Past and present variability in leaf length of evergreen members of Nothofagus subgenus Lophozonia related to ecology and population dynamics

Abstract

SUMMARYThe lengths of fosst! leaves of Nothofagus cunninghamii. and of closely related fossil species from sedimt'nts older than the Middlf Pleistocene showed greater variation than fossil leaves from youngt-r sediments and extant leaves from modern sediment samples and forest Hoor litter. Leaf sizes of samples of the other two extant e\ergreeii species of Nothofagus .subjienus Lophozonio. N. moorei and N. mevziesii, were no more variable than modern A'. cunninghamii. Leaf lengths of modeni and Middle-Late Pleistocene A', ciinninghamn and ot tnodern A . menziesii were more or less unimodal in distribution, whereas the leaves of A', mnnrei follow himodal distributions, and leaves from the Early Pleistocene Regatta Point and the Oligocene Little Rapid River sediments tended to be bimodal and highly variable. Leaf lengths from the Oligo-Miocene Monpeeiyata sediments were also highly variable, but the sample size was too small to determine if they follow bimodal distributions. The bimodality and high variability in the older sites are unlikely to have been due to taphonomif processes. Thus, within population variability appears to have declined during the Late Tertiary-Karly Pleistocene, although the presence nf cryptic species in the fossil record could aisti explain ihe results. Bath a decline in variahility within species, and the extinction of Nothofagus species, are consistent with ii v\e!l documented Late Cainozoic decline in rainforest diversity in Tasmania, Log transformed leaf lengths of modern forest Hoor litter samples of A", cunninghamti were strongly correlated with climatic parameters, particularly summer temperature, suggesting that this parameter is a strong determinant of leaf size in N. cunninghamii. Changes in A', cunninghamii leaf lengths within Middle Pleistocene sediments were associated with floristic changes interpreted as transitions in glacial/interglacial cycles, but these changes were small compared with changes predicted from climatic correlation oi extant samples, probably because of limited genetic variability in local populations at the time of deposition.

A new early pleistocene species of Nothofagus and the climatic implications of co-occurring Nothofagus fossils

“…The separation of the Regatta Point Nothofagus fossils into an extinct species with large leaves, and a population of the extant Tasmanian species, N. cunninghamii, with small leaves data discounts Jordan and Hill's (1994) hypothesis of greater variability in leaf size in the Early Pleistocene. Neither N. pachyphylla nor the fossil N. cunninghamii at Regatta Point are more variable in leaf length than living populations.…”

“…They are, therefore, assigned to N. cunninghamii. All other Pleistocene macrofossils of Nothofagus from Tasmania are consistent with either N. cunninghamii or the other extant Tasmanian species, N. gunnii (Colhoun and van der Geer 1987;Colhoun et al 1989;Fitzsimons 1990;Jordan et al 1991;Jordan 1992;Fowler 1993;Bresnehan 1993;Jordan and Hill 1994;Jordan et al 1995;Rowell 1995). There is no discernible difference between the N. pachyphylla leaves in the RPU2 flora and those in the RPU1 flora.…”

“…by Hill and Macphail (1985) (Jordan and Hill 1994). Jordan and Hill (1994) concluded that either two related species co-occurred near Regatta Point, of which at least one is now extinct, or the variability within populations of N. cunninghamii was greater in the past, possibly due to mixing of warm and cool climate floras. The presence of a highly variable species of Nothofagus would be difficult to interpret climatically, but the presence of a normal population of N. cunninghamii and an extinct species would be much easier, especially in the light of the close relationship between leaf size and climate found in extant populations of N. cunninghamii by Jordan and Hill (1994).…”

“…Fossil leaves were extracted from the sediments by either macerating blocks of mudstone in warm, dilute H 2 O 2 for a few hours, or in approximately 20% HF for one or two weeks, or by splitting the mudstone blocks (Hill and Jordan 1994). The RPU1 and RPU2 collections each contained approximately 1000 Nothofagus leaves and leaf fragments extracted in these ways.…”

“…Oerst. by Hill and Macphail (1985) (Jordan and Hill 1994). Jordan and Hill (1994) concluded that either two related species co-occurred near Regatta Point, of which at least one is now extinct, or the variability within populations of N. cunninghamii was greater in the past, possibly due to mixing of warm and cool climate floras.…”