5.3 Ash Fall Effects On Horticulture
The effects of ash fall are discussed as either physical or chemical effects. Prompt determination of the physical (e.g. particle size) and chemical (composition and reactivity) of ash from an actual event will help to predict its effects and guide mitigation or rehabilitation strategies.
For annual crops, any periods of fallow or cover crops are the most tolerant period, which usually occurs during winter. For deciduous perennial crops, such as pipfruit, kiwifruit and grapes, the winter is their most tolerant time. For evergreen perennial crops, such as avocados and citrus, susceptibility is more uniform through the year due to their persistent foliage cover. These types of crops can be considered more at risk for this reason.
Physical Effects
These effects are most likely to be either physical effects or physiological effects. These are discussed together as they interact. The presence of ash on leaves is akin to the effects of dust. Dust, such as road dust, has been investigated and found to reduce photosynthesis with a subsequent decrease in plant dry weight, and other related effects such as fruit drop (Armbrust, 1986). This is caused by:
- Blockage of leaf pores by dust impeding transport of photosynthetic materials through the leaf.
- Reduced light and energy interception from having a layer of dust on the leaf surface.
- Reduced transpiration causing the leaf to heat up, respire more and impede supply of materials for photosynthesis and plant growth from the roots. The effects of dust on the leaves would be most significant with greater quantities of dust or ash, and longer duration of the leaf being covered. After the Mount St Helens eruption, apple leaves commonly had a 1mm layer of ash on their leaves for up to a week which reduced photosynthesis during the period by 90% (Cook et al, 1981).
Leaves shed dust over time from natural processes, particularly leaf expansion, and wind and rain removing the dust. Growth of new leaves dilutes the impact of the dust. Leaf characteristics affect the rate of this shedding. Smooth shiny leaves shed dust the fastest, and hairy, textured leaves the slowest (Cook et al, 1981) Plants whose leaves were wet at the time of ash fall will stick the dust more firmly, although subsequent water is more likely to wash the ash off leaves that were dry at the time of ash fall. Deciduous plants will of course shed the dust when they shed their leaves each autumn/winter. The least sensitive time for these dust effects is in the winter for deciduous plants. Spring could be the most significant time for ash fall because it is early in the growing season. However, as it is also the time of fastest plant growth, leaf expansion and new leaf development, it is also the time when plants will shed the dust faster. However, annual plants have few stored resources to support the plant during the period of reduced photosynthesis so will be most vulnerable at their seedling stage, usually in spring.
Perennial plants have greater resource to help overcome the period of reduced photosynthesis, so will be better able to withstand a spring ash fall. The Mount St Helens eruption occurred in spring (May of 1980), but after pollination of pipfruit had occurred. The pipfruit crop harvested from the affected areas by ash fall was higher overall than the previous year due to better growing conditions, and the effect of ash by harvest was estimated as a 10% yield reduction. On individual orchards, fruit drop occurred following the ash fall, but some of this was compensated by larger size of the remaining fruit that survived through to harvest (Cook et al, 1981).
Pollination is a particular time that is affected by dust. Transfer of pollen to the receptive parts of the flower is impeded by dust, with resulting reduced fruit set (i.e. fewer fruit), and smaller or deformed fruit. This is relevant to a wide range of fruit and vegetable crops such as pipfruit, kiwifruit, grapes, avocados, tomatoes, peas, sweetcorn, and squash. All these crops would be more sensitive to effects of ash fall dust at the time of flowering and pollination, which is usually in spring or early summer.
Dust may also have significant effects on fruit quality such as delayed fruit maturity, reduced sugar levels and impeded development of colour. (MAF, 1997).
Physical Damage
Physical damage may occur to produce from ash fall. This may be to the leaves or fruit. Surface blemishes may render the produce unsuitable for primary markets and result in lower marketable yield and reduced income. The stage of the crop development and the ash characteristics are important determinants of this effect. Following such physical damage, incidence of disease is likely to increase which will cause crop losses and reduced product shelf life.
Many crops have a period when the fruit surface is especially sensitive to physical damage, usually when the fruit is small. Physical damage then may result in extensive russet or blemishes that downgrade the fruit affected.
"Reworking" of the ash during harvest or post harvest handling of the produce may cause physical damage from abrasion even though the fruit was sound prior to harvest. Even where there is no physical damage, presence of ash on the fruit may be a quality issue either for cosmetic or phytosanitary reasons.
One kiwifruit packhouse near Te Puke was still packing kiwifruit after the Ruapehu ash falls in 1996. The ash was sufficient that people were using car headlights in Te Puke at midday, and there was visible fine dark dust on the ground. The fruit was put over the grader and then back into bins in the coolstore. The brushes on the grader brushed off most of the coarser ash particles. The fruit appeared a bit grey after packing. Fruit samples were sent for chemical analysis and they came back with no problems of residues of concern. Despite the fruit in the trays looking a bit grey, there was no further problem with the fruit. The greyness was sufficiently mild and even across the tray to not be a problem. The grower concerned said "it turned out to be a non-event."
Ecosystem Effects
Dust is generally tougher on predators than on pest species, due to their physical characteristics and greater mobility. (Phillips, 1986. Cook et al, 1981). Thus after ash fall, there may be a change in the pest dynamics and destruction of predator species, thus disrupting integrated pest management of commercial crops. These kinds of pest management programmes are increasing in use on major crops in New Zealand, so this impact is likely to increase in its significance over time.
Pollination will be further affected as ash fall is abrasive to bees, the main pollinating insects. Bees exposed to ash dusted plants following the Mount St Helens eruption died from the physical effects, mainly dehydration, and bee colony numbers were severely reduced (Cook et al, 1981). Moving hives into crops for pollination is a common practice with many horticultural crops, especially kiwifruit. The hives are vulnerable when in a particular crop, and also when in their other locations (especially locations around the central North Island). This could therefore severely impair pollination, as well as reducing production of other bee products such as honey, beeswax and Queen bees. During the 1995 Ruapehu eruptions, there were few bees in the area affected by ash fall. Only small numbers of hives are located in the Central Plateau, mainly during the summer. However, about 30% of New Zealand's hives are usually located in the wider Central North Island area covering the Waikato, Bay of Plenty, Coromandel, East Coast and Central Plateau (M Reid, Personal Communication).
Subsequent to ash fall there will be erosion effects as the ash is redistributed in water. This could affect integrity of drains, and alter natural and man made water courses with consequences of drainage and water logging. Most plants react poorly to water logging due to the consequently reduced aeration in the root zone. The most sensitive plants are affected by short periods. For example, kiwifruit is particularly sensitive, and irreversible effects may occur following four days of root anoxia (Buwalda and Smith, 1990b). Diseases can also follow periods of soil water logging, so are also likely to occur following soil anoxia due to ash smothering. Seedlings are especially vulnerable, as are more sensitive plants such as peas, beans, asparagus, avocados and kiwifruit.
Water logging causing reduced root zone aeration is especially common on poorly structured alluvial soils. These are both the soils more prone to this effect, and are found in the areas where flooding is more common. These kind of aluvial plains areas also tend to be flat and well suited to annual cropping because of their ease of cultivation. The effect of water logging is most significant during the peak of the growing season in summer.
Higher volumes of ash fall have more severe physical effects. Defoliation would be expected to occur due to leaf breakage from about 150mm unconsolidated ash depth. Some plants are more sensitive. For example, alfalfa, a fine leafed legume, was particularly prone to leaf breakage from ash in the area around Mount St Helens (Cook et al, 1981). Stems of non-woody plants may break, causing defoliation. Leaves may break off from the weight of dust on woody plants even when the branches remain intact. The general effects of defoliation are known from experiments, and natural events such as hail and frost as discussed above.
Large branches would be broken by falls of about 500 mm (unconsolidated) ash. The impact of this is far reaching as the branch framework of the tree has a major influence on productivity. It is established during the first few season for a perennial fruit tree. To re establish a branch framework may not be possible, or may result in an inferior tree structure and thus on going reduced productivity. The least sensitive time to branch breakage for a deciduous fruit crop would be in winter prior to pruning. The most sensitive time would be in the period just before harvest when the seasonal costs to grow the crop are almost fully invested, and marketable fruit would be lost with the breakages, and access to the orchard to harvest salvageable fruit may be impeded by broken branches.
Structures such as support structures for vines, greenhouses and crop covers would be broken by heavy ashfalls. The most sensitive time for this would be just prior to harvest, for the same reasons as for the branch breakage. Structures partially broken can be temporarily repaired. However, significant breakage of the structures would be coupled with significant breakage of the vines or trees being supported so the impact would be similar to that of broken branches on unsupported trees.
At thicker depths of ash, burial of plants would kill many small plants due to breakages, smothering and lack of photosynthesis. This would be severe at any time of year when there was actively growing foliage. Access for tending and rehabilitation is likely to be impeded which will further exacerbate the problem. Drifts, and distortion of the landscape by ash would create further problems, including access and safe trafficking problems.
Physical Effects on Soil
Even small depths of ash have physical effects on the soil. The ash tends
to be of low fertility. It may crust on the surface, impeding water absorption. The pale
colour typical of ash leads to cooler soils, which slows plant growth, and can lead to
higher disease levels such as pythium (Cook et al, 1981).
After Mount St Helens, 99% of the ash was found to have reached the soil surface within a
few days of each ashfall (Cook et al, 1981). An ash layer will cause reduced soil aeration
especially where ash is deeper than 500 mm. This reduces suitability of the soil for plant
growth until remedial action is taken. No recovery will occur unless the ash is removed,
mixed with the underlying soil or organic matter added to help soil recovery (Luhr and
Simkin). Ash will compact to 1/3 to ½ its unconsolidated depth with rainfall (Cook et al,
1981).
After even small quantities of ash have been added to soil, the rates for residual herbicides may need to be re calibrated (Cook et al, 1981). These chemicals are used at lower rates on coarser soils, or soils with low organic matter content, so lower rates would need to be used as a precaution, or alternative types of weedkillers used.
Chemical Effects
The chemical effects of the ash would depend on the ash characteristics, particularly acidity (pH), and any reactive chemicals. An acidic ash of say pH less than 3 (MAF, 1997) will cause burning of plant tissue. On leaves, this will have a similar effect to defoliation especially with dew or light rain, and may cause defoliation or even burning of leaf buds which would start to kill the plant. On fruit, acid may cause russet, delayed maturity, fruit drop, or impeded colour development similar to the physical effects of ash discussed above.
Acid rain may occur, causing defoliation and plant death depending on how acid and how prolonged the acid rain was.
Specific chemicals may be an issue for plants. Many plants are especially sensitive to volcanic gases such as fluorine. If gases were to occur during the dormant period for deciduous perennial plants then the effects would be modest. Root uptake of chemicals is regulated by the plant to some degree so would be less damaging than if significant quantities of gas were released into the air around sensitive plants.
Social/Infrastructure
Normal husbandry activities may be disrupted by ash fall. If power supplies are interrupted, rurally located coolstores may be significantly affected as few have standby generators. Power supply may also affect water supply which in turn will affect irrigation and crop spraying for pest control.
Where aviation is affected, aerial pest control may be disrupted. Helicopter spraying is a common means of chemical control for avocados and some field crops. Delays of a week or more may increase crop losses to pests and diseases.
If equipment failure occurs due to increased mechanical wear, the disruption to husbandry activities may cause further crop losses. These would be most severe at critical times of the season for pest control, or during harvest. Most orchard tractors are pretty robust machines. It is more likely that specialised equipment such as harvesting machines for process crops would be most at risk.
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