Assessment of Moisture Effect in Simulating Forestry Biomass Supply Chain Strategy: Case Study of New Brunswick, Canada

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Introduction

Water and aqueous solutions of inorganic and organic ionic and molecular species play a fundamental role in the complex behavior of trees. Electrochemical measurement techniques have demonstrated to be the most suitable tools for gaining information on the physical chemistry of liquids, but are practically not applied in tree research. Also the tensile state of water should be accessible for electrochemical research. Thermodynamic considerations on the state of water in tree trunks during transport to the tree tops suggest a difference of electrochemical potentials between different tree heights. Potential differences were actually repeatedly measured between tissues of living trees and the soil and daily and seasonal changes of the signal were observed ([4], [19], [10], [7], [8], [6]). For a period of 11 years, Toriyama ([19]) studied electrical potentials on Albizia julibrissin trees, observing daily changes in the order of 40 mV. Also seasonal changes were found. Toriyama ([19]) was especially interested in understanding potential anomalies, which he observed before earthquakes. What he apparently observed was probably the effect of groundwater level changes, which should have an effect on the water supply to trees. The study of Fensom ([4]) on an elm tree (Ulmus) yielded typical potential differences between 20 and 40 mV with the highest potential difference observed in the afternoon. He observed a direct correlation between the electrical potentials and the sap flow, and assumed an electrokinetic phenomenon. However, Gibert et al. ([6]) found a significant time delay between sap flow and potential signal in black poplar (Populus nigra) and, consequently, rejected the above hypothesis. The maximum sap flow clearly preceded the maximum potential difference and the potential difference continued in the evening, when the sap flow had already ceased. They consequently assumed the potential difference to be generated by ion diffusion from the xylem sap into the xylem walls establishing a concentration gradient in charge carriers. Gibert et al. ([6]) used steel electrodes in tree trunks. This type of electrodes dissolves iron ions, which become potential determining. For this reason, at least part of the potentials measured must have been caused by concentration gradients of iron. They have to be considered artificial and difficult to interpret. The same is also true for electrodes placed into the ground. They permit electron exchange with humic substances which may shift the potential in an unpredictable way. For this reason, only noble metal electrodes are expected to yield reliable information. Insertion of one of the electrodes into the ground may give information, which is difficult to interpret because of the complexity of inorganic and organic components in solutions present in the ground.

One solution could be the potential measurement of a platinum electrode in contact with xylem water of a tree against a well reproducible, easy to handle reference electrode. Such a system would be the Clark electrode with platinum as the working electrode and a AgCl electrode as the reference with an oxygen transparent membrane (Teflon, Cellophane) in between. The problem we found was that such a AgCl electrode with membrane was difficult to install in the xylem of a tree without damaging the water transporting system under negative pressure. For this reason, we decided to work with platinum electrodes at different heights at the tree trunk. The research strategy assumed here is to use platinum electrodes to facilitate a reliable interpretation of electrical potential measurements and to place both platinum electrodes within the xylem sap environment in different height of the tree stem. In this way a gradient of water potential within the tree should be accessible and measurable. The presence of oxygen in the tree tissues should definitively be measurable and it is expected to vary in dependence of the solar-powered sap transport.

Tree sap transport, indirectly powered by solar radiation, may be a relevant additional mechanism for oxygen supply. It is well known that oxygen enters the tree via the water absorbed by the roots ([5], [16]) and serves for supplying the energy infrastructure of stems. Overnight, during absence of sapflow, the oxygen present in the xylem is largely consumed by plant tissues. Such behavior was systematically studied in long term investigations of a birch (Betula pendula) tree ([1]). In addition, plants occasionally suffer from oxygen deficiency but developed strategies for oxygen transport and distribution.

Consequently, the aims of this study were: (1) to develop an experimental technique for studying the oxygen supply in trees and to correlate it with sap movements, and (2) to provide reliable electrochemical potential measurements in tree trunks using platinum electrodes. These electrodes are introduced into xylem water conduits, which are expected to contain only solutions of inert inorganic ions besides of oxygen.

Material and methods 

For the electrochemical potential measurements a healthy 12 m high small-leaved lime tree (Tilia cordata Mill.) was selected in the garden at the Lise-Meitner-campus of the Helmholtz Center Berlin for Materials and Energy. Platinum electrodes were inserted into the xylem of the tree stem, one at the basis and the second at the height of 5 m. Structure and geometry of the contacts between platinum and the tree tissues are shown in Fig. 1. In order to facilitate a favorable electrical contact with the xylem 3 cm thick vertical wedges were cut into the tree bark. After introduction and fixation of the electrodes the injury was sealed with silicone adhesive. The electrodes were connected via sealed coaxial cables with a grounded voltmeter in order to record the voltage difference. A Hoiki Voltage Logger 3635-24 (Hioki E.E. Corp., Nagano, Japan) was employed with a measure range of ± 500mV and an input impedance of 1.5 MOhm.