How do understory trees deal with small canopy openings? The case of release in growth following drought-induced tree mortality

Abstract

Hotter droughts have become important drivers of change in the structure of forest canopy, and the role of structural loss of overstory trees is gaining attention as an important factor that would trigger changes in understory behavior. The projection of understory individuals, allows us to analyze the potential future of the forests. Through a dendroecological approach, we evaluated growth responses in understory Austrocedrus chilensis trees growing underneath canopy that experienced tree mortality as a consequence of severe droughts occurred in Patagonia Argentina. We analyze the climatic response, tree growth patterns and drought resilience of understory components in three A. chilensis mixed forests. Tree growth was significantly reduced by drought, highlighting the climatic sensitivity of understory individuals; but indirectly, tree growth also showed releasesassociated with the openings due to canopy mortality. Thus, we found that understory tree growth increased over time. Our results demonstrated that growth performance of understory A. chilensis trees become largely modeled by a combination of the releases associated with changes of the canopy and their ability to withstand adverse weather conditions. We were able to show a clear capacity of understory A. chilensis to cope with drought events by increasing its growth, taking advantage of the disturbance in the upper layers of the canopy, while resisting successive dry years. However, the shadier environment underneath canopy did not buffered drought conditions, and understory A. chilensis trees suffers the effects of droughts. Considering an increase in drought frequency and intensity in climate predictions, with the consequent openings due to overstory tree mortality, understory species behavior will result as a complex interaction among a potential increase in understory vulnerability to more severe droughts, and the opportunity of canopy ascension.