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An extreme flood event initiates a decade of stand collapse in Beilschmiedia tawa forest, Turakina Valley, Rangitikei, New Zealand

Redpath, DT, Rapson, GL
New Zealand journal of botany 2015 v.53 no.1 pp. 38-59
Beilschmiedia, Dacrycarpus, canopy, dieback, floods, forests, fungi, hypoxia, indigenous species, introduced species, mortality, recruitment, sediments, seedlings, soil, New Zealand
Impacts on forests of infrequent disturbance events such as floods are not well understood in New Zealand. An extreme flood event in February 2004, Beilschmiedia tawa (tawa Lauraceae) initiated extensive mortality, studied via control and flooded plots in McPherson's Reserve, Turakina Valley. Canopy cover in Flooded plots reduced to < 3% of the Controls by 2006, although native species' richness remained similar. The near synchronous mortality of most tawa individuals in the Flooded plots may be due to prolonged anoxia, probably caused by deposition of fine sediment over flooded soil, perhaps exacerbating fungal infection. Slower dieback of other species and adjacent surviving tawa continues, exacerbated by ongoing canopy collapse and spread of infection by root grafts. Seedlings of tawa and Dacrycarpus dacrydioides were common post-flood, but recruitment was poor, due to disturbance-facilitated invasion of exotics. No comparable flood event has apparently occurred for 200 years. Recovery of the forest and its successional processes, if achievable, will be slow.