A. laxifolia en Duckyls Wood, East Grinstead, Inglaterra, Reino Unido

53 Athrotaxis cupressoides on the Tree Register, to 15m tall, 103 Athrotaxis laxifolia (excluding the Duckyls four, to 21.5m) and 66 A. selaginoides, to 20m high (so it has a long way to go to reach 38m!). I would guess that all are hardy enough to survive in favoured spots on the continent.

It is not impossible that in 3000 years, the largest or at least the tallest Giant Sequoia will be in Europe, considering the excellent growth rates here. See e.g. some specimens on MT. But we can only guess what is the influence of the Global Warming. It is also possible that GS will not reach such ages in Europe due to the moist summers and mild winters.

Regarding the other Athrotaxis spp., I have seen them in some botanical gardens, but they are apparently rare.

The tallest A. selaginoides in Tasmania was 38 m a few years ago.

C.J. Earle does treat A. × laxifolia as hybrid A. cupressoides × selaginoides at The Gymnosperm Database. There is one Image linked at website, showing: "Foliage and cones on an ornamental specimen, Pinetum Blijdenstein, Netherlands [C. J. Earle, 2010.06.11]".

Does anyone know about planted Athrotaxis cupressoides and Athrotaxis selaginoides?

Conifers and Kouta will be right, since many plants (as well as other organisms) will be actually forced to survive at relatively bad environmental conditions by competition. These will grow faster and bigger at better conditions in case stronger competitors are kept away, but had no chance to proliferate as stable population at natural habitats. Some ancient types of conifers have been surviving at locally restrictet, relictous popuations.

Sequoiadendron has been spread by mankind to all continents (except Antarctica), yet it won't have chances to propagate on it's own where the climate is not exactly fulfilling the species needs, and claering fires are missing. But G.S. reaches it's maximal sizes and ages at the Sierra's native groves, other than Athtrotaxis, which apparently got to survive at suboptimal conditions.

I think Conifers' explanation could be the correct one. In the wet lowland regions it will be outcompeted by broadleaf "rainforest" trees (by Australian definition), particularly Nothofagus cunninghamii, Atherosperma moschatum and Eucryphia lucida. In the drier regions fires exclude the fire-sensitive Athrotaxis and favour Eucalyptus, which in turn favour fires as we have recently seen in Portugal.

Maybe they grow where they do in the wild because the climate is survivable (though suboptimal) for them, but not survivable for big Eucalyptus and/or other more vigorous broadleaves? They might theoretically do much better in warmer, drier parts of Tasmania, but get outcompeted and excluded by Eucs there.

I know that the native distribution in Tasmania is restricted to quite high altitudes where they can't grow so fast or tall. These ones are clearly relishing the warmth and shelter of lowland Sussex, though it still puzzles me that none are as vigorous in the far west where the mild, high-rainfall conditions ought to suit them best.

Why so tall in this location, surely drier than the mountains of Tasmania? Is it the same case with Monterey Cypress, once read that they are taller in cultivation than in the wild, is this true?

There are four Athrotaxis laxifolia at Duckyls Wood (where planting began around 1927) which are now considerably taller than any found elsewhere in Britain. They are still growing fast; this one looks certain to reach 30m, which may even be taller than is found in the wild.