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1 Nutrient dynamics and growth of the feathermoss Hylocomium splendens were studied in a subarctic birch woodland. My aims were to estimate the species’ nutrient use strategy (and its determinants: mean residence time (MRT) and annual nutrient productivity (aNP)) and to unravel possible traits related to nutrient conservation and their implications for ecosystem nitrogen flux. Three methods to estimate nutrient losses in bryophytes were evaluated: a conventional growth analysis technique, a retrospective analysis and a 15N-tracer approach.
2 Estimates for nitrogen retention varied between 3 and 10 years as a result of the different N pools considered by the three methods. Growth analysis results depended on the distinction between live and dead tissues, whereas retrospective analysis gave valuable information on N release from decaying segments but did not measure MRT within the living segments. Valid estimates of MRT were obtained by a 15N-tracer approach.
3 MRT and aNP of H. splendens were similar to values typically found in woody evergreen vascular plants. Efficient nutrient recycling and a relatively long segment life span were responsible for the long residence time of nitrogen. Feathermosses show efficient nutrient acquisition, nutrient recycling and acropetal transport; nutrient losses will therefore be small.
4 Dominant bryophytes may retard the nutrient turnover at the forest floor through their production of acidic nutrient-poor organic matter and their negative effect on soil temperature, and they may therefore function as autogenic ecosystem engineers.
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Low temperatures and a short growing season limit plant performance at high altitudes and latitudes. Low temperatures reduce mineralization rates and this – together with a strong microbial competition for free mineral nutrients (Michelsen et al. 1995; Jonasson et al. 1996) – leads to the growth of plants of cold regions generally being nutrient limited. Nevertheless, gradients of nutrient availability exist even within the tundra (e.g. Shaver & Chapin 1991; Giblin et al. 1991; Weih 1998; Weih & Karlsson 1999) and the various combinations of traits exhibited by higher plants in response to such contrasts can be characterized as different strategies of nutrient use (Eckstein & Karlsson 1997). The extremes are represented by fast growing species (herbs and graminoids) with a short retention time of growth limiting nutrients and by slow growing species (mainly evergreens) with a long residence time of these nutrients. The relative performance of plants can be expressed by the index of nutrient use efficiency (NUE) (Berendse & Aerts 1987), which is defined as the annual amount of dry matter produced per unit of nutrient lost. Berendse & Aerts (1987) partitioned NUE into two components: (i) the mean annual nutrient productivity (aNP), which measures the annual dry matter production per unit nutrient in the plant, and (ii) the mean residence time (MRT). Nitrogen productivity depends on photosynthetic nitrogen use efficiency and nitrogen allocation within the plant (Garnier et al. 1995) and represents a measure of the species’ productivity. MRT, which is estimated as the ratio between the annual average nutrient pool and the annual nutrient losses, is an index of nutrient conservation and depends on tissue life span and nutrient resorption (Escudero et al. 1992; Aerts 1995; Eckstein & Karlsson 1997; Garnier & Aronson 1998).
The few available data on the NUE of bryophytes suggest that these plants are either very efficient (Chapin & Shaver 1989) or are intermediate (Oechel & Van Cleve 1986) as compared to various vascular plant life-forms. To my knowledge, there has been no attempt to characterize the nutrient use efficiency sensuBerendse & Aerts (1987) of any individual bryophyte species. There are three reasons why bryophytes do not lend themselves easily to the growth analysis needed for ecophysiological studies. Firstly, the annual dry matter productivity, a central variable in growth analysis, is difficult to estimate in the vast majority of species because they lack innate annual markers of growth. Second, there is no clear-cut distinction between live and dead tissues in bryophytes, nor can these states be defined unambiguously. Third, due to the origin of the gametophytic shoots from microscopic protonema, followed by intense ramification, clonal growth and an unknown degree of clonal integration, it is often difficult to define functional growth units. Therefore, the colony rather than the ‘individual’ is regarded as the functional unit in many bryophyte species (Bates 1998).
Despite these problems, information on the nutrient use strategy of bryophytes is essential in order to understand the role of bryophytes in terrestrial ecosystems. An increasing body of evidence suggests that traits of plants adapted to nutrient-poor or nutrient-rich habitats may have feedback effects on decomposition and ecosystem nutrient turnover (e.g. Hobbie 1992; Berendse 1998) and thus have a stabilizing effect on ecosystem development (Aerts 1999). I studied growth and nutrient use of Hylocomium splendens, an important bryophyte of boreal and subarctic woodlands. My aim was to characterize this species in terms of its nitrogen (N) use strategy and to compare its nitrogen productivity and its ability to conserve N with that of various types of vascular plants. I addressed the following questions:
1 What is the mean residence time of growth limiting nutrients in H. splendens? Mosses characteristic of nutrient-poor habitats should be conservative in terms of their nutrient use. Therefore, I hypothesize that H. splendens will show a long mean residence time of growth limiting nutrients (high nutrient retention) similar to evergreen vascular plants. Since MRT and aNP make conflicting demands on plant performance, a high MRT in H. splendens will be paid for by a low annual productivity per unit nutrient.
Furthermore, given the problems of growth analysis in bryophytes, I used three different methods for estimating the losses and the retention time of N. A traditional growth analysis based on sequential harvests over 2 years was compared with a 15 N-tracer approach where the fate of an initial amount of label was followed through time, and with a retrospective analysis of growth and N and phosphorus (P) content of interconnected annual segments.