Patterns and mechanisms of woody plant encroachment and impacts on ecosystem processes and services

Thesis (PhD)--Stellenbosch University, 2023.

ENGLISH ABSTRACT: Amid global climate change and heightened population growth especially in Africa, proper management of the grassy ecosystems that offer various services that are crucial to human well-being is paramount. Due to various land-use changes, these systems are transforming from a grassy to a woody- dominated system – a phenomenon termed woody plant encroachment (WPE). Driven by a combination of drivers such as fire suppression overgrazing and rising atmospheric CO2 fertilisation, WPE is affecting ecosystem functioning and services in these grassy ecosystems in Africa and worldwide. Despite this, the diverse impacts of WPE have not yet been comprehensively synthesized and the mechanisms behind WPE impacts and explanations of the associated patterns of change remain contested. The broad-scale impacts of encroachment are also likely to vary depending on the socio-economic status of the region and the dependence of the communities on natural resources. For instance, people in the Global South such as in Africa are likely to have a high reliance on ecosystem services when compared to people in the developed Global North such as in North America. This perspective has not yet been explored directly, though some global scale assessments show that WPE is widespread in grassy and savanna ecosystems globally. At the mechanistic level, the main impact of invading woody plants in grassy systems is assumed to relate to light capture by taller woody plants, determined by plant traits relating to the efficiency of light capture and canopy shading. In water-limited savannas, characterized by limited and stochastic rainfall, increasing woody cover can reduce plant available water, streamflow, and groundwater by altering evapotranspiration rates and rainfall partitioning in space, but the ecological relevance of this impact has been little studied. The potential inefficiency of water use by woody encroaching species in semi-arid savannas is a cause for concern and warrants the investigation of the influence of woody plant traits that determine rainfall interception via canopy interception in these systems as an alternative mechanism of woody plant impact to that of light capture, and the potential of these mechanisms for patterns and implications of encroachment. To address these issues, I systematically reviewed the literature on the impacts of woody encroachment on ecosystem services within the three broad categories of Nature’s Contributions to People (NCP), namely: material, non-material, and regulating NCP, in North American and African regions with contrasting socio-economic characteristics (Chapter 2), as a broad scale context for further work on the mechanisms involved at local scales in African savannas, I then narrowed focus in two subsequent chapters, exploring the alternative rainfall interception mechanism potentially involved in WPE in Africa as mentioned above. In Chapter 3 I quantified in a field-based study the altered partitioning of rainfall by two dominant woody plant structural types (fine- and broad-leaved trees) across a local gradient of encroachment in a semi-arid savanna in South Africa. I then further determined plant canopy and leaf traits associated with water loss through canopy capture in a series of controlled experiments conducted in the same field environment Chapter 4. From the review work, material NCP benefits were found to be generally more directly relevant to livelihoods in Africa and constituted the provision of wood materials for fuel, building, and browser forage. I found that the material NCP most adversely affected by woody encroachment was the reduction in herbaceous forage availability, leading to reductions in livestock numbers and products on both continents. Negative impacts on Non-Material NCP such as recreation, tourism and social amenities, herding, and pressure to diversify livestock were also reported for Africa. For regulating services, negative effects were reported on both continents in terms of soil health, habitat availability and quality, and on the regulation of hydrological services. Overall African WPE tended to be driven more by fine-leaved N2 fixing species than in North American ecosystems. In Chapter 3, I found that when averaged across both fine- and broad-leaved woody plant functional types, loss of rainfall through canopy interception and subsequent evaporation approximately doubled with a roughly 13-fold increase in woody cover. Changes in water partitioning comprised fourfold increases in stemflow and a decline in throughfall proportion of about two-fifths. Changes in partitioning were dependent on plant functional type. Rainfall interception by the fine-leaved multi-stemmed shrub Dichrostachys cinerea was almost double that of the broad-leaved tree Terminalia sericea at the highest levels of woody encroachment. Partitioning was also dependent on rainfall characteristics, with the proportion of rainfall intercepted inversely related to rainfall event size, and intensity. The observation of higher water loss by fine-leaved over broad-leaved species was consistent with the findings of Chapter 4, where I found that over and above variability among other species, the fine-leaved species recorded a significantly higher storage capacity than the broad-leaved species at both branch complex and leaf levels. Various canopy traits and leaf traits were more influential in contributing to variability among species. At the branch complex level, traits such as the number of leaf clusters and leaves on each branch as well as leaves in totality contributed to most of the variability in species. At the leaf level, leaf type, structure, apex, and laminar shape had the most influence. Canopy storage capacity had a strong positive correlation with the number of leaves, followed by the number of modules and branches, while at the leaf level, storage capacity had a negative correlation with the number of hairs and the number of waves on the leaf. Overall, this study demonstrated that the mechanism of rainfall interception in underpinning patterns of WPE in semi-arid savannas is at least a viable alternative to the ancillary mechanism of light interception. A major novel finding in this study is that increasing tree cover in African grassy ecosystems reduces the amount of canopy throughfall, especially beneath canopies of fine-leaved species in smaller rainfall events. Rainfall interception traits may thus confer a selective advantage, especially for fine-leaved woody plant species in semi-arid savannas. Understanding the relationships between plant traits and canopy water storage capacity can help guide better management of plant communities in these semi-arid regions where water availability is limited. Leaf and canopy traits that determine rainfall interception, stem flow, and canopy throughfall deserve further attention in mechanistic models of this phenomenon, and possibly in assessing the reasons for distinct types of WPE species between African and North America. Finally, while WPE may be seen as having potential benefits of increased above-ground carbon sequestration by woody biomass, the benefits of carbon sequestration through WPE need to be understood in the context of the diverse NCP impacts involved. The need for critical ecosystem services for African livelihoods such as livestock production, tourism revenue, and water provision is thus presented in the context of recommendations to control and reverse WPE in the final summary Chapter 5.

AFRIKAANSE OPSOMMING: Te midde van wêreldwye klimaatsverandering en verhoogde bevolkingsgroei, veral in Afrika, is die behoorlike bestuur van die grasagtige ekosisteme wat verskillende dienste bied wat van uiterste belang is vir menslike welstand, uiters belangrik. As gevolg van verskillende veranderinge in grondgebruik, transformeer hierdie stelsels van 'n gras tot 'n houtagtige gedomineerde stelsel – 'n verskynsel wat houtagtige plantbesetting genoem word (WPE). Aangedryf deur 'n kombinasie van bestuurders soos brandonderdrukking wat oorbeweiding en stygende CO2-bevrugting in die atmosfeer, beïnvloed WPE die funksionering van die ekosisteem en dienste in hierdie grasagtige ekosisteme in Afrika en wêreldwyd. Desondanks is die uiteenlopende gevolge van WPE nog nie omvattend gesintetiseer nie en bly die meganismes agter WPE-impakte en verduidelikings van die gepaardgaande veranderingspatrone betwis. Die breëskaalse gevolge van inbreukmaking sal waarskynlik ook wissel, afhangende van die sosio- ekonomiese status van die streek en die afhanklikheid van die gemeenskappe van natuurlike hulpbronne. Byvoorbeeld, mense in die Global South, soos in Afrika, sal waarskynlik 'n groot afhanklikheid van ekosisteemdienste hê in vergelyking met mense in die ontwikkelde Global North soos in Noord-Amerika. Hierdie perspektief is nog nie direk ondersoek nie, hoewel sommige wêreldwye skaalbeoordelings toon dat WPE wêreldwyd in grasagtige en savanne-ekosisteme wydverspreid voorkom. Op meganistiese vlak word aanvaar dat die belangrikste impak van die indringende houtagtige plante in grasstelsels verband hou met ligvang deur langer houtagtige plante, bepaal deur planteienskappe wat verband hou met die doeltreffendheid van ligte vang en afdak. In waterbeperkte savanne, gekenmerk deur beperkte en stogastiese reënval, kan toenemende houtagtige bedekking die beskikbare water, stroomvloei en grondwater verminder deur die verdampingsyfers en reënval in die ruimte te verander, maar die ekologiese relevansie van hierdie impak is min bestudeer. Die potensiële ondoeltreffendheid van watergebruik deur houtagtige enroerende spesies in semi-droë savanne is kommerwekkend en waarborg die ondersoek na die invloed van houtagtige planteienskappe wat reënvalonderskepping bepaal via afdakonderskepping in hierdie stelsels as 'n alternatiewe meganisme van houtagtige plantimpak op dié van ligopvang, en die potensiaal van hierdie meganismes vir patrone en implikasies van inbreukmaking. Om hierdie kwessies aan te spreek, het ek die literatuur stelselmatig nagegaan oor die gevolge van houtagtige inbreuk op ekosisteemdienste binne die drie breë kategorieë Natuurbydraes tot mense (NCP), naamlik: materiaal, nie-wesenlik, en die regulering van NCP, in Noord-Amerikaanse en Afrika-streke met kontrasterende sosio-ekonomiese eienskappe (Hoofstuk 2), as 'n breëskaalse konteks vir verdere werk aan die meganismes wat by plaaslike skale in Afrika-savanne betrokke is, het ek die fokus in twee daaropvolgende hoofstukke vernou, ondersoek die alternatiewe reënvalonderskepingsmeganisme wat moontlik by WPE in Afrika betrokke is, soos hierbo genoem. In hoofstuk 3 het ek in 'n veldgebaseerde studie die veranderde verdeling van reënval gekwantifiseer deur twee dominante houtagtige plantstruktuurtipes ( fine- en breëblaarbome ) oor 'n plaaslike helling van inbreukmaking 'n semi-droë savanne in Suid-Afrika. Ek het dan verder bepaal dat plantdak- en blaareienskappe wat verband hou met waterverlies deur afdakvang in 'n reeks beheerde eksperimente wat in dieselfde veldomgewing uitgevoer is, hoofstuk 4. Uit die hersieningswerk is bevind dat materiële NCP-voordele oor die algemeen meer direk relevant was vir lewensbestaan in Afrika en die voorsiening van houtmateriaal vir brandstof, gebou en blaaiervoer uitmaak. Ek het gevind dat die materiaal NCP wat die ergste deur houtagtige oortreding beïnvloed is, die vermindering van die beskikbaarheid van kruidagtige voer was, wat gelei het tot die vermindering van veegetalle en produkte op albei kontinente. Negatiewe gevolge vir nie-materiaal NCP soos ontspanning, toerisme en sosiale geriewe, kudde en druk om vee te diversifiseer, is ook vir Afrika gerapporteer. Vir die regulering van dienste is negatiewe gevolge op beide kontinente gerapporteer ten opsigte van grondgesondheid, beskikbaarheid van habitat en kwaliteit, en op die regulering van hidrologiese dienste. In die algemeen was Afrika-WPE geneig om meer gedryf te word deur fynblaar-N2- vasmaakspesies as in Noord-Amerikaanse ekosisteme. In hoofstuk 3 het ek gevind dat dit gemiddeld in beide fyn- en breëblaar houtagtige plantfunksionele soorte is, verlies aan reënval deur afdak onderskep en daaropvolgende verdamping het ongeveer verdubbel met 'n ongeveer 13-voudige toename in houtagtige bedekking. Veranderings in die verdeling van water het viervoudige toenames in stamvloei en 'n afname in die deurval van ongeveer twee vyfdes behels. Veranderings in verdeling was afhanklik van plantfunksionele tipe. Reënvalonderskepping deur die fynblaar-multi-stam struik Dichrostachys cinerea was byna dubbel dié van die breëblaarboom Terminalia sericea op die hoogste vlakke van houtagtige oortreding. Verdeling was ook afhanklik van reënvalkenmerke, met die hoeveelheid reënval wat omgekeerd onderskep is wat verband hou met die grootte van die reënval en intensiteit. Die waarneming van hoër waterverlies deur fynblaar oor breëblaarspesies stem ooreen met die bevindings van hoofstuk 4, waar ek gevind het dat bo en behalwe die veranderlikheid onder ander spesies, die fynblaarspesies het 'n aansienlik hoër opbergkapasiteit aangeteken as die breëblaarspesies op takkompleks en blaarvlakke. Verskeie afdakseienskappe en blaareienskappe was meer invloedryk om by te dra tot die veranderlikheid tussen spesies. Op die takkompleksvlak het eienskappe soos die aantal blaargroepe en blare op elke tak sowel as blare in totaliteit bygedra tot die grootste deel van die veranderlikheid by spesies. Op die blaarvlak het die blaartipe, struktuur, toppunt en laminêre vorm die grootste invloed gehad. Canopy-bergingskapasiteit het 'n sterk positiewe korrelasie met die aantal blare gehad, gevolg deur die aantal modules en takke, terwyl dit op die blaarvlak is, stoorkapasiteit het 'n negatiewe korrelasie gehad met die aantal hare en die aantal golwe op die blaar. In die algemeen het hierdie studie getoon dat die meganisme van reënvalonderskepping in die onderbou van WPE in semi-droë savanne ten minste 'n lewensvatbare alternatief is vir die aanvullende meganisme van ligte onderskep. 'N Groot nuwe bevinding in hierdie studie is dat die verhoging van boombedekking in Afrika-grasagtige ekosisteme die hoeveelheid afdak verminder, veral onder kanale van fynblaarspesies in kleiner reënvalgebeurtenisse. Reënval onderskep eienskappe kan dus 'n selektiewe voordeel inhou, veral vir fynblaar-houtagtige plantspesies in semi-droë savanne. Die begrip van die verhoudings tussen planteienskappe en die opberging van water kan help om 'n beter bestuur van plantgemeenskappe in hierdie semi-droë streke te lei waar die beskikbaarheid van water beperk is. Blaar- en afdakseienskappe wat reënvalonderskepping, stamvloei en afdak bepaal, verdien verdere aandag in meganistiese modelle van hierdie verskynsel, en moontlik om die redes vir duidelike filogeografiese patrone van WPE tussen Afrika en Noord-Amerika te beoordeel. Laastens, hoewel WPE gesien kan word as potensiële voordele van verhoogde koolstofsekwestrasie bo die grond deur houtagtige biomassa, die voordele van koolstofsekwestrasie deur WPE moet verstaan word in die konteks van die uiteenlopende NCP-impakte wat daaraan verbonde is. Die behoefte aan kritieke ekosisteemdienste vir Afrika-bestaan soos veeproduksie, toerisme-inkomste, en watervoorsiening word dus aangebied in die konteks van aanbevelings om houtagtige plantbesetting te beheer en om te keer in die finale opsomming Hoofstuk 5.

Doctoral