Beyond species richness and community composition: using plant functional diversity to measure restoration success in jarrah forest

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Prepared by Rachel Standish, Aaron Gove, Andrew Grigg and Matthew Daws

Restoration ecologists on a newly restored mine site in the jarrah forest bioregion. Reference jarrah forest in the background. Photo credit: Richard J. Hobbs.

Ecological restoration has matured as a science and a practice since its inception in the 1980s. Mine site restoration has provided important test beds for ecological theory. This is particularly true for restoration of jarrah forest after bauxite mining in south-western Australia, where teams of ecologists have been experimenting with ways to return species and function for decades. Key lessons include the importance of topsoil as a biological resource for restoration and the strong influence of soil nutrients, or rather lack thereof, on forest dynamics.

The jarrah forest bioregion is nested within a global biodiversity hotspot. The number of plant species per unit area rivals many other forest biomes. Consequently, it provides an ideal place to test the contribution of species to ecosystem function. Does the jarrah forest need all those species to function? In our paper, we compiled trait data for over 500 species growing in the restored and reference jarrah forest. We used the data to calculate functional diversity of restored forest at various ages along a 25-year chronosequence and compared it to nearby reference jarrah forest.

Reference jarrah forest, including an iconic grasstree (Xanthorrhoea sp.) on the left. Most of the floristic diversity is in the understory. Photo credit: Rachel J. Standish.

The availability of long-term data on successional dynamics was key to being able to explore the development of function through time and its relationship with species richness. We found that two indices of functional diversity increased with age, and two decreased with age. Species richness declined with age but was similar to reference forest at 25 years. Overall, we concluded that a longer time frame may be needed to assess functional diversity, and that in the meantime, species richness cannot be used to predict functional diversity.

Box-and-whisker plots showing comparison of forest reference and restored forest at 25 years of age. From the original paper.

Wildfire is an important agent of disturbance in jarrah forest. It can re-set successional trajectories because it triggers native plants to recruit from seed. We considered the influence of fire on functional diversity and found that it had mixed effects – functional dispersion, or the spread of species in trait space, increased with time since fire, whereas functional richness, the volume of the trait space, decreased.

Our findings have implications for restoration practice and forest management. Firstly, it is worthwhile pursuing direct measures of functional diversity because these are not related to species richness. Second, that time since fire had mostly neutral effects on restoration measures and therefore additional efforts to manage fire may not be warranted. In the future research, we will measure functional diversity in mature restored forest and old-growth forest.

Pink-flowered myrtle flowering after fire. Photo credit: Rachel J. Standish.

This is a plain language summary for the paper of Rachel Standish, Aaron Gove, Andrew Grigg and Matthew Daws published in Applied Vegetation Science (https://doi.org/10.1111/avs.12607). Rachel Standish wrote the post.