Aerosols have always been part of the atmosphere, and plant surfaces are a major aerosol sink. Given the nutrient content of aerosols and the natural stability of aerosol concentrations over evolutionary time, plants may have developed adaptations to aerosol input. Although little is known about such adaptations, leaf surface micro-roughness appears to play a key role. This review focuses on the deposition and fate of fine aerosols that are less than 2.5 μm in diameter. Most of these aerosols are hygroscopic, and they are often deliquescent (liquid) on transpiring leaves. Such concentrated solutions may be taken up by both the cuticle and stomata, contradicting previous concepts. The establishment of a continuous liquid water connection along stomatal walls affects individual stomata and is a new concept called “hydraulic activation of stomata” (HAS). HAS enables the efficient bidirectional transport of water and solutes between the leaf interior and leaf surface and makes stomatal transpiration partly in...

Hygroscopic particles on leaves: nutrients or desiccants?

A declining, closed-canopy Picea abies (L.) Karst. stand produced as much crown biomass as a healthy stand, although some trees were chlorotic due to magnesium deficiency. The production of wood per unit of leaf area in both stands was related to the foliar magnesium concentration. Although leaf area index and climate were similar at both sites, stemwood production was 35% lower in the declining than in the healthy stand. Nutritional disharmony, rather than a deficiency in a single element, was identified as the mechanism for reduced tree vigor. The role of nutrient stress in forest decline was detected by partitioning the season into three periods reflecting different phenological stages: a canopy growth period in spring, a stem growth period in summer, and a recharge period during the non-growing season. Needle growth was associated with nitrogen supply. Most of the magnesium supply required to meet the demand for foliage growth was retranslocated from mature needles. Magnesium retranslocation was related to concentration of nitrogen and magnesium in those needles before bud break. Retranslocation from mature needles during the phase of canopy production resulted in chlorosis in initially green needles if the magnesium concentration before bud break was low. Nitrogen concentration in 0-year-old needles generally remained constant with increasing supply, indicating that foliage growth was restricted by the supply of nitrogen. In contrast, magnesium concentration generally increased with supply, indicating that magnesium supply for needle growth was sufficient. Much of the magnesium required for wood production was taken up from the soil because stored magnesium was largely used for canopy growth. Uptake at the declining site was probably limited because of restricted root expansion and lower soil magnesium compared to the healthy site. For this reason only wood growth was reduced at the declining site. Because the recharge of magnesium during the non-growing period is dependent on uptake from the soil, it was more limited at the declining that at the healthy stand. However, as nitrogen uptake from the atmosphere may account for an appreciable proportion of the total uptake, and as its supply in the soil at both sites was similar, an unbalanced recharge of nitrogen and magnesium may have occurred at the declining site. If mature needles are unable to recharge with magnesium in proportion to the uptake of nitrogen, chlorosis is likely to occur during the next canopy growth period.

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Performance of two Picea abies (L.) Karst. stands at different stages of decline : VII. Nutrient relations and growth.

Sample preparation (cleaning, drying, homogenization) for trace element analysis in plant matrices

Biomonitoring is a method of observing the impact of external factors on ecosystems and their development over a long period, or of ascertaining differences between one location and another. A monitor suitable for the subject of the investigation must be chosen, and the comparability of the element concentrations thus determined (both within the investigation and with other studies) must be ensured by adhering to precise sampling guidelines. In addition to the small amount of equipment required, the advantages of biomonitoring lie in the ecosystem approach that permits conclusions going beyond the biomonitor itself. Attention has to be given to the factors that may affect element concentrations and which can lead to incorrect statements if ignored.

The use of bioindicators for monitoring the heavy-metal status of the environment

During the summer of 1986, three year-classes of foliage were sampled from approximately 30-year-old Norway spruce [Picea abies (L.) Karst.] trees at 12 sites from S. W. Germany to N. Scotland. At sites in Germany, where trees were showing symptoms of ‘decline’, samples were taken from trees with ‘good’ crown condition and ‘poor’ crown conditon. The distinction between ‘good’ and ‘poor’ was made on the basis of international protocols for defining crown density and foliar discoloration. There was a wide range in nutrient content (percent dry weight) in apparently healthy trees. Current year foliage had ranges of mean values per site: S(0.07–0.13%), N(0.9–1.4%), K(0.5–0.9%), Ca(0.2–0.7%), and Mg (0.05–0.1%). Ranges were greater for 2-year-old foliage: S(0.09–0.18%), N(1.0–1.8%), K(0.4–0.7%), Ca(0.2–1.4%), and Mg(0.03–0.09%). At sites with trees having ‘poor’ crown condition, there were significantly smaller concentrations of Mg and Ca, and larger concentrations of K in 2-year-old foliage from ‘poor’ trees, compared with adjacent ‘good’ trees. Ratios of nutrient content were more significantly related to crown condition within sites than individual nutrients, especially in older needles. ‘Poor’ crowns were associated with larger ratios of N∶Mg, K∶Mg, S∶Mg, K∶Ca and smaller ratios of S∶K and N∶K. A ‘risk index’ is defined for trees showing no visible ‘decline’ symptoms, based upon nutrient content and nutrient ratios, which may be useful in identifying sites liable to experience deterioration in crown condition.

The nutritional status of Picea abies (L.) Karst. across Europe, and implications for ‘forest decline’

Eighteen major and trace elements (Al, As, Ba, Br, Ca, Cl, Co, Cu, Fe, K, Mg, Mn, Na, Sb, Sc, Sr, V, Zn) have been determined in needles ofPicea abies by neutron activation analysis. Trees from 31 sites with different characteristics were investigated. The sampled area was 140 km2 and centered around the city of Winterthur, Switzerland. The effect of washing the needles before analysis was investigated. Washing was found to be essential for a meaningful determination of most elements. The values of the individual elements could be approximated by normal or by lognormal distributions. The width of these distributions varies greatly, being 14% for Cu and a factor of 5 for Mn. Many elements show highly significant positive or negative correlations. Whereas the levels of the major elements as determined here are in the generally accepted range, the values for most of the minor elements show very poor correspondence with published values. For some elements the present data seem to be the only available values. Distinct environmental influences were only manifest for Na, Cl, Br, and these elements show very high values at sites bordering highways.

Th. Keller

Papers

Major and trace element concentrations in needles of Picea abies: levels, distribution functions, correlations and environmental influences