Research

In whole tree ecophysiology, we combine the research of individual processes in different part of trees into holistic understanding of how trees function. The general questions are how individual processes affect each other and the tree as whole, and what are limiting factors for tree growth in different time scales.

Trees get energy and carbon from photosynthesis, and most of the nutrient and water uptake happens in soil. Different parts of trees act as sources or sinks of different resources. Carbohydrates, water, minerals, and secondary metabolites are transported between different parts of the tree via xylem and phloem in stem, branches, and roots.

Trees are highly dynamic organisms, and these processes are controlled by the current and present environmental factors, plant hormones, storage and distribution of resources, and unknown factors. The variety of interactions makes studying these processes and their interactions extremely interesting.

Climate is changing, causing warming temperatures and changes in precipitation and snow cover. High latitude forests are typically temperature limited, and thus considered to be especially sensitive to climate warming. The most dramatic changes are to be expected in winter conditions; characteristic for boreal forest is a long and cold winter, and trees are adapted to these conditions. Our team studies how boreal forests respond to changing climate conditions. We are studying changes in both tree structure and physiology as we emphasize the strong linkage they have.

Process-based growth models based on carbon balance has been our core research activity since early 1980´s.

The forest modelling group

• develops theories and models of tree function, structure, and growth,
• applies these to questions relevant to forest management under changing environmental conditions and alternative management objectives.

The work employs a wide range of advanced mathematical, computational and numerical methods and is carried out in close collaboration with empirical scientists and users of the results. Applications of the models include predictions of timber yield and quality, growth of heterogeneous stands, large-scale effects of climate and weather on productivity and carbon fluxes, and a visual stand simulator for research and teaching (PuMe).

At the field stations, gas exchange of carbon dioxide (CO2) and water (H2O), as well as ozone (O3), nitrogen oxides (NOX, NO, N2O), methane (CH4), small isoprenoids and other different volatile organic compounds (VOCs) in Scots pine, silver birch and trembling aspen is monitored continuously with dynamic shoot and trunk enclosures. To deepen the understanding on their response mechanisms, we also perform short-term laboratory experiments with fast time response instruments.

We estimate carbon exchange of a forest ecosystem directly by micrometeorological methods (eddy covariance) and upscaling enclosure observations of CO2 exchange at shoot and forest floor level over the whole forest stand. The upscaled photosynthesis can explain 90-95 % of the observed variation and predict accurately the annual cumulative photosynthetic production.

Photosynthesis is the process by which solar energy is made available to support life on Earth.  Photosynthesis controls the productivity of crops and forests, supplying us with food, fiber and building materials. It also drives the global carbon cycle, with multiple interactions and feedbacks with the climate.

Photosynthesis can be measured at the leaf, shoot, and forest stand level using IRGAs (InfraRed Gas Analyzers) coupled to chambers or eddy covariance systems. Our goal is to characterize, quantify and model the physical and physiological processes that link optical data and photosynthesis at multiple spatial and temporal scales (from the leaf to the landscape, and from seconds to years).

Water in trees is transported from the soil to the leaves through the mostly dead pipe-like structures in sapwood, and then transpired to the atmosphere from leaf surfaces. Water in the xylem is under negative pressure and cohesive forces between water molecules maintain the water columns intact from the soil to the leaves. The thin layer of phloem tissue between the sapwood and the bark is under positive pressure that is maintained osmotically with assimilated sugars and dissolved minerals.

Movement of either water or phloem sap requires a pressure gradient as transport occurs from higher pressure towards lower pressure. The pressure gradients are influenced by tree structure (e.g. tissue structures), tree function (e.g. gas exchange in foliage) and the environment (e.g. soil water status or CO2 concentration in the air). In leaves, CO2 is taken in for photosynthesis through the same stomatal pores through which water escapes in transpiration, so the leaf stomata need to operate to maximize photosynthetic production and flow from leaves to other tree parts.

We study how uptake, transport and usage of water and carbon are coordinated, and how they are affected by structural properties of trees and by variability in soil and atmospheric conditions. Environmental conditions are expected to change with climate change, and we want to understand how these changes in frequency, duration and severity of drought and freezing stress will affect tree performance and survival.

Our research is based on continuous field measurements of stem diameter variations, sapflow and shoot and canopy transpiration, laboratory experiments in controlled conditions, and modelling.

Roots have a major influence on soil biochemical processes. From the point of view of material fluxes, large quantities of carbon assimilated in photosynthesis is allocated belowground through roots. Carbon compounds in root exudates consist mainly of sugars and are therefore easily available as an energy source for a diversity of soil microbes. Microbial activity is particularly high near roots and mycorrhizal fungal hyphae (mycorrhizosphere). Microbes are responsible of most of the important biochemical reactions in the soil by producing many process-specific enzymes.

In boreal forest, almost all root tips of trees are colonized by symbiotic ectomycorrhizal (ECM) fungi. ECM fungi form large mycelial networks in the forest soil and greatly extend the nutrient-reaching surface area of the root system. In addition to increased nutrient uptake, ECM help trees in many other ways like protecting roots against pathogenic  microbes, affecting their drought tolerance etc.

In the group, the research related to root processes is focused on root-fungal interactions and on how roots and symbiotic fungi affect plants' C allocation. The research is intimately linked to other soil research done in the group (e.g. studies related to the so-called Priming effect).

Soil plays an important role in the sequestration of atmospheric carbon and regulating the ecosystem water balance. Soil is also a substantial storage of organic nitrogen, an important limiting factor for biomass growth. Soil carbon and nitrogen pools are intimately linked, and changes in these pools may have a significant impact on ecosystem productivity and to the radiative balance of the atmosphere. Soils are identified as sources of several trace gases, such as carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) as well as biogenic volatile organic compounds (BVOCs) and volatile organic nitrogen (VON), which have a strong role in the climate change, air quality issues and tropospheric chemistry.

The aim of the soil studies is to quantify and understand the processes underlying the material and energy fluxes in the soil and between the soil and the atmosphere. We are studying the budgets, fluxes and processes of carbon, nitrogen and water within boreal ecosystems with a multitude of measurement techniques (e.g. chambers, gas gradients and eddy covariance). We also work with process-based modeling of carbon, nitrogen and water cycling within the soil and the whole ecosystem. Moreover, we use molecular biological techniques to study the effect of microbial communities, especially ectomycorrhiza, on soil processes.

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Our research is dealing with the effects of disturbances on the biogeochemical cycles in boreal and sub-arctic forests in the circumpolar region. Our multidisciplinary research team consists of forest ecologists and soil scientists and we are collaborating with microbiologists and atmospheric scientists.

Climate change is predicted to increase the vulnerability of forests to various disturbance effects resulted from e.g. drought events, forest fires and permafrost melting. Currently we are study the effects of disturbances (fire, mammalian herbivores (reindeer), wind, etc.) in several projects.

For more information visit the Disturbances and biogeochemistry group web page.

Even though terrestrial and aquatic parts of ecosystems are often studied separately, they are closely connected. For studying the functioning of the carbon cycle, it is necessary to understand how carbon enters and leaves the soil. Globally, the loss of terrestrial organic carbon to rivers is equivalent to 10 % of the net ecosystem production, few streams and rivers have been studied for CO2 fluxes.

We yield new information about carbon transfer in terrestrial-aquatic continuum on landscape level by combining eddy covariance method and CO2 concentration measurements in lake and stream waters and in the littoral zone. We examine the carbon dynamics on both interannual and seasonal scales and pay special attention to the time periods following special weather events.

Our study sites are located at lake Valkea-Kotinen ICP integrated monitoring area in Southern Finland (TransCarbo) and in lake Kuivajärvi at Hyytiälä Forestry Field Station (Vesihiisi). These projects aim at closing the carbon balance of forest and lake at landscape level including both  inorganic and organic parts of carbon cycle.