Pinus cembra L. (arolla pine), a common tree in the inner French Alps since the early Holocene and above the present tree line: a synthesis based on charcoal data from soils and travertines

Abstract

Aim In this study, charcoal‐based data for Pinus cembra L. (arolla pine) were gathered from soil and travertine sequences in order to reconstruct its historical biogeography at the landscape level in the inner western Alps during the Holocene. Location The study sites are located between 1700 and 2990 m a.s.l., in the southern (Queyras Massif and Ubaye Valley) and the northern (Maurienne Valley) parts of the inner French Alps. Methods Charcoal fragments were extracted from sediments by water sieving, using meshes of 5, 2, 0.8 and 0.4 mm. The charcoal mass of P. cembra was determined in each charcoal assemblage. Accelerator mass spectrometry and conventional 14C measurements were used to date the fragments. Results Supported by 40 14C datings, the fragments show that, over 2000 m a.s.l., P. cembra accounts for around 40% (mean value) of identified fragments. Data reveal that arolla pine once extended between 260 and 375 m above the present‐day local tree lines. It was established in the southern and the northern French Alps from at least c. 9000 and 6000 cal yr bp, respectively. Main conclusions While present‐day populations of P. cembra are very fragmented in the inner French Alps, charcoal records indicate large past occurrences of this tree since the early Holocene. Human disturbance since the Neolithic seems to be the main reason for the regression of the arolla pine woodlands. On the south‐facing slopes of the study sites, currently deforested, this species extended up to 2800 m a.s.l. In the northern areas, charcoal records of the P. cembra expansion are consistent with the regional pollen archives, but in the southern massifs charcoal records indicate its presence c. 2600 years earlier than other palaeobotanical studies suggest. This discrepancy highlights the necessity to crosscheck data using several different proxies in order to assess the validity of conclusions regarding tree development in space and time.

Deep soil organic matter—a key but poorly understood component of terrestrial C cycle

“…Charcoal with a 14 C age of several thousand years was isolated by many authors from subsoil horizons (exp. Ali et al 2005). Moreover, black carbon seems to be prone to vertical migration (Dai et al 2005;Rumpel et al 2009) and may therefore accumulate in deep soil horizons.…”

Comparison of different methods of obtaining a resilient organic matter fraction in Alpine soils

“…Charcoal pieces are carbonised fragments of wood. They can give indication about past fire frequency and paleobotanical proxies to reconstruct past vegetation composition and structure (Berli et al, 1994;Carcaillet and Thinon, 1996;Figueral and Mosbrugger, 2000;Ali et al, 2005;) and, since charcoal fragments > 0.4 mm are not masstransported by air more than a few meters from the source of ignition, they can be used to get additional information about landscape dynamics and soil evolution during the last thousands of years BP. In Alpine environments, charcoal fragments cannot be time-stratified, because of the possible soil bioturbation by soil fauna, soil reworking by uprooted trees and freeze-thaw processes (Carcaillet et al, 2001).…”

Trees in the subalpine belt since 11 700 cal. BP: origin, expansion and alteration of the modern forest

“…However, at present, pure stands dominated by this pine are rare and extremely fragmented (review in Ali et al, 2005). The origin of the presentday mixed woodlands dominated by larch (Larix decidua) and…”