What if chill hours change for fruit orchards

How changing winter temperatures, erratic snowfall and spring warmth could affect Kashmir’s fruit orchards

By Mauzam Bakshi

Kashmir’s orchards have a winter requirement that is fundamental to their productivity but largely invisible to the eye. Apple, pear, cherry, peach, plum, almond and walnut trees enter a period of physiological dormancy during winter and require exposure to appropriate cold before their buds can resume normal development.

This process is known as ‘chilling’ and it is an essential part of the biological transition from winter dormancy to spring growth.

For generations, Kashmiri farmers have understood the importance of winter through observation. The character of the winter, the timing of snowfall and the arrival of spring have traditionally been closely watched because they influence what happens in the orchard, months later. Modern plant physiology and climate science now provide the tools to examine that relationship more precisely.

The question confronting Kashmir is whether changes in the duration, timing and variability of winter cold, together with increasingly irregular snowfall and changes in spring warming, are beginning to alter the thermal conditions required by its fruit trees.

This cannot be established from a few unusual winters. Climate trends require long – term records. But the issue deserves serious attention because horticulture is a long – term investment and the biological requirements of an orchard cannot be changed as quickly as its surrounding climate.

The tree does not follow the calendar

Kashmir traditionally divides winter into Chillai Kalan, the 40-day period of severe cold, followed by Chillai Khurd, the 20-day period of lesser winter, and Chillai Bachha, the final 10 days before the transition towards spring.

The tree, however, does not follow the calendar. It responds to the temperatures it experiences.

A winter with prolonged, relatively stable cold is physiologically different from one characterised by alternating warm spells and sudden cold events, even if their seasonal averages are eventually similar. This is why the distribution of temperatures through winter can be as important as the average temperature itself.

The traditional calendar therefore provides an interesting cultural framework, while modern horticultural science asks a more precise question – how much effective cold has the tree actually received?

The traditional chill – hour model counts hours below a specified temperature, commonly 7.2°C. It is useful, but plant response to winter temperature are more complex than a simple count of cold hours.

Different temperature ranges can contribute differently to dormancy release, while periods of warmth may influence the process. Consequently, researchers use more sophisticated approaches, including the Utah Chill Unit model and the Dynamic Model, which expresses chilling accumulation in terms of Chill portions.

This is important when discussing climate change. Saying that Kashmir is “losing chill hours” requires evidence from long – term temperature records and an appropriate chilling model.

The scientifically relevant question is whether the changing winter temperature regime is reducing or otherwise altering effective chilling for particular species and cultivars.

That distinction matters because fruit trees do not all have the same chilling requirements.

Snow is not the same as chill

Snowfall is another important part of Kashmir’s winter environment, and its timing and persistence have appeared increasingly variable in recent years. Whether this represents a long – term climatic trend requires systematic analysis of historical records.

Snow, however, should not be equated with chill. Snow is precipitation, chilling is a physiological response to temperature.

An orchard may receive adequate chilling during a relatively snow – deficient winter if temperatures remain sufficiently low. Conversely, heavy snowfall does not guarantee that a particular cultivar has met its chilling requirement.

Snow nevertheless affects the orchard environment. Snow cover can insulate the soil, influence root – zone temperatures and moisture and affect the timing of snowmelt. Its interaction with temperature therefore deserves attention when assessing changing winter conditions.

The Three Khams and the arrival of spring

There is another aspect of Kashmir’s traditional environmental knowledge that is particularly relevant to the transition from winter – The Three Khams or Khamae, describe the arrival of warmth through a sequence involving earth, water and air.

First comes the warming of the earth. The frozen ground begins to soften and vegetation resumes activity.

Then comes water, as ice retreats and streams and water bodies respond to rising temperatures.

Finally comes the warming of the air and the arrival of “Sonth”, the Kashmiri spring.

The Three Khams should be regarded as traditional ecological knowledge, not as a scientific temperature model. Yet the observation is interesting because soil, water and air have different thermal properties and do not respond to seasonal warming at identical rates.

Today, these transitions can be measured through soil and water sensors, automatic weather stations, snow observations and remote sensing. They can then be compared with the actual timing of bud break and flowering in orchards.

This provides a bridge between Kashmir’s traditional observations and modern phenological research.

Winter chill and spring warmth

Chilling alone does not determine when a tree flowers.

After its chilling requirement has been satisfied, the tree requires sufficient warmth to resume development. Scientists study this second stage through heat accumulation, including growing degree days and other phenological models.

The timing of flowering is therefore influenced by two linked processes – the cold experienced during dormancy and the warmth accumulated afterwards.

A disruption in either can alter the orchard calendar.

Insufficient chilling can result in delayed or uneven bud development in susceptible cultivars. Earlier accumulation of spring warmth can accelerate development once dormancy has been released.

Changes in flowering time can subsequently influence pollination, fruit set and exposure to pests, diseases and adverse weather.

One of the most important consequences may arise from the relationship between earlier spring development and late frost.

If buds and flowers develop earlier because of warmer conditions, they may enter vulnerable stages sooner. If damaging low temperatures continue to occur after that development has begun, the risk to the crop can increase.

The critical issue is therefore not simply whether spring is warmer. It is whether flowering is shifting relative to the last damaging frost events.

This is an area where Kashmir needs much better long – term orchard – level data.

Not all orchards respond the same way

Kashmir’s horticulture comprises species and cultivars with different chilling requirements. Apple, cherry, pear, peach, plum, apricot, almond and walnut cannot be treated as though they respond identically to winter.

The same is true within a species.

This has become particularly significant with the expansion of high – density orcharding, new rootstocks and newer cultivars. A variety that performs well under current conditions may not necessarily remain optimal throughout the productive life of an orchard.

Nor should cultivars described elsewhere as “low chill” automatically be introduced into Kashmir. Their suitability must be demonstrated under local conditions, including winter temperatures, spring warming, frost, water availability, soil, pests and diseases.

The Valley also contains considerable local climatic variation.

Elevation, slope, aspect, topography, snow cover, soil and exposure can produce significant differences in orchard microclimates. Consequently, two orchards within the same district may experience different temperature regimes and chilling accumulation.

Future horticultural planning therefore needs to become more location – specific.

Instead of asking simply whether a district is suitable for apples, the more useful question is – Which crop and cultivar are best suited to a particular site under its present and projected climate?

Building a horticultural climate network

The tools required to begin answering these questions are increasingly available.

A horticultural climate – monitoring network could combine automatic weather stations with measurements of soil temperature, soil moisture, humidity, precipitation and snow conditions. These observations should be linked with records of bud break, flowering, fruit set, maturity and yield.

Geographic Information systems and Satellite data could then combine these observations with elevation, topography and soil information.

Over time, this would allow Kashmir to develop detailed maps of chilling accumulation, spring heat, frost exposure and horticultural suitability.

A Kashmir Horticultural Climate Atlas could ultimately provide growers and planners with location and cultivar – specific information rather than relying solely on historical assumptions.

Preparing rather than predicting crisis

The objective should not be to declare a crisis but to prepare for measurable change.

If research identifies shifts in chilling conditions, adaptation could include better cultivar selection, locally tested breeding material, improved rootstocks, appropriate site selection, frost – warning systems, precision irrigation and stronger pest and disease forecasting.

The introduction of new varieties should be preceded by multi – year trials under Kashmir’s actual climatic conditions. High – density orchards should similarly be assessed not only for productivity but also for their response to changing winter and spring conditions.

Climate suitability should increasingly become part of the decision on what to plant and where to plant it.

Reading the orchard’s biological clock

Kashmir already has a sophisticated traditional vocabulary for the seasonal transition – Chillai Kalan, Chillai Khurd, Chillai Bachha, the Three Khams and Sonth.

What has changed is our ability to measure the environmental processes behind those observations.

We can now record the temperature of the air and soil, monitor water and snow, calculate chilling accumulation, measure spring heat and document precisely when trees break dormancy and flower.

The priority should be to put these measurements together over many years.

Only then will Kashmir be able to determine whether its effective winter chill is changing, which crops and cultivars are most sensitive, and how the horticultural calendar is responding.

The issue is ultimately larger than the number of cold hours recorded in a winter. It concerns the timing and interaction of winter chill and spring warmth that regulate the biological clock of the orchard.

Kashmir’s horticulture has been built around a climate that generations of farmers learned to understand.

The challenge now is to understand, with the same seriousness and with the help of modern science, how that climate is changing and what that means for the orchards of the future.