Relationships between trunk radial growth and fruit yield in apple and pear trees on size-controlling rootstocks

Understanding the mutual co-ordination of vegetative and reproductive growth is important in both agricultural and ecological settings. A competitive relationship between vegetative growth and fruiting is often highlighted, resulting in an apparent trade-off between structural growth and fruit production. However, our understanding of factors driving this relationship is limited. We used four scions grafted onto a series of size-controlling rootstocks to evaluate the relationships between the annual fruit yield and radial growth of trunks, branches and roots. To assess tree radial growth, we measured ring widths on extracted tree cores, which is an approach not frequently used in a horticultural setting. We found that the yield and radial growth were negatively related when plotted in absolute terms or as detrended and normalized indices. The relationship was stronger in low vigour trees, but only after the age-related trend was removed. In contrast, when trunk radial growth was expressed as basal area increment, the negative relationship disappeared, suggesting that the relationship between trunk radial growth and fruit yield might not be a true trade-off related to the competition between the two sinks. The effect of low yield was associated with increased secondary growth not only in trunks but also in branches and roots. In trunks, we observed that overcropping was associated with reduced secondary growth in a subsequent year, possibly due to the depletion of reserves. Our results show that variation in annual fruit yield due to tree ageing, weather cueing and inherent alternate bearing behaviour is reflected in the magnitude of secondary growth of fruit trees. We found little support for the competition/architecture theory of rootstock-induced growth vigour control. More broadly, our study aimed at bridging the gap between forest ecology and horticulture.