A method to quantify canopy changes using multi-temporal terrestrial lidar data: Tree response to surrounding gaps

- A method for crown dynamic quantification using terrestrial laser scanning data.
- Short time scale response (2 years) of adult tree to the removal of competitors.
- The broadleaved species, Sugar maple, shows a stronger response than the coniferous, Balsam fir.
- A strong downward expansion is observed for both species.
- The method is easily generalizable to other species and at the gap scale.

Branch growth and the resulting crown expansion play many roles in tree and forest processes. Crown expansion is difficult to measure on an adult tree due to the crown's complexity and inaccessibility. The present study proposes a method to quantify vegetation changes over time. It was applied to follow the tree crown response to gap formation for broadleaved (Sugar maple) and coniferous (Balsam fir) species. The method was developed using terrestrial laser scanner (TLS) data. It consists of identifying the vegetation boundaries at time t0 and extracting the new material produced or displaced between time t0 and time tx. Changes in vegetation were quantified with four metrics, and vertical profiles of these metrics were analysed. Results show that Sugar maple has a stronger response to gap formation compared to Balsam fir because of the different crown architecture. Both species showed considerable downward space reoccupation within most of the crown two years after the release of competition. These results are probably a consequence of a mechanical rearrangement of the crown and highlight the importance of analysing canopy changes in both the vertical and horizontal directions. The developed methodology can be applied to both individual trees as well as to a group of trees bordering a gap, in order to get insight on how trees recolonize the space that is freed by local and large scale disturbances.