Groner, Vivienne; (2017) Effects of plant diversity on simulated climate-vegetation interaction towards the end of the African Humid Period. Doctoral thesis (Ph.D), University of Hamburg.

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Abstract

This thesis explores the role of plant diversity in simulated climate–vegetation interaction towards the end of the “African Humid Period” (AHP) in models of different complexity, from the conceptual model by Claussen et al. (2013) to the Dynamic Global Vegetation Model JSBACH, the land component of the Max Planck Earth System Model MPIESM1. In the light of recently published pollen data and the current state of ecological literature, the conceptual model by Claussen et al. (2013) reproduces the main features of different plant types interacting together with climate, but it does not capture the reconstructed AHP plant diversity. With a new model version adjusted to AHP vegetation, I can simulate a diverse mosaic-like environment as reconstructed from pollen, and I observe a stabilizing effect of high plant diversity on vegetation cover and precipitation. Plant composition ultimately determines the stability of the climate–vegetation system. The assessment of plant diversity in JSBACH illustrates that the “Plant Functional Type” (PFT) concept is not capable to capture AHP plant diversity and cannot depict mosaic-like environments as reconstructed from pollen. However, offline simulations with different PFT compositions confirm that high PFT diversity can smooth the vegetation response to a – here prescribed linear – precipitation decline. Eventually, the steepness of vegetation decline depends on the composition rather than on the number of PFTs. In coupled ECHAM6/JSBACH simulations, PFT diversity significantly affects land surface parameters, water cycling, surface energy budget, and atmospheric circulation patterns during the AHP as well as the rate and timing of the transition from a wet “green” state to a dry “desert” state. Higher precipitation is not necessarily associated with a higher vegetation cover fraction and a higher stability of the climate–vegetation system, but determined by the properties of prevailing PFTs, thus PFT composition. Despite different underlying assumptions, all considered levels of model complexity lead to the same conclusions: high plant diversity could stabilize a climate–vegetation system, but plant composition is the decisive factor for the climate–vegetation feedback strength and consequently for the system response to changes in orbital forcing. This highlights that the choice of plant types/PFTs and their representation in models significantly affect simulated climate–vegetation interaction during the AHP, the extent of the “green” Sahara, and the timing and rate of transition to the “desert” state. From this I conclude that accounting for plant diversity in future studies – not only on palaeoclimates – could significantly improve the understanding of climate–vegetation interaction and the simulation of the vegetation response to changing climate.

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