Recent increases in Tasmanian Huon pine ring widths from a subalpine stand: natural climate variabiliry, CO2 fertilisation, or greenhouse warming?

Tasmanian subalpine Huon pines from the extreme high-altitude limit of the species distribution provide a summer temperature reconstruction extending back beyond 800 BC. Compared to low elevation Huon pine sites, the subalpine ring-widths exhibit a straightforward direct response to current growth-season temperatures and indicate anomalous warming of 0.33 ± O.06°C from 1967-91. This warming is consistent with Tasmanian instrumental records and with hemispheric and global records.
The possibility that the trees are responding directly to CO2 fertilisation is explored, using a high-precision record of CO2, obtained from air in Antarctic ice and firn, plus direct measurements of air from Cape Grim. The temperature forcing appears capable of explaining the ring-width variations in the alpine trees over the full range of observed periods, whereas CO2 fertilisation would require a more complex interaction and is not supported by other arguments.
Two millennia-long tree-ring reconstructions of summer temperatures from South America do not exhibit the recent warming, nor other features found in the Tasmanian record on decadal to century time-scales. In fact, the South American chronologies bear little resemblance to each other, but do, however, reflect their own regional instrumental records. The Mt Read ring-width chronology, and the instrumental temperature series used for its calibration, also co-vary with climate influences of a distinctly regional character, yet still replicate many of the features reported as hemispheric and global temperatures over the last century.
Spectral analysis of the Mt Read tree-ring data over the full 2792 years suggests that at least part of the recent warming in the instrumental records could be a consequence of "natural forcing" of the record, complicating an interpretation in terms of a greenhouse-forced warming.