Deciduous trees?

I am quite sure that I learned that it is a lack of sunlight which causes a deciduous tree to shed its leaves. Is this true? Can any environmental effect have an influence on deciduous trees?

I’m not sure what you are asking. Certainly air pollution and such factors as acid rain can weaken and eventually kill many kinds of trees, deciduous included. Also drought, insect pests, parasites, diseases.

My understanding is that as longitude (distance from the equator) increases, there is less light available year-round, and particularly in the winter when each hemispere is tilted away from the sun. Leaves absorb energy in the form of sunlight, but they also cost the tree energy to create and maintain. When that cost:benefit equation doesn’t work out in the tree’s favour anymore, it drops the leaves.

But then why don’t coniferous trees drop their leaves?

I’d guess it’s either that:

a) They’re more primitive, and don’t have the cost:benefit response that deciduous trees do

b) They’re more sophisticated – their leaves are so efficient that there’s essentially no reason to have a cost:benefit response.

I’m leaning towards a), but I’m sure a biologist will be along soon to clarify.

Shortened daylength affects leaf drop in deciduous trees, but so do decreases in temperature, and changes in water and nutrient availability.

I was in college in upstate New York one year when we had an ice storm in early October, before the deciduous trees dropped their leaves. As a result, they were carrying a lot of weight in ice, lots of branches broke and there were many power outages. Coniferous trees generally have needles, which can’t carry as much ice. So I think deciduous trees drop their leaves partly as an evolutionary response.

Deciduous trees in Autumn: Combo of light diminishing, temps changing, and soil temp dropping.

Deciduous trees dropping leaves in Spring or Summer: drought…or nutrient definciencies…or insect damage…or nutrient deficiency…or fungus/disease.

Decisuous tree dropping leaves at top of tree only, rest remains healthy (not Autumn) = beginning of the end, which might take many years, maybe decades.

Then how do decidious trees know when to drop their leaves in temperate climates such as southern Florida?

See more than you ever wanted to know re Why do leaves change color in the Fall?

…and a fall poem

wind shivered leaves
arcing on tired stems
blazing gold and crimson
sun sugars changing
breaking free
fluttering, spiraling
skittering past empty streets
brushing the earth
cartwheeling into the wind
on a lemon bright, cloudless day
the sky is opening into the heavens
and I am falling down
to you

Some conifers DO drop their leaves.

IANABiologist, but I think the words ‘primitive’ and ‘advanced’ can be a little misleading here; deciduous and evergreen trees merely employ different survival strategies, essentially; either make expensive, durable leaves, or cheap, disposable ones.

The unasked question first. There are several factors that make it expedient for a plant to be evergreen or deciduous.

Dewey Finn has already mentioned one, and that is the tendency for broadleafed plants with leaved to suffer problems with snow and ice buildup in cold climates that causes physical damage to the tree. Temperate and frigid conifers generally have needles rather than broad photosynthetic surfaces like leaves and so are less affected by this problem.

Another more important factor is water balance. In winter in cold climates there is little or no water available, despite the possible presence of snow, simply because the water is maintained in solid form. Plants that try to retain leaves throughout winter can suffer serious problems due to dehydration. Conifers tend to be much more drought tolerant than broadleaf plants in temperate regions and so have less problems in this regard.

It’s worth mentioning at this juncture that the deciduous habit isn’t restricted to the temperate or frigid regions. There are large communities of deciduous plants throughout the tropical and subtropical regions of the world, and in those cases the deciduous habit evolved to cope with water stress. In many instances these plants drop there leave sin summer rather than winter.

The next factor is predation. Once some plants start to become deciduous for the reasons given above that produces an added incentive for other plants to follow suit. Any individuals or species that attempt to maintain palatable leaves throughout winter when all other species have shed their leaves will be targeted by every folivore in the area. They will probably be defoliated anyway but without the ability to recover any nutrients from the leaves before they are lost. This usually isn’t a problem once a community has settled on a deciduous pattern since the folivorous mammals all tend to hibernate, but in the initial stages it probably plays a major role in selecting for a deciduous habit.

Back to the OP, most plants in temperate regions do drop their leaves in response to daylength. It’s not quite as simple as saying that it’s caused by a lack of sunlight, and in fact plants kept in the dark in autumn usually won’t shed their leaves. The stimulus is the relative length of night and day. However there are plant species that drop their leaves in response to lack of water or temperature, although these are rare.
In tropical regions leaf fall can be trigged by daylength but it’s just as often caused by a lack of water.

So in short environmental conditions can have an influence on deciduous trees, but it’s entirely dependent on species. While some species will respond to daylength and some to other factors I know of no species that are able respond to multiple factors.

BaldTaco hope fully I’ve answered your questions about why most conifers aren’t deciduous. Bear in mind that some conifers like the larches are deciduous, so it’s not as though they can’t be deciduous, it’s just that in general it’s unnecessary because needles don’t; hold ice and because conifers are drought tolerant. It’s worth noting that other drought tolerant broadleaf.taxa such as Eucalyptus that live in snow prone areas are also evergreen.

Blake did a good job at answering this, but i feel like replying.

A few conifers that drop their leaves:

  • Bald Cypress - Taxodium distichum
  • Larch - Larix sp.
  • Dawn Redwood - Metasequoia glyptostroboides
  • Chinese Swamp Cypress - Glyptostrobus pensilis
  • Golden Larch - Pseudolarix

A bit of a stretch - Ginkgo biloba

there are trees that drop leaves due to seasonal changes as well as water stress. the California Buckeye (Aesculus californica) will drop its leaves before fall in mid to late summer due to water stress, most likely evolved due to the seasons in California - a summer dry, winter wet pattern. However if kept watered it will hold its leaves until fall when they drop like typical deciduous trees.

Even with plants that are typically thought of as being deciduous, there are evergreen species, sometimes even growing together. A good example is in California we have both deciduous and evergreen oaks. You’ll find Valley oaks (Quercus lobata - deciduous) growing with Coast Live Oak (Quercus agrifolia - evergreen). Black oak (Quercus kelogii - deciduous) grows among Interior live oak (Q.wilslizenii - evergreen), although Interior Live Oak is limited by snowfall, which permits Black Oak to become dominant where heavy fronts and snows are.

There is even one California oak that is drought deciduous - Quercus engelmannii, which is more allied to Mexican subtropical Oaks. Leaves will stay if kept watered and replaced with next year’s flush of growth, or if drought occurs they shed them.

Quite a few tropical trees are deciduous, and as has been said this is a response to dry/wet seasons. Some trees also tend to become deciduous in response to flowering – they drop them just before flowering, and then grow a new flush after the flowers fade.

Another thing, some trees here in Monterey which are deciduous and on the east coast would drop leaves fully in Autumn sometimes fail to do so here. Liquidambar is one such tree. I’ve seen it keep its leaves until very late winter, finally dropping most, but retaining a good amount until the new spring flush. They never get the full intense fall coloration either.