Pauline Oker-Blom

Effect of grouping of foliage on the within-stand and within-crown light regime: Comparison of random and grouping canopy models

Abstract The effect of grouping of foliage of Scots pine (Pinus sylvestris L.) was studied theoretically and the following results were obtained: 1. (1) grouping of needles and shoots at any level of the canopy compared with the canopy representing horizontal homogeneity decreased the light absorption in the canopy substantially, but; 2. (2) the average light conditions for the lowest part of the canopy were improved by the grouping effect since it enabled more light to pass through the upper layers down to a lower canopy, and; 3. (3) the within-plant shading was in most cases considerably greater than the between-plant shading and, consequently, the light conditions affecting an individual tree were to a high degree determined by the structure of the tree itself, which seemed to be true especially in sparse and/or young tree stands. The silvicultural interpretation of the results regarding the spacing of tree stands is also discussed.

Relationship between radiation interception and photosynthesis in forest canopies: effect of stand structure and latitude

Abstract Interception of radiation and the consequent potential photosynthesis was studied, by sing a simulation model, in structurally different forest stands at latitu udes 40° and 60°. The studied stands were of two different types with respect to the leaf-area distribution: horizontally homogenous canopies and canopies with an aggregation of leaves into individual crowns. The effect of canopy structure on interception of radiation and photosynthesis was studied by varying leaf area index, stand density, and crown size and shape. In none of the studied cases was the relationship between accumulated radiation interception and photosynthesis strictly linear, but on a longer time-scale (one growing season) this non-linearity was not very pronounced. Neither canopy structure nor latitude substantially affected the slope of the relationship. In conclusion, while properties of canopy structure and incoming radiation determine the actual amount of radiation intercepted by the canopy, the conversion efficiency between intercepted radiation and photosynthesis appeared to be rather insensitive to differences in canopy structure and in properties of incoming radiation.

Predicting spatial distribution of direct radiation below forest canopies

Abstract The study explores the effects of latitude, crown shape, stand density and tree arrangement on the horizontal variation of daily irradiance at different depths in the stand. There was a continuous decrease in the horizontal variance of irradiance in Poisson stands, but in hexagonally arranged stands, local minima appeared in the variance profile. At low latitudes with broad crowns there was only one local minimum. With increasing latitude and crown height to crown width ratio, the number of minima increased to two or three, but the minima were less prominent. The results are discussed with reference to the hypothesis that horizontal variance of irradiance regulates the height of understory trees and canopy layering.

Theoretical computations on the role of crown shape in the absorption of light by forest trees

Abstract Maximization of light absorption throughout the whole growing period is used as the criterion in a theoretical study of the optimum crown shape. A tall, narrow conelike crown shape of constant volume maximizes the light absorption when as even as possible light absorption is preconditioned for each moment throughout a single day or the whole growing period.

Structural development of Pinus sylvestrís stands with varying initial density: A preliminary model for quality of sawn timber as affected by silvicultural measures

The effects of stand density, thinning and pruning on the quality of sawn timber of Scots pine (Pinus sylvestris L.) were studied based on a computer model. Procedures for the tree growth and crown structure produce a tree, whose properties in terms of location, dimensions and quality (living, dead) of branches are known for any moment of the selected rotation. Consequently, the size, quality (living, dead) and location of knots in sawn timber are recognizable allowing the grading of sawn pieces. The model computations showed that the natural dynamics of the crown system in narrowly‐spaced stands could yield sawn timber nearly of the same quality as resulted from pruning in widely‐spaced stands. Thinning increased the branch growth and the branchiness of the wood yielding lower grade for sawn pieces, respectively.

Photosynthesis of a scots pine shoot: simulation of the irradiance distribution and photosynthesis of a shoot in different radiation fields

Abstract Using a model of shoot geometry the distribution of irradiance at the needle surface area of a shoot in different radiation conditions is simulated and shoot photosynthesis calculated. It is shown that the distribution of irradiance is very sensitive to radiation field geometry, affecting the efficiency of the utilization of radiant energy in photosynthesis. As a result, the photosynthetic response of a shoot in natural conditions is, for instance, highly dependent on the proportions of direct and diffuse solar radiation, being higher the greater is the proportion of diffuse radiation.

Structural development of Pinus sylvestris stands with varying initial density: a simulation model

The growth and structural development of Scots pine (Pinus sylvestris L.) trees growing at different spacing was simulated using a model based on the dry matter production per needle biomass unit and its allocation to needles, branches and stem. Special emphasis was given to the effect of stand density on the growth of the crown system and its implications on branchiness and timber quality. The simulations showed that the needle biomass culminates considerably earlier than the branch biomass with a time lag inversely related to the stand density. The lengths of living and dead crown were also inversely related to stand density. The resulting differences in branchiness were especially obvious in the early development of the tree stands. In the long run these differences tend to disappear, indicating equal external branchiness independently of the initial spacing for mature stands of Scots pine. The internal branchiness, however, was particularly sensitive to the initial spacing.

Photosynthesis of a Scots pine shoot: The effect of shoot inclination on the photosynthetic response of a shoot subjected to direct radiation

Abstract A set of photosynthetic responses of a Scots pine ( Pinus sylvestris L.) shoot to light was derived from the shoot geometry and the photosynthetic response of a single needle. Computations showed that the shape of the photosynthesis light-curves varies substantially depending on the direction of radiation relative to the shoot position. Differences in the initial and maximum rates of photosynthesis were due to changes in the effective projection area and the irradiated fraction of the shoot, respectively.

Photosynthesis of a scots pine shoot: Test of a shoot photosynthesis model in a direct radiation field

Abstract The photosynthetic light curves of eight Scots pine ( Pinus sylvestris L.) shoots, each with a different structure, were measured in a direct radiation field at different positions relative to the radiation beam. Using a model of shoot geometry, the irradiance distributions on the needle surface area of the same shoots were simulated in each position. Using measurements of shoot photosynthesis, the photosynthetic light curve for a needle surface area element (a small area on the needle surface) was estimated indirectly for each shoot. Shoot photosynthesis was then calculated from the estimated photosynthetic light response curve of a needle surface area element and the irradiance distribution on the surface area of needles. Calculated rates for shoot photosynthesis agreed well with the measured rates. The mean irradiance on the needle surface area was the major component causing variation in the light response of a shoot irradiated from different directions, but the shape of the irradiance distribution on the needle surface area had a considerably smaller effect.

Light regime and photosynthetic production in the canopy of a scots pine stand during a prolonged period

Abstract The light regime and the respective photosynthetic production in the canopy of a Scots pine ( Pinus sylvestris L.) stand is evaluated on the basis of models describing the penetration of radiation into the canopy, the distribution of the needle biomass in the canopy and the characteristics of the incoming solar radiation. Special attention is paid to the introduction of the normal weather pattern into the computations in order to obtain approximations about prevailing light conditions and respective values of the photosynthetic production during a whole growing period. Results are given for values of the shading needle biomass of the canopy system.