Tree regeneration responses in a lowland Nothofagus-dominated forest after bamboo dieback in South-Central Chile

The bamboo Chusquea quila (Poaceae: Bambuseae) is the most abundant understory species of Nothofagus-dominated forests at low elevations in the Chilean Lake District. Species of this genus strongly inhibit the establishment and growth of tree species, especially those of the genus Nothofagus. At intervals of many years, this bamboo flowers synchronously and dies, creating large-scale disturbances. The main objective of this study was to determine the influence of bamboo dieback on tree regeneration, especially of shade-intolerant species. The forest studied is a remnant stand of old-growth forest dominated by emergent individuals of N. obliqua and Eucryphia cordifolia which project about 10 m above a main canopy formed by Aextoxicon punctatum, Laurelia philippiana, and Podocarpus saligna. Tree seedlings that established prior to bamboo dieback were recorded in six gaps densely covered by bamboo. After the dieback event in the early 1990's, recruitment, height growth and survivorship were monitored in four gaps during two growing seasons. After the synchronous mass flowering and death of Chusquea quila, both reorganization of advance regeneration and new seedling recruitment were observed in gaps. The advance regeneration consisted mainly of the shade-tolerant species A. punctatum, L. philippiana, and Amomyrtus luma. These species together with E. cordifolia accounted for over 90% of the total individuals in gaps. During the following two seasons, c. 40% of the advance regeneration either died or was damaged. Root suckers of E. cordifolia and L. philippiana were taller and grew faster than the seedlings of other species. Nothofagus obliqua was unable to establish advance regeneration in gaps formerly dominated by bamboo. New recruitment resulted in the synchronized establishment of tree seedlings, especially less shade-tolerant species such as N. obliqua, Aristotelia chilensis, Rhaphithamnus spinosus, and E. cordifolia. These species recruited into the gaps almost entirely during the first season right after the bamboo dieback. However, by the end of the second season, these species had a lower density due to a combination of lower recruitment rate, shortened recruitment period, and greater mortality rate compared to the shade-tolerant species. New bamboo seedlings grew faster and were more abundant than tree seedlings, except for root suckers (both new establishment and advance regeneration) of E. cordifolia which is more likely to successfully recruit into the canopy. Seedlings of N. obliqua compete poorly with bamboo seedlings; its successful recruitment may require bamboo flowering coincident with a mast seed year for the tree species.