Pertti Hari

Station for Measuring Ecosystem-Atmosphere Relations (SMEAR II)

We planned and implemented the SMEAR II and I measuring systems to measure material and energy fluxes between forest ecosystem and its surroundings and within the ecosystem. In addition, we measured the processes generating the fluxes and environmental factors affecting the processes. We constructed SMEAR I in 1991 and SMEAR II in 1994–1996. The mass and energy fluxes play an important role in our physical and physiological theory of forest ecology. The construction principle of SMEAR measuring stations is coherent with our theory, and this is why the data obtained at SMEAR stations have been useful in the development and testing of our theory.

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.

Stand growth model based on carbon uptake and allocation in individual trees

A stand growth model based on individual tree growth, with a time step of 1 year and a time span equivalent to the rotation period, is presented. The model applies to Scots pine (Pinus sylvestris) in the boreal zone. The individual tree model describes annual net photosynthetic production and its allocation to different growth compartments. The model differs from many related ones in that inter-tree competition is explicitly described through the physical environment. Instead of defining a competition index, the concept of photosynthetic light ratio is applied. This is the ratio between actual photosynthesis produced under shade from other trees and the potential, or unshaded photosynthesis. The model estimates mass flows of the system and describes the differentiation of individual trees during stand development. Canopy closure is defined as the attainment of maximum needle biomass, which is soon followed by a decline in the amount of needles. Stem volume growth declines in the later development of the stand. Biological assumptions underlying these phenomena are discussed. Behaviour of the model is demonstrated with the aid of simulations of a natural stand and a planted stand.

Turbulence Statistics Inside and Over Forest: Influence on Footprint Prediction

Observations of wind statistics within and above a Scots pine forest are comparedwith those predicted from an analytical second-order closure model. The roughnesssublayer (RSL) effects, and the influence of stability on similarity functions, arestudied using observations. The commonly accepted forms of similarity functionsdescribe the influence of diabatic effects above the RSL well. According to earlierstudies they are expected also to apply within the RSL. As an exception, the averagewind speed normalised with friction velocity was found to be invariant with stabilityclose to the canopy top under unstable conditions. Lagrangian stochastic trajectorysimulations were used to evaluate the influence of canopy turbulence profiles onfootprint prediction. The main uncertainty was found to arise from parameterisationof the random forcing term in the Lagrangian velocity equation. The influence ofdiabatic conditions was studied, and it was found that thermal stability affectssignificantly the footprint function above the forest canopy, but significantuncertainty exists because of uncertainties in the formulation of stability functions.

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.

Biotic, Abiotic, and Management Controls on the Net Ecosystem CO2 Exchange of European Mountain Grassland Ecosystems

The net ecosystem carbon dioxide (CO2) exchange (NEE) of nine European mountain grassland ecosystems was measured during 2002–2004 using the eddy covariance method. Overall, the availability of photosynthetically active radiation (PPFD) was the single most important abiotic influence factor for NEE. Its role changed markedly during the course of the season, PPFD being a better predictor for NEE during periods favorable for CO2 uptake, which was spring and autumn for the sites characterized by summer droughts (southern sites) and (peak) summer for the Alpine and northern study sites. This general pattern was interrupted by grassland management practices, that is, mowing and grazing, when the variability in NEE explained by PPFD decreased in concert with the amount of aboveground biomass (BMag). Temperature was the abiotic influence factor that explained most of the variability in ecosystem respiration at the Alpine and northern study sites, but not at the southern sites characterized by a pronounced summer drought, where soil water availability and the amount of aboveground biomass were more or equally important. The amount of assimilating plant area was the single most important biotic variable determining the maximum ecosystem carbon uptake potential, that is, the NEE at saturating PPFD. Good correspondence, in terms of the magnitude of NEE, was observed with many (semi-) natural grasslands around the world, but not with grasslands sown on fertile soils in lowland locations, which exhibited higher maximum carbon gains at lower respiratory costs. It is concluded that, through triggering rapid changes in the amount and area of the aboveground plant matter, the timing and frequency of land management practices is crucial for the short-term sensitivity of the NEE of the investigated mountain grassland ecosystems to climatic drivers.

Relationship between temperature and the seasonal course of photosynthesis in Scots pine at northern timberline and in southern boreal zone

In earlier studies the seasonal dynamics of photosynthetic capacity in northern conifers has been explained as a slow response to the ambient temperature. We tested this concept with Scots pine (Pinus sylvestris L.). We analysed the seasonal dynamics of photosynthetic efficiency in Scots pine at the timberline in Finnish Lapland, and in a southern boreal forest in Southern Finland. The relationship between the daily photosynthetic efficiency and leaf temperature history was determined from continuous measurements of shoot CO2 exchange. The shoot CO2 exchange and photosynthetic efficiency showed similar seasonal patterns in the northern and in the southern locations, following daily mean temperature with a delay. The relationship between the temperature history and photosynthetic efficiency appeared to be near sigmoidal both in the northern and in the southern trees. The relationship was also consistent from year-to-year, thus the seasonal course of photosynthetic efficiency can be predicted accurately from the ambient temperature using a sigmoidal relationship. A rapid decrease of photosynthetic efficiency was observed when daytime temperature dropped below zero or frost had occurred in the previous night. The difference in the rate of acclimation of photosynthetic efficiency between the north and the south was small.

Gas concentration driven fluxes of nitrous oxide and carbon dioxide in boreal forest soil

Nitrous oxide (N2O) and carbon dioxide (CO2) fluxes were measured in a boreal forest during two growing seasons with soil gradient and chamber methods. N2O fluxes obtained by these two techniques varied from small emission to small uptake. N2O fluxes were of the same order of magnitude, however, the fluxes measured by the soil gradient method were higher and more variable than the fluxes measured with chambers. The highest soil gradient N2O fluxes were measured in the late summer and the lowest in the autumn and spring. In the autumn, litter fall induced a peak in N2O concentration in the organic O-horizon, whereas in the spring N2O was consumed in the O-horizon. Overall, the uppermost soil layer was responsible for most of the N2O production and consumption. Soil gradient and chamber methods agreed well with CO2 fluxes. Due to the very small N2O fluxes and the sensitivity of the flux to small concentration difference between the soil and the ambient air, the flux calculations from the O-horizon to the atmosphere were considered unreliable. N2O fluxes calculated between the soil A- and O-horizons agreed relatively well with the chamber measurements.

The Dependence of the Springtime Recovery of CO2 Uptake in Scots Pine on Temperature and Internal Factors

Summary The field data for photosynthetic CO 2 uptake was analyzed and a simulation model for the recovery of CO 2 uptake then developed that expressed the stage of development of the trees in spring. Four different hypotheses were tested concerning the relationship between the rate of maturation and temperature. A certain double sigmoid-curve was found to be best in prediction. Warm periods increased and cool ones decreased the rate of maturation. According to the results this relationship between the rate of maturation and temperature was not the same throughout the whole springtime period, but instead appeared to depend apart from on temperature, on the stage of development. The presented approach allowed the approximation of photosynthetic rate and its control during the whole spring using pure environmental data.

Structural adaptation rather than water conservation was observed in Scots pine over a range of wet to dry climates

Abstract We investigated the functional and structural responses of Scots pine to climate and estimated the importance of the genotype on the traits studied. We analysed 13C isotope discrimination (Δ13C) of various provenances in a common garden experiment and gas exchange characteristics for provenances growing in their natural environment. No clear climatic trend was found in the foliar Δ13C values of common garden trees. Similar results were obtained from estimation of λ (a largely VPD, temperature and light independent measure of intrinsic water use efficiency) from the gas exchange data. The ratio of needle mass to unit stem area and branch area to stem area increased towards south in both experiments and hence, seemed to be genetic. Trees from drier and warmer conditions seemed not to have either lower needle mass or higher intrinsic water use efficiency compared to northern latitudes.