Pollination deficit in apples increases with elevation

Apples are a highly pollinator-dependent crop that require a minimum number of accumulated chill hours to break dormancy in preparation for bud development and blooming. In the Indian Western Himalayas, apple production is shifting to higher elevations to counteract climate change-associated temperature increases. But it is unclear if and to what extent pollinators are able to match these shifts, or how these altitudinal shifts impact pollinator contribution to apple production. To address these knowledge gaps, we manipulated pollination conditions for apple blossom clusters in 13 orchards distributed along an elevational gradient (1680–2360 m) and measured early fruit set. Fruit set was significantly impacted by pollination treatment and elevation, and in some cases by their interaction. Averaged across elevations, fruit set in the pollinator exclusion treatment was only 6% of that observed under open pollination conditions, but fruit set in the open pollination treatment decreased markedly with elevation. Predicted pollinator contribution to fruit set also decreased with elevation, from 0.96 at the lowest modeled elevation to 0.76 at the highest modeled elevation. Fruit set was enhanced by supplementary hand pollination, demonstrating pollination limitation. We found that pollination deficit increased with elevation, ranging from 0.05 at the lowest modeled elevation (1650 m) to 0.24 at the highest modeled elevation (2400 m). Our findings demonstrate that at least some of the productivity gains realized with increased availability of chilling units at higher elevations might be compromised by pollination limitation. Future studies to determine more clearly the extent and underlying causes of elevation effects on pollination deficits are needed.