  Young stellar clusters are believed to inherit the spatial distribution like
hierarchical structures of their natal molecular cloud during their formation.
However, the change of the structures between the cloud and the young clusters
is not well constrained observationally. We select the W40 - Serpens South
region (~ 7 $\times$ 9 pc$^{2}$) of the Aquila Rift as a testbed and
investigate hierarchical properties of spatial distribution of young stellar
objects (YSOs) in this region. We develop a minimum spanning tree (MST)-based
method to group stars into several levels by successively cutting down edges
longer than an algorithmically determined critical value. A total of 832 YSOs
are divided into 5 levels with 23 groups. For describing the hierarchical
properties in a controlled way, we construct a set of synthetic source
distributions at various fractal dimensions, and apply the same method to
explore their group characters. By comparing the $Q$ parameter and the surface
density profiles of the observed and the synthetic data, we find that the YSO
observation matches spatial patterns from multi-fractal dimensions. In the
periphery region where the molecular clouds are more diffuse, the YSO structure
is close to a fractal dimension of 2.0, while in the core regions the fractal
dimensions are close to 1.6 and 1.4 for the W40 and the Serpens South regions,
respectively. Therefore, the YSOs may inherit the fractal pattern of the dense
part of the molecular clouds, but such pattern dissipates slowly in several
Myr.
