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Climate Change Impacts on Vegetation in Humid Asian Mountains
Masahiko OHSAWA
Institute of Environmental Studies
Graduate School of Frontier Sciences
The University of Tokyo
5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8653, Japane-mail: mohsawa@k.u-tokyo.ac.jp

Abstract
Diverse landscapes or ecosystems of mountainous terrain pose a challenge to predicting the response of mountain ecosystems to potential global change impacts. Isolated high mountain areas are
intrinsically fragile habitat-islands surrounded by lowland vegetation that is impassable to highland
biota. In humid East Asia, the more or less continuous extension of high mountain chains exhibits alatitudinal transition of vertical arrangement of forest zones on mountains. Since Humboldt, differences between vegetation zonation in humid tropical and temperate mountains have been described, but
the causal temperature factors responsible for the patterns have not been fully explored. This has madeit difficult to evaluate the impact of climatic change on mountain vegetation. In temperate mountains,
latitudinal transition of vegetation is controlled by increasing temperature seasonality or minimum
winter temperature. In tropical mountains, a year-round decrease in temperature with no seasonality
causes a shortage of energy for forest growth as altitude increases. By distinguishing between these
two temperature factors, most of the structural diversification in terms of life-forms, biomass, stratification, and diversity, among tropical and temperate mountain-forests can be explained. Furthermore, thecritical temperature factors controlling the patterns can be used for elucidating potential globalwarming-induced changes in forest zones and the causal effects, particularly at the interface between
tropical and temperate vegetation in the mid-latitudes.
Key words: evergreen forest, forest limit, temperature sum, tropical mountain, winter temperature

1. Introduction
Since the time of A. von Humboldt, many
researchers have conducted comparative studies onmountain vegetation zonation. Humboldt (1817)
devised isotherms and tried to compare the altitudinalzones of mountain systems from a global perspective(Troll, 1962). Around the middle latitudes, where
tropical zones transition into temperate zones, humid
forest series exist only in South to East Asia where themonsoon climate provides enough rainfall throughout
the year. On the Asian continent, many botanists/
geographers have explored mountainous areas like theHimalayas, upper Burma, the Hengtuan Mountains,
southwestern China, etc. Among them, several researchers, e.g., Troup (1926), von Wissman (1939),
Troll (1959), Schweinfurth (1957), Wolfe (1979), etc.,
proposed mountain-vegetation profiles as the basis forcomparative study of diverse mountain worlds.
During the last several decades, we have been conducting comparative studies on mountain-vegetation
zonation in South (the Himalayas), Southeast, andGlobal Environmental Research
10(1)/2006: 13-20
printed in Japan

East Asian humid mountains to compare the general
pattern of forest zonation in a latitude-altitude matrix(Ohsawa et al., 1985; Ohsawa, 1990, 1995a; Ohsawa& Nitta, 1997a, b; 2002). Recently our comparisonswere extended toward the Canaries where a relic typeof laurel forest developed as the cloud forest sustainedby fog drip provided by trade-wind inversion clouds(Ohsawa et al., 1999), and also toward Georgia, where
the evergreen trees of the Colchic Tertiary relic vegetation, are also sustained by fog/rain from the Blacksea (Kikvidze & Ohsawa, 1999, 2001). These
subtropical/warm-temperate evergreen broad-leaved
forests are interesting from the standpoint of understanding the processes of adaptation, diversification,and evolution of the original tropical montane rain
forest formations of Southeast Asia, which are facing
increasing stresses in the mid-latitudes, i.e. droughts
in the west toward the Mediterranean and low wintertemperatures due to seasonality to the east in monsoon
Asia (Ohsawa et al., 1986; Kikividze & Ohsawa,
2001; Ohsawa & Nitta, 2002).
Global change issues are particularly serious from©2006 AIRIES14

Masahiko OHSAWA

the standpoint of conservation of mountain ecosystems because of their environmental peculiarities.
Mountains are often island-like isolated habitats
among lowland ecosystems or human-dominated landscapes, and it makes it difficult for the mountain biota
to migrate freely. Mountain slopes have a steep temperature gradient, nearly 600 to 1,000 times steeper
than that of horizontal zonation. In general, the temperature changes at a rate of 6°C/km on mountainslopes but 0.01-0.006°C/km of horizontal distance inthe mid-latitudes (Ohsawa & Nitta, 1997a). Thissuggests tight species interactions in mountain environments compared to those of horizontal, latitudinalchanges. At the same time, however, this facilitates
migration of certain mountain biota towards neighboring ecosystems or life-zones; e.g., in the lowlands ofthe humid eastern Himalaya at ca. 500 m a.s.l., we canobserve tropical tree species, e.g., Terminalia
myriocarpa (Combretaceae) coexisting with such atemperate tree species as Betula alnoides (Betulaceae)
in the same community (Ohsawa, 1987a). These aspects of mountain environment act as the migratory
corridor for a certain biota, particularly in temperate
mountains with the seasonal overlapping of temperature conditions between different altitudes (cf. Janzen,1967). Along with this, the complex habitat mosaicsof mountain topography provides the ‘hiding places(refugia)’ for a certain biota from competitively superior species (Ohsawa, 1987b).
The object of the present study is to explore theforest zonation patterns in humid Asian mountains andanalyze the factors controlling the zonation in order toprospect possible impacts of global warming on
mountain ecosystems.

i.e. Taiwan (Su, 1984) and Japan (Ohsawa, 1984a, b).
In contrast to tropical mountains, temperate mountains
have clear zonal transitions of physiognomic forest
zones comprising highly diverse tree life-forms (evergreen, deciduous, and coniferous forests), though
structural (tree height, biomass, etc.) and speciesdiversity traits showed no remarkable changes withaltitude except around the forest limit (Ohsawa,
1995a). All of these data provide us invaluable information on forest zonation in tropical to subtropicaland warm-temperate mountains in humid monsoon
Asia (Fig. 1).
A more or less sharp transition of vegetation zonation from humid tropical to temperate mountains canbe seen at ca. 20°N-30°N in the mountain-vegetation
template for South/East Asia (Ohsawa, 1990, 1995a).
The template depicts the pattern of altitudinal zonation
on humid mountains along latitudinal gradients(Fig. 2). According to a comparative study of zonation
patterns and climatic conditions of these mountains,the main reason for the gradual transition fromtropical, to subtropical, to temperate types of zonations is the increasing seasonality or steep decline inwinter temperatures with no appreciable depression ofsummer temperatures toward high latitudes (Ohsawa,
1990, 1995b; Ohsawa & Nitta, 2002). The transitionzone between tropical and temperate mountainzonation also coincides with the planetary frontal zone

2. Mountain Vegetation Zonation in Humid
Asia and Its Controlling Factors
We have compared the latitudinal-altitudinal diversification of vegetation zones on the basis of several
descriptive studies on mountain vegetation zonation at
different latitudes. On tropical mountains such as Mt.
Kerinci (2°S, Ohsawa et al., 1985) and Mt. Kinabalu(5.6°N, Kitayama, 1991; Ohsawa, unpublished data),
physiognomically all the forest zones rather monotonously consist of broad-leaved evergreen forests up to
the forest limit. Even though tropical mountain forests have such physiognomic uniformity, there are
drastic floristic and structural changes including species diversity that can generally be delineated into
lowland and lower and upper montane zones (Van
Steenis, 1972; Whitmore, 1984; Ohsawa, 1991;
Ashton, 2003). Typical examples of humid tropicalto temperate forest zonation are obtained from slopes
surrounding the mountains of High Asia; the southernslopes in the Himalayas (Numata, 1983; Ohsawa,
1987a; Wangda & Ohsawa, 2006), the eastern slopesin Sichuan (e.g. Mt. Emei, Tang & Ohsawa, 1997),
and some mountainous islands off the Asian continent,

Fig. 1 Altitudinal formation zones in typical high areas ofsouth/southeast/east Asian mountains overlaid on a
mountain vegetation template for humid Asia (Ohsawa,
1990, 1993).
Data sources: Mt. Kerinci, Sumatra (Ohsawa et al.,1985), Mt. Kinabalu, Borneo (Kitayama, 1991; Ohsawa,
unpublished data), Mt. Yushan (Su, 1984), Mt. Namshila(Ohsawa, 1987a), Mt. Emei (Tang & Ohsawa, 1997, Mt.
Miyanoura (Ohsawa, 1984a), Mt. Kirishima, Mt. Fuji
(Ohsawa, 1984b), and Mt. Poroshiri (Ohsawa et al.,1973).

Climate Change Impacts on Vegetation in Humid Asian Mountains

starting around 20°N-30°N (Weischet, 1988). The
northward extension of evergreen broad- leaved treesof tropical origin is blocked by a minimum monthly
mean temperature of –1°C (Iversen, 1944; Ohsawa,1990) around 42°N in East Asia. Beyond this limit,
deciduous and coniferous forests replace the evergreen.
In turn deciduous broad-leaved trees of cooltemperate zone are blocked by an extreme minimum
temperature of – 40°C further north. Both deciduousand coniferous trees in the high latitudes of thenorthern hemisphere have adapted to increasing
seasonality of temperatures, particularly to cold
winters by shedding plant parts or evolving special
means of tolerating freezing temperatures (Sakai,1982). Over geological time scales, deciduous
broad-leaved trees are thought to have evolved to suitan increasing annual temperature range of more than
20°C (cf. Wolfe, 1979), and these temperature conditions start to appear around 30°N in maritime East
Asia at present (Ohsawa, 1990).

15

3. Thermospheres and Corresponding
Forest Zones in Mountain Environments
On the latitude-altitude matrix of temperature conditions for all of humid South/East Asia, there are several thermospheres that are lacking in the other latitudinal ranges. The southernmost or northernmost
thermospheres have reached their distributional limit
in either direction. For example, the thermosphere
for tropical upper montane forests (TUMF) is foundonly in high mountains at latitudes lower than ca.
30°N (Fig. 2, cf. Ohsawa, 1991). The temperature
conditions delineating the TUMF are a temperature-sum (Warmth Index, by Kira, 1948) of
85°C·months for the lower altitudinal limit (at ca.
2,500 m a.s.l. in the tropics), and 15°C·months for theupper limit, which coincides with the forest limit on
tropical mountains (ca. 3,600-4,000 m a.s.l.). On
tropical mountains, a monthly mean temperature of
the coldest month (CMT) of –1°C, which limits evergreen broad-leaved forests on temperate mountains, isFig. 2 Mountain vegetation template with effective boundary temperature factors overlaid on a
mountain profile of humid Asia. Tropical zonations are controlled by the temperaturesum (WI) and temperate zonations, by low temperature, except at the forest limit.

16

Masahiko OHSAWA

located far above the forest limit at ca. 4,500 m(Fig. 2), and the coldest monthly mean temperature at
the forest limit is ca. 5-6°C throughout the year.
Evergreen broad-leaved trees with microphylls in the
TUMF, therefore, reach their energy limit for making
tall forests at the forest limit before reaching their
physiological tolerance limit for low temperatures.
The major evergreen trees of the TUMF zone are theones mainly constituting the understorey layer of thezones below that (lower montane and lowland forests).
The characteristic tree groups are mainly Symplocaceae, Myrsinaceae, Theaceae, Aquifoliaceae, andother ‘symploco-myrsinaceous’ trees, with characteristic features such as microphylls, hypsophyllarybuds, and sylleptic branchings (Ohsawa, 1991;
Ohsawa & Nitta, 1997b, 2002).
Outside the tropics, the isotherm of CMT = –1°C(the lower temperature limit for evergreen forests) is
located at gradually lower altitudes with increase inlatitude. The line of CMT = –1°C crosses the line of
WI=15°C·months (forest limit) at ca. 20°N, where thetemperature conditions stop evergreen forests at
certain altitudes far below the forest limit (Fig. 2).
The isotherm of CMT = –1°C finally reaches sea levelat around 40°N.
Figure 3 illustrates the diversification of the thermosphere in South/East Asia, mainly focusing on
tropical broad-leaved evergreen forests with someFig. 3 Effective temperature factors for the differentiation of forest zones in tropical mountains and the northern
limit areas of evergreen broad-leaved forests around central Japan (based on the data and original diagramof Ohsawa, 1990). Three thermal conditions are proposed as the limiting conditions for evergreen
broad-leaved trees, as follows:▽: altitude at which CMT = –1°C,○: altitude at which CI = –10°C·months,●: altitude at which WI = 85°C·months.
Temperature sum conditions, i.e., WI = 15°C·months for the forest limit and WI = 240°C·months fortropical lowland forests, are indicated only by lines without dots.
CASE 1 and CASE 2 indicate two different scenarios predicting rising temperatures only in winter andsummer, respectively. The range of the rises is not proportional, and only the differences in response pattern
for each quadrant are illustrated.

Climate Change Impacts on Vegetation in Humid Asian Mountains

adjoining thermospheres in the vicinity of their northern limits.
Near the northern limit of broad-leaved evergreen
forests at around 30°N-40°N (around central Japan),the altitudes of CMT = –1°C nearly overlap with thealtitudes of WI = 85°C·months (Fig. 3, cf. Ohsawa,1990). From a wider geographical perspective, theregression curves of the two factors (altitude for CMT= –1°C and WI=85°C·months) crossing at around30°N-40°N, diverge clearly into two toward theequatorial tropics as shown in Fig. 3.
Although two boundary-temperature conditions forevergreen broad-leaved trees are located at aroundthe same altitudes at 30°N-40°N, the biologicalsignificance of the two temperature factors differ. A
CMT of –1°C is effective at limiting evergreen trees
physiologically in temperate latitudes (cf. Iversen,
1944; Ohsawa, 1990), even though the temperaturesum factor in summer is still sufficient for establishingtall forests. On the other hand a temperature sum of
85°C·months may not be a simple factor, but if thecondition is extended toward tropical mountains, it
coincides with the boundary between lower and uppermontane zones at ca. 2,500 m a.s.l. (Ohsawa, 1990).
The boundary also approximately coincides with theupper limit of notophyllous lauro-fagaceous forests,
and above it, canopy dominants are replaced by symploco-myrsinaceous trees which are evergreen microphylls.
Near the northern limit of evergreen forests, thetwo temperature conditions of tropical and temperate
zonation can affect the zonal differentiation. Thus
winter temperatures of CMT = –1°C and a summer
temperature sum of WI = 85°C·months can overlap
and divide the region into four quadrants (Q-A, B, C
and D) as shown in Fig. 3. Around central Japan the
crossings of the two factors and the resultant thermospheric quadrants (within the square in the schematic
thermosphere on the latitude-altitude matrix in Fig. 3)
can be correlated precisely with the extant forest types
around the northern limit of evergreen broad-leaved
forests in temperate mountains as follows (Fig. 3):
Quadrant A (thermosphere > WI 85°C·months and>CMT –1°C), a warm-temperate forest zone
dominated by evergreen notophyllous
lauro-fagaceous trees
Quadrant B (CMT –1 > thermosphere > WI 85),a mid-temperate forest zone dominated by midtemperate deciduous trees of the genera Quercus,
Carpinus, etc., and temperate conifers of Abies,
Tsuga, Cryptomeria, etc.
Quadrant C (CMT –1 < thermosphere < WI 85),
evergreen microphyllous forests dominated bysymploco-myrsinaceous trees (tropical uppermontane forest types)
Quadrant D (thermosphere < CMT –1 and <WI 85),
cool-temperate deciduous forests dominated by
Fagus.
A detailed analysis of the two boundary factors

17clearly indicates a diversification of forest types at thecontact zone between evergreen broad-leaved forestsof tropical origin and deciduous forests of temperate
origin. The pattern also indicates a very limited thermosphere for temperate deciduous and/or relict
conifers (Q-B) in between evergreen and deciduous
forest zones in East Asia (cf. Fig. 3).
It also clearly indicates that the major determinantof the formation type (tree life-form) at the tropical/temperate interface is winter coldness (CMT –1°C),
and evergreen broad-leaved trees are limited their
northward extension by CMT –1°C. On the other
hand the energy level indicated by the WI-value iseffective for differentiating between notophyll evergreens (Q-A) and microphyll evergreens (Q-C),
mainly in tropical mountains (Fig. 3).

4. Global Warming Impacts on Mountain
Vegetation
According to the present analyses, temperature factors that control zonal differentiation of forests differ
between tropical and temperate mountains. While intropical mountains, forest zones are differentiatedthrough temperature sum conditions, winter low temperatures below freezing control the northern limit ofevergreen broad-leaved forests in temperate mountains (Figs. 2 & 3). This brings disparity to their responses to global warming.
Around the mid-latitudes, there are two different
predictions for temperature changes due to global
warming. One is that winter temperatures will risewith no appreciable change in summer temperatures,or at least that the rise in winter temperatures will surpass that of summer (Yoshino, 2006). The other is thattemperatures will rise with no appreciable change inthe seasonal pattern of temperatures (Nakagawa et al.,1998).
If global warming makes winter temperatures rise
to a certain threshold, e.g., CMT above –1°C, thebroad-leaved evergreen trees will be affected directly
and start to move northward beyond their present
northern limit, perhaps replacing the deciduous/coniferous forests. The tropical/temperate interface
affected by the two temperature conditions CMT and
WI could produce the four quadrants shown in Fig. 3.
The two wedge-shaped thermospheres (Q-B andC) both represent fragile vegetation types vulnerable
to temperature changes. Thermosphere Q-B (85°C·
months < thermosphere < CMT –1) will be lost if
winter temperatures rise (CMT > –1) due to climatechange, as in Case 1 of Fig. 3. The forest type which
has developed in thermosphere Q-B is the so-called
mid-temperate zone forest mainly dominated by deciduous broad-leaved trees, e.g., Quercus, Carpinus,
etc., and more importantly Tertiary relic temperateconifers such as Cryptomeria, Tsuga, Abies,
Chamaecyparis and Pseudolarix, which are all valuable timber trees in Japan. Ginkgo, Cryptomeria,

18Masahiko OHSAWA

Taiwania and Chamaecyparis are also vicariant genera
in China and Taiwan.
Replacing these lost trees will be those of the
newly expanding thermosphere resulting from rising
winter temperatures, Q-C (CMT –1 < thermosphere <WI 85°C·months) (Fig. 3, Case 1). Quadrant-C isequivalent to the thermosphere of tropical uppermontane forests, but it does not exist extensively intemperate mountains under the present climatic conditions. As exemplified in tropical mountain zonation,
the evergreen forests which develop under lower energy (temperature sum < 85°C·months) levels are
symploco-myrsinaceous, evergreen, microphyllous
forests, as in the tropical upper montane zone.
This vegetation change scenario is interesting because there are several cases of fossil evidence (hugeamounts of the remains of seeds and fruits) of the occurrence of true Symplocos forests in Central andWestern Europe which was at the northern limit oftropical broad-leaved evergreen forests during the
Tertiary. Those forests became extinct later during the
Quaternary glaciations (Nooteboom, 1975). Thus in
the geological past there may have been forest types
similar to the symploco-myrsinaceous forests of thepresent tropical upper montane forests, near the past
northern limits of the broad-leaved evergreen forests
at ca. 50°N. Even in the present situation at thenorthern limit of broad-leaved evergreen forests intemperate Japan, broad-leaved evergreen tree species
with microphylls and hypsophyllary buds (Ilexcrenata,
Illicium
religiosum,
Daphniphyllum
macropodum, etc.) are invading into the understorey
of cool-temperate deciduous forests dominated by
Fagus, Quercus, Acer, Carpinus, etc. Those broadleaved evergreen trees of the understory are mostly
vicarious species with the trees of the upper montane
zone on tropical mountains (Ohsawa et al., 1985;
Ohsawa, 1991; Ohsawa & Nitta, 1997b).
On the other hand in the case summer temperaturesrise with no change in winter temperatures, thermosphere Q-C will be wiped out and Q-B will expand
drastically (Fig. 3, Case 2). Moreover in this casethe forest limit may also shift upwards as shown in
Fig. 3, Case 2.

5. Conservation of Mountain Ecosystems
The overall temperature rise may shift all the zones
in a parallel way, and this would have a particularly
serious effects on small peaks having a narrow cap ofdistinctive ecosystems at the top. Those isolated
patches may surely be driven off of the peak. Mountains having complex topographies, however, may
somehow harbor the relict vegetation. Several isolated, scattered Pinus parviflora communities (normally distributed above ca. 1,500 m a.s.l.), thrive on
rocky ridges of less than 500 m a.s.l. on the Boso
Peninsula, Chiba Pref., in central Japan. They are agood example of such relic communities from the gla-cial periods (Numata, 1979). These pine stands arenow severely threatened in the area because of theirisolated small populations being subjected to overalltemperature increased, infectious wood nematodes,
and other threats.
In a different case, in which only winter temperatures rise due to global warming with no change insummer temperatures, the situation would be different
at the northern limits of broad-leaved evergreen forests. Only the broad-leaved evergreen forests and/or
deciduous forests would shift upwards with no associated movement of the forest limit (controlled by thetemperature sum), This could suppress the subalpine
forest zone below the forest limit, and the cooltemperate deciduous broad-leaved forests (dominatedby Fagus crenata, Quercus mongolica var.
grosseserrata, etc.) in between the subalpine coniferous and broad-leaved evergreen forest zones.
As already mentioned above, East Asia is one of
the major refugia for Tertiary relic plants, particularlyaround the interface between evergreen and deciduous
forests, ranging from warm-temperate to cool-temperate zones, and those relics can hardly compete withmodern trees appearing in most cases as the dominant
species in both cool and warm-temperate forests(Bond, 1989; Ohsawa, 1987). Most of those relicplants are inferior in competition and grow mainly in
disturbed sites as fugitive pioneers, or as isolated
patchy communities in harsh environments, as in thecase of the relict pine stands on the Boso Peninsula.
There are many similar examples in China, e.g., monotypic genera such as Ginkgo, Tetracentron, Euptelea,
Davidia, Pseudolarix, Cathaya, etc., growing in heavily disturbed sites, on rocky ridges, or at any othersites where modern trees are excluded by stressful
habitat conditions (cf. Bond, 1989). They can sometimes invade into even mesic, fertile sites as pioneers,and can survive by becoming tall emergent trees(Ohsawa, 1987b), but their regeneration is dependent
on the disturbance or topographical complexities on aregional scale.
The only means of conserving those relict or otherdiverse plant communities in mountainous areas is toprotect areas large enough to encompass their diverse
habitats including specific regeneration sites (rocky
ridges, cliffs, flood plains, etc., as well as mesic sites)
as a whole. If the areas are insufficient in scale toinclude all the possible habitat types or the necessary
population sizes to survive, those protected areas
should at least be connected to each other by an ecological corridor that facilitates migration of biota(Ohsawa et al., 1998). In Bhutan, for example, there
are nine protected areas which are inter-connectedwith each other by such ecological corridors tofacilitate conservation of rare wildlife such as tigers,
takin, and snow leopard (Dorjee, 2000). In total, thenine protected areas cover ca. 26% of the land area,
and the conservation corridors contain in total ca.
9.2% of the land area in Bhutan.

Climate Change Impacts on Vegetation in Humid Asian Mountains

6. Conclusion
Humid mountains in South, Southeast and East
Asia provide the best sites for investigating zonal differentiation of forests along temperature gradients on
a continuous latitude-altitude matrix. Such a seriesof vegetational gradation mainly governed by temperature cannot be found which are not subject to dry
spells in other parts of the world. Analyzing the series of mountain forests rigorously, we could identify
two contrasting mountain zonation types, i.e. a tropical zonation type at low latitudes below 20°N-30°N,
and a temperate zonation type at higher latitudes.
The two zonation types are controlled by two contrasting temperature factors, i.e. temperature sum for tropical zonation and minimum winter temperature fortemperate zonation. Only the forest limits of both
tropical and temperate mountains are commonly controlled by the temperature sum. These contrasting
temperature factors cause different responses of zonal
vegetation to global warming. The shifting of vegetation on mountainous terrain will have serious impacts
and the best possible way to conserve mountain vegetation diversity is to connect the isolated mountainenvironments with biological corridors that makes it
possible for the biota to migrate to survive serious
global change impacts.
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(Received 24 July 2006, Accepted 28 July 2006)