Tree ecophysiology

At the field stations, gas exchange of carbon diokside (CO2) and water (H2O), as well as ozone (O3), nitrogen oxygens (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 deepend the understaing 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.

The measured annual carbon sink of the forest at SMEAR II varies between 140 and 260 g (C) m–2 (1997-2006), which is equivalent to about 10 m3 wood production. Roughly half of the annual photosynthetic production goes to biomass increase from which about half is long-lasting wood in stems, coarse roots and branches. The forest floor vegetation, mainly dwarf shrubs and mosses, contributes to 10–15 % of the whole forest stand gross primary production (GPP).

From the respiratory fluxes 60–70% of originate below the ground surface. Annually this means an efflux of 500–600 g (C) m-2 while for the same ground surface area the mean efflux from the Scots pine shoots is about 250 g (C) m-2 and from wood and bark about 100 g (C) m-2.

Management influences the carbon balance. After a clear cut, forest becomes a net carbon source for approximately 10-20 years due to the decaying cutting residue and lowered leaf area and GPP. With stand growth the trends are reversed and COsink recovers. After canopy closure the annual sink varies around the mean depending on weather conditions. Thinnings seem to have no influence on the sink.

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). 

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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).