On the responses of Sitka Spruce and Lodgepole Pine to conditions associated with waterlogged soil
Sanderson, P. L.
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The roots of Sitka Spruce growing in peaty soils subject to waterlogging suffer seasonal die-back which leads to the development of a surface-rooting habit, which in turn renders the trees unstable and susceptible to windthrow. This study was primarily initiated to elucidate which soil factors are instrumental in causing die-back. Possible phytotoxins viz: iron(II), ethylene and hydrogen sulphide were monitored for one year in peaty gley and deep peat soils under a Sitka Spruce stand. 1,4aximum levels were recorded in the period January - March. Unfortunately, 'volatile organic acids (VOA's) which are also potential phytotoxins of waterlogged soil could not be monitored successfully because the problems of separation and analysis, recognized by many workers, were not overcome. One estimate was made of their total concentration, using a titration method. The concentration of manganese(II) was determined on one occasion in the first winter but it proved to be well below the level previously recognized as phytotoxic. The oxygen status of the site hed been assessed in an earlier study but redox potentials were measured during the second winter and proved to be highly correlated with the plant-available iron level. The effects of iron(II), ethylene and acetic and butyric acids on the roots of Lodgepole Pine (Pinus contorta Dougl. ) and Sitka Spruce (Picea sitchensis (Bong. ) Carr. ) growing in regularly aerated nutrient solution at 13°C were examined using the "aeration tank" system devised for this investigation. Plants were allowed to acclimatise for (xiii) one week before root growth was monitored. Toxin treatment began after a further 3 weeks, lasted for 3 weeks and was followd by a3 week recovery period. Measurements of root growth were converted to Relative Growth Rate so that growth during toxin and recovery periods could be easily compared with the original growth rate. The effects of anoxic conditions (once with the addition of sulphide) were studied using the same method. Iron and the organic acids all caused some death of root tips at concentrations of < 100 ppm and severe reduction in root growth at 20 ppm; butyric acid was the most effective toxin. Ethylene (5 - 20 ng/ml) reduced and sometimes halted root growth but did not cause root death. Pronounced swelling of the root, especially in the case of Lodgepole Pine treated with 20 ng/ml ethylene suggested that an induction of root dormancy might be taking place. Sitka Spruce was the more severely affected by anoxic conditions and experiments confirmed that it was the more sensitive to waterlogging. It was also the most sensitive to the volatile organic acids and in some ways, to iron. The modes of recovery of Lodgepole Pine and Sitka Spruce from treatment with zero oxygen or toxins were consistently different. Sitka Spruce tended to produce new roots at the base of the stem or new lateral roots in the upper part of the root system, whereas Lodgepole Pine often recommenced growth at the original root tips or, if these had been damaged by the treatment, laterals in the lower part of the root system. This suggested that Lodgepole Pine root tips were (xiv) receiving a superior supply of oxygen. Using a polarographic technique, the sub-apical oxygen flux from Lodgepole Pine roots was found to be greater than that from Sitka Spruce roots of the same length. A rigorous treatment of the data confirmed that Lodgepole Pine was the better ventilated species. Where roots survive in anaerobic conditions it is thought that the superior internal aeration system of Lodgepole Pine supplies the oxygen necessary to (i) detoxify VOA's (ii) oxidise and therefore immobilize iron(II) ions and (iii) maintain the viability of those distant parts of the roots experiencing an anoxic environment. Thus it is this property which seems to render Lodgepole Pine more able than Sitka Spruce to produce a normal root system in waterlogged soil. The link between internal aeration and tolerance to waterlogging, anoxia and VOA's was extended to embrace wetland and nonwetland species in general. An experiment where 100 ppm acetic acid was applied to the roots of two species known to be capable of extensive aerenchyma formation and to pea, a mesophytic species, provided sound evidence for this hypothesis. The investigation was successful in elucidating the factors causing the observed winter die-back of Sitka Spruce roots. Ethylene, hydrogen sulphide and manganese(II) ions were eliminated as causative agents. Plant-available iron levels in the deep peat trough were high enough to inhibit root growth throughout most of the year and would cause death in winter. Thus, they may account for the stunted tree growth observed in that area but the levels in the peaty gley were insufficient (xv) to account for the root die-back which occurs there. Although low levels of VOA's could play a major role, until extensive field measurements can be made, it must be assumed that anoxia, because its development is likely to precede VOA accumulation, will be the primary factor causing root die-back. The solution to the problem of die-back and subsequent windthrow in Sitka Spruce would seem to lie either in breeding for improved root ventilation or employing soil cultivation techniques which will improve soil aeration.