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

• Published in: Ecosystems (New York) 2011-12, Vol.14 (8), p.1354-1371
• Main Authors: Silva, Lucas C. R., Anand, Madhur
• Format: Article
• Language: English
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Araucaria; Biological and medical sciences; Biomass; Biomedical and Life Sciences; Boreal forests; Carbon; Carbon content; Carbon isotopes; Climate; Climate change; Coniferous forests; Ecology; Environmental Management; Feedback; Fertility; Forest ecosystems; Forest soils; Forestry; Forests; Fundamental and applied biological sciences. Psychology; General aspects; General forest ecology; Generalities. Production, biomass. Quality of wood and forest products. General forest ecology; Geoecology/Natural Processes; Grassland soils; Grasslands; Holocene; Hydrology/Water Resources; Isotopes; Leaves; Life Sciences; Mosaics; Nutrient content; Nutrient uptake; Nutrients; Organic matter; Plant Sciences; Plant species; Riparian forests; Soil; Soil fertility; Soil microbiology; Soil organic matter; Soil properties; Soils; Soils (organic); Synecology; Trees; Tropical environments; Vegetation; Water use; Water use efficiency; Woodland grasslands; Zoology
• ISSN: 1432-9840; 1435-0629
• DOI: 10.1007/s10021-011-9486-y

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 forestgrassland mosaic changed over the past 10,000 years at the southernmost limit of the Brazilian Atlantic forest. We used soil organic matter carbon isotopes (δ¹³C and ¹⁴C) to quantify and date changes in vegetation, examining soil properties and leaf traits of tree species (nutrient content, (δ¹³C, δ¹⁵N, 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 ~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 δ¹⁵N 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 (positive relationships) and in leaf δ¹³C (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.