Light and GP, again

A question that comes up again and again is whether the PACE Turf growth potential (GP) would be improved by also accounting for light. Grass growth—plant growth of any type, actually—is influenced by more than temperature. The primary factors affecting plant productivity are temperature, light, leaf N content, and plant water status.

In the case of turfgrass, two of these factors are somewhat controllable by turfgrass managers—leaf N and plant water status. Nitrogen and water can be linked to growth, or to potential growth, by using tools such as GP or evapotranspiration (ET).

But what about adjusting GP by the amount of light? I don’t think that’s very useful. I’ve considered this many times, over many years, and if you haven’t read these posts yet, or haven’t read them recently, please peruse them to learn more about what to expect when temperature and light are considered together.

I’ve also written about this in my column for Golf Course Seminar magazine, and have made calculations and explained them in those columns. The remainder of this blog post will be excerpts from a couple of those columns.

I’ve used this light index, which I’ve called a DLI index, as a way to adjust the temperature-based GP by the amount of photosynthetic light. To calculate the DLI index, I find the DLI for a day, and put that in the numerator. And I also find the potential DLI for that same day, and place that in the denominator. The resultant DLI index is a number that will also be on a scale of 0 to 1, where a low number means the amount of light was low compared to what was possible, and where a high number means the maximum possible amount of light reached the grass on that day.

Does that make sense? There is some maximum possible amount of light that can reach the grass, and that depends on the location and the time of year. Sydney, Australia has the potential to get a lot more light on January 1 than does Osaka, for example, because of the difference in latitude between those two cities. And comparing January 1 to July 1 at Osaka, because of the earth’s position relative to the sun, and because of the day length difference, there can be more light reaching the grass on a sunny day in July than on a sunny day in January. Of course, what actually reaches the ground is another thing, because if it is cloudy, then some portion of the light will never make it to the grass. By taking the actual measured irradiance, converting it to photosynthetic units of DLI, and then dividing by the DLI that would have occurred on that day if there were no clouds, one can get the DLI index.

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The result of this is a measured DLI of 22.4 mol/m²/day on January 1, 2021, and a potential DLI of 26.2. Dividing the actual DLI by the potential DLI gives the fraction of light that reached the grass: 0.85. This is what I call the DLI index. When people ask me about the effect of light, this calculation that includes actual and potential light is what I recommend.

But I also go back to the temperature, because when it comes to growth in Osaka on that January 1 day, the light was quite good. That’s quite a substantial amount of PAR for cool-season grasses. However, the low temperature was 2.3 °C and the high was 7.6 °C with an average of 4.6 °C. The temperatures are so cool that I don’t expect much growth at all. The cool-season grass GP on this day was 0.02 (or 2%). As one goes through the year looking at light and temperature, I keep finding that although light is interesting to consider, I think temperature is more important.

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Table 1 [see my column in the May 2022 issue of Golf Course Seminar magazine for this table] shows the monthly averages of temperature-based growth potential (GP) and DLI and the various DLI indices and growth indices (the product of multiplying GP by the DLI index) for Osaka in 2021. If one wants to consider both the effect of temperature and light on how the grass has the potential to grow, this is one way to do that. At Osaka in 2021, the C₄ GP ranged from a minimum of 0 in January to a maximum of 0.89 in July. The C₃ GP ranged from a minimum of 0.07 in January to a maximum of 0.93 in May. The temperature, as you can see, goes down to a level where it can basically stop grass growth, and up to an optimum level at which there can be maximum growth. That matches exactly what one sees for actual grass growth in the temperatures of Osaka.

But that’s not what one sees with PAR. Notice that the DLI index, and the adjusted C₃ DLI index, are confined to the range of about 0.5 to 0.8. I would understand that to mean that there is usually from 50 to 80% of the light the grass can use, depending on the season. But temperature can vary so much that it completely controls (stops, or allows) growth. By multiplying the DLI index by the GP, the resultant growth index shows the GP after adjusting for light. At Osaka in 2021, this value identifies May, November, and then June as the top three months for cool-season (C₃) grass growth.

My take home message at this point, after reviewing a lot of the calculations I’ve made, and reviewing a lot of my writing about the adjustment of GP by light, is this:

Although it seems like a good idea to adjust temperature by light, for C₃ grasses, temperature is often a limiting factor for growth; light rarely is. I would encourage those interested in this to work through the calculations and I think you’ll come to a similar conclusion. That doesn’t mean light should be ignored. I recommend considering it as part of an environmental productivity index that incorporates N, light, water, and temperature. But temperature on its own, and GP on its own, does surprisingly well at describing growth potential.