ash and sycamore, how distance effects seedling density.

[lucas logs]

Hello,

 

I need your help for my university project.

This is my graph for ash (blue) and sycamore (red) and the density of their seedlings at 0 - 20 m from the seed source. I need to find which species shows the greatest reduction in density as the distance from the seed source increases. Thanks for taking the time to look, Answers on a postcard.

 

[sloth]

Not exactly sure what you're asking, us to tell you what your results suggest? They are far from conclusive but it would appear (very roughly!) peak density occurs at 6-8m, and there is a 40% drop at 16-18m in both species.

As with any natural system there are going to be many variables and limited factors to consider and note. For example:

- how many trees of each species are surveyed

- the height of the trees, tall trees seeds will go further

- what size of seedlings do you include?

- what are the soil/growing conditions like around the tree?

- how exposed to wind are the trees?

- is there any grazing animals etc which could affect numbers?

- do you count seeds found, or just seedlings?

- do you count all seedlings around the tree, or samples at different compass points and distance?

 

I'm sure there are more factors you can think of! I would suggest eliminate as many limiting factors as you can, and show your graphs as ratios or percentages of distance against tree height. If it is too late to gather more data or change methodology I would say the results are inconclusive, and try to explain why and what could be changed to obtain more significant results. Best of luck! (What course are you on?)

[daltontrees]

I would go with what sloth says plus I would want to consider crown spread opf parent tree, seeds could just fall out of tree on a still day, skewing results towards wide crown trees. On the other hand seedlings under the shade of a parent tree might not do well. Maybe that's one of the things you are trying to show?

Are the graphs a computer generated best fit quadratic? The form of equation you show is definitely a quadratic but I can't think what sort of function in nature it is trying to emulate.

My initial thought is that there would be two main factors at play, firstly how far seeds are broadcast, secondly how well seeds germinate relative to distance from parent. Either of these is a big enough question. The data collection for each might be the same or similar. But a single function to cover both? Good luck!

[daltontrees]

Meant to ask too, why have outliers been removed?

[woodyguy]

Don't understand your graphs or questions. Why the quadratic when you would expect a roughly linear relationship? Why have you drawn a line of best fit when the data doesn't support it ie the r values suggest that only 20-30% of density is explained by distance.

You'd need to look at lots of variables many listed above but including direction of wind for seeds with wings. Perhaps you can define your question more clearly?

[Amelanchier]

I'm not sure the OP needs to consider the variables and control measures - he seems to have been presented with the data and simply needs to draw distinctions between it for his coursework.

 

It might help to ignore the equation / best fit red herrings and take a look at the axis values...

[sloth]

As there is no such thing as a silly question (apparently!) I must ask: what are the axis values?

[Amelanchier]

As there is no such thing as a silly question (apparently!) I must ask: what are the axis values?

 

The number of individuals and the distance from parent tree. Its easier to compare the clustering of the dataset when both are plotted on the same axes. You may need to manipulate the y-axis values to drop the (rather conveniently contrived) order of magnitude out of the data. I could tell the OP how to do that but then where's the learning huh? (PS Excel will do it very simply.)

Edited May 8, 2013 by Amelanchier

[sloth]

Aha, much obliged

[Jon Heuch]

Hello,

 

I need your help for my university project.

This is my graph for ash (blue) and sycamore (red) and the density of their seedlings at 0 - 20 m from the seed source. I need to find which species shows the greatest reduction in density as the distance from the seed source increases. Thanks for taking the time to look, Answers on a postcard.

 

 

Are you plotting density or number of seedlings found at the distances plotted? Either you're not, or you've labelled your y axis incorrectly.

 

I presume your question is: what statistical test do I apply to show that one graph is different from the other? And your specific question, if you really want an answer suggests that you want to normalise both graphs, as you're interested in the rate of reduction from a peak. However, since both graphs have significant outliers (even if you have removed them from Fig 12) which define the peak, I would use a bit of common sense first before attempting to hide behind statistical analysis. Why the outliers? Is your data saying anything at all, or have you chosen a site/picked up data that confounds variables?