- Biometris (WU MAT) (3)
- Mathematical and Statistical Methods - Biometris (3)
- PE&RC (3)
- Resource Ecology (3)
- Earth System Science (1)
- K.P. Kirkman (1)
- K. Kirkman (2)
- H. Kroon de (2)
- F. Ludwig (1)
- H.H.T. Prins (3)
- K.V.R. Priyadarshini (3)
- S. Woodborne (3)
Competition with trees does not influence root characteristics of perennial grasses in semi-arid and arid savannas in South Africa
Priyadarshini, K.V.R. ; Bie, S. de; Heitkonig, I.M.A. ; Woodborne, S. ; Gort, G. ; Kirkman, K.P. ; Prins, H.H.T. - \ 2016
Journal of Arid Environments 124 (2016). - ISSN 0140-1963 - p. 270 - 277.
Savannas support mixed tree-grass communities and interactions between these are typically viewed as being competitive based on studies that focused on grass aboveground production. However, an important plant response to competition and resource limitation is an increase in root reserves. We investigated root characteristics of perennial grasses in the presence and absence of trees as a proxy of competition in South African savannas in three sites that differed in rainfall. We based our study on the hypothesis that competition from trees and water limitation will result in increased storage in roots of grasses under trees. Results indicate no significant effect of variation in rainfall of the different study locations on root characteristics of grasses. Furthermore, trees did not significantly influence most grass root characteristics that we measured. The only exception was nitrogen-content that showed an increase with rainfall and tree presence through potentially higher mineralization rates and nitrogen availability in the under-tree canopy environment. As the study sites are in the drier rainfall range in South Africa, it is likely that trees and grasses in these dry savannas may have a positive relationship conforming to the stress-gradient hypothesis. Alternatively, grasses and trees may be using complementary water and nutritional resources.
Seasonality of hydraulic redistribution by trees to grasses and changes in their water-source use that change tree–grass interactions
Priyadarshini, K.V.R. ; Prins, H.H.T. ; Bie, S. de; Heitkonig, I.M.A. ; Woodborne, S. ; Gort, G. ; Kirkman, K. ; Ludwig, F. ; Dawson, T.E. ; Kroon, H. de - \ 2016
Ecohydrology 9 (2016)2. - ISSN 1936-0584 - p. 218 - 228.
Savanna vegetation is characterized by tree–grass co-existence that can experience intense water limitation, yet the water relations of these savanna plants are poorly understood. We examined the water sources for trees and grasses in different seasons and investigated the importance of hydraulic redistribution in three tree species inhabiting a semi-arid savanna in South Africa. We used natural variation in H and O stable isotope composition of source waters to identify the principal water sources for these plants. We conducted an experiment by labelling deep-soil (2.5-m depth) with a deuterium tracer. Seasonal differences in the stable isotope composition of water in trees and grasses indicated that there was water-source use partitioning as well as overlap. Trees and grasses used water from the topsoil after rainfall indicating overlap of water-source use. All tree species shifted to groundwater or subsoil water use when there was no water in the topsoil indicating partitioning of water use. Grasses always used water from the topsoil. The seasonal changes in water-source use by trees and grasses indicated possible shifts in tree–grass interactions during different periods of the year. The tracer experiment confirmed hydraulic redistribution in all the three tree species and water transfer to grasses via the topsoil. However, this occurred only in the dry season. Our observations and experimental results indicate the potential for facilitation effects by trees to their understory grasses and show that dry season hydraulic redistribution from trees to grasses could be an important facilitative mechanism maintaining tree–grass co-existence in savannas.
Overlap in nitrogen sources and redistribution of nitrogen between trees and grasses in a semi-arid savanna
Priyadarshini, K.V.R. ; Prins, H.H.T. ; Bie, S. de; Heitkonig, I.M.A. ; Woodborne, S. ; Gort, G. ; Kirkman, K. ; Fry, B. ; Kroon, H. de - \ 2014
Oecologia 174 (2014)4. - ISSN 0029-8549 - p. 1107 - 1116.
n-15 natural-abundance - southern african savanna - root activity - patterns - water - availability - vegetation - physiology - isotopes - depth
A key question in savanna ecology is how trees and grasses coexist under N limitation. We used N stable isotopes and N content to study N source partitioning across seasons from trees and associated grasses in a semi-arid savanna. We also used 15N tracer additions to investigate possible redistribution of N by trees to grasses. Foliar stable N isotope ratio (d15N) values were consistent with trees and grasses using mycorrhiza-supplied N in all seasons except in the wet season when they switched to microbially fixed N. The dependence of trees and grasses on mineralized soil N seemed highly unlikely based on seasonal variation in mineralization rates in the Kruger Park region. Remarkably, foliar d15N values were similar for all three tree species differing in the potential for N fixation through nodulation. The tracer experiment showed that N was redistributed by trees to understory grasses in all seasons. Our results suggest that the redistribution of N from trees to grasses and uptake of N was independent of water redistribution. Although there is overlap of N sources between trees and grasses, dependence on biological sources of N coupled with redistribution of subsoil N by trees may contribute to the coexistence of trees and grasses in semi-arid savannas.