Ryan Tangney, a post-doctoral researcher at the University of New South Wales, talks about his recent paper looking at how Seed traits determine species’ responses to fire under varying soil heating scenarios, what prompted this research – and what new question it raises

About the paper

What’s your paper about?

This paper explored the relationships between seed mass, seedling emergence and seed survival through fire (see also this infographic by Carlos A. Ordóñez-Parra). We wanted to understand the range of soil temperatures experienced during fire events and how fire temperatures may impede seedling emergence if soil temperatures surpass seeds lethal temperatures. We were able to describe three distinct seed responses to soil heating- 1) seeds that are able to avoid lethal temperatures through deep emergence depth. 2) Seeds able to tolerate high soil temperatures, but not able to emerge from depths and 3) seeds with limited emergence depth and limited tolerance to elevated soil temperatures.

What is the background behind your paper?

This paper forms the third in a series that examines seed survival through fire. The first paper, published back in 2018 measured soil temperatures in an experimental fire using Distributed Temperature Sensing (DTS). At the time it was the first paper of its kind to use DTS to describe soil temperatures during fire and we used this technology again in this paper. In both instances DTS has provided a wealth of data, well beyond what is provided using classic thermocouples. In this first paper, we measured soil temperatures continuously for 300 m for 7 hours during an experimental burn and captured soil temperatures every 25 cm along that 300 m line every 10 seconds. This gave us great insight into the range of temperatures and duration of heating seeds experienced whilst in the soil seed bank.

The second paper explored lethal temperatures of seeds and established the effects of seed moisture and seed embryo type in determining lethal temperature of seeds. We report that seeds are able to withstand much higher soil temperatures while in a dry state, but as soon as internal seed moisture increases, temperatures required to kill seeds drastically reduces. In the most extreme cases, lethal temperatures decreased by > 30 °C between dry seeds and wet seeds.

With this paper we were able to bring it all together. We now understand the range of temperatures experienced during fire and how much soil heating seeds can tolerate during fire, and now we have a good appreciation of how seeds differ in their emergence strategies and how seedling emergence strategies are separate to their ability to tolerate high temperatures.

What are the key messages of your article?

Seeds have an amazing ability to tolerate soil heating and emerge from remarkable soil depths – the range of seed responses defined in this paper is evidence of that. We now know which seed traits are linked with different aspects of the seed survival and emergence timeline. Fundamentally, we now know that seed responses to soil heating are diverse and even though heavier seeds are able to emerge from deeper within the soil, they are not necessarily better at surviving soil heating because their ability to withstand soil heating may be lower than smaller seeds. 

Does this article raise any new research questions?

We have been able to describe seed responses with dry soils and dry seeds and we know that wet seeds respond differently, but we don’t know whether soil heating in wet soils will lead to temperatures sufficient to kill wet seeds. We also don’t know how seeds are distributed within the soil – particularly to what extent do seeds move once they are dispersed into the soil seed bank.

Another key aspect that could come out of this research is linking these seed emergence responses to other life history traits of species- like plant architecture. You can imagine that perhaps the species that are most sensitive to elevated soil temperatures and are also limited in emergence ability may ensure their seeds arrive in low fuel areas. The reverse could also be true, those seeds that can tolerate high soil heating and can emerge from great depths may benefit if their co-occurring species cannot tolerate as extreme conditions as them – in a similar fashion to the kill thy neighbour hypothesis – where species level flammability may open recruitment possibilities if neighbouring co-occurring species are killed.

About the research

What is the next step in this field going to be?

The next step will be to examine whether these variable responses play out in large scale fires. We need to understand how seedling emergence varies within fires, between fires and between different fire seasons, as well as getting a firmer understanding of how other fire regime elements like fire severity may influence seedling emergence. This will be fundamental in ensuring future fire management maximizes seed bank responses.

About the Author

What are you currently working on?

My current research is a bit of an extension of this project, as it looks to examine post-fire plant responses to fires in different seasons. I am working on a project that compares changes in fire season across Australia to understand the effects of fire season on plant recruitment, reproduction and survival. A primary focus is to compare betwe.en strongly seasonal climate regions and more unseasonal? climate regions. This will allow us to better understand how changes in fire season driven by climate change may effect plant responses, which in turn can promote adaptive management actions to minimize the potential impacts of fire regime shifts.

What’s your current position?

I am currently employed as a post-doctoral research associate at the University of New South Wales,  I am however based in Perth, located on the doorstep of the in biologically diverse South Western Australia. I am currently working in Banksia woodlands and the Jarrah forest as well as spending a lot of time in the seed lab. Read the paper in full here. You can also find an infographicfor the artice on Plant Science Research Weekly