Pinus Uncinata Krummholz

F.-K. Holtmeier, Mountain Timberlines: Ecology, Patchiness, and Dynamics, Advances in Global Change Research 36, 11-28. © Springer Science + Business Media B.V. 2009

critical height of a pine (Pinus sylvestris) should be at least 5 m. Kullman (1987), on the other hand, included birch and spruce higher than 2 m when monitoring tree line in the southern Swedish Scandes, while the corresponding stem height of pine was only 1 m. In the temperate mountains a minimum height of 2 m appears to be adequate to the particular climatic and ecological situation, as a birch or any other tree species growing taller will be exposed to the harsh climatic influence above the winter snow cover, whereas smaller individuals are fairly well protected. However, particularly for this reason and with respect to the varying thickness of the snow cover in the forest-alpine tundra ecotone, the present author (Holtmeier, 1965, 1974) has objected to an absolute minimum height as criterion. Thus, a pine or birch should be considered a tree as soon as it projects beyond the average snow cover typical of the specific site (Table 1; see also Daniker, 1923). At tropical mountain timberline, however, this does not work, because a long-lasting seasonal snow cover is lacking. Yet, also a certain minimum tree height as criterion for defining tree line might make sense in so far as, for example, a 2 m high 'tree' would be more decoupled from the climate near the soil surface and thus would be growing in a different environment than the lower vegetation. Kessler (1995), on the other hand, holds the view that a tree may be smaller than 2 m provided that it shows typical tree habitus (one or several stems and a crown). However, where tall growing forests, such as 9 m high Erica stands on Moun. Kenya, grade into 'low' and 'dwarf forest' and finally into shrub, defining forest limit and tree line turns out to be rather difficult and remains arbitrary (Coe, 1967; Miehe and Miehe, 1994). For further differentiation of the upper forest belt Miehe and Miehe (1994, 1996), for example, make a difference between 'low forests' (>5 to 10 m) and 'dwarf forests' (<5 m). Salomons (1986), on the other hand, calls the 3 to 8 m high stands of Gynoxis, Hesperomeles and some attributed species occurring above the closed forest belt in the Páramo in central Columbia 'Andean dwarf forest'.

To be sure, defining the upper limit of mountain forests by a minimum tree height will result in a 'line' that can easily be correlated to any average temperature or temperature sums, growing degree days, etc. supposed to be controlling factors. However, studying the response of this 'line' will not provide any deeper insight into the ecological situation and spatial and temporal structures (Holtmeier, 1965, 1974; Stugren and Popovici, 1991).

This becomes particularly clear in view of the dynamics in the forest-alpine tundra ecotone. There has been a long discussion as to whether a natural climatic forest limit would be sharper than a transition zone (ecotone) at all. The existence of a transition zone is explained by unfavourable edaphic conditions and/or human impact (Scharfetter, 1938; Ellenberg, 1963, 1966, 1978; Schiechtl, 1967; Nageli, 1969; Mayer, 1970; Kostler and Mayer, 1970; Kral, 1971). This concept has been taken over in textbooks on plant geography (Ellenberg, 1978; Klink and Mayer, 1983). From his own experience in mountain areas in and out of Europe and in the Subarctic the present author does not agree with this opinion, since the mountain and polar timber-line are so heterogeneous that they should not be generalised to such an extent (Holtmeier, 1985b). In fact, in various high mountains that have not, or only randomly, been influenced by humans, the closed forest ends abruptly at its upper climatic limit. However, in many other high-mountain areas as well as in the Subarctic, the climatic timberline forms a more or less wide ecotone, extending from the closed forest to isolated stunted trees within the lower alpine belt or tundra. On gently sloping fjelds in central and northern Finnish Lapland, for example, broad Savannah-like mountain birch forests (Betula tortuosa) with sporadic several metres high trees (Photo 16) form the timberline ecotone. Thus, the position of the tree limit depends on how a tree is defined. There is much evidence that also the original upper birch forests were once open woodland (e.g., Oksanen et al., 1995; Kankaanpaa, 1999; Holtmeier and Broll, 2006). Dead wood, for example, partly found in densely spaced peat hummocks (10-30 cm high) above the present timberline support this hypothesis (Holtmeier and Broll, 2006). The hummocks developed from organic matter accumulated around the stem base of the birches as can be observed below the present timberline.

The existence of these ecotones cannot be primarily and everywhere attributed to human interference and/or unfavourable pedological conditions, but must be explained as the result of the complex influences of the actual and previous climate, fire, biotic factors and site history on tree growth and ecological conditions. In many cases the existence of a timberline ecotone is the result of oscillations of the climate, persistence of tall (mature) trees and regeneration under changing conditions. A general warming may be followed by advance of the forest to greater altitude and northern latitude (Chapter 5), while cooling will cause decay and retreat of the forest in the long-term, followed by change of the site conditions in the former forested area.

Moreover, mountain timberlines are formed by tree species with different ecological properties and requirements. Spruce, for example, is more shade-tolerant than pine or larch and thus forms comparatively dense forests while light-demanding pine or larch forests are usually more open. Some species, such as fir and spruce, are able to reproduce and propagate by layering (formation of adventitious roots) under conditions that would prevent sexual regeneration completely. In this respect most timberline forming pines, for example, are at a clear disadvantage compared to spruce or fir (Holtmeier, 1985b, 1986a, 1993a, Section 4.3.10). Moreover, due to the local and regional history of climate, vegetation and, not least, human impact, tree stands at timberline may be different as to successional stage, age classes, composition and ecological dynamics.

In view of the great physiognomic variety and heterogeneity of mountain timberlines and with respect to the many possible scientific approaches to timberline, it is not surprising that a general precise and practicable definition, meeting all aspects, is hardly possible. Nevertheless, from a global view at least four different types of timberlines can be distinguished (Figure 3).

Definition Scrubline

Figure 3. Main types of timberline. a - abrupt forest limit bordering alpine vegetation, b - transition zone (ecotone), c - true krummholz belt (e.g., Pinus mugo, Pinuspumila) above the upright growing forest, d - gradual transition from high-stemmed forest to crippled trees of the same species bordering alpine vegetation (e.g., Nothofagus solandri var. cliffortioides). Modified from Norton and Schonenberger (1984).

Figure 3. Main types of timberline. a - abrupt forest limit bordering alpine vegetation, b - transition zone (ecotone), c - true krummholz belt (e.g., Pinus mugo, Pinuspumila) above the upright growing forest, d - gradual transition from high-stemmed forest to crippled trees of the same species bordering alpine vegetation (e.g., Nothofagus solandri var. cliffortioides). Modified from Norton and Schonenberger (1984).

Timberline may occur as a line (e.g., Photos 1, 3, 4, 55, 111), or as a more or less wide ecotone (e.g., Photos 2, 32, 38, 51-53, 61, 67, 76, 88, 94, 95, 122; see Table 4 for synonyms). Different types may occur in close proximity to each other, even on a single mountainside (Photos 1 and 2). In other areas the forest gradually merges into alpine scrub formed by wooden species other than the tree species in the forest. Occasionally, high-stemmed forest stands border the alpine vegetation (Figure 3a, Photo 3) while in some areas tree height decreases approaching the upper limit of tree growth, and the most advanced individuals are more or less stunted (Figure 3a; Photo 4).

These climatically shaped individuals (e.g., Photos 24, 29, 38, 58, 59, 64, 66-77, 84, 87, 90, 91, 104) of the normally upright-growing tree species are usually called 'krummholz' in English. This popular term has been introduced from the German language. Originally it meant contorted, gnarled, twisted

Treeline Vista Snow

Photo 1. Abrupt forest limit (Picea engelmannii, Abies lasiocarpa) on the WNW-exposed slope of Goliath Mountain (Mt. Evans area, Colorado) at about 3.500 m (view SW). F.-K. Holtmeier, 19 July 1994.

Photo 2. The same slope as above (view N). In this section, which provides higher soil moisture (being reflected in the distribution of willow shrubs), the timberline occurs as an ecotone located at about 3.500 to 3.540 m. F.-K. Holtmeier, 17 July 1997.

Photo 3. Abrupt limit of Nothofagus solandri var. cliffortioides-forest (evergreen) in the Craigieburn Range (New Zealand, South Island) at about 1.350 m. F.-K. Holtmeier, 24 November 1979.

Pinus Paramo

Photo 4. Abrupt upper limit of Nothofagus pumilio-forest (deciduous) on an east-facing slope (550 to 580 m) on Isla Navarina (Tierra del Fuego). The uppermost trees exhibit dwarfed growth forms. A. Vogel, 3 March 1990.

and prostrate growing species such as Pinus mugo, Pinus pumila, Alnus viridis, Alnus sinuata, and Alnus maximowiczii (Figure 3c, Photos 5, 6; see also Photos 12, 18, 114, 115) the growth form of which is genetically predetermined. Thus, it should not be confused with climatically stunted 'krummholz'. Although krummholz in the proper sense (Holtmeier, 1973, 1981a) does not display tree habitus, Masuzawa (1985, see also Saiko and Masuzawa, 1987) calls the Alnus maximoviczii-belt above the larch forest on Mt. Fuji 'dwarf forest'.

Pinus Mugo Jacobsen
Photo 5. Prostrate mountain pine belt (Pinus mugo) above Swiss stone pine forest (Pinus cembra) in the High Tatra near Strbske Pleso (Slovakia). F.-K. Holtmeier, August 1970.

In the following (see also Holtmeier, 1965, 1974), timberline is understood to be the transition zone between closed forest, the density of which depends on tree species represented and site conditions, and the most advanced individuals of the forest-forming tree species (see also Däniker, 1923; Pfister et al., 1977; Slatyer and Noble, 1992; Heikkinen et al., 1995; Smith et al., 2003). Ecotonal timberlines are characterized by a mosaic of tree clumps, scattered groves, isolated, more or less deformed tree individuals and treeless patches covered by low shrubs, herbs, and grasses. Here, it should be stressed again that, at close sight, abrupt timberlines usually reveal themselves as narrow ecotones. In the temperate and northern mountains, these outliers of tree growth are usually deformed, only a few decametres high and mostly but not everywhere protected by the snow cover from adverse climatic influences in the winter ('scrub line' in the sense of Arno, 1984, dwarf tree or cripple limit in the sense of Schröter, 1926).

Photo 6. Dwarf Siberian stone pine (Pinus pumila) overtopped by several Abies nephrolepis on Lisaja Shg (Sikote Aline, Russia) at about 1.200 m. H. Mattes, 25 August 1997.

Though these climatically stunted spruces, firs and pines will usually not meet the minimum height of a 'tree' the genetic disposition for becoming a tree is inherent, as is evidenced by prostrate-growing individuals having assumed or re-assumed tree-like features (single- or multi-stemmed, crown) or in the event of improved climatic conditions (Photos 72-75, Figure 58). This makes them different from shrubs, which thrive from the base (basitony) and show a sympodial ramification (Strasburger et al., 1991). This is one reason to distinguish genetically predetermined from climatically shaped 'krummholz' (Table 2; Holtmeier, 1973, 1974, 1981a). When the latter is considered in the following chapters it is put in quotation marks. Variation of growth form of tree individuals caused by the effects of oscillating climate is typical of the ecotone (Scott et al., 1997). Consequently, upright stems thriving from a mat-like growing 'tree' above 2 m height should not be lumped together with advance of timberline as it has been usual for several authors (e.g., Kullman, 1986a, 2000b, 2002; Lavoie and Payette, 1992; Lescop-Sinclair and Payette, 1995; see also Section 5.1).

As to the ecological situation, timberline ecotones are completely different from the closed forest and from the treeless alpine belt. While in the treeless alpine belt the microclimatic pattern is a function of the influence of microtopographical structure (convex, concave) on solar radiation and windflow near the soil surface, in the ecotone these climatic elements are influenced also by the scattered stands of trees, an aspect that has hardly

Table 2. Krummholz-terminology and its practical use

True krummholz (genotype)

Environmentally induced krummholz (phenotype)

Growth form

Shrub,scrub

Environmentally shaped growth forms of the forest tree species

Example

Pinus mugo Pinus pumila Alnus viridis Alnus sitchensis Podocarpus nivalis

Flagged trees

Table and flagged table trees, Mat-like growth etc., Identical or similar growth in different tree species at same external influences

Distribution

Usually more or less wide altitudinal belt above the forest, which is formed by other species also common in avalanche chutes

Controlled by the locally varying site conditions (e.g. wind-exposure, snow depth etc.) in the ecotone

English terms

Krummholz1 Scrub2

Subalpine scrub3 Elfin wood4 Dwarf forest5

Krummholz6 Dwarfed krummholz7 Wind-timber8 Dwarf forest9 Dwarf/matted trees10 Crippled trees11 Stunted trees12 Brushwood13

Knieholz15

Latschenbuschwald16

Grünerlenbuschwald17

Legföhrengebüsch18

Alpenerlengebüsch19

Krummholzwald20

Krüppelholz21

Baumkrüppel22 Krüppelbäume23

Krummholz24

References: 'Klikoff (1965); 2Wardle (1973, 1977); 3Wardle (1973, 1977); 4Hansen-Bristow

(1986); 5Masuzawa (1985), Saiko and Masuzawa (1987); 6Wardle (1968, 1973, 1974, 1993),

Lamarche and Mooney (1972), Ives (1973b), Troll (1973b), Pfister et al. (1977), Komarkova

(1976, 1979), Peet (1981), MacMahon and Andersen (1982), Arno (1984), Arno and Hoff

(1989), Crawford (1989); 7Habeck (1969), MacMahon and Andersen (1982); 8Löve (1970);

9Coe (1967), Salomons (1986), Miehe and Miehe (1994), Miehe and Miehe (1996); 10Griggs

(1938), Bliss (1963); "Arno (1984); 12Arno (1984); 13Cuevas (2002); 14Schröter (1926),

Hegi (1958), Braun-Blanquet (1964), Schmidt (1969), Ellenberg (1978), Franz (1979), Klink and Mayer (1983), Strasburger et al. (1991); 15Hueck (1962), Klink (1973), Kuoch and

Schweingruber (1975), Ellenberg (1978); 16Ozenda (1988); 17Ozenda (1988); 18Rübel

(1912); 19Rübel (1912); 20Ellenberg (1978); 21Braun-Blanquet (1964); 22Ellenberg (1978);

23Geiger (1901), Ozenda (1988); 24Marek (1910), Piussi and Schneider (1985).

been considered so far in timberline literature (Section 4.3.12). In the extremely wind-exposed forest-alpine tundra ecotone of the Rocky Mountains, for example, even the prostrate crippled trees cause by their effects on the windmediated relocation of snow a locally varying mosaic of snow-covered and snow-free patches which in turn influence site conditions (cf. Photo 87). The less broken the terrain, the more the wind, snow cover, radiation exchange and thus site conditions are influenced by the distribution pattern of stands of trees and openings. The influence of the mosaic of tree clumps and open areas on snow accumulation, for example, may result in a longer duration of snow cover in the ecotone (cf. Photo 32) compared to the forest (high interception) and the treeless alpine zone (deflation). The finely differentiated local site pattern that appears is partly cause and partly result of the way in which the tree stands are distributed. These ecological conditions are peculiar to the ecotone (Figure 4). In the closed forest, however, the influence of micro-topography on solar radiation, wind velocity and direction is by far less important. Altogether, these effects of microtopography and trees on the patchi-ness of site conditions are by far more important for tree growth, regeneration and survival than altitudinal gradients of air temperature.

Consequently, a better understanding of the ecological situation and spatial and temporal dynamics in the ecotone requires extensive local and regional studies on microsite conditions and microsite patterns specific to the ecotone and cannot be achieved only by investigating physiological responses of mature tree growth to thermal conditions (Holtmeier, 1994b, 1999a). This also holds true for the tropical mountain timberlines. These appear to be as diverse and heterogeneous as the timberlines outside the tropics and obviously more different from each other than can be assumed in view of the tropical type of timberline, which Troll (1959, 1973) compared in a schematic sketch to the 'general' type of timberline in the temperate zones. This generalisation might have been useful for teaching differences in the effects of temperate and tropical climates on timberlines. Thus, it was not by chance that this sketch was adopted by many authors in their textbooks on geography or geobotany (e.g., Price, 1981; Klink and Mayer, 1983; Leser et al., 1991). However, from the landscape ecological view, this generalisation disguised the diversity that is typical of timberlines in the worlds' mountains, and it is this diversity that should be explored (Chapter 1).

Bader et al. (2007), for example, describe timberlines in the tropical Andes and on Haleakala volcano (Hawaii) as being mostly abrupt. However, their physiognomy varies locally. Most of these timberlines are fringed by tall shrubs, ferns or tall grasses (Photo 7). In other places, patchy timberlines occur with sharply-contoured dense tree stands alternating with Páramo vegetation (cf. Photo 113), thus forming a more or less wide ecotone. Moreover, 'gradual' timberlines occur where forest decreasing in height with altitude passes almost seamlessly into the Páramo, although this situation is rather rare. Richter et al. (2008) report considerable variation in the physiognomy of neo-tropical timberline in the Cordillera Real (Ecuador).

Forest

Tímbertinc centone

Radiation transfer in the upper canopy surface

Almost no influence of mierotopogra-phy on solar radiation

Relatively low temperatures

Almost no effect of microtopography on wind velocities and directions

Low wind velocities

High interception of precipitation by the forest canopy

Relatively even depth and length of the snow cover

Relatively high humidity Relatively even distribution of soil moisture

'Forest soils'(m conifer forests mainly Podzols)_

Radiation transfer at the surface of the ground vegetation, tree clumps, scattered trees and al (he soil surface Strong influence of microtopography, tree clumps and scattered trees on solar radiation

Locally varying temperature (High temperatures at sun-exposed and wind-protected sites)

Wind velocities unit directions modified by microtopography, tree clumps and scatlered trees

High wind velocities

Low interception of precipitation by tree clumps, scattered trees and ground vegetation

Irregular depth and length of the snow cover due to the effects of microtopography and scattered trees on wind velocities and directions Locally varying humidity Locally varying soil moisture

Mosaic of different soils related to mi-crotupugntphy and plant cover

Figure 4. Ecological conditions in the forest and in the forest-alpine tundra ecotone. Modified from Holtmeier (1979).

Thus, in tropical high mountains it may be hard to distinguish timberline and tree line in their proper sense, particularly if high-stemmed forests, such as the 9 m high Erica forest on Mt. Kenya for example, gradually merges into 'low forest', 'dwarf forest' and shrub without any change of tree species (Section 4.3.11). In this case a demarcation between tall and lower forest is arbitrary (Coe, 1967; Miehe and Miehe, 1994).

Timberline Ecotone
Photo 7. Abrupt timberline (at about 3.400 m) in the eastern cordillera of southern Ecuador. This timberline has not been influenced by fire for more than 10 years. M. Y. Bader, October 2002.

The great physiognomic and ecological variety and heterogeneity of mountain timberlines is reflected in timberline terminology (Table 3, see also Holtmeier, 1974, 2000). Some terms refer to the location of the timberline only (upper, lower timberline); others refer to the controlling factor or complex of factors (climatic, orographic, anthropogenic timberline) or to both location and causes (alpine, polar/subarctic/arctic, continental, maritime tim-berline). Thus, for example the alpine, the polar (subarctic, arctic, northern) and maritime timberlines are climatic timberlines. While the upper and northern timberlines are caused by heat deficiency, the maritime timberline is caused by strong winds and salt spray that adversely affect tree growth at the seashore (Brockmann-Jerosch, 1928). The maritime timberline also is a lower timberline (see also continental timberline). Consequently, the term 'maritime timberline' should not be confused with the comparatively low al-titudinal timberline in mountains with a maritime climate, as did Pott (1993) for example. Also, the continental timberline is a lower timberline (Brock-mann-Jerosch, 1919) that borders the steppe (Photo 8). Since the continental timberline is caused by insufficient moisture, it is also called dry timberline or drought-caused timberline. In arid and semiarid zones, mountains that are high enough to catch sufficient moisture from the air currents, an upper and a lower climatic timberline ('double timberlines' in the sense of Arno, 1984) occur: the upper timberline caused mainly by heat deficiency, the lower tim-

berline by lack of moisture. The less moisture is available to the forest the higher is the dry timberline located (Schweinfurth, 1957; Troll, 1972; Arno, 1984; Jacobsen and Schickhoff, 1995).

Table 3. Terms related to causes and position of timberline

Terms

(tl = timberline)

(Wgr = Waldgrenze)

Position

Causes

Climatic tl

Alpine tl Alpine treeline ecotone Mountain tl

Obere Wgr Alpine Wgr

Altitudinal limit

Heat deficiency, short growing season (outside the tropics)

Polar tl

Subarctic tl Arctic tl Northern tl Northern cold tl

Subantarctic tl Antarctic tl Southern tl Southern cold tl

Subarktische Wgr Arktische Wgr Nördliche Wgr

Subantarkt. Wgr Antarktische Wgr Südliche Wgr

Horizontal tl, bordering the tundra

Heat deficiency, short growing season

Inverted tl

Valley tl Valley bottom tl Bottom tl

Inversions-Wgr Inverse Wgr

Lower tl in mountain valleys

Inversions with frequent early and late frost, waterlogged soil

Continental tl

Lower tl Drought-caused tl

Dry tl

Kontinentale Wgr Trockengrenze Desertische Wgr

Lower tl in mountain, tl bordering the steppe

Moisture deficiency

Maritime tl

Coastal tl

Maritime Wgr

Lower/horizontal tl at the ocean coast

Influences adverse to tree growth at the coast (e.g. salt spray)

Historic tl

Max. postglac. tl Hypsithermal tl

Historische Wgr.

Wärmezeitliche

Wgr

Altitudinal tl

Heat deficiency, short growing season (outside the tropics)

Potential tl

Hypothetical tl

Potentielle Wgr

Altitudinal tl

Heat deficiency, short growing season (outside the tropics)

Orographic tl

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    What is the definition of timberline?
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