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SYMPTOMS AS BIOINDICATORS
OF DECLINE IN EUROPEAN FORESTS
Peter Schutt
Faculty of Forestry
University of Munich
West Germany
ABSTRACT
Forest decline in Europe is looked upon by most scientists as a
complex disease of forest ecosystems caused or at least
predisposed by air pollution. Symptoms of "Waldsterben," however,
differ from those of classical air pollution with components like
SO2 or HE and also from damages by On, PAN or photochemical
smog. This may be due to effects of hundreds of additional
phytotoxic compounds in the air or to synergistic effects between
them. Pollutants induce higher susceptibility against normal
pathological events (abiotic as well as biotic) so that trees seem
to die, for instance, from bark beetle-attack, fungi or frost. The
consequences for symptomatology of forest decline are
embarrassing: the typical, authentic symptom of the syndrome is
hard to find, and possibly does not exist at all.
Examples are given, which demonstrate the difficulties in situ
to give a reliable answer whether or not a given damage belongs
to forest decline.
INTRODUCTION
The recent changes in condition of forests in central Europe are remarkable for
several reasons:
1) more than 20 species of forest trees and shrubs are affected;
2) the changes in condition of all 20 species first became conspicuous at about the
same time -- late 1970s and early 1980s;
3) some of these changes are familiar but others have rarely been seen before, or have
become evident at different times, places, and stages of forest growth and development;
4) the affected trees are found in an extremely broad range of geographical, climatic,
soil, and elevational conditions; and
5) none of the familiar biotic, edaphic, climatic, and air-pollution causes of stress in
forests (fungi, insects, poor soil conditions, drought, storms of various types, or airborne
sulfur dioxide or fluoride) appear to provide an adequate explanation for the changes
observer! in the forests (Schutt and Cowling, 1985; Schutt et al., 1986~.
In Germany, these remarkable changes in forest condition were given the collective
name "Waldsterben" (forest death, forest decline) or "neuartige Waldschaden" (new types
of forest damages). Forests have a very special place in the life of European people.
119
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For this reason, forest pathologists, tree physiologists, ecologists, soil scientists, and
many other experts in Germany and other countries of central Europe have been called
upon to explain why these changes were occurring So far, none of us have been able
to do so with certainty. The reason is that there is no single, well-defined causal
factor, or simple combination of one or two or even three causal factors, that can
explain all the changes in forest conditions, which have been observed.
A considerable body of evidence points to the probability that airborne pollutant
chemicals are involved. But this evidence is almost entirely circumstantial. Thus, most
well-informed scientists in central Europe share the view that air pollutants are involved,
but we share this view largely because no other set of causal and/or contributing factors
has so far been found to explain the diverse symptoms observed on the trees themselves.
THE IMPORTANCE OF SYMPTOMS AS BIOINDICATORS OF STRESS
In human medicine, in the veterinary sciences, and in the study of disease and
dysfunction in plants, observations of symptoms are almost invariably the first indicators
that something is wrong with the individual organism or the ecosystem in which the
individuals live. To put it another way, symptoms (changes in the physical appearance or
the physiological function of the host organism itself) are the principal foundation for
detection and diagnosis of disease and dysfunction in essentially all living organisms.
In the case of Waldsterben in central Europe, the following three types of symptoms
typify the recent changes that have taken place in many species of trees under a wide
variety of ecological conditions:
1) transparency of tree crowns (loss of crown density);
2) anomalies of tree growth; and
3) degradation of the fine feeder-root system of trees.
We will discuss the significance of each of three these typical symptoms in turn.
1. Analysis of Crown and Foliage Symptoms.
Crown transparency is a general symptom of decline in many tree species. It
occurs under a wide range of edaphic and climatic conditions. Looking closer, however,
this general symptom appears to result from three different processes:
(a) premature casting-off of foliage.--This is a common symptom of decline in many
broad-leaf trees, especially Fagus syZvatica (beech), Acer pseudoplatanus (maple), Betula
pendula (birch), and Sorbus acuparia (mountain ash). It is less common in Quercus (oak)
and Fraxinus (ash) species. Premature loss of foliage often is accompanied by
discoloration of the leaves. It may occur as early as late July or early August.
Frequently, both premature discoloration and casting-off of leaves are interpreted as an
early form of senescence. Premature casting-off of needles is also a common symptom of
decline in several conifers, including Abies alba (silver fir), Picea abies (Norway spruce),
Pinus sylvestris (Scots pine), and Larix decidua (European larch). Especially remarkable
is the active casting-off of leaves while they are still green. This occurs in both
European beech and maple as well as in Norway spruce, Scots pine, and European larch
(Schutt and Cowling, 1985~.
A special kind of needle casting is observed in Picea abies stands at elevations
higher than about 700 m above sea level. Here, a pronounced yellowing of needles is
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observed where needles are frequently exposed to cloud mist.
accompanied by magnesium deficiency.
Often this yellowing is
In both high and low elevation Picea abies and Abies alba stands, needles are often
cast-off after turning brown. This symptom usually originates with the older needles in
the inner parts of the tree crowns. Since these brown needles are sometimes inhabited
by fungal endophytes such as Lophodermium piceae or Rhizosphaera kalkhoffii, a
controversy has developed about whether these fungi have a primary or only a secondary
role in the etiology of spruce decline (Rehfuess and Rodenkirken, 1984; Schutt, 1985b).
We conclude that crown transparency is a common symptom of decline in many
species of trees in central Europe. It often results from premature casting-off of leaves,
it may result from the early induction of senescence processes, and it may be triggered
by contact with cloud water, elevated concentrations of ozone or other airborne pollutant
chemicals, or by fungi.
(b) changes in leaf size and shape.--Decreased leaf size, sometimes in combination
with a lower leaf frequency and distortion of normal leaf form, especially close to the
distal ends of leading shoots, is a common symptom of Waldsterben in Betula, Fagus, and
Carpinus as well as in Pinus. It is not observed in Quercus, Fraxinus, Abies, or Picea,
however. This is a second mechanism by which crown transparency can be increased,
even in cases in which premature casting of leaves is not common. These changes are
also anomalies of tree growth.
(c) changes in leaf position and branching habit.--Normal, healthy tree crowns are
characterized by a dense and very uniform distribution of leaves over both the periphery
and interior parts of the crown. This pattern is based on a well-balanced system of
growth of short-shoots and long-shoots. The balance in number and form of these two
types of shoots is controlled by growth hormones and usually follows the rules of apical
dominance (acrotony). The "rhythm" of development of these two general types of
shoots is what gives rise to the characteristic shape of crowns in various species of
forest trees.
A common manifestation of disturbance of these hormonal control systems is a
reduction in the number of long-shoots and a concomitant increase in the number of
short-shoots. These changes are most easily recognized in Fagus (European beech), in
which, as a consequence, the shape of the whole crown is altered. The crown is turned
into a few very long long-shoots that are covered with large numbers of very short
short-shoots. Thus, the crown becomes very open and transparent, even though the total
number of leaves in the whole crown may not be greatly changed (Roloff, 1986~.
In Quercus, a similar phenomenon is based on an increased intensity of apical
dominance--leaves are formed by only the most distal buds of much foreshortened sets
of branch internodes so that the crown consists of some portions that have a very
bushy appearance and other portions that are almost devoid of leaves.
In Abies, by contrast, almost the opposite changes in branching habit take place.
In this case a loss of apical dominance leads to a very flattened appearance of the crown
which is often referred to as a "stork's nest." This change in morphology of branching
is not uncommon in older Abies, but the "stork's nest" form of crowns is now observed
on very much younger Abies trees (Schutt and Cowling, 1985~.
A recent investigation of healthy and declining Picea abies in the Bavarian forest
also revealed irregularities in branching habit and branching intensity in this species.
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During the last 10 years, declining trees formed only 55 percent as many lateral shoots
as were formed on nearby healthy trees (Schill, 1988~.
We conclude that changes in leaf position due to irregularities in branching habit,
or decreased numbers of lateral shoots, provide both a third mechanism leading to
increased transparency of tree crowns and a conspicuous illustration of anomalies of tree
growth.
2) Anomalies of tree growth.
Anomalies of tree growth also occur under a wide range of ecological conditions.
This second general foliage symptom also appears to result from two different processes:
a.) changes in branching habit (part of process lb (above); and
b.) formation of epicormic shoots.
Formation of epicormic branches.--A common response of trees subject to many
different types of stress is the formation of epicormic branches. These abnormal shoots
are formed by stimulation of otherwise dormant lateral buds or new buds formed in the
cambium. Epicormic branches usually develop on the upper sides of first-order lateral
branches. Formation of such shoots usually occurs only during the year immediately
following the stress. In the case of Waldsterben-affected trees of Picea abies and Picea
rubens, however, epicormic branches continue to be formed year-after-year so that they
gradually replace the normal branches, which tend to lose their foliage by one or more
of the mechanisms described above. As a result, trees with many epicormic branches,
although seriously injured by loss of normal foliage, are often incorrectly diagnosed as
normal because the foliage of epicormic shoots tends to mask the loss of normal foliage.
Thus, formation of epicormic branches year-after-year is a common symptom in
many Waldsterben-affected trees. It is a very significant anomaly of growth, but, in
extreme cases, tends to mask the loss of normal foliage which leads to increased crown
transparency.
3) Analysis of Root Symptoms
Degradation of the fine feeder-root system of trees and decreased abundance of
mycorrhizae are common symptoms of decline in all tree species in central Europe. Root
systems are much more difficult to study than crown and foliage symptoms and are less
well understood as regards timing and causes. Fundamentally, the trees showing
above-ground symptoms also show a conspicuous decrease in abundance of fine feeder-
roots and mycorrhizae. But very few studies have been made to determine if these
changes in root systems precede or follow the occurrence of crown and foliage symptoms
(Schutt et al., 1986).
If degradation of feeder-roots or mycorrhizae were to occur first, it might be
inferred that failure of the root system is a primary cause of the crown and foliage
symptoms. Conversely, if crown and foliage symptoms were to occur first, it might be
inferred that failure of the foliage system is a primary cause of degradation of the root
system. If both types of symptoms occur simultaneously, or if the sequence of these two
events is highly irregular, then possible causal linkages between the two are even more
uncertain.
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DISCUSSION AND GENERAL CONCLUSIONS
The decline of forests in central Europe is a serious pathological, physiological, and
ecological problem. For this reason, forest pathologists, tree physiologists, soil
scientists, and ecologists all have a responsibility to contribute our best skills to the
challenges of detection, diagnosis, and management of the stress factors involved.
Careful study of symptoms with the perspective of discovering clues to the identification
of probable causal and contributing factors is one of the special skills of forest
pathology. In the case of forest decline in central Europe, the detection of reliable
specific symptoms is extremely complicated because of the large number of tree species
involved and the very wide range of climatic, edaphic, ecological, and elevational
conditions under which the symptoms have been observed.
In practice, crown transparency is used as the most general indicator of forest
decline in central Europe. As discussed earlier, however, this general symptom has many
different morphological manifestations, which vary from species to species, and involve
different physiological mechanisms. Thus, with our present meager state of knowledge,
we do not have adequate tools for reliable symptom surveys. For many of the 20 tree
and affected shrub species and several different symptom characters (such as branching
intensity and bud frequency), we do not even know the border between the appearance
of healthy and declining trees. Also, our knowledge about the relationships among the
morphology, anatomy, and the physiology of the trees is very sparse (Schutt, 1985a,b).
These gaps in knowledge will not be closed in the immediate future. For this
reason, significant efforts should be made to develop herbaceous plants and forest tree
seedlings genetically selected for susceptibility to specific airborne chemical and other
stress factors. Such plants would be easily movable and manipulatable and thus valuable
as a aid to diagnosis.
In addition, the general field of comparative symptomatology of forest trees, which
has been severely neglected in recent years, should be reactivated and coordinated on an
international level. If this is not done, we will continue to have problems determining
whether different groups of scientists in different locations are indeed working on the
same or different forest decline problems.
REFERENCES
Rehfuess, K. E., and H. Rodenkirchen. 1984. Uber die nadelrote -- Erkrankung der
Fichte (Picea abies Karst.) in Suddeutschland. Forstw. Cbl. 103:248-262.
Roloff, A. 1986. Morphologische Untersuchungen zum Wachstum und zum
Verzweigungssystem der Rotbuche (Fagus sylvatica Lo. Mitt. d. Dt. Dendr. Ges.
76:5-49.
Schill, H. 1988. Morphologische und okologische Untersuchungen zum Austriebsund und
Verzweigungsverhalten junger Fihten (Picea abies (L.) Karst und Larchen (Larix
decidua (Mill.~. Diss. Forstwiss., Universita Munchen, Munchen, West Germany.
Schutt, P. 1 985a. Vernetzte Problemstellung -- Vernetzte Forschung -- Betrachtungen
zur Waldsterbenforschung. Forstarchiv 56:179-181.
Schutt, P. l985b. Das Waldsterben -- eine Pilzkrankheit. Forstw. Cbl. 105:169-177.
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Schutt, P., and E. B. Cowling. 1985. Waldsterben, a general decline of forests in central
Europe: symptoms, development, and possible causes. Plant Disease 69:548-558.
Schutt, P., W. Koch, H. Blasche, K. J. Lang, H. J. Schunk, and H. Summerer. 1986. So
stirbt der Wald. BLV-Verlag, Mnchen. 127 pp.
Representative terms from entire chapter:
picea abies
