The longevity of decaying eucalyptus hybrid roots in sub-tropical plantation forests

Stephan, Johan Carl Jacobus (2019-02)

Thesis (MScFor)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: The global threat posed by extreme climate change has led to an increase in the amount of climate change related research. It is now more important than ever before to accurately quantify the carbon pools in terrestrial ecosystems, in order to better understand how these pools might influence the carbon cycle. The residence time of carbon in dead coarse roots (i.e. roots greater than 2 mm diameter), an often-neglected carbon pool, are still not well understood. (Fine roots are known to have rapid turnover, rates and was not considered in this study). The decay rate constant of decomposing roots after clear felling in Eucalyptus hybrid stands was determined using a chronosequence sampling approach followed by analysis of the density and carbon contents. The results were subsequently modelled with single component negative exponential model (k = 0.1058). Eucalyptus hybrid root systems in sub-tropical plantations took on average 6.6 years to lose 50% density, 13.1 years to lose 75% density and 28.3 years to lose 95% density. The relationships between root decomposition and root size class (2-10 mm, 10-50 mm, >50 mm diameter roots and tree stump) as well as site productivity (in the form of mean annual increment) were also investigated. Neither root size nor site productivity had significant relationships with root decomposition rate. Coarse root carbon content did not vary with time after felling or site productivity, but rather with root size. The mean carbon concentration for each root size class was 46.8 ± 1.6% (2-10 mm), 48.6 ± 1.9% (10-50 mm), 48.8 ± 1.4% (>50 mm) and 48.6 ± 2.3% (stump). The results showed that Eucalyptus hybrid coarse roots in subtropical plantations in South Africa should be regarded as an important long-term pool of sequestered carbon. The decay model is earmarked for inclusion in a South African forestry carbon calculator that estimates the stock changes of various above- and below ground carbon pools in forest ecosystems over time.

AFRIKAANSE OPSOMMING: Die bewuswording van die erns en spoed van klimaatverandering het gelei tot ‘n groot vermeerdering van klimaatverandering verwante navorsing. Dit is tans meer belangrik as ooit tevore om die koolstof wat vasgevang word in die verskeie ekosisteme se verskillende komponente (koolstof poele), akkuraat te kwantifiseer met die doel om hul invloed op die koolstof siklus beter te verstaan. Die potensiële bydrae van dooie houtagtige dik wortels (wortels groter as 2 mm) tot die koolstof retensie in plantasie sisteme is nog nie volledig beskryf nie. (Fyn wortels is bekend vir vinniger afbraaktempo’s en het nie deel gevorm van die navorsingsprojek nie). Die hoof doel van hierdie studie was om afbraak konstantes te bepaal vir Eucalyptus grandis x E. urophylla dik wortelklasse in subtropiese plantasies na kaalkapping. Dit was ook belangrik om die koolstof konsentrasie van die dikker wortelklasse te bepaal om te sien of dit verskil van waardes wat gereeld in die bedryf gebruik word. ‘n Negatiewe eksponensiële funksie was die bes passende model op die wortel afbraak data, en het ‘n afbraak konstante (k) van 0.1058 opgelewer. Eucalyptus hibriede se wortels het gemiddeld 6.6, 13.1 en 28.3 jaar geneem om 50%, 75% en 95% van hul digtheid te verloor. Die verhouding tussen veranderlikes soos wortelklas grootte en plantasie produktiwiteit, asook wortel afbraak is ook ondersoek. Die resultate toon dat daar geen statisties betekenisvolle verhouding tussen die veranderlikes wortel grootte en plantasie produktiwiteit, of die tempo van afbraak in dik wortelklasse is nie. Verder is daar gevind dat koolstofinhoud van dooie dik wortels nie betekenisvol verander met tyd of plantasie produktiwiteit nie, maar wel met wortel grootte. Die gemiddelde koolstofinhoud van die onderskeie wortelklasse was 46.8 ± 1.6% (2-10 mm), 48.6 ± 1.9% (10-50 mm), 48.8 ± 1.4% (>50 mm) en 48.6 ± 2.3% (stompe). Die resultate het bewys dat die dooie dik wortelklasse van Eucalyptus hibriede in sub-tropiese plantasies ʼn belangrike bron van gesekwestreerde koolstof is. Die afbraak model is beskikbaar vir toekomstige gebruik as komponent van ’n Suid Afrikaanse bosbou koolstof rekenaar model wat veranderinge in koolstof voorraad oor tyd binne plantasie sisteme bereken.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/105677
This item appears in the following collections: