A look at the abiotic and biotic dynamics of a beech forest ecosystem in Arthur’s Pass National Park, located in the South Island of New Zealand.
The Arthur’s Pass National Park extends across the Southern Alps of the South Island of New Zealand, from Canterbury in the east to the West Coast. The high mountains divide the climate of the South Island, with a mixed podocarp rainforest covering the wet western slopes but a dry beech forest to the east. This article examines the features inherent to this beech forest ecosystem.
Multiple abiotic factors influence vegetation types and productivity in the beech forest ecosystem of the Arthur’s Pass National Park, however slope, minimum winter temperature and solar radiation might be considered the most important.
Slope is important in determining vegetation types both directly and for its influences on other abiotic factors. Plant life must be able to cling to the sloping sides of the terrain; in high winds and/or during earth tremors due to the presence of an alpine fault, the sloping terrain would contribute to disturbance events such as tree-fall in the ecosystem. Slope reduces the amount of available solar radiation through shading, increases drainage, thus reducing water supply and restricts soil accumulation and aging and therefore nutrient availability.
The minimum winter temperature is important because it can get very low due to the latitude of the park and its elevation. Add in the wind chill from the predominantly southwesterly winds and the plants that can survive here become a select group.
Solar radiation provides the energy that allows vegetative production and as such would have to be considered one of the most important abiotic factors in any ecosystem containing photosynthesizing organisms. At this latitude and with shading from the mountain slopes reducing periods of availability, it can be considered a limiting factor on plant productivity in the Arthur’s Pass National Park.
Key producers in the beech forest ecosystem are, as to be expected, beech trees (Nothofagus spp.), with mountain beech (N. solandri) predominant in the upper most reaches. As you descend, silver beech or tawhai (N. menziesii) becomes an increasing component of the forest canopy. Below the beech canopy, you might find horopito or pepper tree (Pseudowintera axillaries), pokaka (Elaeocarpus hookerianus) and black mapou (Pittosporum colensoi) growing, with bush rice grass (Microlaena avenacea) and watau or dense forest sedge (Uncinia uncinata) particularly spread near trails and disturbance sites. Korukoru or crimson mistletoe (Elytranthe colensoi) lives on the silver beech as well as an abundance of lichens, fungi and mosses on both beech varieties and throughout the forest.
Vegetative growth begins in early spring with the budding of new leaves from stored energy and nutrient reserves to catch the increasing solar radiation. Trunk thickening and branch extension occurs mainly in early summer and a little in early autumn when there is both sufficient solar radiation and water. There is a hiatus on growth and some leaf shedding late autumn and through winter.
The beech forest acts as a sink for carbon dioxide overall, actively absorbing it in spring summer and early autumn, but in late autumn and winter respiration exceeds photosynthesis and the forest becomes a source. Leaf fall is heaviest at the end of autumn and so is decomposed over winter by fungi and buried bacteria generating enough heat to continue functioning. Thus making the nutrients available again for new growth in spring.
Primary consumers are mainly macro-invertebrates, particularly larval forms. The tree weta (Hemideina spp.), giant weta (Deinacrida spp.) and cave weta (Rhaphidophoridae spp.), although omnivorous, will mainly consume mosses, lichen and leaves. Introduced mammals now play significant roles in the forest ecosystem, particularly red deer (Cervus elaphus) who interfere with forest regeneration by browsing on beech saplings and mice (Mus musculus) that thrive on beech seeds that are not eaten by most native species of macro-invertebrate or bird.
Herbivore activity is dependent on food supply, flowers are available on many plants from early spring to mid-summer, as well as new leaf growth. Fruit and seeds as well as mature leaves are available through to late autumn, early winter. Insects might lay eggs in autumn that won’t hatch into larvae until the next spring. Birds and mammals have their young early summer to give them time to mature before the next winter. Some migration to lower elevations may occur, particularly with red deer, but many species experience a dieback over the winter period, especially small birds and rodents.
Secondary consumers include the predominantly carnivorous ground weta (Hemiandrus spp.) whose back legs allow it to leap on prey, making it a far more effective hunter than other genus of weta. Birds such as the brown creeper (Mohoua novaseelandiae) and the rifleman (Acanthisitta chloris) eat a wide variety of insects, larvae and spiders found amongst the lichen and mosses as well as small fruit. Mice and rats (Rattus rattus) will prey on insects, in turn being eaten by stoats (Mustela erminea). Mice numbers tend to increase dramatically after a beech mast, when the beech trees produce a significantly larger amount of seeds, usually every three to five years. This in turn increases stoat populations who, once mice numbers are reduced, unfortunately turn to native bird species as food sources. The forest-dwelling New Zealand falcon (Falco novaeseelandiae) has slight morphological differences from that living in more open regions, allowing it the agility to hunt within the forest on native and introduced birds and introduced mammals.
Humans (Homo sapiens sapiens) hunt deer for food and trophies and kill mice, rats and stoats through poisons and trapping as part of conservation measures.
Decomposers start with saprophytic macro-invertebrates such as beetles of the Scarabaeidae family, slaters and mites that break down larger sized pieces of dead organic matter supplying more surface areas for bacteria and fungi to operate on. Decomposition is ongoing all year round; an abundance of leaf litter late autumn would be more a boon to fungi than bacteria, which can generally function better at cooler temperatures. In the warmer months the bacteria are likely to take a larger role. All life falls to the decomposers in the end, including their own.
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