Water utilization patterns around isolated Acacia karroo trees in the False Thornveld of the eastern Cape

Abstract

The effect of various vegetation treatments on two soil moisture regimes (i.e. the proportion of the experimental period where the soil had sufficient water for: (a) growth, and (b) to keep the plants turgid in the vicinity of experimentally isolated A. karroo trees) was monitored over a two‐year period. Removal of all vegetation had the greatest effect on soil moisture, increasing the moisture regime by around 200%. Grass removal had the next most significant effect, increasing moisture regimes within 9 m of the tree by around 100%. Removal of the tree had the smallest significant effect, increasing the moisture regime by less than 20%. There was no significant difference in the moisture regime surrounding trees with heights ranging between 1,4 m and 2,5 m, or where various combinations of tree and/or grass defoliations were implemented. We conclude that water supply to the trees is enhanced when soil water extraction is reduced (e.g. during winter or when the sward is harmed) and argue that this may be a mechanism of accelerating bush encroachment in semi‐arid savannas.

Die effek van verskeie plantgroeibehandelings op twee grondvogstelsels (d.i. die gedeelte van die eksperimentele periode waartydens die grond genoegsame vog vir: (a) groei gehad het en (b) om die plante in die omgewing van eksperimenteelgeïsoleerde A. karroo bome se turgordruk te behou) is oor ‘n periode van twee jaar waargeneem. Verwydering van alle plantegroci het die grootste effek op grondvog gehad, met ‘n toename in die vogregime van ongeveer 200%. Verwydering van die grasse het die volgende mees betekenisvolle effek gehad deur die vogregime binne 9 m vanaf die boom met ongeveer 100% te laat toeneem. Die verwydering van die boom het die kleinste betekenisvolle uitwerking gehad, met ‘n toename in die vogregime van minder as 20%. Daar was geen betekenisvolle verskille in die vogregime rondom bome wat ‘n hoogte tussen 1,4 m en 2,5 m gevarieer het, of waar verskeie kombinasies van boom‐ en/of grasontblarings toegepas is nie. Die gevolgtrekking word gemaak dat die watervoorraad vir die bome (wat diep geworteld is) verhoog word wanneer grondwateronttrekking verminder word (soos bv. gedurende die winter of beskading van die grasbedekking). Dit kan moontlik ‘n meganisme wees wat bosindringing in halfdorre grasveld versnel.

Additional index words:

• Competitive relationships
• defoliation
• savanna
• shrub encroachment