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The title of this question is fairly self-explanatory. I am developing a fantasy world, and I want to know what factors determine what kinds of trees I can have, where I can put them, and why.

I want to know what factors (climate, habitat, weather, etc.) will tell me which trees can exist in a given area. For example, maybe a certain type of tree can't exist on a forested mountain. Or in a swamp. Or on the coast.

I want to know what kind of soil I need. I don't need anything too technical, I just need to know if the soil is swampy, rocky, salty (as in from an ocean or something), etc.

And of course, I would like to know the reasoning behind your answers.

EDIT: In order to make this question less broad, and clear up some possible confusion: I am talking about real trees. I do not need need to know the specific kind of trees - if all of the trees in a broad category arise from the same factors, then just use that category. You could also describe the characteristics of the trees (eg, marshy ground usually gives rise to trees like this), maybe with a few examples. This would enable me to make my own trees, which is always more fun anyway. :)

I'm basically trying to make sure I don't put a tree somewhere it isn't supposed to be.

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  • $\begingroup$ This question may be better suited for one of the science stack exchange sites. However, since it is technically about building a world, I thought I would ask it on Worldbuilding. If it is off-topic, please feel free to move it. I apologize for any inconvenience. $\endgroup$ – Thomas Reinstate Monica Myron Apr 25 '15 at 18:07
  • $\begingroup$ Might be better to ask on the gardening site, or just get a gardening reference book (e.g. the Sunset one for the USA) if you're talking about real trees. Or if you know the specific tree, you can (for USA, anyway) look up USDA range maps on the web, for instance plants.usda.gov/core/profile?symbol=sese3 $\endgroup$ – jamesqf Apr 25 '15 at 18:43
  • $\begingroup$ If you provided a list of trees you have designed, we could then place them for you. As it stands, your question is too broad, because we are free to make up our own trees, and therefore our own questions. $\endgroup$ – theonlygusti Apr 25 '15 at 18:48
  • $\begingroup$ @theonlygusti I'm pretty sure he was talking about trees that already exist, otherwise it's unanswerable. $\endgroup$ – Vincent Apr 25 '15 at 18:55
  • $\begingroup$ @Vincent but there's infinitely many types of trees! $\endgroup$ – theonlygusti Apr 25 '15 at 18:55
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This requires a HUGE answer, but I'll try to give you some broad factors. And all of these are generalizations; there are exceptions to every single item, and I look forward to any comments that will include them.

Climate - Rainier climates will see more vegetation, and more competition. Colder wet climates will have hardier tree trunks and leaves. Warmer, wet climates will see thinner trunks and leaves. Dry climates will have plants and trees that maximize their surface area for water collection, or thick, low plants for storage of water. Drier forests tend to have less under-brush, due to being regularly cleared by healthy forest fires.

Altitude - Higher regions are colder and windier and will require large, hardy plants with thick leaves. Pine trees do well in dryer high altitudes than others, and will grow up to the timberline of a mountain.

Fauna - Areas with a lot of competitive bugs and animals will tend to have thicker trunks and waxier leaves for protection.

Soil type - rich, nutritious soil means a very active growth and death cycle, so this is where you find your dense vegetation constantly growing and dying, and usually with lots of our bug friends. A lot of our "nicer smelling" plants, like sage, rosemary, etc. appear to prefer rocky, dry soil.

Floods & droughts - places with annual, healthy floods will see your low growth plants and grasses. Places that experience annual droughts will, as well (think African Savannah - annual dry and wet seasons, lots of grasses and low, hardy trees).

Rain shadow - frequently, on islands as well as mainlands, where there are high mountains near weather systems, one side will be rich with vegetation (where the rains come), but the other side is blocked from receiving the rain.

Sea - most 'trees' in the sea (not talking about corals or other formations in this case; I'm thinking more like the sargasso forests) tends to accumulate closer to shore and in warmer waters, although not exclusively.

Active volcanoes - where lava is flowing, there doesn't seem to be a lot of trees.


EDIT

Competition - this is actually the most important factor. Forests are changing constantly over eras, but one of the drivers is competition. Pine trees, for example, are found in the dry hills where they're able to survive better than their competitors. But you will find them in wet climates, where they are slowly being outcompeted - forests are always changing.

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  • $\begingroup$ Great answer, and one more factor - having seeds available. Species that could survive or thrive in an area must be carried there somehow. Reforestation efforts in India and Africa have shown that forests can be created from savannas if trees are introduced. Species with seeds that can be carried by birds or mammals spread much more quickly and farther than those that spread only by dropping seeds to the ground. $\endgroup$ – user15741 May 14 '18 at 19:08
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There are some tools available to help you determine the vegetation of a specific area.

  • There is the USDA hardiness zone. Basically, it categorize the different areas according to the minimal temperatures in winter. Not only it makes beautiful maps but it tells you what kind of plants you can grow at home. On the bad side, it's more useful for gardening that worldbuilding because it does not indicate the maximum temperature. It only indicates the minimum temperature a plant can survive. Some places can have extreme temperature difference between the summer and winer and this is going to affect what type of plants can grow there. It also doesn't cover the precipitation aspect, so a desert can be classified in the same zone as a temperate rainforest.

  • Other than that, while still general and somewhat confusing to use, there is also the Holdridge life zones system. By following the graphic, it tells you what type of vegetation you should expect depending on the altitude, latitude, and the precipitations. To use it, you need to know the altitude and latitude (that should not be a problem). And you also need to figure out what are the precipitations in the region. I won't cover this here: where does it rain? (I should probably make some clean up there).

    To put it simply, the potential evotranspiration indicate if there is enough water available for a specific vegetation type. Above 1, it's always humid. Under 1, there is not enough water to sustain a dense vegetation. In some cases, large forests are possible. These are monsoon forests were plants accumulates the water during the wet season in order to survive during the dry one.

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  • $\begingroup$ Even if there's enough rain, a lot depends on when that rain comes - that is, whether it's fairly even across the year, or concentrated in a rainy season. Lots of other factors: think about why most of the western US has mostly native conifers (except along streams), while the east has mainly hardwoods. $\endgroup$ – jamesqf Apr 25 '15 at 22:25
  • $\begingroup$ @jamesqf Those factors are exactly the kind of thing I need to know about. I have no idea why the West has conifers and the East has hardwoods. $\endgroup$ – Thomas Reinstate Monica Myron Apr 25 '15 at 23:21
  • $\begingroup$ @Tommy Myron: I don't really know why, either. And to make it more complex, not all conifers are the same, or thrive in the same habitat. For instance, hereabouts (eastern side of the Sierra Nevada) there are different types of 'pines' at different elevations. A bit east, there's the pinyon-juniper woodland, which is limited to between 6-9000 ft elevation in fairly dry mountains. Go west to the coast, and you find redwood (another conifer) in a rain forest. $\endgroup$ – jamesqf Apr 26 '15 at 17:57
  • $\begingroup$ @TommyMyron - I believe part of the answer to your comment here is found in the forest fire cycles. Boreal forests are adapted around fire $\endgroup$ – Twelfth Apr 27 '15 at 19:11
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A few factors not covered in the lovely answers above:

(1) Biological warfare.

Aromatic compunds produced by vegetation have two main functions: attracting pollinators/seed dispersers, and repelling pests. If you want something valuable like cinnamon bark you'll need to add the bugs it's evolved to repel, at least as a vague environmental factor.

(2) Fire cycles.

An environment periodically scrubbed by fire will have a natural progression of the order species come back.

(3) Evolutionary history.

On Earth the conifers evolved first and covered first the easy habitats and then the harsher ones. It's not easy adapting to a harsh habitat but it can be done when you don't have much competition. The superior deciduous trees crowded out the conifers in richer biomes but were unable to establish themselves against the conifers who got there first in poorer environments.

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