Karen Wuyts

Assessing urban habitat quality based on specific leaf area and stomatal characteristics of Plantago lanceolata L.

This study has evaluated urban habitat quality by studying specific leaf area (SLA) and stomatal characteristics of the common herb Plantago lanceolata L. SLA and stomatal density, pore surface and resistance were measured at 169 locations in the city of Gent (Belgium), distributed over four land use classes, i.e., sub-urban green, urban green, urban and industry. SLA and stomatal density significantly increased from sub-urban green towards more urbanised land use classes, while the reverse was observed for stomatal pore surface. Stomatal resistance increased in the urban and industrial land use class in comparison with the (sub-) urban green, but differences between land use classes were less pronounced. Spatial distribution maps for these leaf characteristics showed a high spatial variation, related to differences in habitat quality within the city. Hence, stomatal density and stomatal pore surface are assumed to be potentially good bio-indicators for urban habitat quality.

Nitrogen saturation and net ecosystem production

Arising from: F. Magnani et al. 447, 849–851 (2007)10.1038/nature05847; Magnani et al. replyMagnani et al. found that net carbon (C) sequestration of temperate and boreal forests is clearly driven by nitrogen (N) deposition. From the positive relationship between average net ecosystem production (NEP) and wet N deposition, the authors further conclude that “no signs of N saturation were apparent” in the studied forests and that this is “casting doubts on the risk of widespread ecosystem nitrogen saturation”. Nitrogen additions can clearly alter net ecosystem production, but net ecosystem production cannot be used as an indicator of N saturation.

Comparison of forest edge effects on throughfall deposition in different forest types.

This study examined the influence of distance to the forest edge, forest type, and time on Cl-, SO4(2-), NO3(-), and NH4+ throughfall deposition in forest edges. The forests were dominated by pedunculate oak, silver birch, or Corsican/Austrian pine, and were situated in two regions of Flanders (Belgium). Along transects, throughfall deposition was monitored at distances of 0-128 m from the forest edge. A repeated-measures analysis demonstrated that time, forest type, and distance to the forest edge significantly influenced throughfall deposition of the ions studied. The effect of distance to the forest edge depended significantly on forest type in the deposition of Cl-, SO4(2-), and NO3(-): the edge effect was significantly greater in pine stands than in deciduous birch and oak stands. This finding supports the possibility of converting pine plantations into oak or birch forests in order to mitigate the input of nitrogen and potentially acidifying deposition.

Throughfall deposition and canopy exchange processes along a vertical gradient within the canopy of beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst).

To assess the impact of air pollution on forest ecosystems, the canopy is usually considered as a constant single layer in interaction with the atmosphere and incident rain, which could influence the measurement accuracy. In this study the variation of througfall deposition and derived dry deposition and canopy exchange were studied along a vertical gradient in the canopy of one European beech (Fagus sylvatica L.) tree and two Norway spruce (Picea abies (L.) Karst) trees. Throughfall and net throughfall deposition of all ions other than H(+) increased significantly with canopy depth in the middle and lower canopy of the beech tree and in the whole canopy of the spruce trees. Moreover, throughfall and net throughfall of all ions in the spruce canopy decreased with increasing distance to the trunk. Dry deposition occurred mainly in the upper canopy and was highest during the growing season for H(+), NH(4)(+), NO(3)(-) and highest during the dormant season for Na(+), Cl(-), SO(4)(2-) (beech and spruce) and K(+), Ca(2+) and Mg(2+) (spruce only). Canopy leaching of K(+), Ca(2+) and Mg(2+) was observed at all canopy levels and was higher for the beech tree compared to the spruce trees. Canopy uptake of inorganic nitrogen and H(+) occurred mainly in the upper canopy, although significant canopy uptake was found in the middle canopy as well. Canopy exchange was always higher during the growing season compared to the dormant season. This spatial and temporal variation indicates that biogeochemical deposition models would benefit from a multilayer approach for shade-tolerant tree species such as beech and spruce.

The potential of biomonitoring of air quality using leaf characteristics of white willow (Salix alba L.).

In this study, we assess the potential of white willow (Salix alba L.) as bioindicator for monitoring of air quality. Therefore, shoot biomass, specific leaf area, stomatal density, stomatal pore surface, and stomatal resistance were assessed from leaves of stem cuttings. The stem cuttings were introduced in two regions in Belgium with a relatively high and a relatively low level of air pollution, i.e., Antwerp city and Zoersel, respectively. In each of these regions, nine sampling points were selected. At each sampling point, three stem cuttings of white willow were planted in potting soil. Shoot biomass and specific leaf area were not significantly different between Antwerp city and Zoersel. Microclimatic differences between the sampling points may have been more important to plant growth than differences in air quality. However, stomatal pore surface and stomatal resistance of white willow were significantly different between Zoersel and Antwerp city. Stomatal pore surface was 20% lower in Antwerp city due to a significant reduction in both stomatal length (−11%) and stomatal width (−14%). Stomatal resistance at the adaxial leaf surface was 17% higher in Antwerp city because of the reduction in stomatal pore surface. Based on these results, we conclude that stomatal characteristics of white willow are potentially useful indicators for air quality.

The effect of air pollution and other environmental stressors on leaf fluctuating asymmetry and specific leaf area of Salix alba L.

We aimed at evaluating the effect of low-level air pollution on leaf area fluctuating asymmetry (FAA) and specific leaf area (SLA) of Salix alba L., taking into account other environmental factors. Cuttings were grown in standardized conditions in the near vicinity of air quality measuring stations in Belgium. Variability of SLA and FAA between measuring stations explained 83% and 7.26%, respectively, of the total variability. FAA was not influenced by air pollution or environmental factors such as shading, herbivory, air temperature and humidity. SLA was increased by an increase in shadow, while NO(x) and O(3) concentrations had only a marginal influence. The influence of SO(2) concentration was negligible. Although our data analysis suggests a relationship between SLA and NO(x)/O(3) concentration, the absence of a straightforward relationship between FAA and SLA and air pollution still questions the usefulness of these bio-indicators for monitoring air pollution.

Patterns of throughfall deposition along a transect in forest edges of silver birch and Corsican pine

In two adjacent forest stands in Flanders, one dominated by Corsican pine (Pinus nigra subsp. laricio Maire) and another dominated by silver birch (Betula pendula Roth), throughfall deposition was monitored along a transect per- pendicular to the forest edge exposed to the prevailing wind direction. Throughfall deposition of Na + ,K + ,C a 2+ ,M g 2+ , NH4 + ,N O3 - ,C l - , and SO4 2- was examined on forest edge patterns expressed in the depth of influence of the edge effect (forest edge distance) and the level of enhancement at the edge (forest edge enhancement). In addition, an integrated forest edge enhancement factor was computed that incorporates these two parameters. Our results show that the edge effects on throughfall deposition of Na + ,C l - , the sum of so-called base cations, the sum of potentially acidifying ions, and the sum of inorganic nitrogen (NH4 + +N O3 - ) are more pronounced in the pine stand. The edge zone of the pine stand receives as a result of the edge effect 9.4 times more extra potentially acidifying ions and 12.7 times more extra inorganic nitrogen than the birch stand. We conclude that an appropriate design or conversion of the edge structure, from high-density Corsi- can pine plantations into lower density deciduous forests, can reduce the input of acidifying and eutrophying pollutants in the forest edge.

Former land use affects the nitrogen and phosphorus concentrations and biomass of forest herbs

The colonization rates of understorey plants into forests growing on former agricultural land differ remarkably among species. Different dispersal and recruitment largely account for the contrasting colonization rates, but different effects of the soil legacies of former agricultural land use on plant performance may also play a role. Seven herbaceous forest species were sampled in paired post-agricultural and ancient forest stands to study whether land-use history has an effect on the aboveground nutrient concentrations (N, P and N:P ratios) and biomass of forest herbs and, if so, whether slow and fast colonizing species respond differently. Results showed that P concentrations were significantly affected by former land use with higher concentrations in the post-agricultural stands. N concentrations were unaffected and N:P ratios were significantly higher in the ancient stands. Nutrient concentrations varied considerably among species, but the variation was unrelated to their colonization capacity. Six out of the seven species had higher biomass in the post-agricultural stands relative to the ancient stands, and the degree to which the species increased biomass was positively related to their colonization capacity, i.e., the fast colonizing species showed the strongest increase. Such differential responses to past land use may contribute to the contrasting colonization capacity of forest plants. Land-use history thus affected both the nutrient concentrations and biomass of forest herbs, and only the biomass response was related to colonization capacity.

On the temporal variation of leaf magnetic parameters: seasonal accumulation of leaf-deposited and leaf-encapsulated particles of a roadside tree crown.

Understanding the accumulation behaviour of atmospheric particles inside tree leaves is of great importance for the interpretation of biomagnetic monitoring results. In this study, we evaluated the temporal variation of the saturation isothermal remanent magnetisation (SIRM) of leaves of a roadside urban Platanus × acerifolia Willd. tree in Antwerp, Belgium. We hereby examined the seasonal development of the total leaf SIRM signal as well as the leaf-encapsulated fraction of the deposited dust, by washing the leaves before biomagnetic analysis. On average 38% of the leaf SIRM signal was exhibited by the leaf-encapsulated particles. Significant correlations were found between the SIRM and the cumulative daily average atmospheric PM10 and PM2.5 measurements. Moreover, a steady increase of the SIRM throughout the in-leaf season was observed endorsing the applicability of biomagnetic monitoring as a proxy for the time-integrated PM exposure of urban tree leaves. Strongest correlations were obtained for the SIRM of the leaf-encapsulated particles which confirms the dynamic nature of the leaf surface-accumulated particles.

Spatio-temporal litterfall dynamics in a 60-year-old mixed deciduous forest

IntroductionTo increase our understanding of litterfall dynamics in mixed-species forests, seasonal and annual variations in litterfall mass and nutrient concentrations were assessed for a 60-year-old spontaneously developed forest dominated by silver birch (Betula pendula Roth), pedunculate oak (Quercus robur L.), and northern red oak (Quercus rubra L.) in Belgium.ResultsTotal quantities and seasonal patterns of most litterfall fractions were similar over the 29-month study period, but the species differed in start and duration of their leaf shedding period. The spatial distribution of litterfall persisted over the years for leaves, but not for total litterfall because of the varying spatial pattern of fallen twigs and reproductive structures. Consequently, predicting humus build-up based on short-term litterfall measurements may be difficult in mixed forests. Nutrient concentrations in leaf litter differed considerably between the species and throughout the year, but the seasonal pattern did not depend on the species. Betula returned significantly more nutrients to the soil per mass unit than Quercus, except for sodium.ConclusionAs the present stand conditions only allow recruitment of Q. rubra, Betula is being outcompeted, which decreases the nutrient return to the soil and may negatively affect biogeochemical cycling.