowledge on the aftermath response of trees that were heavily infested, but not exposed to salvage cutting. Growth characteristics, in terms of needle weight, shoot length and tree-ring size were measured on infested and uninfested trees to compare and contrast the spruce growth before, during and after the scale outbreak. The infestation resulted in dwarf annual shoots, stunted needles and thin tree rings. The needle weight returned to normal the following year, whereas shoot length and tree rings required one growing season before full recovery.     

Cold tolerance and water content of current-year red spruce foliage over two winter seasons

Red spruce (Picea rubens Sarg.) in high elevation forests of northeastern North America suffers from frequent and severe winter injury, leading to apical dieback, decreased growth, and high mortality. To examine the role of winter desiccation and freezing injury in winter damage, weekly assessments of cold tolerance and water content were made on current-year foliage collected from native red spruce trees at a high elevation site over two winter seasons. In both years, foliage maintained high water contents and adequate cold tolerance; nonetheless, slight to moderate injury was observed each year on some trees. Despite brief thaw periods each winter, no mid-winter dehardening sufficient to put foliage at risk of freezing injury was evident. These findings suggest that, at least in some years, winter injury to current-year red spruce foliage is produced by a mechanism other than desiccation or absolute low temperatures.     

Variation in the ratio of shoot silhouette area to needle area in fertilized and unfertilized Norway spruce trees

We compared the range and variation in shoot silhouette area to projected leaf area ratio (SPAR) in fertilized and unfertilized (control) Norway spruce (Picea abies (L.) Karst.) trees. We measured SPAR for several view directions of 169 shoots at different depths in the crown of fertilized and control trees. There was an increase in SPAR with depth in the crown in both control and fertilized trees. In the fertilized trees, however, mean SPAR was larger overall, the increase with depth in the crown was steeper, and there was a larger variation in SPAR with inclination and rotation angle of the shoot (relative to the view direction). In particular, shoots in the lower crown of fertilized trees were rotationally asymmetrical ("flat") and had high values of the maximum ratio of shoot silhouette area to projected leaf area (SPAR(max)). Differences in SPAR between fertilized and control trees were explained by changes in shoot structure in response to fertilization and shading. Shoots of fertilized trees were larger and had more needle area than shoots of control trees. However, the ratio of needle area to shoot size was smaller in fertilized trees than in control trees, implying less within-shoot shading and, consequently, a larger SPAR. Also, the increase in SPAR with increased shading (depth in the crown) could be explained by a decrease in the ratio of needle area to shoot size. In addition, because fertilized trees had more needle area than control trees, the effect of shading at a given depth in the crown was more pronounced in fertilized trees than in control trees.     

Photosynthetic capacity of red spruce during winter

We measured the photosynthetic capacity (P(max)) of plantation-grown red spruce (Picea rubens Sarg.) during two winter seasons (1993-94 and 1994-95) and monitored field photosynthesis of these trees during one winter (1993-94). We also measured P(max) for mature montane trees from January through May 1995. Changes in P(max) and field photosynthesis closely paralleled seasonal changes in outdoor air temperature. However, during thaw periods, field photosynthesis was closely correlated with multiple-day temperature regimes, whereas P(max) was closely correlated with single-day fluctuations in temperature. There was a strong association between short-term changes in ambient temperature and P(max) during the extended thaw of January 1995. Significant increases in P(max) occurred within two days of the start of this thaw. Repeated measurements of cut shoots kept indoors indicated that temperature-induced increases in P(max) can occur within 3 h. Although significant correlations between P(max) and stomatal conductance (g(s)) or intracellular CO(2) concentration (C(i)) raised the possibility that increases in P(max) resulted from increases in stomatal aperture, fluctuations in g(s) or C(i) explained little of the overall variation in P(max). Following both natural and simulated thaws, P(max) increased considerably but plateaued at only 37% of the mean photosynthetic rate reported for red spruce during the growing season. Thus, even though shoots were provided with near-optimal environmental conditions, and despite thaw-induced changes in physiology, significant limitations to winter photosynthesis remained.     

Fertilization has little effect on light-interception efficiency of Picea abies shoots

We investigated effects of nutrient availability on shoot structure and light-interception efficiency based on data from control (C) and irrigated + fertilized (IL) trees of Norway spruce (Picea abies (L.) Karst.). The sampling of 1-year-old shoots was designed to cover the variation in canopy exposure within the live crown zone, where current-year shoots were still found. Canopy openness was used as a measure of light availability at the shoot's position. Openness values for the sample shoots ranged from 0.02 to 0.77 on the IL plot, and from 0.10 to 0.96 on the C plot. Among needle dimensions, needle width increased most with canopy openness. At fixed canopy openness, needle width was larger, and the ratio of needle thickness to width was smaller in IL trees than in C trees. Specific needle area (SNA) and the ratio of shoot silhouette area to total needle area (STAR) decreased with canopy openness, so that the combined effect was a threefold decrease in the ratio of shoot silhouette area to unit dry mass (SMR = STAR x SNA) along the studied range of openness values. This means that the light-interception efficiency of shoots per unit needle dry mass was three times higher for the most shaded shoots than for sun shoots. A test of the effect of fertilization on the relationships of SNA, STAR and SMR indicated statistically significant differences in both slope and intercept for SNA and STAR, and in the intercept for SMR. However, the differences partly cancelled each other so that, at medium values of canopy openness, differences between treatments in predicted SNA, STAR and SMR were small. At 0.5 canopy openness, predicted STAR of IL shoots was 6.1% larger than STAR of C shoots, but SMR of IL shoots was 10% smaller than that of C shoots. The results suggest that light-interception efficiency per unit needle area or mass of the shoots is not greatly affected by fertilization.     

Rapid freezing induces winter injury symptomatology in red spruce foliage

Red spruce (Picea rubens Sarg.) suffers frequent and extensive injury to current-year foliage during the winter. Experimental freezing of red spruce foliage at cooling rates > 10 degrees C min(-1) induced visible symptomatology similar to natural winter injury at the branch, needle and cellular levels. Such damage was associated with a low-temperature exotherm near -10 to -12 degrees C, a loss in needle fluorescence, massive cellular disruption, foliar discoloration, and low needle survival. Susceptibility of individual trees to rapid freezing injury was associated with historical winter injury patterns and alterations in foliar nutrition. We conclude that anthropogenic deposition may alter the sensitivity of trees to winter injury caused by rapid temperature changes.     

Rapid freezing injury in red spruce: seasonal changes in sensitivity and effects of temperature range

On calm, cold days in winter, sun-exposed needles of red spruce (Picea rubens Sarg.) may warm 10 to 20 degrees C above ambient air temperature, and undergo rapid (>/= 1 degrees C min(-1)) fluctuations in temperature as light breezes or passing clouds alter the energy balance of the foliage. It has been proposed that the resulting rapid freeze-thaw cycles (freezing stress) cause a type of winter injury in montane red spruce that is characterized by necrosis of sun-exposed foliage. In autumn and winter, we monitored rapid freezing stress response of needle sections from 10 montane red spruce trees by subjecting needles to rapid freezing over the temperature span typically recorded in the field. In autumn, experimental rapid freezing stress produced severe injury only at temperatures considerably lower than expected for that time of year. In winter, rapid freezing caused occasional, moderate injury in fully hardened foliage of trees susceptible to both slow and rapid freezing. Seasonal changes in sensitivity to rapid and slow freezing were correlated, suggesting that environmental factors that are known to affect sensitivity to slow freezing may also affect sensitivity to rapid freezing. Experimental manipulation of the start and end temperatures of rapid freezing stress events showed that moderate to severe needle injury can occur in susceptible trees at temperature spans slightly more extreme than those typically recorded in the field. The extent of injury was similar at different starting temperatures if rapid freezing occurred over the same temperature span. Year-old foliage was consistently less sensitive to rapid freezing stress than current-year foliage, but some year-old foliage was damaged when the rapid freezing stress regime caused severe injury in current-year foliage. We conclude that rapid freeze-thaw cycles can explain light to moderate injury of current-year foliage, but they do not explain the more severe and widespread pattern of foliar damage that has occurred intermittently over at least the last 18 years.     

Reduction in turgid water volume in jack pine,white spruce and black spruce in response to drought and paclobutrazol

Significant reductions in needle water content were observed in white spruce (Picea glauca (Moench) Voss), black spruce (Picea mariana (Mill) B.S.P.), and jack pine (Pinus banksiana Lamb.) seedlings in response to a 10-day drought, although turgor was apparently maintained. When the seedlings were re-watered after the drought, jack pine needles regained their original saturated volume, whereas white spruce and black spruce needles did not. Significant drought-induced reductions in turgor-loss volume (i.e., tissue volume at the point of turgor loss) were observed in shoots of all three species, especially jack pine. Repeated exposure to 7 days of drought or treatment with the cytochrome P(450) inhibitor, paclobutrazol ((2RS,3RS)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-1-yl)-pentan-3-ol), reduced seedling height relative to that of untreated controls in all three species. The reductions in saturated and turgor-loss needle volumes in the paclobutrazol-treated seedlings were comparable with those of seedlings subjected to a 10-day drought. The treatment-induced reductions in shoot and needle water contents enabled seedlings to maintain turgor with tissue volumes close to, or below, the turgor-loss volume of untreated seedlings. Paclobutrazol-treated seedlings subsequently survived drought treatments that were lethal to untreated seedlings.     

Photosynthesis of a Scots pine shoot: a comparison of two models of shoot photosynthesis in direct and diffuse radiation fields

Two models of shoot photosynthesis, the needle surface element model (SEM) and needle volume element model (VEM), were tested against empirical data obtained from measurements of the photosynthetic response of twelve Scots pine (Pinus sylvestris L.) shoots in direct and diffuse radiation. The models assume that shoot photosynthesis is obtained as the integrated response of either all needle surface area elements (SEM) or all needle volume elements (VEM) of the shoot. The models differ in that needles are treated as optically black in SEM, whereas in VEM radiation penetrates into the needle. The photosynthetic response of a surface/volume element was described as a Blackman-type curve and the distributions of irradiance on the elements were derived by computer simulation, based on a model of shoot geometry. The parameters (initial slope and maximum rate) of the Blackman-curve of an element were estimated iteratively by the method of least squares, i.e., by minimizing the residual sum of squares of simulated and measured rates of shoot photosynthesis. The parameter estimation was done separately for direct and diffuse radiation, and the models were evaluated based on the notion that, for the "ideal" model, the estimated parameter values should be the same in direct and diffuse radiation. Both models produced shoot photosynthesis curves that agreed well with measurements, but there was a discrepancy in the estimated parameter values, indicating that differences in the photosynthetic response of shoots in direct and diffuse radiation could not be explained solely on the basis of the simulated irradiance distributions. The agreement was, however, much better for the volume element model, which accounts for penetration of radiation into the needles.