In our latest post, Laura Ortiz Díaz—A PhD student at Universidad Rey Juan Carlos, Spain—presents her new study ‘Functional diversity of experimental annual plant assemblages drives plant responses to biological soil crusts in gypsum systems’. Laura tells us where the idea of the study came from, the importance of biological soil crust for drylands ecology, and how she became hooked on ecology from an early age.
About the paper
Biological soil crusts (BSC) are complex combinations of lichen, mosses, cyanobacteria, algae, and fungi which are intimately associated with the soil surface. These living layers greatly interfere with plants living there, modulating crucial processes such as soil water infiltration and evapotranspiration, carbon cycling, nitrogen fixation, nitrification, etc. In addition to this, BSC are also important for selecting functional trait values (e.g., seed mass, plant height, specific leaf area) throughout the life cycle of plants.
The effects of BSC on plants in drylands have received much attention; however, there are still some questions to be answered. In this study, we tried to shed light regarding the effects of BSC on a prolific annual plant community living in a gypsum system in central Spain. Additionally and importantly, we also looked at how the functional diversity of the plant community can modulate plant responses to this major, and worldwide biotic environmental filter in drylands. Specifically, we aimed to understand the role of BSC conditions and the diversity of plant stature—a main functional trait defining ecological strategies of plants—in a community and its subsequent effect on plant performance (survival, growth and fitness) of coexisting species.
The idea for this experiment came up while conducting other experiments in drylands for my thesis project. In our experimental field station, we noticed that the vegetation differed between plots with and without BSC, and plants in plots where BSC were present were bigger than those growing on bare soil. To understand the multiple effects of BSC—and the extent to which the functional diversity of interacting plants can modulate their effects on the development of coexisting species—we applied an experimental approach by manipulating the initial functional diversity of the entire annual plant community and BSC conditions in a common garden experiment.
Common garden techniques are widely used in plant community ecology because they can provide us with successful methodologies to study responses of entire annual plant communities. In our experiment, we manipulated the initial functional diversity in the community by combining species on the basis of their maximum plant height (MPH, i.e., a major trait strongly determining the competitive ability of plants). Specifically, we selected the species conforming to our experimental assemblages in order to obtain experimental assemblages of annual plants with high initial functional diversity (i.e., coexisting species with different maximum sizes) or with low initial functional diversity (i.e., coexisting species only with large or small maximum sizes). I consider that the most notorious contribution of this paper is the confirmation of our common-garden experimental approach as a novel and effective method to determine causal relationships among functional features of the entire community and plant responses to environmental filtering.
About the research
The information presented in this paper can help us to better understand how plant communities are formed. This study also provides a novel way to study the effects of biotic factors on plant communities, as well as the importance of the own properties of the assemblage (i.e., the initial functional diversity) in the assembly process. Specifically, our results showed that the presence of BSC affects the establishment and development of annual plants. In general, BSC acted as a physical barrier hindering the establishment of annual plants. However, individuals surpassing this early life-cycle obstacle benefitted from the improved microenvironmental conditions provided by the BSC. In addition to this, the characteristics of the species that formed each assemblage (i.e., maximum plant height) also influenced the development of the species. Plants in pots—with species combinations with low height variability (i.e., combinations with only large plants or only small plants)—prioritised growth over reproduction. Conversely, we observed the opposite in pots with high size diversity (i.e., plants of different sizes together). In the first case, inter-species competition seems to be the most important mechanism organizing the community, leading to competitive symmetric responses. In the second case, complementarity between different plant sizes seemed to allow individuals to partition space and resources more efficiently and thus reproduce more successfully.
When you try new methodologies, you always find difficulties. In our case, when we designed the experiment, we did not expect how difficult it would be to create the experimental assemblies. Collecting portions of BSC, seeds of all species, and counting each of them was a true adventure. Furthermore, due to the COVID-19 lockdown in Spain, we had to suspend flowering sampling and we lost information about some species. Despite all of this, we obtained surprising and consistent results that helped us to provide a possible explanation for what we had observed in the field.
In this study we have focused only on one trait (maximum plant height), so we now could/should ask different questions: do plants respond in the same way if we choose other traits? What about looking at a set of traits, which can more broadly represent the functional diversity structure of a community? Moreover, in this study, water availability was not a limiting factor, so we do not know if the response under water stress (a main abiotic filter in our system) would be the same. I believe with this work we have taken a step forward in understanding how a community is formed and how species coexist under such adverse conditions. As I’ve said, I think the next step would be to create experimental communities with functional diversities based on different functional traits or on a set of traits. The same methodology could also be used to study how initial functional diversity affects the establishment of other plant communities in different systems.
About the author
I studied Environmental Science because I was particularly curious to understand how humans interacts with our environment. While I was studying for my degree, subjects related to ecology were those that most caught my attention and the ones that I enjoyed most. This was especially true for the field trips where we were able to put theory into practice. Observing how some plants grew near others, I wondered about plant-plant and plant-soil relationships, and I started asking myself what mechanisms underly such a complex instance of coexistence.
Currently, I am finishing my PhD studies at the Rey Juan Carlos University in Madrid, and I would like to continue contributing to our understanding of plant assembly mechanisms. Indeed, it seems to me that it’s a topic with many unresolved questions. In addition, I feel like this knowledge could help us to predict what will happen to some communities under future climate change scenarios. For any other budding ecologists, if I had to give any advice, it would be to enjoy every step in the research activity. Sometimes, I think about it and I would like to go back and take stock off and relish the small goals because it is only now that I realise how important they were.
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