Nutrient limitation in boreal plant communities and species influenced by scything

Abstract. The nutrient status was studied in permanent plots of four plant communities, two rich-fen communities and two wooded grassland communities, all formerly used for haymaking. The concentrations of N, P and K in plant material of dominant and subdominant species (above- and below-ground) were measured in plots experimentally scythed annually or biennially for two decades, and in plots unscythed for four decades. Three of the communities had an N:P ratio of 14 or less, indicating N-limitation; the most fertile grassland community had particularly low values for the N:P ratio (6–12), as did a majority of the species, including all tall-herb species. A species-rich community of fen-margin vegetation in the lowest productive rich fen, had an N: P ratio of 17–19 in the above-ground biomass, which indicates P-limitation of nutrients. Molinia caerulea and Thalictrum alpinum were found to be the vascular plants with the highest N:P ratio, indicating P-limitation of nutrients. Calculations of N:K and K:P ratios indicated possible K-limitation in the rich-fen communities, especially for Thalictrum alpinum, the species with the highest N:K value. No expected change from N- to P-limited growth was found; in contrast, a reduction in the N:P ratio was found in the annually scythed plots of the rich fens, suggesting that reduced biomass production is mainly a result of disturbance by scything. As expected, a reduction in the concentration of K was detected in the scythed plots.     

Nutrient limitation and nutrient‐driven shifts in plant species composition in a species‐rich fen meadow

Question: We studied the development and persistence of the effects of nutrient pulses on biomass production and species composition in a fen meadow. Location: Nature reserve, central Netherlands, 5 m a.s.l. Methods: Single pulse fertilization with N and P in a factorial design on an undrained central and a drained margin site in a species‐rich fen meadow (Cirsio dissecti‐Molinietum). Biomass production and species composition were monitored during four years. Results: At the central site, N addition boosted biomass production, but only during one year. The species composition was not changed. P fertilization increased the biomass production and changed the species composition from a vegetation dominated by Carex panicea to a grassland community with abundant Holcus lanatus, but not before the second year. At the margin site, P fertilization changed the species composition in a similar way, but biomass production was not increased. N fertilization had no effect. At both sites the P induced shift in species composition persisted for four years although the P effect declined during the experiment. Conclusions: The biomass responses show that N was limiting in the central site. Another nutrient, besides N and P (probably K) must have been limiting in the marginal site. The fast decline of the N effect on biomass is ascribed to increased denitrification and biomass removal. The delay in the P effect on biomass and species composition and the persistence of the P effect on species composition are ascribed to fast immobilisation and subsequent slow release of fertilizer P in the peat soil. Recurrence of the P pulses is expected to cause permanent changes in species composition.     

Importance of assemblage‐level thinning: A field experiment in an alpine meadow on the Tibet plateau

Question: Which fraction of the decrease in species richness under fertilization can be explained by assemblage‐level thinning? Location: An alpine meadow on the eastern Tibet plateau. Methods: 60‐m² plots were randomly assigned to a control or one of four levels of ammonium phosphate fertilizer. Treatments were repeated for three years. The effect of as semblage‐level thinning was decided based on similarity in quadrats within and between fertilizing levels, bootstrap simulation based on random thinning of the high density (low production, low fertility) quadrats and correlation of species’ biomass in low fertility and high fertility. Results: Fertilization increased production, reduced species richness and reduced density of individuals. Heavily fertilized quadrats are more similar in species composition in 2000 but less similar in 2001 and 2002. Rarefaction showed that a decrease in density can account for 32.3‐42.9% decrease of species richness, but the simulated species richness is always significantly higher than the observed one. When production and species richness are similar at two levels of fertilization, species biomass in the higher fertility treatment is positively correlated with biomass at lower fertility. When the two fertilizer levels differed in production and species richness, there was no evidence of correlation in species biomass, suggesting that assemblage level thinning cannot explain all the loss of species. Conclusion: Although a decrease in density could explain much of the decrease (up to 42.9%) in species richness when this alpine meadow was fertilized, other important mechanisms such as interspecific competition cannot be ignored. Future studies should investigate the effect of assemblage level thinning on species diversity, and search for mechanisms responsible for a decrease in diversity.     

Testing the Wiegand–Milton model: A long‐term experiment to understand mechanisms driving vegetation dynamics in arid shrublands

The Wiegand and Milton (1996) simulation model predicts that vegetation dynamics in arid shrublands are characterized by event‐driven stochasticity (weather events), and demographic inertia (persistence of a species in a community) that lead to a lagged response in vegetation compositional change. Slow plant growth is one of the mechanisms driving slow vegetation change. We test this model at the same location (Tierberg Long‐term Ecological Research site) on which the model was based. Three dwarf shrub species, differing in palatability, were tracked over 25 years (1988–2014) at two levels of the past herbivory (pre‐1960) and three levels of the present herbivory (post‐1988). In the period between 1960 and 1988, all sites were grazed at the recommended agricultural stocking rate. For each species, plant density and a number of size attributes (basal diameter, height, canopy area) were surveyed. Analyses using a two‐way Analysis of Covariance (ANCOVA) took initial starting size into consideration. As the model predicted, event‐driven stochasticity (rainfall) resulted in an increase in density of the smaller size classes following a single large recruitment event across all grazing regimes for the palatable and unpalatable species. Size‐class distribution curve types remained unchanged illustrating that population demography remains unaffected for long periods and responses are slow (lagged response). Slow plant growth was evident in that there were no changes in height, canopy area, or density under present grazing regimes over the 25‐year period. Palatable species had a reduced canopy area and density compared to unpalatable species. Our findings provide empirical evidence supporting the predictions of the Wiegand and Milton (1996) model, notably event‐driven stochasticity, demographic inertia, and a lagged response in vegetation change in arid shrublands. In addition, our results support the model assumption of the significance of slow growth in long‐lived plant species and the influence of grazing regime.     

ω‐Turn: A novel β‐turn mimic in globular proteins stabilized by main‐chain to side‐chain CH···O interaction

Mimicry of structural motifs is a common feature in proteins. The 10‐membered hydrogen‐bonded ring involving the main‐chain C?O in a β‐turn can be formed using a side‐chain carbonyl group leading to Asx‐turn. We show that the N? H component of hydrogen bond can be replaced by a C^γ‐H group in the side chain, culminating in a nonconventional C? H···O interaction. Because of its shape this β‐turn mimic is designated as ω‐turn, which is found to occur ～three times per 100 residues. Three residues (i to i + 2) constitute the turn with the C? H···O interaction occurring between the terminal residues, constraining the torsion angles ?_{i + 1}, ψ_{i + 1}, ?_{i + 2} and χ′_{1(i + 2)} (using the interacting C^γ atom). Based on these angles there are two types of ω‐turns, each of which can be further divided into two groups. C^β‐branched side‐chains, and Met and Gln have high propensities to occur at i + 2; for the last two residues the carbonyl oxygen may participate in an additional interaction involving the S and amino group, respectively. With Cys occupying the i + 1 position, such turns are found in the metal‐binding sites. N‐linked glycosylation occurs at the consensus pattern Asn‐Xaa‐Ser/Thr; with Thr at i + 2, the sequence can adopt the secondary structure of a ω‐turn, which may be the recognition site for protein modification. Location between two β‐strands is the most common occurrence in protein tertiary structure, and being generally exposed ω‐turn may constitute the antigenic determinant site. It is a stable scaffold and may be used in protein engineering and peptide design. Proteins 2015; 83:203–214. © 2014 Wiley Periodicals, Inc.     

Cutting reduces variation in biomass production of forage crops and allows low‐performers to catch up: A case study of Trifolium pratense L. (red clover)

• Re‐growth of fodder plants after grazing and mowing drives the profitability of their cultivation and is therefore an important target trait for plant breeding and agricultural engineering. However, for some fodder plants little is known about their re‐growth dynamics in response to grazing or mowing.
• We analysed the native response of plant architecture, leaf morphology and growth performance to experimental cutting in wild Trifolium pratense L. (red clover) plants. A total of 150 potted clover plants were established under controlled field conditions, and half of the plants were cut to 5 cm 3 months after sowing. Each plant was measured every week for 5 months.
• The cut and subsequently re‐grown plants carried fewer main branches (?20%), as well as fewer (?13%) and smaller (?32%) leaves than the control plants. However, the cut plants produced an average of 17% more accumulated leaf area (cut + re‐grown leaf area) than the control plants. This discrepancy was explained by variation in the growth strategy of the plants, where the cut plants invariably expressed a second growth phase, while almost half of the untreated plants did not.
• Our results suggest that cutting acted as an artificial trigger initiating a second growth phase in the cut plants and thereby contributed to yield increase. Exploiting this mechanism may set new goals for breeding and optimisation of the mowing regime.