In this Insight, Dr. Tamir Klein of Weizmann Institute of Science, Israel, talks about this latest paper Stomatal sensitivity to CO2 diverges between angiosperm and gymnosperm tree species.

Not all trees are equal

Most trees on Earth are either broadleaf or conifers. Why did they evolve this way? We think that leaves and needles represent two contrasting solutions to a single challenge: How to maximize photosynthesis, while minimizing water loss. In our recent study, we found that these differences could have far-reaching consequences in a high-CO₂ world.

From one CO₂ experiment to the next, I started getting the feeling that broadleaf and conifer trees do not respond to the increase in CO₂ level the same way. Back in 2007, I was working in a biotech company in Canada, growing pine seedlings for plantations. I saw little growth response of these seedlings when exposed to elevated CO₂. In 2013, I joined Christian Koerner’s lab as a postdoc in Basel, Switzerland, where I led the free-air CO₂ enrichment experiment on mature spruce trees in the forest. Again, we could not find any significant physiological response. This result was different from the observations of reduced stomatal conductance (a parameter of leaf gas exchange) in broadleaf tree species exposed to the same CO₂ level at the same site. In 2016, I was back in Israel leading my own team. Exposing lemon trees to elevated CO₂ levels, we saw a decrease in stomatal conductance.

By 2017, I already had a strong gut feeling that conifers are overall much less sensitive to CO₂ level than broadleaf trees. Yet, my personal experience was insufficient to make this point. Uria Ramon, an undergraduate student who joined my team, agreed to take on the laborious mission of scanning through dozens of research papers, and elucidating the relevant data from experiments with other species. With almost 60 species at hand, we confirmed the hypothesis: Broadleaf trees tend to close stomata at elevated CO₂, while conifers keep ‘business as usual’. How can we explain this difference? In the paper, we test several hypotheses, from morphological to evolutionary considerations.

Consequences might grow with time

Considering the fundamental divergence between broadleaf and conifer trees, forests of each type can respond quite differently, as CO₂ level continues to increase globally. Since stomata control the amount of water being evaporated from leaves, broadleaf forests, but not conifer forests, might use lower amounts of water in the future. In addition, considering the differences in the global distribution of broadleaf and conifer trees, tropical forests might save more water than temperate forests.

In addition to tree physiologists and forest ecologists, we hope that modelers of biosphere-atmosphere interactions read our paper and pick up its messages. Clearly, we cannot handle all forests the same way. We suspect that models that would account for the differences we reported will provide different, more realistic and accurate predictions of water and carbon cycling than those available today.

Still, there is a lot more to be done. We still need to test our hypotheses regarding the underlying causes for the divergence; we still lack important information on responses of tropical tree species; and we still need to verify that differences in stomatal responses translate into differences in tree water-use.

The wind in the willows

Ecology was a natural career choice for me. Growing up in a pine forest in Nazareth, Israel, my parents taught my sisters and me to appreciate, love, and learn from nature. In my spare time, I was reading Graham’s ‘The Wind in the Willows’ and Kipling’s ‘The Jungle Book’ over and over again, fascinated by nature’s harmony.

My lab at the Weizmann Institute of Science is everything I have ever dreamed of in terms of career. I currently lead a group of twenty enthusiastic students. Together, we study various aspects of carbon and water movements in and between trees. We take advantage of our country’s rich biodiversity to study trees in contrasting environments in the field, while running controlled experiments in the greenhouse. Sharing nature’s secrets with scientists, students, and children, is ever so gratifying.

For me, nature has always been a source of power, beauty, and inspiration. A city can be either pretty or ugly, but nature is always beautiful in its unique way. Take a crying baby out to the garden, and the rustle of leaves would calm it down immediately. So it happened with both my sons. My botanist wife shows me new, exciting, things almost every day. Flowers amaze us with scent and color. Darwin taught us that those, not for us were created.

Read Stomatal sensitivity to CO2 diverges between angiosperm and gymnosperm tree species in full or the plain language summary here.