5.2 Impacts from other natural events
Although experience with ash fall is modest, other natural events such as defoliation from frost and hail, indicate likely effects. Some of these are discussed below. Knowledge of these events, and a good understanding of crop husbandry and physiology provide guidance for rehabilitation strategies following the ash.
Cyclone Bola
This cyclone in 1988 caused widespread erosion and silting of plains areas in Gisborne. These observations are taken from discussion with horticultural consultants in the district at that time (G Loudon, S Scarrow personal communication). Cyclone Bola is relevant to this discussion of ash in terms of having a layer deposited on the soil surface, and resulting reduced soil aeration.
Silt, up to 1 metre deep, was washed through orchards and annual crops. The land was covered by silt and water for several days. The silt was difficult to handle initially as it was wet and muddy. Once dry, it was also fine and dusty until it had compacted.
Once the silt was dry it was not a great problem for growers to handle, and the fertility of the silt was good. Orchardists physically removed the silt from around the base of trees and vines to help prevent development of crown rot diseases that could ring bark the tree. Silt is still visible in piles in some orchards a decade later in areas where it was moved from around trees into the grassed area between rows. One of the main on-going effects on surviving plants was from lack of root zone aeration, caused by water logging, which led to diseases and reduced yields. There have been changes in orchard crops grown in the area, partly attributed to the effects of Cyclone Bola but also to crop economics. For example, kiwifruit orchards were replaced by apples, but the better profitability of apples at the time is the most important reason.
On annual crops, growers cultivated the silt into their soil and re sowed crops. Long term the effects were minimal, helped by the fertility of the silt.
Defoliation
Several examples of kiwifruit defoliation have occurred following events such as frost and hail and defoliation experiments. A severe hailstorm in late January of 1987 caused significant defoliation of kiwifruit vines, especially on Belk Road near Tauranga. An unseasonally late frost around the Bay of Plenty occurred in November 1994, which defoliated vines. Experimental defoliation of kiwifruit was carried out at various time of the 1986/87 growing season and the vines evaluated for the following two seasons. (Buwalda and Smith, 1990a)
The kiwifruit defoliation experiment was designed to investigate the role of the leaves in different zones of the vine. However, conclusions relevant to general effects of defoliation can fairly be drawn:
- The most severe effects occurred when a higher proportion of foliage was removed.
- The effects on the current seasons crop was a severely depressed marketable yield, due to fewer and smaller fruit.
- The effects continued into the next season, where return bloom was depressed by 29% to 100%. A higher proportion of leaves removed for more of the season caused the greatest reduction in return bloom.
Kiwifruit Hail
The effects of hail were a once-off partial defoliation. Leaves were either cut off, or severely reduced in effectiveness by tattering. Fruit were marked by the hail stones, rendering more of them unsuitable for sale. The effects of the hail reduced marketable yield significantly in the current season. Return bloom was also affected so the yields did not return to normal levels for a further 1-2 seasons. Fruit size was a little affected but the main impact was unsaleable fruit. There were few disease problems on the fruit because in late January the crop is not especially sensitive.
Kiwifruit Frost
A frost in November 1994 occurred just as kiwifruit were flowering. The frost defoliated vines, the extent depending on the site. Flowers were killed, and some surviving flowers were lost to disease following the physical damage. The damaged and dead leaf and flower tissue also created an innoculum source for disease. The current seasons yield was reduced by crop loss, higher proportions of reject (mainly deformed) fruit. Following the frost, the vines had a growth surge and replaced their foliage. This foliage subsequently proved to be fruitful despite being less mature than fruiting wood that had grown for a full season.
Of interest is that many growers initial reaction was to "tidy up the vines" by pruning which removed more foliage. The best vine recovery occurred where this was not done, and summer leaf area was preserved. Some of the frosted vines had a poor crop the following season - largely due to carrying an excessive cropload of small fruit, while the vine resources were depleted. Preserving the remaining and regenerating leaf area after the frost, and careful management of crop load in the following season were the most successful rehabilitation strategies. Good rehabilitation allowed some orchards to return to normal yields in the first full season following the frost.
Pipfruit Hail
Hail occurred on pipfruit in Hawkes Bay in October 1996, after flowering when the fruitlets were small. The main damage was marking of the fruit. The severity of damage was very dependant on wind direction. Growers were able to manage their losses to some extent by thinning off worst affected fruit. This both helped to overcome the effects of partial defoliation likely to lead to reduced fruit size, and the costs of picking reject fruit which provides little revenue. More recently, several hail events occurred in a single season on pipfruit in Hawkes Bay. As well as the reduced marketable yield from physical damage to fruit, fruit size of the remaining fruit was reduced, and the yield in the following season was also reduced. These latter two effects are largely due to the defoliation, and are a similar response as determined in the defoliation experiments on kiwifruit discussed above.
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