Soil Science Society of America Journal Soil Sci. Soc. Am. J. 78:1753–1764 doi:10.2136/sssaj2013.12.0511 Received 5 Dec. 2013. *Corresponding author (pascal.podwojewski@ird.fr). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. How Tree Encroachment and Soil Properties Affect Soil Aggregate Stability in an Eroded Grassland in South Africa Forest, Range & Wildland Soils T he management of natural grasslands has received much attention glob- ally due to their importance in providing grazing for livestock. Natural grasslands account for 40% of the earth land-surface and almost 1 ´ 10 9 people utilize them (White et al., 2000). Any degradation of grasslands is likely to have a negative impact on local human populations, especially on rural livestock- dependent communities. Tree (or bush) encroachment in climax grasslands is a widespread phenomenon and has been observed for over 140 yr (Van Auken, 2009) in diferent regions of the world (e.g., America, Australia, Africa, and Southeast Asia; Archer et al., 2001). Trees in grassland ecosystems have large impacts on grassland ecology and dynamics and es- pecially on water resources and soil surface features. In grasslands of semiarid areas, tree encroachment has been correlated to higher runof that generates higher inter-rill ero- sion associated with reduced grass cover under trees (Petersen and Stringham, 2008). Te presence of trees is also linked to an increase of evapotranspiration (Scott et al., Pascal Podwojewski* IRD UMR 242 IEES-Paris c/o School of Agriculture Earth and Environmental Science Univ. of KwaZulu-Natal Box X01 Scottsville, 3209 South Africa and IRD UMR 242 IEES-Paris 32 Ave. H. Varagnat 93143 Bondy cedex France Séraphine Grellier IRD UMR 242 IEES-Paris c/o School of Agriculture Earth and Environmental Science Univ. of KwaZulu-Natal Box X01 Scottsville, 3209 South Africa and Univ. of Science and Technology of Hanoi Vietnam Sandile Mthimkhulu School of Agriculture, Earth and Environ. Science Univ. of KwaZulu-Natal Box X01 Scottsville, 3209 South Africa Louis Titshall School of Agriculture, Earth School of Agriculture, Earth and Environ. Science Univ. of KwaZulu-Natal Box X01 Scottsville, 3209 South Africa and Inst. for Commercial Forestry Research P.O. Box 100281 Scottsville, 3209 South Africa Gully erosion and woody plant encroachment are frequently observed in grasslands worldwide. Soil aggregate stability is one of the drivers of gul- ly erosion and needs to be studied, especially in the context of both tree encroachment and gully erosion. This study compared the effects of tree encroachment [paperbark thorn; Vachellia sieberiana (DC.) Kyal. & Boatwr. previously called Acacia sieberiana DC.] and soil surface horizon properties on the aggregate stability (as measured by mean weight diameter, MWD) in a degraded semihumid grassland in the KwaZulu-Natal province of South Africa. Soil properties from under trees (UT) and open areas (OA) of both an Entisol within deep gullies (locally called dongas) and Alfsol from grasslands outside the gullies (grassland). The MWD values were higher in the grassland (3.06 ± 0.21 mm) than in the dongas (1.84 ± 1.03 mm). In the grassland, the MWD was signifcantly higher in the OA than UT, associated with more fne roots in the OA. In the donga, soil properties UT were not signifcantly different than those in OA. The soil aggregate stability showed strong local variability that depends on the age of the donga and the nature of the mate- rial that composes the pediments. Illite was associated with higher MWD values. The presence of interstratifed illite–vermiculite located in active don- ga increases the shrink–swell potential of the aggregates, exacerbating their dispersion refected in the low MWD values measured, especially when C and root contents were also low. These results suggest that the local soil clay mineralogy controls the orientation and extension of gullies rather than soil chemistry, topography, or the size of the drainage area. Abbreviations: CEC, cation exchange capacity; MWD, mean weight diameter of aggregates; OA, open area; PCA, principal components analysis; SW, soil water content; UT, under trees; D b , bulk density. Published August 22, 2014