Annikki Mäkelä

Effects of age and site quality on the distribution of biomass in Scots pine (Pinus sylvestris L.)

The distribution of the above-ground and below-ground biomass of Scots pine in southern Finland were investigated in trees of different ages (18–212 years) from two types of growth site. Secondly, some structural regularities were tested for their independence of age and growth site. Trees were sampled from dominant trees which could be expected to have a comparable position in stands of all ages. All stands were on sorted sediments. The biomass of the sample trees (18 trees) was divided into needles, branch sapwood and heartwood, stem sapwood and heartwood, stem bark, stump, large roots (diameter >20 cm), coarse roots (five classes) and fine roots. The amount of sapwood and heartwood was also estimated from the below-ground compartments. Trees on both types of growth site followed the same pattern of development of the relative shares of biomass compartments, although the growth rates were faster on the more fertile site. The relative amount of sapwood peaked after canopy closure, coinciding with the start of considerable heartwood accumulation. The relative amount of needles and fine roots decreased with age. The same was true of branches but to a lesser degree. The relative share of the below-ground section was independent of tree age. Foliage biomass and sapwood cross-sectional area were linearly correlated, but there were differences between the growth sites. Needle biomass was linearly correlated with crown surface area. The fine root to foliage biomass ratio showed an increasing trend with tree age.

Effect of thinning on surface fluxes in a boreal forest

[1] Thinning is a routine forest management operation that changes tree spacing, number, and size distribution and affects the material flows between vegetation and the atmosphere. Here, using direct micrometeorological ecosystem-scale measurements, we show that in a boreal pine forest, thinning decreases the deposition velocities of fine particles as expected but does not reduce the carbon sink, water vapor flux, or ozone deposition. The thinning decreased the all-sided leaf area index from 8 to 6, and we suggest that the redistribution of sources and sinks within the ecosystem compensated for this reduction in foliage area. In the case of water vapor and O 3 , changes in light penetration and among-tree competition seem to increase individual transpiration rates and lead to larger stomatal apertures, thus enhancing also O 3 deposition. In the case of CO 2 , increased ground vegetation assimilation and decreased autotrophic respiration seem to cancel out opposite changes in canopy assimilation and heterotrophic respiration. Current soil-vegetation-atmosphere transfer models should be able to reproduce these observations.

Assimilate transport in phloem sets conditions for leaf gas exchange

Carbon uptake and transpiration in plant leaves occurs through stomata that open and close. Stomatal action is usually considered a response to environmental driving factors. Here we show that leaf gas exchange is more strongly related to whole tree level transport of assimilates than previously thought, and that transport of assimilates is a restriction of stomatal opening comparable with hydraulic limitation. Assimilate transport in the phloem requires that osmotic pressure at phloem loading sites in leaves exceeds the drop in hydrostatic pressure that is due to transpiration. Assimilate transport thus competes with transpiration for water. Excess sugar loading, however, may block the assimilate transport because of viscosity build-up in phloem sap. Therefore, for given conditions, there is a stomatal opening that maximizes phloem transport if we assume that sugar loading is proportional to photosynthetic rate. Here we show that such opening produces the observed behaviour of leaf gas exchange. Our approach connects stomatal regulation directly with sink activity, plant structure and soil water availability as they all influence assimilate transport. It produces similar behaviour as the optimal stomatal control approach, but does not require determination of marginal cost of water parameter.

CROWN RATIO INFLUENCES ALLOMETRIC SCALING IN TREES

Allometric theories suggest that the size and shape of organisms follow universal rules, with a tendency toward quarter-power scaling. In woody plants, however, structure is influenced by branch death and shedding, which leads to decreasing crown ratios, accumulation of heartwood, and stem and branch tapering. This paper examines the impacts on allometric scaling of these aspects, which so far have been largely ignored in the scaling theory. Tree structure is described in terms of active and disused pipes arranged as an infinite branching network in the crown, and as a tapering bundle of pipes below the crown. Importantly, crown ratio is allowed to vary independently of crown size, the size of the trunk relative to the crown deriving from empirical results that relate crown base diameter to breast height diameter through crown ratio. The model implies a scaling relationship in the crown which reduces to quarter-power scaling under restrictive assumptions but would generally yield a scaling exponent somewhat less than three-quarters. For the whole tree, the model predicts that scaling between woody mass and foliage depends on crown ratio. Measurements on three boreal tree species are consistent with the model predictions.

Optimal function explains forest responses to global change

Plant responses to global changes in carbon dioxide (CO2), nitrogen, and water availability are critical to future atmospheric CO2 concentrations, hydrology, and hence climate. Our understanding of those responses is incomplete, however. Multiple-resource manipulation experiments and empirical observations have revealed a diversity of responses, as well as some consistent patterns. But vegetation models—currently dominated by complex numerical simulation models—have yet to achieve a consensus among their predicted responses, let alone offer a coherent explanation of the observed ones. Here we propose an alternative approach based on relatively simple optimization models (OMs). We highlight the results of three recent forest OMs, which together explain a remarkable range of observed forest responses to altered resource availability. We conclude that OMs now offer a simple yet powerful approach to predicting the responses of forests—and, potentially, other plant types—to global change. We recommend ways in which OMs could be developed further in this direction.

Vertical structure of Scots pine crowns in different age and size classes

Abstract. The pipe model theory postulates a static relationship between foliage mass/area and the cross-sectional area of active pipes in branches and stems. If a regular relationship exists, the theory can be used for modelling growth allocation within crowns, provided that the turnover of active pipes and foliage is understood. The objective of this study was to assess to what extent the assumptions of the pipe model hold true within the crowns of 24 Scots pine sample trees of different age and social position. The results suggest that Scots pine crowns are very regular, but some important modifications to the pipe model assumptions are required. The relative vertical foliage density distribution peaked at about 50% down the live crown regardless of age or social position. The ratio of foliage mass to branch cross-sectional area in the top half of the crown increased from the top downwards. The ratio of cumulative branch cross-sectional area to stem cross-sectional area in the top half seemed to increase with tree vigour or growth rate. The ratio of foliage mass to branch cross-sectional area decreased fast in the lower half of the crown, and this decrease was faster than could be predicted from heartwood formation in the branches. This result may be taken as (1) evidence against the pipe model, or as (2) an indication that active pipes cannot always be identified with sapwood. The latter proposition should be studied further.

Height growth strategies in open-grown trees

This paper analyses the height growth of open-grown trees as a compromise between reducing self-shading by decreasing foliage density, and reducing structural costs by controlling crown dimensions. Self-shading is described with an equation which accounts for the reduction of foliage-specific photosynthesis due to increased volume density of foliage. Tree growth is described using a differential equation based on the carbon balance, where allocation of growth between wood and foliage follows the pipe-model theory. Crown shape is constant and crown length is equal to tree height. In this model, the construction and maintenance costs of wood relative to photosynthetic production increase with decreasing foliage density in the crown. The main result is that optimal height growth for maximizing accumulated net production is a bell-shaped function of time and rather independent of parameter values. As a result of optimal height growth, foliage biomass follows an allometric function of tree height. The allometric exponent is in the range 2–3, depending on the environmental parameters. The model is most sensitive to the fertility of the growth site, such that fertile sites favour low values and poor sites high values of the exponent. The results are compared with Scots pine and red pine trees grown in the open. The empirical material is consistent with the model.

Generating 3D sawlogs with a process-based growth model

Abstract This paper reports the application of a process-based tree and stand growth model to predictions of stem structure, in particular, the 3D geometry of the stem and its internal knots, in Scots pine (Pinus sylvestris L.). The model is an extension of an earlier model which predicted tree growth and competition on the basis of carbon balance, and the vertical profile of the stem and crown on the basis of a dynamic interpretation of the pipe model theory. In this study, statistical models are used to create individual branch information from the vertical profile of branch basal area, and the resulting predictions of 3D stem structure are tested against measured trees. The model predictions of the vertical size distribution of external knots in different size classes of trees are realistic, although the model tends to underestimate knot size in the top of the crown especially in dominant trees. As regards the internal knots, the model tends to slightly underestimate the width of the tight knot zone, especially in suppressed trees. The test results suggest that the model is readily applicable to predictions of the effects of stand management on wood quality distribution. Further research topics include incorporating defects other than branch size and quality in the model, as well as testing the model against stand level quality distributions of timber.

Optimal co‐allocation of carbon and nitrogen in a forest stand at steady state

Nitrogen (N) is essential for plant production, but N uptake imposes carbon (C) costs through maintenance respiration and fine-root construction, suggesting that an optimal C:N balance can be found. Previous studies have elaborated this optimum under exponential growth; work on closed canopies has focused on foliage only. Here, the optimal co-allocation of C and N to foliage, fine roots and live wood is examined in a closed forest stand. Optimal co-allocation maximizes net primary productivity (NPP) as constrained by stand-level C and N balances and the pipe model. Photosynthesis and maintenance respiration increase with foliar nitrogen concentration ([N]), and stand-level photosynthesis and N uptake saturate at high foliage and fine-root density. Optimal NPP increases almost linearly from low to moderate N availability, saturating at high N. Where N availability is very low or very high, the system resembles a functional balance with a steady foliage [N]; in between, [N] increases with N availability. Carbon allocation to fine roots decreases, allocation to wood increases, and allocation to foliage remains stable with increasing N availability. The predicted relationships between biomass density and foliage [N] are in reasonable agreement with data from coniferous stands across Finland. All predictions agree with our qualitative understanding of N effects on growth.

A physiological model of softwood cambial growth

Cambial growth was modelled as a function of detailed levelled physiological processes for cell enlargement and water and sugar transport to the cambium. Cambial growth was described at the cell level where local sugar concentration and turgor pressure induce irreversible cell expansion and cell wall synthesis. It was demonstrated how transpiration and photosynthesis rates, metabolic and physiological processes and structural features of a tree mediate their effects directly on the local water and sugar status and influence cambial growth. Large trees were predicted to be less sensitive to changes in the transient water and sugar status, compared with smaller ones, as they have more water and sugar storage and were, therefore, less coupled to short-term changes in the environment. Modelling the cambial dynamics at the individual cell level turned out to be a complex task as the radial short-distance transport of water and sugars and control signals determining cell division and cessation of cell enlargement and cell wall synthesis had to be described simultaneously.