3.3 Impacts of Ash Fall on Animals and Plants
3.3.1 Impacts on Animals
Mammals Ash fall is unlikely to immediately kill animals except when deposition rates are exceptionally high and thickness is great. Tephra cover on pastures will result in lack of feed for animals. During the 1945 Ruapehu eruption pastures covered by ash were often described as being unpalatable to stock but no significant pasture damage occurred (Johnston & Neall 1995). No stock losses due to lack of feed were reported (Cunningham 1946). Following ash falls from Ruapehu in 1995 and 1996 farmers noted that animals were readily put off their feed by ash deposits of around 2-5 mm thickness. Ash from the 1980 Mount St Helens eruption had little impact on livestock as long as they had access to sufficient feed (Blong 1984). Trials on cows being fed ash at a rate of 1.5 kg per day showed no obvious effects on milk production.
Aquatic life is very susceptible to changes in water conditions such as increases in acidity, turbidity, temperature and concentrations of soluble elements.
Thick ash falls will have a dramatic impact on water conditions in affected areas. Rivers draining such areas will produce sustained high sediment yields due to the availability of readily erodible material as the river flows through the new deposits. Rivers flowing through barren areas that were previously vegetated will suffer from lack of shade, raising water temperatures beyond previous levels. After the 1980 Mt St Helens eruption, water temperatures soared beyond levels for growth and survival of salmon where the Toutle River flowed through the area devastated by the debris avalanche (Lucas 1986). Another problem for such rivers is the lack of leaf litter that is an important food source for aquatic invertebrates. Further away from the mountain little impact was noted on streams in areas with ash thicknesses of 2 cm (Gamblin et al. 1986).
In normal circumstances little sediment is added to rivers by tephra in areas away from stream or river channels, unless the stream channel lies directly below steep slopes. Only during severe rainstorms is tephra readily eroded from the land surface and deposited in streams or rivers. Such events are little different to the behaviour of soils on non-vegetated land during similar severe rainstorms. Keam (1988) notes Dr Hectors observation of the tephra deposits from the 1886 Tarawera eruption. "While rain had certainly washed a great deal of mud off steep slopes, it was showing no tendency to slide .......... the mud was there to stay, unless it was removed by normal erosional processes."
The primary factors causing fish to die in the rivers around Ruapehu after the 1969, 1975 and 1995 lahars were suspended sediments, acidity and concentrations of fluoride. Minor fish kills were also reported in ash-affected rivers after the 1995 eruption but insignificant in terms of the total population (Maxwell 1996). Minor disturbance to the 1995 trout spawning migration was observed but the Tongariro River fishery has generally remained in good condition.
Aquatic floral and faunal invertebrate populations are susceptible to ash suspended in rivers and lakes. Reductions in primary production of planktonic and rooted plants will reduce secondary grazers important as fish food.
Birdlife Ash may cause several problems for birds, with falls of fine ash preventing flight. Widespread ashfall may result in lack of food. Gases from the vent area can kill overflying birds. In the 1886 Tarawera eruption, pigeons, ducks and sparrows were killed in large numbers (Keam 1988). Surviving sparrows were blinded at least temporarily, with eyelids gummed together by the falling mud.
Other living things Ash particles are especially destructive to insects largely due to abrasion of the epicuticular wax layer which causes rapid desiccation and death (Cook et al. 1981). One advantage noted in agricultural areas which received small amounts of Mount St Helens ash was the destruction of insect pests.
3.3.2 Impacts on Plants
Damage to small vegetation and the soils on which they depend will vary with tephra thickness and composition of the ash. The effects in Table 3.4 are based on observations from past eruptions described by Folsom (1986) and Blong (1984).
TABLE 3.4 Impacts on plants and soil from increasing ash thickness.
Thin burial (< 5 mm tephra) |
|
Moderate burial (5 - 25 mm tephra) |
|
Thick burial (25 - 150 mm tephra) |
|
Very thick burial ( > 150 mm tephra) |
|
Eruption impacts on trees are described by Rees (1970) below (Table 3.5).
TABLE 3.5 Impacts on trees of tephra.
Tephra Thickness |
Impact on Trees and Shrubs |
150 - 500 mm |
Slight damage and partial survival of shrubs |
500 - 1500 mm |
Tree damage, large branches were broken, heavy kill of shrubs |
1500 mm |
Total kill zone |
Eggler (1948) noted pines surviving in tephra depths of 1240 mm and 1780 mm. Pine seedlings and small trees were killed as a result of excessive bending and burial while large mature trees suffered from branch breakage under the load of ash. Pines with basal diameters of 100 - 300 mm survived best because their stems were strong enough to resist excessive bending yet sufficiently flexible to dump part of the load and avoid breakage.
Crop damage will result from burial which can kill or damage plants depending on the thickness of the tephra. During the 1995 Ruapehu eruption major losses (~$250 000) to cauliflower crops were reported in Gisborne, 250 km downwind but market gardens were fortunate that many crops were not in the ground at the time of the ash falls. The following table (3.6) was prepared by the Ministry of Agriculture in 1995 following the Ruapehu eruption.
TABLE 3.6 Periods of high crop risk from tephra (from M.A.F. 1995)
Periods when crops are most at risk |
Pea: from emergence until end of flowering. Squash: during the initial stages of growth and flowering. Tomatoes: during seed emergence and flowering stages. Sweetcorn: during the early stages of growth. Pipfruit has three danger periods: - Blossom where severely acidic ash (pH less than 3) could burn plant
tissue and result in poor pollination; Stonefruit is also susceptible at the same times as pipfruit, except that the early fruit development period is 4-6 weeks after blossoming, when sensitive fruit skins could be damaged, and show russet or deformation in severe cases. Kiwifruit is also at risk at, and 6-8 weeks after, blossom. There would also be a problem at harvest time. As kiwi fruit cannot be washed prior to packing, the hairy nature of the fruit would make ash removal very difficult. Grapes have three main periods when damage could occur: - Flowering, when acidic ash could burn plant tissues, reduce pollination
and reduce bunch fill; - Harvest, where ash deposits would be a contaminant with the extra acidity of the ash possibly having a significant impact on wine quality. Ash would have to be removed prior to harvesting by washing and allowing bunches to dry. |
Damage to soils may result from the tephra fall affecting the productive potential of the area. Small amounts may improve soils. A positive impact of the 1995 - 1996 Ruapehu ash falls was to temporarily reduce the sulphur fertilizer requirement for all sheep, beef and dairy farmers within the ash fall area (Cronin et al. 1996). Contamination of water supplies may cause damage to plants and limit production.
Acid rain (and acidic ash) has been reported as causing a number of effects on plants. Blossom drop, poor fruit set, small almost seedless fruit have also been reported in horticultural areas. In most areas where ash fall has occurred, crop damage has only been sporadic. Minor acid burns were reported on some plants on Ruapehu and the Kaimanawa Ranges following the 1995 Ruapehu eruption but most had recovered by late 1996 (Keys 1996).
Plant recovery Most detailed studies of plant growth after tephra fall result from the 1980 Mount St Helens eruption. Algal covers became established on most tephra surfaces within a few weeks, providing a protective surface skin against erosion, although the mat is easily disturbed.
Folsom (1986) showed that the recovery of plants from the trauma of tephra deposition will follow a sequence:-
1. Recovery of surviving not completely buried plants
2. Emergence of surviving buried plants
3. Germination of local seed reserves
4. Colonization from outside seed sources
with individual plant recovery dependent on:-
1. Thickness of ash
2. Degree of continuing disturbance
3. Amount and reliability of rainfall
Chaplin et al. (1986) noted that certain species of plant do not suffer from mineral deficiencies when growing in the nutrient-poor volcanic soils. Such plants will obviously be better suited to recolonization in areas where tephra or surge deposit thicknesses are great or on lava flows. The re-establishment of plants on barren areas influences the colonization of subsequent plants by trapping seeds and changing the microclimate (Dale 1986). After five years specie richness had stabilized in areas where recolonization had occurred (del Moral and Wood 1986). Sites far removed from seed sources were unlikely to be recolonized rapidly.
The recovery of the area devastated around Mount Tarawera in 1886 is well documented (Clarkson and Clarkson 1991). The devastated area remained barren for over 10 years. Where bush had only been partly buried it resprouted quickly. On the lower slopes of the mountain toetoe and tutu shrubs re-established within 20 years and a mat of daisies colonized the middle slopes. After 27 years post-eruption shrubs had established on the middle slopes and daisies and grasses were present high up the mountain. The summit area was still barren. By 42 years after the eruption a young forest was present at the base of the mountain. Higher up the slopes a tutu association was common. After 75 years the forest was dominated by tawa at low altitudes and kamahi at higher altitudes. Near the top of the mountain, shrubs were common and the crater and summit area had widespread grasses, mosses and daisies. In the past 20 years an influx of tutu and scattered introduced conifers has occurred on large areas of the upper part of the mountain.
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