Leaching and Uptake of Ions Through Above-Ground Norway Spruce Tree Parts

Does canopy nitrogen uptake enhance carbon sequestration by trees?

Temperate forest (15) N isotope trace experiments find nitrogen (N) addition-driven carbon (C) uptake is modest as little additional N is acquired by trees; however, several correlations of ambient N deposition against forest productivity imply a greater effect of atmospheric nitrogen deposition than these studies. We asked whether N deposition experiments adequately represent all processes found in ambient conditions. In particular, experiments typically apply (15) N to directly to forest floors, assuming uptake of nitrogen intercepted by canopies (CNU) is minimal. Additionally, conventional (15) N additions typically trace mineral (15) N additions rather than litter N recycling and may increase total N inputs above ambient levels. To test the importance of CNU and recycled N to tree nutrition, we conducted a mesocosm experiment, applying 54 g N/(15) N ha(-1)  yr(-1) to Sitka spruce saplings. We compared tree and soil (15) N recovery among treatments where enrichment was due to either (1) a (15) N-enriched litter layer, or mineral (15) N additions to (2) the soil or (3) the canopy. We found that 60% of (15) N applied to the canopy was recovered above ground (in needles, stem and branches) while only 21% of (15) N applied to the soil was found in these pools. (15) N recovery from litter was low and highly variable. (15) N partitioning among biomass pools and age classes also differed among treatments, with twice as much (15) N found in woody biomass when deposited on the canopy than soil. Stoichiometrically calculated N effect on C uptake from (15) N applied to the soil, scaled to real-world conditions, was 43 kg C kg N(-1) , similar to manipulation studies. The effect from the canopy treatment was 114 kg C kg N(-1) . Canopy treatments may be critical to accurately represent N deposition in the field and may address the discrepancy between manipulative and correlative studies.

An evaluation of forest decline based on field observations focussed on Norway spruce, Picea abies

Forest decline in Europe is centred around areas where air pollution is heaviest. Although statistical relations are still debatable at the stand level, they are a basis for the discussion of mechanisms by which air pollutants affect forest health. The aetiologies of different syndromes of decline are discussed. Exposure to large concentrations of gaseous pollutants appears to have short-term rather than long-lasting effects, whereas pathogens seem to be of only secondary importance. The deposition of sulphur and nitrogen (nitrate and ammonium) pollutants has significantly modified soil chemistry and plant nutrition. In acidic low-pH soils spruce roots, instead of utilising nitrate, preferentially take up ammonium which interferes with the uptake of other cations, notably magnesium. The nitrate remaining in soil solution, as a result of the preferential uptake of ammonium, is leached together with sulphate to groundwater, accelerating soil acidification and further decreasing the calcium and/or magnesium to aluminium ratios in soil solution. Soil solution chemistry affects root development, and thus water and nutrient uptake. Canopy uptake of nitrogen, especially of ammonium, which is additional to root uptake, may occur and appears to stimulate growth inciting a nitrogen to cation imbalance with the consequential production of decline symptoms.

Causal mechanisms by which sulphate, nitrate and acidity influence frost hardiness in red spruce: review and hypothesis

This paper summarizes results from four experiments in which red spruce seedlings (Picea rubens Sarg.) were exposed to simulated acid mist containing SO₄ ^2- , NH_4- , NO₃ ^- and H⁺ ions. Seedlings were grown in compost, with or without fertilizer, in charcoal filtered air in open-top chambers near Edinburgh, Scotland. Plants were sprayed from bud burst between May and November with mist containing different concentrations and combinations of the four major ions to provide a range of doses, which were applied at different frequencies. Reductions in frost hardiness expressed in terms of the temperature which killed 50% of shoots (LT₅₀ ) were significantly correlated with the dose of S received by the seedlings. Differences in foliar S concentrations between the controls and treated plants were correlated with S dose. Absolute S concentrations were, however, of limited use for predictive purposes. Seedlings appear to be more sensitive than older trees to S toxicity because the former have the greatest proportion of newly expanding needles which optimize conditions for foliar uptake. Seedlings are also least well equipped to export SO₄ ^2- ions since they have a smaller resource of older foliage to supplement their assimilate pool. In conditions which promote uncontrolled SO₄ ^2- ion uptake by foliage, i.e. high external SO₄ ^2- concentrations and incompletely formed cuticles, the potential exists for the internal build up of SO₄ ^2- ions. It is proposed that in the absence of sufficient assimilate and K the presence of these high concentrations of SO₄ ^2- ions in the apoplast or cytosol can lead to protein denaturation and loss of membrane integrity. Reductions in frost hardiness appear to result through direct attack by SO₄ ² ions on membrane proteins which impairs their function. Indirect effects on hardiness occur through both an increased consumption of sugars reducing the'pool'available for cryoprotection and a reduction in photosynthetic function, the ability to produce sugars. The presence of NO_3- N mitigates the toxic effects of SO₄ ² because SO₄ ² ions are consumed in assimilation processes which both utilize and are facilitated by the presence of large amounts of fertilizer N. High concentrations of SO₄ ² and H+ are found to be particularly toxic because of the synergistic effects these ions exert on their mutual uptake with devastating consequences for the control of cellular pH. Trees growing at high altitude sites are likely to be particularly sensitive to SO₄ ^2- toxicity because (1) their carbon balance is low, (2) cuticle development is poor and (3) levels of soil available Ca² tend to be low relative to Al³⁺ so that membranes may already be weakened as a result of insufficient Ca²⁺ ions for protein bridging.

Influence of acidic mist on frost hardiness and nutrient concentrations in red spruce seedlings: 2. Effects of misting frequency and rainfall exclusion

Two-year-old red spruce (Picea rubens Sarg.) of Pittston provenance and 3-yr-old plants of Chatham provenance were exposed to acid mist in replicated open-top chambers supplied with charcoal-filtered air near Edinburgh, Scotland. Plants of Chatham provenance had already been exposed to acid mist throughout the previous growing season. The plants were exposed to mist, equivalent to 4 mm rainfall per week, containing an equimolar mixture of sulphuric acid and ammonium nitrate at pH 2.5 or pH 5.0 (1.6 or 0.01 mol m³ ) from May to November. This weekly dose was delivered at a low frequency (2 mm twice a week), or high frequency (1 mm on 4 consecutive days each week) to chambers fitted with ceilings to exclude rain. The low frequency dose was also applied to chambers without ceilings, to examine the effect of natural washing by rain. Frost hardiness, estimated by exposing detached shoots to controlled freezing and then measuring rates of electrolyte leakage, was determined during the misting period at the end of October and in December. Foliar nutrient concentrations were measured during the dormant period after treatment had ceased. At the end of October, plants which had received acid mist were less frost hardy than plants receiving mist at pH 5. The temperature causing 50% shoot death (LT₅₀ ) increased by 6 °C for low frequency application, and by 10 °C at high frequency, relative to the plants receiving mist at pH 5. Exclusion of ambient rainfall had no detectable effect on the frost hardiness response to acid mist. In December, 3 wk after the cessation of misting, all plants were more frost hardy than in October. Significant effects of the acid mist treatment could no longer be detected. Differences in nutrient concentrations were small among treatments, although K⁺ concentrations in the low frequency treatment with acid mist with rain exclusion were 50 % below those in other treatments. Ca concentrations were 50% larger in the acid mist treatment with rain exclusion than without. The data suggested enhanced sulphate uptake resulting from increasing the frequency of exposure, but the increase was not significant. There was no clear relationship between the pattern of frost hardiness and nutrient concentrations except for S, which was 30% smaller in the control plants (pH 5) than in the high frequency pH 2.5 treatments. It is concluded that excluding rainfall, an experimental artifact introduced in evaluating effects of acid mist, does not influence the frost hardiness response of red spruce seedlings. The much greater effect of exposure to the same dose at double the frequency suggests that such experiments may underestimate effects in the field, if those trees are exposed to more frequent episodes of polluted cloud water than experimental plants.