Hydrological conditions predict wood density in Australian riparian plants

Background/Question/Methods

Wood density is a key plant functional trait that integrates the trade-offs characteristic to riparian plant ecological strategies. Although dense woody tissue is costly to construct, it confers mechanical stiffness to stems, which should be useful if a plant must withstand flooding to survive. High wood density is also associated with water stress tolerance and conservative use of resources. For riparian plants, fluctuations in soil moisture driven by surface hydrology should therefore be an important driver of variation in wood density.

We made the following predictions in this study: (1) wood density increases with frequency and magnitude of flooding disturbance (2) wood density increases as water availability in the riparian zone becomes less predictable, and (3) flooding and unpredictability of water availability induce environmental specialisation in wood density as they increase in severity.

We visited fifteen riparian sites along flow-gauged rivers across south-eastern Australia. Coverage of dominant woody species was surveyed and trunk wood samples taken at each site. Abundance-weighted site means of wood density were mapped along gradients of frequency and magnitude of flooding disturbance, and metrics of riparian water availability such as baseflow index, seasonality and inter-annual variability. Trait Gradient Analysis methodology was extended to provide a metric of environmental specialisation.

Results/Conclusions

Metrics describing the largest, most intense flood events throughout a river’s hydrological record were found to be strong positive predictors of mean wood density. Mean wood density was also positively predicted by unpredictability of water availability in the riparian zone. This latter relationship was maximised where patterns of flow were highly seasonal, but the season with which they were associated was not consistent throughout the record.

Using an extended Trait Gradient Analysis, we found that flood magnitude, rise and fall rates, and unpredictability of water availability were positively associated with greater environmental specialisation among woody plants. As hydrological conditions became harsher, species at a given site grew closer in their position along the trait gradient, indicating increasing preference for certain hydrological niches.

We were able to confirm all three of our predictions. Our study highlights the importance of hydrological conditions – particularly disturbance and environmental unpredictability, as determinants of ecological strategy in riparian plants. This is likely to hold important ecological consequences for riparian plant communities in south-eastern Australia, where increasing climatic variability and frequency of extreme events are hallmarks of climate change predictions.