Turf Weeds and Climate Change. 51

Turf Weeds and Climate Change.

Turf Weeds and Climate Change are directly linked, and may well have serious implications on turf management in Australia. Unfortunately, Climate Change and its increases in carbon dioxide and temperature are going to affect herbicide efficacy and weed distribution.

Changes in climate will affect weed distribution due to the impact it will have on many plant functions of turf weeds. Being highly adaptable weeds will undoubtedly adapt to these changes, and this will raise a completely new set of management issues.

Increases in CO₂ concentrations, together with higher temperatures and variable moisture conditions, will not only affect the growth and geographical presence of turf weeds, but also impact the efficacy of herbicides that you use to control them.

C3 and C4 photosynthesis.

Turf weeds like all plants, are categorised as being either C3 or C4 plants in relation to their photosynthetic pathways. Climate Change will result in different reactions to conditions depending on what photosynthetic pathway turf weeds have.

Most C4 plants are warm-season grasses, and generally C4 plants capture CO₂ better as light intensity and temperatures increase.

In contrast, C3 plants capture CO₂ more efficiently under more moderate light conditions, and at lower temperatures, and in fact most broadleaf weeds have C3 photosynthesis.

Differences between C3 and C4 plants.

As you have now figured out depending on the conditions, C3 and C4 are affected differently by the climate.

In brief, C3 turf weeds will benefit from rising CO₂ levels. However, several studies suggest, that rising temperatures will in fact override any beneficial effects of CO₂ on photosynthesis. In fact, once the temperature goes over 25°C, this negatively impacts C3 plants because their photorespiration rate increases¹Jordan, D.B.; Ogren, W.L. The CO2/O2 specificity of ribulose 1,5-bisphosphate carboxylase/oxygenase. Planta 1984, 161, 308–313. [CrossRef] [PubMed].

In contrast, photosynthesis of C4 plants is more effective at higher temperatures, but C4 photosynthesis is usually not affected by increases in CO₂.

As CO₂ levels increase:

• C3 weeds will get bigger more quickly
• C4 weeds will continue to grow at the same rate.

As CO₂ and Temperatures Increase:

• Growth of C3 and C4 plants may become limited.
• Some C3 plants will have a smaller window for treatment.

Water Use Efficiency.

Drought favours C4 weeds because C3 plants have a lower water use efficiency²Singh, R.P.; Singh, R.K.; Singh, M.K. Impact of climate and carbon dioxide change on weeds and their management—A Review. Indian J. Weed Sci. 2011, 43, 1–11..

Because the water-use efficiency of C4 plants is higher than C3 plants, C4 turf weeds are likely to be more competitive than C3 plants under drought  and increased temperatures.

A good example of this is perennial ryegrass, which has a big competitive disadvantage vs C4 weeds like Summer grass and Crowsfoot grass during the Summer.

In the Summer months, high temperatures and increased light, mean that Perennial Ryegrass is not efficient at capturing CO₂. This results in less carbohydrate production.

C4 weeds, take up CO₂ at a higher rates, as temperatures and light intensities increase. The result is that during the summer, C4 weeds accumulate carbohydrates, while Perennial Ryegrass depletes its energy reserves.

In shade the complete opposite occurs. Perennial Ryegrass captures CO₂ better in cool temperatures and at lower light levels.

This explains why you are less likley to find high populations of Summergrass and Crowsfoot grass in the shade.

Table Showing C3 and C4 Turf Weeds.

Survival Mechanisms.

As an environment changes turf weeds can adopt three survival strategies. They can migrate, acclimatise or adapt to the change in conditions³Pautasso, M.; Dehnen-Schmutz, K.; Holdenrieder, O. Plant health and global change—Some implications for landscape management. Biol. Rev. 2010, 85, 729–755. [CrossRef] [PubMed].

Migration: As the climate changes weeds move from one place to another where the conditions better suit it. This process is driven by human activity and the movement of machinery⁴Kubisch, A.; Degen, T.; Hovestadt, T.; Poethke, H.J. Predicting range shifts under global change: The balance between local adaptation and dispersal. Ecography 2013, 36, 873–882. [CrossRef] but extreme weather events like flooding will also play a role.

We have already seen this occur in Sydney. Floods in 2023 lead to Buchan Weed moving onto playing fields that had never seen this weed before.

Acclimation: A change in climate leads to weeds modifying their phenotype, but these modifications are not passed from generation to generation⁵Pearman, P.B.; Guisan, A.; Broennimann, O.; Randin, C.F. Niche dynamics in space and time. Trends Ecol. Evol. 2008, 23, 149–158. [CrossRef] [PubMed].

Adaptation: Traits pass from generation to generation, as weeds respond to changes in their environment. A good example is ragweed which now produces greater amounts of pollen⁶Harlan, J.R.; deWet, J.M.J. Some thoughts about weeds. Econ. Bot. 1965, 19, 16–24. [CrossRef].

Poa is also likely to become more of an issue in the future, as it increases in seed production, under increasing CO₂.

Summary of the Effect of Increasing CO₂ and Temperature.

As it gets warmer, CO₂ increases and weather patterns change, the differences between C3 and C4 plants will become more apparent.

C3 plants tend to respond positively to rising CO₂ levels, whereas C4 plants are better adapted to heat stress and drought (due to their higher water use efficiency).

• Increasing CO₂ levels favours the growth of C3 species over C4 weeds.
• As temperatures increase this favours C4 weeds over C3 weeds⁷Varanasi, A.; Prasad, P.V.V.; Jugulam, M. Impact of climate change factors on weeds and herbicide efficacy. Adv. Agron. 2016, 135, 107–146..
• Top and root growth increases in C4 plants at higher temperatures.
• Drought conditions increase weed competition e.g. Amaranthus sp.

These changes in climate are not just going to affect weeds but are key factors affecting herbicide performance.

Increased CO₂ Levels and Herbicide Efficacy.

As discussed earlier as CO₂ and temperature increases it will stimulate overall plant growth⁸https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1399-3054.2009.01256.x.

Increased top and root growth

More rapid top growth means that the amount of foliar herbicide taken up by weeds is diluted, and so there is less herbicide to kill the plant⁹Holland, J. and Sinclair, P. (2003). Environmental Fate of Pesticides and the Consequences for Residues in Food and Drinking Water. In Pesticide Residues in Food and Drinking Water (eds T. Roberts, J. Miyamoto, D. Hamilton and S. Crossley). https://doi.org/10.1002/0470091614.ch2.

Physiological Effects.

As CO2 levels increase, it can reduce the diffusion of CO₂, water vapor, and O₂, through the stomata of a plant by up to 50%¹²Bunce, J.A. Growth, survival, competition, and canopy carbon dioxide and water vapor exchange of first year alfalfa at an elevated CO2 concentration. Photosynthetica 1993, 29, 557–565..

This means that foliar applied herbicides may give poorer results in the future¹³Alizade, S.; Keshtkar, E.; Mokhtassi-Bidgoli, A.; Sasanfar, H.; Streibig, J.C. Effect of drought stress on herbicide perfor-mance and photosynthetic activity of Avena sterilis subsp. ludoviciana (winter wild oat) and Hordeum spontaneum (wild barley). Weed Res. 2021, 61, 288–297. [CrossRef].

High CO₂ levels also cause leaves to thicken, and this may reduce the efficacy of the post-emergents taken up by the plants.¹⁴Ziska, L.H.; Teasdale, J.R. Sustained growth and increased tolerance to glyphosate observed in a C3 perennial weed, quackgrass Under increased CO2, Rhodes Grass, African Love Grass, and Paspalum have higher survival rates from glyphosate treatment, in comparison to the plants grown under normal CO2 conditions .¹⁵Manea, A.; Leishman, M.R.; Downey, P.O. Exotic C4 Grasses Have Increased Tolerance to Glyphosate under Elevated Carbon Dioxide. Weed Sci. 2011, 59, 28–36. [CrossRef].

Higher CO₂ levels stimulates weed growth, and reduces the time a weed is in the seedling stage. This is when many weeds are susceptible to herbicides, and so reduces the window for herbicide application.

Increasing CO₂ stimulates the growth of roots more than the shoots. This means that weeds that spread underground vegetatively will become more difficult to control.¹⁶Ziska, L.H.; Faulkner, S.; Lydon, J. Changes in biomass and root: Shoot ratio of field grown Canada thistle (Cirsium arvense), a noxious, invasive weed, with elevated CO2: Implications for control with glyphosate. Weed Sci. 2004, 52, 584–588. [CrossRef].

Temperature and Herbicide Efficacy.

Temperature affects herbicide efficacy by impacting plant growth and development, and in turn the translocation and penetration of herbicides.¹⁷Varanasi, A.; Prasad, P.V.V.; Jugulam, M. Impact of climate change factors on weeds and herbicide efficacy. Adv. Agron. 2016, 135, 107–146.

Herbicide movement and the viscosity of cuticle waxes are also directly affected by temperature, and so affect herbicide efficacy¹⁸Price, C.E. The effect of environment on foliage uptake and translocation of herbicides. In Aspects of Applied Biology 4: Influence of Environmental Factors on Herbicide Performance and Crop and Weed Biology; Biologists, A.O.A., Ed.; The Association of Applied Biologists: Warwick, UK, 1983; pp. 157–169.

Higher temperatures increase the breakdown of herbicides, and so reduces herbicide performance on weeds¹⁹Johnson, B.C.; Young, B.G. Influence of temperature and relative humidity on the foliar activity of mesotrione. Weed Sci. 2002, 50, 157–161. [CrossRef].

This increase in the speed of breakdown, is because microbial activity increases as the soil temperature increases²⁰ Long, Y.H.; Li, R.T.;Wu, X.M. Degradation of S-metolachlor in soil as affected by environmental factors. J. Soil Sci. Plant Nutr. 2014, 14, 189–198. [CrossRef].

When the soil temperature increased from 10° to 15°, 25°, and 35°C, the half-life of S-metolachlor decreased from 64.8 to 38.9, 26.3, and 23.7 days respectively.

It’s not all bad.

In some cases, increases in temperature increase the absorption and translocation of herbicides.  Work shows that this may well be the case with the sulfonylurea herbicides like Tribute Selective, Duke Herbicide and Recondo Herbicide in the future²¹Ghazikhanlou Sani, Y.; Yousefi, A.R.; Jamshidi, K.; Shekari, F.; Gonzalez-Andujar, J.L.; Korres, N.E. Weed Response to ALS-Inhibitor Herbicide (Sulfosulfuron + Metsulfuron Methyl) under Increased Temperature and Carbon Dioxide. Agronomy 2023, 13, 2084. https://doi.org/10.3390/agronomy13082084.

Excessive Rainfall Events.

All pre-emergent herbicides require optimum soil moisture for movement within the soil and active absorption by plant roots. While dry soil conditions increase herbicide adsorption to soil particles, heavy rainfall immediately after the application may result in herbicide loss due to leaching.

For example, US work with the pre-emergent herbicide, Indaziflam that this tends to leach more with increasing rainfall.

Weed Futures.

Interestingly, work has been done looking at how some weeds may be impacted by climate change. Weed Futures is a screening tool for managers to assist them in identifying those species for which detailed weed risk assessment and management are needed. The table below gives some examples.