On the responses of Sitka Spruce and Lodgepole Pine to conditions associated with waterlogged soil
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
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
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
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
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
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.