Simon Haberstroh: Cork oaks under shrub invasion behave differently 

In this new post, Simon Haberstroh from Freiburg University, Germany, presents his latest work ‘Plant invasion modifies isohydricity in Mediterranean tree species’. He discusses the capacity of plants to regulate their hydraulic strategies and remembers his long survey nights in Portuguese oak forests. 

About the paper 

Our publication in Functional Ecology deals with plant hydraulic strategies, i.e. how plants regulate their water consumption during different times of the year—this can be crucial to withstand, for example, a drought period. The biggest control plants have over their water losses is through their stomata, which are tiny gaps on the leaves which allow carbon dioxide to enter and water to exit the leaf. In general, there are two extremes when dealing with plant hydraulic strategies: on one side there are plants which almost immediately close their stomata to limit water losses as soon as soils start to dry (isohydric). Plants at the other extreme keep their stomata open as long as possible, which allows for a higher carbon dioxide inflow for photosynthesis, but it also leads to substantial water consumption (anisohydric). You could also say that isohydric species are carefully evaluating their available water to survive a drought period, while anisohydric species are ‘living in the moment’ and do not care so much about the future. Of course, it is not as simple as that and both extremes have their (dis)advantages, but it is quite a neat description for these different behaviours. In reality, plants do not deploy one extreme strategy or the other, but rather a strategy in between—leaning more to one side of the spectrum than the other.  

Cork oaks invaded by the shrub gum rockrose in Portugal (credit: Simon Haberstroh)

Importantly, these behaviours have long been seen as a fixed trait of plants, which is not strongly modifiable. However, there are some indications that this is not entirely true. Similar to humans—who can change and adapt their behaviour in different environments—we wanted to see if plants are able to do the same. Thus, we chose two Mediterranean plant species growing in the same ecosystem with quite different (supposedly) hydraulic behaviours. The first one—cork oak (Quercus suber)—is an isohydric, water saving tree during drought periods. The second—gum rockrose (Cistus ladanifer)—is a native invasive shrub in cork oak ecosystems and belongs close to the anisohydric extreme. We chose to modify the environment for these two plants to assess their responses—especially for cork oaks—in two ways: 1) we induced additional drought by installing a rain exclusion to simulate climate change; and 2) we were interested in how cork oaks behave when they have to compete for water with invasive shrubs.  

The idea for this research paper (and my whole PhD thesis) was born when one of my PhD supervisors measured cork oaks competing with gum rockrose in a naturally dry year and compared the reaction of these invaded trees to cork oaks without shrub invasion (Caldeira et al. 2015). Already then, there was an evidently substantial negative impact of shrub invasion on cork oak water fluxes and cork oak functioning. However, the underlying mechanisms of this negative invasion effect were not quite clear; therefore, the idea was to establish a long-term rain exclusion experiment to induce artificial drought every year. This is when I entered the picture—roughly in the middle of 2017. After the successful establishment of the experiment and 2.5 years of robust data, it was quite clear that cork oaks under shrub invasion evinced lower water fluxes compared to their non-invaded counterparts (discussed in detail elsewhere, Haberstroh et al. 2021). The artificial drought also had an impact, but shrub invasion was the dominating factor.  

The additional drought treatment (credit: Simon Haberstroh)

After scanning the literature, I found more and more publications reporting that water fluxes of different tree species can be modified when growing with other species. For me, this was another indication that plant hydraulic strategies, which control water losses and fluxes, must be flexible and not a fixed trait. I was curious and, as we had the measurements necessary to look closer into the hydraulic strategies of cork oak and gum rockrose, the very first step for this paper was complete. After several more steps, which I won’t describe in detail, it was clear that both species were actually quite flexible in their hydraulic strategies, shifting their behaviour from dry to wet phases and back.  

A number of studies in recent years have also shown that plants do shift their hydraulic behaviour from wet to dry seasons in response to changing soil water resources or meteorological conditions (which actually makes sense—why save water when there is enough water). The novelty of this paper lies in the fact that we demonstrate that the competition for water resources can additionally lead to a modified hydraulic behaviour in cork oaks. This is highly relevant for scientists dealing with hydraulic strategies and how plants, for example, can adapt to climate change effects in the future. Furthermore, our results also have a practical implication: it was quite clear that cork oaks with an altered hydraulic behaviour under invasion also assimilated less carbon, which translated into reduced stem growth and leaf area. Thus, shrub removal is likely to be beneficial for cork oak functioning and growth. Additionally, the understorey is much more diverse in terms of biodiversity when shrubs are not present in the ecosystem. Here, an incentive for cork oak farmers to remove these shrubs on a regular basis might benefit cork oak ecosystems in the long term. 

About the research 

Invasive (plant) species are a ubiquitous issue across the globe. Our research provides insights into the damaging effects of such invasive plants on key species in ecosystems and demonstrates that efforts to remove invasive species are worthwhile.  

However, to get to this point, we had to overcome some challenges. To establish this experiment, we had to consider quite a number of different things. Apart from the manpower needed, the field site was, and is, still being used as a hunting ground, meaning that a lot of wild boars and deer roam the area. Wild boars are especially interested in everything, including soil moisture sensors which are buried in the ground! Thus, we had to build fences to keep the animals out of our plots—not an easy endeavour with bedrock close to the surface (at a depth of 40cm). While we were able to keep the big animals out, some other, smaller “friends” of ours made our lives—especially mine—unnecessarily hard. The protection shielding our sap flow sensors from the sun, rain, etc., was the perfect spot for mice to start a family! And, in the process of family-making, they loved to chew on the cables, representing the main contributor to gaps in our tree water flux data.  

Sap flow sensors to measure tree water fluxes being misused as playground for little mice (credit: Simon Haberstroh)

Apart from field animals, the biggest challenge for this paper was most likely the measurements we had to conduct at night. Pre-dawn leaf water potentials, as the name already describes, are conducted pre-dawn (before the sun rises) usually between 2 and 6am. For our paper, this was an indispensable parameter because it is a reliable plant stress indicator. However, for us it meant getting up early (17 times in total) and handling gas bottles and pressure chambers in a very tired and sleepy manner. You could also say that the pressure was high on those nights! Then again, I will never forget the feeling of watching the sunrise. knowing that we had collected another set of super important and valuable data. 

Sunrise after pre-dawn leaf water potential measurements (credit: Simon Haberstroh)

While the sunrise after a pre-dawn measurement was pretty reliable, there were quite a few surprises. Potentially the biggest surprise was to see how well cork oaks dealt with the additional drought treatment. There were some indications that additional drought would lead to lower water fluxes, but we could not detect any changes in the plant hydraulic strategy for these cork oaks. This is a big question to be answered in the future—whether cork oaks might also change their plant hydraulic strategy under recurrent extreme drought. For invaded cork oaks, it remains uncertain in which direction the ecosystem is developing—will the shrubs out-compete the cork oak trees, or can they withstand the competition in the long term?  

Independent to these questions regarding how this competition will eventually play out, our research clearly demonstrated that plant hydraulic strategies can be flexible. This theory should be tested on more plant species in order to observe what the final consequences are. For cork oak, this shift in hydraulic behaviour seems to have a negative impact, as evident from water fluxes and growth. However, such a shift might potentially be beneficial for other species (e.g., under extreme drought).  

Cork oaks and gum rockrose under natural drought. The evergreen cork oak trees lost the majority of their leaves. How will this competition play out in the future? (credit: Simon Haberstroh)

About The Author 

Simon Haberstroh (credit: Simon Haberstroh).

The path for a career in ecology was paved quite early in my life. I grew up on a beautiful little farm in the Black Forest in Germany, and I was always surrounded by nature and our farm animals. At the age of 2, I already had my first little spot in my grandmothers garden where I proudly started to grow vegetables. After high school, I decided to do a voluntary ecological year in the Black Forest. Being in the forest every day, I was certain that I would like to dig deeper and, thus, I decided to study Environmental Sciences in Freiburg. This decision was also reinforced by the emerging climate change impacts on ecosystems, which were not as strong as nowadays, but already quite substantial back then. This turned out to be the right decision when the opportunity arose to do a PhD in an ecosystem threatened by climate change. For me, it was a highly relevant topic from the beginning to investigate cork oaks under extreme drought and plant invasion, and to try to predict in which direction the ecosystem will move toward in the future. The topic also fascinated me because it is so tangible and has practical considerations, especially for people who actually farm cork (oaks).  

Beautiful Vila Viçosa in Portugal, Alentejo (credit: Simon Haberstroh)

However, for now, I have returned to Germany and currently work as a researcher and lecturer (“Akademischer Rat”) at the Chair of Ecosystem Physiology at the University of Freiburg. But you never know, maybe I will return to continue working with my beloved cork oak ecosystem in beautiful Alentejo, Portugal. Although my time in Portugal—especially the measurement campaigns—was not always easy and fun for me, I have to say that there are a lot of hidden benefits and rewards that are not immediately tangible when completing difficult work. For example, having your papers published! 

Enjoyed the blogpost? Read the research here

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