And what are the implications?

The models created in my research suggest increased seedling elevations and densities are influenced by fire. Dendrochronological studies indicate that large fires in Victorian alpine environments have occurred every 50 – 100 years over the past 400 years. However, under climate change scenarios modeled to the year 2100, fire weather in south-eastern Australia has been predicted to become more frequent and more severe. As fire frequencies increase, the models predict that treeline elevation and density changes will quicken.

However, optimal tree ages that influence seedling establishment as calculated as part of my research were found to be between 24 and 42 years of age. If climate change subsequently reduces fire frequencies to less than (for example) every 20 years, then seedling numbers may actually fall. Repeat burn times that outpace Eucalyptus pauciflora reaching their optimal reproductive maturity age may subsequently have dramatic effects on the Victorian treeline.

The treeline ecotone changes predicted by my research have significant land management implications that should be considered. In particular, the Victorian treeline physiognomy is modeled to become denser under increasing fire frequency due to climate change. Researchers predict that there is a strong relationship between the density of a forest, and the ecosystem services it provides. Increased tree density has been shown to negatively impact habitat quality resulting in trees that are less suitable for food resources, foraging areas, and shelter.

These denser treelines have been shown to be more susceptible to wildfire due to providing additional fuel loads and imposing increased water stress. Fire management in the Australian Alps is recognised as an important strategy to promote natural diversity of the treeline ecotone by application of ecologically suitable fire regimes. Forest management techniques such as tree thinning has also shown to be beneficial to the long term forest structure by reducing fire risk whilst simultaneously improving forest ecosystem services and habitat quality.

Under increased fire frequency and severity scenarios, future changes to the treeline ecotone predicted by my research will require dedicated management to address habitat concerns and wildfire control to ensure alpine conservation is maintained.

So what do these results all mean?

The results of this research suggest that macro level influences are predominantly responsible for seedling establishment at the Victorian treeline following wildfire rather than micro level influences. The level of competition, the age and density of the trees, and low levels of litter are all significant factors that promote seedling recruitment and numbers. This finding supports field evidence gathered that suggests Eucalyptus pauciflora is not particularly physiologically limited at the treeline.

Treelines that are controlled by climatic influences such as low temperatures and shortened growing seasons have generally been shown to harbour trees that are much shorter than at slightly lower elevations. Pinis cembra have registered heights decreased by  40%  over the last 100m to the treeline, whilst it is suggested that trees growing at their climatic limit should not be able to grow much taller than 2-3m. Some transects surveyed in this research did in fact exhibit trees with krumholtz-like characteristics, however many also comprised much larger trees. This vast tree height difference between transects implies that micro level influences are not especially responsible for tree growth at the Victorian treeline.

Furthermore, research of treelines in the Kosciusko National Park in New South Wales concluded that there was no decrease in tree survival in artificially planted seedlings above the treeline. Saplings planted 200m above the treeline reached reproductive maturity suggesting that tolerance limits of Eucalyptus pauciflora extend beyond the current treeline. My research supports these findings, with seedlings found persisting in quadrats well above the treeline over ten years post wildfire.

 

My research suggests that seedling establishment is occurring above the treeline, however probability of occurrence and overall numbers are limited. Therefore, rapid treeline movement is not predicted at the Victorian treeline after infrequent wildfire. Eucalyptus pauciflora is regarded as having poor seed dispersal mechanisms that affects the distance from the tree that seedlings can germinate. Studies at Mt. Hotham showed that encroachment past the treeline was confined to areas of less than 5m from trees, and within overhanging tree cover.

Additionally, studies after the 2003 alpine fires in Kosciuszko National Park concluded that poor uphill seed dispersal was the reason why very few seedlings were located past the treeline, despite an abundance of seed at, and below the treeline.  Results from my research support the theory that Eucalyptus pauciflora has particularly poor uphill seed dispersion as demonstrated by the low seedling probability predictions for elevations above the treeline.

However, model probabilities were determined by including seedlings that were located significant distances above the treeline that were quite obviously a direct result of being close to outpost trees located adjacent to the transect boundary. This suggests that seedling establishment probabilities are even more tightly coupled to the distance from trees than the model may otherwise imply. This is not to say the model probabilities are incorrect, instead it must be recognised that the predicted probabilities are also influenced across their entire elevational range by outpost trees.

One theory of how Australian treelines may significantly increase in elevation is if outpost trees above the treeline can mature into stands from which downslope backfilling may occur. My research suggests that increased seedling establishment probabilities are associated with distance from the treeline (or trees), coupled with the time since last wildfire. As outlined above, outpost trees have already been associated with seedling establishment high above the treeline. It can therefore be envisaged that future fires that stimulate additional seedlings may encourage backfilling from these stands, resulting in a repositioning of future treelines.

 

Previous population age structure investigations of stands of Eucalyptus pauciflora has revealed that three large recruitments over the last 100 years had affected the woodland density structure. At least two of these pulses were suggested to be due to fire, which resulted in conversion of open woodland ecotones into dense mallee-like regrowth. The result of my research also proposes that fire enables a landscape conducive for increased seedling numbers in the sub alpine zone. However, this can only be facilitated in conjunction with the enabling factors of low competition and litter.

Recent studies of sub alpine Eucalyptus pauciflora at Mt. Buffalo have also associated a trend towards increased tree densities with increased fire frequencies. The study demonstrated that open forest structures were slowly being replaced by a crowded stand structure which was being driven by persistent fire. The models created by my research supports this premise, demonstrating that more frequent fires (for example, 20 year recurrence) would significantly increase the number of seedlings, leading to increased density and consequently probable forest structure change.

 

My research identifies litter and competition as the two main factors driving changes at the treeline after a fire disturbance. The effect of plant litter on seedling germination has been shown to have an overall negative effect on seedling establishment. This effect has also been reported in Australian alpine areas where leaf litter from trees and shrubs shaded and smothered the growth of seedlings. The models created in my research substantiate these findings by identifying that larger amounts of litter lead to reduced seedling probabilities.

Competition is acknowledged as a driving influence in shaping plant communities. Various studies have shown that seedling establishment near treelines may be dependent on the level of surrounding competition. Studies have shown that minimum separation distances between tussock grasses and seedlings were required for Eucalyptus pauciflora to flourish, with resource competition driving seedling mortality.

The field surveys I conducted showed that few seedlings were found in quadrats dominated by single elements, such as dense shrub or grass. The models created in my research recognise locations that display the greatest heterogeneity positively influence seedling establishment. My research therefore identifies alpine landscapes that display high levels of ecotone evenness allow for greatest seedling establishment.

Finally: My Research Results

The aim of my research was to investigate the Victorian treeline physiognomy (i.e. the general position, form and appearance of the treeline) after a major disturbance, building on a previous data set collected shortly after wildfire. This research investigated the key questions that might indicate permanent change over the medium to long term. It was assessed by evaluating seedling elevation, density and key factors driving seedling establishment at treeline ecotones burned during the 2003 wildfires, and then re-examining the specific locations initially measured.

Seedlings found in the Victorian Alps after fire are establishing themselves in the alpine zone at higher elevations than the current treeline, and in greater numbers in the subalpine zone in locations where competition is constrained, litter is minimised, and there are adequate amounts of original trees with girths of approximately 50cm. Subsequently, the models suggest that the Victorian treeline physiognomy has slightly changed due to the 2003 alpine fires.

The analysis of increase in treeline elevation after fire was limited by few seedlings establishing above the current treeline. Despite this, a comparison of the burnt and unburnt transect models indicates that seedling establishment probabilities above the treeline are greater after fire compared to locations long absent of fire. This implies an increase in seedlings at elevations higher than the treeline after fire. The model calculates that there is a 31% chance of seedling establishment two metres above the treeline, and an 18% chance five metres above the treeline. Even higher elevations estimate much smaller probabilities. Subsequently, it is likely that only single trees will sparingly establish within the tree species zone, and consequently become outpost trees.

An increase in treeline density after fire is supported by modelling, as seedling numbers are predicted to reach a maximum when fire is a recent occurrence. However, the Time Since Fire predictor variable was found to have 25% less influence than Litter, and 20% less influence than Evenness in maximising seedling numbers as calculated during sensitivity analysis. This suggests that fire may initially facilitate additional seedling numbers, however the magnitude of the density change is more dependent on the amount of competition in the area coupled with the quantity of litter cover. Locations that exhibit evenness across quadrat features are much more likely to support multiple seedlings, most likely due to a reduction in competition, and where a niche space can be realised. Secondly, a reduction in the amount of litter coverage within a location increases the number of seedlings. This is probably because litter acts as a barrier to seedling germination and emergence. In contrast, a decrease in litter allows greater seedling numbers to establish.

Modelling of factors that influence seedling establishment after fire determined that apart from competition and litter, the amount of trees and their age are significant macro level factors influencing subalpine seedling establishment. Seedling counts were highest due to significant influences of competition and litter, and this was verified by models which determined a 91% probability of seedling establishment when competition and litter was minimised.
Tree characteristics were found to be imperative to seedling establishment. Tree girths between 20cm and 50cm are the only circumferences that can result in seedling probabilities above 90%, with substantial probability decreases above and below these girths. By relating girth to the age of the tree, this outcome ultimately defines the optimal reproductive age of the stand to facilitate seedling establishment after fire. Using previously created models that evaluate the relationship of Eucalyptus pauciflora subsp. Niphophila, tree girths of 20cm to 50cm correspond to tree ages of approximately 24 years to 42 years.
Finally, the quadrat must have a sufficient amount of trees to facilitate seedling success. A seedling establishment probability of above 90% was modelled to occur in locations where tree coverage ranges between 45% and 50% of the total area of the quadrat.

The thesis is submitted!

Over a year of work involved in the 115 pages, 31 684 words, and 190 126 characters.

Accounting collides with Botany

I suppose this means we need at least 12 billion of these trees to offset humanities breathe…

Winter at UoM

We are in the midst of quite a cold week in Melbourne, where the maximum temperature has barely made it to 10 degrees C. Some autumn leaves remain around campus, providing for some lovely colour during the winter solstice period where the days are short and grey. The chilly air quickly forces you back inside, to your desk, and contemplating Bayesian network models in trying to predict the probability of a snowgum seedling existing above the treeline, and the factors that facilitated its survival.

Second Last Survey

Transect McKay South 1 was very hard to find – the heath was tall, and the walk was steep and slow. The transect was distinguishable by having a ribbon finger of trees, however it was not as densely packed as other sites. There was pampa to the right hand side of the transect, a watercourse running down the middle of the transect, and tall heath on the left hand side of the transect.

This was a degraded site with many invasive species and signs of deer. In the watercourse areas, there were many maturing willow trees seen, and they were starting to choke up some areas. In the pampa areas there were blackberries starting to sprout. Deer hoof marks and scat were seen throughout.

The striking factor about this transect was the amount of dead trees present. All down the right hand side of the transect (next to the pampa) were dead trees. They were the original old trees that had been burnt in the fires, and not survived. What does that say about the fire and conditions on this side of the transect? The dead trees ran all the way down the transect. The trees at this transect were also very tall – some running as high as 15m. This tends to show that these treelines are not at the trees’ physical limit.

The fieldwork continues

Two more days, two more transects. These ones were located past Nelse North – so quite a hike out from the Watchbed Creek gate – estimated at over 7km one way. Luckily the weather was fantastic as we walked out – high above the clouds in the valley below. The sites were quite simular in that they appeared quite xeric in nature, with low lying heath and grasslands dominating the intra-tree space. There was evidence that quite a few trees had burnt and died at these locations, and very few new saplings were counted. The really interesting feature of site Nelse North 1 was that a stand of trees ran down the left hand side of the transect, and the right hand side was clear with no new saplings evident. This was fascinating, because it demonstrated that there was no sideways movement of trees either, even below the treeline (apart from testing the usual upwards migration above the treeline). So it tends to reinforce the hypothesis that positive feedback dominates at short distances (at the tree edges) and negative feedback at larger distances (between the fingers).

A Long Weekend of Fieldwork

Luckily, I was able to get three other cheery volunteers to help me survey Mt Loch on the Labour Day long weekend. That was because transect Loch North 1 and 2 took almost 2 full days for four people to survey. The two transects are located side by side, with (for the first time) one of the original marker stakes used by Libby found at the base of one of the trees. Trees in these transects were situated in clumps of four or five trees with many short stems. There was a large amount of shale rock that was splintered into small pieces of rock.

The site espoused a harsh climate with stunted vegetation and evidence of new seedlings continually dying and re-sprouting from the roots. There were many new saplings in quadrants 11 and 12, although they were very small. Some had quite thick stems which looked to be dying, whereas new shoots of regrowth was coming thru from the base. It was also the first site with multiple original tree deaths with no lignotuber re-sprout. Possibly this site experiencs harsh winds and a longer winter season. A couple of original trees (that were unburnt in the 2003 fires) were located in the transects above the treeline with some new seedlings noticed in the vicinity of these trees.

As evidence by how different this site was, the average amount of trees found at the other transects was around 70, whereas, the trees surveyed at Loch North measured almost 250! The work was hard and long. But in the end most rewarding as we were lucky with most favorable weather conditions of light winds and blue skies by the last day.