Håkan Rydin

Atmospheric nitrogen deposition promotes carbon loss from peat bogs

Peat bogs have historically represented exceptional carbon (C) sinks because of their extremely low decomposition rates and consequent accumulation of plant remnants as peat. Among the factors favoring that peat accumulation, a major role is played by the chemical quality of plant litter itself, which is poor in nutrients and characterized by polyphenols with a strong inhibitory effect on microbial breakdown. Because bogs receive their nutrient supply solely from atmospheric deposition, the global increase of atmospheric nitrogen (N) inputs as a consequence of human activities could potentially alter the litter chemistry with important, but still unknown, effects on their C balance. Here we present data showing the decomposition rates of recently formed litter peat samples collected in nine European countries under a natural gradient of atmospheric N deposition from ≈0.2 to 2 g·m−2·yr−1. We found that enhanced decomposition rates for material accumulated under higher atmospheric N supplies resulted in higher carbon dioxide (CO2) emissions and dissolved organic carbon release. The increased N availability favored microbial decomposition (i) by removing N constraints on microbial metabolism and (ii) through a chemical amelioration of litter peat quality with a positive feedback on microbial enzymatic activity. Although some uncertainty remains about whether decay-resistant Sphagnum will continue to dominate litter peat, our data indicate that, even without such changes, increased N deposition poses a serious risk to our valuable peatland C sinks.

COLONIZATION–EXTINCTION DYNAMICS OF AN EPIPHYTE METAPOPULATION IN A DYNAMIC LANDSCAPE

Metapopulation dynamics have received much attention in population bi- ology and conservation. Most studies have dealt with species whose population turnover rate is much higher than the rate of patch turnover. Models of the dynamics in such systems have assumed a static patch landscape. The dynamics of many species, however, are likely to be significantly affected by the dynamics of their patches. We tested the relative im- portance of local conditions, connectivity, and dynamics of host tree patches on the meta- population dynamics of a red-listed epiphytic moss, Neckera pennata, in Sweden. Repeated surveys of the species and its host trees were conducted at three sites over a period of six years. There was a positive effect of connectivity, and colonizations mainly occurred in the vicinity of occupied trees. Colonizations were also less likely on strongly leaning trees. Local extinctions sometimes occurred from small trees with low local abundances but were most often caused by treefall. Simulations of the future (100 years) dynamics of the system showed that the metapopulation size will be overestimated unless the increased local ex- tinction rate imposed by the dynamics of the trees is accounted for. The simulations also suggested that local extinctions from standing trees may be disregarded in dynamic models for this species. This implies that the dynamics of N. pennata can be characterized as a patch-tracking metapopulation, where local extinctions are caused by patch destruction.

Effect of water level on desiccation of Sphagnum in relation to surrounding Sphagna

Desiccation of Sphagnum during periods of dry weather and low water table in a mire was followed by measuring the capitulum water content. The lower the water content, the greater the desiccation. For four investigated species desiccation increased in the order S. fuscum < S. rubellum < S. balticum < S. tenellum. The same ranking was found in a laboratory experiment in which the species were exposed to a gradually lowered water table. This ranking is related to the uppermost levels reached by the species on the hummocks of the mire. It is argued that the frequency and degree of desiccation, being more severe in the hollow species, will prevent these from growing on the hummocks. The specific identity of the Sphagnum surrounding the investigated capitulum has a profound effect on the course of its desiccation. Capitula of a species that normally dries out quickly (S. balticum, S. tenellum) benefit from being surrounded by shoots of superior species (S. fuscum, S. rubellum). This may explain the observation that individual shoots of S. balticum can be found at a higher level above the water table in a S. fuscum-hummock than in single-species carpets of S. balticum. This may be interpreted as a case of commensalism, acting in combination with competition.

Transport of carbon and phosphorus compounds about Sphagnum

The bog mosses, Sphagnum, lack any obvious anatomical specialization inside the stem but have a well-developed system of water conduction in capillary spaces among pendent branches around the stem. It has hitherto been assumed that this was the main route for solute transfer too. We describe experiments showing that there is rapid and quantitatively important transport within the stems. The tracers 32P and 14C were supplied to S. recurvum. When applied below the top of the plant they moved to the apex whether external mass flow was upward or downward. Autoradiographs showed high concentration of tracers in the stem. Steaming the stem above and below the site of application prevented tracer movement. An experiment lasting four weeks on S. papillosum showed that after 14C labelling almost all that part of the alcohol-soluble fraction that moved did so from older parts to the apex, with very little transfer to the insoluble fraction, to neighbours, or into the gas phase. For the soluble fraction in the capitulum, the conductance for 14C from below was about the same as that of its loss in respiration. The conductance to the insoluble fraction was about twice as great, and to neighbours about half as much. A longer experiment showed that predominately acropetal transport continued for at least 22 weeks in five species. By that time about 25% of the remaining 14C label was incorporated in the new tissues. The stem of S. recurvum contains a central mass of parenchyma 20-50 cells across. The end walls of the parenchyma cells have perforations approximately 100 nm across at a density of 7-13 μm-2. A single cell wall has about 1500 perforations. They probably house plasmodesmata. These results indicate that Sphagnum has an effective mechanism for retaining and relocating solutes within it. This property, coupled with the ability to grow in a very low supply of solutes, and to make the environment acid by cation exchange, may be seen as causes of the widespread success of Sphagnum.

Species diversity, niche metrics and species associations in harvested and undisturbed bogs

. Competition is considered an important force in structuring plant communities and in governing niche relations, but communities recovering from disturbance, may be less governed by species interactions and less orderly organized. To address this issue, we studied species richness, abundance and patterns of association between plant species at three spatial scales (1 m2, 1/25 m2, 1/625 m2) in two ombrotrophic mires in east-central Sweden. One was at a secondary successional stage following peat extraction 50 yr ago and the other was undisturbed. Peat extraction leads to a change in hydrology which is slowly restored by the formation of new peat. Niche breadth and niche overlap along the gradient of height above the water table were calculated for the five common Sphagnum species occurring at both mires in an attempt to better understand differences in species co-occurrence at each mire. Species cover differed between the mires, and the number of species per plot was higher in the undisturbed community at all scales, suggesting that the degree of species intermingling was greater than at the harvested site. At all scales, the number of non-random associations was higher, and niche overlap lower among ecologically similar species (e.g. hollow Sphagnum species) in the undisturbed mire. These differences indicate that random events are important in colonization, and that biotic interactions between neighbours later result in a higher degree of non-randomness. In addition, we surveyed a number of abandoned peat pit sites to test the effect of disturbance for species composition at a regional scale. Ombrotrophic peat pits contained several Sphagnum species normally associated with minerotrophic mires, and species of wooded mires occurred frequently in peat pits, making them more species-rich than undisturbed bogs. There were also Sphagnum species new to, or rare in, this part of Sweden which indicates effective long-distance dispersal. Even 50 yr after peat extraction had ceased, the vegetation had not recovered to its original composition.

Plant species richness on islands over a century of primary succession: Lake Hjälmaren

Between 1882 and 1886 the water level of Lake Hjalmaren, Sweden, was lowered - 130 cm. As a consequence, a large number of new islands arose. Lists of vascular plant species have been obtained for such islands of size range 50 m2 to 2.5 ha in 1886, 1892, 1903-1904, 1927-1928, and 1984-1985. Species-area relationships were slightly better described by the exponential S = C + z log A than with the power model S = CAz (fitted with nonlinear regression) in all stages of succession. There were no clear trends in the shape of the species-area curve over time. Except for the 1886 survey all slopes of the power model included the theoretically sug- gested value of 0.26 in their confidence intervals, indicating that this value has no obvious significance in relation to equilibrium or succession. On large islands (>0.3 ha) species number had reached an equilibrium at -95 species with immigration of -30 species balancing extinction between the last two surveys. Indications of interactions that led to decreases in species richness at late successional stages are weak. Small islands still continue to accumulate species, mainly due to substrate stabilization, which leads to formation of habitats suitable for plant establishment. A stepwise regression indicates over the whole successional sequence log area as more important for variation in species number than descriptors of island shape, habitat diversity, or location. However, for individual islands the residuals in the species-area regression could be explained by special habitat features.

Plant Characteristics Over a Century of Primary Sucession on Islands: Lake Hjalmaren

Between 1882 and 1886 the water level of Lake Hjdlmaren, Sweden was lowered z 130 cm. As a consequence, a large number of new islands arose. From 30 such islands we have complete lists of vascular plant species with notes on their abundance from 1886, 1892, 1903-1904, 1927-1928, and 1984-1985. The 112 most important species were grouped as pioneers, early-, mid-, or late-suc- cessional, depending on when in the sere they were most abundant or frequent. Several species (referred to as stayers) remained common once they had entered the succession. For the 112 species we collected literature data on height, life form, fecundity, mating system, pollination, seed size, dispersal, and relation to nitrogen and light, and the suc- cessional groups were compared with regard to these attributes. The characteristics of the pioneers were distinct from other groups. Eight of the 13 pioneers were annuals (therophytes), all of which were predominantly autogamous, indi- cating a strong coupling between these life history traits. After the pioneer stage there was no continuous change in these attributes. The seeds in pioneers were not smaller than in early- or midsuccessional species, and seed size did not differ between annuals and perennials. Water dispersal was very important, and therefore dispersal success put no constraint on seed size. Species with the heaviest seeds arrived late; notably they were bird dispersed. Only among wind-dispersed trees and shrubs was there a gradual increase in seed mass over succession. The Ellenberg light indicator value showed a slight decrease over time, as did a variable measuring preference for open habitats. We propose that physiology may be more important than life history for species replacements after the pioneer stage.

Rapid ecosystem shifts in peatlands: linking plant physiology and succession.

Stratigraphic records from peatlands suggest that the shift from a rich fen (calcareous fen) to an ombrotrophic bog can occur rapidly. This shift constitutes a switch from a species-rich ecosystem to a species-poor one with greater carbon storage. In this process, the invasion and expansion of acidifying bog species of Sphagnum (peat mosses) play a key role. To test under what conditions an acidifying bog species could invade a rich fen, we conducted three experiments, contrasting the bog species S. fucsum with the rich-fen species S. warnstorfii and S. teres. We first tested the effect of calcareous water by growing the three species at different constant height above the water table (HWT; 2, 7, and 14 cm) in a rich-fen pool and measured maximum photosynthetic rate and production and difference in length growth as an indicator of competition. In none of the species was the photosynthetic capacity negatively affected when placed at low HWT, but S. fuscum was a weaker competitor at low HWT. In our second experiment we transplanted the three species into microhabitats with different and naturally varying HWT in a rich fen. Here, S. fuscum nearly ceased to photosynthesize when transplanted to low HWT (brown moss carpet), while it performed similarly to the two rich-fen species at the intermediate level (S. warnstorfii hummock level). In contrast to S. fuscum, the rich-fen sphagna performed equally well in both habitats. The brown moss carpet was seasonally flooded, and in our third experiment we found that S. fuscum, but not S. teres, was severely damaged when submerged in rich-fen water. Our results suggest two thresholds in HWT affecting the ecosystem switch: one level that reduces the risk of submergence and a higher one that makes bog sphagna competitive against the rich-fen species.

Diversity and pH changes after 50 years on the boreal mire Skattlösbergs Stormosse, Central Sweden

The Skattlosbergs Stormosse min was reinvestigated in 1995 after 50 years of natural development since the previous investigation. The undrained mire is situated in an area with low anthropogenic deposition. The distribution of 106 plant species was mapped in detail and pH was measured at 251 locations, providinga unique opportunity to quantify long-term mire dynamics. The resultss how decreased pH in the richer (high-pH) parts of the mire, but little or no change in the poor fen andombrotrophipca rts.1 4 species had disappeared while two news pecies were recorded. Most species had a more restricted distribution in the mire area in 1995 than in 1945. Species richness in lOm x 0Omp lots had decreased, especially in plots with higher pH. Most Sphagnum species had unchanged distributions over the mire, while 7 3 % of other bryophyt sepecies and 3 8% of vascular plant species had decreased by more than 20 % in plot frequency. There was a strong relationship between number of species and pH-value. The mean and standard eviation of pH in plots where the species occurred have both decreased since 1945. We interpret the changes in species richness in the richer fens to be mainly caused by acidification. This could partly be an autogenic succession, but may be enhanced by increased atmospheric deposition. The mire represents an almost untouched site which can act as a reference for mires in morepolluted areas.

Climatic modifiers of the response to nitrogen deposition in peat-forming Sphagnum mosses: a meta-analysis

Peatlands in the northern hemisphere have accumulated more atmospheric carbon (C) during the Holocene than any other terrestrial ecosystem, making peatlands long-term C sinks of global importance. Projected increases in nitrogen (N) deposition and temperature make future accumulation rates uncertain. Here, we assessed the impact of N deposition on peatland C sequestration potential by investigating the effects of experimental N addition on Sphagnum moss. We employed meta-regressions to the results of 107 field experiments, accounting for sampling dependence in the data. We found that high N loading (comprising N application rate, experiment duration, background N deposition) depressed Sphagnum production relative to untreated controls. The interactive effects of presence of competitive vascular plants and high tissue N concentrations indicated intensified biotic interactions and altered nutrient stochiometry as mechanisms underlying the detrimental N effects. Importantly, a higher summer temperature (mean for July) and increased annual precipitation intensified the negative effects of N. The temperature effect was comparable to an experimental application of almost 4 g N m(-2)  yr(-1) for each 1°C increase. Our results indicate that current rates of N deposition in a warmer environment will strongly inhibit C sequestration by Sphagnum-dominated vegetation.