How Do Evergreens Conserve Energy without Shedding Leaves?

Here a formerly certified arborist stated that evergreens conserve energy without shedding leaves. So will he kindly tell us how.

I’m not a certified arborist, but from my understanding, coniferous evergreens (there are non-coniferous evergreens and non-evergreen conifers, too) can retain their leaves because they’re adapted to retain their structural integrity in freezing weather. The needles have a very small relative surface area, which minimizes water loss, and are covered by a waxy cuticle, which minimizes it even further.

A major danger in the winter is water loss, because snow and ice don’t exactly flow through the tree easily. This is why broad-leaf evergreens tend to get rarer as you go farther north. They’re NOT adapted as well to freezing temperatures.

While being deciduous is typically a response to daylight length (for trees in temperate areas with snows and freezing temperatures), it has its advantage in preventing damage to the tree. A snow load on a broad leaved evergreen can be damaging.

A good example in California are Quercus kelogii (Black Oak) and Quercus wislizenii (Interior live oak). Black oak is deciduous, and Interior Live Oak is evergreen. They often grow together. Interior Live Oak is common in places where heavy frosts and snows are rare, but it becomes increasingly rare the further inland one goes, and becomes replaced by Black Oak, which then becomes the common tree. Essentially the difference between resources gained by long periods of photosynthesis, and damage due to snow buildup determines where Interior Live Oak grows. The more a tree is damaged beyond what the tree can replace, the oak fails.

Some broad leaved evergreens can make it in harsh winter weather by using strategies such as growing low and shrubby to prevent snow damage, and also the advantage of growing in exposed sites that shed snow (such as cliff faces in Yosemite valley)
Another way to prevent waterloss is to have sclerophyllic leaves (touch, leathery, hardened leaves), as well as keeping stomata closed during times when waterloss is most likely. Most of the evergreen chapparal plants in California have sclerophyllic leaves such as Manzanita (arctostaphylos, which incidentally holds its leaves vertical to further prevent water loss), ceanothus, adenostoma (chamise). Others make use of resins and oils, such as black sage (Salvia melifera), California sagebrush (Artemesia californica). While others have storage organs (such as bulbs) and let the tops die off (in fact most of the garden “spring bulbs” originated from areas around the mediterranean where summers are dry). Some are also drought deciduous, like the California Buckeye (Aesculus californica)

Energy conservation:

As for energy conservation, many evergreen plants simply go dormant during periods when growth would be detrimental. Many of the chapparal plants are like this. You will not see a single plant growing, unless it is a riparian species (where abundant water can afford them growth throughout the year). In mediterranean climates (such as the chapparal), growth halts in summer due to lack of water. It continues when autumn rains come, and also late winter through spring after the winter rains. Being evergreen allows plants that don’t become drought deciduous to continue making food year round. They’ve taken advantage of the relatively mild climates they live in to make food, rather than dropping their leaves.

I’m not sure that I’d even agree that energy conservation is an issue. It’s not like leaves are any more of an energy loss to plant than another living tissue. Water loss is the real issue and many conifers are better at dealing with that than many broadleaf trees.

In deciduous trees it’s the transpiration of water out of the leaves, the polarity of water molecules and osmotic pressure that brings the water and nutrients from the soil up to the top of the tree. Since conifers and other dry adapted plants don’t lose a lot of water via the needles/leaves they must get less in the way of nutrients. Do they grow slower or is there a compensating mechanism to replace the loss of water through leaves?

How about cacti? They obviously have sclerophyllic leaves, but do they make use of resins and oils like sagebrush? Some cacti grow in areas that have snow and they obviously grow low and shrubby.

Thet’s true, but what you need to appreciate is that ‘nutrients’ in this sentence refers to micronutrients and are almost completel unrelated to energy.

Plants don’t catabolise the nutrients they absorb from the soil, they metabolise them. Put more simply plants don’t break down soil nutrients for energy the way animals break down the food they absorb. The only uses plant have for soil nutirenst is to join them to existing organic molecules to produce new compounds. That process of joining soil nutirnts to organic molecules costs energy, it doesn’t produce it.

Plants obtain their energy by combining CO[sub]2[/sub] from the air with water from their roots in the presence of sunlight. Although plants can’t photsynthesise without water they will die due to water stress long before they die through lack of energy in conditions of water deficit.

So as you can see it’s not a conservation issue at all. The problem is one of water availability that has nothing to do with energy per se since the leaves aren’t more of an energy drain than any other living tissue.

It’s important at this juncture to realise that most plants are ‘cold blooded’. Most people don’t really think about that but it’s true, their own internal temperature and hence metabolic rat eis entirely dependant on external heat. Once you factor that in you will realise that almost a plants will grow slower in cold conditions whether conifer or broadleaf. Plants in winter have effectively gone into hibernation, just like all the other cold blooded creatures. Once again you can hopefully see why this isn’t really an energy issue.

To answer the question, no, conifers don’t grow any slower or faster in freezing conditions. Very few plants are capable of any growth in such conditions.

Cacti don’t have sclerophyllic leaves. The leaves in cacti are reduced to spines and because of that they can’t be considered sclerophyllic. Cacti do posess a thick waxy cuticle as do most xerophytes but it’s found covering the photosynthetic stems, not he leaves. Many cold adpated cacti and other cold climate xerophytes have also evolved wooly coats either from spines or from trichomes.

Cacti don’t have sclerophyllic leaves. The leaves in cacti are reduced to spines and because of that they can’t be considered sclerophyllic. Cacti do posess a thick waxy cuticle as do most xerophytes but it’s found covering the photosynthetic stems, not he leaves. Many cold adpated cacti and other cold climate xerophytes have also evolved wooly coats either from spines or from trichomes.

Well no, as Blake said. The “leaves” of cacti are reduced to spines, and they do not photosynthesize. Some cacti, such as the tropical cacti lack spines and their stems are limp and they hang. They also are epiphytes and grow where it is rainy.

Another way cactus conserve water but still take in CO2 is that they use what’s known as CAM (Crassulacean Acid Metabolism). This is apparently a feature of many succulents, although the genus Pereskia, which is a primitive cactus relative (thought to represent a stage before the cactus developed succulency and lost leaves for spines) does not have CAM.

What happens in CAM is that during the night, the cactus opens its stomata, to take in CO2. This is fixed into an acid, Malic Acid and is stored until daytime. During the day, Cacti keep their stomata closed, but process the Malic Acid into CO2 which is then used for photosynthesis.

This process is useful for plants growing where water loss is likely, although it results in slowed growth (since CO2 is limited in the amount a plant can store). Some plants can switch from normal photosynthesis and metabolism to CAM, such as clusia, which is not succulent, but lives in wet/dry season areas. Clusia rosea can go for 4 months without water by using CAM. Some Clusia can do both, regular photosynthesis during the day, and CAM at night, but switch to CAM under water stress.
ANother thng to remember is that not all plants that appear to be cactus are cactus. The Euphorbia of arid Africa look a lot like cactus, even down to Spines, but they often have small leaves, and they lack an organ called an areole, which has growth points for spines, and one for either a flower or a new stem to form.

Doobieus CAM isn’t only found in succulents. It is found in Welwistchia which is either a ver advanced gymnosperm or a very primitive angiosper depending how you want to look at it and is actuallyplced in its own taxon. It’s presence in such a plant indicated that either CAM evolved multiple times or else it was ancestral in flowering plants which either means that its absence in Pereskia is a derived trait or that the process evolved independently in the cacti and other succulents.

True, CAM isn’t only found in succulents. the Clusia genus isn’t succulent, but it’s common to it. But again, Welwistchia is another desert dweller. I’m wondering if CAM was ancestral to flowering plants, that it isn’t something genetic that in most plants is dormant? Either way, it’s an awesome way to prevent dessication, but still be able to get CO2 for metabolization.