Dieback of Nematolepis ovatifolia (Rutaceae), an endemic shrub in the alpine- subalpine heaths of the Snowy Mountains, is facilitated by climate change.

K. Green | 2016

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Abstract

The shrub Nematolepis ovatifolia (F. Muell.) Paul G. Wilson (family Rutaceae) is endemic to the alpine and subalpine areas of the Snowy Mountains, Australia, where it dominates large areas of heath. Mass dieback was observed in the spring/summer of 2012. Damage at first was confined to the tips of branches, a symptom that could be due to frost damage and/or pathogen-induced water stress. Subsequently, whole stems and shrubs died and new areas of chlorosis appeared on smaller shrubs. Surveys of 186 sites covering the geographical range of the shrub in the summers of 2013/14 and 2014/15 found that 59 populations were definitely dieback affected, 92 had early symptoms and 35 were healthy. Two possible causes were investigated: killing frost and pathogens, with insect attack being a further cause of defoliation.

The root rot pathogen, Phytophthora cambivora was isolated from one washed root sample and from one of five soil/root samples. In 2014/15, in five sites where symptomatic plants were monitored, most plants recovered to a condition where they were considered unaffected in March 2015. It is possible that symptomatic plants had in fact suffered frost damage. Hence populations with early symptoms were grouped with healthy populations for analysis of proximity to trails. Compared with these, dieback affected populations were significantly closer to trails. It could not be determined when the pathogen was introduced, as it could have been imported on earthmoving equipment or by subsequent users of trails. It was apparently well spread before spring of 2012 but infection of plants was not evident. Its sudden eruption in spring of 2012 may have been facilitated by two warm and wet La Nina years, with mean growing season soil temperature up to 1.5°C higher than the long term mean and growing season rainfall double the long term mean. Plant death occurred in the hottest year on record in Australia, with the average alpine treeline growing season soil temperature of 9.2°C in 2012/13 being >2.0°C above the long term mean.