Frank Berendse

Regional nitrogen budgets and riverine N & P fluxes for the drainages to the North Atlantic Ocean: Natural and human influences

We present estimates of total nitrogen and total phosphorus fluxes in rivers to the North Atlantic Ocean from 14 regions in North America, South America, Europe, and Africa which collectively comprise the drainage basins to the North Atlantic. The Amazon basin dominates the overall phosphorus flux and has the highest phosphorus flux per area. The total nitrogen flux from the Amazon is also large, contributing 3.3 Tg yr-1 out of a total for the entire North Atlantic region of 13.1 Tg yr-1. On a per area basis, however, the largest nitrogen fluxes are found in the highly disturbed watersheds around the North Sea, in northwestern Europe, and in the northeastern U.S., all of which have riverine nitrogen fluxes greater than 1,000 kg N km-2 yr-1.

Constraints in the restoration of ecological diversity in grassland and heathland communities

Species-rich grassland and heathland communities still occur in low-intensity farming systems in many European countries. Gradually, such systems have either been abandoned or more intensively exploited, with a subsequent decrease in species numbers. Until recently, it was thought that restoration of these communities would be straightforward. However, abiotic constraints (with respect to eutrophication and acidification) have hampered restoration more than previously thought. Moreover, very recent research has revealed that biotic constraints can also be important: many plant species are not present in the soil seed bank and their dispersal is limited in the present fragmented landscape.

Agri-environment schemes do not effectively protect biodiversity in Dutch agricultural landscapes

Roughly 20% of the European Unions farmland is under some form of agri-environment scheme to counteract the negative impacts of modern agriculture on the environment. The associated costs represent about 4% (1.7 billion euros) of the European Unions total expenditure on the Common Agricultural Policy and are expected to rise to 10% in the near future. Although agri-environment schemes have been implemented in various countries for well over a decade, to date no reliable, sufficiently replicated studies have been performed to test whether such measures have the presumed positive effects on biodiversity. Here we present the results of a study evaluating the contribution of agri-environment schemes to the protection of biodiversity in intensively used Dutch agricultural landscapes. We surveyed plants, birds, hover flies and bees on 78 paired fields that either had agri-environment schemes in the form of management agreements or were managed conventionally. Management agreements were not effective in protecting the species richness of the investigated species groups: no positive effects on plant and bird species diversity were found. The four most common wader species were observed even less frequently on fields with management agreements. By contrast, hover flies and bees showed modest increases in species richness on fields with management agreements. Our results indicate that there is a pressing need for a scientifically sound evaluation of agri-environment schemes.

The effect of increased nutrient availability on vegetation dynamics in wet heathlands

A three year fertilization experiment was conducted in which nitrogen (N series: 20 g N m−2 yr−1), phosphorus (P series: 4 g P m−2 yr−1) and potassium (K series: 20 g K m−2 yr−1) were added to a mixed vegetation of Erica tetralix and Molinia caerulea. At the end of each growing season the percentage cover of each species was determined. At the end of the experiment percentage cover of each species was found to be positively correlated with the harvested biomass. In the unfertilized control series the cover of Erica and Molinia did not change significantly during the experiment. In all fertilized series however, especially in the P series, cover of Erica decreased significantly. The cover of Molinia increased significantly in the P series only. In the fertilized series the biomass of Erica and total biomass per plot did not change significantly compared with the control series. In the P series the biomass of Molinia increased significantly. It is suggested that with increasing phosphorus or nitrogen availability Molinia outcompetes Erica because the former invests more biomass in leaves which in turn permits more carbon to be allocated to the root system, which thereupon leads to a higher nutrient uptake.

Reconciling complexity with stability in naturally assembling food webs

Understanding how complex food webs assemble through time is fundamental both for ecological theory and for the development of sustainable strategies of ecosystem conservation and restoration. The build-up of complexity in communities is theoretically difficult, because in random-pattern models complexity leads to instability. There is growing evidence, however, that nonrandom patterns in the strengths of the interactions between predators and prey strongly enhance system stability. Here we show how such patterns explain stability in naturally assembling communities. We present two series of below-ground food webs along natural productivity gradients in vegetation successions. The complexity of the food webs increased along the gradients. The stability of the food webs was captured by measuring the weight of feedback loops of three interacting ‘species’ locked in omnivory. Low predator–prey biomass ratios in these omnivorous loops were shown to have a crucial role in preserving stability as productivity and complexity increased during succession. Our results show the build-up of food-web complexity in natural productivity gradients and pin down the feedback loops that govern the stability of whole webs. They show that it is the heaviest three-link feedback loop in a network of predator–prey effects that limits its stability. Because the weight of these feedback loops is kept relatively low by the biomass build-up in the successional process, complexity does not lead to instability.

Organic matter accumulation and nitrogen mineralization during secondary succession in heathland ecosystems.

SUMMARY (1) Five series of plots in heathlands were selected in which the above-ground biomass and the litter (L) and humus (FH) layer had been removed between one and fifty years ago. Secondary succession in these heathlands was studied by comparing the amounts of soil organic matter, above-gound biomass, below-ground biomass and the annual nitrogen mineralization in plots of different ages. (2) The amounts of organic matter in the L and FH layers increased with age while the communities were dominated by Calluna vulgaris or Erica tetralix. Where dwarf shrubs were replaced by Molinia caerulea the amount of soil organic matter did not further increase. (3) Both above-ground and below-ground biomass in the communities dominated by dwarf shrubs increased with age to about 1300 and 800 g m-2, respectively, but were much lower in the Molinia-dominated plots (c. 400 and 500 g m-2). (4) The annual nitrogen mineralization remained at a low level, or even decreased, during the first ten years after turf removal. Thereafter the mineralization rate increased with an increasing amount of organic matter in the L and FH layers. Multiple regression analysis revealed that the amount of organic matter in the L and FH layers and the biomass percentage of Molinia together explained 84% of the observed variance in the annual nitrogen mineralization. It is proposed that Molinia has a positive effect on the mineralization rate. (5) In communities dominated by Calluna or Erica all the nitrogen that entered the ecosystem appeared to accumulate in the plant biomass and the soil organic matter, whereas losses of nitrogen from the ecosystem appeared to be negligible. In Moliniadominated communities part of the nitrogen that entered the system was lost to deeper soil layers, to the atmosphere or to herbivores.

The influence of savanna trees on nutrient, water and light availability and the understorey vegetation

In an East African savanna herbaceous layer productivity and species composition were studied around Acacia tortilis trees of three different age classes, as well as around dead trees and in open grassland patches. The effects of trees on nutrient, light and water availability were measured to obtain an insight into which resources determine changes in productivity and composition of the herbaceous layer. Soil nutrient availability increased with tree age and size and was lowest in open grassland and highest under dead trees. The lower N:P ratios of grasses from open grassland compared to grasses from under trees suggested that productivity in open grassland was limited by nitrogen, while under trees the limiting nutrient was probably P. N:P ratios of grasses growing under bushes and small trees were intermediate between large trees and open grassland indicating that the understorey of Acacia trees seemed to change gradually from a N-limited to a P-limited vegetation. Soil moisture contents were lower under than those outside of canopies of large Acacia trees suggesting that water competition between trees and grasses was important. Species composition of the herbaceous layer under Acacia trees was completely different from the vegetation in open grassland. Also the vegetation under bushes of Acacia tortilis was different from both open grassland and the understorey of large trees. The main factor causing differences in species composition was probably nutrient availability because species compositions were similar for stands of similar soil nutrient concentrations even when light and water availability was different. Changes in species composition did not result in differences in above-ground biomass, which was remarkably similar under different sized trees and in open grassland. The only exception was around dead trees where herbaceous plant production was 60% higher than under living trees. The results suggest that herbaceous layer productivity did not increase under trees by a higher soil nutrient availability, probably because grass production was limited by competition for water. This was consistent with the high plant production around dead trees because when trees die, water competition disappears but the high soil nutrient availability remains. Hence, in addition to tree soil nutrient enrichment, below-ground competition for water appears to be an important process regulating tree-grass interactions in semi-arid savanna.

Competition in heathland along an experimental gradient of nutrient availability

In a three year field experiment competition between Erica tetralix and Molinia caerulea and between Calluna vulgaris and Molinia was studied at four levels of nutrient (NPK) availability using replacement series. In the monocultures of the unfertilized control productivity of both Erica and Calluna exceeded that of Molinia by a factor two. At the highest fertilisation level the rank order of productivity was Erica 1) and Molinia was outcompeted. Only in the highest nutrient treatment was RYem smaller than 1 and Erica was outcompeted by Molinia. In all nutrient treatments Calluna was the superior competitor (RYcm > 1) and Molinia was outcompeted by Calluna, despite its higher potential growth rate and its greater maximum foliage height. The high competitive ability for light interception of Erica and Calluna with respect to Molinia can be attributed to their evergreen habit, which permits canopy closure early in the growing season. The results emphasize the importance of vertical canopy structure and timing of canopy development in competition for light. However, the abilities to compete for light and mineral nutrients are probably closely inter-dependent.

Hydraulic lift in Acacia tortilis trees on an East African savanna

Recent studies suggest that savanna trees in semi-arid areas can increase understorey plant production. We hypothesized that one of the mechanisms that explains the facilitation between trees and grasses in East African savannas is hydraulic lift (HL). HL in large Acacia tortilis trees was studied during the first 3 months of the dry season during a relatively wet year (1998) and a very dry year (2000). In 1998, we found distinct diel fluctuation in soil water potential (ψs), with increasing values during the night and decreasing again the following day. These fluctuations in ψs are consistent with other observations of HL and in A. tortilis were found up to 10 m from the tree. In 2000, during a severe drought, ψs measurements indicated that HL was largely absent. The finding that HL occurred in wetter years and not in drier years was supported by data obtained on the δ18O values in soil, rain and groundwater. The δ18O of water extracted from the xylem water of grasses indicated that when they grew near trees they had values similar to those of groundwater. This could be because they either (1) use water from deeper soil layers or (2) use hydraulically lifted water provided by the tree; this was not seen in the same grass species growing outside tree canopies. While our data indicate that HL indeed occurs under Acacia trees, it is also true that ψs was consistently lower under trees when compared to outside tree canopies. We believe that this is because tree-grass mixtures take up more water from the upper soil layers than is exuded by the tree each night. This limits the beneficial effect of HL for understorey grasses and suggests that in savannas both facilitation via HL and competition are active processes. The importance of each process may depend upon how wet or dry that particular site or year is.

Competition between plant populations with different rooting depths

SummaryAs an extension of De Wits competition theory a theoretical description has been developed of competition between plant populations with different rooting depths. This model shows that in mixtures of plants with different rooting depths the value of the Relative Yield Total can be expected to exceed one. Moreover, it predicts the frequency-dependence of the relative crowding coefficient of the deep rooting population with respect to the shallow rooting population. The relationship between properties of plant species and the environment required to establish a stable equilibrium turns out to be surprisingly simple. The shallow rooting species has to have a larger competitive ability sufficient to compensate for the extra nutrients that are exploited by the deeper rooting plants. The dependence of equilibrium plant frequencies on the properties of plant species and the characteristics of the environment is discussed.