Spatial patterning of ecosystems can be explained by several mechanisms. One approach to disentan... more Spatial patterning of ecosystems can be explained by several mechanisms. One approach to disentangling the influence of these mechanisms is to study a patterned ecosystem along a gradient of environmental conditions. This study focused on hummock-hollow patterning of peatlands. Previous models predicted that patterning in drainage-dominated peatlands is driven by a peat-accumulation mechanism, reflected by higher nutrient availability in hollows relative to hummocks. Alternatively, patterning in evapotranspiration (ET)-dominated peatlands may be driven by a nutrient-accumulation mechanism, reflected by reversed nutrient distribution, namely, higher nutrient availability in hummocks relative to hollows. Here, we tested these predictions by comparing nutrient distributions among patterned peatlands in maritime (Scotland), humid temperate (Sweden), and humid continental (Siberia) climates. The areas comprise a climatic gradient from very wet and drainage-dominated (Scotland) to less wet and ET-dominated (Siberia) peatlands. Nutrient distribution was quantified as resource contrast, a measure for hummock-hollow difference in nutrient availability. We tested the hypothesis that the climatic gradient shows a trend in the resource contrast; from negative (highest nutrient availability in hollows) in Scotland to positive (highest nutrient availability in hummocks) in Siberia. The resource contrasts as measured in vegetation indeed showed a trend along the climatic gradient: contrasts were negative to slightly positive in Scotland, positive in Sweden, and strongly positive in Siberia. This finding corroborates the main prediction of previous models. Our results, however, also provided indications for further model development. The low concentrations of nutrients in the water suggest that existing models could be improved by considering both the dissolved and adsorbed phase and explicit inclusion of both nutrient-uptake and nutrient-storage processes. Our study suggests that future climate change may affect the ecosystem functioning of patterned peatlands by altering the contribution of pattern-forming mechanisms to redistribution of water and nutrients within these systems.
Nitrogen deposition's role in determining forest photosynthetic capacity; a FLUXNET synthesis
AGUFM, Dec 1, 2011
ABSTRACT There is growing evidence that nitrogen (N) deposition stimulates forest growth, as many... more ABSTRACT There is growing evidence that nitrogen (N) deposition stimulates forest growth, as many forest ecosystems are N-limited. However, the significance of N deposition in determining the strength of the present and future terrestrial carbon sink is strongly debated. We investigated and quantified the effect of N deposition on ecosystem photosynthetic capacity (Amax) with the FLUXNET database, including 80 forest sites, covering the major forest types and climates of the world. The relative effect of climate and N deposition on photosynthesis was assessed with regression models. We found a significant positive correlation of Amax and N deposition for evergreen needleleaf forests in our dataset. We further found indications that foliar N and LAI scale positively with N deposition, reflecting the 2 mechanisms at which N is believed to cause an increase in carbon gain. We can support the hypothesis that foliar N is the principal scaling factor for canopy Amax across all forest types. Deciduous forests are less diverse in terms of climate and nutritional conditions for the included sites and these forests exhibited weak to no correlations with the included climate and N predictor variables. Quantifying the effect of N deposition on photosynthetic rates at the canopy level is an essential step for quantifying its contribution to the terrestrial carbon sink and for predicting vegetation response to N fertilization and global change in the future. The approach shows that eddy-covariance measurements of carbon fluxes at the canopy scale allow us to test hypotheses with respect to the expected nitrogen-photosynthesis relationships at the canopy scale.
Terrestrial vegetation influences climate by modifying the radiative-, momentum-, and hydrologic-... more Terrestrial vegetation influences climate by modifying the radiative-, momentum-, and hydrologic-balance. This paper contributes to the ongoing debate on the question whether positive biogeophysical feedbacks between vegetation and climate may lead to multiple equilibria in vegetation and climate and consequent abrupt regime shifts. Several modelling studies argue that vegetation-climate feedbacks at local to regional scales could be strong enough to establish multiple states in the climate system. An Earth Model of Intermediate Complexity, PlaSim, is used to investigate the resilience of the climate system to vegetation disturbance at regional to global scales. We hypothesize that by starting with two extreme initialisations of biomass, positive vegetation-climate feedbacks will keep the vegetation-atmosphere system within different attraction domains. Indeed, model integrations starting from different initial biomass distributions diverged to clearly distinct climate-vegetation states in terms of abiotic (precipitation and temperature) and biotic (biomass) variables. Moreover, we found that between these states there are several other steady states which depend on the scale of perturbation. From here global susceptibility maps were made showing regions of low and high resilience. The model results suggest that mainly the boreal and monsoon regions have low resiliences, i.e. instable biomass equilibria, with positive vegetation-climate feedbacks in which the biomass induced by a perturbation is further enforced. The perturbation did not only influence single vegetation-climate cell interactions but also caused changes in spatial patterns of atmospheric circulation due to neighbouring cells constituting in spatial vegetation-climate feedbacks. Large perturbations could trigger an abrupt shift of the system towards another steady state. Although the model setup used in our simulation is rather simple, our results stress that the coupling of feedbacks at multiple scales in vegetation-climate models is essential and urgent to understand the system dynamics for improved projections of ecosystem responses to anthropogenic changes in climate forcing.
To study global nitrogen (N) leaching from natural ecosystems under changing N deposition, climat... more To study global nitrogen (N) leaching from natural ecosystems under changing N deposition, climate, and atmospheric CO 2 , we performed a factorial model experiment for the period 1901-2006 with the N-enabled global terrestrial ecosystem model LPJ-GUESS (Lund-Potsdam-Jena General Ecosystem Simulator). In eight global simulations, we used either the true transient time series of N deposition, climate, and atmospheric CO 2 as input or kept combinations of these drivers constant at initial values. The results show that N deposition is globally the strongest driver of simulated N leaching, individually causing an increase of 88 % by 1997-2006 relative to pre-industrial conditions. Climate change led globally to a 31 % increase in N leaching, but the size and direction of change varied among global regions: leaching generally increased in regions with high soil organic carbon storage and high initial N status, and decreased in regions with a positive trend in vegetation productivity or decreasing precipitation. Rising atmospheric CO 2 generally caused decreased N leaching (33 % globally), with strongest effects in regions with high productivity and N availability. All drivers combined resulted in a rise of N leaching by 73 % with strongest increases in Europe, eastern North America and South-East Asia, where N deposition rates are highest. Decreases in N leaching were predicted for the Amazon and northern India. We further found that N loss by fire regionally is a large term in the N budget, associated with lower N leaching, particularly in semi-arid biomes. Predicted global N leaching from natural lands rose from 13.6 Tg N yr -1 in 1901-1911 to 18.5 Tg N yr -1 in 1997-2006, accounting for reductions of natural land cover. Ecosystem N status (quantified as the reduction of vegetation productivity due to N limitation) shows a similar positive temporal trend but large spatial variability. Interestingly, this variability is more strongly related to vegetation type than N input. Similarly, the relationship between N status and (relative) N leaching is highly variable due to confounding factors such as soil water fluxes, fire occurrence, and growing season length. Nevertheless, our results suggest that regions with very high N deposition rates are approaching a state of N saturation.
Critical transitions in peatland development in South Florida
EGU General Assembly Conference Abstracts, Apr 1, 2012
ABSTRACT Terrestrial organic sediments cores from Southern Florida and the Everglades show two di... more ABSTRACT Terrestrial organic sediments cores from Southern Florida and the Everglades show two distinct episodes of initiation of peatland development between 4.5-6.5 ka and 2.0-3.5 ka and a mysterious interrupt separating these peat development phases. As peat development is highly sensitive to changes in groundwater, precipitation and evaporation, the reconstructed pattern of Southern Florida peatland development may provide novel insight in the response of Florida ecosystems to Holocene climate change. In this paper we propose and substantiate three hypotheses to explain these two distinct episodes of peat initiation: (1) a gradual increase in precipitation throughout the Holocene derived from proxies and climate models, (2) decreased drainage due to Holocene sea level rise depending on local topography and, (3) increased climatic variability from mid- to late-Holocene. The three hypotheses were tested in a model of peat accumulation and decomposition by means of specific forcings based upon climatic regional proxy data sets. The model results suggest that long-term average precipitation was sufficient for peat development throughout the Holocene, thereby not explaining the onset of peatland development at 6.5 ka. Although sea level rise and the local topography could explain this first period of peatland initiation, it could not account for the decline in peatland initiation after 4.5 ka. Instead, this period of reduced peatland initiation between 3.5-4.5 ka may be explained by an increase of multidecadal variability in precipitation. Multidecadal droughts decreased simulated hydroperiods and made peatlands vulnerable to erosion and fires. As peatland development is highly non-linear we show that peat heights may suddenly decrease from a dry to a wet low equilibrium by increased precipitation variability. The results further suggest that multidecadal climate variability after 4 ka can explain the second episode of peatland initiation. We conclude that the role of multidecadal climate variability is crucial to understand past and future peatland development in Southern Florida
Dredged cohesive sediment is progressively being used for wetland construction. However, little i... more Dredged cohesive sediment is progressively being used for wetland construction. However, little is known about the effect of plant growth during the self-weight consolidation of this sediment. In order to check the feasibility of such a study, a new experimental setup has been constructed. As an example, the effect of Phragmites australis on the consolidation and drainage of dredged sediment from Lake Markermeer, the Netherlands was investigated. The changes in pore water pressures at 10 cm depth intervals during a 129-day period in a column with and without plants were measured, while the water level was fixed at a constant level. Water loss via evaporation and plant transpiration was measured using Mariotte bottles and the photosynthetic processesincluding plant transpirationwere measured with a LI-COR photosynthesis system. The results show that several processes initiated by Phragmites australis interfere with the physical processes involved in sediment drainage and consolidation. Phragmites australis effectively altered the pore pressure gradient via water extraction, especially between 40 and 60 cm from the bottom of the column. In this zone, daily cycles in pore pressures were observed which could directly be linked to the diurnal cycle of stomatal gas exchange. On average, water loss via evaporation and transpiration of leaves of Phragmites australis amounted to 3.9 mm day -1 , whereas evaporation of bare soil amounted on average to 0.6 mm day -1 . The depth-averaged hydraulic conductivity increased on average by 40% in presence of Phragmites australis. This pilot experiment confirms that the pressures sensors coupled with the new set-up enable to study pore pressure development over time and to link the effect of plant growth with alterations in water pressures profiles. A more systematic study with this set-up will in the future enable to quantify the effects of plant growth on consolidation.
Understanding how the hydrological regime in relation to sediment type interferes with ecosystem ... more Understanding how the hydrological regime in relation to sediment type interferes with ecosystem development is important when wetlands are created with soft muddy material. Especially when plants are used as ecological engineers to promote crest stability and soil formation. We carried out a two-year mesocosm experiment with sediments derived from the Dutch lake Markermeer to identify the effects of the hydrological regime and sediment type on ecosystem functioning in terms of nutrient availability. We measured plant productivity, plant nutrient stoichiometry, and concentrations of N, P, and K in shoots and roots of Phragmites australis and Rumex maritimus and monitored how the clay-rich sediment from lake Markermeer changed into a wetland soil. Plants grown on Markermeer sediments tend to be N limited when periodically inundated and P limited when not inundated at all. The P availability was determined by the hydrological regime, while the N availability was determined by both the hydrological regime and the sediment type. Ecosystem development on created wetlands can be manipulated by adequate management of the hydrological regime, as plant species respond differently to changes in nutrient availability. This should be considered in eco-engineering projects where plants are used as ecological engineers to fasten ecosystem development on wetlands that are to be created from clay-rich material.
In the Vecht river plain, the vegetation diversity has been diminished during recent decades, due... more In the Vecht river plain, the vegetation diversity has been diminished during recent decades, due to hydrological changes and resulting eutrophication. Different options are discussed for hydrological management aimed at abating the eutrophication and at stimulating the characteristic mesotraphent succession phases of the fen vegetation. The ecological impact of the management plans are estimated with the hydro-ecological model ICHORS; this model takes into account the relations between the non-biotic environment and the presence of hydrophytes and phreatophytes. The impact of the hydrological options is assessed, after rearrangement of the predicted values of the species to values of the required fen vegetation. These results are incorporated in the management options with the water balances of the polders. Information on the costs of the options is used as a basis for a cost-benefit analysis. Technical hydrological solutions such as purification of the supplied water might contribute to the eutrophication abatement, but they are on their own unlikely to restore the ecosystems. Contrastingly, options involving the restoration of the original hydrology would probably lead to the characteristic mesotraphent fen vegetation once more, and the financial implications of this are likely to be the more favourable in the long-term.
Five mechanisms blocking the transition towards ‘nature-inclusive’ agriculture: A systemic analysis of Dutch dairy farming
Agricultural Systems, 2022
Abstract CONTEXT As elsewhere in Western Europe, large scale intensive agriculture dominates the ... more Abstract CONTEXT As elsewhere in Western Europe, large scale intensive agriculture dominates the landscape of the Netherlands. Grassland for dairy production occupies more than a quarter of its land surface. The high production intensity on conventionally farmed grassland leads to poor habitat quality, resulting in sharp declines in bird and insect numbers. Nature-inclusive agriculture (NIA) comprises innovations in farm management, technology and resource use that have the potential to address farmland biodiversity decline, but few Dutch farmers implement these. OBJECTIVE We aim to analyze the adoption of NIA practices in the Dutch dairy sector. Specifically, we study the influence of the dominant agri-food regime on the innovation system for NIA. METHODS Innovation Systems Analysis was performed to identify the various structural barriers which hinder adoption. Our study used a multi-method design in which data on NIA in the Dutch dairy sector was collected via a literature study, four workshops and a focus group discussion. RESULTS AND CONCLUSIONS We identified five key blocking mechanisms that hinder adoption of nature-inclusive agriculture in the Dutch dairy sector: (1) insufficient economic incentives for farmers, (2) limited action perspective of many dairy farmers in the Netherlands, (3) lack of a concrete and shared vision for NIA, (4) lack of NIA-specific and integral knowledge and (5) regime resistance, which moreover is connected to each of the previous blocking mechanisms. Our analysis shows that one of the empirical novelties of this paper is that these blocking mechanisms are strongly interlinked in the Dutch dairy sector, thereby perpetuating a situation of lock-in. We conclude that in order to accelerate adoption of nature-inclusive farming practices, problems need to be addressed in conjunction with one another, and therefore holistic approaches are key. A second important conclusion is that in order to foster growth of the innovation system around NIA, the focus should not only be on innovation, but also on transforming current regimes, in particular the currently dominant economic paradigms of growth and yield maximization. SIGNIFICANCE By unraveling strongly interlinked blocking mechanisms, this paper provides intervention points to accelerate the transition towards NIA in the Netherlands. These intervention points are not only located within the innovation system, but should preferable be sought for in the broader structures and institutions of the dominant agri-food regime.
Background and aims Understanding the potential effects of iron toxicity on plant development is ... more Background and aims Understanding the potential effects of iron toxicity on plant development is important when constructing new wetland from iron-rich sediment. We aim to study plant species-specific effects of iron toxicity when grown in the iron-rich sediments of lake Markermeer (the Netherlands). Methods Using three sediment sources that varied in total Fe and Fe-P concentrations, we performed a greenhouse experiment to study the development of three wetland species that differ in their tolerance to iron and utilization capacity of Fe-P: Rumex maritimus, Phragmites australis and Eupatorium cannabinum. Results Phragmites australis was the only species that developed an epidermis-damaging iron plaque on its roots. Plaque formation mainly depended on the Fe(III) and Fe-P concentration of the sediment, which led to different nutrient imbalances in leaves. All three species showed reduced growth compared to the control substrate, which could not be linked to indirect Fe toxicity. In contrast, direct Fe toxicity following the uptake of Fe could not be excluded as a mechanism potentially explaining our results, and this result warrants further examination in longer-term experiments. Conclusions Our results highlight the importance of considering the Fe and Fe-P availability in sediments, as these properties may constrain plant performance and delay the development of pioneer ecosystems in wetland construction sites.
The surface of bogs commonly shows various spatial vegetation patterning. Typical are "string pat... more The surface of bogs commonly shows various spatial vegetation patterning. Typical are "string patterns" consisting of regular densely vegetated bands oriented perpendicular to the slope. Here, we report on regular "maze patterns" on flat ground, consisting of bands densely vegetated by vascular plants in a more sparsely vegetated matrix of nonvascular plant communities. We present a model reproducing these maze and string patterns, describing how nutrient-limited vascular plants are controlled by, and in turn control, both hydrology and solute transport. We propose that the patterns are self-organized and originate from a nutrient accumulation mechanism. In the model, this is caused by the convective transport of nutrients in the groundwater toward areas with higher vascular plant biomass, driven by differences in transpiration rate. In a numerical bifurcation analysis we show how the maze patterns originate from the spatially homogeneous equilibrium and how this is affected by changes in rainfall, nutrient input, and plant properties. Our results confirm earlier model results, showing that redistribution of a limiting resource may lead to fine-scale facilitative and coarse-scale competitive plant interactions in different ecosystems. Self-organization in ecosystems may be a more general phenomenon than previously thought, which can be mechanistically linked to scale-dependent facilitation and competition.
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Papers by Martin Wassen