Evidence of threshold temperatures for xylogenesis in conifers at high altitudes |
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Authors: | Sergio Rossi Annie Deslauriers Tommaso Anfodillo Vinicio Carraro |
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Institution: | (1) Treeline Ecology Research Unit, Dipartimento TeSAF, Università degli Studi di Padova, viale dell’Università 16, 35020 Legnaro, PD, Italy |
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Abstract: | Temperature is the most important factor affecting growth at high altitudes. As trees use much of the allocated carbon gained
from photosynthesis to produce branches and stems, information on the timing and dynamics of secondary wood growth is crucial
to assessing temperature thresholds for xylogenesis. We have carried out histological analyses to determine cambial activity
and xylem cell differentiation in conifers growing at the treeline on the eastern Alps in two sites during 2002–2004 with
the aim of linking the growth process with temperature and, consequently, of defining thresholds for xylogenesis. Cambial
activity occurred from May to July–August and cell differentiation from May–June to September–October. The earliest start
of radial enlargement was observed in stone pine in mid-May, while Norway spruce was the last species to begin tracheid differentiation.
The duration of wood formation varied from 90 to 137 days, depending on year and site, with no difference between species.
Longer durations were observed in trees on the south-facing site because of the earlier onset and later ending of cell production
and differentiation. The threshold temperatures at which xylogenesis had a 0.5 probability of being active were calculated
by logistic regressions. Xylogenesis was active when the mean daily air temperature was 5.6–8.5°C and mean stem temperature
was 7.2–9°C. The similar thresholds among all trees suggested the existence of thermal limits in wood formation that correspond
with temperatures of 6–8°C that are supposed to limit growth at the treeline. Different soil temperature thresholds between
sites indicated that soil temperature may not be the main factor limiting xylogenesis. This study represents the first attempt
to define a threshold through comparative assessment of xylem growth and tissue temperatures in stem meristems at high altitudes. |
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Keywords: | Alps Cambial activity Cell differentiation Treeline Tree ring |
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