ECR feature: Anne Thomas

Anne is a postdoc at the Laboratoire d’Ecologie Alpine, CNRS, France. She is an ecologist studying at the intersection of phylogenetics, biogeography, and climate change. Both in prose and verse, Anne shares the history of New Zealand’s largest plant radiation.

Anne hugging a hebe in the subalpine tussock grassland of the Rock and Pillar range in Otago, NZ.

Personal links. Twitter | Substack

Institute. Laboratoire d’Ecologie Alpine, CNRS, France.

Academic life stage. Postdoc.

Recent JBI paper. Thomas, A., Meudt, H. M., Larcombe, M. J., Igea, J., Lee, W. G., Antonelli, A., & Tanentzap, A. J. (2023). Multiple origins of mountain biodiversity in New Zealand’s largest plant radiation. Journal of Biogeography, 50(5), 947–960.

Uplifting Plants: How Mountains Generate Plant Diversity.

Mountains all over the world are known as biodiversity hotspots. New Zealand’s largest group of endemic plants, flowering shrubs called hebes (genus Veronica), has over 120 species, most of which live in mountain habitats in New Zealand’s Southern Alps. DNA evidence suggests the group is only around 6 million years old—relatively young on an evolutionary timescale—but hebes have surprisingly diverse forms. They range from small trees with long, narrow leaves, to dense shrubs, to cushion plants that only grow in the high alpine zone. Can their preferred mountain habitats explain how hebes evolved so much diversity in so little time?

I explored this question in my PhD research, but also through poetry. Before delving further into the scientific take on mountain diversity, here’s a poem to give us a look at the diversity of hebes in situ.

Field Guide to New Zealand Veronica

Hunting hebes,
you climb east-facing cliffs
scramble rocky river gorge
hike to treeline through sparse mountain scrub.

Phyllotaxis: decussate
that is, look for
leaf pair
rotate right angle
leaf pair
again, again, again,
squared-off spiral up the stem
jazzy ladder
to diamond leaf bud
waiting to spring and spread
into more rungs.

Find rock shelf, crevice, seepage
pick your way up scree slope
look for limestone outcrop
or margins of ephemeral alpine pool.

Inflorescence: simple lateral raceme of
crowded, spiralled,
pedicellate flowers

that is, find
fountain and froth of flowers
four white lobes
framing demurely the shock
of hot pink anthers
where the pollen calls
and the green-nestled ovules
waiting to swell
into capsuled fruit.

Range through fellfield, herbfield
streamside, rockslide
tussock grassland, cloud forest
coastal bluff, bare greywacke
road cutting, bog, sand, beech shade, snowbank—

Take the leaf-ladders
and the froth-flowers
and the rock-hound roots
shrink them to cushions
spin them out to long-leafed trees
round them down to springy shrubs
press them into rawhide whipcords
spread them through golden grass
tuck them into cracks—

huddle in the cold, reach for the light
wander alone between rock walls

for six million years
and find
variations on a theme.

If you loved rock and light like a hebe
with all New Zealand to hide in,
where would you go?
What would you be?

Phyllotaxis: decussate

Some of the hypothesized reasons for high mountain diversity in places like the Andes and Himalayas are centred on the habitat: high heterogeneity, barriers to gene flow between populations, and novel, harsh conditions that allow suitably adapted plants to escape competition. These conditions could encourage speciation within mountains. However, the diversity could also come from surrounding areas, transferring to the mountains through colonization. In New Zealand, the estimated origin of the Southern Alps is several million years after that of the hebes, with relief beginning to form around 4 million years ago and a persistent alpine zone around 1.9 million years ago. When the possible ancestor of the hebes arrived in New Zealand, the land was most likely still very flat (a “peneplain”). This makes the hebes a useful case study for teasing apart in situ diversification and colonization as drivers of mountain diversity.

To do this, I first inferred the evolutionary relationships of hebe species with a time-calibrated phylogeny estimated from dozens of genes. This poem gives an idea of what that process is like:

Phylogeny, or A Leaf Has a Long Memory

Look at this leaf
   firm on its springy stem
      squeezed where a seed lodged
         in a limestone crack after a capsule 
             popped on the plant rooted in rock above
                 where a bee brought pollen from over the cliff
and the chromosomes found each other.

Pluck this leaf
   to harvest chromosomes coiled with
      leaf-code, stem-code, rock-rooting code
         an inheritance latticed with accidents: 
            A flipped to T, G slipped to C, refolded proteins 
               or silent jots-become-tittles tell the story
to be laid open in the leaf.

Look at this leaf
   crushed to dust in the test tube
      ready for the chemical alchemy
         of centrifuge and pipette, enzyme and heat:
             essence unfurled, swirled, chopped and copied
               until a mountain is made of a molehill
of leaf-letters.

Trace the branches
   spun from crushed leaves and leaf-letters
      and mathematical model ticking back time
         pressing into lines the slow drama of glaciers
            calving and halving ranges, the bee-flow of pollen,
               chromosomes doubling, leaves finding new shapes
and new branches on the family tree.

Look at the letters
   aligned and inscrutable on the screen
      pieced together by computer algorithm
         sleuthing the deep-buried footnotes of leaf—
            ribosome, hormone, the space between—
               leaf-cousin by leaf-cousin, their cascade of edits
sifted side-by-side into snippets of sense.

A leaf has a long memory.
We do our best to tease it free.

After estimating when species formed, the next step is to reconstruct where they might have formed. With the phylogeny telling us about the past and a map of species telling us about the present, we can do some math to predict the most likely path the ancestral species would have taken through the landscape to arrive at their present distributions. This is historical biogeographical modelling.

My models showed that the core group of hebes, to which most of the species belong, didn’t arise until their common ancestor colonized the newly rising mountains, followed by a surge of speciation. Their evolution was indeed linked to the new mountain habitat.  The surprising thing was that even though the mountains kept rising, the rate of new species didn’t. When the mountains rose high enough for there to be a treeless high alpine zone, most species successfully expanded their range to live in that new habitat. However, only a few species split off into new specialist alpine species, and the rest were happy continuing to live in both the subalpine and the alpine zones without forming many new species. This could indicate that the first successful species outcompeted any new species that tried to form.

This pattern could also have implications for the future. I’ve been talking about broad evolutionary time: millions of years. There was a lot of change in this time—not only the mountains rising but also fluctuations in climate. There were warm and cold periods and glacial advance and retreat, which created extra challenges and opportunities for hebes in the mountains. Clearly, a lot of them were able to survive, but there were likely others that didn’t. Now we’re facing a period of unprecedented climate change. In another few million years, the New Zealand flora will inevitably look different. But the question for the immediate future is, how much evolutionary potential in the diversity we have now might be lost to the speed of environmental change? The generalists with wide ranges might survive, but they also might push out the species specially adapted to the coldest zones, the cushion plants and whipcords. Or they might be pushed out themselves by plants from even warmer areas.

I’ll leave you with a poem that imagines the perspective of one of those alpine plants.

Alpine Elegy

I am alpine.
I hunch and hug the ground
where the wind sears
and summer is too cold for trees.
I like it here.
I’m found nowhere else.

My ten-millionth-great grandparents
clung to coastal rocks
when the land was low and warm
blanketed in beech shade.
Lifting land meant shifting luck
for their children:
new rocks, wind, and light.
They climbed.

After mountains rose, 
glaciers descended.
My ancestors were ice-dodgers
as the crush heaved down and up
their home-slopes.
They sent out seeds
with luck and pluck.
Some survived.

It’s been quieter.
For ten thousand generations
my kin and I have calibrated
to this high band of land,
its stable chill, its harsh peace.
The alpine made us.
We made it our own.

But I sense change in the wind—
winter losing its edge
snow blanket growing bare
shady strangers creeping in
no longer kept at bay by freeze.
I will my children upslope.
I hope I gift them lucky genes.

And when they find only sky?

Hebes, Veronica section Hebe species. Flowers (top left), showcases fruits after seeds have been released (bottom left), species with a whipcord habit (centre), apical leaf bud (top right) and tree habit and unripe fruit (bottom right).

Published by jbiogeography

Contributing to the growth and societal relevance of the discipline of biogeography through dissemination of biogeographical research.

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