Enset: Ecology and EnvironmentWhere is enset grown and what is its range of environmental adaptation?
How does the production of enset affect the environment?
Domesticated enset is planted at altitudes ranging from 1,200 to 3,100 meters. However, it grows best at elevations between 2,000 and 2,750 meters. Most enset-growing areas receive annual rainfall of about 1,100 to 1,500 milllimeters, the majority of which falls between March and September. The average temperature of enset growing areas is between 10 and 21 degrees centigrade, and the relative humidity is 63 to 80 percent.
Detailed studies on the effects of environmental constraints such as temperature and water availability have not been conducted on enset. Therefore, comments about the range of adaptation and the effects of environment on such characteristics as plant growth, time to maturation, yield, and pest management are from preliminary observations. Enset is not tolerant to freezing. Frost damage on upper leaves is commonly observed above 2,800 meters above sea level, and serious stunting is seen above 3,000 meters. For a certain range below 1,500 meters, the constraint to enset plant growth probably is more related to available water than to high temperatures. In most areas of Ethiopia below 1,500 meters, the total rainfall and the length of the rainy season decrease, and the potential water use by plants increases because of the greater evaporative demand. Most enset plantings below 1,500 meters have supplemental irrigation or are small enough in size that household waste water may be applied.
There has been much speculation about the drought tolerance of enset. Farmer interviews suggest that those populations dependent upon enset have never suffered from famine, even during Ethiopia's tragic drought and famine prone decades of the 1970s and 1980s. Several authors (e.g., Bayush, 1991; Shigeta, 1990) have noted that enset tolerates short season droughts that have seriously damaged annual crops, especially cereals. During the dry spell, only the edge of older leaves and the outer leafsheath are visibly affected, and the plant resumes normal growth after the onset of the rainy season.
Characterization of enset drought tolerance is a vital issue in clarifying the role of enset in Ethiopian food security. Observed drought tolerance and its attributes must be carefully interpreted. Will enset grow and produce successfully where the average annual rainfall is less than 1,100 millimeters, or where the dry season has an average length greater than where it is currently grown? Research data are needed to answer these questions. It is hypothesized here that, once enset is established, it can tolerate occasional years of unusually low total rainfall or a short rainy season. During that stress year, enset plants may gain little additional weight, but they can survive and provide an all-important food source (which can also be stored for months and years) when there is failure of crops that produce an annual harvest. In environments where enset is adapted, enset can serve a vital role similar to that of livestock, i.e., providing food "on the hoof" for famine years. However, enset will fail in environments of consistently low rainfall or short rainy seasons.
Enset is not affected by occasional heavy rainfall. This resilience is attributed to the plant's stiff leaves, which resist large rain drops. In fact, one of the main attributes of enset is that it protects the soil from erosive rainfall. The main danger of heavy rains to enset is that roots and the corm do not tolerate water-logging for long periods. For that reason, enset is usually grown in soils that do not have high water tables and are well drained.
Enset grows well in most soil types, if they are sufficiently fertile and well drained of water. Cattle manure is used as the main organic fertilizer. Manure increases water holding so that soil water endures longer into the dry season, and reduces the negative effects of the high clay content of vertisols. The ideal soils in enset growing areas are moderately acidic to alkaline (pH 5.6 to 7.3) and contain two to three percent organic matter.
Observations in areas that have been planted with enset for many years suggest that native soils have been altered positively by the long-term application of manure. Compared to native soils that have not been similarly treated, these modified soils are likely to be more fertile and have better physical characteristics, such as water holding capacity. Enset's perennial canopy of leaves and the abundant accumulation of litter also reduce soil erosion and organic matter decomposition to a minimum. Because enset production improves soils, particularly with adequate manure, many enset fields have been in continuous production for decades, if not centuries. A current fear is that significant increases in human population and decreases in animals and manure may cause reductions in crop yields and soil fertility, thereby reducing the long-term sustainability of the enset system. Increased use of fertilizer may not compensate for the manure loss because of the multiple roles that manure plays in improving soils biologically, chemically, and physically.
Enset affects the physical environment around houses where it is most commonly grown. Enset serves in the same role as trees, providing people, other plants, and animals with protection from wind and sun. Having a field that partially encompasses the homestead is considered aesthetically desirable by enset-based societies; enset beautifies the Ethiopian landscape by its thick, dark green foliage (Plate 1).
Enset is also likely to affect the macro-environment of an area in a positive manner. It has been commonly observed that species like enset, with deep roots and leaf canopies of long duration, improve the hydrological dynamics of an area, as can easily be measured at the watershed level. As the proportion of these species increases with respect to annual species, water infiltration increases and surface runoff decreases, resulting in more water in the soil and aquifers. The result is increased water availability and greater volume and duration of discharge to springs, decreasing the effective length of the dry season.