PhD candidate Jianhong Zhou aimed to better understand whether and how plant species adapt to environmental changes. She developed two databases that she used to analyze how easily or difficultly plants adapt to changing conditions. Zhou defended her PhD thesis on 4 September.
Some plant species can adapt to changing conditions much more easily than others. For example, leaves may become larger or thicker over years or generations, or they may contain more nitrogen or phosphorus, the rate of photosynthesis may change, and so on. The less a plant species can adapt to new conditions, the greater the chance of extinction.
Plants in dry steppes struggle to adapt
Ecologist Jianhong Zhou demonstrated in her PhD research at the Institute of Environmental Sciences Leiden (CML) at Leiden University that there is a significant variation in the adaptability of different plant species. 'For instance, trees and shrubs adapt more easily than herbaceous plants. This explains, for example, why researchers worldwide are seeing shrubs encroaching on some grasslands. The lowest adaptability was observed in plant species from dry steppe regions and tropical savanna climates with dry winters.' These are precisely the areas where rapid climate change is occurring, according to the researcher.
A major task
It was a major task to make her discoveries. Zhou reviewed extensive literature to compile two databases that allowed her to analyze the adaptability of plants. Her supervisor, professor of Environmental Biology Peter van Bodegom, praises Zhou's perseverance: 'There are sometimes errors in scientific literature, which need to be corrected. And people measure things in different ways, leading to varying results. The data that researchers used to achieve their results are not always accessible.'
Data on 2,064 plant species
Zhou resolved all these issues. For her largest database, she gathered data on 2,064 plant species and eleven functional traits within them. This database contains the extent to which traits such as leaf size and thickness, nitrogen and carbon content in leaves, and root length vary within all these species. This intraspecific trait variation (ITV) database also sheds light on how these traits vary across all these species.
Thicker leaf? Then it lives longer
Using this database, Zhou also examined relationships between traits. 'A well-known example of such relationships is the leaf economic spectrum of plants observed between species.' The leaf economic spectrum covers six traits, including leaf mass per area and the average lifespan of leaves. If you measure these traits across different plant species, you'll see that a high leaf mass per area is associated with a longer leaf lifespan. 'This shows that if a plant or species invests more in a leaf, that leaf also lives longer.'
She continues: 'We observed that the relationships between most Leaf Economic Spectrum traits existing between species were also maintained within species. So also within a species, you can see that the leaves of an individual with thicker leaves live longer on average. This indicates that these correlations are genuine plant strategies to optimize adaptation to their environment.'
No true strategy after all
It can be different as well. A surprising result, according to Zhou: 'We observed a positive relationship between leaf nitrogen content and specific leaf area (SLA, the ratio of leaf area to leaf dry mass, ed.). But only between species, not within species. This means that the correlation between these traits is not a true strategy, as researchers previously thought.'
The connection between nitrogen content and specific leaf area is likely caused by certain environmental conditions that different plant species in a specific area experience, such as high nitrogen levels in the soil and low light availability. 'If those conditions change, for instance, due to climate change, the connection between leaf nitrogen content and specific leaf area might disappear as well.'
A comprehensive overview of knowledge on plant species adaptation
Zhou investigated much more and established various connections. According to her supervisor Van Bodegom, her research has led to a comprehensive overview of knowledge on plant species adaptation. 'Jianhong has analyzed patterns and presented them in an accessible manner. This is crucial for predicting biodiversity in the future and optimizing conservation efforts.' Thus, the importance of establishing ecological corridors as "escape routes" for certain plant species increases as their sensitivity to extinction grows. 'An article about the ITV database has now been published in New Phytologist, a leading journal in our field.'
For Zhou, the email in which New Phytologist confirmed they would publish her article was a highlight of her PhD journey. 'It was on the day of the Mid-Autumn Festival in 2021, a Chinese holiday. A beautiful coincidence, a memorable moment for me.'
Jianhong Zhou defended her PhD thesis Phenotypic plasticity and genetic adaptation of plant functional traits on global scales on 4 September.
Text: Rianne Lindhout
