Sept. 14, 2021
Aug. 26, 2021
An interesting marshland study in the Yangtze Estuary shows the difficulty in extrapolating general predictions of increased vegetative growth, under climate change-induced air warming. Local environmental factors in specialized ecosystems, such as salt marshes, can complicate plant growth patterns to give C3 plants an advantage over C4 plants in warmer climates. Moreover, population-level effects can override individual plant performance for parameters like leaf area index.
Like other natural ecosystems, wetlands are also being cleared for agriculture. People cut off tidal influences in the land that they want to use for farming in areas called “reclaimed land”. Changes in wetland species composition are likely results of these land-use changes.
In the Yangtze Estuary, Phragmites australis is the predominant perennial grass in natural salt marshes. Phragmites australis is important for several migratory and resident bird communities. Upper tidal reclamation for agriculture, has seen an increase in the numbers of the weedy and problematic Imperata cylindrica. In several cleared salt marshes, I cylindrica has changed the marsh plant community by becoming sub-dominant and reducing the ecosystem services that marshes provide.
Climate change-related air warming could exacerbate this situation, since it is expected to increase the production of C4 grasses, such as Imperata cylindrica. C4 plants are more responsive to increased warmth and improve their photosynthesis, compared to C3 plants like Phragmites australis.
However, due to a lack of studies on marsh grasses, the effect of climate change is unknown. Other factors, such as nutrients in soil, water availability, or salinity in marshes could affect the growth of plants with limited predictability.
To see how climate warming could affect the growth and distribution of the C3 and C4 grasses in reclaimed marshes, a team of marine fisheries scientists in China, conducted an in-situ experiment.
Zhong, Gong, Wang, and Zhang laid twelve plots in the reclaimed marshes of Dongtan of Chongming Island in the Yangtze Estuary. The usual yearly average temperatures here are 15.3oC, and the average precipitation is 1004 mm.
In eight of the plots, the scientists set up open-top chambers (OTCs) to carry out a manipulative air warming experiment, as can be seen in Figure 1. In these plots, they exposed both P. australis and I. cylindrica to elevated air temperature of 1.5 ± 0.2oC for two years, and to temperatures of 2.3 ± 0.3oC in the third year in eight plots (ET). The remaining four plots were used as control (CON).
After three years, the scientists measured the effect of warmer temperatures on growth-related traits such as morphology, photosynthesis, and biomass accumulation, in all 12 plots. The scientists also looked for possible compounding effects of associated changes in soil temperature, moisture, nutrients, and salinity due to air warming.
Figure 1.: “Appearance of the open-top chambers (OTCs) applied in the simulated warming experiment conducted in the reclaimed salt marsh,” Zhong et al. 2014. (Image credits: Aquatic Botany 117, 18-26. https://doi.org/10.1016/j.aquabot.2014.04.001 )
Air temperature in the OTCs was measured by the temperature probe of a data logger. Gas exchanges of three randomly chosen leaves in each species were recorded in the morning with a photosynthesis system to get net photosynthetic rate, transpiration rate, stomatal conductance,
intercellular CO2 concentration, and leaf vapor pressure deficit. From these, water use efficiency and intrinsic water use efficiency were calculated.
Net photosynthesis responses to changes in solar radiation intensity were also measured by changing PPFD inside the OTCs gradually from 1500 to 1200, 1000, 800, 600, 500, 400, 200, 150, 100, 50, 20, 0 mol m−2 s−1. The leaves studied in this experiment were used to measure leaf nitrogen content by the colorimetric method.
The scientists measured morphological traits, eleven times in ET and control plots for comparison, starting from the second year. The traits measured were: plant height, stem basal diameter, mean leaf inclination, and the leaf area of three randomly selected shoots per species.
Measuring the leaf area of the entire plant can be difficult if done manually; traditional methods like grid counting and the gravimetric method are tedious and destructive. More modern techniques using mathematical models and image processing are non-destructive, but still time-consuming.
The scientists wanted a non-destructive and rapid means of measuring the numerous leaves of the marsh grasses, as the same shoot needed to be used for biomass estimation. The team decided to use a leaf area meter, manufactured by CID Bio-Science Inc.: the CI-203 Handheld Laser Leaf Area Meter. The meter can be used to calculate leaf length, width, perimeter, and area for leaves for varying shapes and sizes with a scanning speed of 200 mm/second. The scientists needed to only sweep the measuring wand of the leaf area meter over the leaf; the tool scanned the leaves with a laser and used preloaded formulae to calculate the leaf area instantly. During the scanning process, curled leaves are flattened so measurements are accurate.
After taking the leaf area measurements, the different parts of the shoot were separated into leaves and stem, to get dry weight through the oven method. The scientists added the leaf and stem biomass, to get the total aboveground biomass. Specific leaf area was calculated as leaf area/ leaf biomass.
Shoot densities (SD) of the two grass species were calculated from two permanent quadrants in each plot. Leaf area index (LAI), total leaf biomass, total stem biomass, and total above-ground biomass were calculated by multiplying SD with shoot leaf area, leaf biomass, stem biomass, and above-ground biomass.
Soil temperature, volumetric moisture, electrical conductivity, and nitrogen levels were recorded once a month for the entire duration of the experiment. To estimate root biomass, two soil samples 20 cm deep were taken in each plot three times in the last year. Roots of the two species were separated and oven-dried to measure their biomass.
Air warming was not observed to have any effect on the soil temperature, moisture, or nitrogen content. It did, however, increase salinity by 119%, which played a vital role in shaping the responses of the two grass species.
Figure 2. “The net photosynthetic rate-photosynthetic photon ﬂux density (PN-PPFD) curves of P. australis (a) and I. cylindrica (b) in CON and ET in the peak growing season. CON, control plots; ET, elevated temperature plots,” Zhong et al. 2014. (Image credits: Aquatic Botany 117, 18-26. https://doi.org/10.1016/j.aquabot.2014.04.001 )
The scientists found that air warming significantly reduced the net photosynthetic rate by 28% in C3 plants- P. australis, see Figure 2 (a). This decreased leaf area by 22%, and the aboveground biomass and relative growth rate of aboveground biomass by 28% and 36% respectively at the shoot level. An increase was seen in specific leaf area, at shoot level, of 12%.
However, at the population scale, P. australis shoot density increased by 142% due to warming treatment. The leaf area index increased by 87%. Both these together boosted aboveground shoot biomass by 69% at the population level. There was no difference in root biomass to a depth of 20 cm, indicating the plants reserved their resources for aboveground parts to improve competitive advantage.
The scientists found a reduction in stomatal conductance and lowered water use efficiency. The reduction in photosynthesis was likely due to the process being affected by higher temperatures, as transpiration and CO2 levels remained similar in warm and normal environments. The rise in warmth also increased vegetative propagation through rhizomes, as P. australis is tolerant to increased salinity brought about by air warming. The increase in the number of plants, was able to compensate for lower individual shoot leaf area, and the population level leaf area index was higher in air warmed plots.
Air warming did not have the expected growth enhancement in I. cylindrica. There was no difference in net photosynthetic rate (See Figure 2 b), and morphological or growth traits at the shoot level in warmed environments, compared to ambient conditions. At the population level, the effects of warming were negative, as shoot density was reduced by 49%, leaf area index by 45%, and aboveground biomass by 47%.
The scientists argue that the lowered CO2 content in leaves indicated that I. cylindrica did manage to fix more carbon but didn’t increase Rubisco activity, so the net photosynthetic rate remained low. Moreover, I. cylindrica thrives in areas of low salinity, so the negative impacts of an increase in salinity are more than the positive effects on photosynthesis due to air warming.
I. cylindrica was a subdominant species in the experimental area, so the rise in population of the dominant P. australis, gave the latter an additional competitive advantage. The higher leaf area index of P. australis crowded other grasses and cut I. cylindrica access to sunlight; in other words, the rhizomatous propagation of P. australis suppressed I. cylindrica multiplication.
The scientist concluded that despite being a C3 plant, the native P. australis, which is better adapted to highly saline marshes, will be able to hold its own when temperatures rise by 1.5 °C, in the future. I. cylindrica’s C4 physiology is not enough to overcome the higher salinity or the interspecific competition from an enhanced P. australis.
Time and again, research has shown that leaf area can be an important indicator and factor of plant growth and productivity. Being one of the largest organs, and one through which the plant interacts with the environment, this is not a surprise. Having a precise leaf area meter can be crucial, given the vital role that specific leaf area and leaf area index can play, as in this experiment, to explain the functioning of ecosystem dynamics.
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture
|CI-203 Handheld Laser Leaf Area Meter|
Zhonga, Q., Gong, J., Wang, K., & Zhang, C. ( 2014). Effects of 3-year air warming on growth of two perennial grasses (Phragmites australis and Imperata cylindrica) in a coastal salt marsh
reclaimed for agriculture. Aquatic Botany 117, 18-26. https://doi.org/10.1016/j.aquabot.2014.04.001
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