  Spatial association of clumps from different tracers turns out to be a
valuable tool to determine the physical properties of molecular clouds. It
provides a reliable estimate for the $X$-factors, serves to trace the density
of clumps seen in column densities only and allows to measure the velocity
dispersion of clumps identified in dust emission. We study the spatial
association between clump populations, extracted by use of the GAUSSCLUMPS
technique from $^{12}$CO (1-0), $^{13}$CO (1-0) line maps and Herschel
dust-emission maps of the star-forming region Rosette, and analyse their
physical properties. All CO clumps that overlap with another CO or dust
counterpart are found to be gravitationally bound and located in the massive
star-forming filaments of the molecular cloud. They obey a single mass-size
relation $M_{\rm cl}\propto R_{\rm cl}^\gamma$ with $\gamma\simeq3$ (implying
constant mean density) and display virtually no velocity-size relation. We
interpret their population as low-density structures formed through compression
by converging flows and still not evolved under the influence of self-gravity.
The high-mass parts of their clump mass functions are fitted by a power law
${\rm d}N_{\rm cl}/{\rm d}\,\log M_{\rm cl}\propto M_{\rm cl}^{\Gamma}$ and
display a nearly Salpeter slope $\Gamma\sim-1.3$. On the other hand, clumps
extracted from the dust-emission map exhibit a shallower mass-size relation
with $\gamma=2.5$ and mass functions with very steep slopes $\Gamma\sim-2.3$
even if associated with CO clumps. They trace density peaks of the associated
CO clumps at scales of a few tenths of pc where no single density scaling law
should be expected.
