# Technologies for Forecasting Tree Fruit Load and Harvest Timing--From Ground, Sky and Time
## 2 Measurement of Fruit Number
### Correlative Methods  
#### 2.5.1 What Determines Tree Fruit Load?  
Correlative methods aim to avoid the direct count of fruit through use of a relationship between a relatively easily measured attribute and fruit load. To inform the choice of attribute, the determinants of tree fruit yield are briefly reviewed. For some crops, there is a direct allometric relationship between the harvested component and the biomass of the whole crop, such that crop yield can be estimated by 'indirect' measurements of more easily assessed attributes. For example, sugarcane crop height and density are correlated to can yield [61]. For tree fruit, a given tree will have a genetically and physiologically determined maximum fruit biomass that it will tend towards. For example, [PERSON] et al. [62] reported that the removal of a proportion of mango panicles at the anthesis stage resulted in the set of more fruit on the remaining panicles, such that the yield was unaffected.  
The final tree fruit load, however, reflects a series of events that include flowering, pollination, fruit set, fruit retention and fruit growth (sizing). Each step is affected by an interplay of endogenous and exogenous factors, with limitations not always compensated, as in the example of [PERSON] et al. [62]. Further, as a perennial crop, management and environmental conditions of previous seasons can impact the yield of the current season,  
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and some genotypes have a strong two-year cycle in yield. The yield potential of a given tree is therefore often not reached, with the extent of influencing effects varying between seasons, between orchards, between trees in an orchard and even between branches on a tree. Management actions operations that do not impact tree size or shape can also alter the yield, such as the chemical thinning of fruit or timing and extent of irrigation. Thus, the relationship between fruit load and other characters of the tree can vary - in brief, bigger trees do not necessarily have more or larger fruit. The difference between the crop potential and the actual load in a given year has been termed a 'yield gap'.  
A number of studies have attempted to relate fruit load to the number of flowers or blooming intensity, e.g., [PERSON] et al. [63], [PERSON] et al. [64], but such relationships are dependent on a range of conditions. In the case of mango, each stem terminal has the potential to convert from the vegetative to reproductive mode, producing a panicle. Thus, the reproductive potential of a tree is set by the number of branch terminals on the tree that are of reproductive age. However, the conversion to the reproductive mode requires a period of 'cold' weather, i.e., temperatures under 18\({}^{\circ}\)C, or applications of chemicals such as KNO\({}_{3}\) and paclobutrazol [65; 66]. the timing of pruning is also important, as juvenile stems (<4 weeks of age) are not able to convert to the reproductive mode. Further, only a proportion of the panicles that form on a mango tree set fruit, and of those, only a proportion will hold fruit through to harvest, with these proportions varying between trees and seasons as pollination and growth conditions vary. If these factors are not optimal, a yield gap will exist between the actual and potential yield.