How do trees die following low intensity fires: Exploring the hydraulic death hypothesis

 

Show simple item record

dc.contributor.advisor West, Adam en_ZA
dc.contributor.author Nel, Jacques Adriaan en_ZA
dc.date.accessioned 2015-05-27T04:10:05Z
dc.date.available 2015-05-27T04:10:05Z
dc.date.issued 2014 en_ZA
dc.identifier.citation Nel, J. 2014. How do trees die following low intensity fires: Exploring the hydraulic death hypothesis. University of Cape Town. en_ZA
dc.identifier.uri http://hdl.handle.net/11427/12921
dc.description Includes bibliographical references. en_ZA
dc.description.abstract The mechanism by which trees die following a low intensity fire is poorly understood. Traditionally, cambial necrosis (Carbon starvation hypothesis) has been used to explain post-fire tree mortality, however, this does not explain why some trees die within days following a fire. To address this, the hydraulic death hypothesis argues that post-fire tree mortality is a result of a reduction in hydraulic conductance rather than the necrosis of cambium. There are a number of proposed mechanisms by which hydraulic failure can occur during a fire: firstly, plume-induced cavitation as a result of high vapor pressure deficit (VPD) in a fire-plume has been shown to reduce hydraulic conductance. Secondly, changing surface tension as water is heated has also been shown to increase the chance of cavitation. The final mechanism is a reduction in conductance as a result of direct xylem vessel deformation due to the visco-elastic properties of xylem walls (lignin). In order to determine the relative importance of each proposed mechanism, stems of Kiggelaria africana and Eucalyptus cladocalyx were exposed to 70 and 100„aC in two treatments designed to isolate the effect of each mechanism. An oven treatment was used as a surrogate for a fire-plume in order to demonstrate VPD-induced cavitation and a water bath treatment (transpiration inhibited) was used to demonstrate xylem deformation (along with microscopy). This was possible because post-treatment flushing was indicative of the initial cause of the reduction as cavitation is reversible while deformation is permanent. The data was then explored using a Hydraulic Death model we created based on a xylem conductance model from literature. The results showed that VPD-induced cavitation as well as deformation are able to reduce hydraulic conductance in trees exposed to fire, however, E. cladocalyx showed higher loss of conductance at 65„aC than K. africana and deformation was only seen to occur in water bath treatments and only in K. africana. Here we propose that a chain of events provides a mechanism for slowing the rate of heating in branches exposed to a fire-plume and that cavitation plays a protective role. Model exploration implied that vulnerability segmentation is responsible for preventing fire-plume induced runaway cavitation. This is in agreement with the ¡§safety valve hypothesis¡¨, however, rather than drought stress, it is a fire-plume which causes the cavitation. It was also found that E. cladocalyx was able to prevent deformation because of xylem vessel characteristics (thick vessel walls) and not bark properties. We propose that the necrosis of cambium and phloem leading to the inability to refill cavitated vessels is the actual cause of mortality in trees exposed to low intensity fires. The ability to refill is dependent on water availability and carbohydrate content, which is highly sensitive to drought. As resprouters store water and carbohydrates in lignotubers and stems, they are less sensitive to pre-fire conditions. However, the survival of cambium and phloem is essential to the refilling process and thus the mechanism for reducing heat transfer, bark properties as well as xylem characteristics work in combination to determine persistence after a fire. en_ZA
dc.language.iso eng en_ZA
dc.subject.other Conservation Biology en_ZA
dc.title How do trees die following low intensity fires: Exploring the hydraulic death hypothesis en_ZA
dc.type Bachelor Thesis
uct.type.publication Research en_ZA
uct.type.resource Thesis en_ZA
dc.publisher.institution University of Cape Town
dc.publisher.faculty Faculty of Science en_ZA
dc.publisher.department Department of Biological Sciences en_ZA
dc.type.qualificationlevel Honours
dc.type.qualificationname BSc (Hons) en_ZA
uct.type.filetype Text
uct.type.filetype Image
dc.identifier.apacitation Nel, J. A. (2014). <i>How do trees die following low intensity fires: Exploring the hydraulic death hypothesis</i>. (Thesis). University of Cape Town ,Faculty of Science ,Department of Biological Sciences. Retrieved from http://hdl.handle.net/11427/12921 en_ZA
dc.identifier.chicagocitation Nel, Jacques Adriaan. <i>"How do trees die following low intensity fires: Exploring the hydraulic death hypothesis."</i> Thesis., University of Cape Town ,Faculty of Science ,Department of Biological Sciences, 2014. http://hdl.handle.net/11427/12921 en_ZA
dc.identifier.vancouvercitation Nel JA. How do trees die following low intensity fires: Exploring the hydraulic death hypothesis. [Thesis]. University of Cape Town ,Faculty of Science ,Department of Biological Sciences, 2014 [cited yyyy month dd]. Available from: http://hdl.handle.net/11427/12921 en_ZA
dc.identifier.ris TY - Thesis / Dissertation AU - Nel, Jacques Adriaan AB - The mechanism by which trees die following a low intensity fire is poorly understood. Traditionally, cambial necrosis (Carbon starvation hypothesis) has been used to explain post-fire tree mortality, however, this does not explain why some trees die within days following a fire. To address this, the hydraulic death hypothesis argues that post-fire tree mortality is a result of a reduction in hydraulic conductance rather than the necrosis of cambium. There are a number of proposed mechanisms by which hydraulic failure can occur during a fire: firstly, plume-induced cavitation as a result of high vapor pressure deficit (VPD) in a fire-plume has been shown to reduce hydraulic conductance. Secondly, changing surface tension as water is heated has also been shown to increase the chance of cavitation. The final mechanism is a reduction in conductance as a result of direct xylem vessel deformation due to the visco-elastic properties of xylem walls (lignin). In order to determine the relative importance of each proposed mechanism, stems of Kiggelaria africana and Eucalyptus cladocalyx were exposed to 70 and 100„aC in two treatments designed to isolate the effect of each mechanism. An oven treatment was used as a surrogate for a fire-plume in order to demonstrate VPD-induced cavitation and a water bath treatment (transpiration inhibited) was used to demonstrate xylem deformation (along with microscopy). This was possible because post-treatment flushing was indicative of the initial cause of the reduction as cavitation is reversible while deformation is permanent. The data was then explored using a Hydraulic Death model we created based on a xylem conductance model from literature. The results showed that VPD-induced cavitation as well as deformation are able to reduce hydraulic conductance in trees exposed to fire, however, E. cladocalyx showed higher loss of conductance at 65„aC than K. africana and deformation was only seen to occur in water bath treatments and only in K. africana. Here we propose that a chain of events provides a mechanism for slowing the rate of heating in branches exposed to a fire-plume and that cavitation plays a protective role. Model exploration implied that vulnerability segmentation is responsible for preventing fire-plume induced runaway cavitation. This is in agreement with the ¡§safety valve hypothesis¡¨, however, rather than drought stress, it is a fire-plume which causes the cavitation. It was also found that E. cladocalyx was able to prevent deformation because of xylem vessel characteristics (thick vessel walls) and not bark properties. We propose that the necrosis of cambium and phloem leading to the inability to refill cavitated vessels is the actual cause of mortality in trees exposed to low intensity fires. The ability to refill is dependent on water availability and carbohydrate content, which is highly sensitive to drought. As resprouters store water and carbohydrates in lignotubers and stems, they are less sensitive to pre-fire conditions. However, the survival of cambium and phloem is essential to the refilling process and thus the mechanism for reducing heat transfer, bark properties as well as xylem characteristics work in combination to determine persistence after a fire. DA - 2014 DB - OpenUCT DP - University of Cape Town LK - https://open.uct.ac.za PB - University of Cape Town PY - 2014 T1 - How do trees die following low intensity fires: Exploring the hydraulic death hypothesis TI - How do trees die following low intensity fires: Exploring the hydraulic death hypothesis UR - http://hdl.handle.net/11427/12921 ER - en_ZA


Files in this item

This item appears in the following Collection(s)

Show simple item record