The importance of spatiotemporal scale in understanding agroforestry management effects on grassland vegetation

Background/Question/Methods

Many ecological phenomena are scale-dependent, and these relationships can change among habitats and with disturbance. We studied the importance of spatiotemporal scale in understanding the effects of afforestation and livestock grazing in Campos grasslands of southern South America. Afforestation is a common land-use change across this region; in Uruguay alone, ~19% of the land base has been or will be impacted by afforestation. Livestock grazing was widespread before afforestation and continues in and around plantations. Our study had two objectives: 1) compare species richness for four treatment types (grazed grassland, ungrazed grassland, grazed pine forest, and ungrazed pine forest) at various spatial scales, and 2) investigate time since exclosure effects between grazed and ungrazed habitats. We calculated species richness in modified-Whittaker nested plots of varying sizes (1, 4, 9, 36, and 144 m²) for each treatment type at 5 sites. Data were collected in spring and fall for two years after exclosures were established. Each season was analyzed separately, testing the effects of afforestation status (forest or grassland), management (grazed or ungrazed), time since exclosure, and their interactions on species richness for each plot size using generalized linear mixed models.

Results/Conclusions

Afforestation and grazing had interactive effects on species richness: richness was greater in ungrazed than grazed pine forests but lower in ungrazed than grazed grasslands, especially at large scales. At small scales, afforestation status was most important: richness was greater in grasslands than forests, and the effects of grazing were only significant for grasslands (richness was greater in grazed than ungrazed). Species richness was greater in spring than fall, but effects of grazing on richness were greater in fall (grazed compared to ungrazed habitats at time exclosure was built and 1 yr later). Our results illustrate the importance of spatial and temporal scale when examining habitats and disturbance regimes. Similar to previous research conducted in other regions, we found scale-dependent relationships, and that to fully understand the effects of complex land-use change, multiple spatiotemporal scales must be examined.