Chapter 19
Long-Term Fates of Declining Forests Richard M. Klein and Timothy D. Perkins
The Science of Global Change Downloaded from pubs.acs.org by YORK UNIV on 12/03/18. For personal use only.
Forest Decline Program, Botany Department, University of Vermont, Burlington, VT 05405
Contemporary forest declines were initiated about 1950-1960, virtually simultaneously throughout the industrial world at the same time as damage to aquatic systems and structures became apparent. A broad array of natural and anthropogenic stresses have been identified as components of a complex web of primary causal factors that vary in time and space, interact among each other, affect various plant growth and development systems and may result in the death of trees in mountainous ecosystems. As these ecosystems decline, the alterations in forest ecology, independent of the initial causal complex, become themselves additional stress factor complexes leading to further alterations. In 1968, reports from Sweden, subsequently confirmed in other industrial countries, noted that shallow lakes with low concentrations of divalent cations were becoming more acidic with consequent decreases in aquatic plants and animals. In severely affected lakes and ponds, only acidophilic algae survived. Increased acidity and the runoff of solubilized aluminum and other metal ions from surrounding watersheds are now known to be primarily responsible for formation of these almost sterile bodies of water. Shortly thereafter, reports appeared in European forestry journals that plantation-grown silver fir and Norway spruce were exhibiting a decline syndrome characterized by loss of foliage, reduced growth and susceptibility to other stresses (2). Termed Waldsterben in Germany, by the late 1970's the phenomenon was evident in montane coniferous forests of upper New England, New York and adjacent Canadian Provinces. The most severely affected tree species was, and still is, the red spruce, a previously dominant tree in upper elevation forests of affected areas of New England and down the Appalachian mountain chain to North Carolina. The other dominant conifer, balsamfir,was not affected, but mature heart-leaved white birch showed decline symptoms and, in hardwood areas of lower mountain slopes, maple species, American beech and other trees seemed to be declining.
0097-6156/92/0483-0360$06.00/0 © 1992 American Chemical Society
19.
KLEIN & PERKINS
Long-Term Fates of Declining Forests
361
Research on Camels Hump In 1979, the Forest Decline Laboratory of the Botany Department of the University of Vermont conducted a quantitative evaluation of forest health of ecosystems on Camels Hump mountain in Vermont's Green Mountains. Camels Hump is a major summit in the Green Mountains, peaking at a bit over 1200 meters. The lower slopes from 550 to 730 meters are a typical New England sugar maple-beech-yellow birch association. Above a narrow transition zone, the upper elevations from 850 to about 1100 meters are forested with red spruce-balsam fir-white birch above which the forest is almost entirely balsam fir. A small arctic alpine tundra area above tree line is a relict of the last Ice Age, containing plants now found in upper Canada. Cutting and fire damage occurred up to 1950 in the hardwood zone, but the conifer areas have been relatively undisturbed by human activity. Our study area is on the west-facing slope of the mountain along the three-mile long Burrows Trail (Figure 1). Rectangles represent permanently marked survey plots along each of the 60 meter elevational transects. We have studied Camels Hump since 1963 when a doctoral candidate conducted a comprehensive ecological study including quantitative and qualitative identification of all plant species, their location, density and growth plus data on weather patterns, soils and geology. The 1979 survey results confirmed our suspicions; severe declines in numbers and vigor of deciduous and coniferous tree species in both hardwood and conifer areas had occurred since 1965. We resurveyed in 1983, 1986 and 1990 to complete a full 25 year record of forest decline, now one of the most comprehensive vegetation data bases in the United States (Figures 2 and 3). Although balsam fir was not affected up to 1983, it had begun to decline in 1986. Red spruce has continued to be the most seriously affected tree. Several species of maple are declining as is beech and mountain ash. Declines are not the result of the natural death of older trees; all age classes are affected. Forest decline is real and devastating. In most particulars, the symptomology of red spruce decline on Camels Hump corresponds to the pattern seen in eastern Canada, in New England and New York and down the Appalachian Mountain chain to the Carolinas. It also matches the pattern of decline of Norway spruce in Europe (2). Whether the pattern exists in our western mountains is still controversial. Tree ring analyses and other studies suggest that initiation of forest decline in the northeastern United States, adjacent Canada, and central Europe all date to about 1950 1960. Examination of lake sediments also gives the same date range for the damage to both European and North American fresh waters. While making correlations is a dangerous game, this was the same period when alkaline fly ash was eliminated, when construction of tall stacks on power plants allowed pollutions to be widely dispersed, when the high-compression automobile engine resulted in increases in nitrogen oxide emissions, when leaded gasolene became common, and when smelting and refining operations were greatly expanded (3,
362
THE SCIENCE OF GLOBAL CHANGE
e
VV /v\\(//////"7? Ν.
λ
%
—
ϊ !ο
LEGEND - « » - Elevation Major brook Intermittent brook Town road Hiking trail Building —
ο
Vegetation sampling plot (Whitney) Lysimeter station Fog collector Precipitation gauge/collector Throughfall collector Stand identification number
Vegetation sampling plot (Siccama) SCALE
CONTOUR INTERVAL 100 FEET O N E A C R E
^
O N E
METERS
H E C T A R E
FEET
Figure 1. Map of the Burrows Trail on the southwest-facing slope of Camels Hump mountain showing location of permanently-marked study plots.
19.
KLEIN & PERKINS
Long-Term Fates of Declining Forests
1979
1965
1983
363
1986
20 19
_ Sugar ΜαρΙ·
18 17 16 15 14
^
.
+
13 12 11 10 9
American Beech
8 7 6
Yellow Birch
.—
—^
7
1
u
-—C
5 4 3 2 1 0
Mountain Maple 1
1
1965
1_
L
I
1
1
1
1979
1983
1986
1990
Figure 2. Changes in dominant species of trees in the lower elevation, hardwood ecosystem of Camels Hump based on density (upper graph) and on basal area (lower graph).
364
THE SCIENCE OF GLOBAL CHANGE 2.2 Balsam Fir
2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6
Red Spruce
0.4
Paper Birch
0.2 0.0
1
_J
I
L_
1965
1979
1983
1986
1990
1979
1983
1986
1990
Year
17 16
Balsam Fir
15 14 13 12 11 10 9
Red Spruce
8 7 6 5
Paper Birch
4 3 2 1 0 1965 Year
Figure 3. Changes in dominant species of trees in the upper elevation, coniferous ecosystem of Camels Hump based on density (upper graph) and on basal area (lower graph).
19.
KLEIN & PERKINS
Long-Term Fates of Declining Forests
365
The facts that contemporary forest declines a) were initiated simultaneously throughout the industrial world; b) that they simultaneously affect a number of tree species rather than a single species; c) that forested lands in North America, Europe and Asia are declining; d) that the declines of forests parallel in time and space to the well-documented damage to aquatic ecosystems and to structures; e) that they are found in association with a large number of other damaging alterations in many components of affected ecosystems; and f) that they can be correlated in time and space with changes in human life styles and industrial activities are each cogent arguments that anthropogenic causal factor complexes must be involved in forest decline (4). Primary Causes of Forest Decline Obviously, one looks for causes. That declines in one or another species have natural factor etiologies is unequivocal. The demise of American elms and of the chestnut were due to natural factors. Insect infestations, bacterial and fungal diseases, hurricanes, floods, freezes, droughts and many other stresses can cause extensive tree death (5). But in such declines typically only a single species is affected or climatic events caused decline in a delimited area. In almost all declines caused by natural events, the causal factors can be identified; we know their precise etiologies. Natural events are always part of the natural environment and must be factored in when evaluating forest declines (Table I). Table I. Natural events and factors that stress forest trees (cf. 4, 6, 7). CLIMATIC EVENTS Drought Flooding Soil erosion High temperature stress Low temperature stress High winds Global warming Increased UV-B radiation
ROLE IN FOREST DECLINE Low in mountains Low in mountains Low to moderate Low in mountains High for red spruce Moderate to high Unknown Unknown
BIOTIC FACTORS Viruses/bacteria Fungal disease Insect infestation Mineral recycling Indigenous metals
Unknown to low Usually moderate Moderate as co-stresses Moderate as co-stresses Low without acid input
366
THE SCIENCE OF GLOBAL CHANGE
Some of the chemical and biological anthropogenic causal factors that have, individually and in concert, been demonstrated in both field and labora tory to damage tree growth, development and survival are given in Table Π. They may interact synergistically or additively, interface with natural causal factor complexes, are strongly affected by site characteristics, weather conditions. They, and the natural factors, vary in time and space with one factor predomin ating today and, with a change in the weather or shifts in the wind direction, another factor being dominant tomorrow. There is no possibility that we will ever be able to pin down THE or even A cause of forest decline (4, 8). Using what little epidemological evidence is now available on declines, the probability that anthropogenic factors are deeply involved is very high (12). The consensus is that a large share of the factors that cause forest decline are anthropogenic is very high (4, 7). Contemporary forest declines are getting worse and have a bewildering and complex of natural and anthropogenic causal factors (3, 4,12, 13, 14). Upper elevation coniferous forest trees live on the edge of disaster under continuous natural biotic and abiotic stresses. Soils are shallow to bedrock, nutritionally poor and wet, wind speed has been measured at close to 100 miles per hour, and winters are usually severe. These forests are immersed in clouds for many days each year. Droplets of this cloud water condense on foliage and drop to the ground. Indeed, this cloud water or fog accounts for up to 70% of the water received by montane coniferous ecosystems. It is many times more acidic than rain or snow with average annual weighted pH values of 3.6 -3.7 compared to the 4.1 - 4.2 of precipitation (75). These acidity levels are sufficiently great to cause direct damage to foliage, cause leaching of cations, sugars, amino acids and proteins from foliage (16) that affect photosynthesis. Cloud water also contains higher concentrations of metal ions than precipitation. Consequences of Primary Forest Decline Using a physiological-ecological perspective, we analysed alterations in forest communities occurring as a result of forest decline. Since there are neither roads nor electric power lines on Camels Hump, all monitoring equipment runs on DC batteries recharged by solar panels (Figure 4). Real time data are col lected on light flux, soil and air temperatures, wind speed and direction, soil moisture, relative humidity, precipitation, cloud water and ozone from both gap and canopied sites for computer analyses (16). We have found that gap forma tion has resulted in increased insolation with attendent increases in air and soil temperature, as well as changes in relative humidity. Wind patterns are shifting, there are alterations in competition among plant species, and there are many other modifications of the initial micro and macro environment. These en vironmental parameters are compared and correlated with quantitative measure ments of changes in the vegetation. The initial causes of forest decline - natural and anthropogenic - have resulted in a forest decline syndrome that is, per se, a new series of causal factors whose consequences are themselves new causes for ecosystem alteration.
19.
KLEIN & PERKINS
Long-Term Fates of Declining Forests
367
Table II. Anthropogenic factors known from laboratory andfieldstudies to cause stresses in forest trees (cf. 3, 4, 9,10,11,12,13,14). STRESS AND CONSEQUENCES
FACTOR ACIDITY (H S04/HN0 ) Foliage 2
3
Root systems
Soils
METAL IONS Soils Roots
GASEOUS POLLUTANTS O,
S 0 and N O 2
x
Organics (PAN, etc.)
Damage to epicuticular waxes Altered photosynthesis Increased water loss Accumulation of acidic anions Leaching of ions, sugars, etc. Mineral imbalances Altered metabolism Increased susceptibility to winter freezing injury Death of fine roots Destabilization of trees Reduced water/mineral uptake Reduced water uptake Cations leached below roots Accumulation of acidic anions Altered structure/texture Altered microflora Reduced litter decomposition Altered Ν transformations Solubilization of metal ions Accumulation in soil solutions Altered microflora Reduced litter decomposion Metal ions accumulate Reduced water/mineral uptake Reduction in mycorrhizae Reduced vigor/resistance Altered photosynthesis Altered cell metabolism Damage to foliage cuticles Damage to epicuticular waxes Many foliage effects Altered reproductive patterns Poorly investigated
ASPECT:
SLOPE :
ELEVATION:
190°
900m
TOWER
1 4m
INTENSIVE RESEARCH SITE
1
GAP TOWER
Figure 4. Diagramatic representation of the environmental monitoring units installed in gaps and adjacent canopied sites of the montane coniferous ecosystem of Camels Hump.
CANOPY
19.
KLEIN & PERKINS
Long-Term Fates of Declining Forests
369
There is, then, a complex cascade of causes and consequences whose end is not yet in sight. Let me illustrate this with examples from our researC.H. Field surveys showed that very few red spruce seedlings are present in the declining ecosystems (16). One factor is, of course, that there are very few remaining mature red spruce trees to provide seed. Additionally, germination and seedling development of red spruce seed from Camels Hump are low (17). These limitations on natural regeneration are, of course, primary effects of the conditions that result in forest decline. But there are other considerations. Litter on the forest floor is not being degraded because of precipitation acidity, metal toxicity and death of those insects that normally chew litter into small fragments - also primary consequences of forest degradation. The increased depth of litter and forest duff suppresses red spruce seedling root growth down to mineral soil, a secondary effect. Litter and forest duff contain allelopathic substances extractable with synthetic precipitation that inhibit red spruce seed germination and repress red spruce seedling growth (19). The gaps formed by tree death become ideal habitats for ferns whose fronds contain allelopathic substances that interfere with seed germination and seedling growth of red spruce, but not balsam fir; this, too, is a secondary effect that will have tertiary effects and consequences. Based on all these findings, one can speculate that even were all pollution taps turned off today, the chances of red spruce again becoming a dominant member of the coniferous forest is minimal, at least for the forseeable future. Although seen only occasional during the first half of this century, winter injury of first-year red spruce needles has become an annual event in the coniferous montane forest area, resulting in the formation of red-brown first year needles that subsequently desiccate and are shed (20, 21). The loss of foliage reduces photosynthesis and the obligatory accumulation of carbohydrate in the twigs and root systems. There is some evidence that this phenomenon involves both natural and anthropogenic causal factors. Another gap invader is seedling birch (22). We are using these thickets to estimate how rapidly gaps are increasing in area as trees on gap edges die. The gaps are, indeed, increasing in size at rates averaging over one m per year. Since the birch in these gaps is a short-lived species, their more permanent replacement is still in doubt, but the birch invasion will undoubtedly set into motion another whole series of causes and consequences. The changed wind patterns in gaps is causing flagging of the tree tops of surrounding balsam firs. Flagging, easily recognized as an assymetiy of the crown due to branch breakage and loss of foliage on the upwind side of the tree, is the one of the first signs of fir decline and has become increasingly evident in the past five years (23). Clearly, fir decline is not a primary response to the natural and anthropogenic factors that caused initial forest decline. It is a secondary consequence that will initiate another cascade. We must also consider the consequences of forty years of ecosystem pollution loading. Compared with soil analyses of heavy metal concentrations made in 1965, cadmium, copper, lead and zinc levels are now elevated (24) to the point where laboratory studies have shown that red spruce root and shoot growth is reduced, growth of obligatory mycorrhizal fungi is repressed and
THE SCIENCE OF GLOBAL CHANGE
KLEIN & PERKINS
371
Long-Term Fates of Declining Forests
e
1 CO
i
