In self-thinning or intraspecific competition (more relevant for trees I guess) why does logarithm of Biomass vs logarithm Density give you a slope of -1/2 whereas logarithm of Weight vs logarithm of Density gives you a slope of -3/2? Biomass is mass (or quantity) of an organism per volume(or area), weight is mass scaled by gravitational acceleration which is a constant, unless the organism you are dealing with is very very tall. So the plots can be shifted accordingly but shouldn't slopes be more or less the same ?

source: Ecology: Concepts and Applications by Manuel Molles www.mheducation.com/highered/product/ecology-concepts-applications-molles/M0077837282.html

Ch 13 Competition. enter image description here

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    $\begingroup$ Welcome to Biology.SE. This is a question of statistics, not biology. You should give it a try on Stats.SE. $\endgroup$
    – Remi.b
    Commented Sep 20, 2016 at 17:49
  • $\begingroup$ I would not expect the slope of y~x to be equal to the slope of log(y)~log(x) (to the exception maybe of when the slope is 1?). Consider this R code for example. x=rnorm(100,40,7);y=x/6+rnorm(100,0,1);plot(x=x,y=y);lm(y~x);lm(log(y)~log(x)) $\endgroup$
    – Remi.b
    Commented Sep 20, 2016 at 17:53
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    $\begingroup$ @Remi.b I understand log plots. For example, if you know Stephan's law of radiation; radiated energy E is proportional to fourth power of temp T, in this case, if we plot log(E) and log(T) we will get slope to be 4. But that (value) is not my concern, why are they different is my concern. $\endgroup$ Commented Sep 20, 2016 at 18:42
  • $\begingroup$ @Remi.b I don't understand how is this a Stats question, I think definition of biomass and physics are more relevant here ! $\endgroup$ Commented Sep 20, 2016 at 18:44
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    $\begingroup$ @Remi.b I know that this is not exactly a Biology question, it is more about the understanding of those quantities, physics behind them and the mathematical analysis of it. I am not exactly sure where to post it. I googled definitions of Biomass and weight and crosschecked if there was any misinterpretation, but there wasn't any. And as far as I understand it should only offset ( y = mx + c, I mean c by offset ) the plot and not change the slope. BTW I still don't get it how is it a Stats question XD. $\endgroup$ Commented Sep 20, 2016 at 19:20

1 Answer 1


Biomass here refers to the total mass of all the organisms in the population (in a unit of area), added together.

Weight here refers to the average weight or mass of an individual.

If biomass decreases, that could be because the individuals got lighter, or it could be because there are fewer of them, or both.


I think I see what's happening.

Let $W$ be the average mass of an individual, $N$ be the number of individuals, and $B$ be their total biomass. Then of course,


Let's start with:

  • $W$ is proportional to $N^{-\frac{3}{2}}$
  • $N$ is proportional to $N$ (duh). Let's rewrite that though, as:
  • $N$ is proportional to $N^1$

But that means:

$B=WN$ is proportional to $N^{-\frac{3}{2}} \cdot N^{1} =N^{-\frac{3}{2}+1} = N^{-\frac{1}{2}}$

Density increases with density (N). Weight per individual decreases with the -3/2rd power (W). So their product, biomass decreases with the -3/2 + 1 = -1/2nd power (B=WN).

  • $\begingroup$ Plants do not get lighter, the loss of leaves is not considered in this theory and it would apply to all individual in the stand, only the stem density is discussed. $\endgroup$ Commented Jul 5, 2018 at 10:16

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