Effects of understory light conditions and intraspecific competition on development of established natural regeneration in a mixed multiaged forest of Italian Eastern Alps

The current trend of silviculture aiming to manage forest as complex forest ecosystems, justified the attention researchers have devoted over the years to the characterization of light below canopy cover. Explicit measurements of light gradients allowed to study the interactions between different canopy structures (e.g. heterogeneous vs homogeneous) and light transmittance providing managers with crucial information on how to control understory light availability by manipulating overstory and predict successional dynamics, such as natural regeneration processes. On the other hand, the presence of natural regeneration represents one of the main conditions for promoting natural evolutionary process, providing continuity, enhancing resilience and resistance of forest ecosystems and maintaining heterogeneous conditions in terms of structure and light availability. The objective of this P.h.D thesis is to improve the current knowledge on how the light affects two major processes involved in the natural regeneration of mixed multi-age forests (competition among recruited trees and their growth) as a function of structural attributes of the stands. A mixed silver fir (Abies alba Mill.) and Norway spruce (Picea abies L.) forest located in the mountain belt of eastern Italian Alps was used as a study case. Both species were considered and their behavior compared. Relationships between overstory canopy structure and radiance transmitted in the understory were also analyzed. The following questions were addressed: i) which structural attributes mainly affect the light availability in the understory of a multi-aged mixed forest? ii) which understory light levels allow the coexistence of natural regeneration of Norway spruce and silver fir? iii) do the two species use the same growth strategies to access to the available light resource or do silver fir and Norway spruce seedlings differ in morphological responses to low light availability? Results suggested that: i) radiation below canopy in heterogeneous stands with continuous canopy cover depends more on the varying dendrometric attributes within the stands than their mean values. Vertical structure exerts a clear effect on transmission of radiance. In particular, crown size characteristics, such as crown length (CL), crown base (CB) and height, and in particular their standard deviations are good proxy for light transmission, even more in mixed species forest stands. iii) and iv) significant differences in terms of light availability characterize the site where the seedlings of the two species grow. Levels of radiance ranging from 20 to 30% of total transmittance allow the coexistence of fir and spruce seedlings. At those light conditions competition stimulate species to develop different growth strategies. While silver fir take advantage in terms of apical growth, increasing in light availability promote more effects in terms of radial growth in spruce. Neither light availability nor the competition indexes applyed significantly affected the apical dominance ratio (ADR) and the live crown ratio (LCR) of the seedlings. A careful overstory trees selection or a fine adjustement of light permeability of overstory is of particular importance for silver fir and Norway spruce regenerating in the understory of heterogeneous mixed forest. Both species apparently add to shade tolerance a rather good reactivity to light condition change.