Fall Colors: Science, Pigments, Weather, Tree Adaptations
This podcast delves into the science of fall colors, explaining how leaf pigments like chlorophyll, carotenoids, and anthocyanins interact to create vibrant hues. It highlights photosynthesis as the process that makes leaves green during the growing season, and how its cessation reveals underlying colors.
Key factors influencing the autumn display are discussed, including the length of night (shorter days) and weather conditions, such as temperature and moisture. Optimal conditions for brilliant reds, purples, and crimsons involve warm, sunny days and cool, non-freezing nights, which promote anthocyanin production by trapping sugars in the leaves.
The overview also covers tree adaptations for winter, explaining why deciduous broad-leaved trees shed their leaves to conserve water and protect against frost damage, entering a state of dormancy. Different species like maples, oaks, aspens, and hickories are noted for their characteristic fall colors and varying timing of change. The importance of fallen leaves in replenishing soil nutrients and supporting the forest ecosystem is also touched upon.
0.000000 4.400000 Welcome to Everyday Explained, your daily 20-minute dive into the fascinating house and
4.400000 6.260000 wise of the world around you.
6.260000 10.080000 I'm your host, Chris, and I'm excited to help you discover something new.
10.080000 11.080000 Let's get started.
11.080000 12.720000 OK, let's unpack this.
12.720000 17.560000 I've ever stood beneath a canopy of fiery red maples or those shimmering golden aspens
17.560000 21.920000 and just felt this sense of wonder, thinking, how does that actually happen?
21.920000 25.560000 It's arguably nature's most spectacular annual art show, isn't it?
25.560000 27.800000 Unfolding, right before our eyes every autumn.
27.800000 32.600000 So today, we're doing a deep dive into the fascinating intricate science behind fall
32.600000 33.600000 foliage.
33.600000 36.520000 Our mission is really to pull back the curtain on this natural masterpiece.
36.520000 40.160000 We want to understand why leaves change color, what influences those specific, sometimes
40.160000 43.760000 breathtaking hues, and why they eventually fall off.
43.760000 48.640000 We're going to be guided by some fantastic sources of drawing insights from pop scientific
48.640000 49.640000 institutions.
49.640000 53.720000 I think U.S. Forest Service, UNC Chapel Hill, Pacific Science Center, the Smithsonian,
53.720000 54.720000 S-A-N-Y-E-S-F.
54.720000 55.720000 Yeah.
55.720000 62.120000 What's truly fascinating here is how just a few key factors, they seem simple, maybe,
62.120000 66.120000 but it's complex biochemistry, right, plus environmental signals.
66.120000 70.280000 They all kind of work together to create this absolutely dazzling display.
70.280000 71.680000 It's not just pretty though.
71.680000 77.240000 It's a remarkable, deeply ingrained survival strategy, an annual reset for the tree, and
77.240000 79.360000 really for the whole forest ecosystem.
79.360000 80.560000 That makes perfect sense.
80.560000 81.920000 So let's start with what we all know.
81.920000 82.920000 Green leaves.
82.920000 83.920000 Right?
83.920000 84.920000 For most of the year.
84.920000 85.920000 There.
85.920000 86.920000 But what's in the mechanism?
86.920000 88.560000 How does it break down so fast on the fall?
88.560000 90.480000 And what exactly does that break down?
90.480000 91.480000 Well, unmask.
91.480000 92.480000 Right.
92.480000 97.080000 So during the long days of spring and summer, leaves are basically the tree's tiny, super
97.080000 99.120000 efficient food factories.
99.120000 103.800000 They're just constantly making sugars that's the tree's fuel through photosynthesis.
103.800000 106.520000 And that whole process is powered by chlorophyll.
106.520000 111.840000 It absorbs the sunlight, and crucially, it's what gives leaves that dominant green color.
111.840000 116.520000 The chlorophyll is actually being produced and broken down all through the growing season.
116.520000 118.240000 But there's just so much of it being made.
118.240000 124.440000 It completely masks any other pigments that are also hanging out in the leaf, just waiting.
124.440000 129.040000 So these other pigments, they're always there.
129.040000 133.240000 Even when the leaf looks completely green, like you said, an underpainting, just waiting
133.240000 134.840000 for the top layer to disappear.
134.840000 135.840000 Precisely.
135.840000 136.840000 Yeah.
136.840000 139.000000 While chlorophyll is king, you've got these other pigments present the whole time.
139.000000 140.320000 Mostly we're talking about carotenoids.
140.320000 141.320000 They are like that underpainting.
141.320000 144.320000 They give you those beautiful yellows, oranges, even some browns.
144.320000 148.800000 You see them all the time, like corn, carrots, daffodils, bananas, buttercups, they're stable.
148.800000 151.320000 They're always there just, you know, hidden by all that green.
151.320000 152.320000 Right.
152.320000 156.880000 So when you see a tree turned that brilliant yellow, that color didn't just magically appear.
156.880000 160.080000 It was there the whole summer just waiting for its moment.
160.080000 161.080000 Exactly.
161.080000 162.080000 But then.
162.080000 163.080000 Yeah.
163.080000 166.680000 Then you get the showstoppers, the reds, the purples, the crimson's.
166.680000 170.760000 These really dramatic colors, they come from a totally different group of pigments
170.760000 176.120000 anthocyanins and unlike carotenoids, these are not present all season long.
176.120000 177.120000 Nope.
177.120000 181.200000 They are actually newly produced, synthesized right there in the leaf in the autumn, triggered
181.200000 183.000000 by specific conditions.
183.000000 187.960000 You know, these colors from transberries, red apples, conquered grapes, blueberries, cherries,
187.960000 188.960000 that sort of thing.
188.960000 193.360000 And what's really fascinating is that anthocyanins aren't just for looks.
193.360000 197.360000 These bright pigments also act kind of like a natural sunscreen for the leaf.
197.360000 200.440000 They protect it from too much sun during its final days.
200.440000 204.060000 This actually helps the tree pull back more of those valuable nutrients before the leaf
204.060000 205.060000 drops.
205.060000 206.060000 It's a hidden benefit.
206.060000 207.060000 Really vital.
207.060000 208.060000 Okay.
208.060000 209.920000 So we've got pigments hanging around and new ones being made.
209.920000 211.720000 But what's the actual trigger?
211.720000 214.920000 What's the signal that tells the leaf, okay, showtime?
214.920000 218.520000 The main trigger, the big signal, is the increasing length of night.
218.520000 221.960000 That steady decrease in daylight as autumn progresses.
221.960000 224.880000 As the days get shorter, the tree gets the message.
224.880000 228.680000 It slows down chlorophyll production in the leaf, then stops it completely.
228.680000 233.000000 And eventually, all the chlorophyll that's left breaks down and gets resorbed by the tree.
233.000000 235.400000 So boom, the green is gone.
235.400000 239.900000 And that unmasks those carotenoids, the yellows and oranges that we're hiding.
239.900000 243.520000 Now for the brilliant reds and purples, the anthocyanins.
243.520000 247.480000 Their production gets kicked into high gear by a combo of bright sunlight during the day
247.480000 250.560000 and excess plant sugars that get trapped inside the leaf.
250.560000 255.720000 See, as the tree starts preparing for winter, it forms this special separation layer, the
255.720000 260.760000 abcision layer at the base of the leaf stem and this layer starts pinching off the veins.
260.760000 264.520000 So sugars made during the day can't get out of the leaf and back into the tree.
264.520000 269.080000 This sugar buildup plus sunlight is the recipe for making lots of anthocyanins.
269.080000 273.480000 So the green basically just packs its bags and leaves, right, revealing the party colors
273.480000 274.480000 underneath.
274.480000 276.600000 It really is like a plant of performance every year.
276.600000 279.240000 But okay, that brings up a really interesting question.
279.240000 285.320000 If it's all about these pigments and lighten sugars, why isn't every single tree a fiery
285.320000 286.320000 red?
286.320000 287.760000 Why do some go gold?
287.760000 288.760000 Some crimson?
288.760000 291.440000 And let's be honest, some just turn a kind of drab brown.
291.440000 292.440000 Right.
292.440000 298.240000 That's where you see this beautiful mix of genetics and environmental factors.
298.240000 301.040000 Certain colors are characteristic of particular species.
301.040000 302.360000 It's baked into their genes.
302.360000 306.160000 So for instance, oaks, they usually turn red brown or russet.
306.160000 310.880000 Hickories often give you that lovely golden bronze, aspens and yellow poplars, famous for
310.880000 314.320000 golden yellow, dogwoods get that distinctive, purplish red.
314.320000 317.920000 Each tree often goes soft, light tan, and then you have trees like sour wood and black
317.920000 318.920000 two below.
318.920000 320.920000 They're known for a really striking crimson.
320.920000 324.760000 And I've definitely noticed, even within the same type of tree, like maples, there's just
324.760000 325.760000 a huge range.
325.760000 329.000000 You get brilliant scarlet, but sometimes almost no color at all.
329.000000 332.080000 If it's genetic, how does that work, that variability?
332.080000 334.480000 That's a really sharp observation.
334.480000 335.920000 Maples are a perfect example.
335.920000 338.440000 Red maples can be brilliant scarlet.
338.440000 340.680000 Sugar maples often lean towards orange red.
340.680000 344.280000 Black maples tend to be a glowing yellow, but then yeah, you get striped maples
344.280000 349.600000 that are almost colorless, or maybe even a red maple that just looks dull that year.
349.600000 354.240000 So while the potential to make certain pigments is genetic, the actual expression of those
354.240000 358.280000 pigments, especially the anthocyanins, is really influenced by the weather.
358.280000 362.640000 The type of pigments a tree mainly makes, plus its knack for trapping sugars, that dictates
362.640000 364.120000 its signature color.
364.120000 368.560000 Maples, genetically, are set up to make lots of anthocyanins, hence the potential for fiery
368.560000 369.560000 reds.
369.560000 373.920000 Oaks, on the other hand, they don't make as much anthocyanin, so you get more browns
373.920000 378.760000 and resets from the tannins, mixing with leftover carotenoids, birches and poplars.
378.760000 381.240000 They rely more on their carotenoids, giving you yellows.
381.240000 385.520000 They just don't produce much anthocyan, so that dull red maple, it might just not have
385.520000 389.400000 had the perfect weather combo, those sunny days and cool nights, to really crank out the
389.400000 390.680000 anthocyanins that year.
390.680000 393.360000 Okay, and the timing, is that genetic too?
393.360000 397.800000 Because some trees seem to jump the gun on fall color, while others hold out until it's
397.800000 398.800000 much colder.
398.800000 399.800000 Exactly.
399.800000 403.160000 The timing varies a lot by species, and yes, that's largely inherited too.
403.160000 405.320000 It's like an internal clock.
405.320000 409.520000 Sourwood, for example, down south, they can turn incredibly colorful in late summer, way
409.520000 412.760000 before most other trees even think about changing?
412.760000 415.800000 Oaks, conversely, they're often late to the party.
415.800000 420.680000 They put on their colors long after many other trees have already dropped their leaves.
420.680000 425.600000 And interestingly, you'll often see a specific tree species, say at the same latitude, show
425.600000 429.280000 color around the same time, whether it's up in the cool mountains or down in the warmer
429.280000 433.280000 lowlands, really highlights that strong genetic control over timing.
433.280000 436.000000 We've talked about the beauty, but you also mentioned survival.
436.000000 438.720000 So why do trees actually shed their leaves?
438.720000 440.240000 It seems kind of drastic.
440.240000 443.040000 Letting go of all those food factories they work so hard to build.
443.040000 445.920000 It does seem drastic, but it's absolutely critical.
445.920000 451.400000 It's a life-saving adaptation for these broad-leaved trees and places with cold winters.
451.400000 455.600000 As the days shorten and nights get longer, the tree starts building that special layer
455.600000 460.080000 of cells, the obsession layer, right at the base of the leaf stalk.
460.080000 463.080000 This layer slowly but surely closes off the veins.
463.080000 466.720000 The ones carrying water and nutrients into the leaf and sugars out.
466.720000 470.240000 So this traps those sugars, like we said, helping make anthocyanins, but its main job is
470.240000 473.480000 to seal the connection, prepping the leaf to detach cleanly.
473.480000 477.280000 Okay, but still, growing all those leaves takes up a ton of energy.
477.280000 479.240000 What's the big payoff for just ditching them?
479.240000 482.000000 Ugh, there are several huge payoffs.
482.000000 486.920000 First, those broad tender leaves, they'd just freeze solid and die in the winter, shedding
486.920000 488.560000 them prevents that damage.
488.560000 491.600000 Second, winter often means frozen ground.
491.600000 493.680000 Trees can't easily suck up water through their roots.
493.680000 496.840000 If they kept their leaves, they'd lose tons of water through transpiration and basically
496.840000 498.160000 dry out.
498.160000 501.400000 So shedding leaves conserves precious water.
501.400000 505.120000 Third, think about heavy snow or ice claimed big leaves.
505.120000 507.120000 The weight could easily snap branches.
507.120000 510.400000 No leaves, less surface area, less risk of breaking.
510.400000 513.920000 And finally, it lets the tree go dormant, kind of like hibernation.
513.920000 518.560000 It slows down all its metabolic processes, saving energy until spring comes around again.
518.560000 521.360000 It's a really smart, strategic energy saving move.
521.360000 522.360000 Right.
522.360000 523.360000 But then you've got the evergreens.
523.360000 524.360000 Don't you?
524.360000 525.360000 Pine, spruces, furs.
525.360000 526.360000 Yeah.
526.360000 527.360000 They stay green all year.
527.360000 530.120000 How do they handle winter without dropping their leaves or needles, I guess?
530.120000 531.440000 Good point.
531.440000 535.280000 Evergreens have a totally different strategy, but just as effective.
535.280000 540.360000 Their needles or scales have a much smaller surface area, so they lose less water.
540.360000 544.800000 Plus they're coated in a thick wax, which helps even more with water conservation.
544.800000 549.120000 And the fluid inside their cells often has natural, anti-freeze properties, helping them
549.120000 551.380000 resist freezing temperatures.
551.380000 555.000000 They do lose their needles eventually, but it's usually because the needles get old
555.000000 557.600000 maybe every two to four years or so.
557.600000 561.600000 It's not a seasonal shed triggered by day length like the broadleaf trees.
561.600000 563.440000 They just replace old ones gradually.
563.440000 567.360000 Okay, and how long do the colorful leaves actually stick around?
567.360000 571.080000 Once the fall color season feels like it's over in a blink, other times it seems to last
571.080000 572.240000 for ages.
572.240000 574.240000 What determines how long they hang on?
574.240000 578.160000 Well the main thing is the completion of that obsession layer we talked about.
578.160000 582.480000 Once that layer fully seals the leaf off, the connection is weak.
582.480000 586.280000 Then it just takes a bit of wind, or even the leaf's own white to make it fall.
586.280000 587.760000 But yeah, there's definitely variation.
587.760000 591.360000 Some trees, especially certain types of oaks, sometimes hold onto their dead, brown
591.360000 594.360000 leaves right through the winter, it's called marsh sessence.
594.360000 598.440000 You might not fall until the new buds start pushing them off in spring.
598.440000 603.080000 So the timing of the drop varies by species, and can also be affected by weather like if
603.080000 606.960000 there's no strong wind or rain for a while, the leaves might just hang on longer.
606.960000 611.600000 You know, some years the fall colors are just legendary, absolutely stunning.
611.600000 616.040000 In other years, they're kind of, hmm, hmm.
616.040000 617.040000 What's going on there?
617.040000 619.700000 Is there like a perfect recipe for a great fall color year?
619.700000 620.700000 There definitely is.
620.700000 623.960000 Weather is the master artist here, making every autumn unique.
623.960000 627.760000 The brilliance of the colors, especially those reds and purples, really depends on the weather
627.760000 631.360000 before and during the time the chlorophyll is fading.
631.360000 635.400000 Temperature and moisture, those are the two main conductors of the fall color symphony.
635.400000 637.160000 So what is the secret sauce?
637.160000 641.800000 What conditions give us those really intense jaw-dropping reds and purples?
641.800000 646.520000 Okay, the ideal setup, the perfect recipe for spectacular color, particularly those reds
646.520000 649.600000 and purples from anthocyanins, is this.
649.600000 655.520000 A string of warm sunny days, followed by cool, crisp nights, but, and this is important
655.520000 660.760000 not freezing nights, why that combo, well, the warm sunny days keep the tree making lots
660.760000 663.060000 of sugar through photosynthesis.
663.060000 668.060000 But the cool nights, along with that developing obsession layer closing off the veins, they prevent
668.060000 671.640000 those sugars from moving out of the leaf, so you get this build-up of trapped sugars
671.640000 675.800000 in the leaf, and that, combined with the bright sunlight, is what really ramps up the
675.800000 678.680000 production of those vibrant anthocyanin pigments.
678.680000 683.320000 Ah, okay, so if the yellows and golds are from carotenoids that are always there, does
683.320000 687.360000 that mean those colors are generally more reliable year after year, less dependent on
687.360000 688.360000 weird weather?
688.360000 692.880000 Yes, that's generally right, because carotenoids are just always present.
692.880000 697.400000 The yellows and golds tend to be more consistent, they're pretty dependable, it's really the
697.400000 702.400000 reds and purples, the anthocyanin colors that are the divas, they're the ones most sensitive
702.400000 706.200000 to the weather fluctuations because their production is the variable part.
706.200000 711.840000 So when you hit one of those truly epic fall color years, it's not just random luck, it's
711.840000 716.960000 this amazing alignment of the trees, genetics, and just the right environmental cues hitting
716.960000 717.960000 at the right time.
717.960000 722.840000 That Goldilocks sweet spot of warm days, cool nights, enough moisture makes those years special.
722.840000 724.080000 And what about the flip side?
724.080000 728.880000 What kind of weather can really mess things up and turn a potentially amazing fall into
728.880000 730.480000 just brown and quick?
730.480000 734.360000 Oh yeah, adverse conditions can definitely put a damper on the show.
734.360000 739.140000 It's spring freeze or a really bad summer drought, can stress the trees, maybe it's a laying
739.140000 744.120000 the color onset, a warm spell during the fall, especially if it stays warm at night.
744.120000 747.560000 That really lowers the intensity of the colors because the sugars don't get trapped as well
747.560000 749.880000 so less anthocyanin gets made.
749.880000 753.360000 And probably the worst spoilers are too much rain right when colors should peak, which
753.360000 757.160000 can just knock the leaves off, or an early hard frost.
757.160000 761.160000 A frost can kill the leaf tissue, turning everything brown prematurely and cutting the
761.160000 763.400000 whole display short, it's a delicate balance.
763.400000 764.520000 It really is.
764.520000 769.720000 So summing up the perfect recipe then, you want a good warm, wet spring for growth, a decent
769.720000 773.920000 summer, nothing too extreme, and then the magic window.
773.920000 779.040000 Warm sunny fall days paired with cool, crisp nights, nature's Goldilocks moment.
779.040000 780.040000 That's it exactly.
780.040000 783.500000 And because there are just countless ways those factors, temperatures, sunlight, moisture
783.500000 787.880000 can combine, it guarantees that no two atoms are ever identical.
787.880000 791.800000 Makes long range forecasting tricky, but it also makes you appreciate the unique show
791.800000 793.880000 each year puts on, doesn't it?
793.880000 794.880000 Absolutely.
794.880000 795.880000 Okay.
795.880000 799.920000 So once these leaves have done their amazing technical or thing and they finally fall, what
799.920000 800.920000 happens next?
800.920000 802.880000 Do they just disappear or are they just waste?
802.880000 804.120000 Oh, not at all.
804.120000 805.720000 Not wasted in the slightest.
805.720000 810.640000 Those fallen leaves and evergreen needles too play an absolutely vital role back in the
810.640000 812.080000 ecosystem.
812.080000 817.280000 Once they hit the forest floor, they start to decompose, break down, and as they do,
817.280000 820.320000 they return essential nutrients to the soil.
820.320000 822.040000 And trans the tree took up in the first place.
822.040000 824.160000 It's a perfectly cycling system.
824.160000 829.660000 This decomposition process is also key to building up that rich, spongy, humus layer you
829.660000 831.760000 find in healthy forests.
831.760000 835.720000 That layer is fantastic at soaking up and holding rainwater, preventing erosion, and keeping
835.720000 836.720000 moisture in the soil.
836.720000 837.720000 Wow.
837.720000 838.720000 So they're not just dead leaves.
838.720000 842.760000 They're actually becoming food and shelter, building the very foundation of the forest floor.
842.760000 843.760000 Precisely.
843.760000 847.240000 Fallen leaves become a critical food source for a whole universe of soil organisms.
847.240000 850.960000 We're talking microbes, fungi, earthworms, insects.
850.960000 854.600000 All these critters are essential workers, constantly breaking down the leaves and keeping
854.600000 856.240000 the nutrient cycle going.
856.240000 858.160000 They're the unsung heroes of forest.
858.160000 859.160000 How?
859.160000 862.000000 It's pretty clear why shedding leaves benefits the individual tree survival through winter,
862.000000 863.280000 like you said.
863.280000 867.160000 But the benefit to the whole forest is maybe more subtle, isn't it?
867.160000 868.160000 More interconnected.
868.160000 869.160000 It is.
869.160000 870.160000 Absolutely.
870.160000 873.000000 You could argue the forest couldn't really sustain itself in the long run without this
873.000000 875.520000 annual leaf fall and replenishment.
875.520000 879.640000 Constant cycle leaves falling, decomposing, returning nutrients.
879.640000 880.800000 It's fundamental.
880.800000 885.400000 It keeps the soil fertile and allows the entire community to thrive season after season.
885.400000 886.560000 Nothing is wasted.
886.560000 888.400000 Every leaf feeds the future.
888.400000 889.560000 So there you have it.
889.560000 892.560000 The secret life of fall leaves.
892.560000 896.320000 From understanding why they changed that unmasking of hidden pigments and the creation of
896.320000 901.040000 new ones, to how species and weather dictate the specific colors and timing.
901.040000 903.960000 And then the survival strategy behind why they fall.
903.960000 907.800000 And how that final act is actually crucial for the whole forest renewal.
907.800000 908.800000 It's quite a story.
908.800000 909.800000 It really is.
909.800000 912.280000 And it raises a question for you, doesn't it?
912.280000 916.720000 Next time you're out walking in the fall, enjoying the display, what new things will you
916.720000 917.720000 notice?
917.720000 922.400000 Maybe you'll start spotting which trees turn which colors or thinking about the recent weather
922.400000 926.000000 and how it might be playing out in the leaves you see right there in your own neighborhood
926.000000 927.000000 or park.
927.000000 928.000000 Exactly.
928.000000 932.320000 It definitely makes you appreciate that fleeting beauty even more, knowing the incredible
932.320000 935.120000 science and strategy behind it all.
935.120000 939.360000 These beautiful interrelationships in the forest, they just leave us with so many fascinating
939.360000 941.480000 puzzles still to figure out.
941.480000 946.560000 Makes you wonder what else looks like just a pretty sight but is actually this complex,
946.560000 949.200000 vital dance of survival and renewal.
949.200000 950.600000 Something to ponder.
950.600000 952.640000 Thanks so much for joining us on this deep dive.
952.640000 954.960000 Keep exploring the amazing world around you.
954.960000 957.960000 And that wraps up today's episode of Everyday Explained.
957.960000 961.440000 We love making sense of the world around you five days a week.
961.440000 965.520000 If you enjoyed today's deep dive, consider subscribing so you don't miss out on our next
965.520000 966.520000 discovery.
966.520000 968.560000 I'm Chris and I'll catch you in the next one.