In Insights we discover the story behind a recent publication in Functional Ecology: what inspired the authors to do the research, how did the project develop and what wider impact might the work have?

This week Bjorn talks to Angela Prendin about her article,  Axial xylem architecture of Larix decidua exposed to CO2 enrichment and soil warming at the tree line. Angela is affiliated with the Department of Land, Environment, Agriculture and Forestry (TESAF) of the University of Padova, Italy. With her collaborators at the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), she studied the xylem architecture of Larch in response to CO₂ fertilisation and soil warming. You can also read the free plain language summary here: The treetop is the hotspot determining growth in larch trees

Your research is performed in the context of the effects of climate change. Why is it important to understand the response of xylem to environmental drivers?

Trees in our forest are not only spectacular, but also very important organisms for our planet. They provide many ecosystem services, such as sequestering CO₂ from the atmosphere, protecting against erosion and avalanches, providing fuel and timber, and many more. Rapid climate change jeopardises these services. Because trees are sessile and often live for centuries, a young tree today may experience very different climate conditions when it reaches maturity. It is thus very important to understand how trees function throughout life and under different environmental conditions. Tree functioning depends both on physiological processes and structural properties, because structure fits function. Among the most important processes in tree life is water transport from the roots to the leaves. This long-distance transport is mainly performed in specialized water-conducting cells, the xylem conduits. Investigating how the xylem or wood responds to environmental drivers is thus key and can help us to develop a mechanistic understanding of tree growth.

You are affiliated with the University of Padova, a long way from where your experiment was performed (but very close to the beautiful Dolomites). Is there a particular reason why you choose the Davos tree-line experiment to study your questions?

Yes, you are right, the Dolomites are amazing mountains but unfortunately, in Italy there is not such a unique long-term free air CO₂ enrichment (FACE) experiment. The Stillberg experiment, located near Davos, Switzerland, represents the only existing in situ CO₂ enrichment study of an alpine tree-line ecosystem. Additionally, this study is unique because of the simultaneous manipulation of the CO₂ concentration and the soil temperature.

The CO₂ fertilisation and soil warming treatments were in place for almost 10 years before you started your particular study. Earlier research had shown an effect from one of these treatments (CO₂ enrichment) on tree growth. How did you come to study the functional anatomy?

Tree growth in a seasonal climate often shows in distinct tree ring: a wide ring means a favourable year for tree growth and a narrow ring means an unfavourable year. Tree rings are thus often used as natural archives for past environmental conditions. However, tree rings alone are not able to shed light into the mechanisms behind tree growth responses. For this, we think it can help to go deeper as we did in this study. Since the xylem structure is linked to xylem function, the intra-ring anatomical characteristics provide us high-resolution and long-term insights into past functional xylem responses. This, but also a lack of fundamental understanding how functional xylem anatomy changes within the trees from roots to stem apex and throughout life, motivated us for this study.

The results from your study suggest that some traits related to the functional anatomy of the trees xylem are under apical control, whilst others are much more plastic and responsive to environmental change. You describe this as a trade-off to maintain hydrological efficiency. Can you explain in simple terms how this works?

Yes, I can try. The xylem architecture of trees shares a common design, according to which the conduit diameter within each tree ring increases from the stem apex to the base following a power-like trajectory. This allows minimizing the negative effect of the increasing tree height on the total hydraulic resistance from roots to leaves. Our results show that this trend is stable during ontogeny and with different treatment. We think this stability indicates that the maintenance of hydraulic efficiency is of high priority compared to other wood functions. Since this trend in conduit diameter initiates at the apex, the increase in hydraulic conductivity at the apex has important consequences for the whole transport system with the final effect of promoting height and ring growth.

Your work focussed on CO₂ fertilisation and soil warming. An obvious climate variable that would alter hydrological conditions, and thus has an impact on the tree hydrological efficiency, are droughts. How would you expect drought to play out on the functional traits you have studied?

Limited resources to form wood and differing biophysical constraints imply trade-offs between the xylem functional needs, as demonstrated by the competing axial structural adjustments observed in our study. In the case of drought, tree-ring growth is reduced. I would expect anatomical adjustments aiming at increasing the xylem conductance to partly compensate the reduced water transport capacity provided by the narrower tree rings to maintain leaf transpiration. Conduit diameter should therefore be increased to support assimilation necessary for tree growth, while, other functional traits linked to mechanical support and metabolic xylem functions would be deprioritized. Wider conduits under drought should also be favoured because the carbon construction costs per unit of water conductance are lower than with narrower conduits. In reality, this may often go along with thinner and thus less robust cell walls. In other words, in general, I would expect trees to prioritize hydraulic efficiency over safety.

And moving away from the science, one last question: when you are not in the Alps studying the effects of enviro-climatological drivers on tree anatomy, where can we find you and what do you like to do for fun?

When I am not studying anatomical features of alpine conifers, I enjoy a lot spending time with outdoor activities. I love being in the field hiking, skiing, climbing with friends but what I enjoy most is kayaking. You can find me paddling in white water rivers or in the sea. As I recently discovered, the latter can also be a lot of fun.

More information on Angela’s research can be found at: https://www.researchgate.net/profile/Angela_Prendin