Autumn in terminal decline?

In amongst the nasty consequences of global warming – sea-level-rise-ocean-acidification-violent-storms-heat-waves-large-scale-extinctions (if you say it quickly and rock back and forth at the same time it doesn’t seem quite so bad) – a decline in the intensity of autumn leaf colour really doesn’t draw the eye.  Bar a few unusual plants like the red-fruited kurrajong and the Antarctic beech, most deciduous plants around here aren’t even natives.  So who cares, eh?

But as I huddle in my chilly house on its shady south-facing hillside, waiting for the leaves on the neighbour’s looming liquidambar to fall, the impact of climate change on deciduous trees seems like a tremendously pressing question.

I’m not the only one gripped by this crucial topic.  The latest  Trends in Ecology and Evolution has roundly denounced the scandalous neglect of autumn. Spring gets its own live feed on BBC TV, but even scientists get depressed by extended discussions of leaf senescence, it seems.   Garnering less than half as many articles as its greener sibling, autumn, according to the indignant authors, is a “neglected season in climate change research”.

Well, neglected no longer!  Not here in the backyards of Berowra.  Right here, right now we bring you…. in the prophetic words of Gallinat and her outraged colleagues… “the future of autumn research”.

As we march boldly into fall’s future, I’m cling to the hope that photoperiod (that’s the day-length to you and me) will rescue me from climate change, sending that winter-sun-blocking foliage promptly into the compost bin regardless of how roasting hot it is. And it’s not a vain hope – the amount of light a deciduous plant receives does seem to help many decide whether it’s time to shed their leaves or not.

In the case of liquidambar, long days or lots of light delay dormancy, as you can see from these nifty pictures of a specimen down the street, well illuminated day and night so as to minimise deaths on a local pedestrian crossing, clinging to its leaves long after its neighbours have shrugged their own.

Depressingly, it does seem that sweetgums need cooler weather to finally ditch their leaves, even in the short days of midwinter.   Photoperiod matters most near the poles – but for trees at the lower latitudes (like Sydney, curse it) temperature is the clincher.

This raises interesting questions about the future of the veggie garden. Around the winter solstice it lurks in the shade of our dawn redwood, a living fossil that grew across the temperate Arctic when dinosaurs stomped the earth, and was dramatically rediscovered in the 1940s in a single isolated valley in China.  Will its gorgeous copper needles still fall in time to give my broadbeans a decent run-up to spring when we’re wearing shorts all winter?

In the words of a Facebook status update, “it’s complicated”.  Could this be why climate scientists, like nervous singles, are staying well clear?

For instance, warmer springs lead to earlier bud burst, which can sometimes mean earlier leaf-fall.  And deciduous trees in general tend to lose their leaves more readily in dry weather.  “On average”, according to Estiarte and Peñuelas (2015) “climatic warming will delay and drought will advance leaf senescence”.  Work that one out.

And that’s not even throwing nutrient availability into the mix.  For instance, what if trees start going ballistic with all that carbon dioxide in the atmosphere?  This vision of a greenhouse planet jungle awash with joyful plants growing at breakneck speed sounds like something out of a climate denialist fantasy, doesn’t it?  “More open cut mines, pretty please!” beg the earth’s desperate forests “my future is coal!”

Sadly for the wind-farm haters, it mostly doesn’t work like that.  Carbon dioxide can give trees a flying start but eventually the nitrogen supply conks out, or drought and too much CO2 do the leaves in.  Even with the help of globe-trotting survivors like sweet gums and dawn redwoods, coal (and copious quantities of greenhouse gases) won’t make the world greener.  Let’s just hope, even gardening in our bikinis, we can still find gold.

References

  1. Estiarte, M and Peñuelas, J (2015) “Alteration of the phenology of leaf senescence and fall in winter deciduous species by climate change: effects on nutrient proficiency” from Global Change Biology 21(2) 1005-17
  2. Flexas, J, Loreto, F and Medrano, H. (2012) Terrestrial photosynthesis in a changing environment: a molecular, physiological and ecological approach, Cambridge, Cambridge University Press
  3. Gallinat, AS, Primack, RB, Wagner, DL (2015) “Autumn, the neglected season in climate change research” from Trends in ecology and evolution 30(3)
  4. Warren JM, Jensen AM, Medlyn BE, Norby RJ, Tissue DT, (2015) ‘Carbon dioxide stimulation of photosynthesis in Liquidambar styraciflua is not sustained during a 12-year field experiment’, AOB Plants, vol.7, Article no.plu074
  5. Warren, JM, Norby, RJ, Wullschleger SD (2011) ‘Elevated CO2 enhances leaf senescence during extreme drought in a temperate forest”.  Tree Physiology 31, 117-30
  6. Worrall, J (1993) “Temperature effects on bud-burst and leaf-fall in subalpine larch” Journal of Sustainable Forestry 1(2)
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9 thoughts on “Autumn in terminal decline?

  1. Very interesting that you should write a post about this as I have been noticing that the the exotic Chinese Elm trees in my neighbour’s yard have been losing their leaves later in the last two years. Rather than start to lose them in late autumn they don’t begin now until winter and this year they still have about half of their leaves. I join you in your quest to put autumn research out there! Especially since my clothes won’t dry and my verandah is cold from the trees shedding late this year! Brrr… It is not something we hear much about really, except when it comes to the agricultural side of things. Love the gorgeous colous and the light streaming through the leaves in your pics. Thanks again for another entertaining and thought provoking post. 🙂

    • Thanks Jane! I’m glad I’m not the only one who’s been trying to make autumn leaves on adjacent properties fall through force of will alone! The reasons for the neglect of autumn seem to be quite interesting – it’s easy to get people to spot the first migratory bird they see or the first spring flower, but a bit harder to notice the last falling leaf or the last time you heard a koel! I guess the seasons really should have different names here anyway – the Dharawal seasonal calendar (that’s from south of Sydney but I can’t seem to find a Dharug or Gadigal one) seems to have about six seasons. I wrote a little bit about it last year ( https://berowrabackyard.wordpress.com/2014/09/13/sprinter/ ) though I’d definitely like to find out more…

      • That’s very interesting about the Dharawal seasonal calendar. I will have a look at the post. It is kind of strange that we use the European seasons here! Thanks. 🙂

  2. The potential affects of climate change are certainly very complicated – one small consolation about living through this time is that we’re all part of a fascinating global experiment. Thanks for a thought- provoking post, and for making me jealous of your dawn redwood.

    • Thanks Paula – we certainly live in “interesting times”…. gulp. Yes, I feel very lucky to have a metasequoia in my garden. At first I thought it was a swamp cyprus (nifty in itself) but was very chuffed when I worked out that it was the Wollemi pine of the 1940s. I wrote a post about it last year ( https://berowrabackyard.wordpress.com/2014/09/16/a-fossil-in-the-garden/ ) though possibly none of that is news to you if you are an enthusiast! It’s looking very gorgeous at the moment thanks to a week or so of chilly weather this month (chilly for Sydney anyway), though my broccoli will be happy when the lovely coppery leaves finally fall!

  3. Love this tribute to autumn! Am watching the bright leaves on my liquid amber only now reluctantly beginning to fall, and still waiting for the magnolia to bloom.

    • Thanks Kathryn! I love magnolia. Lots of them here, waiting to do their beautiful leafless thing. I’m tempted to try to plant one but apparently possums love them! With my track record it would be munched for sure! Thanks so much for reading…

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