Rien Aerts

Nutrient resorption from senescing leaves of perennials: Are there general patterns?

1 Possible patterns in nutrient resorption efficiency (% of the leaf nutrient pool resorbed) from senescing leaves of perennials were examined at both the intra- and the interspecific level. Most of the data used originated from studies with evergreen and deciduous shrubs and trees. 2 Combining all data, mean nutrient resorption efficiency was 50% for N (n = 287) and 52% for P (n = 226). N resorption efficiency of evergreen shrubs and trees (47%) was significantly lower than in deciduous shrubs and trees (54%), whereas P resorption efficiency did not differ significantly between these growth-forms (51 and 50%, respectively). Although nutrient resorption is an important nutrient conservation mechanism at the species level, it does not differ strongly between growth-forms. 3 Mean N and P concentrations in leaves of deciduous shrubs and trees were about 60% higher than in evergreen species. There were only small differences in mean resorption efficiency and nutrient concentrations in leaf litter of deciduous species were therefore much higher than in evergreens. This implies that, in comparison with deciduous species, the low nutrient concentrations in mature leaves of evergreens contribute far more to nutrient conservation than does nutrient resorption. 4 Relations between leaf nutrient status and leaf nutrient resorption were absent or very weak. Assuming that leaf nutrient status reflects nutrient availability, this implies that nutrient resorption is only weakly controlled by nutrient availability. 5 At the intraspecific level, nutrient resorption was not very responsive to increased nutrient availability. There was no response in 63% of the experiments analysed (covering 60 spp.), whereas in 32% there was a decrease in N resorption in response to increased nutrient availability. For P (37 species analysed) there was no response in 57% of the cases and in 35% of the cases P resorption decreased upon enhanced nutrient supply. Evergreen shrubs and trees showed especially low responsiveness. 6 This review shows that there are no clear nutritional controls on nutrient resorption efficiency. Future research should focus on the biochemical basis of variation in nutrient resorption efficiency and on the factors, other than nutrient availability, that control nutrient resorption efficiency.

The advantages of being evergreen

Recent research shows that the dominance of evergreen species in nutrient-poor environments can be explained by their low nutrient loss rates. From this work It appears that the plant traits that are associated with low nutrient loss rates lead to low maximum-dry-matter production and to low rates of litter decomposition. This suggests a positive feedback between the evergreen habit and low nutrient availability. The growth characteristics of evergreens lead to a low responsiveness to environmental changes. As a result, global warming may lead to changes in the distribution of evergreens.

Carbon respiration from subsurface peat accelerated by climate warming in the subarctic

Among the largest uncertainties in current projections of future climate is the feedback between the terrestrial carbon cycle and climate. Northern peatlands contain one-third of the world’s soil organic carbon, equivalent to more than half the amount of carbon in the atmosphere. Climate-warming-induced acceleration of carbon dioxide (CO2) emissions through enhanced respiration of thick peat deposits, centuries to millennia old, may form a strong positive carbon cycle–climate feedback. The long-term temperature sensitivity of carbon in peatlands, especially at depth, remains uncertain, however, because of the short duration or correlative nature of field studies and the disturbance associated with respiration measurements below the surface in situ or during laboratory incubations. Here we combine non-disturbing in situ measurements of CO2 respiration rates and isotopic (13C) composition of respired CO2 in two whole-ecosystem climate-manipulation experiments in a subarctic peatland. We show that approximately 1 °C warming accelerated total ecosystem respiration rates on average by 60% in spring and by 52% in summer and that this effect was sustained for at least eight years. While warming stimulated both short-term (plant-related) and longer-term (peat soil-related) carbon respiration processes, we find that at least 69% of the increase in respiration rate originated from carbon in peat towards the bottom (25–50 cm) of the active layer above the permafrost. Climate warming therefore accelerates respiration of the extensive, subsurface carbon reservoirs in peatlands to a much larger extent than was previously thought. Assuming that our data from a single site are indicative of the direct response to warming of northern peatland soils on a global scale, we estimate that climate warming of about 1 °C over the next few decades could induce a global increase in heterotrophic respiration of 38–100 megatonnes of C per year. Our findings suggest a large, long-lasting, positive feedback of carbon stored in northern peatlands to the global climate system.

Growth-limiting nutrients in Sphagnum-dominated bogs subjects to low and high atmospheric nitrogen supply

The effects of increased nitrogen or phosphorus supply on the productivity of Sphagnum-dominated ombrotrophic bogs in northern and southern Sweden were studied. Atmospheric nitrogen deposition in souther Sweden (high-N site) exceeds that in northern Sweden (low-N site) by about tenfold. Vertical height growth of the Sphagnum carpet was measured by the crankedwire method. Length growth of individuals was measured by autoradiography after labelling with 14 CO 2 . The results of both methods were significantly correlated, but the cranked-wire data were systematically lower. Productivity of Sphagnum at the low-N site increase almost fourfold after additional nitrogen supply (4 g N m −1 year −1 ), but no increase was found after additional phosphorus supply (0.4 g P m −2 year −1 ) (...)

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.

PLANT-MEDIATED CONTROLS ON NUTRIENT CYCLING IN TEMPERATE FENS AND BOGS

This paper reports on patterns in plant-mediated processes that determine the rate of nutrient cycling in temperate fens and bogs. We linked leaf-level nutrient dynamics with leaf-litter decomposition and explored how the observed patterns were reflected in nutrient cycling at the ecosystem level. Comparisons were made among growth forms (evergreen and deciduous shrubs and trees, graminoids and Sphagnum mosses) and between mire types (fens and bogs). A literature review showed that the predominant growth form was more important as a determinant of leaf-level nutrient-use efficiency (NUE) than mire type (fen vs. bog). Evergreens had the highest N and P use efficiency. The growth form differences in NUE were mainly determined by differences in N and P concentrations in mature leaves and not by differences in resorption efficiency from senescing leaves. Sphagnum leaves had lower N and P concentrations than the other growth forms, but because of a lack of data on nutrient resorption efficiency the NUE of thes...

The relation between above- and belowground biomass allocation patterns and competitive ability

SummaryIn a 2-year experiment, the evergreen shrubsErica tetralix andCalluna vulgaris (dominant on nutrient-poor heathland soils) and the perennial deciduous grassMolinia caerulea (dominant on nutrient-rich heathland soils) were grown in replacement series in a factorial combination of four competition types (no competition, only aboveground competition, only belowground competition, full competition) and two levels of nutrient supply (no nutrients and 10 g N+2 g P+10 g K m−2 yr−1). Both in the unfertilized and in the fertilized treatmentsMolinia allocated about twice as much biomass to its root system than didErica andCalluna. In all three species the relative amount of biomass allocated to the roots was lower at high than at low nutrient supply. The relative decrease was larger forMolinia than forErica andCalluna. In the fertilized monocultures biomass of all three species exceeded that in the unfertilized series.Molinia showed the greatest biomass increase. In the unfertilized series no effects of interspecific competition on the biomass of each species were observed in either of the competition treatments. In the fertilized mixtures where only belowground competition was possibleMolinia increased its biomass at the expense of bothErica andCalluna. When only aboveground competition was possible no effects of interspecific competition on the biomass of the competing species were observed. However, in contrast with the evergreens,Molinia responded by positioning its leaf layers relatively higher in the canopy. The effects of full competition were similar to those of only belowground competition, so in the fertilized series belowground competition determined the outcome of competition. The high competitive ability ofMolinia at high nutrient supply can be attributed to the combination of (1) a high potential productivity, (2) a high percentage biomass allocation to the roots, (3) an extensive root system exploiting a large soil volume, and (4) plasticity in the spatial arrangement of leaf layers over its tall canopy. In the species under study the allocation patterns entailed no apparent trade-off between the abilities to compete for above- and belowground resources. This study suggests that this trade-off can be overcome by: (1) plasticity in the spatial arrangement of leaf layers and roots, and (2) compensatory phenotypic and species-specific differences in specific leaf area and specific root length.

Nutrient use efficiency in evergreen and deciduous species from heathlands

SummaryThe nutrient (N, P) use efficiency (NUE: g g−1 nutrient), measured for the entire plant, of field populations of the evergreen shrubs Erica tetralix (in a wet heathland) and Calluna vulgaris (in a dry heathland) and the deciduous grass Molinia caerulea (both in a wet and a dry heathland) was compared. Erica and Calluna are crowded out by Molinia when nutrient availability increases. NUE was measured as the product of the mean residence time of a unit of nutrient in the population (MRT: yr) and nutrient productivity (A: annual productivity per unit of nutrient in the population, g g−1 nutrient yr−1. It was hypothesized that 1) in low-nutrient habitats selection is on features leading to a high MRT, whereas in high-nutrient habitats selection is on features leading to a high A; and that 2) due to evolutionary trade-offs plants cannot combine genotypically determined features which maximize both components of NUE.Both total productivity and litter production of the Molinia populations exceeded that of both evergreens about three-fold. Nitrogen and phosphorus resorption from senescing shoots was much lower in the evergreens compared with Molinia. In a split-root experiment no nutrient resorption from senescing roots was observed. Nutrient concentrations in the litter were equal for all species, except for litter P-concentration of Molinia at the wet site. Both Erica and Calluna had a long mean residence time of both nitrogen and phosphorus and a low nitrogen and phosphorus productivity. The Molinia populations showed a shorter mean residence time of N and P and a higher N- and P-productivity. These patterns resulted in an equal nitrogen use efficiency and an almost equal phosphorus use efficiency for the species under study. However, when only aboveground NUE was considered the Molinia populations had a much higher NUE than the evergreens.The results are consistent with the hypotheses. Thus, the low potential growth rate of species from low-nutrient habitats is probably the consequence of their nutrient conserving strategy rather than a feature on which direct selection takes place in these habitats.

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.

Highly consistent effects of plant litter identity and functional traits on decomposition across a latitudinal gradient.

Plant litter decomposition is a key process in terrestrial carbon cycling, yet the relative importance of various control factors remains ambiguous at a global scale. A full reciprocal litter transplant study with 16 litter species that varied widely in traits and originated from four forest sites covering a large latitudinal gradient (subarctic to tropics) showed a consistent interspecific ranking of decomposition rates. At a global scale, variation in decomposition was driven by a small subset of litter traits (water saturation capacity and concentrations of magnesium and condensed tannins). These consistent findings, that were largely independent of the varying local decomposer communities, suggest that decomposer communities show little specialisation and high metabolic flexibility in processing plant litter, irrespective of litter origin. Our results provide strong support for using trait-based approaches in modelling the global decomposition component of biosphere-atmosphere carbon fluxes.