| Résumé: |
This work tests the hypothesis that deglaciation and climate change interact to condition the water sources available to trees in revegetating proglacial forefields. To test the hypothesis, we cored trees in the proglacial margin of the Olguin glacier (Torres del Paine National Park, Chile) over the last three decades. Cores were obtained from three zones. Three forest sites were chosen with regards to presumed water supply sources, respectively Z1 (a proglacial stream proximal zone), Z2 (a proglacial stream distal zone), and Z3 (an intermediate zone). For trees within each zone, we determined annual growth rates from the cores and also determined the δ2H signature associated with each annual growth. We found that the growth histories of each zone were statistically distinct. Whilst tree growth was mainly dependent on precipitation and temperature, we found that the growth rates in Z1 were significantly different from Z2 and Z3. Using meteorological data (precipitation and temperature), available for a 30-year period we found that Z1 growth rates were strongly correlated with temperature, Z2 with precipitation and that Z3 showed a shift from precipitation (i.e. Z2) to temperature correlation (i.e. Z1). The average isotopic signature of δ2H was lowest at Z1, suggesting that tree growth involves depleted water most likely of glacial origin. It was highest at Z2 suggesting a predominantly meteoric water supply. At Z3, isotopic signature was initially higher but declined through time, suggesting a shift from meteoric water supply to glacial water supply. We argue that these data show how the increased water supply associated with temperature-driven glacier recession may compensate for decreasing water supply from precipitation to influence tree growth. The extent to which this happens is likely to be a function of the spatial organisation of the subsurface flux of glacial melt and lead to different revegetation processes to those envisaged in the classical chronosequence model of vegetation in response to glacier recession. |