Periodic Reporting for period 1 - THrESholds (Thresholds in past relative Humidity and Ecosystem Structure in Africa)
Periodo di rendicontazione: 2021-09-01 al 2023-08-31
Tropical forests and savannas, vital for one-fifth of global human population, face challenges in predicting their responses to changing conditions like rainfall and relative humidity. These ecosystems function as alternative stable states due to feedback loops involving fire and vegetation. Understanding the thresholds in environmental conditions that trigger transitions between these vegetation states is crucial, but current data is limited due to the extended timescales involved.
Fossil records in lake sediments spanning millennia offer valuable data, though their use presents methodological challenges, particularly when reconstructing variables like vegetation, rainfall, and RH. The present project aimed to address these limitations by participating to the development of a quantitative RH proxy based on the triple oxygen isotope analysis of phytoliths and identifying the RH thresholds responsible for transitions between forests and savannas in Africa over the past 5,000 years.
Historically, the ecological dynamics of these ecosystems were not adequately considered in vegetation models, leading to mismanagement strategies such as fire suppression and afforestation in savannas. These strategies, while aimed at mitigating climate change, have adverse effects on biodiversity and local populations. To address this, it is essential to integrate the long-term ecological functioning of tropical forests and savannas into vegetation models, making the quantification of RH thresholds critical.
Fossil records in lake sediments spanning millennia offer valuable data, though their use presents methodological challenges, particularly when reconstructing variables like vegetation, rainfall, and RH. The present project aimed to address these limitations by participating to the development of a quantitative RH proxy based on the triple oxygen isotope analysis of phytoliths and identifying the RH thresholds responsible for transitions between forests and savannas in Africa over the past 5,000 years.
Historically, the ecological dynamics of these ecosystems were not adequately considered in vegetation models, leading to mismanagement strategies such as fire suppression and afforestation in savannas. These strategies, while aimed at mitigating climate change, have adverse effects on biodiversity and local populations. To address this, it is essential to integrate the long-term ecological functioning of tropical forests and savannas into vegetation models, making the quantification of RH thresholds critical.
A team, led by the supervisor, has pioneered a new method using the 17O-excess of phytoliths, silica particles found in plants and used as a proxy for past vegetation, to quantify changes in relative humidity (RH). But essential questions remained before applying this new proxy in lacustrine sediments; this is what we addressed in the present project.
First, we determined the time frame in which the 17O-excess of phytoliths accurately reflects RH. We examined the 17O-excess in rainfall, soil water, grass and tree phytoliths at various sites in Benin and Senegal. We demonstrated that the 17O-excess of phytoliths accurately records RH of the growing season (+/- 6%), corresponding to the rainy season for C4 grasses, dominating in savannas, and to the entire year for evergreen and semi-evergreen trees in tropical dry and moist forests of West Africa.
We also explored in which tree tissues were produced globular granulate morphotypes found in large quantities in forest soils and characteristic of tropical trees. We sampled phytoliths from different parts of trees (bark, wood and leaves) and found that partial dissolution modifies morphotypes assemblages from tissues to soils and sediments. Identifying the source of globular granulate phytoliths proved challenging, as not all tree species produce this morphotype.
We then assessed whether phytoliths originating from riparian vegetation around Lake Guiers (Senegal) introduced a bias in 17O-excess measurements of the total sedimentary phytoliths assemblage. We found that phytoliths from this vegetation, as well as other grass species, recorded RH from the growing season and did not introduce significant bias.
We also examined how phytoliths were transported and deposited into these lake sediments. We compared top-core phytolith assemblages with soil samples from different land-cover types in the lake's catchment area, and with a land-cover analysis at different time period around Lake Guiers. The analysis revealed a shift toward more degraded and open vegetation in the watershed, associated with increased agriculture and population density.
After constraining this new proxy, we applied it to long-term paleo-sequences from Lake Guiers and Lake Ngofouo. At Guiers, there was a substantial drop in RH that coincided with vegetation changes c. 2000 BP, suggesting that a threshold of 65% RH was associated with the vegetation transition. Phytolith analysis of Lake Ngofouo suggests that around 2000 BP, the region was likely covered by a large swamp forest, the RH was high above 80%, with no recorded fires. The swamp forest started shrinking around 1500 BP, leading to increased watershed contributions and erosion episodes. At 1500 BP, fires emerged, causing forest fragmentation and the appearance of savanna, coinciding with a decline in RH. In this case, it seems that the transition from forest to savanna was not associated with a change in RH, but rather with an increased fire activity
These findings have been presented at conferences and discussed during radio interviews. The research resulted in one submitted paper, with three more in preparation. The project beneficiated from the framework, technical support and collaborations of two ANR projects, HUMI-17 (2017-2022) and PAST-17 (2023-2027).
First, we determined the time frame in which the 17O-excess of phytoliths accurately reflects RH. We examined the 17O-excess in rainfall, soil water, grass and tree phytoliths at various sites in Benin and Senegal. We demonstrated that the 17O-excess of phytoliths accurately records RH of the growing season (+/- 6%), corresponding to the rainy season for C4 grasses, dominating in savannas, and to the entire year for evergreen and semi-evergreen trees in tropical dry and moist forests of West Africa.
We also explored in which tree tissues were produced globular granulate morphotypes found in large quantities in forest soils and characteristic of tropical trees. We sampled phytoliths from different parts of trees (bark, wood and leaves) and found that partial dissolution modifies morphotypes assemblages from tissues to soils and sediments. Identifying the source of globular granulate phytoliths proved challenging, as not all tree species produce this morphotype.
We then assessed whether phytoliths originating from riparian vegetation around Lake Guiers (Senegal) introduced a bias in 17O-excess measurements of the total sedimentary phytoliths assemblage. We found that phytoliths from this vegetation, as well as other grass species, recorded RH from the growing season and did not introduce significant bias.
We also examined how phytoliths were transported and deposited into these lake sediments. We compared top-core phytolith assemblages with soil samples from different land-cover types in the lake's catchment area, and with a land-cover analysis at different time period around Lake Guiers. The analysis revealed a shift toward more degraded and open vegetation in the watershed, associated with increased agriculture and population density.
After constraining this new proxy, we applied it to long-term paleo-sequences from Lake Guiers and Lake Ngofouo. At Guiers, there was a substantial drop in RH that coincided with vegetation changes c. 2000 BP, suggesting that a threshold of 65% RH was associated with the vegetation transition. Phytolith analysis of Lake Ngofouo suggests that around 2000 BP, the region was likely covered by a large swamp forest, the RH was high above 80%, with no recorded fires. The swamp forest started shrinking around 1500 BP, leading to increased watershed contributions and erosion episodes. At 1500 BP, fires emerged, causing forest fragmentation and the appearance of savanna, coinciding with a decline in RH. In this case, it seems that the transition from forest to savanna was not associated with a change in RH, but rather with an increased fire activity
These findings have been presented at conferences and discussed during radio interviews. The research resulted in one submitted paper, with three more in preparation. The project beneficiated from the framework, technical support and collaborations of two ANR projects, HUMI-17 (2017-2022) and PAST-17 (2023-2027).
This project significantly advanced the state of the art in several key areas.
1.Development of a New Quantitative Proxy. Historically, there have been very few proxies capable of quantitatively reconstructing variables such as rainfall, RH, or tree cover. The innovation here lies in utilizing the triple isotope composition of oxygen in phytoliths to achieve this quantitative reconstruction. In particular we demonstrated that phytoliths 17Oexcess records RH of the growing season when transpiration and silicification occur. The distinction between how this process functions in tropical trees and grasses is crucial and we found that C4 grasses record RH during the rainy season, while trees capture mean annual RH. This knowledge is crucial, considering that grasses dominate in savannas and trees in dry and moist forests, and that depending on the dominating ecosystem we do not record the same RH.
2.Improving our understanding of taphonomic processes. By comparing sedimentary phytoliths records with phytoliths from riverine vegetation and soils from different land-cover types, we showed that this riverine vegetation did not impact the overall contribution of the phytoliths from the watershed.
3.Reconstruction of Past RH in Different Ecosystems: We reconstructed past RH at two distinct sites, Lake Guiers (Senegal) and Ngofouo (Republic of Congo). We showed the critical role of RH changes in triggering shifts in vegetation, particularly at Guiers. Conversely, at Ngofouo, fire regime changes were a key factor in influencing ecosystem changes, despite subsequent increases in RH that were compensated by increased fire activity. This demonstrates the complex and dynamic nature of these ecosystems and the importance of considering feedback loops, particularly those involving fire.
4.Implications for Savanna Management: The project's results have significant implications for savanna management, suggesting that fire is a crucial factor in maintaining savanna distributions. Indeed, we showed that even if climate becomes more humid, fire continues to play a vital role in limiting woody encroachment and preserving savanna ecosystems. This insight offers a more comprehensive view of savanna functioning over long time scale.
Overall, this project has advanced the field's knowledge and methodologies in reconstructing past climate variables, interpreting phytoliths assemblages and oxygen isotopes composition, and understanding the dynamic interactions within tropical ecosystems. It enhances our ability to make more accurate paleoclimatological interpretations and informs strategies for the sustainable management of savanna environments.
1.Development of a New Quantitative Proxy. Historically, there have been very few proxies capable of quantitatively reconstructing variables such as rainfall, RH, or tree cover. The innovation here lies in utilizing the triple isotope composition of oxygen in phytoliths to achieve this quantitative reconstruction. In particular we demonstrated that phytoliths 17Oexcess records RH of the growing season when transpiration and silicification occur. The distinction between how this process functions in tropical trees and grasses is crucial and we found that C4 grasses record RH during the rainy season, while trees capture mean annual RH. This knowledge is crucial, considering that grasses dominate in savannas and trees in dry and moist forests, and that depending on the dominating ecosystem we do not record the same RH.
2.Improving our understanding of taphonomic processes. By comparing sedimentary phytoliths records with phytoliths from riverine vegetation and soils from different land-cover types, we showed that this riverine vegetation did not impact the overall contribution of the phytoliths from the watershed.
3.Reconstruction of Past RH in Different Ecosystems: We reconstructed past RH at two distinct sites, Lake Guiers (Senegal) and Ngofouo (Republic of Congo). We showed the critical role of RH changes in triggering shifts in vegetation, particularly at Guiers. Conversely, at Ngofouo, fire regime changes were a key factor in influencing ecosystem changes, despite subsequent increases in RH that were compensated by increased fire activity. This demonstrates the complex and dynamic nature of these ecosystems and the importance of considering feedback loops, particularly those involving fire.
4.Implications for Savanna Management: The project's results have significant implications for savanna management, suggesting that fire is a crucial factor in maintaining savanna distributions. Indeed, we showed that even if climate becomes more humid, fire continues to play a vital role in limiting woody encroachment and preserving savanna ecosystems. This insight offers a more comprehensive view of savanna functioning over long time scale.
Overall, this project has advanced the field's knowledge and methodologies in reconstructing past climate variables, interpreting phytoliths assemblages and oxygen isotopes composition, and understanding the dynamic interactions within tropical ecosystems. It enhances our ability to make more accurate paleoclimatological interpretations and informs strategies for the sustainable management of savanna environments.