Benjamin Hafner: Split it – disentangling functional mechanisms in forest ecology research

Benjamin Hafner of Technische Universität München & Cornell University, discusses his work Water potential gradient, root conduit size and root xylem hydraulic conductivity determine the extent of hydraulic redistribution in temperate trees” shortlisted for the 2020 Haldane Prize for Early Career Researchers and draws on how a lifelong love of nature led to a career in forestry research.

Setting trees into pots in the greenhouse was fun, especially when their root system was as long as the legs of the researcher.
Setting trees into pots in the greenhouse was fun, especially when their root system was as long as the legs of the researcher

I have always been fascinated by nature and trees. To me, there is no more impressive and majestic sight than an old oak tree on a glade in the forest. But I also learned early on that natural ecosystems are threatened by human activities and climate change, so I decided to study forestry to learn more about forests and take part in sustaining their future. During my studies I was especially fascinated by the mechanisms and functions trees use to cope with all different imponderables. So, when I was preparing a poster on hydraulic redistribution for a seminar with my later PhD supervisor this effect drew me into the world of ecology research.

Hydraulic redistribution (HR), the passive reallocation of water along moisture gradients, e.g. in the soil through plant roots, is widely described, however its extent and driving factors, especially in temperate trees, remain unclear. In this paper, for the first time, we quantified hydraulically redistributed water in five ecologically and economically important temperate tree species. We found that species redistribute significantly more water along higher soil-moisture gradients, larger root xylem conduit diameters and especially with higher root xylem hydraulic conductivities. This paper emphasizes that the amount of water species redistribute will likely increase in a hotter and drier future, stressing the importance of HR for temperate forests and improving our mechanistic understanding of HR and tree species’ reaction to climate change. In addition to its mechanistic importance to tree functional ecology, the paper demonstrates that trees with high xylem hydraulic conductivity also have a high HR capacitance, potentially making them valuable ‘silvicultural tools’ to improve forest stability.

In a similar experiment I have shown that neighboring plants receive up to 80% of their water from HR plants, stressing the importance of this effect in plant ecosystems. These results have to be validated in field research at various forest sites. I am reviewing data from a long-term drought stress experiment, where I could show HR by mature beech trees and quantify the amounts of HR water to evaluate the relevance of HR for beech and neighboring spruce trees. In an attempt to expand the knowledge on HR with an emphasis on its stabilizing function on ecosystems, I applied for further research projects on HR in old growth forests, forest plantings and agroforestry systems.

Split-root system with oak trees. Finding oaks with two equally developed roots was a difficult task!
Split-root system with oak trees. Finding oaks with two equally developed roots was a difficult task!

I really enjoyed the growth chamber component of this study as I think the design of the split-root system allowed us to separate two soil compartments and interconnect them via tree roots. The “split-root tree” can therefore be manipulated in multiple ways and the encouraging results of this study have convinced me to plan for future split-root studies. I also enjoyed examining roots from an anatomical perspective, we used a laser ablation tomography technique that yielded very pretty images.

Cross section of an oak root, ablated with laser tomography. This technique yields very nice images!

We discovered that some species were more amenable to the split-root setup than others. For this study I initially planned to have pine trees included, but they apparently did not like the split-root set-up at all and ended up dying shortly after planting, so I replaced them with Douglas fir – a species that proved to like the set-up the most. A challenging split-root candidate was also oak, since oaks develop a strong tap root with only few side roots that could be split between two pots. I got lucky having a friend who plants trees. I asked him to send me all oaks that developed two or more equally strong tap roots out of his 3000 plants order – and got a total of 15.

Setting up a split-root system with a Douglas fir tree. Separating the root system was easy for this species.

Disentangling these differences between species -tap root or multiple seminal roots, conifer or broadleaved- and still finding common ecological principles makes the life of an ecologist so fascinating – and challenging. It is most exciting to find basic functions and mechanisms that describe plant behavior but so easy to get lost in the myriad of parameters necessary to only get a grasp of the full picture. As we strive to describe ecosystems as holistic as possible, I have learned that it always pays off to reach out to colleagues in related disciplines, interdisciplinarity is helpful, fruitful and necessary to understand ecology. To me, that is what makes the life of an ecologist so exciting!

When I am not studying trees in the forest or greenhouse, I can still enjoy them on hiking or biking trips, especially in the mountains or appreciate their thermic energy when sitting with friends around a cozy fireplace on a mild summer night.

Read the article in full here.

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