SAEON Ndlovu Node Manager receives PhD from Colorado State University

Tony Swemmer, Manager of the SAEON Ndlovu Node, has been awarded a PhD in Ecology by the Colorado State University, Fort Collins, Colorado, USA (Picture © Mitzi du Plessis) Tony’s dissertation looks at the effects of intra-seasonal variation in precipitation on the productivity of grasses and grasslands (Picture © Mitzi du Plessis)

SAEON is proud to announce that Tony Swemmer, Manager of the SAEON Ndlovu Node, has been awarded a PhD in Ecology by the Colorado State University, Fort Collins, Colorado, USA.

With global warming fast becoming one of the greatest challenges mankind will be facing in this century, Tony’s research deals with an aspect of climate crucial for our survival – precipitation.

Tony’s dissertation investigates the effects of intra-seasonal variation in precipitation on the productivity of grasslands. Above-ground productivity, a fundamental process in terrestrial ecosystems, varies greatly between years, particularly in grasslands. While annual precipitation is considered to be the primary determinant of above-ground net primary productivity (ANPP) in grasslands, much of the inter-annual variation in ANPP cannot be explained by annual precipitation alone.

Tony examined the hypotheses that the timing of rainfall (i.e. intra-seasonal variation) can account for some of the unexplained variation in ANPP, and that two other factors known to affect grassland productivity (grass species composition and grazing) can alter the relationship between productivity and rainfall.

To test the importance of intra-seasonal variation in rainfall, he analysed long-term productivity data for three South African grassland sites. The results showed that intra-seasonal variation in precipitation can explain as much of the inter-annual variation in grassland productivity as annual rainfall, with different aspects of intra-seasonal variation (the size, number and number of days between rainfall events), more important at different sites.

Tony also conducted a field experiment at seven grassland and savanna sites to investigate differences in growth patterns for common grass species, and how grazing might alter these patterns. He found that co-occurring grass species generally respond differently to rainfall in the absence of grazing, apparently a result of different species utilising soil water from different depths. However, with regular clipping the same species tended to respond more uniformly, indicating that grazing can alter the way grasslands will respond to future changes in precipitation.

In a final experiment, Tony investigated eco-physiological mechanisms responsible for differences in tolerance of water stress between the two dominant grasses of the tallgrass prairies in the USA.

Overall, the results of the dissertation indicate that intra-seasonal variation in rainfall can be an important determinant of grassland productivity, and that any effect of grass species composition on the relationship between rainfall and productivity, for a given grassland, is likely to be reduced by heavy grazing.