Physiological ecology of understorey trees in low impact silvicultural systems
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Date
2009Author
Bertin, Sophie
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Abstract
Continuous cover forestry (CCF), an alternative forest management approach
to clearfelling, is increasingly being adopted in the UK. It aims at enhancing
stand structural diversity and favouring natural regeneration and subsequent
seedling growth below the existing canopy of plantation forests. One area of
limited knowledge is the critical level of below-canopy light for the growth of
naturally regenerating seedlings. In addition, plant growth beneath canopies is
influenced by other factors (e.g. herbivory). Picea sitchensis (Bong.) Carr. (Sitka
spruce) seedlings under canopies have been observed to be severely damaged
by Elatobium abietinum (Walker) (green spruce aphid) attacks. The combined
effects of light availability and insect attack on seedling growth are not well
understood, however, this understanding is crucial in order to ensure successful
management of regeneration within CCF systems.
A controlled experiment, which mimicked different natural understorey light
levels along with repeated artificial aphid infestation, was conducted over two
years to look at seedling performance through structural and physiological
(chlorophyll fluorescence) measurements. Aphid population assessments showed
significant increased population density under shaded conditions. Nevertheless,
aphid impacts were mainly localised in extent while the impact of light was
the major component that described seedling growth. Light was the primary
factor affecting the whole-plant biomass, whereas aphids had only localised effects
on the total dry weight of older needles and roots, and on leader extension
growth. A significant interaction between light levels and presence/absence of
aphid infestation was found for main leader extension growth of the seedlings
during the second year of the experiment, with lower values at low light levels
under infestation. Plant biomass allocation was affected strongly by light, while
aphid presence did not result in significant changes. At the leaf physiologylevel,
the light environment was found to be the main driving factor affecting
photosynthetic response, whilst aphid presence had only a short-term localised
effect on photosynthesis.
The impact of light levels and aphid presence on seedling growth were also
determined at an experimental field site where plots were located across two
light regimes typical of CCF conditions in upland UK coniferous forests. The
comparison of the photosynthetic response of the seedlings in both the controlled
and field experiment highlighted the importance of considering the temporal
heterogeneity of the light environment experienced by understorey seedlings in
CCF stands, while aphid and aphid x light interaction effects could not be
determined due to very low aphid levels during the year.
Finally, a pre-existing light model was parameterised to predict the understorey
light environment required to promote successful seedling growth in CCF
stands. Sensitivity and validation analyses were also performed.