Growth and physiological responses of canopy tree species to selection harvests in a northern hardwood forest

Description

The basal area increment (BAI) response of trees to gap creation was generally positive; however, there was a lag in BAI response which typically lasted between one and three years. Tree size was the best predictor of both pre-harvest BAI and BAI response to gap creation. Large trees had the greatest BAI but did not respond as positively to gap creation as smaller trees. Unexpectedly, it was observed that the BAI response of the more shade tolerant species was more positive than the less tolerant species. This may be related to species differences in canopy structure, with the deep crowns of shade tolerant species providing increased light interception following gap creation.The management of tolerant hardwood forests in central Ontario is typically done using the selection harvest system where no more than one third of the stand basal area is removed in each harvest while a balanced distribution of tree sizes and species is maintained to ensure continual stand development. Because regeneration is achieved through the manipulation of the understory light environment, the response of tree seedlings and saplings to changing light availability has been well studied. However, little is known about either the physiological acclimation processes or the growth responses of canopy trees to gap creation. This thesis examines the physiological acclimation and growth responses of large trees to selection management and addresses the factors that determine the temporal pattern and magnitude of the growth response.In order to better understand growth responses of large trees to gap creation an experiment examining canopy physiological and morphological acclimation to gap creation was conducted. Leaves within the lower portions of the canopy experienced gradual increases in area-based maximum photosynthetic rates, stomatal conductance, and leaf nitrogen. No change in these variables was observed that could not be accounted for by changes in leaf mass per unit area. Both the growth and physiological studies point to the importance of increases in incident light in the lower portions of the canopy, in conjunction with slow acclimation responses, as drivers of whole tree carbon gain and growth following selection harvests.

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