Effects of burning of slash piles of Acacia spp. and Eucalyptus camaldulensis biomass on soil physicochemical properties within Western Cape riparian and terrestrial areas

Thesis (MScConsEcol)--Stellenbosch University, 2016.

ENGLISH ABSTRACT: Removal of woody invasive alien plants from riparian and terrestrial fynbos ecosystems usually leads to the accumulation of large volumes of plant biomass. The three study species, Acacia mearnsii, Acacia saligna and Eucalyptus camaldulensis are well known for their ability to considerably increase above-ground biomass production. While some biomass may be removed for use as timber, or chipped for export, in cases where there is no alternative use for this biomass, or where site accessibility is an issue, burning of biomass in the form of piles or rows is practiced as a way of destroying biomass in situ. However, this approach has been reported to produce high temperatures, which may lead to altered soil properties and destroyed soil stored seed banks. Burn scars may develop on soil surfaces that were exposed to the burning of slash piles, which may remain unvegetated for extended periods of time. As a consequence, restoration may be patchy, uneven or delayed in post-clearing landscapes. The aim of this study was to evaluate the seasonal and spatial effects of burning of slash piles of Acacia spp. and Eucalyptus spp. biomass on soil physicochemical properties. Four riparian study areas (Hermon and Robertson, both dominated by Eucalyptus camaldulensis prior to clearing, and Wit River and Rawsonville, Acacia mearnsii dominated) and one terrestrial study area (Blaauwberg; Acacia saligna) were selected within the fynbos biome. Burning was conducted in spring 2014 (Hermon and Blaauwberg) and winter 2015 for all other study areas. Acacia mearnsii and A. saligna piles had a volume of between 21.01 and 88.17m3 and E. camaldulensis stacks had a volume of between 93.93 and 116.68 m3. Soil samples were collected from the topsoil layer, 0-10 cm depth, prior to burning, post-fire and three subsequently seasons, from within the burn scars (in the centre, an intermediate position, i.e. between the edge and centre, and the edge), from the soil matrix (about 2 m from the edge), from a recovering reference site and from an invaded reference site. The collected samples were subjected to laboratory analyses for pH, electrical conductivity (EC), total carbon (C) and nitrogen (N), available N, available phosphorus (P), exchangeable cations and hydrophobicity. At all study areas, soil pH (water), EC, available P and exchangeable cations increased significantly immediately after burning and had returned to pre-fire levels within one year of sampling, with the exception of soil pH, which persisted longer. This was with the exception of the Wit River riparian study area, where soil pH increased significantly and had returned to pre-fire within 3-4 months of sampling and soil EC was not affected at all. Total C and N responded differently across study areas, where it remained unchanged at Hermon and decreased significantly at Rawsonville. Available N was not initially affected by fire at any of the study areas, but later showed higher levels within fire scars in Acacia invaded areas. No such difference emerged within fire scars of Eucalyptus camaldulensis invaded areas, suggesting that nitrogen may be more readily available in fire scars of riparian Acacia invaded areas. Hydrophobicity increased only at Rawsonville (Acacia mearnsii) as a result of fire and was not affected by fire in other areas. At the terrestrial site, soil pH, EC and available P increased significantly, but returned to pre-fire levels after a few seasons, with the exception of pH, which remained significantly higher. The results from this study indicate that certain parameters such as soil pH, EC, available P and cations generally increase immediately after fire. In addition, the response of other properties including total C and N, available N and hydrophobicity may be governed by the characteristics of the ecosystem, soil type, burn fuel and seasonal variations. The implications of the study are that using fire as a tool for biomass management in post clearing landscapes may introduce unwanted soil physicochemical alterations, which may impact recovery, especially of native species.

AFRIKAANSE OPSOMMING: Die verwydering van houtagtige indringerplante in rivieroewers en terrestriële gebied in die fynbos-ekosisteem lei gewoonlik tot die opeenhoping van groot volumes plantbiomassa. Hierdie studie fokus op drie hoof indringerplantspesies in die Fynbos ekosisteem, naamlik Acacia mearnsii, Acacia saligna en Eucalyptus camaldulensis. Hierdie spesies is bekend vir hul vermoë om die bogrondse biomassa te verhoog. Verder, waar die verwydering van hierdie indringerplante vir kommersiële doeleindes, soos die verkoop van hout nie moontlik is nie, of in gevalle waar daar geen alternatiewe gebruik vir die oorblywende plant biomassa is nie, en waar daar die toeganglikheid van die ooblywende biomass problematies word, word die verbranding van hope of rye biomassa gebruik as 'n manier om van die biomassa ontslae te raak. Hierdie benadering lei tot ’n toename in temperature, wat kan lei tot die veranderinge in grondeienskappe en die vernietiging van die saadbank. Brandletsels kan ontwikkel op grond oppervlaktes weens die impak van vuur. Die impak van vuur verhinder die regenerasie van plante, en areas met brandletsels kan braak le vir lang tye. Die verlies van saadbank na die vuur impak dien as n moontlike faktor hoekom die plant nie kon groei in die gebrande areas nie. Gevolglik kan regenerasie kolsgewys, ongelyk of vertraagd plaasvind in post-restorasie landskappe. Die doel van hierdie studie was om die seisoenale en ruimtelike gevolge van die brand van Acacia en Eucalyptus biomassa op grondeienskappe te evalueer. Vier rivieroewer areas (Hermon en Robertson, gedomineer deur Eucalyptus camaldulensis voor die verwydering van biomass, en Witrivier en Rawsonville, gedomineer deur Acacia mearnsii) en een terrestriële studiegebied (Blaauwberg, Acacia saligna) is gekies in die fynbos bioom. Die brand in Hermon en Blaauwberg is uitgevoer in durende die lente in 2014, terwyl die brand in ander areas in die winter in 2015 uitgevoer is. Die volume van die biomassahope van Acacia mearnsii en A. saligna was tussen 21,01 en 88.17m3, waar van die volume in E. camaldulensis tussen 93,93 en 116,68 m3 was. Grondmonsters is geneem voor die brand, na die brand en drie seisoene opeenvolgend na die brand. Die monsters bevat grond vanaf die boonste grondlaag (0-10 cm diepte) en die monsters is geneem vanuit die volgende areas: (i) in die middel, (ii) by die intermediêre areas (die areas tussen die rand en die middle), (iii) die rand van die biomassahope, en (iv) die grond matriks (sowat 2 m van die rand). ’n Herstelde verwysings area en ’n indringerverwysings areas is ook in die studie ingesluit. Die versamelde monsters is onderwerp aan laboratorium ontleedings vir pH, elektriese geleiding (EG), totale koolstof (C) en stikstof (N), beskikbare N, beskikbare fosfor (P), uitruilbare katione en hidrofobisiteit. Al die areas het getoon dat vlakke van grond pH (water), EG, beskikbare P en uitruilbare katione beduidend toegeneem het na die brand. Al die chemiese eienskappe het teruggekeer na die vlakke voor die vuur, en wel binne ‘n periode van een jaar, met die uitsondering van grond pH, wat langer hoog gebly het. Dit is in teenstelling met die Wit river studiearea, waar grond pH toegeneem het na die vuur en teruggekeer het na vlakke voor die vuur binne 3-4 maande, sonder om die grond EG te affekteer. Elke studie area het verskillende tendense in terme van die totale C en N getoon, byvoorbeeld, die Hermon area het minimale verandering getoon waar Rawsonville ’n statisties beduidende verskil getoon het. Die beskikbare N was aanvanklik nie geraak deur die brand nie, maar later is daar beduidend hoër vlakke binne die gebrand areas van Acacia gevind. Hierdie bevindinge is nie getoon in areas wat beinvloed was deur die brand van Eucalyptus camaldulensis biomassahope nie. Hierdie resultate beteken dat N na die brand meer beskikbaar was in brandletsels in Acacia ingedringde rivieroewerareas. Slegs die Rawsonville (Acacia mearnsii) area het n toename in hidrofobisities na die brand getoon, ‘n tendens wat nie by deur ander areas getoon is nie. By die terrestriële studiearea het die grond pH, EG en beskikbare P aansienlik toegeneem, en al die faktore het teruggekeer na die vlakke voor die vuur na 'n paar seisoene, met die uitsondering van grond pH wat aansienlik hoër gebly het. Die resultate van hierdie studie dui daarop aan dat sekere parameters soos grond pH, EG, beskikbaar P en katione onmiddelik toeneem na ’n brand. Verder, die reaksie van ander eienskappe, insluitend die totale C en N, beskikbaar N en hidrofobisiteit mag moontlik beheer word deur die invloed van die ekosisteem, grondtipe, brandstof en seisoenale variasies. Die implikasies van hierdie studie is dat die gebruik van vuur om oorblywende plant biomassa te beheer (na die verwydering van indringerplante), mag moontlik ongewenste fisiese en chemiese veranderinginge veroorsaak wat die herstel van inheemse spesies kan beinvloed.