Tropical Environment PDF

Tropical Environment The tropics or equatorial zone refers to the area roughly centred on the equator within latitudes 30° N and 30° S. Within this zone, the atmospheric pressure is relatively low with heavy and abundant rainfall associated with thunderstorms. * about 30° North and South of the equ...

Tropical Environment The tropics or equatorial zone refers to the area roughly centred on the equator within latitudes 30° N and 30° S. Within this zone, the atmospheric pressure is relatively low with heavy and abundant rainfall associated with thunderstorms. * about 30° North and South of the equator- sub tropical climate * 50° - 60° North and South of the equator- mid latitudes (temperate climate) * Polar regions- polar climate General features/ characteristics of tropical climate: The characteristics of tropical climate include the following: 1. Low atmospheric pressure. 2. High/ abundant rainfall associated with thunderstorm. 3. Considerable sunshine. 4. Temperature rarely exceeds 35° C with a daytime maximum of about 32° C and minimum temperature not lower than 22° C. Temperature throughout the year is therefore ˃18° C. 5. Abundant cloud cover at night which prevents loss of heat. 6. Seasons are distinguished by variation in rainfall and cloudiness 7. The greatest rainfall occurs when the sun is overhead at noon (midday). On the equator, this occurs twice in a year (March and September). Therefore, at the equator, there are two wet and two dry seasons. Away from the equator, the two wet seasons merge into one, with one wet and one dry season. Weather: Weather describes the condition of the atmosphere (including such variables as rainfall, temperature, humidity, sunshine, cloudiness etc) of a place at a given time. Weather varies from place to place. It could be hot, sunny, dry here now but cloudy, rainy in other places. Change in one weather factor affects the others. Weather is what you see or experience in any particular day. So it could hot or cold, sunny or cloudy etc. So, you hear or read the weather forecast for a particular day (reflecting the expected temperature, rainfall, cloudiness etc). Therefore, weather reflects short-term condition of the atmosphere. It can change from minute-to-minute, hour-to-hour, day-to-day or season-to-season. Climate: This describes the average condition of weather of a place over a long period of time and space. So, you can expect harmattan at a particular time etc. The difference between climate and weather lies in time/ duration. Weather is a short-term effect. Climate is a long-term effect. Differences in climate arise due to variation in planetary pressure belts, the resulting system of winds and their associated air masses. Climate has a profound influence on agriculture. This influence is through its effects on soil characteristics, natural vegetation, the choice of crops that can be grown, the cropping system and the type of farming that can be practiced. Types of Tropical Climate The climate of the tropics is influenced by (a) Inter Tropical Convergence Zone (ITCZ). The ITCZ is an area of low pressure and marks the point of convergence of Trade winds. The location of the ITCZ varies throughout the year. Though it remains near the equator, overland it drifts farther north or south due to variations in land temperatures compared with the oceanic temperatures. (b) Land and seas (with regards to temperature) and altitude. Tropical climate is highly diverse and this leads to variation agricultural potential of the different parts of the tropics. Within each type of tropical climate, there are internal variations due to differences in altitude or presence of great water bodies. 1. Equatorial climate/ Tropical rainforest: This occurs within latitudes 5° north and south of the equator. It is characterized by heavy and frequent rainfall for most parts of the year. Total precipitation range between 2000 – 3000 mm. Every month of the year has at least 60 mm of precipitation; high humidity; high average temperatures 25° C – 30° C with minimal seasonal variations; crops include rubber, oil palm, plantain/ banana, coffee, coconut, cacao, yam, cassava, cocoyam. 2. Tropical monsoon climate: This occurs in areas within 5° and 15° north and south of the equator. Mean monthly temperatures ˃18° C with minimal variations in the course of the year; precipitation ˃1000 mm per annum; there is alternation between rainy and dry seasons with differences in the number and duration of these seasons per annum and in the amount of seasonal rainfall; this climate tends to have at least on very dry month. Variants of tropical monsoon climate include: - Areas with up to 1000 – 2000 mm of rainfall per annum which fall into two rainy seasons, with a short/ less pronounced dry season separating them. Such areas are suitable for perennial crops, and two annual crops can be grown per annum. - Areas with two short rainy seasons and a pronounced intervening dry season. Rainfall ranges from 600 – 1250 mm. The area is less suitable for perennial crops except the fairly drought tolerant ones such as cashew and sisal. High altitudes such as Kenya with a rainfall of 1500 – 1800 mm are good for perennial crops such as coffee and tea. The area may support two annual crops and have good quality grassland for cattle rearing. - Areas with one fairly long rainy season and a long dry season. Rainfall range from 750 – 1250 mm over 5 to 6 months. High altitudes in this area with up to 1250 mm can support tea and coffee. The area is generally suited for annual crops like maize, yam and cotton. The long dry season affects the availability of pasture. - Areas with one short rainy season and a long dry season. Suitable for short term annual crops such as sorghum, millet, groundnut and sesame. - Wet coastal climate which occurs along the coast of West Africa, Central America and northern parts of South America. Precipitation occurs in all months of the year with a few drier months, cool nights and temperature range of 24° C – 27° C. This area is good for perennial as well as annual crops such as coconut, sugar cane, sweet potato, maize, rice, yam, cocoyam etc. 3. Tropical dry climate: This occurs within latitudes 15° and 30° north and south of the equator. The area is described as arid owing to limited rainfall. Areas with this climate are hot, covers extensive deserts and of limited agricultural use. This climate is found in many parts of north Africa, Iran and south America Effects of some climatic factors on crop production 1. Rainfall: Rainfall makes water available for plant growth and productivity. Crops that can be grown in any area depend on rainfall. Therefore, agricultural potential of any area in the tropics is largely determined by the amount, duration, distribution and reliability of rainfall. High intensity  rainfall, as experienced in most parts of the tropics, lead to reduced infiltration, increases runoff and soil erosion. These lead to loss of soil nutrients and a reduction in crop productivity. Rainfall also increases relative humidity which may affect the rate of transpiration in plants. 2. Humidity: Atmospheric humidity varies with the amount and distribution of rainfall. It is uniformly high throughout the year in the wet equatorial and monsoon climates and falls to very low levels where the dry season is severe. Humidity reduces the rate of transpiration in crops and this in turn affects water uptake. High humidity coupled with high temperature over long periods favour the growth of fungal diseases of crops and growth of moulds on stored products. 3. Temperature: In the tropics, daytime temperatures are usually greater than 20 °C throughout the year although local variations due to altitude do occur. Plant growth within a minimum and maximum temperature regime. In between these two extremes is the optimum temperature required by a plant at which plant growth proceeds with rapidity. The minimum, optimum and maximum temperatures are collectively known as cardinal temperatures. The cardinal temperatures may vary with stage of crop development. In the tropics, low temperature is not a limiting factor to crop production except in areas with high altitude where marked reduction in temperature could occur. The daily variation and range of temperature is known as diurnal temperature range or thermo-periodicity. Diurnal temperature range is low in the tropics. Generally, temperature influences: plant growth and development, dry matter production and accumulation, flowering and fruiting in some plants, crop yield, soil water and nutrient uptake, and transpiration rate in plants. Temperature also affects the rate of decomposition of organic matter. 4. Solar radiation/ Sunlight: Solar radiation is a set of electromagnetic radiation emitted by the sun. Simply put, it is the energy from the sun. Sunlight supplies the energy needed for photosynthesis. The productivity of a crop depends on the ability of the plant cover to intercept solar radiation. The part of the plant that intercepts solar radiation is the leaf. Therefore, the amount of leaf area in a canopy per unit ground area (known as Leaf Area Index, LAI) is very important. It should be noted that not all solar radiation is useful. The proportion of the total solar radiation used for photosynthesis is termed Photosynthetically Active Radiation (PAR). When other conditions, such as nutrients and moisture, are not limiting, the amount of dry matter produced by a crop is linearly related to the PAR intercepted by the crop. Apart from its effect on the rate of photosynthesis, sunlight affects the rate of transpiration due to its effect on stomatal opening. Adequate sunlight promotes the growth many plant species. Low sunlight, due to high cloud cover, leads to the growth of shade tolerant crops. 5. Photoperiod: The length of a day is termed photoperiod. The response of a plant development to day length (photoperiod) is termed photoperiodism. Around the equator (equatorial and wet tropical monsoon regions), day length varies little about 12 hours throughout the year. In tropical areas, the difference between the shortest and longest day does not exceed 3 hours. In high altitudes, this difference could be up to 8 hours. Photoperiod affects flowering in plants. On the basis of response to day length, plants are classified into: short-day plants, long-day plants, and day-neutral plants. Short-day plants are plants that require a long period of darkness. Short-day plants form flowers only when the day length is less than 12 hours. Examples of short-day plants include sorghum, soyabean, sweet potato, sugar cane, tobacco etc. Long-day plants are plants that require a short night (short period of darkness) to flower. Long-day plants flower only when the day length is greater than 12 hours. Examples of long-day plants include barley, water melon, spinach, winter wheat, sugar beet, radish etc. Day-neutral plants are plants that flower regardless of day length. Examples include tomato, cucumber, cotton etc  Photoperiod also influences vegetative growth, tuber and bulb formation, leaf shape, nature and extent of branching, abscission and leaf fall (e.g. simultaneous leaf fall in deciduous treea), pigment formation, root development, pubescence, dormancy and death. Photoperiodism affects natural distribution of plants. Generally, plants that originate from low altitudes are short-day plants, while plants from high altitudes are long-day plants. Photoperiodism has many practical applications. For example, the selection of a plant variety for a given locality requires knowledge of its interaction with day length. Within the same plant species, it is possible to have varietal differences for day length. Consequently, types adapted to growth at different latitudes can be selected and bred. Crop Domestication and Evolution The early man was a wanderer. He moved from one place to another gathering his food from the wild. He later settled down in a place and started to cultivate his crops which were previously in the wild. The process of bringing wild plants/ organisms (species) under human management is termed domestication. Domestication involves the modification of the genetic constitution of an organism. A plant population is said to be domesticated when it has been substantially changed/ altered from the wild state to the extent that it is no longer able to survive in the wild. Domestication is a continuous process and can be considered as a reflection of the ever changing demand of human societies for new and improved agricultural products. Each crop must have originated from somewhere. Evidence shows that cultivated plants were not distributed uniformly throughout the world. Even today, some areas show far greater diversity than others in the forms of certain crops and their wild relatives. Vavilov (1951) proposed that crop plants evolved from wild species in the areas showing great diversity. These areas are termed ‘Primary Centres of Origin’. Areas where some crop species showed considerable diversity of forms although they did not originate from there are termed ‘Secondary Centres of Origin’. Based on Vavilo’s concept, 12 Centres of Origin are recognized: 1. Mediterranean 7. Chile 2. Abyssinian 8. Brazil-Paraguay 3. Indo-Burma 9. Mexico 4. Siam-Malaya-Java 10. Persia 5. China 11. Afghanistan 6. Peru 12. United States of America Vavilov’s concept was later modified by Harlan (1975) who recognized that evolution has spread over large areas. Harlan therefore proposed that Vavilov’s concept of unique Centres of Origin should be replaced by regional or multiple Centres of Origin. With Harlan’s classification, West Africa is now recognized as an important area for the evolution of at least 6 crops namely: Sorghum, Cowpea, Coffee, Yam, Kola and African rice. Peasant farmers and plant breeders are responsible for domestication and evolution of crop plants. Originally, these were solely in the hands of peasant farmers. Peasant farmers select plants in their fields to propagate the next crop thereby improving the mean performance of the population of their crops. Science-based plant breeding has tremendously contributed to crop improvement. The role of peasant farmers in crop domestication has declined over the years while that of plant breeders continue to increase. Natural and Artificial Selection under domestication Selection is defined as any process, natural or artificial, that allows an increase in the proportion of particular genotypes or group of genotypes in succeeding generations. That is, the phenomenon of some genotypes or group of genotypes in a population leaving more progenies than others. The genotype(s) that leave behind more progenies in the next generation are selected for while others are selected against. Selection be it natural or artificial, leads to several distinct changes in the characteristics of domesticated species. Natural selection: This occurs when environmental/ natural factors such as temperature, soil, weather, diseases, pests etc cause more progenies to be left behind by some genotypes to the detriment of others. The genotypes that are more suited/ adapted to the environment leave behind more progenies than the less adapted ones. The process of natural selection is in degrees and rarely stops/ prevents a genotype from producing any progeny. Thus, considerable variation is retained under natural selection. Natural selection was more important in earlier times. Artificial selection: This results from the conscious decision of man to keep the progeny of certain plants with traits of interest in preference to others. Under artificial selection, only selected plants are permitted to reproduce. The progeny from the remaining plants are generally discarded. The process of artificial selection progressively reduces variability in the population. Artificial selection has assumed more importance than natural selection in recent times. Changes in crop plant traits in the course of evolution: Significant changes in the characteristics of plants have occurred under domestication and evolution. Some of these important changes include: 1. Elimination of / reduction in shattering of pods, spikes e.g. grain amaranth, millet, sorghum etc. 2. Decrease in toxic and undesirable substances as in the case of mango, yam (alkaloids), cassava (cyanogenic glucosides), grain legumes (trypsin inhibitors) etc. 3. Elimination of / reduction in seed or tuber dormancy. 4. Life cycle has become shorter in some plant species e.g. cassava, pigeon pea, cotton etc. 5. Reduction in plant size associated with a change from indeterminate to determinate growth habit e.g. millet, maize etc. 6. Increase in economic yield of crop species. 7. Development of attractive colours e.g. maize, grain amaranth, tomato etc. 8. Altered photoperiodism, from day length sensitive plants to day neutral plants. 9. Reduced spininess e.g. pineapple, okra, egg plant etc. 10. Modification in plant type, with cultivated plants showing altered tillering, branching, leaf characters, size of inflorescences, etc. Table 1. Families and area of domestication of selected tropical crops S/N Crop Family Area of Domestication 1. Citrus Rutacea South East Asia 2. Peppers Solanaceae Central and South America 3. Tomato Solanaceae Western South America 4. Tobacco Solanaceae Andes 5. Egg plant Solanaceae India 6. Potato Solanaceae Andes 7. Kola Sterculiaceae West Africa 8. Cacao Sterculiaceae North Western South America 9. Jute Tiliaceae India 10. Carrot Umbelliferae South West Asia 11. Groundnut Fabaceae Central/ South America 12. Pigeon pea Fabaceae India 13. Soybean Fabaceae China 14. Lima bean Fabaceae Central/ South America 15. Cowpea Fabaceae West Africa 16. Onion Liliaceae Central Asia 17. Garlic Liliaceae Central Asia 18. Okra Malvaceae Africa/ India 19. Upland cotton Malvaceae Central America 20. Hemp Moraceae Central Asia 21. Bananas Musaceae South East Asia 22. Coconut Palmae South East Asia 23. Arabica coffee Rubiaceae East Africa 24. Canephora coffee Rubiaceae West Africa 25. Water melon Cucurbitaceae Africa 26. Dioscorea alata Dioscoreaceae South East Asia 27. Dioscorea esculenta Dioscoreaceae South East Asia 28. Dioscorea rotundata Dioscoreaceae West Africa 29. Dioscorea cayanensis Dioscoreaceae West Africa 30. Rubber Euphorbiaceae Amazon 31. Cassava Euphorbiaceae South America 32. Oryza sativa Poaceae India/ China 33. Oryza glaberrima Poaceae West Africa 34. Bulrush millet Poaceae West Africa 35. Sorghum Poaceae West Africa 36. Maize Poaceae Central America 37. Amaranth Amaranthaceae Central America 38. Mango Anacardiaceae North India 39. Taro Araceae India 40. Tanier Araceae Tropical America 41. Pineapple Bromeliaceae Central/ South America 42. Tea Camelliaceae South East Asia 43. Sunflower Compositae North America 44. Sweet potato Convolvulaceae Western South America