Does an extended growing season really mean greater plant productivity?

There are four main limiting factors in plant growth: sunlight, water, temperature and nutrient availability. Improving any one of these only shifts the balance and in terms of crop production, nutrient availability is the problem. Lots of people think that increasing atmospheric carbon automatically improves productivity, but this is bullshit since it is tied to soil nitrogen and phosphorus as well as temperature. And then if water is limited, it doesn't matter how much extra N and P is in the soil or CO2 is in the air because the plants need water to absorb N and P from the soil (with some exceptions). The decrease in rubisco means less nitrogen is required, but this may still be a net increase to increase productivity (albeit lower N:C ratios). The solution to this for agriculture is to irrigate (reducing environmental flows) and add fertilisers, which release... NxO greenhouse gasses!! So there is no way increasing productivity will balance other effects on natural ecosystems and agricultural systems are likely to just worsen the problems without technological innovations to offset these issues.

Carbon fixation is only the energy source for plants, they need the other minerals for protein synthesis. Energy is used to drive protein synthesis and reproduction but without enough water and nutrients there is no need for extra energy, so photosynthesis is reduced. Some studies have also shown that increasing CO2 is only beneficial while temps remain below certain thresholds. So the whole idea that warmer climates and more CO2 is good for productivity is... to steal from climate realist... plant food.

FACE experiments used increases in CO2 without increasing temperature effects to measure the influences, and found they are mainly beneficial but regions with low nitrogen in soils do not benefit as much as increasing temps offsets the effects of carbon fertilisation.

Ainsworth and Long (2005). What have we learned from 15 years of free-air CO2 enrichment (FACE)? A meta-analytic review of the responses of photosynthesis, canopy properties and plant production to rising CO2. New Phytologist Volume 165, Issue 2, pages 351–372. http://onlinelibrary.wiley.com/doi/10.1111/j.1469-...

Leakey et al (2009). Elevated CO2 effects on plant carbon, nitrogen, and water relations: six important lessons from FACE. Journal of Experimental Botany, Vol. 60, No. 10, pp. 2859–2876. http://www.bnl.gov/pubweb/alistairrogers/linkable_...

This can be a just right question and beyond the scope of this discussion board. Fannylight makes a just right . In 2010, I ran a sunlight greenhouse scan with the stated intention of keeping photosynthetic endeavor by means of the winter months for biosequestration of excess vitamins and minerals from municipal wastewater. The crops that had very high productivity in the summer months didn't develop good in the winter, despite the surplus of nutrients and day temperatures 30 C above ambient. My collaborator (a biologist) suggested that a seasonal crop rotation utilising crops adapted for low light within the iciness could increase total photosynthetic productivity. Because of funds constraints, we did not experiment that hypotheis. I suppose the hindrance posed is an open query in science with the intention to be settled by scan.

This is a good question and beyond the scope of this forum. Fannylight makes a good start. In 2010, I ran a solar greenhouse experiment with the stated goal of maintaining photosynthetic activity through the winter months for biosequestration of excess nutrients from municipal wastewater. The plants that had very high productivity in the summer months did not grow well in the winter, despite the surplus of nutrients and day temperatures 30 C above ambient. My collaborator (a biologist) suggested that a seasonal crop rotation using plants adapted for low light in the winter might increase overall photosynthetic productivity. Due to budget constraints, we did not test that hypotheis. I think the problem posed is an open question in science that will be settled by experiment.

Since our emissions of CO2 have not been blamed on affecting the length of day, at least not yet, then it appears that the point of the article is that warmth isn't necessarily going to help plants that much. Clearly plants need lots of light. My garden explodes in the summer with the long days. Even if it remained warm through out the winter, the lack of light would hinder their growth. My tomatoes suck in the late fall or winter because they don't get enough light. Temperature is just one factor. It seems to me that with warmth and water, my plants are going to do very well in the summer 9 times out of 10. At least the warmer fall should reduce the chances for an early frost. It should help crop productivity but it isn't going to be proportional to temperature. Other factors such as water might be more critical.

Carbon budgets regarding vegetation are largely ephemeral and an illusion in longer time scales in my opinion. I just don't see why it is a good sink. The ocean can absorb vastly more CO2 than we emit and then precipitate that carbon as limestone (or various other biologic mechanisms). With the biosphere, it might grow a bit and incorporate a bit more carbon here and there but that carbon will soon be returned to the atmosphere except when it is buried which isn't typical.

At some point, if farmers can raise two crops each summer instead of just one, then their productivity is (ceterus paribus) doubled.

The global carbon budget is a different matter. So far plants seem to be, on net, removing part of the human-caused long-term CO2 increase (as are oceans). We should not assume that their absorption capacity suffices to continue being net sinks long term. http://www.science20.com/news_releases/where_does_...

Haven't heard much about carbon-eating trees lately. We could use a few, say, about ten million of "Avatar" size, and dropping diamonds instead of leaves. After you lavishly-funded scientists finish your daily plotting of galactic carbon tax Armageddon, contradicting the King James Inhofe bible where Jesus says "blessed are the stupid liars", and procuring gold-plated toilet seats, could you please take a little time off from yachting, golf, and ivory tower bungee-jumping, and invent us some?

http://www.realclimate.org/index.php/archives/2008...

Certain plants require a thermoperiod for (maximum) growth. I'm banking that study was on C3 trees. So yes, in lieu of carbon you now have oxygen fixing. Under monitored and controlled situations.

What and where do wish to plant? C3 or C4 plants. Though~85% of all terrestrial plants are C3, it still remains a regional distinction concerning environmental conditions.

I don't think there is a general answer globally. There must be many factors like local climate, the type of crop, the latitude, etc. If climate change causes changes to the hydrological cycle (e.g. more precipitation in one area and less in another), then I would think that different regions would be affected differently.

So what would be the overall effect? Sounds to me that that would be difficult to assess. And I doubt there would be agreement among experts.

Don't forget the effects of drought.

It can. In some cases, like an alfalfa crop, it can be harvested three times regularly rather than where it is now two.

In some cases, however, like wheat, the hardier wheat comes from areas with shorter crop seasons. The wheat from Canada and North Dakota are highly prized.

So you just can't make a blanket statement.

Tedious lol. yes very clever, but remember the law of diffusion. Parts of the planet where plant-life conditions are ideal will remove more co2 making more co2 diffuse into that area - basic chemistry. It's a scientific fact, done as practical experiments throughout thousands of schools that proves a lack of co2 is the current limiting factor for plant growth. - nice try lol.