The Abiotic Environment

Aboveground

Abiotic factors have traditionally been viewed as dominating the ecology of high altitudes, including the alpine forest. Sunlight, temperature, water, and gas-phase nutrients (e.g., CO2 and O2) can vary substantially with altitude, regional climate, and orographics (e.g., maritime

Table 1 Biogeography of alpine forest areas worldwide

Latitude

Mountain range

Altitude (m)

Climate

Life form

Dominant tree genera

Western Hemisphere

50-60° N Northern Rockies,

55-580 N

45-500 N

18-250 N 9-110N

100 N-200 S

230 S-500 S

USA/Canada Scottish Highlands,

UK Northern

Appalachians, USA Pacific Coast Mtns,

USA, Canada Middle Rockies, USA Sierra Nevadas, Spain Sierra Nevadas, USA Southern Rockies, USA

Sierra Madres, Mexico Talamancas, Costa Rica

Tropical Andes, Costa

Rica-Peru South America, Temperate Andes

Eastern Hemisphere 60-700 N Skanderna,

500 N 440 N 46-430 N

430 N 380 N

35-360 N

280 N

360 S

420 S

430 S

Scandinavia Altai, Mongolia Tien Shan, Asia Alps, Europe

Caucasus, Georgia

Pamir, Tajikistan, Asia Japanese Alps/Fuji,

Japan Great Atlas, Northern

Africa Himalayas, Asia

Maoke Mtns

East Africa Highlands,

Africa Australian Alps,

Australia Tasmania

Southern Alps, New Zealand

2600-2900

600-800

1500

To 3300

2900-3300 1950

3000-3500 3300-3800

4000 3000

3150-4700 1100-1500

700-900

2000 3000

1600-2300 2200 3000

1950-2400 2850

3800-4500

3000-3600 4050

1800-1950 1200-1260 1200-1500

Continental

Oceanic

Continental

Oceanic

Continental Continental Oceanic Continental

Oceanic Oceanic

Oceanic

Oceanic

Continental

Continental Continental Continental

Continental

Continental Oceanic

Continental Oceanic

EN DB

EN EN

DB DB

Abies, Picea, Pinus, Larix Pinus, Juniperus, Betula Abies, Picea

Tsuga, Pinus, Abies, Picea,

Chamaecyparis Abies, Picea, Pinus Quercus, Pinus Pinus, Tsuga, Juniperus Abies, Picea, Pinus

Pinus Quercus

Polylepis

Podocarpus, Nothofagus

Betula

Pinus, Larix Picea

Abies, Picea

Betula, Rhododendron, Pinus Picea

Abies, Larix, Pinus, Tsuga

Cedrus, Juniperus

Betula, Rhododendron, Picea, Larix,

Juniperus, Tsuga Podocarpus Erica

Eucalyptus

Athrotaxis, Eucalyptus, Nothofagus Nothofagus

DN, deciduous needleleaf; EN, evergreen needleleaf; DB, deciduous broadleaf; EB, evergreen broadleaf.

vs. continental mountain ranges). In addition, many factors influencing leaf energy balance and temperature may also vary with elevation, including solar and long-wave radiation, wind, and ambient humidity. Probably, the best-known abiotic change with increasing elevation is the decline in air temperature in response to lower ambient pressure. Ambient pressure decreases by over 20% at 2 km and over 50% at 6 km, leading to a maximum, dry adiabatic lapse potential of 1.0°C/100m. Simulated dry (8.0° C km ) versus wet (3.0°Ckm ) lapse conditions resulted in a more rapid decline in air temperature with altitude for both winter and summer temperatures. Also, dry lapse conditions in summer generated similarly cold air temperatures at higher elevations (>4km) that were very near values computed for wet lapse conditions during winter (Figure 5b). Similar dry and wet lapse rates of 7.5°Ckm-1 and 5.5°Ckm-1, respectively, have been used previously to evaluate transpiration potential for plants growing on mountains of temperate and tropical zones.

Rocky

Rocky

Scottish

Highlands

Mountains

Pacific Coast Range Northern Appalachians

Sierra Nevadas

Sierra Nevadas

Sierra Madri

Talamancas

TSha

^ Deciduous needleleaf (DN) ^ Evergreen needleleaf (DN) § Deciduous broadleaf (DB)

Evergreen broadleaf (DB)

^ Deciduous needleleaf (DN) ^ Evergreen needleleaf (DN) § Deciduous broadleaf (DB)

Evergreen broadleaf (DB)

Sierra Madri

Talamancas

TSha

Skanderna

/-"A (

Urals

Alps Altais

440 Caucasus Tien Shan Atlas Mtns Pamirs

Himalayas

^Bpr pHL

Japanese t

f-—» *

Alps

J p #

__

Maoke Mtns

Tasmania Southern Alps

MO 0

-

Figure 3 Global biogeography of alpine forest areas. Italicized mountain ranges have no corresponding information in Table 1.

Figure 4 Individual alpine forest tree in the high-altitude treeline (3306 m) at a wind-exposed site. At this high-altitude limit of tree growth, extreme distortion in tree structure results in the classic krummholz mat growth form showing the presence of flagged branches on the downwind edge. The prevailing winds are from the right in this photo.

Figure 4 Individual alpine forest tree in the high-altitude treeline (3306 m) at a wind-exposed site. At this high-altitude limit of tree growth, extreme distortion in tree structure results in the classic krummholz mat growth form showing the presence of flagged branches on the downwind edge. The prevailing winds are from the right in this photo.

Another fundamental change in abiotic factors of increasing altitude is the unique and colligative property of decreasing atmospheric pressure and, thus, the partial pressures of gas-phase molecules such as CO2 and O2. In contrast, the amount of water vapor in the air at saturation is dependent only on temperature and, thus, strongly influenced by the lapse rate in air temperature described above. Because ambient CO2 concentration can have a strong, direct influence on plant photosynthesis via the leaf-to-air concentration gradient (driving force for diffusion), it has often been assumed to be a limiting factor for carbon gain and growth at high elevation. For plants, where the diffusion process is the primary mode of gas exchange, a lower ambient CO2 concentration with altitude could result in a corresponding decrease in the leaf-to-air gradient, assuming a constant CO2 concentration inside the leaf. For this reason, mountain ecosystems have been considered as natural field models for evaluating the effects of natural differences in atmospheric CO2 concentrations. However, because molecular diffusion is more rapid at lower ambient pressure, a substantial compensatory effect on CO2 uptake potential occurs with greater elevation. In agreement with this physiochemical property, little evidence has been found supporting the idea that lower partial pressures result in diffusion limitations at higher altitudes, at least for systems depending on the diffusion process for physiological gas exchange. Although quantitative evaluations showing these compensating effects on photo-synthetic CO2 uptake exist in the literature, there are few comprehensive studies incorporating all of the potentially important factors influencing diffusional gas exchange at higher altitudes. Similar concerns for animal O2 uptake at high altitude form a vast literature, although animals, depend primarily on bulk supply mechanisms for enhancing gas exchange. However, diffusion effects on animal

Within herbaceous cover cold nights, intermediate water stress, shade

90% survival *

Exposed, on bare soil warmer nights, least water stress, full sunlight 44% survival

Exposed, on bare soil warmer nights, least water stress, full sunlight 44% survival

In opening in herbaceous cover cold nights, greatest water stress, full sunlight 19% survival

In opening in herbaceous cover cold nights, greatest water stress, full sunlight 19% survival

Figure 5 Microsite alteration experiment showing effects of facilitation vs. competition on survival of new (first-year) seedlings of Picea engelmannii Parry ex. Engelm. (Engelmann spruce) in an alpine treeline ecotone, southeastern Wyoming. Greatest survival (90%) occurred for seedlings growing in vegetative ground cover that resulted in low sky exposure and incident sunlight the following morning, intermediate water stress, and relatively cold nights. Removing all vegetation well away from a seedling reduced competition for soil water (higher xylem water potentials), but increased sky exposure, resulting in significantly lower survival (44%). The highest mortality occurred when only proximal vegetation was removed to increase sky exposure, while maintaining boundary layer effects, lower minimum needle temperatures, and competition for water (as validated by higher water potential values). Higher photosynthetic carbon gain due to less low-temperature photoinhibition of photosynthesis was also associated with greater survival. Thus, facilitated reduction in sky exposure (day and night) appeared to have a greater influence on photosynthesis and survival, compared to low temperatures or competition for water with neighbors, although all three stress factors had significant impact. From Germino MJ, Smith WK, and Resor C (2002) Conifer seedling distribution and survival in an alpine-treeline ecotone. Plant Ecology 162: 157-168.

ecophysiology at high elevations (e.g., eggs, burrowing and subnivian animals) are not well studied, except for a large literature dealing with human physiology under hypoxic conditions.

Other abiotic factors such as the known increases in sunlight due to a thinner, unpolluted atmosphere, lower ambient humidities, high wind regimes, and decreased long-wave radiation from the sky (downwelling) have been studied less thoroughly, and for only a few mountain systems. In particular, the decrease in downwelling radiation can result in lower minimum temperatures at night that are often freezing even in summer. The influence of snow accumulation has been shown to be critical for winter survival of evergreen plants, preventing potentially lethal wind damage and desiccation via cuticle abrasion, as well as exposure to the cold sky and lower air temperatures above the snowpack. Though most studies have considered changes in single, or a few, abiotic factors, none have considered the concerted influence of multiple stress factors on the different habitat types of the alpine forest environment; for example, only a few studies have incorporated multiple abiotic factors to evaluate effects of high elevation on such important physiological processes as evapotranspiration, even though water diffuses rapidly from all evaporating surfaces, both plants and animals, compared to sea level.

Belowground

The soil environment of the alpine forest, as for many other communities and ecosystems, is strongly dependent on the prevailing moisture regime, including the seasonal timing and physical nature of the precipitation (rain vs. snow). Although the winter season can result in snow even in tropical mountains, there can also be important impacts due to the occurrence of dry seasons, sometimes twice per year. Regardless, rainfall of a melting snowpack results in quite different impacts on soil nutrients based on the accompanying temperature regimes. Warmer periods with rainfall will result in the release of soil nutrients, while colder periods with snow accumulation will involve dormant periods for surface soil organisms and, thus, decomposition and nutrient release. In addition, the growth activities of the important mycorrhizal fungi of plant root systems are strongly limited by soil temperatures well above freezing (up to 7°C). Because more tropical alpine forests receive the majority of precipitation as snow during a relatively brief winter season, or wet season, snowmelt often occurs quickly and is followed by a rapid drying of surface soils where roots are found. Thus, plants must take up soil nutrients during a very abbreviated time period which can be limited by persistently cold soils that lag behind air temperatures on a daily and seasonal basis. Soils' freeze and thaw cycles at high altitudes can also create distinct patterns in the microtopography of the soil surface (e.g., polyhedrons), providing important microsites for seedling establishment and small-scale differences in plant distribution patterns.

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