The content of organic N has been shown in many studies to increase during the passage of rain water through forest canopies. The source of this organic N is unknown, but generally assumed to come ...from canopy processing of wet or dry-deposited inorganic N. There have been very few experimental studies in the field to address the canopy formation or loss of organic N. We report two studies: a Scots pine canopy exposed to ammonia gas, and a Sitka spruce canopy exposed to ammonium and nitrate as wet deposition. In both cases, organic N deposition in throughfall was increased, but only represented a small fraction (<10%) of the additional inorganic N supplied, suggesting a limited capacity for net organic N production, similar in both conifer canopies under Scottish summertime conditions, of less than 1.6
mmol
N
m
−2
mth
−1 (equivalent to 3
kg
N
ha
−1
y
−1).
Experimental addition of inorganic N to conifer canopies shows limited production of organic N in throughfall.
The effects of ozone and other photochemical oxidants on individual trees have been studied for several decades, but there has been much less research on the potential effects on entire forest ...ecosystems. Given that ozone and other oxidants affect the production and subsequent fate of biogenic volatile organic compounds that act as signalling molecules, there is a need for more detailed study of the role of oxidants in modifying trophic interactions in forests. Deposition of fine particulates to forests may act as a source of nutrients, but also changes leaf surface properties, increasing the duration of surface wetness and modifying the habitat for epiphytic organisms, leading to increased risks from pathogens. Even where this pathway contributes a relatively small input of nutrients to forests, the indirect effects on canopy processes and subsequent deposition to the forest floor in throughfall and litter may play a more important role that has yet to be fully investigated.
Secondary air pollutants may have indirect effects on forest ecology that have not yet been fully explored.
Vehicular emissions of NO
x
and NH
3 result in elevated concentrations of nitrogen at roadside verges. To determine the extent that vehicular nitrogen emissions, disturbance and management affect the ...vegetation composition of road verges, a survey of 92 verges in Scotland was carried out with sites stratified by background nitrogen deposition and road type. NO
x
and NH
3 concentrations were monitored at 15 key sites for a year, and showed a decreasing gradient with increasing distance from the road. Ellenberg fertility indices of the vegetation communities also showed a general decrease with increasing distance from the road, but there was no straightforward correlation with NO
x
and NH
3 air concentrations between sites. Cover of bare ground, ruderal species and salt-tolerant species were highest at the verge edge. The proximity of the verge to traffic is important both in terms of NO
x
and NH
3 gradients, but also for deposited salt, grit and physical disturbance.
NO
x
, NH
3 and road verge vegetation Ellenberg fertility indices decline with distance from traffic.
Passive diffusion tubes have been widely used in Europe for spatial and temporal measurement of NO
2
concentrations. The method is cheap, simple, and provides concentration data in most circumstances ...that are sufficiently accurate for assessing exposure and compliance with air quality criteria. Tube-type diffusion samplers are prone to several sources of uncertainty, arising from the materials of construction, the absorbent used, the methods of preparation, the details of their deployment (including the exposure time) and the analytical methods used to establish the concentration of nitrite ion absorbed. This review considers the major sources of uncertainty, and reports on the many experiments aimed at identifying and minimizing uncertainties, including modifications to the simple open tube devices that were originally developed in the 1970s.
global exposure of forests to air pollutants Fowler, D; Cape, J.N; Coyle, M ...
Water, air and soil pollution,
11/1999, Letnik:
116, Številka:
1/2
Journal Article, Conference Proceeding
Recenzirano
The tall, aerodynamically rough surfaces of forests provide for the efficient exchange of heat and momentum between terrestrial surfaces and the atmosphere. The same properties of forests also ...provide for large potential rates of deposition of pollutant gases, aerosols and cloud droplets. For some reactive pollutant gases, including SO sub(2), HNO sub(3) and NH sub(3), rates of deposition may be large and substantially larger than onto shorter vegetation and is the cause of the so called "filtering effect" of forest canopies. Pollutant inputs to moorland and forest have been compared using measured ambient concentrations from an unpolluted site in southern Scotland and a more polluted site in south eastern Germany. The inputs of S and N to forest at the Scottish site exceed moorland by 16% and 31% respectively with inputs of 7.3 kg S ha super(-1) y and 10.6 kg N ha super(-1) y super(-1). At the continental site inputs to the forest were 43% and 48% larger than over moorland for S and N deposition with totals of 53.6 kg S ha super(-1) y super(-1) and 69.5 kg N ha super(-1) y super(-1) respectively. The inputs of acidity to global forests show that in 1985 most of the areas receiving > 1 kg H super(+) ha super(-1) y super(-1) as S are in the temperate latitudes, with 8% of total global forest exceeding this threshold. By 2050, 17% of global forest will be receiving > 1 kg H super(-1) ha super(-1) as S and most of the increase is in tropical and sub-tropical countries. Forests throughout the world are also exposed to elevated concentrations of ozone. Taking 60 ppb O sub(3) as a concentration likely to be phytotoxic to sensitive forest species, a global model has been used to simulate the global exposure of forests to potentially phytotoxic O sub(3) concentrations for the years 1860, 1950, 1970, 1990 and 2100. The model shows no exposure to concentrations in excess of 60 ppb in 1860, and of the 6% of global forest exposed to concentrations > 60 ppb in 1950, 75% were in temperate latitudes and 25% in the tropics. By 1990 24% of global forest is exposed to O sub(3) concentrates > 60 ppb, and this increases to almost 50% of global forest by 2100. While the uncertainty in the future pollution climate of global forest is considerable, the likely impact of O sub(3) and acid deposition is even more difficult to assess because of interactions between these pollutants and substantial changes in ambient CO sub(2) concentration, N deposition and climate over the same period, but the effects are unlikely to be beneficial overall.
During two field campaigns (OP3 and ACES), which ran in Borneo in 2008, we measured large emissions of estragole (methyl chavicol; IUPAC systematic name 1-allyl-4-methoxybenzene; CAS number 140-67-0) ...in ambient air above oil palm canopies (0.81 mg m−2 h−1 and 3.2 ppbv for mean midday fluxes and mixing ratios respectively) and subsequently from flower enclosures. However, we did not detect this compound at a nearby rainforest. Estragole is a known attractant of the African oil palm weevil (Elaeidobius kamerunicus), which pollinates oil palms (Elaeis guineensis). There has been recent interest in the biogenic emissions of estragole but it is normally not included in atmospheric models of biogenic emissions and atmospheric chemistry despite its relatively high potential for secondary organic aerosol formation from photooxidation and high reactivity with OH radical. We report the first direct canopy-scale measurements of estragole fluxes from tropical oil palms by the virtual disjunct eddy covariance technique and compare them with previously reported data for estragole emissions from Ponderosa pine. Flowers, rather than leaves, appear to be the main source of estragole from oil palms; we derive a global estimate of estragole emissions from oil palm plantations of ~0.5 Tg y−1. The observed ecosystem mean fluxes (0.44 mg m−2 h−1) and mean ambient volume mixing ratios (3.0 ppbv) of estragole are the highest reported so far. The value for midday mixing ratios is not much different from the total average as, unlike other VOCs (e.g. isoprene), the main peak occurred in the evening rather than in the middle of the day. Despite this, we show that the estragole flux can be parameterised using a modified G06 algorithm for emission. However, the model underestimates the afternoon peak even though a similar approach works well for isoprene. Our measurements suggest that this biogenic compound may have an impact on regional atmospheric chemistry that previously has not been accounted for in models and could become more important in the future due to expansion of the areas of oil palm plantation.
1 Atlantic oak woods are of high conservation value and contain many rare lichens and bryophytes. The effects of nitrogen pollution on these epiphytic communities have not been previously studied. We ...investigated the composition of Atlantic oak wood epiphytic communities in relation to atmospheric N deposition in order to identify N indicator species and propose a critical load for such communities. 2 The epiphytic communities of seven Atlantic oak woods receiving estimated total nitrogen deposition in the range 10-53 kg N ha-1year-1were surveyed. Exposure of epiphytes to atmospheric N over 7 months was measured in terms of stemflow flux and concentration and airborne NH3. 3 Redundancy analysis (RDA) related the species to stemflow chemistry and bark pH. Different groups of species were found at (i) a coastal low N deposition site (Graphina ruiziana, Lecania cyrtella, Lobaria pulmonaria, Opegrapha atra, Orthotrichum affine, Melanelia fuliginosa ssp. glabratula and Pertusaria hymenea), (ii) inland low N deposition sites (I. myosuroides, F. tamarisci, Plagiochila atlantica, Cladonia chlorophaea, C. squamosa, Hypotrachyna laevigata and Thelotrema lepadinum) and (iii) high N deposition sites (Hypnun andoi, Hypnum cupressiforme, Calicium viride, Chrysothrix candelaris, C. coniocraea, Hypogymnia physodes, Parmelia saxatilis, Phyllospora rosei and Usnea subfloridana). 4 RDA analysis showed that bark pH and NH4
+concentration in the stemflow explained the greatest amount of variation in the species composition among the sites. The sites separated into two groups (low and high NH4
+concentration) leading to an estimate for the critical load for N deposition for epiphytes in Atlantic oak woods of 11-18 kg N ha-1year-1. 5 Analysis of the occurrence of species against N levels suggested that I. myosuroides and F. tamarisci are indicators of small N inputs, whereas H. andoi, C. coniocraea, P. saxatilis and H. physodes are tolerant of increased N loads. 6 Different forms of N ($\text{NO}_{3}^{-},\text{NH}_{4}^{+},\text{NH}_{3}$) were found to significantly affect the occurrence of different species. Some species were sensitive to total flux while others were sensitive to concentration.
A first step in interpreting the wide variation in trace gas concentrations measured over time at a given site is to classify the data according to the prevailing weather conditions. In order to ...classify measurements made during two intensive field campaigns at Mace Head, on the west coast of Ireland, an objective method of assigning data to different weather types has been developed. Air-mass back trajectories calculated using winds from ECMWF analyses, arriving at the site in 1995–1997, were allocated to clusters based on a statistical analysis of the latitude, longitude and pressure of the trajectory at 12
h intervals over 5 days. The robustness of the analysis was assessed by using an ensemble of back trajectories calculated for four points around Mace Head. Separate analyses were made for each of the 3 years, and for four 3-month periods. The use of these clusters in classifying ground-based ozone measurements at Mace Head is described, including the need to exclude data which have been influenced by local perturbations to the regional flow pattern, for example, by sea breezes. Even with a limited data set, based on 2 months of intensive field measurements in 1996 and 1997, there are statistically significant differences in ozone concentrations in air from the different clusters. The limitations of this type of analysis for classification and interpretation of ground-based chemistry measurements are discussed.
Manure application to managed grassland is a common agricultural practice. There are, however, limited studies looking at the fluxes and interactions of reactive N compounds and aerosols following ...fertilisation with manure. In this study, state-of-the-art chemical analysers (GRAEGOR, QCLAS, PTRMS) were used to investigate concentrations, fluxes and chemical interactions of reactive nitrogen containing trace gases (NH3, HNO3, HONO) and aerosols (NO3−) above a grassland fertilised with 164kgNha−1 of cattle slurry. Emissions of NH3 peaked at >67μgm−2s−1, based on a 30min average. The estimated overall loss of total ammoniacal nitrogen (TAN) from the applied slurry through NH3 emissions in the first 5 days was 33.5%. The average trimethylamine flux in the first 31h following the first slurry application was 40ngm−2s−1 and amounted to 0.38% of the NH3-N emissions. Apparent nitrate aerosol emissions were observed following the slurry application peaking at 13.0ngm−2s−1. This suggests formation of NH4NO3 from reaction of the emitted NH3 with atmospheric HNO3, consistent with the observation of gaseous concentration products exceeding the dissociation constants of ammonium nitrate. Fluxes of total nitrate (HNO3+NO3−) were bi-directional and positive during the mid-day period after fertilisation, suggesting that the slurry acted as a net source for these compounds. There is evidence of small HONO emission following fertilisation (up to 1ngm−2s−1), although the production process is currently not identified. By contrast, all compounds showed deposition to the adjacent unfertilised grassland.
The regulation of the emissions of ‘traditional’ primary air pollutants (fluorides, sulfur dioxide) has changed the pattern of exposure of ecological systems, with greatly reduced exposure close to ...sources, but with a smaller effect in some remote areas. Measurements show that recovery is occurring at some sites, in fresh water chemistry (reduced acidity) and in sensitive biota (sustainable fish populations). However, the pattern of change in exposure has not always been simply related to emission reductions. An understanding of responses to recent changes will improve our predictions of the response to future emission changes, both locally and globally.
As exposure to ‘traditional’ pollutants is reduced, the potential for other pollutants to have effects becomes more evident. In the aqueous phase, we need to understand the role of soluble and suspended organics, but this also means explicit recognition of the possibility of phase exchange, and the role of photolytic reactions on plant, soil, and water surfaces. Do highly reactive free radicals in the atmosphere, formed by the action of sunlight on volatile organic compounds, have direct effects on plants? Organic compounds and heavy metals may be bioactive as gases and particles, but for many potentially toxic compounds, the experimental evidence for biotic response is very limited.
To evaluate the potential effects of pollutants, we need to understand the pathways by which airborne pollutants enter and react within ecosystems. For vegetation, we have to consider bidirectional fluxes, and distinguish among uptake through stomata, through leaf surfaces, or through roots. There are several challenges for the future. (1) Can we devise experiments that permit exposure of vegetation to gases, particles, and/or aqueous pollutants at ‘realistic’ concentrations? (2) Can we include the potential interactions with photolytically derived free radicals, and the dynamics of exchange? (3) How do we allow for responses to pollutant mixtures, or the simultaneous exposure to pollutants in gas, particle, and aqueous phases? The recognition of the importance of the dynamic exchange of pollutants between phases will be the key to the development of effective experimental approaches to evaluating cause–effect relationships between pollutant mixtures and ecosystem responses.
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