# Technologies for Forecasting Tree Fruit Load and Harvest Timing--From Ground, Sky and Time
## 2 Measurement of Fruit Number
### Correlative Methods
#### 2.5.6 Prediction Based on Multiple Season Attributes  
A time series across seasons of yield data from one orchard may display autocorrelation, reflecting the size and health of trees. Such a time series can therefore be useful in the prediction of future yield, adjusted for current conditions. The fruit yield of an orchard can be expected to trend upwards in the establishment years of an orchard, and downward during the decline of an over-mature orchard, and in relation to seasonal limitations ranging from pollination to water stress. Tree fruit yield also sometimes demonstrates a marked fluctuation between seasons. A biennial bearing pattern can be described by a bearing pattern index [81], defined as:  
\[I=\sum_{i=2}^{n}\frac{|Y_{i}-Y_{i-1}|/(Y_{i}+Y_{i-1})}{(n-1)} \tag{6}\]  
where \(Y_{i}\) is the \(i\)th observed yield over \(n\) years, and \(|Y_{i}-Y_{i-1}|\) is the absolute difference in yield between successive years. The index, \(I\), can vary between 0 and 1, where 0 is associated with equal yield in alternate years and 1, zero yield in alternate years. For example, the fruit yield of 18 mango trees reveals a level of bi-annual variation in fruit load at an individual tree level, with enough synchronicity between trees such that the 18-tree average demonstrates a biennial cycle, mirroring variation in the packhouse count for the orchard (Figure 8). [PERSON] et al. [76] note that climatic events (such as extreme temperatures or drought) can act to synchronize the beginning of a period biennial bearing, markedly so at a local level but increasingly less so moving from local to regional scales if the climactic event is patchy. Thus, the relevance of alternate year bearing is lessened at wider scales. An example was described for table grapes in which the initiation of a 7-year period of synchronous biennial bearing of vines was ascribed to an abnormally hot period during bud formation that resulted in a low fruit yield in all vines in the following year, with an increase in carbohydrate availability during the 'off' year proposed to have supported a strong flowering in the next year, which in turn resulted in depleted reserves and a weak flowering in the following year [82]. It was postulated that over time, the physiology of the vines varied, such that synchronicity was dampened or lost. Figure 8: Fruit load per tree of 18 mango trees from one orchard, over 6 years. **Top panel**: trees demonstrating synchronicity in biennial bearing, with ‘on’ years in odd years; **middle panel**: trees demonstrating biennial bearing with ‘on’ years in even years or a non-biennial bearing pattern; Colored lines represent individual trees in top and middle panel; **bottom panel**: average fruit load of 18 trees and the packhouse count for the orchard (494 trees). An empirical model can be fit to existing time series data and used in extrapolation to forward predict the yield of the next season. For example, [PERSON] et al. [83] presented a method based on nonlinear time series analysis for a forward prediction of yield of citrus in the next year from relatively short duration time series sets based on evidence of description of deterministic chaos behind the alternate bearing pattern. Using an ensemble dataset of fruit counts on 48 citrus trees followed for 6 years, they were able to forward predict the final three seasons' yield for each tree based on previous years' counts for the ensemble with acceptable accuracy, while correctly predicting the biennial bearing pattern of the orchard. 5.7 Prediction Based on Yield Across Multiple Seasons, Climatic Variables and Canopy Characteristics  
The production of tree fruit crops generally increases over a half decade or more as trees mature and canopy size increases, allowing yield to be predicted based on tree age and tree spacing. For example, almond yield increased each season for the first seven years post planting, then stabilized, albeit with year-to-year fluctuations [84].