Spur dynamics: The key to understanding cropping in almond trees

The majority of the crop in mature almond (Prunus dulcis) orchards is borne on short, proleptic shoots called spurs. Spur dynamics were studied in commercial almond orchards in two experiments; in 1995-1998 and again in 2001-2006. In the first experiment 2185 spurs were initially tagged and in the second experiment 2400 spurs were tagged and followed for the duration of the experiments. The first experiment involved trees in three different late season water stress treatments and in the second experiment the tagged spurs were on trees in four irrigation/nutrient regime treatments. The primary lessons learned from both experiments involved insights into the dynamics of individual and collective spur behaviors over multiple years. In both studies less than 20% of the tagged spurs bore fruit in a given year and significant numbers of spurs died each year (5-27%) after the trees reached full productive maturity. On average, fewer than 10% of the spurs flowered in two sequential years. Spur productivity and mortality in a given year was positively and negatively correlated with previous year spur leaf area, respectively. Spur fruiting tended to increase chances of subsequent spur death. Both decreased subsequent year flowering and increased chances of spur death after fruiting appeared to be linked to a negative effect of spur fruiting on the development of leaf area of fruiting spurs. Decreased leaf area on fruiting spurs appeared to be caused by competition for resources between simultaneously developing fruit and vegetative growth (leaves) shortly after bud-break in spring. This was exacerbated in almonds because flowers came out prior to vegetative growth and developing fruit subtended spur extension and leaf growth. Alternate bearing was apparently not a major problem at the whole tree or orchard level in highly productive orchards, in spite of strong tendencies for alternate bearing among individual spurs, because of an overall spur population dynamic in which a majority of spurs do not bear fruit in any given year and thus accumulate critical amounts of leaf area and are available to flower and set fruit in subsequent years. Interestingly, spur death and turnover was greater in the orchard treatments that received more water and nutrients, presumably because of increased internal canopy shading. Based on this research it is clear that successful management of almond orchards should be focused on maintenance of dynamic populations of healthy spurs. It appears inevitable that a significant percentage of spurs die each year and thus growers must insure that modest amounts of vegetative shoot growth occur each year to provide renewal sites for new spurs to replace dead spurs but excessive growth can increase the rate of spur death by the promotion of internal canopy shading. Thus, it appears that excesses should be avoided in almond orchards as in life.