Why Plants Are Where They Are

Why are there different plants in different places? If you revisit a place decades later, will the same plants still be there, only having grown larger? How do plants get where they are? All of these questions (and more) are asked by plant biogeographers. Many students of biology who pursue the subject in school out of a childhood fascination with natural history will be stunned to find out that they have sit through lecture after lecture about photosynthesis and chloroplasts, the parts of the flower, plant genetics and other topics before the course may (or may not) eventually get around to talking about plant distribution.

Eugenius Warming

Eugenius Warming

The descriptive stage of plant biogeography was largely accomplished in the 19th century by explorers like Alexander von Humboldt and Charles Darwin (on the voyage of the HMS Beagle). Interpretive plant biogeography took the raw data of the explorers and tried to find patterns in it.

Danish botanist Eugenius Warming taught the first university course on ecology at the University of København in the 1880s and presented a summary of the world’s biomes in his landmark work Plantesamfund. Biomes are large-scale collections of ecosystems that have similar environmental conditions (e.g., rainfall), but generally different plant (and animal) species on different continents. The dynamic stage of plant biogeography involves the study of the development and interaction among plants, including the idea of plant succession, the change in the composition of plant communities over time.

Through the late 19th and into the early 20th century, an entire vocabulary was invented to describe newly understood phenomena. Physical environments were described as xeric, mesic, or hydric, depending on the amount of rainfall they received and the amount of water they retained. The plant communities varied accordingly. Series of communities that developed in these environments were called xeroarchs, mesarchs and hydrarchs, respectively.

If the plants themselves were seen as the driving force in succession, then the evolution of communities was said to be

Pioneer community

Pioneer community

autogenic. That is, the plants were modifying the inorganic environment to make it more suitable for them. For example, pioneer species of plants may secrete chemicals that physical break down rock to begin the process of soil formation. When the physical environment is in control of plant distribution it is referred to as an allogenic plant succession. Locations at environmental extremes (polar, desert) are likely to be examples of this phenomenon.

The landscape may be denuded of plants in several ways precipitating the recovery of vegetative cover through the process of succession. The surface of the land may be rendered by barren by erosion (by water or ice), or on steep slopes by mass wasting (i.e. landslides). Geological causes include faulting, mountain-building and volcanism.

Climatic causes include droughts, flooding, snowiness, protracted windiness (which removes soil), and freezing and thawing. Lightning and the ensuing fires can reduce thousands of acres to ash. Other biota may remove some or all plants from the landscape. Grazing by livestock eliminates species that cannot withstand the constant clipping. Insects (locusts, gypsy moths) may defoliate entire plant communities. Beavers may drown acres of plants. Pathogens like chestnut blight may selectively remove dominant species from a community and cause reshuffling of dominance in the remaining species. Plants move into an area in the form of seeds, or as partially pieces of plant that can establish itself vegetatively.

Ecesis is the process of germination, growth and reproduction. Once individual plants are established in this manner, many pioneer species spread by aggregation, spreading like carpets over the landscape. The first plants will compete aggressively for light, water, nutrients and pollinators. Eventually the community reaches a state where it shares the available resources and can perpetuate itself indefinitely. Frederic Clements referred to this as the climax stage.

Beech-maple climax forest

Beech-maple climax forest

Clements originally conceived of each region has having a single (mono-) climax community. Subclimax stages were recognized as being stalled on the way to climax by some limiting factor (wind, water or the lack of an important nutrient). A disclimax stage was produced when an invading factor (e.g. chestnut blight) reversed the direction of seral progression. Clements and his followers eventually recognized polyclimaxes, in which the nature of the climax community was affected by various factors. The concept of an inevitable climax community has been largely abandoned since the 1950s as the ideas of Henry Gleason became accepted. See separate entry.

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