Physiological ecology of understorey trees in low impact silvicultural systems
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.