Sometimes in urban forests here, we have near monocultures of Periwinkle (Vinca minor).  Some tree seedlings seem to manage, but there's little else in there. I hope that helps

There are many examples, but here are two.

1. European buckthorn (Rhamnus cathartica) is a deciduous understory shrub to small multi-stemmed tree with a spreading crown. It leafs out early shading the competition. Rhamnus cathartica is problematic in many parts of North America.

http://www.dnr.state.mn.us/invasives/terrestrialplants/woody/buckthorn/index.html

http://dnr.wi.gov/topic/invasives/fact/commonbuckthorn.html

2. Coral ardisia (Ardisia crenata) is a small shrub that can form dense communities in deep shade in Florida forests. 

http://plants.ifas.ufl.edu/node/42 (see the short video)

I assume you ask for trees/shrubs? E.g. Miconia calvescens in the Pacific islands (Hawaii, Tahiti). Leucaena leucocephala in Australia and other tropical places. Mimosa pigra in riverine forests, e.g. Cambodia. Tamarix also in riverine forests in the USA. Australian acacias in the kerangas of Brunei. Rhododendron in Britain (this one is mostly invading in heath/open woodlands, but I've also seen it inside closed forest). Most invasives (also Lantana) readily spread inside forests mostly with at least some level of disturbance (e.g. ground fires, flooding and gap formation, herbivory, etc.).

A good species comparison would likely be Chromolaena odorata, which shows a relatively similar growth form to Lantana camara. Both these species are invasive in the sub-tropical regions of the KwaZulu-Natal province of South Africa, however C. odorata seems to be far more pervasive. The species forms dense monotypic stands in ecotonal areas as well in forested regions. There is a fair amount of literature on the species, its invasiveness and its management.

Triffid weed/ Chromolaena odorata in South Africa, we also have lots of problems with Lantana camara itself, as do you in Australia.What about Duchesnia indica? 

Kahili ginger (Hedychium gardnerianum) forms monocultures in Hawaii's montane rainforest, thereby preventing regeneration of native tree and other species (Minden et al. 2010a, b). On the long term, a degradation of the ecosystem is a possible scenario.

If you looking for a different lifeform example in Queensland, then the impact of Cenchrus ciliaris buffel grass on native  grasslands is a good example. Look to papers by R.J. Fensham, A.Franks, R.J.Fairfax or D. Butler

Yes, Hedychium  can form monocultures not only in Hawaii, but in the Azores, and it is now spreading on Madeira. But I have seen no published work on the effects of this.

Dear Robert, I have not been to the Azores but went to Madeira because I had heard about its spread there. It is somewhat invasive (disturbed areas, sometimes in forests along paths) but I have not seen any stand  that could be compared to the dramatic situation in Hawaii´s National Park, or anything that suggests it would outcompete native species in Madeira. However, that was in 2010; maybe we should keep an eye on it.

The situation in the Azores is much worse than on Madeira, though it is aggravated by the fact that much native forest has been replaced by Cryptomeria plantations

Ageratina adenophora in China

Yes, Phragmites australis in North America. Also hybrid cattails. These are both in wetland habitats of North America.

To name just a few we have issues with doing these exactly things here in Florida: coral ardisia (Ardisia crenata), Mexican petunia (Ruellia simplex), Brazilian pepper-tree (Schinus terebinthifolius), cogongrass (Imperata cylindrica).

An excelent site with good images, information  and access to research is: Center for Aquatic and Invasive Plants, a part of the UF. They have a great website. The web address is:  http://plants.ifas.ufl.edu/

Good luck with your research. I think you will find that instead of being a problem having too little material you are going to find too much information.

Melaleuca quinquenervia (paperbark tree), introduced from Australia, can become a monoculture in South Florida wetlands. Japanese climbing fern, Lygodium japonicum, is a major problem in central Florida and much of the southeast U.S. There are too many to list here. Try this website: www.invasivespeciesinfo.gov/plants/main.shtml‎CachedSimilar

In Britain, especially the mild, humid west, Rhododendron ponticum is a major problem - especially in woodland but also on moorland.  British plants are not actually pure R. ponticum, but a complex hybrid with various other species including R. maximum and R. catawbiense.  This hybridisation seems to have increased their vigour and frost tolerance.

Rhodo forms a very dense, tall (up to about 5m) understorey that shades out pretty much all other plant species.  It's probably the most important threat to the temperate rainforests of western Scotland, which are of international importance for their lichen and bryophyte communities.

For more information see http://www.snh.gov.uk/land-and-sea/managing-the-land/forestry-and-woodlands/looked-after/rhododendron/

If you want more information on history, impacts or control please let me know - I have lots!

Scotch broom Cytisus scoparius in Australia, New Zealand and North America. There is a considerable literature on the impacts of broom monocultures, especially in Australia and New Zealand. For example, Sinden and Cacho researched the impact of broom at UNE. A good place to start would be http://www.weeds.org.au/WoNS/brooms/

Camphor laurel (Cinnamomum camphora) is a great local example .  See http://www.dpi.nsw.gov.au/agriculture/pests-weeds/weeds/profiles/camphor-laurel

Chromolaena odorata in tropical forest (attached); Arundo donax in wetland (e.g. Namibia)

Article Efficacy of Chromolaena odorata control in a South African c...

As for ground cover vegetation, Euonymus fortunei (winter creeper) is an invasive vine that is used in plantings along sidewalks, as it is very tolerant of trampling. It has escaped into many urban parks in North America, and I have seen parks where it covered almost an entire hectare in a solid mat  30cm thick. It also grows up any tree or shrub. It completely grew over a 60ft tall Red Oak at a site I frequent. 

Also, Ranunculus ficaria (Yellow Figowort Buttercup) is rapidly becoming a severe issue in riparian areas around Louisville and a few other locales I've heard of in Kentucky. It spread vegetatively, and a single plant can create a mat 10m across in as little as 2years time. It crowds out even other invasives. It is a problem in riparian areas as it drops seed pods and tubercles into streams and during times of flood, and they have a high success rate for colonization.

As for shrubs, someone mentioned Lonicera spp. already but specifically Lonicera maackii (Amur honeysuckle) is the worst, using allelopathy and light competition to reduce ground cover vegetation to a fraction of background levels. It also suppresses tree seedlings and other shrubs. Lonicera morrowii (Morrow's honeysuckle) is nearly as bad. Both spread out from urban areas primarily. L.maackii has also been shown to reduce seed bank diversity over time.

Some citations for L. mackii include Gorchov and Trisel 2003; Hartman and McCarthy 2004, 2008; McEwan et al. 2010; Miller and Gorchov 2004

In the Midwest of the United States Rhamnus cathartica and R frangula ( Frangula alnus) under some circumstances can form dense monoculture understories. An herbaceous species that can is Alliaria officinalis, garlic mustard.

Hi.

A recently introduced plant in Tenerife (Canary Islands), Pluchea ovalis, is invading many areas of the south and southwest of the island, mainly around roads and tracks, but also the beds of small gorges (called "barrancos" in our islands). This plant is woody, can reach more than 2 m high and produce hundreds of seeds in a short time, so it is very dangerous for the environment and form dense scrub vegetation, sometimes almost avoid of indigenous plants. For the moment we have detected this species only in one protected area, but it is spreading fast and in the next years for sure it will be present in more protected sites of special interest for endemic plants. I consider that P. ovalis is at this moment one of the most serious threats to natural ecosystems of lowland areas in the south of Tenerife. 

https://link.springer.com/chapter/10.1007/978-3-030-45367-1_5

Hello Kathryn; As Hein notes, Arundo donax creates dense monocultures along low elevation streams and mesic woodlands in California. Best regards, Jim Des Lauriers