nvasive Plants, Climate and CO2: Implications for Agriculture
Dr. Lewis Ziska, research plant physiologist, USDA-ARS, Crop Systems and Global Change Lab, Beltsville, MD
So what happens if CO2 goes up? An indirect effect of rising carbon dioxide is warmer temperatures. For example: No H2O and CO2? Surface temperature of Earth would be -18o C on average. With H2O and CO2? Surface temperature averages 15o C. A direct effect of rising CO2: Stimulation of plant growth. Any change in light, water, nutrients, or carbon dioxide will alter plant growth.
What would the rise in CO2 alone mean for agriculture? Couldn’t there be positive effects? As an example, the difference in yield with genetic variability in soybean varieties changes with elevated atmospheric CO2 (710 ppm). In nine varieties of soybeans, yield increases ranged from 22% to 85%. So it can be quite variable within a plant species.
Green is not always good, as demonstrated by an image kudzu almost completely covering an abandoned car. CO2, climate change, and plant biology can affect these issues:
- Food security
- Crop/weed competition
- Invasive plant species
- Weeds and public health
An invasive weed is a weedy species, usually non-native for a given region, whose introduction results in wide-spread environmental or species degradation.
Invasive weeds cost us two ways:
- Financial Cost: In the billions to eradicate and in lost crop production
- Environmental Cost: loss of genetic diversity
Canada thistle is one of the top noxious weeds, and a good example of potential impacts. Canada thistle is common throughout most of North America. The exception is southeastern US. Historically it has been a troublesome weed in the Northeast. General Burgoyne unwittingly brought Canada thistle from eastern Canada into the Upper Hudson Valley of New York during that Campaign of the Revolutionary War in 1777 in the hay that they fed their horses.
Canada thistle responded to elevated CO2, 350 ppm over ambient air levels (Ziska, 2004. Weed Sci. 52:584-588), in a 4-year experiment (2000-2003) by increasing shoot production by 140% and root (and rhizome) growth by 270% on average. When glyphosate was sprayed on the thistle grown in an elevated CO2 environment, less control was achieved due to the dilution effect created by the larger root system that could absorb the systemic weed control chemical and render its concentration much lower. The study indicates that carbon dioxide–induced increases in root biomass could make Canada thistle and other perennial weeds that reproduce asexually from below-ground rhizomes harder to control in a higher CO2 world.
In another experiment, where 3 rates of nitrogen fertilizer were applied to Canada thistle seedlings at 2 different levels of elevated CO2 in the air, it was found that both CO2 and N increased biomass, relative growth rates, and leaf area. Root : shoot ratio was increased by CO2 but decreased by increasing N. However, N supply did not effect the relative response to CO2 for any measured vegetative parameter up to 77 days of growth. Due to the relative stimulation of shoot biomass, total above-ground N increased at elevated CO2 for all levels of supplemental N, but NUE did not differ as a function of CO2.
Overall for Canada thistle, there is a strong response to rising carbon dioxide with differential response between root and shoot growth, root (and rhizome) growth much greater. The response to recent changes in carbon dioxide appears independent of N concentration.
Of six noxious weeds tested, Canada thistle is by far the most aggressive grower in an enriched CO2 environment. It total biomass (roots and shoot) has increased 180% since CO2 has increased from 285 ppm to 380 ppm. Perennial sowthistle is a distant second at 110%, yellow star thistle is third at 105%, leafy spurge is fourth at 95%, field bindweed is fifth at 83%, and lastly spotted knapweed at 80%.
Does CO2 preferentially select for invasive species within plant communities? The answer was yes from among the 5 invasive species studied so far for this factor. Only yellow star thistle was found not to be favored in a California grassland. Much more work needs to be done on this factor since there are over 600 invasive species in North America alone.
Summary so far: Invasive weeds and CO2 / temperature
- Canada thistle, the “worst” invasive, shows a strong response to CO2, but greater below ground, relative to above ground growth. Response to CO2 is nitrogen independent.
- Suggestion that CO2 may select for invasives within plant communities, such as pastures.
Is the rise in CO2 the same everywhere? The answer is no; it tends to be higher in cities than in rural areas. An example is from Maryland where Baltimore has a CO2 level of 456 ppm while a rural organic farm’s CO2 level is 386. A corollary question then is, “Is the increase in temperature the same?” This answer too is no; it tends to be higher in cities than rural areas. The change in average daily temperature (o C) from downtown Baltimore to an organic (rural) farm (2002) was 20.7 to 18.6. With this existing CO2 and temperature gradient in a localized area, it allows us to study climate change effects now, rather than wait to see the outcome years down the road.
Taking advantage of this existing gradient, Lew set up an experiment using Baltimore and the rural organic farm by setting up plots that used the same soil and seed bank at both locations. Each site was watered to replace soil water lost by evapotranspiration. The first year at the rural farm re-growth on the fallow soil plot was +90% lambsquarters, 6-8 feet in height at maturity. First year response, urban Baltimore, re-growth was 80% lambsquarters, 10-12 feet in height. More annual weeds present here. After 5 years, he found that the plant succession occurred much faster in the urban setting of Baltimore. Trees had started to grow in the plots there, while in the rural setting the plots were still mostly annual weeds (Ed. note: Plot location looked remote to a tree seed source, however.).
It appeared that the difference in rate of succession was caused by a big difference in plant litter deposition on the soil surface between the two locations. Greater litter deposition from the urban site (high carbon dioxide, temperature) prevents germination of small seeded annuals (e.g. ragweed), while promoting larger seeded (usually perennial) species. The city site after 5 years had almost 8 times more plant biomass on it than did the rural site. Eighty-five percent of the city biomass was from invasive species.
Where do we go from here? Invasive plants are invasive, in part, because they lack predators to keep populations in check. Climate therefore, will be a major factor in limiting population establishment and spread of invasive species. Differential response to CO2 may also favor invasive species. As climate and carbon dioxide change, invasive weed populations will change.
These questions still need answers:
- Which ones are more likely to increase their range? Why?
- How will climate and CO2 alter competition between native and invasive plants?
- How can we manage invasive species in the future?
- WHAT ELSE? WHAT DO YOU THINK?