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Long-term physiological responses of herbaceous plant species from contrasting functional groups and environments to centennial climate change

Final Report Summary - LEAFISOTRENDS (Long-term physiological responses of herbaceous plant species from contrasting functional groups and environments to centennial climate change)

The aim of this project was to assess possible century-long physiological responses of different herbaceous plant species from different functional groups and different habitats across Switzerland to past changes in climate. Such investigations are important as they allow a species- and/or functional groups specific assessment of long-term processes of acclimation to global environmental change in plants. The project involved the carbon, nitrogen and oxygen isotopic analysis of 3334 herbarium specimens from the Herbaria at the University of Basel. The isotopic analysis of archived plant material offers the exceptional opportunity to reconstruct the physiological activity of plants in the past and thus, to asses possible plant physiological responses to environmental changes occurred during the last centuries. The carbon and oxygen isotopic composition of plant tissues constitute an integrated indicator of plant stomatal conductance and photosynthetic assimilation rates that occurred during the formation of plant material. The nitrogen isotopic composition of plant material can be used as an integrator of the nitrogen cycle.
The 3334 analyzed specimens belong to 85 herbaceous plant species that were collected at several locations within Switzerland and from year 1820 until today. The selected plant species belong to different plant functional types (grasses, sedges, legumes and forbs) and have contrasting habitat preferences and mycorrhizal status. In addition, specimens were collected in locations with contrasting climate, elevation and N deposition levels. The influence of all these factors on the long-term physiological responses of herbaceous plants to changes in climate can be tested due to the large amount of samples analyzed in this project.
We found that herbaceous plants have increased their intrinsic water use efficiency with time in response to increasing atmospheric CO2 concentration since 1850. Increased photosynthesis and reduced stomatal conductance may be responsible for this change. Interestingly, the increment of plant intrinsic water use efficiency was more pronounced at higher elevations. This can be explained by the known higher efficiency of CO2 assimilation of alpine plants compared to plants from lowlands. There were also differences among functional groups, with grasses and forbs showing higher increments than legumes and sedges. In addition, sedges increased their leaf C/N and decreased their leaf N concentration with time, indicating that an enhancement of photosynthesis with higher atmospheric CO2 may have been limited by N availability in sedges. Imbalances in plant physiological responses to climate changes across functional types and locations may be responsible for changes in biodiversity and plant communities’ composition.
The data also show a positive correlation between plant intrinsic water use efficiency and leaf oxygen stable isotope composition (across time, locations and plant functional types). This is an important finding that shows how tightly coupled are the responses of the carbon and water cycles to global environmental changes. Increased intrinsic water use efficiency of plants with time is not only related to changes in photosynthetic rates associated to higher atmospheric CO2 concentrations but is also driven by decreased stomatal conductance.
In addition, herbaceous plants showed a decreasing trend with time in their N isotopic composition. Plant N isotopic composition is an integrated indicator of changes in the fractionating processes that occur along the N cycle until the N is assimilated by the plant. A decreasing trend with time in the N isotopic composition of plants may reflect changes in the source of N used by plants and in the intensity of their mychorrizal associations (which also influence the N isotope composition of plants). It may indicate a progressive tightening of the N cycle due to higher biological activity in the soil with increasing atmospheric CO2 concentration. Interestingly, we found that plant N isotope composition decreased more in fertile habitats, where fast growing species may be tightening the N cycle even more rapidly. In addition, N deposition intensity did not significantly influence the trend of plant δ15N values with time. We did not find evidence that the decreasing trend in the N isotope composition of plants is driven by changes in the intensity of their association with mychorriza, which has been a long-debated question: leaf N isotope composition decreased as well, and in a more pronounced manner, in plants that do not show mychorrizal associations.
The results from this project corroborate some of the findings of other short-term experiments that assessed plant responses to climate change and validate the possibility to extrapolate their results to make long-term predictions. However, the results from this project also highlight the fact that plant physiological responses differ by plant functional type and environmental conditions. This project will help that these factors are taken into account in global change models.