In this post Gillis Horner, Shaun Cunningham, James Thomson, Patrick Baker and Ralph Mac Nally discuss their recent paper ‘Recruitment of a keystone tree species must concurrently manage flooding and browsing

Floodplain forests are threatened by the three-pronged attack of land-use change, river regulation and climate change. Establishing new seedlings – a fundamental component of any strategy to sustain these vital forests – depends mainly on flooding. The pulse of water and nutrients coming with floods gives the young trees a head start. However, young, nutrient-rich plants are delectable to browsers – native kangaroos but also to introduced rabbits, cows and sheep. Our new work shows that successful regeneration of a keystone floodplain tree depends on the concurrent management of flooding and browsing. If either is ignored, a new generation of seedlings is lost, jeopardizing the future of these forests.

Widespread mortality of floodplain trees
Widespread mortality of floodplain trees has occurred along the Murray-Darling Basin floodplains, mostly due to reduced flooding frequency (and magnitude) and lower availability of ground water from river regulation.

Natural resource managers and ecologists long have recognized that drought and browsing each affect native ecosystems, with sediment and water salinity being another threat. However, management of threatened ecosystems often takes a triage approach, dealing with the most obvious or overarching threat first. The iconic river red gum (Eucalyptus camaldulensis) forests on the floodplains of Australia’s longest river, the Murray River, has been severely affected by the combination of river regulation and water extraction, and drought for all but two of the past 19 years. There is widespread dieback – groves of dead trees are common along the Murray River – and new trees are a rare sight. The Millennium Drought or more aptly titled ‘Big Dry’ (1997–2010) motivated government agencies to embark on a program of managed flooding to arrest the decline of these floodplain forests. However, less obvious threats to these ecosystems may interact with managed floods to reduce the successful establishment of new seedlings and saplings.

Condition map for Eucalyptus camaldulensis
Condition map for Eucalyptus camaldulensis forests at Lindsay Island, where most of our sites were located. Good condition forests (dark green) are restricted to the river and permanent creeks. The river and creeks are indicated in blue.
Managed flooding
Managed flooding is achieved by pumping river water into dry floodplain creeks, and is retained by levees and regulators.

Along with regional and state natural resource managers, we tested these ideas by artificially flooding some tributaries along the Murray River (see images below). We fenced out browsers from half of the flooded sites and from half of the unflooded sites. This allowed us to identify the separate effects of flooding and browsing, and to isolate their interactive effects on tree regeneration.

Seedling plot at Walpolla Creek
Seedling plot at Walpolla Creek during and after managed flooding.

We showed that flooding had a strong positive effect on seedling survival and height growth and that browsing had strong negative effects. We found that the positive effects of flooding effectively were cancelled by the negative effects of browsing. The river red gum clearly requires flooding to regenerate, so that controlled floods are a good thing. However, if managers do not deal concurrently with browsing pressure, then the artificial floods will not have a chance to stimulate the production of saplings and then new trees. Dealing with the interaction between browsing and flooding is critical to the future establishment of new river red gum trees and the diverse ecosystems they support.

Managed floodwaters
Managed floodwaters travel for many kilometres along creeks, inundating fringing trees and wetlands.
creek bed
Salt-encrusted creek bed due to rising saline groundwater, with footprints of Eastern grey kangaroo (Macropus sp.) (left) River red gum seedling needs both flooding to initiate germination and protection from browsers to survive (right).