How plant traits determine where native and alien species occur along rivers

Ever wondered why there are so many plant species – and so much weed invasion – along rivers?

At first glance, one might attribute this to the lush conditions of riparian ecosystems: lots of nutrients + loads of water = ideal conditions for plants. This coupled with the fact that riparian systems are on the interface of aquatic and terrestrial ecosystems, so are home to a rich mix of wet- and dry-loving species? Makes sense.

 I have much respect for this tree!
Balancing on a rock can’t be easy. I have much respect for this Mekong River tree!
(Source: JAC)

However, when you really think about the conditions of riparian ecosystems – physical disturbance from floods, inundation and desiccation, ice scour in cold places – then they perhaps don’t seem quite so hospitable. Like rocky intertidal platforms, presumably the species that live in riparian zones need to have some pretty specialised adaptations to be able to cope.

In a recently published Tansley Review in New Phytologist, Roland Jansson and I focus on the key structuring forces of riparian zones, plant ecophysiological traits and mechanisms of species coexistence to resolve the apparent conundrum between the high floristic diversity of riparian ecosystems and their challenging environmental conditions.

OLYMPUS DIGITAL CAMERA
Would you drop your offspring in there?
(Source: JAC)

We describe 35 traits that enable plants to cope with riparian conditions. These include traits for tolerating or avoiding anoxia and enabling underwater photosynthesis, traits that confer resistance and resilience to hydraulic disturbance, and attributes that facilitate dispersal, such as floating propagules. This diversity of life-history strategies illustrates that there are many ways of sustaining life in riparian zones, which helps to explain high riparian biodiversity.

Using community assembly theory, we examine how adaptations to inundation, disturbance and dispersal shape plant community composition along key environmental gradients, and how human actions have modified communities. Dispersal-related processes seem to explain many patterns, highlighting the influence of regional processes on local species assemblages.

Treating alien plant invasions like an (uncontrolled) experiment in community assembly, we use an Australian and a global dataset to examine possible causes of high degrees of riparian invasion. We found that high proportions of alien species in the regional species pools have invaded riparian zones, despite not being riparian specialists, and that riparian invaders disperse in more ways, including by water and humans, than species invading other ecosystems.

You can find the paper details here and a free pdf copy here.

Please get in touch if you have any comments or questions – I’d love to hear them.

How do riparian plants even begin to survive a nordic winter?  Read our paper to find out!
How do riparian plants even begin to survive a nordic winter?!
Read our paper to find out!
(Source: Johanna Engström)

New paper: Predicting novel riparian ecosystems in a changing climate

Image
A bit of a stretch, but I couldn’t resist including this image!

I was fortunate to attend a workshop hosted by the National Climate Change Adaptation Research Facility last year that focused on riparian ecosystems under climate change. Among the various discussions at the workshop, some colleagues and I started discussing how riparian ecosystems might be affected by climate change and ways in which their abiotic and biotic characteristics are likely to change. It soon became clear that envisioning future ecosystems is no easy task, so we set about trying to come up with an approach by which to do so. 

We present our approach in a paper that has recently been published in the journal Ecosystems. Based around four recommendations, we present the approach in the first part of the paper.  We then use four case studies from contrasting environments to illustrate the approach and to determine:

–        Whether certain characteristics make some ecosystems more susceptible to climate-induced shifts in community structure than others; and

–        Which aspect of climate change seems to have the greatest effect on community structure and therefore should be a research priority.

Focusing on changes in community structure, we use qualitative process models to predict likely abiotic and biotic changes in four case study systems: tropical coastal floodplains, temperate streams, high mountain streams and urban riparian zones. We concentrate on functional groups rather than individual species and consider dispersal constraints and the capacity for genetic adaptation.  Our scenarios suggest that climatic changes will reduce indigenous diversity, facilitate non-indigenous invasion (especially C4 graminoids), increase fragmentation and result in simplified and less distinctive riparian ecosystems.

Compared to models based on biota-environment correlations, process models built on mechanistic understanding (like Bayesian belief networks) are more likely to remain valid under novel climatic conditions. We posit that predictions based on species’ functional traits will facilitate regional comparisons and can highlight effects of climate change on ecosystem structure and function. Ecosystems that have experienced similar modification to that expected under climate change (e.g. altered flow regimes of regulated rivers) can be used to help inform and evaluate predictions.

While the paper centres on Australian riparian zones experiencing climate change, the approach can be applied to ecosystems in other biomes that are subject to environmental change.

The paper is now online early; you can find the abstract and link here.

Please drop me a line if you’d like a copy.

Newsletter article: Causes, impacts and ways to manage exotic plant invasion along the River Murray

Dec 2011 – Condensing a rather large topic down to a single page, I discuss some of the processes that lead to high levels of invasion in riparian zones in this article. I also present  information on why we should be concerned about riparian plant invasion, as well as some of the management approaches that are available. Published in the Inland Rivers Network News, you can find the article on page 9 of the Summer 2012 edition.

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