Positive Chill Units - June

Certain biennial plants which have a dormant (i.e. rest) season during winter may require a period of accumulated minimum temperatures below a threshold in order to stimulate growth, develop leaves, flower or set fruit. Deciduous trees, for example, need a certain period of winter chilling for completion of their seasonal dormancy. Insufficient winter chilling may result in delayed foliation, reduced fruit set (including fruits remaining small and being misshapen as they ripen) and reduced fruit quality (including reduced firmness, and/or greener fruit coloration). The required amount of chilling for completion of the rest period varies between species, cultivars and different locations. Many chill accumulation models have been formulated, most of them requiring hourly temperatures to have either been observed or estimated. The Linsley-Noakes PCU model was used to produce maps of chill units in this Atlas. The techniques by which PCUs were computed and then mapped are described in the shaded sections which follow.

Derivation of hourly values of temperature for the determination of PCUs hourly values of temperature were determined from the 50 year time series of daily maximum and minimum temperatures generated at each of the 429 700 raster points covering South Africa, using the techniques described by Schulze and Maharaj (2004). For each daytime and night-time cycle the sine-log equations derived by Linsley-Noakes et al. (1995) were employed. In these equations the estimation of hourly temperatures is based on the observation that the daytime solar cycle follows a sine curve from sunrise to sunset, whereas night-time cooling may be described by a logarithmic equation.

部分二年生植物会在冬季经历休眠（即休整）期，此类植物通常需要一段低于临界温度的累积低温时段，方可启动生长、展开叶片、开花或坐果。以落叶乔木为例，其完成季节性休眠需要一定时长的冬季低温积累。若冬季低温积累不足，可能导致展叶延迟、坐果率下降（包括果实发育瘦小且成熟时畸形），以及果实品质降低（包括硬度下降、果皮颜色偏绿）。不同物种、品种以及种植区域所需的低温积累量存在差异。目前已提出多种冷量累积模型，其中多数模型需要使用实测或估算的逐时气温数据。本图集采用Linsley-Noakes冷量单位（PCU）模型绘制了冷量单位分布图，该模型的计算与制图方法详见后续的阴影栏章节。
用于PCU计算的逐时气温数据，是通过覆盖南非的429700个栅格点生成的50年日最高、最低气温时间序列，采用Schulze与Maharaj（2004）提出的方法推导得到。针对每个昼夜周期，研究人员采用了Linsley-Noakes等人（1995）提出的正弦-对数方程。该类方程的逐时气温估算基于以下观测规律：日间日照循环自日出至日落呈正弦曲线分布，而夜间降温过程则可通过对数方程进行描述。