Gerrit W. Heil

DETERMINANTS OF LONG-DISTANCE SEED DISPERSAL BY WIND IN GRASSLANDS

Long-distance seed dispersal is an important topic in ecology, but notoriously difficult to quantify. Previous modeling approaches have failed to simulate long-distance dispersal, and it has remained unclear which mechanisms determine long-distance dispersal and what their relative importance is. We simulated wind dispersal of grassland plant seeds with four mechanistic models of increasing complexity and realism to assess which processes and which attributes of plants and their environment determine dispersal distances. We compared simulation results of the models to each other and to data from field dispersal experiments. The more complex and realistic models predicted short-distance dispersal more accurately and were the only models able to simulate long-distance dispersal. The model comparisons showed that autocorrelated turbulent fluctuations in vertical wind velocity are the key mechanism for long-distance dispersal. Seed dispersal distances are longest under high wind velocity conditions, when mechanically produced turbulent air movements are large. Under very low wind velocity conditions seeds are dispersed farther when there is more surface heating, but never as far as during strong wind events. Model sensitivity analyses showed that mean horizontal wind velocity, seed release height, and vegetation height are crucial determinants of dispersal potential and dispersal distances. Between plant species (but not within a species), seed terminal velocity is an additional important determinant of long-distance dispersal. These results imply that seed release height is the most important plant-controlled dispersal parameter for grassland plants, and that the structure of the local vegetation greatly affects dispersal distances. Thus, management plans for grasslands should take into account that changes in vegetation structure, e.g., due to eutrophication, can reduce the seed dispersal ability of wind-dispersed plant species.

Atmospheric deposition and canopy exchange processes in heathland ecosystems

The aims of the present study were to determine canopy exchange processes and to quantify total atmospheric deposition of sulphur and nitrogen in heathland. The study was carried out in dry inland heath vegetation, dominated by Calluna vulgaris, in two nature reserves in the eastern part of the Netherlands. Atmospheric deposition was determined with throughfall-stemflow measurements, adapted for low vegetation. Throughflow measurements (sum of throughfall and stemflow) in artificial Calluna canopies showed co-deposition of SOx and NHy upon heathland vegetation. In the real Calluna canopy, a significant part of the deposited ammonia/ammonium was directly assimilated by the Calluna shoots, especially in wet periods. The concentrations of potassium, calcium and magnesium in throughflow, after passage through the Calluna canopy, increased significantly compared with bulk precipitation. The amount of cations lost from the canopy were in good agreement with the observed ammonium uptake by the Calluna. A field experiment demonstrated that losses of the above-mentioned cations can be doubled by application of ammonium sulphate. It was shown that interception deposition is an important component of the atmospheric deposition of sulphur and nitrogen upon Calluna heathland; bulk precipitation amounted to only c. 35-40% of total atmospheric input. Total atmospheric deposition of sulphur and nitrogen in the investigated heathlands was 1.5-2.1 (27-33 kg S ha(-1) yr(-1)) and 2.1-3.1 kmolc ha(-1) yr(-1) (30-45 kg N ha(-1) yr(-1)), respectively. It is concluded that the present atmospheric nitrogen deposition is a continuous threat for the existence of heathlands in Western Europe.

Low investment in sexual reproduction threatens plants adapted to phosphorus limitation

Plant species diversity in Eurasian wetlands and grasslands depends not only on productivity but also on the relative availability of nutrients, particularly of nitrogen and phosphorus. Here we show that the impacts of nitrogen:phosphorus stoichiometry on plant species richness can be explained by selected plant life-history traits, notably by plant investments in growth versus reproduction. In 599 Eurasian sites with herbaceous vegetation we examined the relationship between the local nutrient conditions and community-mean life-history traits. We found that compared with plants in nitrogen-limited communities, plants in phosphorus-limited communities invest little in sexual reproduction (for example, less investment in seed, shorter flowering period, longer lifespan) and have conservative leaf economy traits (that is, a low specific leaf area and a high leaf dry-matter content). Endangered species were more frequent in phosphorus-limited ecosystems and they too invested little in sexual reproduction. The results provide new insight into how plant adaptations to nutrient conditions can drive the distribution of plant species in natural ecosystems and can account for the vulnerability of endangered species.

Light partitioning among species and species replacement in early successional grasslands.

Abstract We studied canopy structure, shoot architecture and light harvesting efficiencies of the species (photon flux captured per unit above-ground plant mass) in a series of exclosures of different age (up to 4.5 yr) in originally heavily grazed grassland in N Japan. Vegetation height and Leaf Area Index (LAI) increased in the series and Zoysia japonica, the dominant in the beginning, was replaced by the much taller Miscanthus sinensis. We showed how this displacement in dominance can be explained by inherent constraints on the above-ground architecture of these two species. In all stands light capture of plants increased with their above-ground biomass but taller species were not necessarily more efficient in light harvesting. Some subordinate species grew disproportionally large leaf areas and persisted in the shady undergrowth. Some other species first grew taller and managed to stay in the better-lit parts of the canopy, but ultimately failed to match the height growth of their neighbours in this early successional series. Their light harvesting efficiencies declined and this probably led to their exclusion. By contrast, species that maintained their position high in the canopy managed to persist in the vegetation despite their relatively low light harvesting efficiencies. In the tallest stands ‘later successional’ species had higher light harvesting efficiencies for the same plant height than ‘early successional’ species which was mostly the result of the greater area to mass ratio (specific leaf area, SLA) of their leaves. This shows how plant stature, plasticity in above-ground biomass partitioning, and architectural constraints determine the ability of plants to efficiently capture light, which helps to explain species replacement in this early successional series. Nomenclature: Makino (1962); Ohwi (1965). Abbreviations: LAI = Leaf area index; LAR = Leaf Area Ratio; LMR = Leaf Mass Ratio; PPFD = Photosynthetically active photon flux density; SLA = Specific Leaf Area.

Acidification effects on vesicular-arbuscular mycorrhizal (VAM) infection, growth and nutrient uptake of established heathland herb species

A greenhouse experiment was carried out to determine whether the decline of Arnica montana L. in heathland vegetation in the Netherlands could be caused by a detrimental effect of soil acidification on vesicular-arbuscular mycorrhiza of this species. Arnica montana and two non-declining species from the same habitat, Hieracium pilosella L. and Deschampsia flexuosa (L.) Trin., were grown with and without the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus fasciculatum (Thaxter sensu Gerdemann) Gerd. and Trappe in pots with an extremely nutrient-poor, sandy soil. They were percolated weekly with nutrient solution with different pH values, viz. 5.5, 4.5, 3.5 and 2.5. At intervals of three weeks and up to 12 weeks, measurements were made on growth, nutrient uptake and VAM infection.In the most acid treatments growth and nutrient uptake were reduced in all species. VAM infection decreased only slightly with decreasing pH of the treatments. Without VAM, Arnica montana died and Hieracium pilosella hardly grew at the most acid treatments. Therefore it is concluded that VAM decreased the stress caused by the most acid treatments. Leachate from the most acid treatment had a pH of approximately 4, and contained considerable amounts of aluminium, dissolved from the solid phase of the soil. This might have played a role in the detrimental effects on the plants in the case of the most acid treatment. No evidence was found in this experiment that the decline of Arnica montana was due to detrimental effects of soil acidification on VAM of this species.

“Calluna”, a simulation model for evaluation of impacts of atmospheric nitrogen deposition on dry heathlands

Air pollution can cause a decline in species through eutrophication of the habitat. In the Netherlands eutrophication largely results from atmospheric nitrogen deposition. A simulation model has been developed for evaluation of the impacts of atmospheric nitrogen deposition on dry heathlands. The model has been applied to three deposition scenarios. The results of the model show that atmospheric nitrogen is the trigger for the change of heatland into grassland. Moreover, the results implicate that a further transition of heathlands into grasslands will be ineviable in the Netherlands under scenario 1 (an average deposition of 2200 eq N·ha−1·yr−1). Scenario 2 (1400 eq N·ha−1·yr−1) meets the target that Calluna will be the (co)-dominant species 25 years after sod cutting for 14 out of 17 areas in the Netherlands. Scenario 3 (1230 eq N·ha−1·yr−1) meets this target for all areas. It is argued that uncertainties of the model are little, because the model outcomes are sufficiently supported by quantified data. It is concluded that the process of decline of heathlands can only be reversed if eutrophication is reversed.

AFFOREST sDSS: a metamodel based spatial decision support system for afforestation of agricultural land

Abstract.The concept and structure of the Spatial Decision Support System AFFOREST sDSS dealing with environmental performance (EP) of afforestation on agricultural land in northwestern Europe, is presented. EP is defined in terms of three environmental impact categories: (1) carbon sequestration (2) groundwater recharge and (3) nitrate leaching. The core of the sDSS is a raster based geographical database which allows for queries addressing 14 types of questions on where, how and how long to afforest in order to reach a desired EP or change in EP due to afforestation of the agricultural land. First the study area is differentiated according to the site conditions (based on soil texture, soil drainage, initial land use, yearly average precipitation, and yearly average N deposition. Then the EP for every site class is computed as a function of time using the VSAM metamodel. VSAM results from a conceptual simplification of an existing mechanistic point model, the forest process model SMART2. Input data for the metamodel are limited to the classified site conditions, the tree species used for afforestation, the afforestation strategy and the evaluation time. Besides limiting the data requirements, the metamodel approach allows for rapid and flexible computations on large numbers of pixel classes. Finally, depending on the type of question, the sDSS creates georeferenced outputs based on SQL-type spatial or attribute queries and more advanced multiple goal programming techniques.

Time-dependent, species-specific effects of N:P stoichiometry on grassland plant growth

N and P have different eutrophication effects on grassland communities, yet the underlying mechanisms are poorly understood. To examine plant growth in response to the varying (relative) supply of N and P, we conducted a two-year greenhouse experiment. Five grasses and three herbs were grown with three N:P supply ratios at two overall nutrient supply levels. During the first year the plant growth was relatively low at both high and low N:P supply ratios, whereas during the second year the growth was especially low at a high N:P supply ratio. This second-year low growth was attributed to the high root death rate, which was influenced by a high N:P supply ratio rather than by the nutrient supply level. Species responded differently, especially in P uptake and loss at a high N:P supply ratio. Each species seemed to have a different strategy for P limitation, e.g. an efficient P uptake or a high P resorption rate. Species typical of P-limited grasslands had neither better P uptake nor better P retention at a high N:P supply ratio. This study quantitatively demonstrates an increased plant root death triggered by strong P limitation. This finding indicates a possible extra effect of N eutrophication on ecosystem functioning via changed N:P stoichiometry.

Habitat suitability study for the conservation of the volcano rabbit (Romerolagus diazi)

The endangered Mexican volcano rabbit (Romerolagus diazi)-habitat relationship was investigated. Data, obtained from 137 sampling units in 14 patches of the geographical distribution areas of the rabbit, included information on floristic composition, vegetation structure, terrain characteristics and human activities. The abundance of the volcano rabbit was measured in 685 subsampling units by pellet-counts and estimations of pellet-coverage. Two questions have been addressed in this paper: what is a good indicator of habitat type and which are the habitat factors that explain best the distribution of the volcano rabbit? Thirteen plant communities were identified. Terrain and landscape units were distinguished through aerial photograph interpretation and were verified in the field. By means of classical (parametric and non-parametric) and multivariate (Canonical Correspondence Analysis) statistical analyses, the most suitable, suitable and unsuitable habitat classes were detected. The results indicated that plant communities (F = 4,14; P < 0.0001) and landscape units (F = 7,29; P < 0.001) were adequate levels to distinguish habitat types for the rabbit. Plant communities of Festuca tolucensis and Trisetum altijugum-Festuca tolucensis attained the largest abundance of rabbits, followed by the communities Muhlenbergia quadridentata-Pinus hartwegii, Festuca tolucensis-Pinus hartwegii and Pinus sp.-Alnus firmifolia. The abundances of the volcano rabbit in the remaining plant communities were significantly less than in the communities mentioned above. These results suggest that the volcano rabbit shows strong preferences for subalpine habitat types. Soil moisture (r = -0.840), altitude (r = -0.756), grazing (r = -0.423) and burning (r = 0.494) correlated significantly with the distribution and abundance of the volcano rabbit. These results are discussed in light of their role in the conservation of the volcano rabbit.

Effect of the air pollution component ammonium sulphate on the VAM infection rate of three heathland species

Three heathand species, Antennaria dioica, Arnica montana and Hieracium pilosella, were artificially rained with ammonium sulphate solutions at increasing concentrations in a greenhouse experiment. The same species were also artificially rained with increasing ammonium sulphate solutions under field conditions. Dry weights of the plants in the field experiments did not change with increasing ammonium sulphate applications. Nor did the dry weights of plants in the greenhouse experiments change with increasing ammonium sulphate concentrations, except for Arnica montana, which showed an increase in dry weight. VAM infection percentage of Antennaria dioica increased in both the greenhouse and the field experiment. The results of the field experiment show that VAM infection rates are reduced after two years of artificial rain in the plant species Arnica montana, which grows naturally under nutrient poor conditions and is presently declining in its natural habitat in the Netherlands. In the greenhouse experiment, VAM infection of Arnica montana did not change with increasing ammonium sulphate concentrations. VAM infection rates of Hieracium pilosella, which presently is not declining, did not change with increasing ammonium sulphate concentrations.