Mechanisms of Araucaria (Atlantic) Forest Expansion into Southern Brazilian Grasslands

Recent studies have shown that tropical and subtropical forests expanded during the late Holocene, but rates and mechanisms of expansion are still unknown. Here, we investigate how a forest-grassland mosaic changed over the past 10,000 years at the southernmost limit of the Brazilian Atlantic forest. We used soil organic matter carbon isotopes (delta C-13 and C-14) to quantify and date changes in vegetation, examining soil properties and leaf traits of tree species (nutrient content, delta C-13, delta N-15, and specific leaf area-SLA) to describe potential mechanisms of expansion. Our results show that after several millennia of stability, forests have been expanding over grasslands through continuous, but very slow, border dynamics and patch formation (< 100 m since similar to 4,000 YBP). This process of expansion coincided with past changes in climate, but biotic feedback mechanisms also appear to be important for the long-term persistence and expansion of forests. Soil fertility and microbial biomass match current rather than past vegetation distribution, increasing progressively across the gradient: grasslands < isolated trees < forest patches < forests. Foliar delta N-15 values of trees that are able to colonize the grassland are consistently lower across this vegetation gradient, suggesting an increasingly greater reliance on symbiotic nutrient uptake from grasslands to forests. No significant relationships were found between soil and leaf nutrients, but SLA explained variation in leaf N, P, and K (positive relationships) and in leaf delta C-13 (negative relationship). These findings suggest that a tradeoff between tree growth and water use efficiency is an important regulator of forest-grassland dynamics in the study region.