= Solution
Under the fitted <compound-symmetry covariance> model, an orchard mean based on $k$ trees has <variance>
$$
v_k=c+\frac ak.
$$
With the current estimates this falls from $v_{30}=2+180/30=8$ to $v_{45}=2+180/45=6$. The numbers of orchards contributing to each marginal remain unchanged, so both marginal <treatment-contrast variances> fall by a factor $6/8=3/4$: pruning differences have estimated <variance> $3$ and the spray difference has estimated <variance> $2$.
\b[Adding trees gives a predicted 25% reduction in these <variances>, but no extra independent orchard <replication>.] It requires 50% more tree measurements and leaves six orchard residual <statistical degrees of freedom>. Increasing $k$ reduces the $a/k$ term but cannot remove the shared orchard component $c$, so gains eventually diminish.
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