Needs["PlotLegends`"]

b1BlSmall = {4.17891*^-1, 5.59816*^-1, 7.09626*^-1, 4.17891*^-1};
b1BlLarge = {3.15389*^-1, 4.73084*^-1, 6.85972*^-1, 3.62698*^-1};
b2BlSmall = {9.74949*^-1, 9.74949*^-1, 9.74949*^-1, 9.74949*^-1};
b2BlLarge = {9.74949*^-1, 9.74949*^-1, 9.74949*^-1, 2.00000};

C2B = 2.0
pftColor = {Green, Darker[Green], Darker[Darker[Green]], Magenta};

size2bl[dbh_,ipft_,a_:1,b_:1,c_:1]:=If[ipft!=4,[Piecewise] b1BlSmall[[ipft]]/C2B*dbh^b2BlSmall[[ipft]] dbh<dbhAdult[[ipft]]
b1BlLarge[[ipft]]/C2B*dbh^b2BlLarge[[ipft]] True

,[Piecewise]   (a*b1BlSmall[[ipft]])/C2B*dbh^(b*b2BlSmall[[ipft]]) dbh<dbhAdult[[ipft]]
(a*b1BlLarge[[ipft]])/C2B*dbh^(c*b2BlLarge[[ipft]]) True
];

putzbl[dbh_] := (0.0856*dbh^2 - 0.376)/C2B;

Manipulate[
Grid[{{"short range","long range"},{
Plot[{putzbl[dbh],Evaluate@Table[size2bl[dbh,ipft,a,b,c],{ipft,1,4}]},{dbh,0.0,11.0},PlotStyle->{Black,pftColor}],
Plot[{putzbl[dbh],Evaluate@Table[size2bl[dbh,ipft,a,b,c],{ipft,1,4}]},{dbh,0.0,90.0},PlotStyle->{Black,pftColor}]}}],
{{a,1.},0.8,1.2,Appearance->"Labeled"},{{b,1.},0.8,1.2,Appearance->"Labeled"},{{c,1.},0.8,1.2,Appearance->"Labeled"},FrameLabel->{{None,None},{None,"Leaf Biomass(DBH)"}}]