Willow – a sequel

In a time long ago (or only a few years ago, depending on how you look at it) in land far, far away (or close by depending on where you are reading this post from) tundra willows have been eking out an existence on the warmest edge of their environmental niche. Willows from the North and willows from the South, from high up in the mountains or out on the Arctic coast, are growing together in a common and warmer environment. But over the past eight long years, what has this adventurous yet labourious experiment taught us about the shrubification taking over tundra as the climate warms? Read on to find out.

In 2018, we wrote up a blog post providing some interim results about our tundra willow common garden experiment and we even made a movie trailer!

The trailer from our 2018 blog post about our common garden experiment entitled Willow. editing credit: Gergana Daskalova

But what is going on with the willows now? What have we learned from our eight-year long experiment. We will be working on writing up these results over the coming year, but we thought we could give you all a sneak preview in this blog post in the form of a mini scientific paper.

Title

Tundra shrubs take over when moved down mountains,

but not AS much when Moved south

or

Tundra shrubs grow rapidly in warmer conditions,

but local alpine willows outpace northern Arctic willows

Abstract

Tundra shrubs can grow rapidly in warmer environments reaching heights as high as over a metre in less than eight years of growth. However, Arctic tundra willows don’t grow as fast as local alpine willows when grown in a warmer environment in the boreal forest. This suggests that tundra shrubs genetically vary across latitudes in the timing of their growth, the way that they grow and the size to which they grow. And that we should expect rapid, but not uniform rates of shrub expansion across the tundra biome with warming.

The common garden!

Introduction

A major unknown in tundra ecology is whether shrubs will rapidly respond to changing climate conditions or whether plants are genetically adapted to their current environment, thus limiting future vegetation change. We designed a tundra willow common garden experiment to investigate the variation in tundra shrub growth across the Yukon Territory.

Our Research Question:

How does growth vary in southern alpine versus northern Arctic willows when grown in a common and warmer environment?

Methods

We collected willows from the alpine tundra of the Kluane Region (61°N) to the arctic tundra of the Yukon North Slope (70°N) and planted them in a warmer boreal forest environment.

The tundra ecosystems have average summer temperatures of around 10°C and growing seasons of around 60 days whereas the boreal forest site has temperatures of around 15°C and a growing season of closer to 80 days. We established the garden in 2013 and planted willows each year until 2018.

The garden is planted with three species: Salix pulchra (diamond-shaped willow) and Salix richardsonii (Richardson’s willow) and Salix arctica (Arctic willow) from the Kluane Region and Qikiqtaruk – Herschel Island.

The willows were planted across the garden in different beds to account for variation in the microtopography and soil conditions within the garden.

The common garden experiment from the air. photo credit: Iain Myers-Smith

Results

Here are the headline findings including the latest data from the 2021 field season:

  • Tall willows from the alpine and same latitude grew faster and taller than willows from 1000 km to the north in the Arctic. The alpine plants from the two tall willow species were 2 to 6 times taller and twice to three times as fast as the Arctic plants reaching average heights of 1 m and growing at rates of on average 5 cm and up to 20 cm per year in eight years since planting.
Figure 1. Difference in willow growth between species for the two different willow source populations the Kluane alpine (green) and Qikiqtaruk Arctic tundra (blue) for all willows that remain alive in the experiment with age (top plots) and in 2021 (bottom plots).
  • The tall willows from the Arctic have smaller and lighter leaves with a greater specific leaf area (the ratio of area to mass) relative to the alpine willows. The leaves from the Arctic plants are four times smaller and lighter than the alpine willows.
Figure 2. Difference in willow traits including height and leaf traits between species for the two different willow source populations the Kluane alpine (green) and Qikiqtaruk Arctic tundra (blue) in the common garden (bright colours) when compared to data from the source locations (faded colours). Common garden leaf trait data are from 2017 and canopy heights are from 2021, source location data are from 2013 – 2017.
  • The dwarf willow species Salix arctica from the alpine and Arctic have grown equally well with no difference in canopy height, plant size or growth rates. The size of the plants and their leaves is similar between the Arctic and alpine plants after six years since planting.
Measuring the height of Salix arctica (arctic willow). photo credit: Gergana Daskalova
  • Arctic willows don’t grow for as long as alpine willows each summer. The leaves tall Arctic willows start to turn yellow in mid-July each year, three weeks earlier than the alpine willows.
An Arctic willow in the common garden experiment turning yellow in mid July with green alpine willows behind it.

So what does this all mean and why does it matter? How can the findings from this experiment help us understand tundra shrubification with climate change?

Discussion

Through this experiment, we now know that rates of shrub growth can be very rapid in a warmer boreal forest environment. Growth rates in the experiment were more rapid than we currently see in willows growing in warming tundra locations with shrub heights reaching over a metre in only eight years – that is more than 10 cm of vertical growth per year! That is pretty rapid when compared to the couple of centimetres of growth seen in naturally growing Arctic and alpine tundra willows.

We also found that willows from the north that are moved south are genetically adapted to the midnight sun and different light conditions up north. In Kluane, the willows from the Arctic begin to change colour in the middle of the summer and the willows are not able to grow for as long. Those northern willows grew more slowly and reached smaller plant sizes across the experiment in the two tall willow species that we studied, but surprisingly not in the third dwarf willow species Salix arctica.

Salix arctica is one of the most widely distributed tundra willows and grows as far north as the top of Ellesmere Island and Greenland at over 80°N. For this species, growth rates were similar between the Arctic and alpine plants suggesting that some willow species might be better able to take advantage of changing environmental and light conditions even when moved long distances.

Salix arctica on Ellesmere Island. photo credit: Anne Bjorkman

Conclusion

The take-home message is that tundra shrubs can grow very rapidly in a warmer environment, and thus we should expect further rapid shrub expansion with climate warming. But, we shouldn’t necessarily expect different shrub species to respond in the same ways north to south across the tundra biome. Shrubs that are adapted to their local conditions and are also able to take advantage of the warming climate will likely respond most rapidly. And there could be some surprises in store with future shrubification where the species that we least expect, like the cold-loving dwarf willow Salix arctica, could potentially thrive the best under changing climate conditions.

Meticulous measurement collection in the common garden experiment. video credit: Noah Bell

Acknowledgement

Thanks to everyone who has contributed to the countless hours it has taken to set up and maintain this experiment over the past eight years. It has been and continues to be a labour of love! Garden party to celebrate anyone?

By Isla with 2021 common garden data collected by Madelaine and Beth

Written for the 2021 Yukon University Northern Studies Field Methods Course

A common garden tea party!

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