Temporal shifts in the interaction between woody resprouters and an obligate seeder tree during a post-fire succession in Patagonia

Question: The study of temporal variation of plant-plant interactions has received little attention despite its importance in community dynamics. Successional processes triggered by disturbances such as fire may be determined by changes in key interactions between dominant woody plants. Taking advantage of a 95-yr-old post-fire natural experiment we assessed temporal shifts in post-fire interactions between woody resprouters and the obligate seeder tree Austrocedrus chilensis. We hypothesized that during the initial post-fire period, resprouters may facilitate A. chilensis trees' growth, while at advanced time since fire the interaction between A. chilensis trees and resprouters may turn to competition. Location: Northwestern Rio Negro province, Patagonia, Argentina. Methods: We selected pairs of A. chilensis trees and woody resprouters growing 80 cm apart (growing in association) and both A. chilensis trees and resprouters growing at more than 3 m apart from any individual (controls). We cored all individuals, measured ring widths and calculated basal area increment (BAI) and the relative interaction index (RII). Results: During mid-early stages of post-fire succession, A. chilensis trees' BAI was suppressed for at least 15 yr when growing in association with resprouters, and accordingly displayed a negative RII. Strikingly, during the same period, resprouters associated with A. chilensis trees showed higher BAI than control resprouters, and a positive RII. At advanced time since fire, RII showed a neutral interaction, and according to model estimations, the BAI of A. chilensis trees in association may eventually exceed control BAI whereas resprouters' BAI in association may decline earlier than that of the controls. Interestingly, A. chilensis trees and resprouters' growth was less correlated with climate when growing in association than when growing in isolation. Conclusions: Competition of resprouters over A. chilensis treess' growth during intermediate post-fire stages may be slowing the successional process. However, the association appears to be at the same time mutually beneficial as it buffers against adverse climatic conditions. Studying how plant growth varies along the succession is essential to understand community functioning and to predict possible alterations associated with changes in disturbance regimes due to climate change.