Abstracts

Savannas cover a broad geographicalrange across continents and are a biome best described by a mix ofherbaceous and woody plants. The former create a more or lesscontinuous layer while the latter should be sparse enough to leavean open canopy. What has long intrigued ecologists is how these twocompeting plant life forms of vegetation coexist. Initiallyattributed to resource competition, coexistence was considered thestable outcome of a root niche differentiation between trees andgrasses. The importance of environmental factors became evidentlater, when data from moister environments demonstrated that treecover was often lower than what the rainfall conditions would allowfor. Our current understanding relies on the interaction ofcompetition and disturbances in space and time. Hence, theinfluence of grazing and fire and the corresponding feedbacks theygenerate have been keenly investigated. Grazing removes grasscover, initiating a self-reinforcing process propagating tree coverexpansion. This is known as the encroachment phenomenon. Fire, onthe other hand, imposes a bottleneck on the tree population byhalting the recruitment of young trees into adulthood. Sincegrasses fuel fires, a feedback linking grazing, grass cover, fire,and tree cover is created. In African savannas, which are the focusof this dissertation, these feedbacks play a major role in thedynamics. The importance of these feedbacks came into sharpfocus when the notion of alternative states began to be applied tosavannas. Alternative states in ecology arise when different statesof an ecosystem can occur under the same conditions. According tothis an open savanna and a tree-dominated savanna can be classifiedas alternative states, since they can both occur under the sameclimatic conditions. The aforementioned feedbacks are critical inthe creation of alternative states. The grass-fire feedback canpreserve an open canopy as long as fire intensity and frequencyremain above a certain threshold. Conversely, crossing a grazingthreshold can force an open savanna to shift to a tree-dominatedstate. Critically, transitions between such alternative states canproduce hysteresis, where a return to pre-transition conditionswill not suffice to restore the ecosystem to its original state. In the chapters that follow, I will cover aspects relating to thecoexistence mechanisms and the role of feedbacks in tree-grassinteractions. Coming back to the coexistence question, due to theoverwhelming focus on competition and disturbance another importantecological process was neglected: facilitation. Therefore, in thefirst study within this dissertation I examine how facilitation canexpand the tree-grass coexistence range into drier conditions. Forthe second study I focus on another aspect of savanna dynamicswhich remains underrepresented in the literature: the impacts ofinter-annual rainfall variability upon savanna trees and theresilience of the savanna state. In the third and final studywithin this dissertation I approach theAdvisors/Committee Members: Jeltsch, Florian (advisor).