Climate and Competition: Gas Exchange in Two Studies of Wild Species

Climate change studies are focusing on the possible effects of higher temperature on vapor pressure, soil water availability, and light quality on plants. In native species adapted to milder climates, altered patterns of evapotranspiration could lead to increased stomatal closure and decreased photosynthesis. On the other hand, weed species that usually thrive in warm and marginal environments could extend their range. Higher temperatures that improve the growth rate of weeds could increase their advantage in a crop field or natural ecosystems, stressing primary species. The only way to find out what is happening is through research.

How will Shrublands Cope?

About 30% of terrestrial carbon is stored in the plant tissue and soils of natural ecosystems. This includes not just tall forests, but also shrublands and grasslands. A reduction in carbon assimilation can be critical for their function as carbon sinks.

Several process-based models, which are in use to estimate plant biomass accumulation, must factor in the changes that could be occurring in natural habitats due to climate change, to correctly predict their carbon sequestration potential.

Climate scientists from Waco, Texas, Adhikari and White, decided to check how climate change was impacting growth in managed and restored shrublands, using the 3-PG model, which uses “Physiological Principles in Predicting Growth.”

The study area was situated in Lower Rio Grande Valley (LRGV) Wildlife National Refuge, Texas, a crucial reserve for small wild cats such as jaguarondis and ocelots. The LRGV is a large area that covers a gradient of aridity, so it was divided into four climatic zones, each with characteristic monthly temperature, rainfall, and vapor deficits.

Climate Change Study

The 3-PG model measures growth based on the absorption of photosynthetically active radiation (PAR), which is calculated from leaf area index (LAI) and global solar radiation. Productivity of the ecosystem—gross primary production (GPP) and net primary production (NPP)—was calculated while taking into consideration the soil water, soil fertility, and atmospheric vapor pressure deficit that affect gas exchange in plants.

The climate scientists used a previous study based on remotely sensed data to get current estimates of stand density and biomass. These were correlated with site-specific parameters collected in this experiment.

The parameters estimated in the current study were related to specific leaf area of different species. Moreover, they took stomatal conductance measurement and canopy conductance measurement, of sixty shrubs over the course of three years. These were averaged over species. To measure conductance, the climate scientists used the CI-340 Handheld Photosynthesis System, manufactured by CID Bio-Science Inc. The device is an infrared gas analyzer connected to a leaf chamber. Stomatal conductance measurement is based on transpiration rate as a function of leaf temperature. It shows how open stomata are to determine the amount of carbon dioxide entering into the plant for photosynthesis.

The climate scientists also collected spatial data (resolution of 800 meters) for thirty years of meteorological conditions. It included monthly maximum and minimum temperatures, precipitation, number of frost days, and dew point. Soil fertility and texture were also plotted for the same spatial resolution.

For the current study, the scientists used vegetation databases where the planting dates and replanting densities were known. Then, the scientists randomly selected forty-four tracts, which were distributed equally over the four climate zones. They ran the model for the forty-four tracts that were replanted between 1991 to 2008, using monthly averages for spatial meteorological data for thirty years up to 2010.

The climate scientists first simulated aboveground biomass (AGB) and LAI for shrubland after twenty years of replanting in current climate conditions based on actual data. This, they referred to as baseline data.

Then, they compared three climate change scenarios that could occur in 2050 by creating temperature and precipitation data to depict the following:

• B1- low carbon emissions
• A1B- medium emissions
• A2- high emissions

The meteorological data considered were based on the 4th assessment report of the IPCC. They then compared the AGB and LAI that could result from the three scenarios.

Climate Change Effects are Complex

The 3-PG model was able to predict current shrubland biomass and productivity correctly. There was a high correlation between the prediction of the model and the biomass estimations made from remotely sensed data.

The model also predicted that there would be a decrease in biomass production even in lower (B1) emission scenarios, as shown in Figure 1. The accuracy of the 3-PG model depended on the values of species and site-specific measurements of parameters; many of these parameters are difficult to estimate over a large area.

The climate change study found that future biomass will depend on climate, and vapor pressure deficit (VPD) is the defining factor, as it affects photosynthesis. According to the 3-PG model, VPD will be more important than soil water availability because the latter will have a similar decline in all three scenarios, as shown in Figure 2.

The difference between the four climate regions showed biomass decreasing due to aridity except in zone 2, which defied the trend. The climate scientists were able to find that the reason for higher biomass in zone 2 was due to high soil fertility. Thus, the implications are two-fold–climate change effects in the future can be expected to be higher in areas that have lower soil fertility, and carbon accumulation can be asymptomatic of temperatures in high fertility areas.

The climate scientists concluded that carbon emissions alone cannot accurately predict how ecosystems will respond to climate change. Factors such as interactions between atmosphere, land, and ocean can create atypical local climate conditions. This was the case in zone 1, which was closer to the ocean and had more biomass than predicted by the model, based on climate factors.

The climate scientists warn that shifts in species composition should be expected and that could affect the many endangered species of animals that live in specialized niches in the shrubland. They advise increasing the number of species used in replanting to maintain the carbon assimilation levels of the shrubland.

The study shows the limitations of any model that can take into consideration only a limited number of factors. There will always be many local or large-scale factors that affect a biological system in nature, which will interact to produce surprises for scientists. However, these types of studies are necessary to show what should be expected even if ecosystems adapted to arid conditions.

Could Temperature Impact Light Quality?

Looking for other factors’ interactions, a team of biologists investigated the quality of light. Studies examining the combined effect of temperature and light quality are rare.

The biologists, Qaderi, Slauenwhite, Reid, and MacKay, wanted to find if temperature affected light or if light quality can affect temperature. The quality or color of light influences various physiological processes in plants. Changes in plant growth could affect light quality, as the high R:FR ratio in the upper canopy leaves can be moderated in the lower canopy by the shade of neighboring plants or by upper canopy leaves.

They decided to focus on Velvetleaf (Abutilon theophrasti), a common weed in agricultural lands and disturbed sites; see Figure 3. Weeds are expected to become more aggressive due to global warming. Their growth could get a boost from higher temperatures and weeds could become a greater risk for crop plants and native plant species in nature.

The biologists collected velvetleaf seeds from maize fields and germinated them in Petri dishes. Seedlings were then transferred and grown in pots. Each of the six treatments had nine pots with a single seedling in them.

There were three treatments with low temperature (24/20OC): one with a low R:FR ratio, the second with a normal R:FR ratio, and the third with a high R:FR ratio.

Three other treatments had higher temperatures of 30/26OC, in combination with low, normal, and high R:FR ratio, respectively.

Photoperiod, intensity of light/ photosynthetic photon flux density (PPFD), and relative humidity levels were similar in all treatments. After growing the weeds for fifteen days in the laboratory, three plants were used for morphological (leaf number, area, and moisture, and petiole length), and leaf, stem, and root biomass estimation. Six plants were used for other data collection. Plant height was measured when the weeds were potted and then once in five days. Stem diameter was measured at the end of each experiment.

Gas exchange measurement of net carbon dioxide assimilation and transpiration was done using three leaves from each of the six treatments. A rapid, portable, and precise infrared gas analyzer, CI-340 Handheld Photosynthesis System, was used for the purpose. The difference in carbon dioxide in the air entering and leaving the leaf chamber is used for photosynthetic measurement. Transpiration rate is calculated as the amount of water vapor in the air before and after it leaves the leaf chamber. Gas exchange measurements were made between 10 to 14 hours.

The biologists also determined chlorophyll fluorescence, ethylene evolution, and the amount of chlorophyll and carotenoids.

The entire experiment was repeated twice to get three data sets.

Light is More Crucial Than Temperature

Of the twenty-four morphological and physiological parameters the biologists studied, three were affected by temperature and seventeen by light.

This shows there is no interaction between temperature and light quality, and their effects are independent of each other on velvetleaf.

The weeds growing under higher temperatures had greater root mass but lower water use efficiency due to higher transpiration rates. However, the water availability was not reduced enough to affect the photosynthetic rate. Even a change in the chlorophyll a:b ratio under higher temperatures did not reduce carbon assimilation, plant growth, or biomass accumulation.

The weed plants grown under R:FR ratios that were higher or lower than normal both produced taller plants. Temperature also increases the height of plants, regardless of light quality.

Plants grown under a low R:FR ratio had significantly less plant biomass due to fewer and thinner leaves and lighter roots and shoots than plants grown under a normal R:FR ratio; however, the leaf moisture content was higher. Light quality also affects the accumulation of leaf pigments and the rate of photosynthesis in weeds grown in both higher and lower R:FR than weeds grown under normal F:FR; see Figure 4.

Weeds grown in higher than normal R:FR had lower root mass, chlorophyll concentrations, specific leaf mass, and leaf mass ratio.

Only one parameter, chlorophyll fluorescence, was significantly affected by both temperature and light. Overall, it means that the velvetleaf is more sensitive to light quality than temperature. Even an increase by 6oC will not harm the weed. This could be because the weed has evolved to survive and thrive in dry and hot climates. Therefore, climate change will not affect it negatively or positively.

The plant will continue to be a strong competitor to crops. However, since there is no significant increase in biomass or physiology due to temperature, it also does not pose any extra risk. This result may be considered unsurprising since the weed they experimented with is a hardy, early succession species that grows in a wide range of environmental conditions. The increase in temperature is well within its natural range.

Temperature Alone Cannot Predict All Changes

Both the experiments, one on a native species and the other on weeds, show that temperature is not the only factor that is important to determine the growth and success of a species. Other environmental factors, as well as site characteristics, can influence the impact of a temperature rise, either moderating or amplifying its influence. Also, effects will differ on the individual plant or habitat levels. As the study on shrubland shows, increasing species diversity makes the ecosystem more resilient, because it increases the range of plant traits in play, many of which may be useful in dealing with rising temperature and dryness.  

Both studies are impressive in the range of attributes they test, making the results more robust. This also sheds light on the importance of having efficient, precise, rapid, and portable tools that can measure these physiological and morphological parameters with ease.

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Vijayalaxmi Kinhal
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture

Feature photo courtesy of NY State IPM Program

Sources

Adhikari, A., & White, J. D. (2016). Climate change impacts on regenerating shrubland productivity. Ecological Modelling, 337, 211-220. doi:10.1016/j.ecolmodel.2016.07.003

Brelsford, C., Trasser, M., Paris, T., Hartikainen, S., & Robson, T. (2019). Understory light quality AFFECTS leaf pigments and LEAF phenology in different plant functional types. doi:10.1101/829036

Mahr, S. (n.d.). Velvetleaf, Abutilon theophrasti. Retrieved from https://hort.extension.wisc.edu/articles/velvetleaf-abutilon-theophrasti/

Qaderi, M.M., Slauenwhite, K.L.I., Reid, D.M., & MacKay, R.M. (2015). Does temperature regulate light quality effects on Abutilon theophrasti?. Acta Physiol Plant 37, 125. https://doi.org/10.1007/s11738-015-1878-8