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Serendipitous opportunity to study exotic hybrid establishment


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From top: Figure 1. Matt Larcombe collects capsules from an adult E. perriniana.

Figure 2. Map of population (see below for enlargement)

Figure 3.  Justin Bloomfield assists with E. perriniana coppice measurements.

Figure 4. One of 26 E. perriniana seedlings that have established after fire at Strickland, central Tasmania.

Since 2006 researchers at the University of Tasmania have been monitoring gene flow from Eucalyptus nitens plantations near Strickland in Tasmania’s central highlands, to one of the State's three populations of E. perriniana (a species that is listed as rare under the State Threatened Species Protection Act 1995) (see related article in BioBuzz 5, April 2008). The monitoring program was initiated in collaboration with industry, the Tasmanian Forest Practices Authority, the Threatened Species Section (of what was at that time the  Department of Primary Industries and Water, DPIW) and the CRC. The monitoring program was integrated into the Forest Practice Plan for establishing plantations in the landscape surrounding the E. perriniana population to facilitate long term adaptive management and to act as a case study for the management of gene flow into small isolated populations of conservation significance.

The degree to which hybridisation occurs between two species depends on the presence and strength of a number of physical and biological barriers (Potts et al. 2003). In the case of E. perriniana, there were few expected barriers to gene flow from E. nitens. The species have almost complete overlap in their flowering time (Barbour et al. 2006), they are relatively closely related and hybridise readily (Barbour et al. 2005). The monitoring program aimed: (i) to assess whether hybridisation was occurring; then (ii) to monitor the level of hybridisation as the plantation estate surrounding the population matured; and (iii) determine the effectiveness of a 500 m buffer zone around the native population (see related article in BioBuzz 5, April 2008). This has been done through annual monitoring of flowering time overlap between the population and the plantation, and by assessing the frequency of hybrid seedlings occurring in open pollinated (OP) seed collected from 100 E. perriniana trees each season (see related article in BioBuzz 5, April 2008). Currently the only source of E. nitens pollen is from a small E. nitens plantation established in 1998 which started flowering in the 2005-2006 season. Low levels of hybridisation have been found in the OP seed (0.28 % of 18,625 seedlings), and the level is currently lower than the levels of natural hybridisation with other native species in the area (1.7 % of 18,625 seedlings).

Over the past few years the plantation estate in the area surrounding the E. perriniana population has increased considerably within a 10 km radius. At this stage only the original plantings are reproductively mature, but as the newer plantations mature and E. nitens pollen levels rise in the landscape, levels of hybridisation with the E. perriniana population may increase. Seed collections are under way at the moment, covering the 2007-2008, 2008-2009 and 2009-2010 capsule crops.

The monitoring to date has focused on effective pollen flow and seed set (i.e. E. nitens pollen reaching and fertilising E. perriniana flowers such that seed is produced). However, effective gene flow depends on hybrid seedlings becoming established and reaching reproductive maturity, so that back-crossing and introgression can occur. Thus, understanding barriers to seedling establishment will be important in managing gene flow in a forestry context.

Eucalyptus perriniana grows in a harsh environment.  It experiences water logging and frost in winter and drought in summer. Such extreme conditions coupled with fire stimulates E. perriniana to take on a mallee habit, allowing individuals to survive fire through subterranean lignotubers; the underground stems are protected from the extreme heat of a fire and produce vegetative shoots after a fire has removed the above-ground foliage. Mallees can be very long lived because  their ability to regenerate vegetatively allows them to survive multiple fire events. The environmental fluctuations in the habitat of E. perriniana are probably responsible for the species showing very low levels of recruitment from seed. Hence, when approximately one third of the Strickland population was burnt in 2008, there was an excellent opportunity to assess survival, search for seedlings establishing post-fire, and determine if any of those seedlings were hybrids with E. nitens.

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Figure 2. Map of the Strickland E. perriniana population showing the extent of the 2008 fire, an earlier fire, and the distribution of the 100 study trees. Figure by Richard Schahinger (DPIPWE), Grant Williamson and Matthew Larcombe.


This year Matt Larcombe (Figure 1) and co-workers mapped the fire boundary (Figure 2) and assessed survival of the 100 study trees. They also surveyed the burn area for seedlings.  All 33 study trees that were burnt in the fire have survived, and are coppicing from lignotubers (Figure 3). Although approximately one third of the population (that comprises an estimated 1000-2000 trees) was burnt, and the fire was followed by good rainfall in 2009, seedling recruitment was very low and extremely localised. A total of 26 seedlings were located, 21 of which were establishing in a 20 x 20 m area around a single isolated E. perriniana adult. This tree was one of the study trees being monitored and produced no exotic hybrids in the OP seed collections from the 2005-2006 and 2006-2007 flowering seasons.  All 21 seedlings establishing around this tree and the other five seedlings found in the study population had pure E. perriniana seedling morphology (Figure 4).  The seedlings will be monitored as they mature to ascertain the survival rate of pure E. perriniana seedlings in this tough environment.

References

Potts BM, Barbour RC, Hingston AB,  Vaillancourt RE (2003) Genetic pollution of native eucalypt gene pools - identifying the risks. Australian Journal of Botany 51: 1-25.  [read]

Barbour RC, Potts BM, Vaillancourt RE (2005) Gene flow between introduced and native Eucalyptus species: crossability of native Tasmanian species with exotic E. nitens. Australian Journal of Botany 53: 465-477. [read]

Barbour RC, Potts BM, Vaillancourt RE, Tibbits WN (2006) Gene flow between introduced and native Eucalyptus species: Flowering asynchrony as a barrier to F1 hybridisation between exotic E. nitens and native Tasmanian Symphyomyrtus species. Forest Ecology and Management 226: 9-21. [read]

For more information, contact:

Matthew Larcombe
School of Plant Science
University of Tasmania
Biobuzz issue twelve, August 2010