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Additional.html
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<h4>Why are there different methods for calculating winter chill?</h4>
<p>Many models of winter chill have been developed using the observed effects of
temperature on dormancy breaking. The <b>Chill Hours</b> model (Weinberger,
1950) was the first to be developed and estimates winter chill based on hourly
temperatures between 0 & 7.2°C. The <b>Chill Units</b> model (Richardson
et al., 1974) is slightly more complicated. It incorporates the understanding
that temperatures vary in how much they contribute to winter chill and that high
temperatures can have an adverse effect. In this model, temperatures below
1.4°C do not contribute to chill accumulation, temperatures between 2.4 and
9.1°C make the greatest contribution and temperatures above 15.9°C have
a negative impact. </p>
<p> Knowledge of temperature effects on winter chill has since expanded and the
<b>Dynamic chill model</b> (Erez et al. 1990) is the <b>current best
practice model, </b>especially in warmer climates. It calculates chill in
<b>Chill Portions</b>, based on hourly temperatures. The Dynamic
model has many features that capture known temperature-winter chill
relationships that are lacking in other models including the Chill Hours model.
</p>
<p> The Dynamic model uses a two-step process. In the first step, an
intermediate product is created following a bell-shape response function to
temperature with an optimum at 6°C, tapering to zero at -2 and 14°C.
This intermediate product can then be destroyed by subsequent warm temperatures.
The second step is where the intermediate product is banked as a chill portion
once a threshold is reached. A chill portion cannot be destroyed regardless of
subsequent temperatures. Summing chill portions over autumn and winter provides
an estimate of accumulated winter chill.</p>
<h4>What are the chilling requirements of apple and pear cultivars?</h4>
<p>Knowing how much winter chill different crops and cultivars need for healthy
flowering and strong yields is essential for growers when deciding what to plant
in their region. Failure to suitably match the chilling requirement of a crop or
cultivar with the local winter chill will result in reduced productivity. And
yet chilling requirements are unknown for many cultivars of apples and pears.
</p>
<p>Cultivars are generally ranked in broad groups of 'high', 'medium' and 'low'
chill often based on observations of flowering time in the climate in which they
have been grown. Gala apple and Williams' pear are generally considered 'high'
chill, Cripps Pink apple and Packham's pear 'medium' chill and tropical apple
cultivars such as Anna are in the 'low' chill category. Gaps in understanding of
the physiological processes involved in dormancy breaking and a lack of
necessary research means that putting actual numbers on these chilling
requirements remains difficult. </p>
<p>Some reported chilling requirements include:</p>
<ul>
<li>34 chill portions for Cripps Pink (Darbyshire et al. 2016)
<li>50 chill portions for Golden Delicious <a href="http://fruitsandnuts.ucdavis.edu/Weather_Services/chilling_accumulation_models/CropChillReq/">See this site at UC Davis</a>
<li>1041 chill units for Granny Smith and 1115 chill units for Gala.
</ul>
<h4>How will climate change impact winter chill?</h4>
<p>Warmer temperatures in future years are likely to mean less winter chill for
most growing regions of Australia.</p>
<p>Summary of mean values in chill portions (1 March 31 August) for 2030 and 2050 using a moderate
(RCP4.5) to worst case scenario (RCP8.5) modelling approach</p>
<table border="0" cellspacing="0" cellpadding="0" width="600">
<tr>
<td nowrap="nowrap" rowspan="2" valign="bottom">
<p>
Average Chill Portions
</p>
</td>
<td nowrap="nowrap" rowspan="2" valign="bottom">
<p>
Present
</p>
</td>
<td nowrap="nowrap" colspan="2" valign="bottom">
<p>
2030
</p>
<p>
</p>
</td>
<td colspan="3" valign="bottom">
<p>
2050
</p>
<p>
</p>
</td>
</tr>
<tr>
<td nowrap="nowrap" valign="bottom">
<p>
RCP4.5
</p>
</td>
<td valign="bottom">
<p>
RCP8.5
</p>
</td>
<td colspan="2" valign="bottom">
<p>
RCP4.5
</p>
</td>
<td valign="bottom">
<p>
RCP8.5
</p>
</td>
</tr>
<tr>
<td nowrap="nowrap" valign="bottom">
<p>
Applethorpe
</p>
</td>
<td nowrap="nowrap" valign="bottom">
<p>
72 (62-83)
</p>
</td>
<td nowrap="nowrap" valign="bottom">
<p>
63 (48-75)
</p>
</td>
<td nowrap="nowrap" colspan="2" valign="bottom">
<p>
60 (44-73)
</p>
</td>
<td nowrap="nowrap" valign="bottom">
<p>
56 (41-69)
</p>
</td>
<td nowrap="nowrap" valign="bottom">
<p>
51 (36-65)
</p>
</td>
</tr>
<tr>
<td nowrap="nowrap" valign="bottom">
<p>
Shepparton
</p>
</td>
<td nowrap="nowrap" valign="bottom">
<p>
84 (73-93)
</p>
</td>
<td nowrap="nowrap" valign="bottom">
<p>
75 (63-85)
</p>
</td>
<td nowrap="nowrap" colspan="2" valign="bottom">
<p>
74 (62-86)
</p>
</td>
<td nowrap="nowrap" valign="bottom">
<p>
71 (57-81)
</p>
</td>
<td nowrap="nowrap" valign="bottom">
<p>
67 (56-79)
</p>
</td>
</tr>
<tr>
<td nowrap="nowrap" valign="bottom">
<p>
Manjimup
</p>
</td>
<td nowrap="nowrap" valign="bottom">
<p>
67 (55-82)
</p>
</td>
<td nowrap="nowrap" valign="bottom">
<p>
57 (43-76)
</p>
</td>
<td nowrap="nowrap" colspan="2" valign="bottom">
<p>
55 (42-75)
</p>
</td>
<td nowrap="nowrap" valign="bottom">
<p>
50 (36-72)
</p>
</td>
<td nowrap="nowrap" valign="bottom">
<p>
47 (33-68)
</p>
</td>
</tr>
<tr>
<td nowrap="nowrap" valign="bottom">
<p>
Huonville
</p>
</td>
<td nowrap="nowrap" valign="bottom">
<p>
105 (94-113)
</p>
</td>
<td nowrap="nowrap" valign="bottom">
<p>
98 (81-112)
</p>
</td>
<td nowrap="nowrap" colspan="2" valign="bottom">
<p>
97 (78-110)
</p>
</td>
<td nowrap="nowrap" valign="bottom">
<p>
93 (74-105)
</p>
</td>
<td nowrap="nowrap" valign="bottom">
<p>
90 (71-104)
</p>
</td>
</tr>
<tr>
<td nowrap="nowrap" valign="bottom">
<p>
Orange
</p>
</td>
<td nowrap="nowrap" valign="bottom">
<p>
100 (90-110)
</p>
</td>
<td nowrap="nowrap" valign="bottom">
<p>
94 (85-104)
</p>
</td>
<td nowrap="nowrap" colspan="2" valign="bottom">
<p>
93 (82-101)
</p>
</td>
<td nowrap="nowrap" valign="bottom">
<p>
90 (81-99)
</p>
</td>
<td nowrap="nowrap" valign="bottom">
<p>
87 (78-94)
</p>
</td>
</tr>
<tr>
<td nowrap="nowrap" valign="bottom">
<p>
Mount Barker
</p>
</td>
<td nowrap="nowrap" valign="bottom">
<p>
84 (67-93)
</p>
</td>
<td nowrap="nowrap" valign="bottom">
<p>
74 (55-88)
</p>
</td>
<td nowrap="nowrap" colspan="2" valign="bottom">
<p>
73 (53-88)
</p>
</td>
<td nowrap="nowrap" valign="bottom">
<p>
70 (53-84)
</p>
</td>
<td nowrap="nowrap" valign="bottom">
<p>
65 (47-80)
</p>
</td>
</tr>
</table>
<p>
</p>
<H4>What are the symptoms of inadequate winter chill?</H4>
<p>Winter dormancy, although an effective protective measure, can pose
production challenges during mild winters when fruit trees do not accumulate
sufficient winter chill to meet chilling requirements. <b>Light and variable
flowering and a protracted flowering period may result from insufficient
accumulation of winter chill</b>. These conditions can markedly affect fruit yield
and quality through poor pollination, increased risk of frost damage or by
increasing the variability in fruit maturity, making picking more difficult and
extending the length of harvest.</p>