Hyper-emergence in eucalypts

Tropical savanna in the Northern Territory
(Photo: John Morgan)
 

Sub-tropical rainforest, Dorrigo National Park
(Photo: John Morgan)

Q: What's the most obvious difference between a rainforest you'd find in the tropics and a eucalypt forest you might find in temperate areas?

The difference between these two is so obvious, most of us don't even notice it.

And it struck me recently that the explanations for it are also poorly understood.




Mountain Ash - Eucalyptus regnans
The tallest flowering tree on the planet.
(Photo: John Morgan, Wallaby Creek, 2006)

One of the most obvious differences is that Eucalyptus (and this would include some closely related genera such as Corymbia), are characteristically emergent trees of many vegetation types in Australia. Typically, they are considerably taller than all other species in these communities, often by an order of magnitude or two. As such, the trees tower over all the other plants. Rainforests, by contrast, tend not to have such obvious tiering or separation of the canopy and the mid-under storey, with lots of different species comprising the canopy. Emergent trees, when they do occur, do not tower over the rest of the plant community. This seems a really obvious distinction, but have you ever stopped and asked "why"?

A recent review sheds some light on why eucalypts grow so tall.

Tng et al. (2012) New Phytologist 196: 1001-1014, while focusing on the reasons for the evolution and occurrence of gigantic eucalypts (i.e. those species that grow >70 m tall, a fascinating topic for another post),  highlight that 'hyper-emergence' of eucalypts is a trait that extends across climates and clades and can be found in many Australian vegetation types such as heath, mallee, dry sclerophyll, subalpine and savannah communities. In some cases, some giant eucalypt trees are >60 m taller than the underlying canopy. The nearly ubiquitous nature of hyper-emergence in eucalypts suggests that this trait is an ancestral feature of the eucalypt lineages, and if this is true, would have arisen >60 million years ago!

In some respects, growing really tall above your competitors seems like a dumb idea. There's the risk of wind damage (or worse still, lightning strike!), tall trees need to sustain their own weight once bent, there's a lot of investment into woody structures that are diverted away from reproduction, and of course there is the not insubstantial problem of getting water up to great heights. So, why would trees - and eucalypts in particular - bother growing so tall? What are the benefits of far exceeding the heights of competitors?

Alpine Ash (E. delegatensis) are hyper-emergent
trees at high altitudes in the Australian Alps
(Photo: John Morgan, near Mt Hotham)

Tng et al. suggest a couple of reasons, but clearly this is an area where more research is needed.

First, tall trees tend to have very rapid early growth (relative to lots of other woody plants in the same community), allowing them to escape a 'fire trap' such that this growth would allow saplings (or resprouts) to reach heights that allow them to avoid the effects of high intensity ground fires. Such processes could apply to eucalypts in general (e.g. such as those of savannah and forest) but may be less applicable to eucalypts where fire return intervals are very long (decades to centuries). Indeed, in Mountain Ash forest, infrequent but high intensity canopy fires are actually necessary for stand replacement.

Second, most eucalypts are shade-intolerant. Intense intra- and inter-specific competition provides a strong selection pressure for tall growth, hence allowing them to overtop their neighbours. By overtopping slower growing, shade-tolerant trees, often quickly after disturbance, early reproductive maturity can be assured.

While both explanations undoubtedly play a role, they are not particularly satisfactory answers when asking the question "why grow so much taller than all the other plants in your community"? I think this is a fascinating topic that really needs a bit more thought. So next time you're in the bush, marvel at the eucalypts that undoubtedly overtop all else and ponder why this might be so.


Further reading
Tng et al. (2012) Giant eucalypts - globally unique fire-adapted rain-forest trees? New Phytologist 196: 1001-1014.
Larjavaara (2014) The world's tallest trees grow in thermally similar climates. New Phytologist 202: 344-349.





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Endemic plant species on restricted soil types: 'early victims' or 'hardy survivors' of climate change?

One of the greatest challenges that land managers face today is anticipating how climate change will affect the diversity and composition of ecological communities to develop effective strategies for adaptation and mitigation. The direct effects of climate change on species via changes in temperature and precipitation have been the focus of many studies. Many conclude that altitudinal and latitudinal shifts in distribution will be necessary to survive the impacts of predicted climate change. 

 

Little attention, however, has been given to how plant species on 'restricted' soil (i.e. very infertile) will respond to climate change. Here, suitable habitats for such species are patchily distributed, and the dispersal distances required to move to newly suitable habitat are large, making successful migration unlikely. Are species confined to low-nutrient soils, which may reflect their tolerance of such conditions and intolerance of other biotic factors such as competition, particularly vulnerable to climate change?  Some studies suggest that soil specialists may be at less risk than species on 'normal' soils due to their stress-tolerant functional traits, but there is also contrary evidence.

 



Conceptual model of how present and future climate changes will affect (a) soil generalist and (b) soil specialist species differentially. Blue represents the current range of a species, Green is the future range of a species and Red is the area of overlap between the current and future ranges. Patchy suitable habitat for the soil specialist creates fewer colonization opportunities among the current and future ranges than for the soil generalist with contiguous suitable habitat.

Image: Damschen et al. (2012) Journal of Ecology 100: 1122-1130.



Plant communities on low-nutrient soils have two distinctive attributes that may cause them to respond uniquely to climate change, and I don't think we've really thought about these factors as they pertain to climate change responses.

 

First, they are often found in discrete areas making them more spatially isolated from one another than species on ‘normal’ soils that tend to be more contiguous. Hence, this spatial isolation may make it much more difficult for soil-specialist species to successfully migrate under climate change because suitable soils are isolated (or embedded in a hostile matrix to borrow an analogy from landscape ecology).

 

Second, because these species are on unproductive substrates, they may differ from communities on ‘normal soils’ in terms of limiting resources, functional traits, and the relative importance of disturbance, competition and other ecological processes. Plants in these special soil habitats often have traits associated with tolerance of drought and nutrient-limitation [e.g. small stature, low-specific leaf area (SLA), high allocation to roots relative to shoots] because nutrient availability is limited, water can be scarce, and soils may have additional unusual chemistries (e.g. particularly acidic pH). Special soil communities are often more strongly water-limited than others; therefore, they may be especially responsive to changes in available precipitation. On the other hand, because plants on special soils already have adaptations for stress tolerance, they may be particularly well-suited to withstand climatic changes.



Prostanthera galbraithiae (Wellington Mint-bush) is an erect to

spreading small shrub and is restricted to sandy soils of the Holey Plains State Park, Victoria

Asking questions about the fate of soil specialists in response to climate change is important for plant species such as the Wellington Mint Bush (Prostanthera galbraithiae), a vulnerable species in Australia. The species is endemic to a very small area of the Gippsland region of Victoria, restricted to sandy podzol soils typically low in macronutrients (especially N, P and K) and subject to long periods of soil moisture stress. Importantly, the sandy, nutrient-poor soils where Wellington Mint-bush occurs are restricted and embedded in a matrix of clay-based, more fertile soil types.

What are the potential responses to climate change of endemic plant species like the Wellington Mint-bush when soil factors appear to limit their current distribution?



To understand the role of climate factors on the fitness of soil specialists, it is necessary to compare the plasticity to water and temperature stress of the endemic Mint-bush to that of more widepread species (such as Prostanthera lasianthos, P. rotundifolia) to test the hypothesis that soil specialists are already well-adapted to environmental stress and hence, they may be particularly well-adapted to withstand climatic changes. Such studies, and life history study more generally, allows us to understand how plants respond to stress, and provides a powerful tool for making informed decisions about which species may need active intervention to ensure their persistence.

For species found in highly variable environments (such as those areas with frequent but unpredictable drought) where soil factors accentuate the magnitude of drought, a history of climate variability might confer rather general plasticity or tolerance of  future climate variation. Coupled with having to persist on infertile soils, some species might show more resilience to change in climate than those species from much more stable/predictable climates (i.e. where factors such as rainfall are more evenly distributed through the year) because they already have to exhibit 'stress-tolerance' to survive their specialised soils. Hence, it would be useful to ask whether range-restricted, soil specialists like the Wellington Mint-bush are more plastic in their response to climate variability than widespread species (where local adaptation might have developed – i.e. the ‘provenance’ concept), or are they simply better adapted to environmental stress.  

 

Managing sites for biodiversity based on plot-level data

A simple, but powerful realisation hit me the other day while I was away on a field course with my 3rd Year Botany students. We went to Cape Conran in the east of Victoria to study fire and diversity relationships in sandplain heathlands. We set the students some questions for which they had to design a research project. They did a great job, so much so that it made me think about how we use scientific data to inform conservation management actions.

One of the questions we asked was: what is the relationship between species number and time-since-last-fire? This is a very 'old' question in ecology, with lots of evidence that species number initially increases after fire, then plateaus off, before declining with increasing time. As such, it is recommended that to maintain diversity in heathlands, they need to be burnt at frequent intervals. Reading the literature, it is evident that fire return intervals of 10-25 years are recommended.



Botanists often quantify the number of species in quadrats of fixed size.
This is SPECIES DENSITY. Often only one or two quadrats per 'site' are sampled. An individual site here is
a heathland with a known date of  Year-Since-Last-Fire

 

Land managers take advice about the ways in which to manage biodiversity from scientists, so it is important that the advice we give is based on solid data from well-executed studies.

At Cape Conran, we asked whether species number declines with long fire-free intervals to test the well-established understanding of heathland ecology. But we decided to take a different approach to that which is usually employed.

In many of the 'fire in heathland' papers I read, I see that botanists compare the number of species in a quadrat (this is known as Species Density) across sites with known fire histories. Importantly, in these studies it is usual that the same sized quadrat is employed across sites to generate Species Density estimates. But this might be a flawed approach. Imagine you set up a 4 x 4 m quadrat in recently-burnt heathland. The plants are very small and many of them can 'pack' into the quadrat. As plants age, they become bigger and hence, it is likely that fewer of them will fit into your 4 x 4 m quadrat. In long-unburnt heathland, heath species are as large as they can grow and even fewer will fit into your (what now seems small) quadrat.

So, while Species Density might inevitably decline with time-since-fire in your small quadrat, this is not the same as saying Species Richness declines across the site. Species Richness is the number of species you find in a defined site - in our case, a heathland - and it is this scale that a manager manages. Hence, is there a mismatch between the scale of evidence that botanists have used to assess fire impacts (the plot level) and what managers actually need to know - how do I manage heathland to maintain their diversity?

To answer this question, we set out to sample Species Richness across sites that contrasted in time-since-fire. We used lots of small 1 m2 quadrats (n = 25) at each of three sites (that differed in time-since-fire: recent to long-unburnt) rather than relying on one or two big quadrats as is usually done. These were spread out across each heathland and all species were recorded. The results were VERY surprising!




Here, students are also quantifying species density
in smaller quadrats, but doing lots
of them across a site

As expected, Species Density was highest in the most recently-burned areas compared to long-unburnt areas (15 vs. 8 species per m2). But then it got interesting. The total number of species observed in each site across our 25 quadrats was the same - 39 (all of them native). So, despite there being many more species in the small quadrats in the recently burned areas, unburnt areas supported the same number of species across the site. Yes, they were 'rarer' and some were different species, but if the aim was to maintain species richness at the Site Level, the level at which managers would be most interested, then the need for fire was not so clear.

And, when using some fancy statistical techniques to assess how much more biodiversity would be present if we had sampled more quadrats (the procedure is called 'rarefaction'), we actually expected more species in the long-unburnt heathland than the recently-burned heathland (60 vs. 41).

It became apparent that frequent fire might actually hold-up succession of sandplain heathland to more complex, diverse systems rather than maintain it. Frequent fire would likely knock out species that have long primary juvenile periods or that take time to colonise after fire. Far from allowing such species to be members of the community, frequent fire might reduce spatial heterogeneity and hence, reduce habitat complexity. Such thinking hasn't been applied to heathland ecology before in south-east Australia and while it is too early to suggest we change our current practices (of frequent fire), this student research hints that maybe a rethink is in order.

And, as I said at the start, it highlights that the relationship between Plot-level data and Site-level responses are not always as clear-cut as might be imagined. Stay tuned........

Thanks to Luke O'Loughlin, David Cameron and Botany students Holly Fiske, Nicole Baboucek, Tony Hampton, Jasmine Thum, Darragh O'Sullivan and Kate McWhinney for collecting and interpreting this interesting dataset from the annual Botany 3 undergraduate trip to Cape Conran.

New study points to the global significance of the Plains-wanderer

Can you spot Australia's most unique bird?
Terrick Terrick National Park
Photo: John Morgan

The Plains-wanderer has for some time been known to be a member of Australia’s ancient avifauna and its nearest, albeit distant, relatives are seedsnipe from South America.  It is the sole member of a Family of birds called the Pedionomidae. It's a species typically confined to native grassland habitats in eastern Australia and, unfortunately, one of the most endangered species of those grasslands. It should be a flagship for conservation and new research tells us why!

Recently, Jetz et al. (2014) published a major review of the world’s 9,993 recognised bird species to determine which species we can least afford to lose in the current extinction crisis if maximum global phylogenetic diversity is to be maintained.  Phylogenetic diversity is a measure of biodiversity which incorporates phylogenetic difference between species and phylogenetic analyses have become essential to research on the evolutionary tree of life. The concept of phylogenetic diversity has been rapidly adopted in conservation planning.

Jetz et al. (2014) developed a hierarchy based on how isolated a species is on the phylogenetic tree which they termed ‘evolutionary distinctness’.  They also included global geographic range, and global endangerment in their metrics.  The summary metric that Jetz et al. (2014) used to rank the world’s birds combines evolutionary distinctness and extinction risk. 

By their calculation, the Plains-wanderer is ranked:
 #1 among Australian birds and #4 in the world!!

As such, these analyses highlight we can ill-afford to lose the species, yet current data suggest that significant declines are being observed, and it's not entirely clear why.

The two strongholds of the Plains-wanderer are the semi-arid (or xeric) native grasslands of the Riverina region of NSW and Victoria’s Northern Plains.  Monitoring in NSW during 2001-2012 has found that the population size has declined by 75% during droughts, then recovered slightly during benign years, and was then recorded at record low levels during the very wet years of 2011-12.  The population has remained at very low levels for over a decade, and this is cause for considerable concern. In Victoria there has been monitoring on Terrick Terrick NP and nearby private land over five years (2010-14).  Numbers declined by >90% during 2011-12 in the wet years (perhaps because breeding was negatively affected, while thickening of grasslands has reduced occupyable habitat) and the numbers have remained at historically low levels.

If ever there was a need to monitor the dynamics of a species of conservation concern, whilst also monitoring its habitat suitability and key determinants of mortality risk (e.g. predation),  then the Plains-wanderer would seem an essential candidate species. Good, basic scientific research is needed to answer simple questions: how long do birds live; are population dynamics cyclic; can suitable habitat be successfully created from scatch? In some respects, a metric of the success of grassland conservation and management will be that species like the Plains-wanderers can be maintained in their habitat, and that their numbers grow rather than decline.

Thanks to David Baker-Gabb for alerting me to the evolutionary distinctiveness of the Plains Wanderer, and for providing information on the population trends of this species.

Reference

Jetz, W, Thomas, G H, Joy, J B, Redding, D W, Haartmann, K and Mooers, A.  2014.  Global distribution and conservation of evolutionary distinctness in birds.  Current Biology (2014), http://dx.doi.org/10.1016/j.cub.2014.03.011.

A new population curve for prehistoric Australia

The arrival of Aboriginal people in Australia marked an important change in the continent's ecology. The loss of the mega-fauna and a rise in the use of fire are common themes that resonate when discussing the role of Aboriginal people in shaping the nature of Australia.

Up until now, most of the discussion has been about when aboriginal people arrived. The numbers fluctuate between 40,000 and 100,000 yrs ago, with a convergence of opinion in recent years (because of better carbon dating techniques) that it was probably about 52,000 yrs BP.

An interesting paper by Williams take this insight one step further.

Using radiocarbon dating techniques on >1700 sites from all over Australia, Williams has tried to reconstruct/assess population growth rates in Australia up until the time of European contact.

Using some modelling assumptions about the initial founder effect (that is, how many people first colonised), he interprets three key things:

1) Australia was settled by thousands, not just a handful, of humans, suggesting deliberate rather than accidental colonisation of the continent. The research suggests that it probably would have taken 1000 to 3000 people to reach the numbers of Aboriginal people observed at time of European contact.
2) the population size grew very slowly - constrained by glacial periods. It wasn't until the Holocene (from about 10,000 yrs BP) that the population grew substantially. The study shows during the glacial maximum 18,000 to 21,000 years ago, the population fell dramatically. The data suggests 60% of the population was lost during this time, a period of extreme dry and cold. It took 9000 years for the population to recover to the same levels.
3) the maximum population size before European settlement was approx. 1.2 M people (assuming a founder population of 1000-2000). This occurred only 500 years ago.



Distribution of archaeological sites contributing radiocarbon data to the study by Williams.
You can see there is a pretty good coverage across Australia.



You can see this population reconstruction clearly here in this figure from the paper. The effects of 'founder size' (how many people might have first colonised Australia) has a big effect on population size at the time of European contact, as does climate. The two graphs show the outcomes based on different model assumptions - they give pretty much the same results. They suggest that population growth really took off at about 12,000 yrs BP; this corresponds with increasing climate stability

This paper (along with others - see here) hints that the big changes in vegetation and mega-fauna extinctions observed in the last 50,000 yrs were probably driven by climate. Landscape burning by Aboriginal people has been linked to significant changes in the geographical range and demographic structure of many vegetation types but there is now an emerging lack of congruence between human activity and fire records during the period 20-40 kya. During a period of consistently low human populations, as posited by Williams, it is difficult to reconcile how Aboriginal people could have had the profound impacts that have been speculated by many.

I can't vouch for the veracity of the work reported by Williams, but it provides interesting food for thought when interpreting the recent history of Australia.


References
Williams (2013) A new population curve for prehistoric Australia. Proceedings of the Royal Society B 280, 20130486.

Sakaguchi et al. (2013) Climate, not Aboriginal landscape burning, controlled the historical demography and distribution of fire-sensitive conifer populations across Australia. Proceedings of the Royal Society B 280: 20132182