Biomass allocation An increase in the leaf fraction and decrease in the stem fraction was observed from 2,000 m to 2,200 m. Branch fraction variations with altitude seem to differ in the two species (Fig. 2). Dry matter partitioning in Larix decidua Mill. and Pinus cembra L. in Italian treeline ecotones Claudio Fior, Tommaso Anfodillo, Alessandro Masotto, Giorgio Pielli, Vinicio Carraro Treeline Ecology Research Unit, TeSAF Department University of Padova, Agripolis, Viale dell'Università 16, 35020 Legnaro (PD) Italy e-mail: claudio.fior@unipd.it Treeline is certainly determined by low temperatures, but the physiological process affected is still under discussion. Tree height and biomass partitioning are reported to vary with altitude but very few observations are available. Studies on biomass allocation can provide further information in order to test a recent hypothesis of treeline formation. P.cembra L. decidua Materials and Methods 45 plants of L. decidua were collected in the Eastern Italian Alps (Fig. 1) near Cortina d’Ampezzo (Belluno) at 1,500, 2,000 and 2,200 m a.s.l.; in the same area 51 P. cembra plants were sampled at 1,700, 2,000 and 2,200 m. Sites were selected for past and current low anthropogenic pressure. Trees had a fairly homogenous base diameter (< 6 cm). Plant parameters were measured (Tab.1) and the whole trees extracted. The dry weight of the different parts was measured after 24 h of oven-drying at 80°C. Biomass allocation was expressed as the ratio between the biomass invested in a part and total biomass. Statistical treatment of the data was made with ANOVA type III analysis and Kruskal- Wallis. Conclusions Plant growth and biomass allocation seemed to be inconsistent with the carbon limitation hypothesis (Cavieres et al., 2000), which foresees a direct limitation in the plant carbon balance at higher altitudes. The results instead agree with the growth limitation hypothesis (Körner, 1998) wich foresees a limitation on plant growth related to new cell formation in the cambial zone. References Bernoulli M, Körner C (1999) Dry matter allocation in treeline species. Phyton 39: 7-12 Cavieres LA, Rada F, Azocar A, Garcia-Nunez C, Cabrera HM (2000) Gas exchange and low temperature resistance in two tropical high mountain tree species from the Venezuelan Andes. Acta Oecologica 21: 203-211 Körner C (1998) A re-assessment of high elevation treeline positions and their explanation. Oecologia 115: 445-459 Li MH, Yang J, Kräuchi N (2003) Growth responses of Picea abies and Larix decidua to elevation in subalpine areas of Tyrol, Austria. Canadian Journal of Forest Research 33: 653-662 Stevens GC, Fox JF (1991) The causes of treeline. Ann Rev Ecol Syst 22: 177-191 2.200 2.000 1.700 m -1.500 Italy Fig. 1 - Site location Plant growth A significant reduction in annual height growth with altitude was observed. L. decidua had longer roots and deeper root systems with increasing altitude (Fig. 3, 4). Fig. 3 - L. decidua main allometric traits Root system deep +200% Longest root +150% 1500 m 2000 m 2200 m Longest root +210% Plant height -29% Root system deep +180% L. decidua 14th Federation of European Societies of Plant Biology Congress 23-27 August 2004 Discussion Results are consistent with studies on small plants (Bernoulli and Körner, 1999), but differ from measurements on mature (Körner, 1999) or taller plants (Li et al., 2003). This different behaviour may be due to the more favourable micro- ecological conditions of small plants due to the atmosphere boundary layer close to the soil (Li et al., 2003). Height growth decreases with altitude but not biomass production. The growth limitation does not appear to be related to insufficient primary production. There is a lower proportion of heterotrophic tissues at higher altitude, so respiration could not be expected to increase. Fig. 4 - P. cembra main allometric traits Plant height -42% 1700 m 2000 m 2200 m Plant height -66% P. cembra Fig. 2 - Biomass allocation; in yellow significantly different fractions than at other altitudes in the same species. L. decidua 1500 m 2000 m 2200 m 20% 30% 40% 50% 60% 70% 80% 90% 100% P. cembra 1700 m 2000 m 2200 m Crown Branches Stem Roots 19% 31% 24% 30% 15% 40% 32% 29% 27% 35% 23% 14% 13% 20% 21% 33% 32% 26% 33% 30% 27% 24% 9% 15% Base stem diameter Height Total biomass Age Longest branch Number of branches Longest root Root system depth Needle biomass Branch biomass Stem biomass Root biomass Tab. 1 - Parameters measured