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

Stem growth reduction in mature Sitka spruce trees exposed to acid mist

An eighteen-year-old clone of Sitka spruce (Picea sitchensis (Bong.) Carr) growing in the field was used to evaluate the whole tree response of 'mature' Sitka spruce to acid mist treatment. The mist, an equimolar mixture of H(2)SO(4) and NH(4)NO(3) at pH 2.5 with or without particles (soda glass ballotini < 20 microm diameter), was applied twice weekly (equivalent to 4 mm precipitation week(-1)) throughout the growing season, May-November 1990-1992. The annual dose of S, N, H applied as mist (at 51, 48 and 3.3 kg ha(-1), respectively) was 2.5 times that measured in the Scottish uplands. Throughout the experiment there was no evidence of visible injury symptoms, yet there was a highly significant reduction (p < 0.02) in the stem-area increment relative to the stem area at the start, measured using vernier dendrometer bands. There was no significant difference between the (acid mist + particle) and the acid mist only treatments. The mean relative stem-area increment over two complete growing seasons (1991-1992) was 65% for control trees, but only 53% for acid-misted trees.

Effects of acid rain on growth and nutrient concentrations in Scots pine and Norway spruce seedlings grown in a nutrient-rich soil

The effects of artificially applied acid precipitation on growth and nutrient concentrations of Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies [L.] Karst.) seedlings were investigated in a long-term acid irrigation experiment in field conditions. Seedlings of northern and southern origin were planted in boxes containing peat and composted soil rich in nutrients, and sprinkler irrigated with water acidified with nitric and sulphuric acids to pH 3 or pH 4 for periods varying from two to three and a half growing seasons during 1986-1989. Water irrigated (pH 5.4-7.6) and non-irrigated groups of seedlings were also included in the experiment. At the end of the experiment needles, main and lateral shoots and roots were collected from the seedlings for the determination of height growth and biomass partitioning, and for the analysis of S, N, Mg, P, K, Ca, Mn and Fe concentrations. The treatment effects compared to the irrigated control were studied using multivariate analyses of variance and covariance. In the pine seedlings the total dry matter production increased by 25-70% compared with the irrigated controls when the total wet deposition to the seedlings exceeded 67 kg S ha(-1) and 36 kg N ha(-1) (e.g. after two growing seasons' exposure of the pH 3 treatment). The increase was mainly due to an increase in needle dry weight (54-72% greater at pH 3) and root weight (20-65% greater at pH 3), whereas the height growth or shoot weight growth were less affected. The northern provenance pine seedlings responded more clearly to the pH 3 irrigation than the southern ones. The treatments had no consistent effects on any of the growth variables studied in the spruce seedlings, however. The pines had higher root and foliage Ca concentrations as a result of the acid irrigation, whereas in spruce, acid rain decreased the Ca concentration in needles and shoots. Root Mn and Fe concentrations were higher in both species as a result of the pH 3 treatment. A higher soil conductivity and Ca concentration resulted from the prolonged pH 3 treatment. The results strongly support the hypothesis that the long-term growth and nutrient allocation response of conifers to acid precipitation is dependent both on the tree species and on the nutritional status of the soil.

Effects of acid mist on needles from mature Sitka spruce grafts. Part II. Influence of developmental stage, age and needle morphology on visible damage

Mature grafts of five Sitka spruce (Picea sitchensis (Bong.) Carr.) clones were exposed to simulated acid mist comprising an equimolar mixture of H(2)SO(4) and NH(4)NO(3) (1.6 and 0.01 mol m(-3)) at pH 2.5 and 5.0. Mist was applied to potted plants growing in open-top chambers on consecutive days, four times a week, at a precipitation equivalent of 1 mm per day. The total exposure to polluted mist was equivalent to three times that measured at an upland forest in SE Scotland. The aim of the experiment was to characterize the response of juvenile foliage produced by physiologically mature grafts (on seedling root stock) and compare it with the behaviour of juvenile foliage on seedlings. Development of visible foliar damage was followed through the growing season. Measurements of needle length, diameter, weight, surface area, surface was weight and wettability were made on current year needles to determine whether particular foliar characteristics increased susceptibility to injury. Significant amounts (> 10%) of visible needle damage was observed on only one of the five clones. Damage was most severe on the clone with the most horizontal branch and needle habit, but over the five clones there was no relationship between angle of branch display and damage. Likewise no combination of needle characteristics (length, width, area, amount of wax) was indicative of potential susceptibility. A comparison with previous acid misting experiments using seedlings suggests that juvenile foliage on physiologically mature trees is equally susceptible to visible injury as juvenile seedling foliage. Data of budburst differed among clones, and in this experiment exerted the over-riding influence on development of injury symptoms. Foliage exposed to a combination of strong acidity and high sulphate concentrations over the few weeks immediately following budburst suffered most visible injury. The absence of significant amounts of visible damage in UK forests probably reflects the general low susceptibility to visible injury of Sitka spruce exposed to acid mist.

Exposure of red spruce seedlings to acid mist: importance of droplet composition just prior to drying periods

Two-year-old red spruce (Picea rubens Sarg.), raised from seed collected at five locations from Maine to North Carolina, were exposed in a greenhouse to either sulphuric or nitric acid mist (median pH 2-85). Exposures were performed three times a day, four days a week over an eleven-week period during the growing season. Each exposure consisted of 4 h of misting followed by a 3.75 h drying period. Immediately before each drying period, acidic droplets were either left to dry directly on the foliage or were removed by a 15 min deionized water (diw) rinse. When sulphuric acid droplets dried directly on the foliage, visible injury developed and to a lesser degree the same was true for nitric acid. When acidic droplets were replaced with dim before each drying period, foliar injury did not develop despite misting for hundreds of hours at pH values well within the range that has caused visible injury in previous experiments. When acidic droplets dried directly on foliage, needle dry weight was reduced, but differences between sulphuric and nitric acid exposure were not found. No treatment effects were found on terminal shoot length. Treatment responses among seedlings from different seed resources were similar, with greater variation found within provenances than between provenances. We conclude that the composition of liquid on foliage at the end of wet deposition events and the processes occurring during drying periods are important factors in determining the response of red spruce to acidic cloudwater.

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.