Greetings honorable forum.

 

I have a particular stats problem which I feel would benefit from some
inputs from the group so I thank you in advance for any feedback you
might have.

                First some background. I have two vegetation maps...one
developed from a historical map at time 1 (t1) and one developed from a
current map (t2). We are interested in assessing whether there has been
an elevational shift in the trailing and leading edge of specific veg
types between t1 and t2. Additionally, the maps span many degrees of
latitude and consequently we want to include latitude as a covariate to
account for latitudinal effects on distribution boundaries. 

 

We have currently, sampled the map on continuous latitudinal transects.
At each transect we determine the minimum and maximum elevation at which
veg type X is encountered for both t1 and t2 maps. Consequently, our
sample unit as it currently stands is at each lat transect. There are a
couple issues that come to mind. First, if we assume that the transect
is the sample unit, than this seems to be a form of repeated sampling
that will require a mixed effects model. Second, there is likely to be
spatial autocorrelation in the data. Does including latitude as a
covariate account for this given that we expect a trend in min and max
elevation with latitude? Lastly, any inputs on specifying such a model
with lme is much appreciated. Thanks. Solomon. 

 

 


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