removing intercept from lm() results in oddly high Rsquared

Can anyone help me understand why an lm model summary would return an 
r.squared of ~0.18 with an intercept term, and an r.squared of ~0.98 
without the intercept?   The fit is NOT that much better, according to 
plot.lm: residuals are similar between the two models, and a plot of 
observed x predicted is almost identical.

Thanks,

-Joseph

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Joseph P. LeBouton
Forest Ecology PhD Candidate
Department of Forestry
Michigan State University
East Lansing, Michigan 48824

Office phone: 517-355-7744
email: lebouton at msu.edu