Effects of prescribed burning on plant available nutrients in dry heathland ecosystems

Heathland management is an important tool with which to modify ecosystem impacts caused by atmospheric nutrient deposition. Since changes in nutrient availability as a result of management measures affect the outcomes of heathland succession and species competition, studies on this issue are important from both a nature conservation and management point of view. This study reports the effects of prescribed burning on nutrient availability in dry heathland soils and the nutrient content of the two competing heathland species Calluna vulgaris and Deschampsia flexuosa, with particular reference to N and P. We hypothesise that winter prescribed burning leads to additional N availability, which enhances the importance of P in the context of nutrient limitation in heathland ecosystems. In the nature reserve “Lueneburg Heath” (NW Germany) we examined the availability of nutrients in the humus horizons and in the leachate as well as the relevant C:element ratios in Calluna and Deschampsia before and after a burning experiment. Our results show that prescribed burning resulted in drastically increased NH₄⁺ availability in the O-horizon. We observed only short-term effects (for NO₃⁻, PO₄³⁻, Mg) and insignificant effects on the availability of other nutrients (K, Ca). As a consequence of an increased nutrient availability in the humus horizons and a limited nutrient uptake by plants after burning, leaching increased significantly for N, Ca, K, and Mg after burning treatment. No significant changes were found in the foliar C:N ratios for either species after prescribed burning, although Deschampsia showed an increased deficiency for all the other nutrients, particularly for P, as expressed by increased foliar C:P and N:P ratios. By contrast, the nutrient content of Calluna did not change significantly, suggesting that prescribed burning favours the competitive capacity of Calluna as against Deschampsia. We assume that water shortage as a result of changes in the microclimate was mainly responsible for the deterioration of the nutrient content of Deschampsia. This gives Calluna a competitive advantage, enabling it to out-compete Deschampsia on burned heathlands, with respect to the key factor P-limitation.     

Root production and root turnover in two dominant species of wet heathlands

Summary Root biomass production, root length production and root turnover of Erica tetralix and Molinia caerulea were estimated by sequential core sampling and by observations in permanent minirhizotrons in the field. Root biomass production, estimated by core sampling, was 370 (Erica) and 1080 (Molinia) g m^-2 yr^-1. This was for both species equal to aboveground production. Assuming steady-state conditions for the root system, root biomass turnover rates (yr^-1), estimated by core sampling, were 1.72 (Erica) and 1.27 (Molinia). Root length production of both species, estimated by minirhizotron observations, varied significantly with observation depth. Root length turnover rate (yr^-1) of both species did not vary significantly with observation depth and averaged 0.92 in Erica and 2.28 in Molinia. Reasons are given for the discrepancy between the results of the two types of turnover measurements. The data suggest that the replacement of Erica by Molinia in a wet heathland, which occurs when nutrient availability increases, leads to an increased flow of carbon and nutrients into the soil-system. Therefore, there may be a positive feedback between dominance of Molinia and nutrient availability.     

Nutrient use efficiency in evergreen and deciduous species from heathlands

Summary The 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.     

A laboratory assessment of the relative susceptibility of Molinia caerulea (L.) Moench and Calluna vulgaris (L.) Hull to a range of herbicides

There is a concern that Molinia caerulea (L.) Moench may be increasing in upland moorland communities at the expense of Calluna vulgaris (L.) Hull and other ericaceous species. In order to develop a strategy for the control of Molinia, laboratory studies were carried out to determine the dose-response relationships of seven graminicides and glyphosate on both Molinia caerulea and Calluna vulgaris. Plants were grown under laboratory conditions and sprayed with increasing doses of herbicide using a precision sprayer. Results were highly variable, indicating the great morphological plasticity of these two species, making determination of the ED₅₀ difficult. Glyphosate, quizalofop-ethyl and sethoxydim successfully reduced various measures of Molinia growth to 50% of control levels (ED₅₀ estimates ranged from 0.41–0.67 kg a.i. ha^-1, 0.18–1.20 kg a.i. ha^-1and 0.37–0.49 kg a.i. ha^-1respectively). In contrast, only glyphosate reduced Calluna growth significantly (ED₅₀s ranged from 0.025–0.45 kg a.i. ha^-1), whilst selective herbicides left it undamaged. A comparison of the results for Molinia and Calluna suggests that there is no application rate of glyphosate which will reduce Molinia effectively whilst leaving Calluna undamaged. Recommendations for field testing of selective herbicides are discussed.     

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.     

The influence of ammonium nitrate on the root growth and ericoid mycorrhizal colonization of Calluna vulgaris (L.) Hull from a Danish heathland

Conversion of European heathlands to grassland has been reported as a response to increased nutrient availability, especially of nitrogen; a direct effect upon mycorrhizal colonization has been proposed as an likely explanation.This hypothesis was tested in a random block experiment with four blocks and four replicates on a Danish inland heath, Hjelm Hede. Ammonium nitrate was applied (0, 35, 50 and 70 kg N ha^–1 year^–1) to a stand of Calluna vulgaris (L.) Hull four times annually for 2 years. Calluna roots were sampled on four occasions in the 2nd year of the nitrogen treatment. The extent of ericoid mycorrhizal colonization was determined by direct observation of the roots using a line-intersection method. The nitrogen content of the current-year shoots of Calluna increased when they were treated with nitrogen. Nitrogen fertilization had no significant effects on ericoid mycorrhizal colonization of Calluna nor on root biomass. The seasonal variation in mycorrhizal colonization of the Calluna roots was highly significant. The spatial variability of mycorrhizal colonization, both in replicated plots and in the two contrasted soil horizons – the mor layer and the bleached sand – within the plots, were considerable. I conclude that heather decline under enhanced nitrogen input is unlikely to be caused by a direct impact on the ericoid mycorrhizae of Calluna. Received: 1 December 1998 / Accepted: 7 December 1999     

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

Summary In 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.     

Wood growth response of Calluna vulgaris (L.) Hull to elevated N deposition and drought

Ring diameters and ring number counts were recorded from Calluna stem cross-sections harvested during an N manipulation experiment. The application of dendrochronological methods permitted accurate ageing of shrubs growing in a heathland of known age. Linear regression analyses demonstrated a highly significant association between increasing levels of N input and stem diameter in both watered and droughted plots (droughted in 1997 only). N inputs were identified as the most significant environmental stressor, appearing to speed ageing vis the Calluna growth cycle. Drought sensitivity was noted in the high N treatment plots (120 kg N ha⁻¹ yr⁻¹) with decreased stem diameters. This may have implications in the response of Calluna growth under current climate change scenarios.     

The effect of increased nutrient availability on leaf turnover and aboveground productivity of two evergreen ericaceous shrubs

Summary Leaf turnover and aboveground productivity in relation to nutrient availability were studied in the evergreen shrubs Erica tetralix and Calluna vulgaris. In monospecific stands of these species four levels of nutrient (NPK) availability were created during three growing seasons. Percentage survival and life expectancy of Erica leaves decreased with increasing nutrient availability. For Calluna there was no effect. Winter mortality of Erica leaves was smaller than growing season mortality. These was no difference for Calluna. The timing of leaf mortality of both species was not affected by nutrient treatment. At the end of the experimental period current year leaf biomass, total biomass and current year second year and third year biomass of both species showed a significant increase with increasing nutrient availability. The relative increase was greater for Calluna, except for second and third year biomass. Stem production and stem mortality of both species increased with increasing nutrient availability. The increased stem mortality resulted also for Calluna in an increased leaf turnover (per unit ground area) with increasing nutrient availability. Nutrient cycling in ecosystems dominated by these species will increase with increasing nutrient availability, because of increased leaf and stem turnover and productivity. This phenotypic effect is similar to the effect of the shift in dominance between different species which occurs along natural gradients of nutrient availability.     

15N estimates of nitrogen fixation by white clover (Trifolium repens L.) growing in a mixture with ryegrass (Lolium perenne L.)

The ¹⁵N isotope dilution technique and the N difference method were used to estimate N₂ fixation by clover growing in a mixture with ryegrass, in a field experiment and a controlled environment experiment. Values obtained using N difference were approximately 25% lower than those estimated using ¹⁵N isotope dilution. In the field experiment there was a measured N benefit to grass growing with clover, equivalent to 42.7 kgN ha^-1. The grass in the mixture had a lower atom %¹⁵N content and a higher N content than grass in a monoculture; therefore values for N₂ fixation were different depending on choice of control plant i.e. monoculture or mixture grass. In the controlled environment experiment there were no significant differences between either the atom %¹⁵N contents or the N contents of monoculture grass and grass growing in a mixture with clover. It is concluded that there is a long term indirect transfer of N from clover to associated grass which can lead to errors in estimates of N₂ fixation.     

Fitness of resprouters versus seeders in relation to nutrient availability in two Plantago species

Two contrasting strategies of plants from disturbed areas are reported to depend on nutrient availability. Resprouters, investing into storage and capable of vegetative regeneration after disturbance, are predicted to be enhanced in nutrient poor environments. This contrasts to seeders, which invest preferentially into seed production and regenerating only from seeds, and are thought to prevail in nutrient rich environments. To test such predicted dichotomy, we set up an experiment with two facultative resprouters with contrasting nutrient demands and assessed the fitness of individuals regenerated from seeds and root fragments in differently productive environments. We hypothesized that 1) plants with higher nutrient demands have a higher fitness as seeders irrespectable of nutrient availability and/or 2) both species will have a higher fitness as resprouters under lower nutrient availability and as seeders when nutrient availability is higher. Nutrient availability was also manipulated prior to and after disturbance to evaluate the impact of changing nutrient availability on the strategy of resprouting. The results of our pot experiment with Plantago lanceolata and Plantago media supported the first but not the second hypothesis. Moreover, high nutrient availability prior to disturbance negatively affected resprouting success, but the growth and fitness of successfully regenerated individuals were enhanced under higher nutrient availability. We concluded that resprouting from roots after disturbance is affected by nutrient availability, but this effect considerably differs between individual life-history stages.     

Competitive relationships between two contrasting but coexisting grasses

Leymus chinensis (Trin.) Tzvelev and Phragmites communis Trin. are co-dominant grasses in both nutrient-poor and rich steppes in the Songnen Plains of northeastern China. A replacement series experiment was conducted along a five-level nutrient gradient to test for changes in the competitive relationships of the species with nutrient availability. There were significant effects of nutrient level and species proportion on mean plant height, biomass and rhizome length. Facilitation was found in these plant attributes for both species when grown in mixture compared with monoculture, especially at high nutrient levels. For example, L. chinensis grew taller and intercepted more light when in mixture. The aboveground dominance of P. communis did not decrease in mixture, possibly because it benefited from belowground interactions with L. chinensis. Biomass allocation was apparently influenced by both above and belowground competition. For example, L. chinensis allocated much more biomass to shoots and less to roots when neighbor number was increased, suggesting that aboveground competition occurred. Root: shoot ratios of L. chinensis in monoculture decreased gradually with increasing nutrient availability, whereas the ratio in mixture declined rapidly, implying intense competition for light at high levels. Our results support the hypothesis that nutrient competition is more important than light competition when soil nutrients are scarce. Although the interactions between these species benefit both, P. communis is more likely to displace L. chinensis at high nutrient availability.     

Contrasted Responses of Two Understorey Species to Direct and Indirect Effects of a Canopy Gap

Positive associations between adult trees and understorey species have been explained either by direct or indirect facilitation. We tested both models by comparing the performance of two understorey species with contrasted stress-tolerance abilities Galium odoratum and Deschampsia flexuosa. Individuals of both species were transplanted in the four combinations of two treatments (gap and removal of an herbaceous competitor, Molinia caerulea). Our experiment demonstrated that direct facilitation of adult trees may explain the restricted occurrence of the shade-demanding Galium within closed forest communities. In contrast, the shade-tolerant Deschampsia was subjected to additional competition within the forest, likely because adult trees had a higher negative effect on light availability and a similar negative effect on nitrogen availability within the forest than did Molinia in the gaps.     

Community interactions between the filamentous alga Cladophora glomerata (L.) Kuetzing,its epiphytes,and epiphyte grazers

Summary Interactions between epiphytes, epiphyte grazers and the filamentous green alga Cladophora glomerata (L.) Kuetzing were explored with smaples from rivers in Montana. Extracts of C. glomerata lowered photosynthetic rates of Nitzschia fonticola Grunow (an epiphytic diatom). Nutrient enrichment showed that C. glomerata from the Madison River was N deficient and its epiphytes were P deficient on 2 dates and N deficient on one date, while no nutrient deficiencies were detected in samples from 3 other rivers; this implies there was little nutrient competition between the epiphytes and C. glomerata. Epiphytes lowered drag on C. glomerata tufts and current velocity inside the tufts, apparently by decreasing the effective surface area. Lower drag may decrease detachment, but lowering current velocity from 8 to 0 cm s^-1 resulted in a 100 % decrease in photosynthesis. Light absorption by epiphyte pigments may lower photosynthetic rate of C. glomerata when irradiance is below 200–500 E m^-2 s^-1, and protect against photoinhibition above this irradiance range. Invertebrate grazers (predominantly Baetis tricaudatus Dodds, Trycorythodes minutus Traver and Brachycentrus occidentalis Banks) at high densities removed 75% of epiphytes and B. occidentalis grazed on C. glomerata. Invertebrates regenerated a mean of 0.16 mol NH _inf4 ^sup+ individual^-1 d-1 which could have enhanced growth of downstream C. glomerata. Competition and grazing were not the only interactions in the C. glomerata community, positive (mutualistic) interactions were also important.     

Size-selective predation by fourhorn sculpin,Myoxocephalus quadricornis (L.) (Pisces) on Mesidotea entomon (L.) (Crustacea,Isopoda)

During three different seasons Mesidotea entomon specimens from fourhorn sculpin stomachs were analyzed, and compared with the M. entomon population in the field. The field samplings were carried out in the northern Bothnian Sea. Feeding experiments revealed fourhorn sculpins to be highly selective when feeding on M. entomon.The fourhorn sculpin most preferably selected large M. entomon in the field as well as in the laboratory experiments. The preference for large M. entomon remained after correction for availability of differently sized M. entomon. In summer the actual sizes of M. entomon eaten by fourhorn sculpin were smaller than in autumn and winter. During all three seasons the oldest M. entomon were the ones most preferred. Ultimate effects of predation by fourhorn sculpin on the life-history of M. entomon are discussed.     

Dissolution of ferric phosphate by alfalfa (Medicago sativa L.) root exudates

Alfalfa (Medicago sativa L.) was grown in hydroponic culture to investigate adaptation to Fe-deficiency. Root exudates released into the nutrient solution from Fe-deficient plants were trapped and condensed on an amberlite XAD-4 resin column. The diethyl ether fraction of these exudates dissolved ferric phosphate remarkably. The dissolving capability was about 62 times higher than that of root exudates obtained from Fe-sufficient plants in complete nutrient solution. The Fe-dissolving compound was separated and identified. It was a new natural compound with molecular formula C₁₄H₁₀O₅ and was identified as 2-(3,5-dihydroxyphenyl)-5,6-dihydroxybenzofuran by means of mass spectrometry and ¹H-nuclear magnetic resonance. This new compound worked as a phytoalexin and inhibited completely the fungal growth of Fusarium oxysporum f. sp. phaseoli.     

Variability and structure of natural populations of Elymus caninus (L.) L. and their possible relationship with Hordelymus europaeus (L.) Jess. ex Harz as revealed by AFLP analysis

The AFLP method was used to study the inter-population variability of eight populations of Elymus caninus (L.) L. as well as three populations of Hordelymus europaeus (L.) Jess. ex Harz. In these studies a clear distinction was found between two phenotypes of E. caninus collected in the same locality. It also appeared that two populations of E. caninus representing the “pauciflorum” morphotype were clustered together, similarly as two populations of E. caninus exhibiting morphotype “caninus”. Additionally, the populations of the “pauciflorum” type were clustered together with all samples of H. europaeus. Furthermore, the same approach was applied to analyze the intra-population variability of E. caninus. The populations ranged from nearly uniform to as diverse as the samples collected from different localities. In some populations of this species the presence of off-type plants was revealed. Our data indicate the predominantly self-pollinating character of E. caninus and the possible genetic relationship between of E. caninus and H. europaeus. It is the second paper from the series: Biodiversity of wild Triticeae (Poaceae) in Poland, the first is: Mizianty M. 2005. Variability and structure of natural populations of Elymus caninus (l.) L. based on morphology. Pl. Syst. Evol. 251: 199–216.     

Linking community and ecosystem development on Mount St. Helens

In the two decades following the 1980 eruption of Mount St. Helens in Washington State, the N₂-fixing colonizer Lupinus lepidus is associated with striking heterogeneity in plant community and soil development. We report on differences in nutrient availability and plant tissue chemistry between older, dense patches (core) of L. lepidus and more recently established low density patches (edge). In addition, we conducted a factorial nitrogen and phosphorus fertilization experiment in core patches to examine the degree of N and P limitation in early primary succession. We found that there were no significant differences in N or P availability between core and edge L. lepidus patches during the dry summer months, although nutrient availability is very low across the landscape. In the high density patches we found lower tissue N content and higher fiber content in L. lepidus tissue than in the younger edge patches. The addition of nutrients substantially altered plant community composition, with N addition causing an increase in other forb biomass and a corresponding competition-induced decline in L. lepidus biomass. The majority of the positive biomass response came from Hypochaeris radicata. In the second year of the fertilization experiment, the addition of N significantly increased total community biomass while L. lepidus biomass declined by more than 50%. The response of every species other than L. lepidus to N additions suggests that N may be the macronutrient most limiting plant production on Mount St. Helens but that the gains in productivity were somewhat offset by a decline of the dominant species. By the third year of the experiment, L. lepidus began to increase in abundance with P addition. This result suggests co-limitation of the community by N and P.     

Foliar Iron Fertilization of Peach (Prunus persica (L.) Batsch): Effects of Iron Compounds, Surfactants and Other Adjuvants

During a survey of nitrogen-fixing Burkholderia associated with sugarcane in Tamil Nadu, some endophytes were isolated on PCAT medium. Isolation was based on the use of the selective PCAT medium. Four isolates were studied, all belonging to the genus Burkholderia. One of them, MG43 was consistently more active in reducing acetylene and was identified as Burkholderia vietnamiensis. This isolate was used to inoculate micro-propagated sugarcane plantlets in a comparison with two other diazoptrophs, viz. Gluconacetobacter diazotrophicus^T and Herbaspirillum seropedicae^T. Inoculated plants and uninoculated controls were used in a pot experiment followed by two field experiments under different rates of nitrogen fertilisers. MG43 and G. diazotrophicus performed best in sugarcane, their natural host. Biomass increase due to MG43 inoculation reached 20% in the field. Inoculated plants were heavily colonised by the inoculated bacterium (up to 115,000 CFU g⁻¹ root fresh weight). Inoculation by a combinaison of the three strains performed less well than inoculation by a single MG43 suspension. Ecological implications are discussed, as well as the potential of these bacteria to provide a feasible alternative to higher N fertilisers rates in a low input and long term sustainable rural economy.     

Interactive effects of mycorrhization and elevated carbon dioxide on growth of young pedunculate oak (Quercus robur L.) trees

Pedunculate oak (Quercus robur L.) was germinated and grown at ambient CO₂ level and 650 ppmv CO₂ in the presence and absence of the ectomycorrhizal fungus Laccaria laccata for a total of 6 month under nutrient non-limiting conditions. Mycorrhization and elevated atmospheric CO₂ each supported the growth of the trees. Stem height, stem diameter, and dry matter accumulation of pedunculate oak were increased by mycorrhization. Elevated atmospheric CO₂ enhanced stem height, stem diameter, fresh weight and dry weight, as well as lateral root formation of the trees. In combination, mycorrhization and elevated atmospheric CO₂ had a more than additive, positive effect on tree height and biomass accumulation, and further improved lateral root formation of the trees. From these findings it is suggested that the efficiency of the roots in supporting the growth of the shoot is increased in mycorrhized oak trees at elevated atmospheric CO₂.Abbreviations DW dry weight - FW fresh weight - RWC relative water content