Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!agate!shelby!polya!Polya.Stanford.EDU
From: ray@Polya.Stanford.EDU (Ray Baxter)
Newsgroups: sci.bio
Subject: Re: Is Mowing your Lawn bad for the Environment?
Summary: Depends on if you use a noisy gasoline mower.
Keywords: lawns, oxygen production
Message-ID: <9444@polya.Stanford.EDU>
Date: 24 May 89 21:08:10 GMT
References: <1989May23.190505.18229@utpsych.toronto.edu> <8323@pyr.gatech.EDU>
Sender: Ray Baxter <ray@polya.Stanford.EDU>
Reply-To: ray@Polya.Stanford.EDU (Ray Baxter)
Distribution: usa
Organization: Biological Sciences, Stanford University
Lines: 53
In-reply-to: steve@revolver.gatech.edu (Poppa Smurf)


In article <1989May23.190505.18229@utpsych.toronto.edu> raymond@psych.toronto.edu (Raymond Shaw) writes:

> Would the lawn produce more oxygen (which I presume is good for the 
> environment) if I didn't mow it, and just let it grow?

In article <8323@pyr.gatech.EDU>, steve@revolver (Poppa Smurf) replies:

>     You can stop worrying.  Grass is biomass and perennial.  It draws its
>energy and nutrients from the sun and the soil.  When it dies, the elements
>return from whence they came.  No net gain or loss in oxygen.

This response and another reponse by Ashwin Ram assume that the original 
question has to do with atomic oxygen, ie. atoms of O.  I assume that it
is more probable that the question is about oxygen gas, O2, this being one
of the products of green plant photoysnthesis.

In fact, grass (and all other green plants) do produce O2 and consume
CO2 and H20 and solar energy.  The reaction is:

                        CO2 + H20  ---->  CH20 + 02
where CH2O is generalized carbohydrate.

This does not answer the original question which I interpret as asking how
the height of grass influences the rate of O2 production.  The answer to
this is that it depends.  Cutting the grass almost surely causes a
transient decrease in the rate of oxygen (O2) prodution.  Beyond that, it
pretty much depends on how short the grass is cut.  Empirically, as the
surface area of plants (leaves only) increases from 0 to three times the
surface area of the ground that it is covering, the rate of production over
the whole stand increases.  Once you get beyond a ratio of three,
essentially all of the availible solar energy is captured and so there is
no increase in photosynthesis and hence no change in the rate of O2
production.

On the flip side, the whole plant is constantly consuming O2 as it 
respires.  This reaction, which is merely the reversal of the 
equation above, yields energy to the plant rather than consuming solar
energy.  The rate of this reaction is more or less proportional to the
mass of the plant.  

As the ratio of leaf area to ground increase above three, the mass of the
plant and so the rate of O2 consumption continues to grow.  (Much of
this additional mass is allocated to supporting structures.)  Putting
the last several paragraphs together, you can see that the rate of O2
production will be a reasonably complicated function of the height of
the grass.  Cutting the grass can increase, decrease, or not change the
rate of O2 production.  (The time derivative of this rate is left as an
exercise for the reader.)


   Ray Baxter
   ray@polya.stanford.edu