Time scales, behavior, and extreme events: the story of a backyard garden

Plants may have all kinds of behavior – or maybe they do not, depending on your definition of behavior.

In his documentary, The Private Life of Plants, David Attenborough demonstrates the way they do at least respond to cues from their environment. This we all know: that some windowsill plants will tend to grow toward the light. I learned recently that the majestic saguaro cacti, like barrel cacti and others, grow distinctly differently on their south and north sides, as so in transplanting one, care should be taken to orient it the same way as originally planted.

My point is that plants move and respond to their environment. Sometimes quickly, like the veuns fly trap, which I was able to watch in its native range near Wilmington, North Carolina, at the end of December. Stroking a stick along the hairs of open traps, as seen below, caused them to quickly glide shut, as is the one at the bottom of the frame.

Winter senescence in a Venus fly trap.  Photo credit: Neill Prohaska
Winter senescence in a Venus fly trap.
Photo credit: Neill Prohaska

But sometimes plants move more slowly – important competitive and survival behaviors may take place as they grow, but on timescales too slow for us animals to register them. This is where time lapse cameras like Attenborough’s come in. I decided to take a stab at plant behavior in my backyard.

Last November, my housemate moved a big planter behind the house, and started a garden. I placed a relatively robust wildlife camera in the planter to capture the movement and growth of her lettuce, beets, and radishes. The bright reflection off soil during the day meant I had to time my photos only for infrared detection at night, giving the scenes a spooky air. But then something important happened:

The weekend of El Tour de Tucson, a 100 mile bike race around the perimeter of our city, it rained. Not a nice gentle cooling rain for a few hours or even a whole night, the kind that makes me run outside and dance in the puddles because it rains so infrequently here. This was a downpour. And it was cold. Like living in Portland, or Seattle, or anywhere but Tucson, really. This mattered to me (and 9,000 other cyclists signed up for the race) because we almost never ride in the rain – if it’s raining in Tucson, you usually just wait and ride tomorrow instead because it will be sunny and dry. [Note – this applies only to casual cyclists, otherwise known as sane or relatively normal people. Real cyclists are not deterred by snow or hurricanes.] It was also the first time it had rained – really rained – for the race, which has been going on for decades.

So this storm dumped an unusual amount of water. What’s the big deal for the garden, or the plants and animals in general? There are two main takeaways from this for ecologists and population dynamics analyses:

(1) Infrequent events require long time scales to ensure a capture. If I were a plant ecologist studying winter annuals, and I only had time in my PhD to collect 3 years of data and was trying to extrapolate the growth trajectories of a population over the next 50 years, how would I know to include a storm of this magnitude if I never observed one? Only sampling 3 years from any of the last 30 years would rarely turn up a rainstorm like this. I assumed my nice rugged weatherproof camera would be safe getting rained on in the outdoors for a few months. Based on my experience in the last four years in Tucson, I did not expect the garden to turn into a swimming pool, which was not safe for my camera.

(2) But if you happen to capture one in a short window, it will really mess with your estimates of how a system usually behaves. This was only my second time riding El Tour de Tucson, so now I have one memory of a beautiful ride, and one memory of a truly horrific near-hypothermic sufferfest. Weighted equally in terms of frequency in my experience. If my 3 year window included this event, I might naively conclude this system was pretty highly variable – that an event like this might happen every couple of years.

This was an issue I explored in my undergraduate thesis (with Diane Thomson at Scripps College), but that still continues to be an interesting problem today.

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