The Different Ecosystems Within The Bunya Mountains, Australia
The Bunya Mountains, named after the Bunya tree (Araucaria bidwillii ), in Queensland, Australia is an amazing representation of distinctly different ecosystems in one area. The three main ecosystems that make up this are rainforests, savannas and grassland bands. These are further categorized further into microclimates which have distinct abiotic and biotic features and conditions that define them. These microclimates include, microphyll, mesophyll and dry rainforests, notophyll vine thickets, grassland balds, and sclerophyll eucalypt woodland. The Bunya Mountains are not the only area in the world to hold such an array of habitats, and these have been heavily researched in an attempt to further understand the origin and how such vastly different microclimates can coexist adjacent to each other. This paper will take a look into some of that research and take a further look into the biotic factors that separate and categorise these habitats.
Bunya Mountains General Climate
The weather in the Bunya Mountains is generally mild, experiencing temperatures between freezing and 30C, with four distinct seasons. Even in summer the weather is still 7-10C cooler than other areas and has annual rainfall is 1050mm with common mists and heavy fogs (Bunya Mountains Accommodation Centre, 2018). Grassland Bald Grassland balds are treeless areas that apparently disclimax, that occur all over the world (Fensham & Fairfax, 1996), with the Bunya Mountains’ Tussock Grassland Balds showcasing some of the most studied balds in the world. The grassland balds in the Bunya Mountains are surrounded or adjacent to eucalypt forests and rainforests at both low and high altitudes on ridgetops, steep slopes and bottoms of valleys (Fensham & Fairfax, 1996) and have been described by Webb (1964) to ‘have a belt of one or two two E. ikinsoniana trees in width’ and is next to vine woodland, while the tall woodland of the ridge tops is transitional with vine woodland. The area varies largely in size, from 0. 1ha up to 44ha and is surrounded by both rainforest and eucalypt forest 52% of the time. 47% of the time they are surrounded by only rainforests and are only surrounded by eucalypt forests 7% of the time.
At lower altitudes combination Eucalyptus tereticornis forest and dry rainforest (vine thicket) surround the balds and in the upper lands it is notophyll rainforest and combinations of Eucalyptus eugenoides and Eucalyptus tereticornis forests, and (Fensham & Fairfax, 1996). It is believed that fire has played a huge role in the formation of the grassland balds and are maintained by fire. It was suspected that this ecosystem was once a rainforest that suffered severe fires and patches were destroyed during the Holocene, which was suspected to be followed by severe droughts (Butler, 2014).
Fire-Resistance and Fire-Suppression Thresholds
In 2012(a), Hoffman proposed a theory that the interaction fire plays in shaping savannas and forests is governed by two critical thresholds as well as interactions between climate, resources and species traits.
These two thresholds are as follows; the fire-resistance threshold is reached when individual trees have accumulated sufficient bark to avoid stem death, and the fire-suppression threshold is reached when an ecosystem has enough canopy cover to supress fire by excluding grasses. For either ecosystem to exceed either threshold depends on how long the fire-free intervals are the level of resources at the site. For a savanna to become a forest it must be at a high resource site and it will reach the suppression threshold quicker and vice versa. When considering the fire-free intervals, Hoffman (2012a) found the transition of a savanna into a forest would need a long fire free interval, and the opposite shift from forest to savanna would require a severe drought. This drought brings about intense fires within the savanna and cause substantial top kill of fully-grown trees, destroying the canopy cover and when left undisturbed the forest becomes flammable (MacDermott et al, 2016).
Sclerophyll Eucalypt Woodland (Savanna) Savanna ecosystems are defined by the coexistence of trees and grasses in the landscape (Sankaren et al, 2005). They are where trees and grasses interact forming a biome that is neither forest or grassland (Scholes & Archer, 1997). The tree’s thick bark is what allows them to persist and reproduce in an environment that is prone to fires (MacDermot et al, 2016). This bark feature provides insulation to the the cambial tissue and stem buds of the vegetation against the heat of the fires allowing the survival of the stems, surpassing the fire-resistance threshold (Hoffman, 2012a). The flammable understory growth of the savanna is promoted by the open canopy cover of the dominant species and the positive feedbacks between the trees and the frequency of the fires act together to stabilise the system and prevent it from converting back to a forest (MacDermot et al, 2016). During the day these microclimates tend to have a higher wind speed and a higher air temperature, and lower relative humidity and fuel moisture than forest (Hoffmann 2012b). Hoffman (2012b) found that the increased flammability was not due to the climate of these ecosystems being windier and drier but was actually from the presence of the flammable grasses.
There are many different types of rainforests and Webb (1958) describes that for a forest to be considered a rainforest it must be a closed forest with closely spaced trees that are arranged in several or more continuous stories, where the canopy level can be either even or uneven. The microclimate tends to be damp, cool and still, and grasses that are shade intolerant and flammable are not present, making them a fire-resistant habitat, completely opposite to the savanna (MacDermot, 2016). The presence of vegetation such as epiphytes and lianes, of complex flora provides further identification, and depending on the following criteria is what kind of forest it is.