I just uploaded this, and I’m not sure yet if the tree is a hickory or an ash, which are in different orders, so it may make a difference. I took three pictures of the tree, then noticed that the edges of leaflets are a different color than the rest where they overlap adjacent leaflets, and took another picture. Do you know anything about this?
This tree is at the edge of both a power line easement and a private road, so it gets more light than a tree in dense woods, but less than one in the open.
The autumn colours of the leaves come from protective pigments (anthocyanins for red/purplish hues, anthoxanthins for yellow). their main job is in filtering out harmful light (and also helping collect vawelengths not collecdted by clorphyll). Areas shaded have naturally less of these pigments as they are not needed for protections, so when protective pigments become apparent after chlorphyll levels start dropping in autumn, shaded leaves (or shaded portions of leaves) generally appear green for longer the more exposed parts.
@ljazz Why do you think northeastern North America has such vibrant autumn colours in the trees (like a forest fire), whereas western North America has mostly dull yellows, dull greens, etc.?
I don’t mean HOW. We know the mechanics of HOW leaves change colour. I mean WHY. In evolutionary terms. What is the advantage or the disadvantage to the tree in doing this?
This looks more like sooty mold than autumn color to me. There’s probably scale insects or other hemipterans on branches above these leaves, and the honeydew that drips down is only falling on exposed leaf surfaces, and then the mold grows from the honeydew.
It looks to me like where the leaflets overlapped on this particular leaf, the ones that were previously under are now over, so those green edges are freshly exposed to sun, drips, etc.
I think eastern forests have a greater diversity of deciduous trees, resulting in more variety and abundance of fall colors. Where I live in the US Southwest the only native tree in abundance at higher elevations that has a nice fall color is aspen. Maples are not common. At lower elevtions we have cottonwoods and some willows and I admit the cottonwoods can be pretty spectacular in golden color.
I am from Europe, and have limited knowledgo of NA ecology, but;
I think it is mainly due to western north America being boreal/oceanic/mediterranean, due to being on the western side of a continent and it being shielded by a huge mountain range from continental influences from central plains and polar intrusions.
This mountain shield and oceanic/mediterranean climate means that historically, climate shifted less during glaciations.
A more stable, and generally milder climate are good for long term survival of evergreen broadleaf species in areas where it is warm enough during the summer, but oceanic influences also mean damp and cool summers up north, which is good for conifers.
(and Western north america is much more mountainous and at higher elevations, conifers tend to prevail)
So overall, I would guess Western north America is less suitable for evolution of deciduousness, as it does not confer such a big advantage; or broadleaf trees are outright outcompeted by conifers. I would guess there is a lower phylogenetic diversity of decidouous trees in WNA, compared to ENA
Eastern north America is far more continental (zones go boreal/nemoral/warm temperate subtropbical), where there is a strong benefit in losing leaves in winter; and lineages where this trait evolved survived and diversified more that those that did not. Also during glaciation, I guess many evergreens went extinct or only survived in refugia in what is now the subtropical belt in the American South.
So in ENA, there is both proportionally more decidouous trees, as well as bigger phylogenetic diversity of them, so the effect they have on the appearance of the landscape is larger, as well as the spectrum of colour they are able to produce.
But that is just my educated guess.
(another one would be; WNA probably has autumns that start slowly and taper of longly into the winter so the leaf turn is slow and uneven, and plants have less cold-protective pigments because day/night temps do not swing so much ; in ENA this change is more abrupt, and day/night temartures swing a lot more, so plants have more cold-protecting pigments, leading to more vivid colours)
I’m not sure how you are defining “western North America”, but a fair portion of the western half of the continent is either a) occupied by said mountains or b) natively consists of prairie or even desert with nary a tree in sight. (There is a good map of the ecological zones here.)
As someone who grew up in what is culturally referred to as the “West” (Great Plains/Rocky Mountains), I will venture that grasses are not known for their brilliant fall colors. On the other hand, aspens (Populus tremuloides), while they generally do not produce reds and oranges, do turn a beautiful clear yellow that is no less a local fall event on the Front Range than the turning of the oak and maple forests of the east coast.
(This Coloradan would not describe her experience of the local climate as either particularly sheltered by the mountains – except insofar as the volume of precipitation is concerned – or characterized by gradual transitions between the seasons. Summer temperatures one day and a load of snow the next is more-or-less “normal weather”.)
Yes, sorry, I was refering to forests along the coasts and in the mountains on the western side; I think that is also roughly how WWF sees it (terrestrial biomes, in the west and east there are forests, center grasslands - I am aware this is a simplification, that there are pockets of forests in the cetntre, and that there are several mountain ranges. With sheltering I mainly meant the extreme western side of the continent, although, one could argue the basins and valleys of the rockies are also sheltered (from rain, but also from intrusions of polar air)).
As I said, I am from Europe, I do not know where your cultural lines are, I am only vaguely aware of biogeographical divisions due to occupational interest. I assumed the person who asked me was refering to biogeography, as they asked about evolutionary processes.
@ljazz based on your answer, I got some guesses. To restate the question more clearly, for anyone following along:
First, let’s set the stage of tree diversity:
The uplift of the Rocky Mountains started around 40 million years ago. Before this, much of North America had a more uniform, subtropical climate. We have fossils of trees related to those in Toronto (like hickories and elms) from Oregon and Washington.
But as the western mountains rose, some things happened:
Aridification: The new, young, high Rocky Mountains blocked moisture from the Pacific Ocean, casting the western part of North America into a rain shadow. The climate became drier and more continental.
Extinction: This climate change wiped out the majority of the moisture-loving, broadleaf deciduous genera that couldn’t adapt. They were replaced by drought-tolerant conifers (pines, firs, junipers) and tough, fast-growing deciduous trees that could handle the harsh climate (aspen, cottonwood, etc.).
Resulting biodiversity: Because of this history, Eastern North America is a global hotspot for temperate deciduous tree diversity. It boasts a huge number of species of maples, oaks, hickories, etc. that don’t exist in Western North America.
The problem: Bright sunlight in autumn, especially when combined with cold temperatures, can overwhelm the photosynthetic machinery of a dying leaf. This creates damaging reactive oxygen species (free radicals) that can destroy the cellular equipment responsible for nutrient resorption. If this equipment is damaged, the tree fails to pull back its valuable nutrient investment before the leaf drops.
The solution: Anthocyanins (red pigments that the tree has to manufacture) act as a powerful sunscreen. They absorb excess light, protecting the delicate cellular machinery underneath. This allows the tree to continue resorbing nutrients efficiently for a longer period.
Maples and Oaks in Toronto say, "I have the genetic ability to produce red sunscreen (anthocyanins). In this sunny yet cold climate, the benefit of protecting my nutrient resorption machinery for an extra week outweighs the energy cost of making this sunscreen. I will turn red.
In addition, the transition to winter in Toronto is somewhat gentle and predictable. By investing in anthocyanins, which act as a mild antifreeze, I can lower the freezing point of my leaf cells. This buys me crucial weeks to finish resorbing nutrients."
It’s a strategy to maximize nutrient resorption in a wetter and gentler climate.
Poplars in Calgary say, "I do not have the genetic ability to produce red sunscreen efficiently. The sun is damaging my photosynthetic machinery. The best strategy is to finish the job as fast as possible before more damage occurs. I will accelerate senescence, pull back what nutrients I can, and drop my leaves quickly.
In Calgary, the risk of a brutal snap freeze (that kills most leaves) is extremely high. Investing energy in antifreeze for a leaf I’m about to discard is foolish, because a -10°C night will destroy it anyway. The only winning strategy is to drop the leaves before that hard freeze arrives. Speed is my only defense."
This is why an aspen grove in the Rockies can go from green to yellow to bare in a few days. They use a strategy of speed and avoidance, not protection and prolongation.
Now, let’s talk about Eastern trees signalling fitness to pests:
Because there is higher deciduous tree diversity in Toronto, there are also more sucking insects that have co-evolved with those trees for millions of years.
The signal: The intensity of the red colour might indicate the tree’s health and its chemical defenses. A healthier tree can afford to produce more metabolically expensive anthocyanins. It also likely has more chemical defenses (like phenols) to poison or deter insect larvae in the spring.
The insect’s choice: An insect, seeing (or chemically detecting or whatever) a brilliantly red tree, would know that it is a well-defended, poor-quality host for its offspring. It would therefore choose to lay its eggs on a less-vibrant, weaker tree. This gives the trees that produce bright colours a reproductive advantage by starting the next season with a lower pest load.
To summarize, there are basically three things to consider: the risk of hard frosts, who has the genetic ability to make sunscreen (red leaves), and pest signalling.
is the change from green to not green more a function of slower breakdown of chlorophyll in shade or of more red and yellow pigments in parts exposed to sun?
that’s sort of what i was thinking. i can see chlorophyll breaking down faster with more exposure to sunlight. but i’ve never seen a green or brown leaf where i could tell there was a variation in the underlying distribution of red or yellow pigment due to uneven shade.
Then there are the Bigleaf Maples in Vancouver. In Vancouver, the onset of fall is also the onset of the rainy season, with many overcast days where sunscreen is superfluous. So, even though the transition to winter is somewhat gentle and predictable, the lack of a need for sunscreen means a lack of a need for orange and red colors. They can start changing color as early as the beginning of September, as soon as the day length gets noticeably shorter.
Now here’s what confuses me:
In eastern North Carolina, fall is also a time of sunny days, and the onset of winter is prolonged. Yet orange and red colors are not common – the Red Maple and the Sweetgum do it, and a vine, Virginia Creeper, but yellow and brown are much more typical of autumn. But then there are the Crape Myrtles. According to Wikipedia, “Lagerstroemia indica is native to the subtropical regions of the Himalayas, southern China, mainland Southeast Asia, the Philippines, and Japan.” This sounds like a similar type of climate to North Carolina in the northern portion, extending into regions without any winter in the southern portion. Yet while our native forests are as of today still in transition from green to yellow, the crape myrtles are in spectacular orange, red, and purple shades, and actually peaked last week. They are at least as spectacular in fall color as they are in flower, which may be one of the reasons cities plant so many of them.
