  The transient dimensional changes during \textit{hygro}-expansion and
\textit{hydro}-expansion of freely and restrained dried, softwood and hardwood
sheets and fibers is monitored, to unravel the governing micro-mechanisms
occurring during gradual water saturation. The response of individual fibers is
measured using a full-field global digital height correlation method, which has
been extended to monitor the transient \textit{hydro}-expansion of fibers from
dry to fully saturated. The \textit{hygro}- and \textit{hydro}-expansion is
larger for freely versus restrained dried and softwood versus hardwood
handsheets. The transient sheet-scale \textit{hydro}-expansion reveals a sudden
strain and moisture content step. It is postulated that the driving mechanism
is the moisture-induced softening of the so-called "dislocated regions" in the
fiber's cellulose micro-fibrils, unlocking further fiber swelling. The strain
step is negligible for restrained dried handsheets, which is attributed to the
"dislocated cellulose regions" being locked in their stretched configuration
during restrained drying, which is supported by the single fiber
\textit{hydro}-expansion measurements. Finally, an inter-fiber bond model is
exploited and adapted to predict the sheet-scale \textit{hygro}-expansion from
the fiber level characteristics. The model correctly predicts the qualitative
differences between freely versus restrained dried and softwood versus hardwood
handsheets, yet, its simplified geometry does not allow for more quantitative
predictions of the sheet-scale \textit{hydro}-expansion.
