FACTORS INFLUENCING THE FLOWERING OF THE TARENGO LEEK ORCHID (PRASOPHYLLUM PETILUM) Nicholas Wilson, Julian Seddon and Greg Baines Conservation Research Technical Report 36 November 2016 Technical Report 36 Factors influencing the flowering of the Tarengo Leek Orchid (Prasophyllum petilum) Nicholas Wilson, Julian Seddon and Greg Baines Conservation Research Environment Division Environment, Planning and Sustainable Development Directorate November 2016 ISBN: 978-1-921117-52-7 © Environment, Planning and Sustainable Development Directorate, Canberra, 2016 Information contained in this publication may be copied or reproduced for study, research, information or educational purposes, subject to appropriate referencing of the source. This document should be cited as: Wilson, N., Seddon, J. And Baines, G. 2016. Factors Influencing The Flowering Of The Tarengo Leek Orchid (Prasophyllum petilum). Technical Report 36. Environment, Planning and Sustainable Development Directorate, ACT Government, Canberra. http://www.environment.act.gov.au Telephone: Access Canberra 13 22 81 Disclaimer The views and opinions expressed in this report are those of the authors and do not necessarily represent the views, opinions or policy of funding bodies or participating member agencies or organisations. Front cover: Prasophylum petilum and the Hall Cemetery. All photographs: Conservation Research. Contents Executive Summary .................................................................................................................... ii 1 Introduction ........................................................................................................................ 1 2 Methods .............................................................................................................................. 2 2.1 Data Collection ................................................................................................................. 2 2.2 Statistical Analysis ............................................................................................................ 3 3 Results ..................................................................................................................................... 4 3.1 Total flowering individuals over 25 years ........................................................................ 4 3.2 Total flowering individuals over 21 years by patch ......................................................... 6 3.3 Frequency of flowering in individuals over 21 years ....................................................... 8 4 Discussion ................................................................................................................................ 9 4.1 Warm winters favour flowering ....................................................................................... 9 4.2 Complex temporal variation .......................................................................................... 10 4.3 Implications and further research ................................................................................. 11 4.4 Conclusion ...................................................................................................................... 12 Acknowledgments.................................................................................................................... 12 References ............................................................................................................................... 12 Appendix 1 ............................................................................................................................... 14 Appendix 2 ............................................................................................................................... 15 Factors Influencing The Flowering Of The Tarengo Leek Orchid (Prasophyllum petilum) i Executive Summary The Hall Cemetery, on the northern border of the Australian Capital Territory, is host to the second largest population of the endangered Tarengo Leek Orchid (Prasophyllum petilum). Since the Hall Cemetery population was identified in 1991, it has been monitored annually, providing 25 years of population statistics. The population at the Hall Cemetery has fluctuated substantially from year to year, particularly during the earlier surveys. Fluctuations in the population have often been attributed to climatic variation, though no specific associations have been suggested. There has also been spatial variation in the number of P. petilum counted during annual surveys of the Hall Cemetery population. The 25 years of P. petilum monitoring data were analysed to determine how the population has changed temporally and spatially, and what climatic variables might have influenced these changes. We used generalised additive models (GAMs) to predict the total population at Hall Cemetery, the population by location within the Hall Cemetery, and the probability of counting an individual plant. In addition to the monitoring data containing the date and location where a plant was observed, basic climatic variables including maximum and minimum temperature, and rainfall over one, three, six and twelve month periods prior to each survey were used as explanatory variables. Nights ≤ 0oC and ≤ -4oC in the preceding calendar year were also used as explanatory variables. The total number of P. petilum counted at the Hall Cemetery generally increased until the late 1990s, after which it remained relatively stable. However, the population appears to be changing differently within different parts of the site. There has been a general decline in most parts of the Hall Cemetery, however, the population in some locations has increased. Part of this variation appears to be explained by the number of nights ≤ -4oC which have a strong negative association with the number of P. petilum counted across the site, and within different parts of the site. The proposed mechanism for this association is that frost damages the aboveground parts of the plant before it flowers, resulting in the plant not being counted during the survey. The probability of counting a plant after it has first been identified declined rapidly to less than 0.1 within the first three years – a useful statistic when interpreting the findings of annual monitoring surveys. This research indicates that the Hall Cemetery P. petilum population is stable. Understanding how the number of nights ≤ -4oC affects the number of individuals present will help to inform management and conserve the species. ii Factors Influencing The Flowering Of The Tarengo Leek Orchid (Prasophyllum petilum) 1 Introduction Prasophyllum petilum D.L.Jones & R.J.Bates (Tarengo Leek Orchid) was first described in 1991 and is only known to occur at five locations in south-eastern New South Wales (NSW) and the Australian Capital Territory (ACT) (Department of Environment 2010). Growing up to 400 mm tall, P. petilum produces 5-20 pale brown to green flowers on an inflorescence that develops within the folded leaf (Jones et al. 2008). The species is listed as endangered nationally as well as in the ACT and NSW. All of the known populations occur in lower parts of the landscape in grassy woodland or natural grassland (Department of Environment 2010). This is characteristic of the genus, which is known to commonly occur in moist depressions (Jones 1988). Prasphyllum petilum persists as a tuber during late summer and autumn, sprouting in May before flowering in late spring (Department of Environment 2010). Prasphyllum species are usually pollinated by bees and wasps, that are attracted by perfume and nectar produced by the flowers, though other insects are also attracted (Jones 1988). The number of flowering individuals (individuals cannot be identified unless flowering) at Hall Cemetery has fluctuated year to year (Environment and Planning 2016), a trend consistent with other populations (Department of Environment 2010). To date, little is known about the conditions that influence flowering (ACT Government 2003). Fire can have a range of effects on other species of the Prasphyllum genus (Jones 1988). However, fire has been consistently absent from the Hall Cemetery for some time – and is likely to remain so – indicating that P. petilum is not dependent on fire, and that fire has not played a role in influencing the observed fluctuations. Prasphyllum petilum is also considered to be sensitive to grazing (NSW OEH 2012), a disturbance that is in part replicated by mowing at the Hall Cemetery site. Although mowing is now managed to avoid damage to P. petilum before flowering is complete, this has not always been the case, and may explain some of the past variation in flowering individuals. Competition with other plants may also influence the number of flowering individuals (NSW OEH 2012). Coates et al. (2006) find that the emergence of Prasophyllum correctum – a species with a similar life cycle to P. petilum – was negatively associated with autumn/winter rainfall in the previous year. Fluctuations in the number of flowering plants have been similar between the Hall Cemetery and Tarengo Travelling Stock Reserve sites (Department of Environment 2010), indicating flowering may be influenced by climate rather than site specific environmental factors. The absence of information about the ecological requirements of P. petilum makes management and conservation of the species difficult. At the time of writing, there are data containing information about the number of flowering P. petilum at Hall Cemetery over