In forested ecosystems in high northern latitudes, climate warming may influence establishment and survival of tree seedlings and alter forest composition, especially at species range margins. Seedling survival depends on maintaining a positive carbon balance through adequate carbon assimilation and allocation to growth, defense, storage, and metabolic maintenance. In the shaded forest understory, plants operate closer to their compensation points and may be particularly sensitive to changing temperature conditions. Research suggests that low-light survival mechanisms include maintaining low dark respiration (Rd) rates and high stored carbon reserves; both parameters are subject to change under elevated temperature conditions. We conducted a greenhouse experiment to examine survival and non-structural carbohydrate storage and use in low light under two temperature environments (25oC/30oC). Study species included three temperate (Pinus strobus, Quercus macrocarpa and Acer saccharum) and three boreal tree species (Populus tremuloides, Pinus banksiana and Abies balsamea). After germinating and growing seedlings in full light conditions, we imposed a light treatment whereby seedlings were shaded below their light compensation points. We hypothesized that, after shading, seedling survival would depend on total non-structural carbohydrate (TNC) reserves and the ability to down-regulate respiratory carbon use. We expected that TNC reserves would decline through time, as the seedlings relied on TNCs instead of new assimilate to fuel metabolic processes. We hypothesized that plants in 25oC would survive longer in cooler temperatures due to lower Rd rates, independent of the ability to down-regulate respiration.
Results/Conclusions
All species survived significantly longer (P<.0001) in the cooler environment. Pinus banksiana survived 57 days on average in 25oC, but only 36 days in 30oC and A. saccharum survived 96 days on average in 25oC, but only 38 days in 30oC. After shading, Rd declined in both temperatures, but remained higher in 30oC across species. Longer survival was related to higher TNC reserves and temperature. The most shade tolerant species had the highest stored reserves and survived the longest, and survived longer in 25oC than in 30oC. TNCs declined through time for all species after shading, supporting the hypothesis that seedlings rely on stored carbon reserves as a low-light survival mechanism. Our data suggest the compounded stresses of elevated temperature and low light will decrease the shade tolerance of understory tree seedlings.