The effect of fire on the root carbohydrate concentration and growth of encroaching terminalia sericea in a semi-arid savanna woodland in the Waterberg plateau park, Central Namibia
Lutibezi, Siphiwe Patience
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Fire is an ecological disturbance that may kill or topkill woody plants and control the biomass of woody species in disturbance prone areas, thus maintaining the co-existence of trees and grasses in savanna ecosystems. Relatively recently, savannas have been altered worldwide by a phenomenon known as bush encroachment, which is the suppression of grasses by woody species due to the increase in woody biomass. Terminalia sericea is a woody encroaching species which occurs in woodland savannas on dystrophic sandy soils; it often forms dense thickets and is considered as an encroacher on the Waterberg Plateau Park. The ability of encroaching woody species to resprout after topkill by fire, particularly in savanna ecosystems, is due to the already established and functioning carbohydrate reserves stored in the roots. From a management perspective, fire may be used as a preventative measure to combat encroachment of woody plants or less successfully as a symptom treatment. It may be possible to reduce the regrowth of woody encroaching species through the more frequent use of fire, if it is known how fast the root carbohydrate concentrations of resprouts return to pre-fire concentrations. The aim of the study was to investigate how time since last burn, in a semi-arid savanna woodland at the Waterberg Plateau Park, influences the growth of the encroaching T. sericea, in particular the replenishment of root carbohydrates subsequent to fire. The Waterberg Plateau Park is on the south-western side of the woodland to the north of the country on deep Kalahari sands. The study used the space for time substitution approach as described by Pickett (1989), to investigate the effect of fire on the resprouting ability, growth rate and the replenishment of root total non-structural carbohydrates of T. sericea. In October 2014, thirty T. sericea resprouts were collected from each of the four fire treatment blocks which were last burned 1, 2, 14 and 24 years ago, respectively. These treatments have a mean fire return interval ranging from every 6 to 19 years. The BECVOL method (Smit, 1996) was used to estimate aboveground components (tree height, leaf mass and canopy volume). In May 2015, the roots of twelve of the measured resprouts were excavated in fire blocks when the time since last burn was 2, 3, 15 and 25 years ago, respectively. Three discs were sawn from the top 50 cm of the main root and used for non-structural carbohydrate analysis and for approximate age determination. Material used for root total non-structural carbohydrate (TNC) analysis were oven dried, milled and sieved. For TNC analysis, oligosaccharides, polysaccharides and starch were extracted using various laboratory methods as described by Rohwer, E. A (2014). Age was determined for 33 of the samples by counting age rings using the LinTab apparatus. Fire had a strong and positive effect (p < 0.05) on the growth rate of the aboveground components by causing a rapid growth increase within the first two years which then slows ii down between 2 and 14 years after a fire. The recovery of T. sericea is rapid with a relative growth rate of 60 % for tree height, 140 % for canopy volume and 82 % for leaf mass, within a period of one year, between years one and two. In the third year after, there is the highest root TNC concentration. The rapid recovery of root TNC correlates well with aboveground growth. After a fire, mobilisation of root carbohydrates leads to the recovery of aboveground components that leads to an increase in leaves, photosynthesis then increases, which leads to the replenishment of root TNC, and this occurs within 2 to 3 years after a fire. In the study area T. sericea can be classified as a fire tolerant species because it coppices relatively fast and this leads to the rapid replenishment of root reserves. Controlled fire can be used to supress the regrowth of encroaching T. sericea once the period of root TNC replenishment is determined. Based on the carbohydrate reserves subsequent to fire, the current fire regime on the plateau could not suppress the growth of T. sericea.
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