The effect of water-absorbing synthetic polymers on the stomatal conductance, growth and survival of transplanted Eucalyptus microtheca seedlings in the Sudan

Summary

In arid and semi-arid regions over-exploitation of trees for fuel wood and clearance for agriculture are two important factors which have led to widespread deforestation. In the Sudan, 3.6% of the total woodland production is estimated to removed annually, and this has led to soil erosion and desert encroachment. Trees raised under irrigation, e.g. Eucalyptus microtheca (a non-native species) are particularly susceptible to the breakdown of irrigation systems. Any measure which aids tree survival or establishment in the nursery or the field will therefore be important and may be also relevant to enhancing survival of native tree seedlings in reforestation programmes.

This study assessed the effects of two water-absorbing polymers applied to 1 -year-old transplanted seedlings of E.microtheca on survival and performance.

Study site: The experiments were conducted at the Institute of Environmental Studies, Khartoum, Sudan. The climate is semi-arid with mean monthly temperatures ranging between 23 and 24°C (maximum 51°C, minimum 19°C) and annual rainfall of 170 mm concentrated in July and August.

Experimental design: On 2 February 1985, 1-year-old E.microtheca seedlings were transplanted from plastic tubes (9 cm diameter x 12 cm high) containing a 1:1 mix of sand and river silt, into porous clay pots (18 cm diameter x 25 cm high) containing 5 l soil comprising 3 parts sand to 1 part silt (to increase the water stress). Soil was removed from the seedling roots on transplanting to ensure early contact between the roots and the three polymer treatment soils: soil without polymer, soil with polyacrylamide at 0.2% v/v and soil plus polyvinylalcohol at 0.2% v/v. The polymers were mixed thoroughly in each pot. Each was watered with 1 l of water and four irrigation treatments were initiated: no further water; 0.5 1 every day; every third day; and every sixth day. The pots were arranged in a randomized block design with seven replicates. Each day survival was assessed. The experiment ended after1 month when fresh and dry (105°C for 24 h) weights were determined for each surviving tree.

The pattern of leaf water loss was determined on 6 and 12 February 1985. Diffusive resistances of leaves were recorded on each of three seedlings per polymer treatment from the daily irrigation regime at intervals of approximately 1 h between dawn and dusk. Air and leaf temperatures were recorded.

On 7 February 1985, concentrations of the two polymers were increased to 0.5% v/v and three irrigation treatments were applied: no further water after transplanting; 0.5 1 each day; and 0.5 1 every third day. Black polythene pots (16 cm diameter x 20 cm high, 5 1 volume) were used (rather than clay) with five replicates of each. The total number of leaves, the number of healthy leaves and the number of wilted leaves were counted daily. Fresh and dry weights were determined at the end of the experiment on 6 April 1985.

Both polymers aided seedling survival and growth. All trees died in treatments receiving no water subsequent to transplanting with all control trees dead after 6 days but the two polymers extended the period of apparent survival, the polyacrylamide almost doubling it because of the significantly slower death rate.

The most dramatic effects of the polymers were seen when water was added every sixth
day. Control trees were all apparently dead after 5 days. However, the significantly slower wilting of trees in the polymer treatments allowed 44% of the trees to survive until the first irrigation event on the sixth day. Following this irrigation, recovery took place of some apparently dead seedlings, so that between 57 and 71% of the trees were surviving after 1 month, whereas no control seedlings recovered.

The trees grown in the polymer treatments had slightly greater dry weights than the
Controls and the trees grown in both polymers had significantly greater fresh weights.

Stomatal conductance measurements showed stomatal closure due to transplant shock and gave early indications of the longer term patterns of tree survival in relation to the various irrigation and polymer treatments.

Conclusions: In this experiment both polymer types added to the sandy soil aided E.microtheca seedling survival and growth. This technique is applicable also to the establishment of native species e.g. the survival period of Acacia senegal seedlings was increased by factors of 3-5 by adding polymer (Callaghan et al. 1988). Addition of the polymers equated to between £0.01 - 0.016 per tree and would seem a cost effective way of increasing tree planting success. The impact of polymers is likely to be restricted to early growth as root systems will soon grow out of the polymers. However, these early stages are the most critical for survival and once the seedling has become established continued survival is far more likely.


References
Callaghan T.V., Abed El Nour H. & Lindley D. K. (1988). The environmental crisis in the Sudan: the effect of water-absorbing synthetic polymers on tree germination and early survival. Journal of Arid Environments, 14, 301-317.


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