Natural regeneration

 

Natural regeneration is the term used to describe the growth of plants from seed naturally distributed to the site. Natural regeneration relies on existing seed sources, such as soil or canopy stored seed, or seed transported to the site by water, wind or animals in the area to be revegetated. This method of re-establishing vegetation is especially worthwhile for individuals and groups with limited resources.

 

The advantages of natural regeneration are that it is relatively cheap and native plants that grow from this method are likely to be well adapted to the site. The disadvantages are that it can take a long time to achieve success and can be patchy. The time taken and the unpredictability, can actually increase the risk and cost for land managers, making natural regeneration less cost-effective than other methods such as direct seeding (Dorrough et al, 2006).

 

Natural regeneration is the best technique for use where there are scattered natural trees or shrubs, where there is native pasture or the area is able to be lightly cultivated, where there are few or controlled stock and on rocky or steep sites where other techniques are difficult. If there is a good source of seed, natural regeneration can result in high species diversity, representing the original range of plant species. Natural Regeneration is most effective where good quality remnant bush is adjacent or near the site, where disturbances to the site were minimal (for example, the site was grazed but not cultivated), and where people are readily available for following up and gradually expanding weed control actions year after year.

 

To be successful, natural regeneration requires:

 

  1. a source of healthy seed,
  2. a receptive seed bed,
  3. suitable climatic conditions,
  4. protection from grazing, and
  5. low competition from weeds or parent plants.

In nature, regeneration does not occur every year for most plants. This is because the ideal conditions require a combination of the elements listed above, which may only coincide every 10-20 years. Many Australian species are also triggered to regenerate by fire, which releases seed from the canopy, creates a nutrient-rich seedbed and eliminates competition. Fire is now actively, excluded from many landscapes, further reducing the chances of natural regeneration events occurring for these species.

 

We can however, manipulate some of these conditions to increase the chance and frequency of natural regeneration.

 

Healthy seed

 

For natural regeneration to occur, there needs to be a source of healthy seed. The seed can be stored on the plant (canopy seedbank) or in the soil (soil seedbank). Some plants, such as eucalypts, can retain seeds in their canopy for long periods, only releasing them environmental factors dictate. Other species, such as some Banksia, release their seeds after a fire. Other species release their seeds soon after ripening, where they fall onto the soil. At this point they may germinate, remain on the surface, be incorporated into the soil or predated. Ants are common seed predators. The viability of seeds usually declines once released from the parent plant, with some species losing seed viability rapidly (e.g. Bursaria, Dianella), while others remain viable in the soil for a very long time (e.g. Acacia).

 

To produce a large crop of healthy, viable seeds, most plants need:

  1. pollination (the more pollinators the better),
  2. sufficient parent plants in the population to allow outcrossing (for obligate out-crossers),
  3. suitable conditions for growth (rainfall, protection from predators and disease, nutrients).

See Florabank's References and Resources for more on seed quality.

 

Receptive seed bed

 

Most seeds need contact with a moist, weed-free mineral earth if they are to germinate successfully. Once they have germinated, new seedlings are vulnerable to desiccation, so any variation in the surface of the soil that reduces wind speed or holds moisture will increase their chance of survival.

 

We can increase the chances of successful natural regeneration by manipulating the condition of the seedbed. Usually this will involve removing weeds and cultivating or disturbing the soil to increase the chance of seeds contacting bare, moist soil. Usually a light scarification is sufficient to create a seed bed for many trees and shrubs. Young seedlings need to rapidly push their roots into the soil to access more reliable sources of moisture. This process can be impeded by compacted soils from livestock trampling and may need to be alleviated by ripping. Avoid ripping beneath the crowns of the parent trees as this is likely to damage the parents and allelopathic effects means germination is unlikely under the crown.

 

Some species require an ash-bed before they will successfully germinate, needing either the nutrients from the ash or the chemicals from smoke to trigger germination. A fire on the site may be necessary to create a suitable ash bed for these species. Use caution with fire to prevent the fire getting out of control. Test on small areas first to ensure there are no unwanted effects on other plants. Alternatively, smoke chemicals can be applied using commercially available 'smokewater' treatments.

 

Scalping is a technique that will both expose mineral earth and remove weeds from the soil seedbank. It is not suitable if the species you want to regenerate have their seeds stored in the soil.

 

Climatic conditions

 

Seeds have particular moisture, light and temperature requirements before they germinate. Most plants time their seed release to ensure the highest chance of getting these conditions. While we cannot manipulate these conditions to increase natural regeneration we can make sure that when these conditions are present we manipulate other factors such as seedbed and weed competition to make the most of them.  On the Northern Tablelands of NSW, germination of many eucalypt species is favoured by above average summer rainfall. This fact sheet (Northwest 98/2) gives a case study of regeneration after such an event.

 

Protection from predation and grazing

 

 Ants and other insects are major predators of seed, responsible for removing or destroying the majority of seed that falls. Pathogens, such as fungi are also responsible for many seeds failing to germinate. Once the seeds germinate they are susceptible to browsing and grazing by all sorts of creatures from the microscopic (e.g. red-legged earth mites) to the macroscopic (e.g. cattle and horses). While we can do little to control seed predation or seedling browsing by micro-beasties, we can reduce the number of seedlings lost to larger animals.

 

Some of the major predators of seedlings include rabbits, sheep, cattle, other domestic livestock,  kangaroos and wallabies and possums. Fencing that excludes these animals is the best strategy to reduce damage. See Revegetation techniques for ideas for grazing exclusion. If you are preparing an area to increase the chances of natural regeneration it will pay to control rabbit numbers prior to cultivating and fencing. Once fenced the sites can still be crash-grazed with livestock, providing you check for the presence of seedlings prior to opening the gate.

 

As a rule of thumb, the area you exclude from grazing should extend one and a half tree heights downwind from the trees you are trying to regenerate. For scattered trees it may be more efficient to fence a large area containing many trees rather than fence individual trees.

 

Time control grazing, where paddocks are subject to high-intensity, low-frequency browsing has been shown to increase natural regeneration in many instances. In this case total exclusion is not necessary, which results in less cost for fencing and less land taken out of production. Once regeneration has occurred it will be necessary to observe the impacts from livestock that have access to the seedlings.

 

Reduced competition

 

Like any newly established plants, competition for moisture will determine success. If moisture is limited, new seedlings will not be able to compete with other established plants and will die. Weed control is therefore critical to the success of natural regeneration. Weeds also reduce the amount of mineral earth available for seeds to land on, use moisture in the soil that seeds would imbibe for germination and compete with young seedlings.  

 

If sites are fenced to exclude grazing and cultivated to encourage grazing, it may still take several years for the right climatic conditions to occur. During this period it is essential to conduct regular weed assessments and control weeds as they occur, particularly during periods of ideal climatic conditions.

 

The parent plants and other species in the bushland you are regenerating are also likely to compete with young seedlings for moisture, sometimes actively releasing chemicals that are hostile to the young seedlings (allelopathy).

 

Summary

  • Choose an appropriate site, with healthy parent trees,
  • Control rabbits and exclude or control grazing animal access,
  • Observe seed crops and prepare site when present and ripe.
  • Control weeds from before seed fall until plants are well established.
  • Cultivate site or prepare seed bed with fire, ripping or scalping.

Some useful resources

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