| 水生野草：外來的水生野草(註1) 能對河川、湖泊等水文系統造成顯著的危害。在非洲破壞性最大且最常見的野草是水生風信子（大批漂浮於水面上的野生植物）。水生風信子的破壞性包括: 通過蒸騰使得水分流失（水面有野草覆蓋的湖泊和水庫，水分流失的速度是沒有野草覆蓋的湖泊和水庫的6倍），降低水質，藏匿著藉水攜帶、傳播的病菌，以及減少魚種的種類和數量。
水生風信子於1902年首次出現在南非，1950年出現在剛果，1960年出現在辛巴威，到二十世紀八十年初代出現在尚比亞。這種自由漂流的水生植物有著很廣的根部網路，而且每隔15天就能繁衍出另一子株。這些野草同時還威脅著維多利亞湖和非洲其他水路。水壩加速了這種植物的蔓延，一是因為水庫的水流速太慢，二是大壩使得下游水流速度變小。Kariba大壩水庫的五分之一，也就是超過1000平方公里的水面已被水生植物覆蓋。
全球暖化：愈來愈多的研究文獻專注於南非氣候變化的現狀，同時這些研究在科學家圈內也具有許多爭議。在二氧化碳含量是正常情況的兩倍的環境裏，降水的時間，強度和變化方向都具有很大的不確定性。在地形對降雨情況的影響非常大的南非，這種情況尤其突出。
最近的一些預測指出，全球暖化會使非洲南部發生下列變化：
▲南非中部內陸地區的夏季降雨量將減少10-20%
▲洪水和旱災的強度和頻率都將增加
▲隨著二氧化碳濃度的上升，溫度將平緩而線性地升高。到2050年，平均氣溫將達到會比目前增加攝氏1. 5度，並隨之出現更加頻繁的高溫期。
對流域與集水區的其他的影響效益包括：
▲非洲南部各地的土壤水分蒸發蒸騰率將增加5-20%；
▲該地區東部一些地方的地表水流量將大幅度增加，並同時使那裏地表水流量的可變性增高，以及變得更難預測；
▲生界（biomes）也將發生變化，由於溫度升高，那裏的草原將會被熱帶稀樹草原型的植被大規模取代。
上述這些變化可能會明顯地造成該地區土地的退化的嚴重性，這是因為高溫和低降雨量二者都與土壤和植被的高度退化密切地聯繫在一起。
由全球暖化所引起的變化已經在影響著人類。最近的研究表明，本世紀內整個南部非洲的年平均氣溫已經有所增加，而且由於造成溫室效應的氣體不斷的被排放，氣溫有可能還將繼續上升。雨量減少之後的降雨模式也已受到人們的關注。在1980年之前，降雨一直以18年為一期的週期性變化：大約每九個濕年後有九個乾年。然而，二十世紀70年代這一比較潮濕的十年之後，整個南部非洲從1980年至1994年始終極度乾旱。儘管非洲南部的降雨量並未像撒哈拉地區那樣急劇地減少，但是記錄顯示，該地區的一些地方在仲夏季節（12月至次年2月）的降雨量已經減少了大約5-10% 。
在某些地區，年降雨總量的減少程度還要嚴重得多。
(註1):"Weed" 在此建議將之譯為 "野草" 而非 "雜草"。西方多位環境哲學家 (Muir, Leopold等) 主張生界萬物本來沒有 "好草壞草、益蟲害獸" 之分，是人們以是否利於人類的角度來決定某一物種是作物或雜草、是家畜或野獸。文中描述水生風信子於1902年首次出現在南非，所以為外來物種，並不是非洲南部的原生植物。近年的研究指出，人為引進加上溫室效應的助長，使得非洲外來植物的適應性、競爭力、與繁殖率都比原生植物強，生態消長與平衡因此遭到極大的影響，在此的風信子即是一例。
（資料來源：《1999年南非的土地退化》，作者為Hoffman 等）
版權歸屬Earth Day Network，環境信託基金會（金霞譯，吳國玢，陳維立 審校）
中英對照全文：http://news.ngo.org.tw/issue/water/issue-water-irn00120401.htm
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Aquatic Weeds: Aquatic weeds can do major harm to waterways such as rivers and lakes. One of the most destructive and common weeds in Africa is the water hyacinth. The water hyacinth increases water loss through evaporation (loss of water in a weed-covered water body can be up to six times higher than from weed-free lakes and reservoirs), lowers water quality, harbours water-borne diseases and contributes to the reduction in the number of fish species and population.
Water hyacinth first surfaced in South Africa in 1902, the Congo in 1950, Zimbabwe in 1960 and Zambia in the early 1980s. The free floating vegetative plant has an extremely wide root network and is able to double itself every 15 days. The weed is also threatening Lake Victoria and many other African water- ways. Dams have increased the plant's spread, both in the unnaturally slow waters of reservoirs and because of decreased flow in rivers downstream of dams. At one point, one-fifth of the surface of the Kariba Dam's reservoir - more than 1,000 square kilometers - was smothered by aquatic plants.
Global Warming: There is a growing and often contradictory literature on climate change in Southern Africa. Considerable uncertainty exists concerning the timing, intensity and direction of change of rainfall in an environment experiencing doubled rates of CO2. This is especially true in South Africa, where the topographical influence on rainfall patterns is so large.
Some of the more recent predictions suggest the following global-warming-caused changes to Southern Africa:
▲A 10-20 percent decrease in summer rainfall over South Africa's central interior
▲An increase in the intensity and frequency of floods and droughts
▲A gradual and linear increase in temperatures with rising CO2 levels, reaching 1.5 degrees C hotter than present by the year 2050 with an associated increased frequency of higher-temperature episodes.
The implications for catchments include:
▲Increased evapotransporation rates of 5-20 percent across Southern Africa;
▲A major increase in runoff in eastern parts of the region with an associated increase in the variability and therefore reliability of runoff.
▲A shift in biological communities (biomes), with grasslands being largely replaced by savannah vegetation as a result of increased temperatures.
These changes are likely to significantly increase land degradation in the region since both high temperatures and low rainfall are closely related to high levels of soil and vegetation degradation.
Changes from global warming may already be upon us. Recent studies suggest that there has been an increase in mean annual temperature over Southern Africa in this century, and it is likely to rise even more as a result of greenhouse gases. Patterns of reduced rainfall, too, have been noted. Up until 1980, rainfall followed an l8-year pattern, with roughly nine wet years and nine dry years. However, following the relatively wet decade of the 1970s, the years 1980-94 have been exceedingly dry throughout Southern Africa. Although not as dramatic as the reduction in rainfall in the Sahelian region, the record suggests that there has been an approximately 5-10 percent reduction in midsummer rainfall (Dec.-Feb.) in parts of Southern Africa. In some localized regions the reduction in annual rainfall totals has been far greater.
(Source: Land Degradation in South Africa 1999, by Hoffman et al.) |
