OOS 18-2 - Drought legacy effects on plant phenology: exploring how shifts in plant communities alter vegetation green-period

Background/Question/Methods
Extreme climate events, such as drought, have substantial effects on ecosystem functioning and services. Understanding how vegetation within ecosystems recover after these events is critical for meeting land management objectives, such as maintaining wildlife suitability. Management options such as fire may play a role in how these systems recover after drought. In 2015 and 2016, an exceptional drought swept through much of southern Africa. In a savanna grassland in Kruger National Park (KNP), South Africa, this drought was particularly intense, being an order of magnitude more severe than the next worst drought during the last 100 years. We used a combination of on-the-ground measurements of plant communities before and after the drought, and remote sensing data to address two major questions: first, how do plant communities change under extreme drought and how do these responses relate to fire frequency. Second, how do these plant community shifts affect phenology patterns throughout the growing season, which are critical for supporting a diversity of wildlife.

Results/Conclusions
We found that frequently burned areas in KNP experienced a switch from perennial grass dominance to annual forbs directly after the drought, effectively reducing the palatability of the landscape in these areas. In contrast, areas burned less frequently switched from less palatable grasses to more palatable grasses after the drought. Generally, green-up patterns in all areas post-drought occurred earlier compared to pre-drought green-up. Yet, frequently burned areas tended to senesce earlier than less frequently burned areas, likely because of greater abundances of annual species. Earlier senescence dates could be due to either intrinsic early senescence by annual forbs or the absence of litter drawing down soil moisture earlier in the growing season. These findings highlight the important of considering secondary impacts of management decisions, such as fire frequency, on the sustainability of ecosystems services under more frequent and extreme climatic events.