Dense woody vegetation: thickets

As a side project to my PhD research, I have spent some considerable time over the last two and half years researching dense woody vegetation, or ‘thickets’. There has been recent interest in this area due to their increased occurrence in the landscape. This post gives a brief overview of dense stands.

What is a dense stand (thicket)?

A dense stand usually refers to patch of similar aged woody plants (often eucalypt trees) that are growing much more densely than would normally occur in an undisturbed ‘natural’ setting.

Dense eucalypt stand in central Victoria

Dense eucalypt stand in central Victoria

How do they arise?

There are four things usually required to create a dense stand:

  1. Space – cleared land free from heavy plant competition
  2. Source – a seed source such as a paddock tree or roadside vegetation
  3. Freedom – no/little grazing by insects or mammals
  4. Rain – sufficient rainfall to trigger a mass germination event

These are precisely the conditions that we see on agricultural pastures. Grazing animals will usually eat seedlings and so these stands won’t occur in heavily stocked pastures, but if the stock are removed or the numbers reduced and rainfall stimulates the mass recruitment of seedlings then these are ideal conditions for creation of dense stands. Some great examples of this were recently posted by Ian Lunt.

Dense stands are becoming more common in the landscape due to changes in land management practices. Some landholders are grazing less, some are fencing off parts of properties for environmental management (e.g. fencing off creeks), some are leaving and being replaced by ‘lifestyle’ farmers who are less focused on intensive agriculture.

A mass recruitment event creating a thicket around a large paddock tree (Eucalyptus microcarpa - Grey Box).

A mass recruitment event creating a thicket around a large paddock tree (Eucalyptus microcarpa – Grey Box).

What effects do they have? Are they bad?

The two primary reasons why land managers care about dense stands are: reduced tree growth, and suppressed understorey vegetation, both as a result of competition for resources.

Trees in dense stands grow much slower, have fewer large boughs, have smaller canopies, and produce fewer flowers later. Reduced tree growth is therefore an issue for the provision of resources by large trees, i.e. hollows, flower (nectar) production, and litter/logs.

Suppression of the understorey vegetation is a serious concern for provision of habitat, food, soil stability and structure, and amenity. If the understorey is suppressed for long enough, long-term detrimental effects can be caused by erosion of topsoil (not stabilised by plants) and loss of the soil seed bank (loss of recovery potential).

Naturally, for both of these issues with dense stands, there is a continuous scale of effects. ‘Some’ reduced tree or understorey growth from open stands might be ok, whereas ‘lots’ from very dense stands would be highly undesirable. Different environments will also respond differently to these stands. It is therefore very difficult to know how much is too much, how much is bad, and when we might want to do something about it. Studies that attempt to use good quality, relatively undisturbed, examples of vegetation to estimate what the vegetation ‘should’ be like in these systems (e.g. Phil Gibbons et al. in 2008 and 2010), can be useful for setting targets for management.

Suppression of the understorey vegetation beneath dense eucalypts

Suppression of the understorey vegetation beneath dense eucalypts

Do they self thin? Is there a need for management?

This is a great question and the answer comes down to time. Yes, dense stands do self thin, as competition for resources becomes so intense that some individuals die off. As some mortality occurs those resources are taken up by the remaining individuals. Eventually, this is likely to result in a more natural vegetation state, however this may take many decades or even centuries to occur. Management (such as thinning) is likely to be important if we want to achieve some desired state or attributes sooner.

In future posts on this topic I will present some of the research I have been doing with some people from the QAECO lab. Much of which focuses on the understorey suppression aspects of dense stands, and how it responds to management. Stay tuned…

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Evaluating riparian vegetation condition

After a brief interlude to write posts about some of the things I just find interesting, I am returning to some work from my thesis.

In post #1 I described the effect of livestock grazing on the cover of different vegetation life forms. Here I focus on the amount of cover at these sites, and if it is a good amount or not.

Vegetation change in itself is certainly not a bad thing at all. Vegetation communities are constantly changing and it would be ridiculous to think that we could ever manage a completely static environment. Change becomes a negative when the vegetation is no longer able to provide the functions that are expected of it, e.g. habitat or food for fauna. Negative changes such as this occur naturally in the landscape from events such as fires, floods, droughts, etc. and there is little we can do to influence these naturally occurring events. Our primary concern, as researchers and managers are the negative effects that caused by humans.

We can only tell if vegetation has been negatively impacted if we know what relative condition it is in. Previously I wrote a post about what is vegetation condition, which described the use of benchmarks against structured sampling methods to evaluate how different the vegetation of a given site is from what it ‘should’ be in the absence of human disturbance. I am referring to the Victorian government’s standardised process known as Vegetation Quality Assessment. Accompanying the standard assessment method and the calculation process of condition scores, is the set of benchmark attributes for each vegetation type (or specifically, Ecological Vegetation Class, EVC) in Victoria.

So if these benchmarks of vegetation condition are freely available, can I use them to evaluate the condition of my study sites, even though I used a different survey method? Well, yes, and I did. Using a different survey method makes it impossible to generate the condition score based on the standardised methods, but I can still use the benchmark attributes to asses specific native vegetation attributes of my sites. For this post I will focus on vegetation cover.

This site looks ok but does it have the amount and diversity of vegetation that would be expected of a high quality site?

This site looks ok but does it have the amount and diversity of vegetation that would be expected of a high quality site? Does it have nearly enough of some things and far too few of other things?

To determine the condition of the native vegetation attributes at my sites I needed to do 3 things:

1. Determine the cover of each attribute for each site

2. Determine the vegetation type (EVC) for each site

3. Compare observed cover values against benchmark values

COVER: So firstly I needed to find out the cover of different attributes within my sites. I surveyed 180 sites along creeks in northern Victoria (for some more background on my study look here, and more on survey methods look here). I separated each site into an inner (near the creek) and outer (away from the creek) zone to explore differential effects  with distance from the water. The results for the cover of different life forms, bare ground, litter and tree cover are shown below. The spread of data is similar in the inner and outer zones, but there were commonly large differences between zones for a specific site (as indicated by lines). This information is interesting in itself and tells us that average cover for many of these life forms was low – but is that ok? How much cover should there be? Comparing these values against the benchmarks can help us evaluate this.

Percentage cover of ground layer, native understorey and midstorey life forms, and tree canopy across all sites. Each point indicates cover within a site, where closed circles (•) are cover within the inner zone and open circles (ο) within the outer zone. Black plus signs indicate the median values. Grey lines connect cover from the inner and outer zone within the same site. Points without connecting lines occur for sites with insufficient data in either zone to generate a cover estimate. Data are presented on a logit scale. I added 0.0025 to scores of zero, equating to one hit every 400 points, to enable plotting on the logit scale. Vertical lines are medians. Note change of scale for litter and tree cover.

Percentage cover of ground layer, native understorey and midstorey life forms, and tree canopy across all sites. Each point indicates cover within a site, where closed circles (•) are cover within the inner zone and open circles (ο) within the outer zone. Black plus signs indicate the median values. Grey lines connect cover from the inner and outer zone within the same site. Points without connecting lines occur for sites with insufficient data in either zone to generate a cover estimate. Data are presented on a logit scale. I added 0.0025 to scores of zero, equating to one hit every 400 points, to enable plotting on the logit scale. Vertical lines are medians. Note change of scale for litter and tree cover.

BENCHMARKS: The next step in comparing vegetation to the benchmarks is to determine vegetation type or EVC for each site. Normally this would be done on delimited patches (‘habitat zones’) of similar vegetation but in this study I determined an EVC for each zone (inner and outer) within each site, based on statewide EVC mapping. If you want to know more about how I did this just let me know.

COMPARISON: Benchmark Comparisons were made by subtracting the benchmark cover value from the observed cover from my surveys. So anything below ‘0’ meant that the cover was below the benchmark level, i.e. below what it ‘should’ be in a good quality site. Small shrubs, litter, and tree cover were the only attributes that were generally at or above the benchmark levels, while all others were below. Medium tufted graminoids (such as Austrostipa scabra) and medium herbs (such as Calocephalus citreus) were by far the worst performing. Interestingly, despite the differences in cover between inner and outer zones at many sites, the difference to benchmark levels was roughly equivalent across attributes. This suggests that the different benchmark levels for each zone were appropriately accounting for these differences.

Difference in percentage point cover between observed and EVC benchmark cover of ground layer, native understorey and midstorey life forms, and tree canopy cover. Box plots show the range of cover differences across all sites. White boxes indicate inner zone covers; grey boxes indicate outer zone covers. Boxes indicate the median and inter-quartile range and whiskers extend to the most extreme data point which is no more than 1.5 times the inter-quartile range from the box.

Difference in percentage point cover between observed and EVC benchmark cover of ground layer, native understorey and midstorey life forms, and tree canopy cover. Box plots show the range of cover differences across all sites. White boxes indicate inner zone covers; grey boxes indicate outer zone covers. Boxes indicate the median and inter-quartile range and whiskers extend to the most extreme data point which is no more than 1.5 times the inter-quartile range from the box.

It is clear that some of these vegetation attributes are well below the desired levels within my study area. This information can help inform management of the current status of these sites, suggest which life forms should be targeted, and the amount of change that might be required to restore the sites.

This is just a small part of the picture and it needs to be evaluated in the context of land management at each site, but I think it is a neat use of existing resources to help evaluate vegetation condition at a site to inform management.

Cheers.

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Birds of Albert Park: Quiz

Well I thought I’d go out on a limb (excuse the pun) and see if anyone is paying any attention to these bird posts by seeking some audience participation.

Many of the birds in Albert Park are quite striking, or common, or large; all attributes that makes their identification relatively straight forward. Venturing out to look for birds you are undoubtedly going to come across what many would call a ‘little brown bird’, and Albert Park has its own.

These little birds were very agile flyers, and were in a large group of about 8-10 individuals chasing each other around the trees when I saw them in May this year.

Anyone know what they are called?

'Little Brown Birds'

‘Little Brown Birds’

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Birds of Albert Park: Part 2

This is the final collection of bird photos from this Albert Park series; now we are moving from the lake to the rest of the park. As explained in the introduction, there are many different vegetation types within the park that support many different birds. Some birds spend the majority of their time digging in the grass, while others are most often in the trees. The flowering eucalypts are a great food source for many birds and they are a great place to target your twitching.

I will break this post up into sections: common native species, parrots, and exotic species.

COMMON NATIVE BIRDS

There are many native birds within Albert Park, many of which we might not consider native, such Crested Pigeons and Magpie Larks. Another good example is the native Noisy Miner (Manorina melanocephala), although considered a pest in many places with large populations, it should not to be confused with the introduced Common Myna (Sturnus tristis). All of these birds are common, but some are nevertheless very difficult to photograph, such as the Welcome Swallow (Hirundo neoxena) which is often in flight and moves so quickly.

Welcome Swallow (Hirundo neoxena)

Welcome Swallow (Hirundo neoxena)

PARROTS

These are the birds from within the order Psittaciformes, which includes ‘true’ parrots and Cockatoos. The parrots spend much of their time in the trees and can often be seen in flowering eucalypts. By far the most common of these are the Rainbow Lorikeets  (Trichoglossus haematodus), which form large colonies and can be deafening when they are all calling in the same tree. The Cockatoos and Galahs in the park are often seen browsing through the lawns for tubers, roots and insects.

Eastern Rosella (Platycercus eximius)

Eastern Rosella (Platycercus eximius)

Rainbow Lorikeet (Trichoglossus haematodus)

Rainbow Lorikeet (Trichoglossus haematodus)

Musk Lorikeet (Glossopsitta concinna)

Musk Lorikeet (Glossopsitta concinna)

Sulphur-crested Cockatoo (Cacatua galerita)

Sulphur-crested Cockatoo (Cacatua galerita)

Long-billed Corella (Cacatua tenuirostris)

Long-billed Corella (Cacatua tenuirostris)

Galah (Eolophus roseicapillus)

Galah (Eolophus roseicapillus)

EXOTIC SPECIES

There are few introduced species in the park, but they are common and very familiar. Two of them are doves or pigeons, the others are in the genus of starlings.

So this is the final collection of birds from my photographic adventures in Albert Park. I will continue to take the camera down and will update these posts now and then when I get more photos. I may have one final post, a quiz, for next week. Stay tuned.

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Birds of Albert Park: Part 1

WATER BIRDS of Albert Park

So following on from the introduction last week, here is the first batch of photos.

And where better to start this collection of images than birds spending their time on or around the lake itself. The lake is not a continuous body of water with uniform sides; there are two small vegetated islands with sloping banks (one heavily treed: Gunn Island), and two fenced off sections with shallow water or exposed mud, and patches of dense reedy vegetation (Phragmites, Juncus, Carex). Although not a pristine natural environment, this diversity of habitat types supports a wide range of native and introduced species. There are two species I will focus on here, one for its iconic relationship with the park, and the other for its population size.

The Black Swan (Cygnus atratus) is the avian icon of Albert Park lake. They are large birds (~4-9kg, 2m wingspan). Many of the swans in the Park have identification collars on them, which is done for research through the University of Melbourne by Raoul Moulder’s research group. They breed once a year and chicks (cygnets) are a common site on the lake.

Black Swan (Cygnus atratus)

Black Swan (Cygnus atratus)

The Eurasian Coot (Fulica atra) is the most numerous resident of the lake. These are small birds (~0.5-1kg) with partially-webbed feet. They spend much of their time grazing on the lawn surrounding the lake in large groups. You are guaranteed to see these guys if you ever visit the lake. It is distinguished from the similar species, the Dusky Moorhen (Gallinula tenebrosa), by the colour of the beak (white versus red and yellow).

Eurasian Coot (Fulica atra)

Eurasian Coot (Fulica atra)

The rest of these photos are a collection of birds that feed in, on, and around the lake. Some are permanent residents and others more transient. The Spoonbill in particular was mostly solitary and only occasionally present at the lake, I was very pleased to be able to eventually get it.

Stay tuned for some terrestrial birds next week.

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Birds of Albert Park: Introduction

Although my heart still lies with plants, my first bird post was a roaring success (for my standards) so I thought I’d do some more. This time my focus is on Albert Park. I live a stone’s throw away from the park and have spent quite a bit a time walking, running, riding, and driving through it. Together with the lake, many trees, and large grassy areas, it is recreation, food source, and home to many bird species.

Albert Park is a large area (225ha) of public open space in the heart of Melbourne, Victoria. It was opened in 1864 and as such contains many old European trees, such as Elms and Poplars, but has an increasing representation of native and endemic species. It has a golf course, driving range, many sports fields, and its key feature is the lake, around which the Australian F1 Grand Prix is run each year. The park is managed by Parks Victoria and receives approximately 5 million visitors per year.

One of many ovals within the park. A mix of exotic and native tree species.

One of many ovals within the park. A mix of exotic and native tree species.

Two sections of the park, along the light rail track and off St Kilda Junction, have been fully revegetated with native species. Both of these sections contain a much more diverse understorey and midstorey structure than the rest of the park which is mostly trees and lawn. This creates additional habitats that are exploited by different species and it is great that they are maintained.

Native revegetation site near St Kilda Junction.

Native revegetation site near St Kilda Junction.

The lake itself is 49ha and approximately 5km around. It was originally a natural lagoon (or more accurately part of a lagoon system) that was dredged and shaped into the lake by 1880. It is now home to recreational sailing, rowing and fishing – and HEAPS of birds.

Albert Park Lake

Albert Park Lake

Over the past couple of months I have taken my camera down to the park to try to photograph as many of the avian residents of Albert Park as I can. I am sure there are many more that I have not yet captured but this is a start. I will update these posts as I take more photos.

There are far too many birds for a single post so I have decided to break this post into sections. This week is just the introduction, the first collection of birds will be posted next week, but here are some teasers to get you started.

Rainbow lorikeets

Rainbow Lorikeets (Trichoglossus haematodus)

Eurasian coots

Eurasian Coots (Fulica atra)

Long-billed Corellas on a light pole

Long-billed Corellas (Cacatua tenuirostris) on a light pole

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Eucalyptus, Corymbia and Angophora = eucalypts

Q: When is a eucalypt not Eucalyptus? A: When it is Corymbia or Angophora

This post was inspired by an ID mistake I made recently. Hopefully I have now learned to not do the same in the future.

Most people are very familiar with eucalypt trees. In fact, most Australians would see one every day.  They occur in almost all vegetation types across the continent in coastal, arid, tropical and alpine environments. We all have a fairly good idea of what a eucalypt looks like despite the vast array of growth forms, bark types and fruit shapes and sizes, but very few of us have a good handle on their names.  If you asked the average person on the street to tell you the name of a eucalypt, many would start by suggesting it was called Eucalyptus something-or-other, and with over 700 species they will most likely be right.

However there are also a number of trees that are commonly refered to as eucalypts, and they look like Eucalyptus for the most part, but they are not. Technically there are 7 genera that occur within the tribe Eucalypteae (Wilson et al. 2005), however I am focusing on the two most commonly observed and misidentified genera: Corymbia and Angophora. These genera are both in the same family (i.e. Myrtaceae) and tribe (i.e. Eucalypteae) as Eucalyptus  and are indeed very similar, but they have a few distinct spotting characters that can help you to tell them apart.

Angophora: This genus contains only 10 species and are often difficult to distinguish from Eucalyptus. Perhaps the most common distinguishing features are 1) the lack of an operculum (or cap) on the fruits and ribs along the fruits, and 2) the occurrence of opposite leaves on mature individuals. Since the operculum in Eucalyptus species is formed from the petals and or sepals, Angophora species are one of few eucalypts to have visible petals. Opposite leaves occur in many Eucalyptus species but typically only in the juvenile stages.

Angophora subvelutina (Rough-barked Apple) young fruits. Note the absence of an operculum.

Angophora subvelutina (Rough-barked Apple) young fruits. Note the absence of an operculum, the ribs along the capsule, and the pale petals.

Corymbia: These are mostly ‘gums’ due to the bark type, and there are currently 113 species in the genus (Parra-O et al. 2009). Their primary distinguishing character is the inflorescence, which occurs as a ‘corymb’ i.e. a compound umbel, similar to the inflorescence commonly seen in the family Apiaceae. They possess an operculum on the fruits, as do Eucalyptus species. Some common species planted in suburban areas are the Red-flowering Gum, Lemon-scented Gum and the Spotted Gum.

Corymbia ficifolia (Red-flowering Gum). Note the corymb arrangement of the inflorescence. PHOTO: Bidgee (Wikimedia Commons)

Corymbia ficifolia (Red-flowering Gum). Note the corymb arrangement of the inflorescence. PHOTO: Bidgee (Wikimedia Commons)

References:

Parra-O C. et al. (2009). Phylogeny, major clades and infrageneric classification of Corymbia (Myrtaceae), based on nuclear ribosomal DNA and morphology. Australian Systematic Botany, 22, 384–399

Wilson P. et al. (2005). Relationships within Myrtaceae sensu lato based on a matK phylogeny. Plant Systematics and Evolution, 251: 3–19

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