Explanations, please.
I know that this is a very broad question. I need as much information as possible.
Thank you!
How are plants affected by droughts and floods?
Yes it is a very broad question, that might get more answers if asked in the biology section. I will attempt to help you out with the perspective of an earth scientist and someone who studies paleoecology.
Plants often have evolved to specialize in a certain environment, so average rainfall and tolerance of drought or flood are often built into the species by their adaptation. Many plants respond to drought conditions by slowly moving (by reproduction methods) into other wetter habitats. Those that remain in dry areas over generations of the species have evolved various coping mechanisms. Cacti for example not only retain as much water as possible within the plant, but they often do not flower until they have detected sufficient rainfall to make reproduction likely. For this reason, the flowering of many cacti will suddenly follow a period of rainfall and is not tied directly to the season. Other plants, such as juniper trees have actually evolved in such a manner that rainfall falling on the tree is redirected toward the base of the tree. (see: http://72.14.253.104/search?q=cache:Ex35...
Plants like the Agave americana, commonly called the Century plant, wait for years to flower, and do not flower until they have stored enough nutrients to enable them to flower. This is related, at least indirectly, to the drought conditions that this plant experiences in its typical 25 year lifetime. It only flowers once and then dies. Other dry-climate plants like creosote have developed methods of actually poisoning the soil around the plants roots that inhibit competing plants. In addition to this, they are so efficient at absorbing soil moisture that there are effectively dead zones around each plant. One creosote bush in the Mojave desert of California has been dated to be 11,700 years old.
http://www.hcn.org/servlets/hcn.Article?...
Some plants actually depend on the effects of drought conditions for reproduction. Fires, a product of drought, often trigger reproductive cycles in some plants. The Giant Sequoia tree is believed to be one of these. I quote: "Hartesveldt and Harvey (1967) reported giant sequoia seeds on the ground after a prescribed fire at a concentration of 7,500 per m2. This high concentration was attributed to the heating of the cones of a large sequoia near an adjacent burn pile." From: http://www.nps.gov/history/history/onlin...
Many other trees and plants have some type of response to fire, and some seeds will not germinate until there is a fire, while other plants grow special cones that open in a fire and appear to have seeds more tolerant of post-fire conditions:
http://www.ingentaconnect.com/content/kl...
The effect of these plant adaptations over time is that the plant species either has evolved along with climate change, or that it has moved its habitat to follow climate patterns. One interesting example of how plant species have moved as a result of changing climate conditions is that many species that now live only in the colder portions of North America are found in sediments in Florida and Alabama that date back about 10,000 years or more. (see: http://geology.geoscienceworld.org/cgi/c... ) In some places in Alabama (I believe this is referenced in a book titled "Lost Worlds in Alabama Rocks A Guide to the State’s Ancient Life and Landscapes" ) there are microclimates within protected canyons that are still inhabited by plant species found nowhere else in this southern latitude. These isolated plant communities are believed to have survived since the last glacial-maximum.
Flooding has different effects on plants, as the primary problem it creates for plants is that respiration is restricted. Most plants have evolved to live in a soil that has some oxygen available, and as flooding fills this with water, the plants respond by changing their metabolic rate. Over the long-term plants will adapt to a frequently flooded environment in much the same way that they adapt to a drought environment: by moving to habitats that they are better suited for, or by evolving new adaptive characteristics. Plants that have adapted well to flooding conditions typically live in swamp environments. Bald Cypress trees are a good example of this "flooding" adaptation.
See: http://dcm2.enr.state.nc.us/wetlands/Coa...
This tree is an ancient tree, first seen in the Triassic period, over 200 million years ago and was adapted to swamps of that period.
See: http://www.reuters.com/article/scienceNe...
You can see some very large scale changes to vegetation patterns on NASA's Earth Observatory pages. You can build animations covering several years here:
http://earthobservatory.nasa.gov/Observa...
I would also suggest you visit the Natural Hazards image database on the NASA Earth Observatory. Here you can see the satellite images of floods and drought:
http://earthobservatory.nasa.gov/Natural...
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