Climate & Vegetation

Climate is basically (entirely?) the product of two simple variables: heat and precipitation: how much heat, and how it is distributed throughout the year, and how much precipitation, and how it is distributed throughout the year.

Both of these things have everything to do with latitude, with the arrangement of continents in relation to oceans, with altitude/elevation, and with the height and position of mountain ranges.  These are the physical factors/parameters that translate into climate, the things that determine, by your location, whether your climate is Mediterranean or Monsoon, whether you live among palm trees or polar bears.

Starting at 0 degrees latitude—the equator: the sun is pretty much directly overhead all year, so it is warm all year; there isn’t much of anything you could call “seasons”.  At the equator it rains all year, so the air is humid.  And because the air is humid, there is as little difference in temperature between night and day as there is between January and July—it’s always warm.  This constant heat and humidity is what makes tropical rainforests; it means your neighborhood will have a lot of banana and papaya “trees”, coconut palms, and royal poincianas.

More-or-less all trees are evergreen (broadleaf evergreen, not conifers).

If there happen to be high mountains on your part of the equator, you will notice a rapid shift in climate and vegetation as you move upward, just as you do in northern mountains, but tropical mountains have climate and vegetation zones very different from mountains at higher latitudes, and for the same reason that the corresponding lowlands differ so much: the constant, year-round, directly overhead sun.  Tropical mountains have no seasons, or rather their seasons are repeated every 24 hours.  Temperatures are cooler, both day and night, but as you move upward above the zone of cloud forest (cool and damp, still evergreen), the air becomes drier and so the nights become colder, until . . . 

The wet tropics are in a belt on or near the equator, including Amazonia, central Africa, southeast Asia/Oceania.

As you move away from the equator in either direction, toward the poles, there is a gradual increase in seasonality, with more heat in summer, less in winter.  There is also a corresponding seasonality of rainfall, with most precipitation occurring in the warm months.  The extreme of this is the so-called Monsoon climate, with a long drought for much of the year, ended by torrential, prolonged, rains at the height of the summer.  Such a climate produces a tropical or subtropical “dry forest”, with most of the trees losing their leaves in response to drought (rather than cold, as is the case farther north or south.  Vegetation is sparser than in the wet tropics, and the trees are shorter, often thorny, and often members of the Pea Family (acacia, mesquite etc.).  The flora can also be rich in succulents.  This climate/vegetation type is widespread through the subtropical parts of the world, covering much of East and South Africa (including the familiar savanna country); parts of Mexico and Central America, northward to include the Sonoran desert; and parts of Madagascar and Australia and India etc.

Continue on through subarctic climate/vegetation type and somehow work in descriptions of tropical and temperate montane climates—very different from each other.

Idea:  Since mountains in temperate northern hemisphere have floras reflecting those of latitudes farther north, then do the very different floras of the tropical (equatorial and southern hemisphere) mountains derive from the mostly long-gone Antarctic flora (now confined to Patagonia and New Zealand)?

Evergreen sclerophylls of Mediterranean climates have leaves hardy enough to stand the moderate winter cold of those areas, and are able to start photosynthesizing as soon as the rains begin in fall—a time when temperate-zone plants are losing their leaves (and so their ability to photosynthesize).  In a Mediterranean climate, winter-deciduous plants are only able to grow for a short period in spring, after it has become warm enough to be safe for tender new leaves and while there is still some moisture left in the ground.

In fact, all Mediterranean plants are fairly cold-hardy, since they all begin their growth with the first rains, even the annuals and bulbs, and so have to endure the coldest part of winter at the most vulnerable stages of their development, i.e. while putting out tender new growth.

What is the origin (tropical evergreen? or temperate deciduous) of the non-Ericaceous evergreen genera of Mediterranean climates?  Do the southern hemisphere Mediterranean areas derive more-or-less all their evergreen species from tropical evergreen ancestors? (as it seems: Euphorbiaceae, Myrtaceae, Fabaceae, Proteaceae—except Ericaceous South African heathers).

Ericaceae—why does this family have so many evergreen species at such high latitudes (and altitudes)?  These plants are capable of withstanding far more extreme cold than any? other evergreen plants except conifers.

Do the Northern Hemisphere Mediterranean areas get theirs from a more-or-less even mix?  Umbellularia and Laurus and Ceratonia, and palms and olive and Nerium on the tropical side, and oaks and pines and Ceanothus? And Rosaceae and Lamiaceae?  Should Ericaceae be considered some sort of oxymoronic “temperate evergreen” family?

Asteraceae (small-leaved)

Chenopodiaceae (small-leaved=all?) Atriplex

Fabaceae (small-leaved, especially desert)

Lamiaceae (small-leaved, especially desert) 

Equivocal groups difficult to determine whether evergreen or deciduous, especially those with tiny leaves, especially desert/Great Basin/dry Mediterranean plants.  Especially difficult? in the case of mainly-non-woody families.   

Is Mediterranean flora related to tropical mountain flora?

High tropical mountains cannot have cold-deciduous plants because every night is winter, therefore cold-deciduousness belongs to high latitudes in seasons.

Therefore northern mountains have more in common with northern lowlands than they have with tropical mountains!

Southern hemisphere conifers are subtropical, more-or-less primitive, broad-needle types (Podocarpaceae and Araucariaceae, etc.).

Northern hemisphere conifers are more-or-less temperate, recent, small-needle types (Pinaceae, Taxodiaceae, Taxaceae, Cupressaceae).

Therefore southern hemisphere conifers often grow with palms, tree ferns, evergreen dicots.  Northern hemisphere conifers mostly grow with deciduous dicots.   

Tropical (equatorial) mountains have: 

Evergreen cloud forest (but no pine-oak?)    

Heath/moorland 

“puna” grassland 

ice cap          

Temperate mountains have:

Mixed deciduous/conifer forest

conifer-only belt

“tundra”

ice cap