Periodic heavy flowering of New Zealand flax (Phormium, agavaceae)

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Abstract
The numbers of flowering stems of Phormium tenax in and near Wellington City fluctuated up to 75-fold between good and bad years. P. tenax flowered prolifically over most of central New Zealand in the summers of 1978/79, 1981/82, and 1985/86 but produced few flowers in the summers of 1976/77, 1979/80, and 1982/83. P. cookianum flowering fluctuated 10-fold between good and bad years. Poor and heavy flowering years for P. tenax and P. cookianum coincided but P. cookianum produced additional flowering peaks in 1977/78 and 1980/81. Light, moderate, and heavy summer flowering of P. tenax was inversely correlated with wet weather the previous February (r = -0.789) and high absolute air temperatures during the previous April to June (r = 0.969). The spectacular flowering of P. tenax in the summer of 1981/82 was probably induced by exceptionally high air temperatures on 9 May 1981. Phormium tenax in Wellington flowered synchronously with alpine tussock (Chionochloa) in Nelson and some Nothofagus species. The synchronous flowering of these three genera over many years supports Connor's suggestion of a quasi-triennial cycle triggered by high air temperatures the previous summer and autumn.