The CLIMATE section of MatchClover provides information on the important aspects of weather and climate which affect selecting and managing clovers and other forage species.
The environment plants experience depends on climate and the variation of weather events within the climate. Climate refers to the long-term history of temperature, precipitation, and radiation for a given region. Climate is the principal factor affecting suitability of forage species or cultivars for a given location. In contrast, weather includes day-to-day and short-term extremes in temperature, precipitation as rain, snow or hail, relative humidity, wind, and solar radiation at a given site. Forage producers need to be aware of the climate, i.e. the average year, when selecting adapted species and cultivars to plant and weather events that alter year-to-year productivity and influence day-to-day management decisions.
Many attempts have been made to classify the climates on Earth into a comprehensive and comprehensible system. One of the earliest began with Aristotle and his discussion of Temperate, Torrid, and Frigid Zones. The system that is in almost universal use now is the Köppen system, developed in 1928 by German climatologist and amateur botanist Wladimir Köppen.
The Köppen System
The modified Köppen system uses letters to denote the six major climate regions and their 24 sub-classifications. These regions are based on average monthly temperature and precipitation values. While it does not take full account of factors such as cloudiness, solar radiation, wind, or even extremes in temperature, it still remains a useful system. Although the individual zones are shown (Figure 1) with clearly delineated boundaries, it is important to note that the system is just a guide to average climate trends and that the areas between zones represent a gradual transition between climates.
Köppen spent time updating and modifying his system of climate classification right up until his until his death at 94 in 1940. Since then it has been modified by a number of geographers, most notably the late Glen Trewartha of University of Wisconsin, whose version is probably in the widest use today.
The modified Köppen classification uses six letters to divide the world into six major climate regions, based on average annual precipitation, average monthly precipitation, and average monthly temperature:
Each category is further divided into sub-categories based on temperature and precipitation (Table 1).
For example, the U.S. states located along the Gulf of Mexico are designated as "Cfa." The "C" represents the "mild mid-latitude" category, the second letter "f" stands for the German word feucht or "moist," and the third letter "a" indicates that the average temperature of the warmest month is above 72°F (22°C). Thus, "Cfa" gives us a good indication of the climate of this region, a mild mid-latitude climate with no dry season and a hot summer.
Köppen also used vegetation to aid in climate classification, including tropical rainforest, tropical wet and dry season vegetation, low-latitude steppe, low-latitude desert, Sclerophyll forest, mid-latitude deciduous forest, Boreal forest, and tundra vegetation.
More recently, climate station data has been used to create spatial models of climate elements including precipitation, minimum and maximum temperature, relative humidity, and solar radiation. These models have typically used distance weighting approaches that work well for flat conditions. Mountainous areas with detailed topography and coastal regions, however, have been poorly modeled by these purely statistical approaches. PRISM was developed to address these issues and allow for detailed, accurate modeling of real-world conditions (Daly et al., 2002). The climate spatial layers used in this project were developed with PRISM.
Table 1. Köppen climate classification chart.
|A||Tropical humid||Af||Tropical wet||No dry season|
|Am||Tropical monsoonal||Short dry season; heavy monsoonal rains in other months|
|Aw||Tropical savanna||Winter dry season|
|B||Dry||BWh||Subtropical desert||Low-latitude desert|
|BSh||Subtropical steppe||Low-latitude dry|
|BWk||Mid-latitude desert||Mid-latitude desert|
|BSk||Mid-latitude steppe||Mid-latitude dry|
|C||Mild Mid-Latitude||Csa||Mediterranean||Mild with dry, hot summer|
|Csb||Mediterranean||Mild with dry, warm summer|
|Cfa||Humid subtropical||Mild with no dry season, hot summer|
|Cwa||Humid subtropical||Mild with dry winter, hot summer|
|Cfb||Marine west coast||Mild with no dry season, warm summer|
|Cfc||Marine west coast||Mild with no dry season, cool summer|
|D||Severe Mid-Latitude||Dfa||Humid continental||Humid with severe winter, no dry season, hot summer|
|Dfb||Humid continental||Humid with severe winter, no dry season, warm summer|
|Dwa||Humid continental||Humid with severe, dry winter, hot summer|
|Dwb||Humid continental||Humid with severe, dry winter, warm summer|
|Dfc||Subarctic||Severe winter, no dry season, cool summer|
|Dfd||Subarctic||Severe, very cold winter, no dry season, cool summer|
|Dwc||Subarctic||Severe, dry winter, cool summer|
|Dwd||Subarctic||Severe, very cold and dry winter, cool summer|
|E||Polar||ET||Tundra||Polar tundra, no true summer|
|EF||Ice Cap||Perennial ice|
Plant growth responses to solar radiation can be separated into those due to quality (wavelength or color), density (intensity), and duration (photoperiod). Radiation in the visible range is most active in photosynthesis and is referred to as photosynthetically active radiation (PAR; 400-700 nm). Photoperiod responses such as flowering are controlled by the relative ratio of radiation in the red and far-red regions of the spectrum. Radiation density is measure in energy units (umol photons m-2s-1). Although plant growth rate of plants with adequate nutrient and water supplies is directly related to radiation density, forage growth rate is more often related to percent radiation interception by leaves. A full canopy of leaf blades is needed to intercept the maximum amount of radiation. Thus, moderate defoliation is optimal in promoting plant growth.
Duration of the photoperiod (the time from sunrise to sunset) changes with latitude and season due to the tilt of the earth relative to its orbital path around the sun. Minimal seasonal change occurs at the equator whereas large changes occur at the poles. Most temperate grasses and legumes flower during long photoperiods, with perennial grasses also requiring induction caused by cold temperatures (a process called vernalization).
Growth rate and other processes depend on the temperature pattern, including diurnal variation. Daytime temperatures should be near optimum for photosynthesis and growth, whereas lower temperatures at night conserve energy by reducing respiration. Cool-season forages have optimal growth temperatures near 70F but can still grow slowly near 35F. Warm-season forages have growth optima around 90F and grow little below 60F. Higher temperatures increase the rate of plant development and decrease time from seeding to flowering. This is one reason forage yields of cool-season species such as alfalfa and red clover are lower during hot summer periods.
Stress occurs when temperatures are above or below the optimal range. High-temperature stress often occurs concurrently with moisture stress. Excessively high temperatures can induce flower sterility, especially pollen abortion, leading to poor seed production.
Low-temperature stress can cause chilling injury in warm-season grasses and some legumes, but most cool-season legumes and grasses are not sensitive to above-freezing temperatures. Most cool-season grasses accumulate the storage carbohydrate fructan in cell vacuoles, whereas legumes and warm-season grasses store starch in chloroplasts. At low temperatures, both synthesis and breakdown of fructan occur more readily than for starch.
Forage species differ widely in their ability to withstand cold temperatures. Differences within species (among types and cultivars) also exist. Overwintering species gradually develop cold resistance with the onset of the shorter days and colder temperatures of autumn. Cold resistance is lost much faster than it is acquired. Most winter killing in the northern areas occurs during late winter and early spring when snow cover has disappeared and plants are exposed to severe temperature fluctuations above and below freezing.
Seasonal distribution patterns of precipitation, total quantity of precipitation, and evapotranspiration demands affect water availability and adaptation of forage species. In addition, available water in the soil depends on soil texture and plant rooting depth. Transpiration is driven primarily by solar radiation.
Limited water. Shoot growth slows well before water stress becomes severe enough to cause stomatal closure and a decline in photosynthesis. Sugars from photosynthesis often accumulate during mild to moderate drought stress because growth is slowed. Some forage species gain drought tolerance by accumulating osmotically active solutes like sugars, amino acids, and ions that hold water in plant tissues and prevent injury.
Forage species differ in response to drought stress. Many plants reduce shoot growth but maintain root growth under moderate drought stress conditions. Deep-rooted plants avoid drought stress due to greater access to water in the soil profile.
Excessive water. Poorly drained soils provide an unfavorable environment for growth of many forage species, especially legumes. (See Soils section)
Climate information slightly revised from: Volenec, Jeffrey J. and C. Jerry Nelson. 2003. Environmental Aspects of Forage Management. p. 99-124. Chapter 4 In: Robert F. Barnes, C. Jerry Nelson, Michael Collins, and Kenneth J. Moore (eds.) Forages: An Introduction to Grassland Agriculture. 6th ed. Vol. 1. Iowa State Press.