An
observation by Rex Bushell
Native flora restoration in cities is under
constant pressure from hybrids, selections of genetic variations and selective
breeding done by commercial nurseries targeting new and varied cultivars of
native origin for the domestic urban market. These commercial variances will
cross with many genetically pure species that may be planted by restoration
groups or cross with old established canopy trees that have existed in the city
for many years. Some species like flax, manuka, lancewood, kowhai and totara
are particularly vulnerable.
Over time, depending on the life cycle of a
particular flora species, this will create a genetic evolution within the
native forest reserves in or close to, a city.
Is this good or bad? Either way it is inevitable.
It can be slowed down by planting eco sourced plants but it cannot be stopped.
For the flora purist this may not be something to look forward to.
To look at it from the native fauna angle,
they will happily eat the berries or drink the nectar of a modified native. In
fact, their participation is often the cause of genetic variation by
transferring the pollen from a domestic selection to a native.
Because of the large variation of flora
species within a city there is an opportunity to target non native plants that attract
- as opposed to just feeding – a targeted fauna species such as the tui and bellbird. This is why Banksia integriflora is being promoted. There is plenty of
opportunity to plant Banksia integriflora
in parks around a city where exotic trees are planted and in domestic gardens
without compromising the integrity of native restoration projects. The gardens of the University of Waikato would
be an excellent example where natives and exotics are planted along side one
another including Banksia integriflora resulting in a high population of tui.
Banksia
integriflora may have a propensity to be invasive
in some parts of New Zealand but this is not the case in the Waikato. Bunny and
John Mortimer had a tree growing at their Taitua Arboretum and only ever got
one seedling. Maxine Frazer has a QE11 block at Te Pahu and has a Banksia integriflora growing by her house and has never seen a seedling. On the road verge by 90 Wellington Street in
Hamilton there is a very fine specimen that must be all of 100 years old with
no proliferation of seedlings in the area.
To
summarise, in city environments exotic flora tends to dominate over
natives. Because of this management
plans have to be more adaptable if our target is to reintroduce or sustain
native fauna species within our cities.
2 comments:
Yes native hybrids and cultivars do pollute the genetic pool of our restoration plantings; however there are some important genetic processes that are on our side. The main ones are genetic introgression and backcrossing.
Firstly, progeny may not necessarily survive because they are not as fit for local environments as ‘pure’ plants which may out-compete them. If they do survive they tend to regress to the ‘pure’ features in subsequent generations because these are the stable features; these features have been selected for over time and have proven to promote the species’ survivability. Backcrossing of the hybrid with a ‘pure’ plant is also likely which increases the presence of ‘pure’ features. This is a very good reason to have a dominant population of ‘pure’ natives because the hybrid progeny are more likely to cross with the ‘pure’ plants than a few ‘unpure’ ones.
There is also variation in the susceptibility of different species to genetic pollution. For example, species that have specialist pollinator relationships are less likely to be polluted than those that receive wind-dispersed pollen across long distances.
It is useful to remember that the exotic gully ecosystems that we are trying to restore have had 150 or so years to develop to their current state, this includes the development of an exotic seedbank, deciduous nutrient cycling regimes and overwhelming exotic dominance. Therefore, we must expect that it will take a significant amount of time to win the battle and restore dominance of appropriate native species. Key processes which need to be turned around include those related to seed rain and the seed bank. Recent work by Overdyck & Clarkson (NZ J. Ecology 2012) shows that it takes about 20 years to reach a threshold where native regeneration becomes abundant. Another paper by Overdyck et al. (Rest. Ecology 2013) shows how novel approaches like broadcast seeding with clay balls can potentially speed things up.
Timescale is also relevant to the idea of planting Banksia species. They are a useful tool to attract and feed fauna while suitable native species are still establishing, but they should not be thought of as the end point because there is a full suite of native plant species which have co-evolved with our fauna and can provide all required services. This paper by Clarkson et al.: Indigenous vegetation types of Hamilton Ecological District (2007), provides a list of the species that once dominated our Waikato landscape. Currently we are probably using as little as 20% of the native flora which could or should be utilised. A lot of these species are not available commercially but with time I believe that we will have a greater selection to choose from… just keep asking that nurseryman!
Gully restoration, or indeed any restoration seeking to bring indigenous nature back into a city, is truly an intergenerational endeavour.
Thanks Catherine. This is good quality feed back.
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