BOREAL FORESTS,TUNDRA & ALPINE BIOMES |
I. BorealConiferous Forest ('Taiga' in Eurasia)
A. Location and Ecology
Coniferous forests are mainly found in broad circumpolar belt across the northern hemisphere and on mountain ranges where low temperatures limit the growing season to a few months each year; thus too unfavorable for most hardwoods.
Extends from Alaska to New Foundland. Furthest north extension at 69° in NW Canada. Extends from eastern Scandinavia across northern Asia to the Pacific Ocean. Furthest north extension at 72° in Siberia. The geographic position of the southern continents precludes the development of similar forests at high latitude.
Almost all of the coniferous species are evergreen with the exception of Larix (larch); subshrubs (often scandent) are common.
Soil is 'spodozol' [podzol], heavily leached and acidic.
Many of these areas have been recently glaciated so the boreal forest is very young and intimately connected with fens and bogs. The boreal forest is connected to cold montane coniferous forests even south of the boreal forest, but the species are often quite distinct. There is a poleward decrease in species diversity, as expected.
B. Climate
Average daily temperature of 10°C is found in fewer than 120 days and cold seasons last 6 months. Tundra will form at the northern edge when there are 30 days only of average daily temperature of 10°C and cold season lasts up to 8 months.
Considerable variation is climate from location to location. Cold oceanic boreal forests show considerably less fluctuation in seasonal temperature relative to cold continental boreal forests of Eurasia.
C. Biome Regions
1. North America
a. boreal forest
Picea glauca (white spruce) : only species to range across N. America
Picea mariana (black spruce), Pinus (pines), Abies (fir), Larix (larch), Tsuga (hemlock), Thuja (white cedar), Juniperus (junipers)
non-conifers: Alnus (alder: N2 fixing!), Betula (birch), Populus (aspen)
subshrubs/herbs: Vaccinium (blueberries), Maianthemum (wild lily of the valley), Linnaea (twinleaf), Pyrola (shinleaf), Rubus (raspberry), Lycopodium (club moss), Pteridium (bracken fern)b. sub-alpine forest
Picea glauca (white spruce) extends into Rocky Mts.
Pinus contorta (lodgepole pine), Pinus ponderosa (ponderosa pine), Abies lasiocarpa (subalpine fir)2. Eurasia
a. taiga
floristically poorer than North America with 2 species dominate (Picea abies: Norway spruce, Pinus sylvestris: Scot's pine). species pairs with North America common: (e.g., Larix decidua, larch).
II. Tundra Biomes
A. Locations
Tundras are characteristic of Arctic or Alpine regions where the severity of environmental conditions excludes tree growth. 30 days of 10°C ave. temperature and 8 mos cold season.
Arctic tundra occurs north of the boreal forest or taiga and thus form a treeless ring south of the zone of permanent ice (North America, Greenland, Eurasia).
In the Antarctica, tundra area is very small because of the lack of large continental masses. Develops only on certain small Antarctic islands such as South George and MacQuarie Island on several spots on the most northerly extension of Antarctica proper (only 2 flowering plants! Deschampsia antarctica [Poaceae] and Colobanthus quitensis [Caryophyllaceae cushion plant].
Alpine regions include Rocky Mountains, European Alps, Himalayas, and Austral-antarctic area (South Island, also Tasmania, Snowy Mts. in Australia). Exclude tropical 'puna' in Andes and similar high elevation peaks in East Africa (will deal with later). Links of Austral-antarctic region with puna (Azorella, Umbelliferae).
B. Characteristics
Low precipitation; less than 400 mm per year.
A short vergetative period of generally less than 50 days between spring and sutumn frost.
Permanently frozen sub-soil. Permafrost of variable depth. Consequences are physical barrier to roots, low temperatures suppress decomposition and promotes peat, and retard water percolation and promotes swampy or boggy conditions = very slow growth.
C. Zones
High arctic: not continuous cover: mosses, lichens, Dryas (avens), Saxifraga (saxifrages), Salix herbacea (least willow), Salix arctica (arctic willow)
Middle arctic: continuous cover: dwarf-shrub communities, peaty swamps with Carex (sedges) and Juncus (rushes), lichen, Arctic bell heather
Low arctic: scrubby willows and birches, dwarf shrubby heath (Ericaceae); merges into taiga ('wooded tundra') with spruce, larch, pine
Alpine: show many similarities to tundra regions of high latitude; they do not form continuous expanses but are best characterizes as islands.D. Life forms
chamaephytes (incl. subshrubs) and hemicryptophytes by far the dominant forms, often cushions (e.g., Abrotanella, Tasmania)
grasses and sedges dominate (e.g., North America: Poa arctica, alpine meadow grass; Carex bigelowii, rigid sedge)
vegetative reproduction common (bulbils or vivipary; eg. Polygonum viviparum, alpine bistort)
apomixis, wind and fly/bee pollinated (incidence of open bowl flowers with conspicuous marks increases towards the arctic)
E. FloristicsCircum-boreal often the case, Amphi-atlantic, Amphi-pacific often the case as well (Eric Hulten) [Rhododendron lapponicum, lapland rosebay]
Alpine vegetation shows very close resemblances both in life-forms and in species composition to that of the Arctic tundra. A whole group of species are common to both and referred to as 'Arctic-Alpine' species.
Families of importance
Ericaceae (Vaccinium: blueberries, Rhododendron, Empetrum: crowberries)
Caryophyllaceae (Silene, Cerastium)
Saxifragaceae (Saxifraga: saxifrages)
Rosaceae (Dryas: arctic avens, Acaena)
miscellaneous: Polygonum (arctic bistort), Primula (primroses), Gentiana (gentians)
Grasses (Poaceae) and sedges (Cyperaceae)
Shrubs: birches (Betula), willows (Salix), alders (Alnus)
F. Origins
Will discuss more in detail later; but forests grew at high latitudes across North America and Eurasia until the beginning of the Pleistocene epoch (2mya)! Fossil remains of dawn redwood, swamp cypress, Ginkgo, and other broad leaved genera are common throughout the Canadian Arctic and Eastern Siberia. Alaska (most of it) switched over from coniferous forest to shrubby and herbaceous vegetation during the late Pliocene (3mya).
Glacial activity over the last 2 million years has caused some uneven distributions.
Bipolar distributions occur because of high elevation zones in mountain range running through North and South America.
Most of the alpine flora in southern Hemisphere appear to be relictual from an extensive tundra flora in Antarctica prior to be covered by ice.
