Leaf Color Changes Remind Us What We Already Know: It’s Been a Long, Hard Summer... and Change is Here
Southwest Yard & Garden by Dr. Marisa Thompson
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Bright yellow mulberry
leaf in the Los Lunas bosque on September 2, 2020. Question: In my yard and around town, I have noticed tree leaves
turning yellow in different areas on the tree and falling sporadically. It is
primarily older mulberry trees. Is this due to nutrition deficiency, disease,
drought, or all of the above? -J.
White, Las Cruces Answer: I’ve noticed the same in my own mulberry trees, and my wisteria vines too. I think the prolonged heat and drought are taking their toll, and the stress is observable. Early color change in leaves is a known stress symptom. Last year the bosque up here around Los Lunas flooded for several weeks in June and July, and I was worried that the native golden currant shrubs (Ribes aureum) I’d been tracking might not make it with their roots inundated for so long. As soon as it was dry enough, I went back to check. I was relieved to find that they were still alive and amazed to see that leaves had changed color completely to a deep red in response to the stress. As of yesterday, these same shrubs were still mostly green, with only some leaves showing off their famous fall colors.
As NM State Climatologist Dr. Dave DuBois (@NMClimate) and others have
been tweeting all summer, the temperatures and precipitation levels have been both
record- and heart-breaking. In a phone conversation with my predecessor Dr.
Curtis Smith months ago, he warned
that as our climate continues to warm, this summer and in coming years, we
should expect many more calls and emails about sickly plants, and pest problems
too. Lots of pests and diseases are known to be secondary stressors that strike
plants with underlying
conditions like water stress.
While walking in the Albuquerque bosque last week, yellowing tips
in some cottonwood canopies kept catching my eye. I wondered about the water
table levels and how thirsty those giant trees must be. It was so hot that day
it was hard to imagine that the seasons are still changing. |
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Cottonwood leaf in the Albuquerque bosque on August 26, 2020, practically shouts, “It’s been too hot and dry!” and also promises that fall is on the way. Photo credit M. Thompson. |
In November 2018, I wrote a fun column on fall foliage colors and
why tree leaves go straight from green to brown in some years but turn bright
gold in others. In that column, I delved into the details, but it all boils
down to chemistry and how different plant pigments respond to environmental
signals. I wrote: “A general rule is that while temperature tends to affect the
intensity of leaf color, it’s the shorter days and longer nights that trigger
overall color change.” CLICK HERE to link to that 2018 column and scroll down down down to the bottom of this page for a slew of leaf color photos I’ve
collected over recent years.
While stress is a good bet as we try to explain yellowing leaves
all over, I can’t help but also wonder how much of this color change is a
seasonal norm. Time has been incredibly wonky this year. In April, my friend
Matt Cohen reflected, “Since COVID started, there are only three days of the
week: yesterday, today, and tomorrow.” It’s hard to imagine what the trees were
doing at this time last week, let alone last year. During quarantine, people
have been much more observant too. How early is this color change compared to
other years? Could it be that we are so exhausted from heatstroke and everything
else that it just seems too early this year?
I asked Albuquerque BioPark Botanic Garden volunteer Karl Horak what
he thought. Horak and a team of dedicated Nature’s Notebook volunteers have
been tracking phenology in the bosque throughout recent years. He shared a remarkable
graph depicting cottonwood (Populus
deltoides) leaf color change observations throughout the state for 2018,
2019, and year-to-date 2020. Horak remarked, “It looks like 2020 is very similar to previous years: a
tiny blip or two in the spring (likely aphids), a bigger blip (heat?) in
mid-summer, and then the big fall color change starting about now.” I’ll keep
you posted as the season—and the data—progress.
Horak added, "Although the NN obs for fall 2020 will be months away from being finalized, here's an article from earlier this year about spring: https://natureecoevocommunity.nature.com/posts/spring-2020-started-with-a-bang-how-did-it-wind-up."
Find details about how these data are collected each year in the Fall 2018 and Fall 2019 issues of the Rio Grande Phenology Trail Cottonwood Correspondence Newsletter.
Side note: Nature's Notebook is a national, online program with
the USA National Phenology Network that uses amateur and professional
naturalists to record plant and animal observations in a given location over
time. The steps for becoming a volunteer are straightforward. And you can set
your backyard as a location or pick a public space and get a group to sign up
together—with masks and safe distancing, of course (https://www.usanpn.org/natures_notebook).
At any rate, what a cool relief we had last week across the state. A colleague in Santa Fe said it got down to 39°F one morning. The low temperature at my house on Tuesday was 55°F. The briskness made me giddy, and I briefly considered hanging Christmas lights out by my mailbox to make my neighbors (and myself) laugh. Since then, temps have already rebounded right back up to the high 90s. And forecasts show another expected drop down into the low 40s in a few days (low 50s in southern New Mexico).
Together, we’re learning tough lessons about climate and stress,
but also about adaptation and resiliency. Selecting landscape trees and shrubs
that can handle each season’s challenges is as difficult as it is important.
more pics and captions coming soon! ...
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Send gardening questions to Southwest Yard and Garden - Attn:
Dr. Marisa Thompson at desertblooms@nmsu.edu, or at
the NM Desert
Blooms Facebook page (@NMDesertBlooms)
Please
copy your County
Extension Agent (http://aces.nmsu.edu/county/) and
indicate your county of residence when you submit your question!
For more gardening information, visit
the NMSU Extension Horticulture page at Desert Blooms (http://desertblooms.nmsu.edu/) and the NMSU Horticulture
Publications page at http://aces.nmsu.edu/pubs/_h/.
Marisa Y. Thompson, PhD, is the Extension
Horticulture Specialist, in the Department of Extension Plant Sciences at the
New Mexico State University Los Lunas Agricultural Science Center.
Edible New Mexico Magazine article crosslink -- GROWING YOUR FUTURE FAVORITE TREE
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