Transient effects of nitrogen addition and rainfall suppression on Vachellia karroo growth under grass competition in a southern African savanna

Abstract

It is not clear how woody species, especially nitrogen fixers will respond to the combined effect of increased N fertilisation and reduced rainfall amount in savannas in a changing environment. A field experiment was set up at a southern African savanna site to investigate the interaction effects of increased N deposition and reduced rainfall amount on the growth of Vachellia karroo saplings in the presence of grass competition. Rainout shelters were erected around experimental plants to mimic the projected decrease in rainfall in southern Africa while N was added as ammonium nitrate over four growing seasons. The experiment uncovered significant but transient effects of rainfall suppression alone (F₁ = 5.171, P = 0.031) and its interaction with N fertilisation (F₁ = 6.369, P = 0.017) on the height of V. karroo saplings in the second growing season but not in the first, third and fourth season (P > 0.05). Rainfall suppression significantly reduced sapling height. The interaction of fertilisation and rainfall suppression increased stem height of the study species. In contrast, the interaction effects of N supply and rainfall suppression significantly (F₁ = 4.213, P = 0.049) increased diameter of saplings during the first season but not thereafter. Conversely, grass competition did not significantly influence the growth of V. karroo either alone or in interaction with the main treatments though saplings growing in competition with grass had relatively higher growth than the control. Overall, results suggest that N fertilisation may cancel out the predicted negative effect of rainfall decrease on woody species growth thereby enabling the persistence of these species under global environmental changes.

Keywords:

• Rainfall reduction
• Nitrogen addition
• Vachellia karroo
• southern African savanna

PUBLIC INTEREST STATEMENT

We manipulated rainfall and nitrogen to understand how juvenile Sweet thorn plants will respond to the expected changes in rainfall and nitrogen deposition in savannas of Zimbabwe over four years. The height of plants was affected by rainfall alone as well by the combination of rainfall and nitrogen in the second season but not in the first, third and fourth growing season. Rainfall suppression reduced tree height while the combination of fertilisation and rainfall suppression increased the stem height of target plants. Nitrogen supply and rainfall suppression increased the diameter of juveniles during the first season but not thereafter. In contrast, grass competition did not influence the growth of juveniles either alone or in interaction with rainfall suppression though plants growing in competition with grass had relatively higher growth than the control. Overall, results suggest that N fertilisation may cancel out the predicted negative effect of rainfall decrease on woody species growth thereby enabling their persistence under global environmental changes.

Competing Interest

No conflicts of interests were declared.

Additional information

Funding

This work was supported by the University of Zimbabwe through the Vice Chancellor's Research Grant.

Notes on contributors

Munyaradzi D. Shekede

Munyaradzi D. Shekede was born on the 9^th of May 1981 in Marondera, Zimbabwe. He completed his BA Honours degree in Geography at the University of Zimbabwe in 2003 and a Masters degree in Environmental Policy and Planning from the same university in 2005. Currently, Munyaradzi is studying for a PhD at the University of Zimbabwe focusing on the effects of nitrogen deposition and precipitation decline on tree-grass interactions in southern African savannas. From 2007 to date, he has been working as a lecturer at the University of Zimbabwe, Department of Geography and Environmental Science. Some of the courses he teaches include Remote Sensing, Geographic Data and Analysis, Global Environmental Change and Biodiversity Conservation. Munyaradzi is interested in understanding global changes on society and the environment including flood modelling. He has published more than fourteen peer-reviewed articles and three book chapters.