Exotic tree invasion alters community dynamics and ecosystem function in succesional pampean grasslands

M. I. MIRANDA; E. J. CHANETON

Ciudad de S.C. de Bariloche, Argentina.

Congreso; VI Southern Connection Congress; 2010

UNCOMA, CONICET, INTA

<!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman";} @page Section1 {size:612.0pt 792.0pt; margin:70.85pt 3.0cm 70.85pt 3.0cm; mso-header-margin:36.0pt; mso-footer-margin:36.0pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> Exotic tree invasions in the Pampas are driving wholesale ecosystem changes from the pre-Columbian perennial tussock grasslands to modern savanna-like grasslands and woodlands. We predicted that tree encroachment in open successional fields would modify herbaceous community composition and functioning, by selecting for species with complementary resource-use patterns. We studied tree invasion gradients in two old fields with contrasting history and dominant woody species (Ulmus pumilia and Gleditsia triacanthos) in the Inland Pampa, Argentina. We focused on four stages of invasion: non-invaded grassland, isolated adult trees, low-density woody patches, and high-density woody patches. We found that light and soil water availability and herbaceous primary productivity decreased across woody invasion gradients, with the impact depending on the amount of basal area accrued by trees. Effects on herbaceous communities were already apparent at intermediate stages of invasion, as we detected a shift in functional composition towards relative dominance by cool-season grasses. The final stages of invasion were characterised by low total herbaceous productivity and a significant decline in topsoil organic carbon. Our findings reveal major ecosystem-level impacts of exotic tree invasion in old-field communities including forage production and carbon storage in soil. Tree-invaded Pampa grasslands constitute a dramatic case of the ongoing emergence of novel ecosystems worldwide.