Help! Is this a mixed model ANCOVA?

#1
Hi, I could really use some help.

I am conducting a study looking at the different growth rate of two species of trees (100 of each) randomly places in a 10km^2 urban area. Their average growth rate is recorded. In addition to the species, the locals water the individual plants (in addition to rainwater) and it is recorded. Therefore I am going to use ANCOVA looking at the if there is a difference in growth rate between the two species, accounting for the effect of water (covariate). What I am confused about is whether water is a random or fixed effect? Can someone please help me??
 
#2
im voting for fixed. That would be standard ANCOVA model anyway. If you made the effect of water random you would have a 'random slope' wouldn't you? I don't think that is productive in this case,

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