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What happens to tree seeds at the Millennium Seed Bank?

Through our partnership with Kew Gardens on the UK National Tree Seed Project (UKNTSP), we manage over 80 amazing volunteers who collect the seed from woodland across the UK and send it to the Millennium Seed Bank (MSB). After our last blog opened the flood gates with even more questions, we take a closer look at what happens to the seeds there.

Cherry seeds sent in to the Milllennium Seed Bank by our volunteer Seed Collection Champions (Credit: Louise Taylor)
Cherry seeds sent in to the Milllennium Seed Bank by our volunteer Seed Collection Champions (Credit: Louise Taylor)

Which trees are you targeting in your collections and why?

As part of the UKNTSP, which is generously funded by the players of People’s Postcode Lottery, we are targeting UK native woody tree and shrub species. For Phase 1 of the project, this encompassed 70 species including many of the Sorbus microspecies (whitebeams, rowan and service trees) as well as ash, beech, alder, elm and juniper. Now that we are moving on to Phase 2 of the project we are targeting the Salicaceae (willows and poplars) and the remaining native woody species such as hazel and wayfaring tree. This project is unique in that we are aiming to capture as much of the genetic diversity of each of these species as possible. There are many threats facing our UK native woodland, and this collection will serve not only as a research and conservation resource, but also as a backup against future unforeseen threats.

What is the first thing that happens to seeds when they arrive at MSB?

This depends on what type of seeds they are and how much cleaning they have had before arrival. But generally, the first thing to happen to the seeds is that they are dried and cleaned. For more information on the drying process see our previous blog.

Processing cherry fruits. (Credit: RBG Kew)
Processing cherry fruits. (Credit: RBG Kew)

Some trees such as crab apple, blackthorn and hawthorn have a fleshy fruit around their seed, do you have to remove this before storing the seeds?

Yes. The first thing that happens to these fruits on arrival is that they are cleaned to separate the fleshy fruit from the seeds. How this is done varies depending on the species and what state they arrive in. The picture shows a collection of cherries where the fruit is being removed from the seeds. During the 2017 season we received a record 21 collections of crab apples across the country and due to the volume of apples, the team used an apple press to speed up the apple crushing process.

Crab apples going into apple press. (Credit: John Adams)
Crab apples going into apple press. (Credit: John Adams)

Ash is under threat from disease, are the ash keys easy to store and do they last for a long time in storage?

Due to the threat of ash dieback, ash was one of the first species targeted through the UKNTSP. Because of the potential of ash to transport disease, all ash collections are quarantined. This means the collections are all processed separately to other species and triple packed to prevent cross-contamination.

X-ray image of an ash collection showing a high infestation rate. (Credit: RBG Kew)
X-ray image of an ash collection showing a high infestation rate. (Credit: RBG Kew)

Once the ash keys arrive at the MSB, they go to an initial dry room where they are stored for a minimum of three months. After this initial drying, the seeds are removed from their keys, then put through an aspirator to remove the lighter, empty seeds. This process aims to remove as much of the dead seed and debris from the collection as possible.

Once the collection has been cleaned, a subset is then x-rayed to check the quality. This allows us to see inside the seeds to check how many of them are empty or infested. For example, the x-ray image here shows a collection of ash seeds which has a high infestation rate. After all this, the seeds are sealed into glass containers ready for banking.

Splitting the ash collection into an active and base section, ready to be stored in the bank at -20°C (Credit: Matt Reed)
Splitting the ash collection into an active and base section, ready to be stored in the bank at -20°C (Credit: Matt Reed)

Oak is an iconic British tree. How do you prepare and store acorns?

Acorns are classed as recalcitrant. This means that they cannot tolerate the drying required for long-term conservation in a seed bank (over 50 years). The standard storage process at the MSB involves drying seeds down to between 3-7% moisture content prior to storage at -20°C, at which acorns do not survive. Some seed banks specialising in the distribution of seeds for the forestry sector perform partial desiccation of the whole acorn down to 20-30% moisture content prior storage at temperatures around 0°C, which can extend acorn longevity up to three years. As an alternative, Royal Botanic Gardens Kew is conducting research into the cryo-preservation of the embryos of oak seeds at liquid nitrogen temperatures, which could extend oak germplasm longevity for centuries. Because of the difficulty in storing acorns they have not been targeted for collection through the UKNTSP.

Cryopreservation of the embryos of oak seeds in liquid nitrogen (Credit: Bethan Hobbs)
Cryopreservation of the embryos of oak seeds in liquid nitrogen (Credit: Bethan Hobbs)

How do you know if your stored seeds are still healthy and viable?

The collections team at the MSB monitors the seeds in storage. All seeds are viability tested within months of them being first banked, this means checking their potential to germinate, usually by germinating a small sample of seeds. This gives us a baseline estimate of the percentage of seeds within a collection which should be able to germinate given the correct conditions and an absence of seed dormancy. The collections are then re-tested, usually every 5-10 years depending on the species. If a collection looks like the viability is reducing, they will be tested more frequently. Once a collection drops to less than 85% of its initial viability, then we look into either making a re-collection or regenerating the seed. This monitoring process helps us make sure that all collections within the seed bank have the potential to germinate.

Are the seeds of UK trees easy to germinate?

Many of our tree species have some form of seed dormancy which prevents them from germinating until conditions for seedling establishment and growth are optimal. There are various types of seed dormancy, for example, the seed coat may provide a physical barrier (it stops the water necessary for germination entering the seed), environmental conditions may inhibit the chemical reactions necessary for the shoot to emerge, or the embryo inside the seed may not yet be fully mature. The correct environmental conditions break dormancy and allow germination to occur when conditions are optimal and the chance of seedling survival is maximised. This is beneficial to the long term survival of the plant. However, it can make germinating seeds in a seed bank challenging. Ash seeds display a complex dormancy mechanism. To break their dormancy the seed must experience a warm spell (to mimic summer conditions) which allows the immature embryo to grow, followed by a cold spell (to mimic winter conditions) which weakens structures of the seed and allows germination to occur.  

Which tree seeds last the longest? Why do they last so long?

All of our tree seeds are dried down to between 3-7% moisture content before banking and stored at -20°C, as seed longevity roughly doubles for every 1% reduction in moisture content and also roughly doubles for every 5°C drop in temperature (known as Harrington’s rule of thumb). This maximises the life span of the seeds in storage. But as the oldest collections of UK woody species we have stored at the MSB were collected in the 1980s, our seed collections have not been stored long enough yet to answer definitively which tree seeds last the longest.

Whitebeam leaves and fruits. (Credit: WTML)
Whitebeam leaves and fruits. (Credit: WTML)

Whilst we have no real-time ageing examples at the MSB for UKNTSP species, research has been undertaken over those 30 years using artificial ageing experiments to comparatively rank the species to say which are more likely to be short-lived. Results suggest that a number of the Sorbus species (whitebeams, rowan and service trees) are likely to be amongst the most long-lived of the UK tree species. But so far the oldest UK collection we have is a rowan collection from 1989, so maybe ask again in a couple of hundred years!

Want to get involved in important research like the UKNTSP?

Check out our volunteering opportunities