documentclass{article}

usepackage{amsmath}
usepackage[showframe]{geometry}
usepackage{qtree}

begin{document}

Tree [.A B C [.D E ] ]
Tree [.F [.G H ] I ]
Tree [.J [.K [.L M M ] N ] O ]
Tree [.F [.G H ] I ]

end{document}

documentclass{article}

usepackage{amsmath}
usepackage[showframe]{geometry}
usepackage{tikz}
usepackage{tikz-qtree}

begin{document}

begin{center}
begin{tikzpicture}
    Tree [.A B C [.D E ] ]
end{tikzpicture}
begin{tikzpicture}
    Tree [.F [.G H ] I ]
end{tikzpicture}
begin{tikzpicture}
    Tree [.J [.K [.L M M ] N ] O ]
end{tikzpicture}
begin{tikzpicture}
    Tree [.F [.G H ] I ]
end{tikzpicture}
end{center}

end{document}

tikzset{align at top/.style={baseline=(current bounding box.north)}}

Tree [.A B C [.D E ] ]
Tree [.F [.G H ] I ]
Tree [.J [.K [.L M M ] N ] O ]
Tree [.F [.G H ] I ]

newenvironment{spreadTrees}{%
    expandafterdefexpandafterpgfpictureexpandafter{expandafterhfillpgfpicture}%
    center}{hspace*{fill}parendcenter}

newenvironment{spreadTrees*}{%
    %noindent % (if not only used inside floats)
    expandafterdefexpandafterpgfpictureexpandafter{expandafterhfillpgfpicture}%
}{hspace*{fill}}

documentclass{article}
usepackage[showframe]{geometry}
usepackage{tikz}
usepackage{tikz-qtree}

newenvironment{spreadTrees}{%
    expandafterdefexpandafterpgfpictureexpandafter{expandafterhfillpgfpicture}%
    center}{hfillnullendcenter}
newenvironment{spreadTrees*}{%
    %noindent % (if not only used inside floats)
    expandafterdefexpandafterpgfpictureexpandafter{expandafterhfillpgfpicture}%
}{hfillnull}

begin{document}%
begin{spreadTrees}
Tree [.A B C [.D E ] ]
Tree [.F [.G H ] I ]
Tree [.J [.K [.L M M ] N ] O ]
Tree [.F [.G H ] I ]
end{spreadTrees}

hrule

begin{figure}[h]
begin{spreadTrees*}
Tree [.A B C [.D E ] ]
Tree [.F [.G H ] I ]
Tree [.J [.K [.L M M ] N ] O ]
Tree [.F [.G H ] I ]
end{spreadTrees*}
caption{(Figure above)}
end{figure}
end{document}

documentclass{article}

usepackage{amsmath}
usepackage[showframe]{geometry}
usepackage{tikz}
usepackage{tikz-qtree}

begin{document}


begin{center}

hello\[-baselineskip]
raisebox{-height}{begin{tikzpicture}
    Tree [.A B C [.D E ] ]
end{tikzpicture}}
raisebox{-height}{begin{tikzpicture}
    Tree [.F [.G H ] I ]
end{tikzpicture}}
raisebox{-height}{begin{tikzpicture}
    Tree [.J [.K [.L M M ] N ] O ]
end{tikzpicture}}
raisebox{-height}{begin{tikzpicture}
    Tree [.F [.G H ] I ]
end{tikzpicture}}\
world
end{center}

end{document}