Growth of Betula pubescens and Pinus sylvestris Seedlings in a Subarctic Environment

Abstract

The relative growth rates of seedlings of the deciduous tree Betula pubescens ssp. tortuosa (Ledeb.) Nyman and the evergreen tree Pin us sylvestris L. were compared in a subarctic environment using material grown in pots. The species showed similar growth rates under low nutrient conditions, while Betula grew faster under high nutrient conditions. The higher growth rate of Betula under high nutrient conditions is apparently a function of a larger leaf area per unit plant mass combined with a larger proportion by weight of roots. Intraspecific variation in the growth rates of both species was mainly caused by differences in unit leaf rates. The unit leaf rate was higher for Pin us than for Betula under nutrient poor conditions but there was little difference when species were growing under nutrient rich conditions. The leaf productivity per unit leaf nitrogen was similar for both species when grown under nutrient poor conditions but Betula was more productive per unit N under high nutrient conditions. The species showed similar relations between nitrogen uptake and root mass. Betula lost 5-25% of the whole plant nitrogen at leaf abscission, while Pin us lost 0-2.6 % of the nitrogen in the leaves which were shed during a year. The plasticity in dry matter partitioning was relatively small for both species when compared with woody plants in more southern environments. Key-words: Betula pubescens ssp. tortuosa, Pinus sylvestris, seedlings, relative growth rate, dry matter partitioning, unit leaf rate, nitrogen economy, deciduous, evergreen, subarctic Correspondence: Dr P.S. Karlsson, Department of Plant Ecology, University of Lund, 0 Vallgatan 14, S-223 61 Lund, Sweden. Introduction In the subarctic parts of Fennoscandia the deciduous Betula pubescens ssp. tortuosa (Ledeb.) Nyman is the dominant tree. In some areas, scattered stands of the evergreen Pinus sylvestris L. coexist with B. pubescens (Sonesson & Lundberg 1974). Mature trees of both species have been found to produce viable seeds (K.O. Nordell & M. Sonesson, unpublished) but seedlings are rarely found in situ (K.O. Nordell, in preparation). The seedling phase of the life cycle, i.e. the establishment and survival of the seedlings, may therefore be critical in determining the dominance relationships between these two species. A high growth rate decreases the time spent at a vulnerable size (ef Grime & Hunt, 1975; Calow & Townsend, 1981; Fitter & Hay, 1983), and the probability that a particular seedling will survive might thus increase with increasing growth rate (Sarukhan, Martinez-Ramos & Piner, 1984). The growth performance at the seedling stage is therefore likely to be important for maintaining distribution patterns of the two species. Evergreen and deciduous plants differ in a number of characteristics important for growth; subarctic evergreens generally have a larger proportion of leaf biomass than deciduous species (Kjelvik & Kdrenlampi, 1975; Svoboda, 1977). The leaf area ratio in evergreens is comparatively low due to the correspondingly lower specific leaf area (Chabot & Hicks, 1982). In addition, the turnover of mineral nutrients (Monk, 1966; Small, 1972, 1973; Reader, 1978) and photosynthetic rates (Chabot & Hicks, 1982; Mooney & Gulmon, 1982) are generally slower in evergreen than in deciduous leaves. In this paper the relative growth rates of seedlings of Betula pubescens ssp. tortuosa and Pinus sylvestris are compared and related to dry matter partitioning and plant nitrogen economies. More specifically, an attempt was made to determine which of those plant characteristics which determine growth rate differ between these species and how these characteristics can be modified by environmental changes. This was done using the relationships between relative This content downloaded from 207.46.13.57 on Fri, 09 Sep 2016 04:18:47 UTC All use subject to http://about.jstor.org/terms 38 growth rate (R), unit leaf rate (EA) and plant dry P. S. Karlsson & matter partitioning (for abbreviations see Table 1): K. 0. Nordell R = EA X FA, where the leaf area ratio, FA, is related to the specific leaf area (LA/Lw) and the weight proportion of leaves (Lw/W): FA = LA/Lw x Lw/W (Causton & Venus, 1981). Materials and methods