Shelterbelts are used in New Zealand kiwifruit orchards to help reduce wind damage. However shelterbelts can also reduce fruit yield (fruit numbers and fruit size) on the vines closest to the shelter. Tall shelter blocks sunlight and shelter roots compete with the vines for water and nutrients. Shading by shelter also has more indirect effects. For example, shading may decrease bee activity and therefore reduce pollination.

Successful pollination can be measured by the number of seeds in the fruit. Fruit size in kiwifruit is proportional to the number of seeds. Therefore reduction in yield can be linked to poor pollination by measuring the seed numbers.

HortResearch scientists, lead by Dr Snelgar, counted seeds from fruit randomly selected from middle-row and shelter-row vines. A total of 15 orchard blocks, located in 4 orchards were studied.

The researchers found no difference in seed numbers between kiwifruit picked from middle rows and fruit from shelter rows. However, there were large differences in seed numbers between blocks, between orchards and even within an individual row, so it was difficult to calculate the shelter effects. Sampling more blocks, but fewer fruit per row may give a clearer picture of how seed number differences reduce yields.

In all the orchards tested, high seed numbers corresponded with larger individual fruit weights. Between 1100 and 1350 seeds were required to produce a 100g fruit, and this did not vary between shelter-row and middle-row vines.

Fruit weight increases with the number of seeds per fruit at an average rate of 4.5 g per 100 seeds.

Differences in flowering time of the male and female vines did not seem to affect the numbers of seeds per fruit.

Table 1.
Variation in treatments and seed numbers on the different blocks.

Year	Shelter species	Row	Date at which 50% of flowers reached full bloom (November)			# of seeds per fruit	Mean fruit weight (g)	# of fruit per m2 of canopy
			Male	Female	Diff (days)
1988	Poplar	Shelter	13	26	13	880	82	10
		Middle	14	22	8	1140	89	29
	Casuarina	Shelter	24	28	4	870	90	10
		Middle	18	23	7	900	93	13
1989	Popular	Shelter	12	16	4	990	91	24
		Middle	11	14	3	1110	97	44
	Casuarina	Shelter	15	18	3	1250	99	27
		Middle	13	15	2	1200	97	41
	Willow(1)	Shelter	12	14	2	1300	97	23
		Middle	10	13	3	1240	101	38
	Willow(2)	Shelter	17	25	8	1150	99	23
		Middle	17	21	4	1200	99	36

As can be seen in Table 1, the position of the row did not generally have a large effect on seed numbers, but yield was always higher in the middle rows.

It was surprising that pollination was not a major cause of the low fruit yields on vines near the shelter. This result may have been because the sampling methods were not precise enough to show small differences in seed numbers. It could also be because the results are based on one season only. In another season, or climate, the outcomes may be different.

Based on these findings poor pollination, as measured by seed numbers, is not strongly linked to shelter belt shading. Although there is some scope for increasing orchard yield by improving pollination, areas of poor pollination need to be carefully identified.

Acknowledgment
HortResearch acknowledges the New Zealand Kiwifruit Marketing Board for their support.

Reference
SNELGAR W.P., P.J. MARTIN and P.J. MANSON. 1991. Influence of shelterbelts on pollination of kiwifruit. Acta Horticulturae 297 : 263-268