The nitrogen economy of mountain birch seedlings: implications for winter survival

1 Seedlings of mountain birch Betula pubescens ssp. czerepanovii were grown outdoors, under different environmental conditions, during their first growing season at Abisko, northern Sweden. Winter survival of seedlings was studied in relation to their growth and nitrogen (N) acquisition rate during the previous growing season.
2 Effects of fertilization and soil temperature on seedling growth and N acquisition were analysed in a factorial experiment including seven fertilizer levels and two temperature treatments. Effects of shading and neighbours ( B. p. ssp. czerepanovii and Empetrum hermaphroditum ) on seedling growth and N economy were evaluated in another experiment including five different neighbour interaction treatments.
3 An increase in either soil temperature or fertilization rate caused the birch seedlings to take up more N and grow faster. The relative growth rate and rate of N accumulation during the 12-week growing season were closely related to winter survival: at a relative growth rate of 2.0% and 3.8% day⁻¹, the winter survival of mountain birch seedlings was estimated to be 5% and 95%, respectively. This range corresponded to a relative N accumulation rate between 2.4% and 4.3% day⁻¹.
4 The relative N accumulation rate was clearly reduced by shading and by the plant–plant interactions studied. The effects of shading and birch neighbours but not of Empetrum on the rate of N accumulation could be explained by lower soil temperature.
5 Nutrient supply, soil temperature, vegetation shade and, presumably, allelopathy affect the N acquisition of first-year mountain birch seedlings, and thus also influence their winter survival. Soil temperature might be the major influence on the survival rate, due to its strong influence on the root N uptake rate.     

Growth and nitrogen utilization in seedlings of mountain birch (Betula pubescens ssp. tortuosa ) as affected by ultraviolet radiation (UV-A and UV-B) under laboratory and outdoor conditions

 Growth patterns and nitrogen economy were studied in pot-grown seedlings of mountain birch subjected to different ultraviolet radiation under both laboratory and outdoor conditions at Abisko in northern Sweden. In the laboratory, nutrient supply, temperature, humidity, ultraviolet radiation-A (UV-A, 320–400 nm) and B (UV-B, 280–320 nm) were controlled, while photosynthetically active radiation (PAR, 400–700 nm) and photoperiod varied naturally. Under outdoor conditions nutrient supply was controlled, and the irradiation treatments were ambient and above-ambient UV-B using additional fluorescent lamps. Mountain birch nitrogen economy was affected by increased ultraviolet radiation, as reflected by a changed relationship between plant growth and plant nitrogen both in the laboratory and outdoors. In the laboratory enhanced UV-A decreased leaf area per unit plant biomass (leaf area ratio) but increased biomass productivity, both per unit leaf area (leaf area productivity) and per unit leaf nitrogen (leaf nitrogen productivity). Low levels of UV-B affected growth patterns and nitrogen economy in a similar way to enhanced UV-A. High levels of UV-B clearly decreased relative growth rate and nitrogen productivity, as leaf area ratio, leaf area productivity and leaf nitrogen productivity were all decreased. Under outdoor conditions above-ambient levels of UV-B did not alter growth or biomass allocation traits of the seedlings, whilst nitrogen productivity was increased. Mountain birch seedlings originating from different mother trees varied significantly in their responses to different ultraviolet radiation. Received: 10 April 1997 / Accepted: 19 September 1997     

Systematic within-tree variation in mountain birch leaf quality for a geometrid, Epirrita autumnata

Abstract.

• 1 We studied within-tree variation in leaf quality of the mountain birch, Betula pubescens ssp. tortuosa, for larvae of the autumnal moth, Epirrita autumnata.
• 2 The purpose of the study was to determine the possible occurrence of systematic differences in larval growth on short shoot leaves (i.e. leaves of the same age): among leaves facing in different compass directions, between leaves of lower and upper branches, among leaves on different positions within a branch and among leaves of different sizes within a short shoot. We also measured larval growth between short shoot and long shoot leaves (i.e. between leaves of different age).
• 3 The larvae grew best on leaves on the north side of trees and most poorly on south side leaves, the east and west sides being intermediate. Leaves from the upper branches supported larval growth better than leaves from the lower ones. The larvae grew better on the smallest leaf of each short shoot and were able to utilize it more efficiently than the two larger leaves. Short shoot leaves from the basal and middle parts of the upper branches of the trees were of better quality for the larvae than short shoot leaves from the tip part of the branches. The larval growth rate did not differ between short shoot and long shoot leaves. In general, within-tree variation in the larval growth rate was lower than variation among different trees.
• 4 Damage to leaves can decrease leaf quality for herbivores in the same year (rapidly inducible responses) or the following year(s) (delayed inducible responses). Our results show that systematic within-tree variation in larval growth can be as great as the effects of rapidly inducible responses and that variation among individual trees can be as great as the mean effects of delayed inducible responses.

     

Comparative analysis of leaf trichome structure and composition of epicuticular flavonoids in Finnish birch species

The morphology, ultrastructure, density and distribution of trichomes on leaves of Betula pendula, B. pubescens ssp. pubescens, B. pubescens ssp. czerepanovii and B. nana were examined by means of light, scanning and transmission electron microscopy. The composition of flavonoids in ethanolic leaf surface extracts was analysed by high pressure liquid chromatography. All taxa examined contained both glandular and non-glandular trichomes (short and/or long hairs) but differed from each other in trichome ultrastructure, density and location on the leaf. Leaves of B. pubescens were more hairy than those of B. pendula, but the latter species had a higher density of glandular trichomes. Of the two subspecies of B. pubescens, leaves of ssp. pubescens had more short hairs on the leaf surface and four times the density of glandular trichomes of leaves of ssp. czerepanovii, whereas, in the latter subspecies, short hairs occurred largely on leaf veins, as in B. nana. The glandular trichomes were peltate glands, consisting of medullar and cortical cells, which differed structurally. Cortical cells possessed numerous small, poorly developed plastids and small vacuoles, whereas medullar cells had several large plastids with well-developed thylakoid systems and fewer vacuoles. In B. pubescens subspecies, vacuoles of the glandular cells contained osmiophilic deposits, which were probably phenolic, whereas in B. pendula, vacuoles of glandular trichomes were characterized by the presence of numerous myelin-like membranes. The composition of epicuticular flavonoids also differed among species. The two subspecies of B. pubescens and B. nana shared the same 12 compounds, but five of these occurred only in trace amounts in B. nana. Leaf surface extracts of B. pendula contained just six flavonoids, three of which occurred only in this species. In summary, the structure, density and distribution of leaf trichomes and the composition of epicuticular flavonoids represent good taxonomic markers for Finnish birch species.     

Cotyledon damage at the seedling stage affects growth and flowering potential in mature plants

Seedling herbivory is an important selective filter influencing patterns of plant community composition. Nevertheless, while many of the mechanisms governing seedling selection by herbivores are well established, the effects of tissue loss at the seedling stage on subsequent plant development are poorly understood. Here we examined how the removal of 50 or approximately 100% of cotyledon area from 7-d-old chalk grassland seedlings affected subsequent plant growth and flowering over a 100-d period. Cotyledon damage had a significant effect on growth during the establishment phase for six of the nine species. For two species, significant effects on plant growth were manifest in 100-d-old plants. Of the five species that flowered, three developed fewer inflorescences or flowered later as a consequence of cotyledon damage suffered as a seedling. Our results show that, in addition to the direct effect of herbivory on seedling mortality, more subtle sublethal effects may also influence plant establishment. Reduced growth as a result of cotyledon damage may have implications for plant competition during the establishment phase, and on subsequent reproductive success at maturity.     

Performance of moth larvae on birch in relation to altitude, climate, host quality and parasitoids

We studied topographical and year-to-year variation in the performance (pupal weights, survival) and larval parasitism of Epirrita autumnata larvae feeding on mountain birch in northernmost Finland in 1993–1996. We found differences in both food plant quality and parasitism between sites ranging from 80 m to 320 m above sea level. Variation in food plant quality had particularly marked effects on larval survival. The advanced phenology of the birches in relation to the start of the larval period reduced pupal weights. Parasitism rates were different between years and between sites. The clearest site differences were in the proportions of different parasitoid species: Eulophus larvarum was most abundant at the lowest-altitude sites, and Cotesia jucunda at the highest. Differences in the performance of E. autumnata were related to temperature conditions: at higher temperatures, survival and the egg production index were lower, and larval parasitism was higher than at lower temperatures. The higher parasitism at higher temperatures was probably due to greater parasitoid activity during warmer days. In the comparison of different sources of spatial and annual variation in the performance of E. autumnata, the most important factor appeared to be egg mortality related to minimum winter temperature, followed by parasitism and, finally, the variation in food plant quality. If, as predicted, the climate gradually warms up, the effects of warmer summers on the outbreaks of E. autumnata suggest a decrease in outbreak intensity. Received: 4 January 1999 / Accepted: 22 March 1999     

Wound-induced oxidative responses in mountain birch leaves

AIMS: The aim of the study was to examine oxidative responses in subarctic mountain birch, Betula pubescens subsp. czerepanovii, induced by herbivory and manual wounding. METHODS: Herbivory-induced changes in polyphenoloxidase, peroxidase and catalase activities in birch leaves were determined. A cytochemical dye, 3,3-diaminobenzidine, was used for the in situ and in vivo detection of H2O2 accumulation as a response to herbivory and wounding. To localize peroxidase activity in leaves, 10 mm H2O2 was applied to the dye reagent. KEY RESULTS: Feeding by autumnal moth, Epirrita autumnata, larvae caused an induction in polyphenoloxidase and peroxidase activities within 24 h, and a concomitant decrease in the activity of antioxidative catalases in wounded leaves. Wounding also induced H2O2 accumulation, which may have both direct and indirect defensive properties against herbivores. Wound sites and guard cells showed a high level of peroxidase activity, which may efficiently restrict invasion by micro-organisms. CONCLUSION: Birch oxidases together with their substrates may form an important front line in defence against herbivores and pathogens.     

Treeline advances along the Urals mountain range – driven by improved winter conditions?

High‐altitude treelines are temperature‐limited vegetation boundaries, but little quantitative evidence exists about the impact of climate change on treelines in untouched areas of Russia. Here, we estimated how forest‐tundra ecotones have changed during the last century along the Ural mountains. In the South, North, Sub‐Polar, and Polar Urals, we compared 450 historical and recent photographs and determined the ages of 11 100 trees along 16 altitudinal gradients. In these four regions, boundaries of open and closed forests (crown covers above 20% and 40%) expanded upwards by 4 to 8 m in altitude per decade. Results strongly suggest that snow was an important driver for these forest advances: (i) Winter precipitation has increased substantially throughout the Urals (~7 mm decade^?1), which corresponds to almost a doubling in the Polar Urals, while summer temperatures have only changed slightly (~0.05 °C decade^?1). (ii) There was a positive correlation between canopy cover, snow height and soil temperatures, suggesting that an increasing canopy cover promotes snow accumulation and, hence, a more favorable microclimate. (iii) Tree age analysis showed that forest expansion mainly began around the year 1900 on concave wind‐sheltered slopes with thick snow covers, while it started in the 1950s and 1970s on slopes with shallower snow covers. (iv) During the 20th century, dominant growth forms of trees have changed from multistemmed trees, resulting from harsh winter conditions, to single‐stemmed trees. While 87%, 31%, and 93% of stems appearing before 1950 were from multistemmed trees in the South, North and Polar Urals, more than 95% of the younger trees had a single stem. Currently, there is a high density of seedlings and saplings in the forest‐tundra ecotone, indicating that forest expansion is ongoing and that alpine tundra vegetation will disappear from most mountains of the South and North Urals where treeline is already close to the highest peaks.