Metla Project 3394

The effects of the timing of soil frost thawing on growth and physiologyof roots and shoot in trees

Duration: 2004-2007 Keywords: Climate change, impedance spectroscopy, nutrients, phenology, physiology, root, soil frost, soil temperature, stress, tree growth, water uptake
Research project group: Distinct projects 1 - Structure and function of forest ecosystems

Objectives

The aim is to study growth responses of trees induced by the changes in the circumstances at the root zone area. The root conditions will be changed experimentally, followed by measurements of growth, physiology, phenology and anatomic structure of roots and shoots. The study will focus on the events taking place in trees at the time of soil frost thawing in spring. A new non-destructive method for monitoring of root growth will be developed.

Results

Effects of soil frost thawing on physiology, phenology, anatomical structure and growth of forest trees (subproject 1)

The delay of soil frost thawing by two weeks may cause irreversible damage and even kill Scots pine (Pinus sylvestris L.) seedlings. Different degrees of injuries and growth losses may occur, however, even in the case of a much shorter delay of soil frost thawing.
According to the experiments in controlled conditions in Joensuu dasotrons the dark adapted chlorophyll fluorescence (Fv/Fm), chlorophyll a/b-ratio and water potential of needles declined as a result of delayed soil frost thawing. At the same time, extracellular resistance first decreased, but then - as a consequence of the drying of needles - remarkably increased. The impedance spectra of trunks were abnormal in the seedlings that were exposed to delayed soil frost thawing. This proved disorders in the water transport system. Starch content of needles started to increase at first in those seedlings where the soil thawing took place during the artificial winter. In those seedlings shoot and root elongation started earlier than in the others. New root tip formation and root elongation was faster if the soil was already thawed before onset of the artificial spring than thawed just after some delay.
An increase in soil water content is the first expression of the initiation of soil frost thawing. Thereafter the soil temperature typically remains close to 0 °C for a while before increase. Until now we do not know exactly how effectively roots may fulfil water deficit due to demand of evapotranspiration in needles and stems in those conditions.
A preliminary field trial was carried out in a 30 years old stand of Norway spruce. Soil frost thawing could be delayed artificially by several weeks. Consequently, we could observe changes in bud burst of trees and in some physiological characteristics of needles.

Modelling the effects of soil frost thawing on trees (subproject 2)

According to the dasotron trials the difference in the temperature sum between soil (threshold for the daily mean temperature 0 °C) and air (threshold for the daily mean temperature 5 °C) was negative in those instances where the soil thawing was delayed. The longer was the thawing delay, the more negative was the differential temperature sum (unit degree days, d.d.). When the thawing started coincidentally with the increase of air temperature the differential temperature sum declined to a value of -60 d.d. As a result of two weeks delay in thawing the corresponding value was -210 d.d., however. In the former case clear stress symptoms were observed in the seedlings but they recovered. In the latter case the seedlings suffered strong damage.
In the field trial in Kangasvaara (Sotkamo, Finland) negative values for the differential temperature sums were observed in forest but not in the clear cut area. In the forest, the differential temperature sum was negative in four out of ten years between 1993-2002. It was at the biggest -50 d.d. This corresponds well with the treatment in the dasotron trial where there were reversible stress symptoms.

Development of the method for root growth sensing (suproject 3)

In the development of the method for root growth sensing we aim to find out the effects of different factors in the root and the growing substrate on the electrical impedance. The aim is to develop an electrical model for the system and according to the model parameters to sense root growth.
In the first phase of the methodological development we simplified the measurement system by raising and measuring the seedlings in hydroponic culture. Willow cuttings were used as the test material. According to the results the impedance spectra of willow cuttings changed with root growth. The impedance spectra could be modelled by a distributed model. A resistance parameter of the model correlated significantly with the root mass.
The impedance properties of soil as the growing substrate differ from those in the hydroponics. Obviously the electrochemical phenomena on the surface of roots and the electrode are more complicated in the soil than in the hydroponics. Those items have been investigated by measuring the impedance spectra of sandy soil with different moisture contents. According to those measurements have been developed an electrical model for the soil, and investigated the changes in model parameters as a function of soil moisture content.

Project leader: Repo, Tapani
The Finnish Forest Research Institute, Joensuu Office, PL 68, FI-80101 JOENSUU, FINLAND
Phone: +358 29 532 3136
E-mail: tapani.repo@metla.fi

Other researchers: Finér, Leena, JO (2007), Sutinen, Sirkka, JO (2004-07)

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Updated 12.06.2012
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