Tropical rainforest plant water relations and vegetation dynamics

View from rainforest canopy crane by Deborah Apgaua

Deborah Apgaua – Ph.D F/Time at James Cook University.

Many scientists fear that increasing drought or climate change events will threaten the future of tropical rainforest communities.

In an earlier research project in 2014 supported by the Skyrail Foundation titled Functional Traits and Water Transport Strategies in Lowland Tropical Rainforest Trees, we used a multidisciplinary approach to examine how eight different rainforest trees exhibit different strategies for transporting water. Although trees constitute large part of the diversity in rainforests, other plant lifeforms such lianas and vines are also important ecological components parts of tropical rainforests.

It would be quite an impossible task however to examine the individual strategies of every one of the thousands of plant species in tropical rainforests. Approaching this using plant guilds such as plant lifeforms or plant ecological groups, we can obtain useful insights as to how important components of the forest differ in form and function.

With the support of the Skyrail foundation, we returned once again in 2015 to Cape Tribulation, to the Daintree Rainforest Observatory, where we collected samples from 90 plant species differing in light preferences and lifeform, thus forming six ecological groups: mature-phase trees, understorey trees, pioneer trees, understorey shrubs, pioneer shrubs and vines.

We examined wood samples from these species to examine anatomical features related to water transport, such as vessel sizes, frequency and distribution.  At the same time, we also sampled leaves from each of the 90 species to quantify leaf intrinsic water use efficiency through carbon isotope ratios. Leaf intrinsic water use efficiency reflects plant physiological performance because it measures how much carbon could be taken up by photosynthesis for a given amount of water lost to transpiration.

We found a diverse spectrum of wood characteristics, and therefore ecological strategies from the 90 study species. Lianas, for instance, had often large solitary vessels and low wood densities, indicating an ecological strategy that maximizes water transport at the expense of mechanical support. Conversely, understorey shrubs and trees had high wood densities and numerous small vessels, reflecting greater investment in structural support, and the reduced need for high water transport. Vessel size was a strong predictor of water use efficiency across the study species, indicating that plants with larger vessels, and thus a higher potential for water transport, are able to fix more carbon per unit of water transpired.

We conclude that a plant functional group approach is a useful way to obtain a better understanding of plant performance and tropical rainforest ecology. The data can also serve as a basis for us to model how rainforests will change during climate change scenarios.

Student funding was released by the Skyrail Rainforest Foundation for the purpose of rainforest research, preservation and education.