Geography 140
Introduction to Physical Geography

Lecture: Biomes Dominated by Trees

  V. Major biomes of the earth.  Biomes are a way of describing associations 
     of plants, or vegetation, according to dominant life forms in them.
     A. Forests
        1. An association of plants dominated by the tree life form (and what 
           did you learn in college today?  That forests are dominated by 
           trees.  Do tell.).
        2. The trees form a continuous cover.
        3. Usually there are several layers, or stories, or canopies, of 
           vegetation.
           a. This can include up to four stories of trees.  Some of these
              stories may be discontinuous or grouped, but the trees as a 
              whole form a continuous cover overall, so that you generally 
              don't get to see much ground when you fly over a forest.
           b. There may a discontinuous or grouped shrub story.
           c. There may be other undergrowth, including forbs or grasses, 
              fungi, and byrophytes. There may also be lianas, epiphytes, and 
              lichens as well. 
           d. The forest biome, then, includes the most different types of 
              life form, but it is dominated by trees, which together form 
              continuous coverage.
        4. There are many types of forest, depending on locality.  The major 
           types are closely correlated with particular climate types, so you 
           might want to review the climate lecture and map of world climates.
           a. Tropical rainforest (sometimes called "selva," a Brazilian 
              Portuguese word, since Brazil has so much of the world's 
              tropical rainforests):  
                i. These forests have a huge variety of species!
                   a. A typical mid-latitude forest might have somewhere 
                      between 2 and 6 different species of tree per hectare, 
                      where a tropical rainforest routinely has more than 100 
                      different tree species per hectare!  There are some 
                      places that have more than 400 species per hectare!!! 
                      (oh, a hectare is about 2.5 acres, or 10,000 square 
                      meters)
                   b. This astounding biodiversity is echoed in other life 
                      forms (and in animals, too).
                   c. There are very intricate food webs in the tropical 
                      rainforest, with often microfine ecological niches 
                      worked out in the competition among different species.  
                      Disturbance in one area can cause the collapse of quite 
                      a few links in the food web, as each species gets 
                      knocked out of its narrow range of tolerance.  You can 
                      get waves of extinctions in an area radiating out from a 
                      local disturbance that appears trivial!  
                   d. So, while these forests are very stable through time, 
                      they are not very resilient to change in the narrow 
                      range of conditions that supports them.  
               ii. This kind of forest typically includes virtually all kinds 
                   of life forms, generally all of them evergreen.
              iii. There are normally three tree canopies and sometimes as 
                   many as four:  The uppermost one tends to consist of a 
                   discontinuous cover of gigantically tall emergents (over 30 
                   m tall); the next one or two are mostly continuous; and 
                   there may also be a fourth layer that is discontinous or 
                   grouped in coverage.

                   

               iv. This tree coverage is so continuous that the forest floor 
                   is very shady, to the point of dark and gloomy.  
                   a. It is so dark on the forest floor that, for the most 
                      part, there are few shrubs and forbs down there:  The 
                      forest floor tends to be very clean of undergrowth and, 
                      indeed, of litter in mature rainforest.
                   b. The only exceptions occur where there are breaks in the 
                      forest cover, perhaps created by a blow down of some 
                      tall trees and especially along any river ("riparian 
                      vegetation," means along the river).  Here, the unusual 
                      exposure of the ground to sunshine permits a very heavy 
                      and luxuriant undergrowth of forbs and shrubs and small 
                      trees (some of them young rainforest trees) and what 
                      have you.  This profusion of low growth is termed 
                      "jungle," and this is pretty much impassable.  Early 
                      explorers, seeing all this along the rivers they were 
                      navigating, concluded that the Amazon and Congo 
                      were covered with thousands of miles of "impenetrable 
                      jungle."  Actually, once you get past the first 30-60 
                      meters, you find yourself in classical rainforest, and 
                      the going is much easier. 
                v. Tropical rainforest is associated with the heavy rainfall 
                   and even, warm temperatures of the tropical humid climates 
                   and the tropical monsoon climates.  You can see them on the 
                   map in the climate lecture as the Af and Am climates in the 
                   Köppen system.  So, that would include places like the 
                   Amazon Basin of Brazil, the Congo Basin of central Africa, 
                   and the Indonesian archipelago.

                   

               vi. Because of the climate, the soils supporting this forest 
                   are generally extremely poor (unless they form over recent 
                   volcanic material).  They are generally dominated by oxides 
                   of iron and aluminum, because the soil nutrients are very 
                   quickly washed down way down into the subsoil and out of 
                   the reach  of plant roots.  In fact, if the vegetation is 
                   removed (as by slash-and-burn agriculture or lumbering or 
                   for cattle-raising operations), the few nutrients wash away 
                   and, in the intense sunshine of the tropics, the oxides 
                   will often form a concrete-like substance called "laterite" 
                   (after the Latin word for "brick," which this stuff 
                   resembles). This makes the area pretty much useless for 
                   agriculture and animal husbandry and, worst of all, hinders 
                   the natural succession process that would normally restore 
                   small patches of rainforest to their original condition.  
                   This is a problem of grave significance in the world today, 
                   since deforestation is accelerating in the humid tropics.
              vii. How can these forests look so luxuriant on such a poor soil 
                   base?  They do it by exploiting another climate 
                   characteristic:  Things rot really fast in the tropical 
                   rainforest, because it is so warm and humid.  You don't 
                   want to suffer a hiking accident out here, folks -- in a 
                   very short while, there wouldn't be enough left of you to 
                   ship home in a box!  Any organic matter that falls to the 
                   forest floor (leaves, branches, monkeys who missed a vine, 
                   hapless hikers) decomposes super fast, releasing the 
                   nutrients in them.  Rainforest plants' roots aggressively 
                   absorb them and take them back up into the canopy.  So, 
                   there is very fast and thorough nutrient cycling, and most 
                   of the nutrients in these environments resides in the leaf 
                   canopies!  Clearing of the vegetation is problematic, 
                   because it allows the nutrients to be drained down into the 
                   subsoil and out of reach of plants, rather than recycled 
                   immediately.
           b. Subtropical conifer forest.
                i. A continuous cover made up of various conifer trees (e.g., 
                   loblolly pine, yellow pine, short leaf pine in the American 
                   Southeast; Araucaria pine in southern Brazil; Kauri Pine on 
                   New Zealand's Auckland Peninsula).
               ii. There's an understory of evergreen broadleaf shrubs (e.g., 
                   rhododendrons, azaleas, blueberries). 
              iii. This vegetation is associated with Köppen's Cfa 
                   climate (humid subtropical climate), which is mapped in 
                   the online lecture on climates.
               iv. But this type of forest needs something else than the humid 
                   subtropical climate to develop:  It also requires sandy, 
                   fast-draining soils.  If the soils are heavier, then 
                   another type of forest outcompetes the subtropical conifer 
                   forest there:  The temperate broadleaf and mixed forest 
                   (about which more later).  It is only in sandy, fast-
                   draining, and generally nutrient-poor soils that the 
                   conifers can outcompete the deciduous-dominated woods.
                v. This ability of the subtropical conifer forest to dominate  
                   the poorer soils of the humid subtropical climate has to do 
                   with a symbiosis, called a mycorrhyza, that pines strike up 
                   with fungi.  In mycorrhyzae, plant roots are infected with 
                   a fungus, and the fungus extracts carbohydrate food from 
                   the pine's roots.  In exchange, the mycelia (filaments that 
                   fungi send out in a large area around them) alter the 
                   acidity of the soil and draw in mineral nutrients for the 
                   pine's roots to absorb.  Because of this symbiosis, both 
                   partners in the mycorrhyzal relationship can exist in areas 
                   that would otherwise be off limits to them (acidic, sandy, 
                   fast-draining, mineral-poor soil).
               vi. Places with the subtropical conifer forest vegetation:
                   a. The Pine Barrens of New Jersey
                   b. The Pineywoods of the American South
                   c. Southern Brazil's Araucaria forests
           c. North American West Coast marine forest:
                i. This features a continuous tree cover made up of various 
                   conifers (e.g., Douglas fir, redwoods and sequoias, spruce, 
                   hemlock, Western red cedar).

                   

               ii. Sometimes there are two distinct stories of trees: Giants 
                   (60 m+), such as the coastal redwoods, the big tree 
                   sequoias, ponderosa pines, and Doug fir), with merely tall 
                   trees below (>25 m), including hemlock, spruce, and cedar). 
                   There are few species found in any one area, however, 
                   perhaps two to ten species per hectare, but the particular 
                   species change, depending on local circumstances.
              iii. There is an understory of evergreen and deciduous broadleaf 
                   leaf shrubs (e.g., blackberries, blueberries, 
                   rhododendron), ferns (such as bracken ferns), forbs (e.g., 
                   devil's club or Alaskan ginseng, skunk cabbage, manzanita).

                   

               iv. There is also an herbaceous understory of forbs and ferns 
                   (and sometimes some grasses). 
                v. In some of the more humid sites, there can also be a lot of 
                   bryophytes, lianas, and loads of mushrooms.
               vi. This forest varies in its openness and number of tree and 
                   understory canopies, depending on proximity to the Pacific 
                   Ocean, windward or leeward aspect on a mountain, and 
                   adrêt or ubac aspect on a mountain:  In some areas 
                   inland, in the Sierra or Cascades, it opens up almost into 
                   a woodland, while along the Pacific Coast in Northern 
                   California, Oregon, Washington, British Columbia, and 
                   southern Alaska, it can get so lush and complexly 
                   stratified that it's dubbed the "temerate rainforest," 
                   complete with draping lianas.
              vii. This forest is associated with the west coast marine 
                   climate, or the Cfb climate in the Köppen system.  It 
                   can also be found in some of the bordering Mediterranean 
                   Csb, Csbn, and Csc climates and the Dsb Mediterranean 
                   highland climate.
             viii. It is more narrowly distributed than the Cfb and adjacent 
                   climates, however:  It is found only on the North American 
                   Pacific Northwest Coast.
           d. Temperate broadleaf and mixed forest (sometimes called the 
              summergreen forest).
                i. Continuous, but not dense forest cover (some variants are 
                   open enough to be considered woodlands rather than true 
                   forests; others are very thickly treed), usually with only 
                   one tree canopy.

                   

               ii. The trees are medium to tall in height, but never very tall 
                   in the way you see in the rainforest or West Coast marine 
                   forest types:  These trees are usually somewhere between 8 
                   and 30 m in height. They generally comprise one layer but 
                   sometimes there's a second tree canopy made up of shorter 
                   species, such as dogwod and redbud.
              iii. There are usually very few species that dominate an area:  
                   Perhaps two to six species will make up, say, 80 percent of 
                   the trees in the area.  As in the West Coast marine forest, 
                   however, the particular species change as you move around 
                   in the forest, as conditions change in relation to the 
                   tolerances of particular species.  You might, for example, 
                   find beeches, oaks and magnolias in northern Florida and 
                   the Gulf Coast states; farther north, it might, instead, be 
                   oaks and hickories to the west or oaks and chestnuts and 
                   walnuts and poplars to the east; farther north yet, it 
                   could be maple and beech or basswood, perhaps mixed in with 
                   such conifers as hemlocks and pines.  If you moved up in 
                   elevation, too, you might see similar zonation, with 
                   deciduous trees on the lower hillsides and conifers on the 
                   hilltops.
               iv. Because there is usually only the one tree canopy, and that 
                   canopy is dominated by deciduous trees, there is enough 
                   sunshine on the forest floor to support undercover.
                   a. There is normally a shrub layer, which can be 
                      discontinuous or grouped in cover.  This includes 
                      blueberries, rhododendrons, raspberries, and laurel. 
                   b. There are often lianas around, too, such as wild grape                       
                      and poison ivy or poison sumac ("leaflets three, leave 
                      it be!").  This picture shows poison ivy on the left 
                      from the summergreen forest biome -- and our own poison 
                      oak to its right -- with a picture of the sores you can 
                      get for your edification!  To become an expert in 
                      recognizing poison oak, visit this gallery to see many 
                      of its seasonal variations: 
                      http://www.hanskellner.com/photos/2004/05/PoisonOak/

                       
                      
                   c. There is also very commonly a ground layer of 
                      bryophytes, mosses, fungi, and lichens.
                v. This is a profoundly seasonal forest, and the activities at 
                   each season are set by the condition of the deciduous tree 
                   canopy:
                   a. In fall, you get the famous fall color display, as the 
                      trees' leaves turn red (e.g., maple) or yellow (e.g., 
                      birch) and every color in between.  This is really well 
                      worth a trip Back East in September or early October, 
                      folks!  After the trees have shed out but before the 
                      weather gets really cold and snowy, some forbs will 
                      bloom and set seed, taking advantage of the sunnier 
                      conditions.
                   b. In winter, it really looks awfully dead, with all the 
                      bare branches of the deciduous trees and shrubs, the 
                      only visual relief being the occasional conifer trees.
                   c. Comes spring, sunshine hits the forest floor, and forbs 
                      go nuts, developing very rapidly, flowering, and setting 
                      seed in the very few weeks before the tree canopy has 
                      leafed out:  They have to complete their life cycles 
                      before they're shaded out in summer by the leaves of the 
                      trees and shrubs.
                   d. Summer is actually a sort of dappled light and shadow 
                      effect produced by the trees' and shrubs' leaves:  It 
                      can be too dark for a lot of sun-loving forbs, so they 
                      are less apparent at this time.  You often see ferns, 
                      though.
               vi. This forest type is associated with Cfa, Cfb, Dfa, and Dfb 
                   climates:  Humid subtropical and milder humid continental 
                   climates. See map in the climate lecture.
              vii. The mere existence of the climate is insufficient to 
                   produce the summergreen forest, however:
                   a. Soils are important, and the temperate broadleaf and 
                      mixed forest is generally absent from areas with sandy, 
                      fast-draining, nutrient-poor soils.
                   b. Evolutionary history is important, too:  The Pacific 
                      Northwest is something of a relict zone, an area with 
                      living fossils, an area perhaps protected by various 
                      barriers from the spread of deciduous species.  So, the 
                      Pacific Northwest remains nearly as dominated by 
                      conifers as the landscapes known by dinosaurs!
             viii. Some places you can find this forest type:
                   a. North American South and East
                   b. Western Europe 
                   c. Southern Chile
                   d. Northern China, the Koreas, and Japan

                   

           e. Taiga or boreal forest is basically a conifer, mostly evergreen 
              needleleaf forest.
                i. In North America, it's dominated by pines, spruces, and 
                   firs; in northern Eurasia, there are lots of pines and the 
                   Siberian larch (which is a weird deciduous conifer!).
               ii. This forest is associated with the Dfc, Dfd, Dwc, and Dwd 
                   climates (see map in the climate lecture):  Extremely cold 
                   climates in winter and only tepid in summer.
              iii. So, trees are approaching the limits of their physical 
                   tolerance for extreme cold in such a climate:
                   a. There are long, wickedly cold winters, and a short 
                      summer growing season, which produces relatively short 
                      trees (mostly <15 m. 
                   b. High winds and the brittleness of the branches in winter 
                      mean that the trees take on this really odd look:  Often 
                      really short branches that make the trees look like 
                      fuzzy telephone poles.

                      

               iv. The trees are widely spaced and only provide a continuous 
                   canopy along the lower end of trees. 
                v. Even though a lot of sunshine can get below the trees in 
                   this situation, there is still pretty sparse undergrowth, 
                   though, because of the density of the litter layer built up 
                   by years of needles falling to the ground and being too 
                   tough and resinous to rot in the very short warm season.  
                   This thick carpet of needles, dead branches, and trees 
                   shows how slowly decay proceeds in such an environment. The 
                   litter stifles new growth by cutting off seedlings from 
                   soil contact (if they're on top of the litter when they 
                   germinate) or from sunshine (if they're on the ground below 
                   the litter).
               vi. You will see undergrowth wherever the forest cover is 
                   broken sufficiently, as in the bogs that formed in the 
                   messed-up drainage patterns created by glaciers 18,000 
                   years ago or so).  This typically includes  sedges 
                   (grasslike plants), shrubs, and mosses.
              vii. The big hazard here is huge fires. Fires clear the forest 
                   floor and allow sun-loving species, such as deciduous 
                   aspens, some alders, and birches in.  These guys change the 
                   soils, making them a little more basic, which makes it 
                   easier for baby conifers to re-establish themselves.  The 
                   conifers are tolerant of shade, but baby aspens, alders, 
                   and birches are not tolerant of the shade created by their 
                   parents.  So, the deciduous trees change the fired 
                   environment in such a way that they are no longer the 
                   fittest competitor in the new conditions!  So, when the 
                   short-lived deciduous trees die, the baby conifers replace 
                   them in the forest.
             viii. This is not stable, though, because the conifers change the 
                   environment in a direction outside their own tolerance, 
                   given enough time!  They acidify the soil!  So, they weaken 
                   and die and may be replaced by more sun-loving deciduous 
                   trees!
               ix. Another way the conifers change the environment in a way 
                   that frustrates their own propagation is by building up 
                   that litter level and thereby setting themselves up for the 
                   next conflagration.
                x. The taiga makes us realize that the climax concept in 
                   vegetation, which holds that certain climatic and soil 
                   conditions always result in certain climax vegetations 
                   after disturbance, needs to be modified.  Classic 
                   succession theory typically held that a disturbance, such 
                   as a fire, was some outside (or exogenous) force (the 
                   proverbial act-of-god) that messed things up.  Taiga, the 
                   so-called climax for boreal climates, in fact, creates the 
                   conditions for catastrophe!  Fire isn't some weird fluke:  
                   It is built into the system -- it is endogenous.
     B. Woodland
        1. A woodland is another biome dominated by the tree life form, 
           similar to a forest.
        2. Unlike forests, however, woodlands have a discontinuous or a 
           grouped tree cover.
        3. This means that, under the trees, there's enough sunshine for 
           shrubs or grasses to grow.
        4. I'll discuss three examples:
           a. Tropical deciduous woodland: 
                i. This is associated with the tropical wet and dry climate,                    
                   the Aw climate in the Köppen system (see map in the
                   climate lecture). 
               ii. The trees are smaller than rainforest types, commonly 5-15 
                   m tall.
              iii. Because of the pronounced dry season in this climate, most 
                   trees show deciduousness in winter, or else they develop 
                   "xerophytic" adaptations (e.g., thick bark; small, 
                   sclerophyllous or pulpy leaves, and thorns). Examples of 
                   such trees include teak in Southeast Asia, acacias in 
                   southern and eastern Africa, eucalyptus and acacia in 
                   northern Australia. 
               iv. With the open tree canopy, there's an understory of grasses 
                   and/or shrubs.
                v. In some versions of this forest, there may be pretty even, 
                   if discontinuous coverage of trees, and even up to two 
                   canopies of trees, particularly as you approach the 
                   tropical rainforest.  If there are two canopies, the taller 
                   one tends to be broadleaf evergreens, while the sorter one 
                   is made up of deciduous or otherwise xerophytic trees. You 
                   may even see lianas and the occasional epiphyte.
               vi. In other versions, the trees' distribution becomes more and 
                   more clumpy or grouped, with discontinuous to continuous 
                   cover of grasses or mixed grasses and shrubs in the areas 
                   between groups of trees.  This is more common as you 
                   approach the transition between tropical deciduous woodland 
                   and savanna. Some of these trees are really pretty thorny.
           b. Mediterranean woodland
                i. This is typically dominated by mid size oaks with 
                   sclerophyllous leaves and various other xerophytic 
                   adaptations.  Most are evergreen, but some are winter 
                   deciduous (if they have access to underground water during 
                   the summer).  Sometimes other trees are mixed in with the 
                   oaks, such as pines, pistachios, Chilean pepper trees, 
                   eucalyptus, and olives, depennding on the area and its 
                   history.
               ii. In California, there are 21 species of oak, such as the 
                   coastal live oak (as opposed to deciduous), interior live 
                   oak, canyon live oak, valley oak, black oak, Engelman oak, 
                   Douglas oak, and white oaks, which are sometimes mixed in 
                   with pines. 
              iii. There is an understory, usually of grasses, though it can 
                   also include some shrubs, too.  Often, you will see forbs, 
                   too, particularly in the spring, when they do their 
                   frenzied blooming thing (e.g., California poppies, blue 
                   lupines, and coriopsis daisies).
               iv. This woodland is associated with the Mediterranean 
                   climates: Csa and Csb in the Köppen system (see map in 
                   the climate lecture). 
                v. It usually also requires gentle slopes and hills with deep, 
                   well-drained soils.
                   a. On steep slopes with skeletal or unstable soils, 
                      Mediterranean scrub tends to dominate.
                   b. On flat terrain, usually grasslands tend to win out 
                      (though the Great Central Valley has valley oaks on the 
                      bottomlands in some places).

                      

               vi. Places you can find this vegetation:
                   a. Locally, you often find it in the hilly terrain of 
                      Orange County and Ventura County, and it is found in 
                      much of California around interior valleys and in 
                      coastal canyons.
                   b. You can find it in similar circumstances around the 
                      Mediterranean basin, in central Chile, and in Australia 
                      in the southwest and in the southwest parts of the 
                      eastern peninsula (where eucalyptus is the group of 
                      trees that have risen to the Mediterranean woodland 
                      niche needs).
           c. Desert woodland
                i. Contains a discontinuous or grouped coverage of smaller 
                   trees (usually 5 to 15 m tall).
               ii. These form a single tree canopy.
              iii. Beneath the trees will be desert shrubs and, in good years, 
                   some grasses and forbs, too.
               iv. This vegetation is a sort of transition from desert 
                   vegetation to montane forest vegetation: In California, 
                   this "ecotone" (ecological transition zone) contains 
                   piñon, juniper, and Joshua trees, with Artemisia and 
                   creosote bush among the desert shrubs in the understory.  
                   As with the true desert, there will be many ephemeral 
                   forbs, too, notably, California poppies and blue lupines.
                v. You tend to find it on higher hillslopes overlooking 
                   desert.  These higher hillslopes receive a bit more rain or 
                   snow, and they don't see salt buildups, so conditions are a 
                   little better for the tree life form.

This lecture briefly described forests and woodlands, which are biomes 
dominated by the tree life form.  The difference between a forest and a 
woodland is that a forest has a continuous tree cover, while a woodland has a 
discontinuous or grouped tree cover.  I described five subtypes of forest:  
Tropical rainforest, subtropical conifer woodland, West Coast marine forests, 
temperate broadleaf and mixed forests, and taiga.  In each case, there were 
clear connections with Köppen climates, sometimes augmented by soil 
conditions.  I also described three examples of woodlands:  Tropical deciduous 
woodlands, Mediterranean woodlands, and desert woodlands.  Again, the 
Köppen classification is relevant.

In the next lecture, I'll take up shrub-dominated biomes.

Document and © maintained by Dr. Rodrigue
First placed on web: 11/06/00
Last revised: 07/02/07