Abstract of dissertation submitted to the Senate of University of Aberdeen in fulfilment of
the requirements for the degree of Doctor of Philosophy

Effects of Periodic Drought on Acacia mangium Willd. and Acacia auriculiformis A. Cunn. ex. Benth Growing on Sand Tailings in Malaysia

by

Ang Lai Hoe
1996

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SUMMARY

        Peninsular Malaysia has large tracts of sand tailings because of mining activities since the 1930s, which remain unproductive until this day. Water deficit is identified to be one of the main constraint factors in limiting tree establishment on high sand tailings. High sand tailings which is situated more than 4 m above standing water table level  (a.s.w.l.), is constantly subjected to water deficit. Other studies have shown that Acacia auriculiformis and Acacia mangium are potential nitrogen fixing tree species for rehabilitation of sand tailings at < 1.5-m a.s.w.l. However, Acacia auriculiformis had been reported to have greater diameter and height growth than Acacia mangium on sand dunes at 8-m a.s.w.l. Little is known on the fluctuations of water status and its effect on the early growth and adaptability of the two acacia species on high sand tailings.

        This study aims to: 1.) quantify  the  factors affecting the changes of site water status of the high sand tailings, 2.) investigate the growth and physiological responses of Acacia mangium and Acacia auriculiformis to the fluctuations of site water status on high sand dunes, and 3.) identify characteristics, which may allow the trees to adapt to and survive on high sand tailings.

        In both shade and open plots on sand tailings, diurnal air water deficits would be anticipated particularly in sunny days and late afternoon in cloudy days. The results from dipwell showed that the main source of soil water supply to the sand tailings is solely dependent on the rainfall and not from the surrounding mining pool. Furthermore, at 6 days after rainfall < 37 mm, sand tailings at 0-15 cm depth had low sand suction showing soil water deficit to plant uptake.  Hence, the rainfall distribution would determine the growth of acacias in sand tailings.

        Dry periods significantly reduced (P<0.05) the shoot length and stem diameter growth rates and leaf area of both acacias. Dry period also significantly increased leaf and branch number shedding rates of both acacias.  Generally, shading did not improve the growth rates of both acacias in dry periods but significantly delayed leaf shedding of the two acacias in dry periods. Acacia auriculiformis had significantly  higher shoot length  and stem diameter growth rates as the result of higher leaf and shoot numbers. The superiority of Acacia auriculiformis compared to  Acacia mangium in adapting to high sand tailings was explained by its growth patterns that had characteristics of drought avoidance, and/or  tolerance. Greater vegetative growth was observed in Acacia auriculiformis than Acacia mangium in the wet periods.

        The effects of atmospheric and soil water fluctuations on some physiological responses of  the two species were also examined. The two acacia species in open and shade plots reduced their  diurnal gas exchange activities following dry periods and in response to atmospheric drought. Shading significantly (P<0.05) reduced the net photosynthesis (A_net) and (water use efficiency = net photosynthesis/transpiration) WUE but not (transpiration) E of both species. In addition,  in the open plots only, drought significantly reduced the WUE of both species at full capacity by reducing the A_net in the subsequent wet days. The reduction of A_net was possibly due to the  damage of photosynthetic machinery during drought. Acacia auriculiformis had significantly higher A_net at full capacity and WUE than Acacia mangium in the wet periods. Furthermore, Acacia auriculiformis showed a higher soil-to-leaf hydraulic conductance than Acacia mangium at 6 months after planting in a wet sunny day. The Acacia auriculiformis was significantly having higher net photosynthesis and water use efficiency but lower leaf temperature than  Acacia mangium  at similar range of leaf water potential on wet sunny days. Hence, Acacia auriculiformis has significantly better physiological  responses than Acacia mangium  to the fluctuations of site and atmospheric water status on the sand tailings. This partly explains the better growth of Acacia auriculiformis than Acacia mangium in drought-prone sand tailings.

A comparison between the leaf temperature characteristics of both acacia species showed that  Acacia mangium had significantly higher leaf temperature than Acacia auriculiformis in wet or drier sand tailings. This is confirmed by the higher Idso's index of Acacia mangium when approaching solar noon during the sunny day. However, Acacia auriculiformis developed a similar water stress as Acacia mangium during the period of mild soil drought coupled with high air vapour pressure deficit (VPD). High leaf temperature was also closely related to the size of  leaf area exposed to the irradiance. The irradiance interception of the leaves depended on leaf size and leaf orientation. The results showed that on average, the leaf angles of both species did not change much during the day. However, the orientations  of the leaf angles of both species did reduce the average fractional irradiance interception of both species to approximately 50%. Hence, Acacia mangium had significantly higher leaf temperature than Acacia auriculiformis  mainly due to its larger leaf size.  Hence, it is more likely for Acacia mangium to develop  more severe leaf temperature stress than Acacia auriculiformis during  prolonged dry periods.

        An assessment on the leaf and root characteristics of  the two acacias species showed the leaf of both acacia species did not acquire more drought tolerant properties in the shade plots at four months after planting. This means that in prolong dry periods, the acacia species in shade plots  would be adversely affected by water deficits as those in the open plots. Acacia auriculiformis had acquired significantly more leaf drought tolerant  properties than Acacia mangium in the open plots. This suggests that Acacia auriculiformis would be able to maintain  positive turgor during drought. Hence, Acacia auriculiformis is able  to maintain its physiological activities better than Acacia mangium during drought. In addition, the superiority of Acacia auriculiformis  to Acacia mangium in drought was further  affirmed by its more developed and deeper root system. The high mechanical impedance (MI) in sand tailings, and/or, rooting habits are most probably  the main factors that restrict  the root  development of  both species in the sand tailings. Consequently, these would restrict their root from accessing to deeper available water.

        A phytotron experiment in controlled conditions were carried out to determine the growth responses of the two acacias species to soil water drying with MI. Soil water deficit alone without the interactions of other drought factors such as high VPD and high irradiance would have similar effect on both acacias though it may not be in the same level of stress as observed in the field. High mechanical impedance (2.3 MPa) reduced vegetative growth and biomass partitioning of the acacias. In addition, high MI aggravated the water deficit effect by preventing the root of both acacias reaching to deeper available water.

        Lastly, the early growth and physiological responses of Acacia mangium and Acacia auriculiformis to fluctuation of site water status in the field are discussed along with the finding in the controlled environments. This study concluded that Acacia auriculiformis is better adapted to soil and atmospheric droughts than Acacia mangium on high sand tailings. The reasons are  due to: 1.) Acacia auriculiformis is more evidently possessed the property of drought avoidance in vegetative growth patterns than Acacia mangium, 2.) Acacia auriculiformis has a more drought tolerant leaf and a more developed root system than Acacia mangium, 3.) Acacia auriculiformis has a smaller but thicker leaf  than Acacia mangium which contributing to the prevention of thermodamage, and  lastly, 4.) It has more effective efficiency mechanisms to conserve water than Acacia mangium in the high sand tailings, which frequently subjects to soil and atmospheric drought.

Citation:
Ang, L. H. 1996. Effects of periodic drought on Acacia mangium Willd. and Acacia auriculiformis A. Cunn. ex. Benth growing on sand tailings in Malaysia. Ph.D. Thesis. University of Aberdeen. 281p. (http://www.oocities.org/EnchantedForest/Palace/1170/abang1.html)

Published on the internet by courtesy of Dr. L. H. Ang.