Temperature and Moisture (and How They Affect Plants)
The two main limiting factors on types of plant forms including overall habit, leaf form, seasonality, deciduous vs. evergreen, conifer vs. broadleaf, relative plant density, etc.
***Moisture—The important factor is not total average rainfall in a year, but timing of rainfall; in particular, whether most of it comes during the growing season or the dormant season. Also the form of precipitation (fog, snow, humidity, etc.).
***Temperature—Again, the average temperature through the year is relatively unimportant compared with the timing of highest and lowest temperatures, whether in the wet or the dry season, the growing or the dormant season, the daily fluctuations between day and night and from day to day.
*Wind patterns are another factor related mainly to temperature.
*Soil is an important factor, but more important floristically than as a determiner of “types” and “forms” mentioned in the heading. Such variables as pH, grain size, nitrogen, and other specific chemical levels are the main significant factors in soil.
*Elevation above sea level is important, but more as a primary determiner of more basic variables like temperature and moisture than for their more basic hallmark, which is air pressure.
*Photoperiod (i.e., Latitude) is crucial, but again more in terms of phenology and floristics than in the basic structure of plants. Obviously high latitudes, with extreme fluctuations of photoperiod, produce a much more boom and bust strategy than in the tropics.
**Animal factors in a major way influence the morphology of plant (i.e., Predation, Pollination, etc.) organs, and strategies of growth and reproduction, (i.e., Ecology). In fact, this may be the single greatest determiner (or Competition with other plants) of plant morphology after temperature and moisture.
Witness some of the bizarre moa-adaptations in New Zealand, for example.
Getting Down to Specifics:
Some common water/temperature regimes around the world, and the types of plants such places “invent” to populate them.
- Wet tropics, or “rainforests”. Warm/hot all year, wet all year. Landscape dominated overwhelmingly by woody vegetation, generally tall. More or less all broad-leaved evergreen. Mainly big-leaved, including palms and banana-like plants. Little seasonality. Lots of epiphytes, which are the main herbaceous component of the flora. Amazonia, Southeastern-most Asia, Central Africa.
- Dry tropics, or “monsoon” climates. Warm/hot all year, but wet only in a (generally summer) wet (i.e., rainy) season, which becomes the de-facto growing season. Lands more or less dominated by dry-deciduous woody vegetation, shortish to tall. Finely compound leaves (mainly legume trees) and other small-leaved deciduous species dominate, and flowering/fruiting is concentrated in the growing (i.e., rainy) season. Some types of desert as well as dry subtropical forest fit this pattern, and there are usually a lot of succulents as well as dry-deciduous trees, etc. India, Sonora Desert, Costa Rica coast.
- Warm/hot rainy summer and cold snowy (dry) winter, i.e., “North Temperate” climates. Mainly conifers and perennial herbs, the latter the only component in tundra and at altitudes above tree line. Eastern North America, East Asia, Northern Europe.
- Dry-summer temperate, or “Mediterranean” climates. Warm to hot but dry summer and fall, moderate and rainy winter, which becomes the growing season. Mix of small-leaved sclerophyll trees and shrubs; grasslands, etc., mixed in with “winter annuals”, and lots of bulbs (the world’s headquarters for bulbs), together with perennial herbs and grasses forming the considerable herbaceous component of this flora. Mediterranean, California, Chile, Southwest Australia, Southwest Africa.
- Moderate, moist summer and cold snowy (dry) winter, i.e., “montane” climates. Mainly conifers and perennial herbs, the latter the only component in tundra and at altitudes above tree line. All high mountains, taiga and tundra.
- Cold and dry (snowy) all year. No vegetation. Polar.
- Hot and dry all year. No vegetation. Sahara.
