Gmj. Hall et Ms. Mcglone, Forest reconstruction and past climatic estimates for a deforested region of south-eastern New Zealand, LANDSC ECOL, 16(6), 2001, pp. 501-521
Predictions of species biomass from a forest simulation model were compared
with pollen percentages for seven peatland sites in an area of Southland-O
tago, New Zealand, now depleted of forest cover. Comparisons were made for
the recent past 700-800 cal. yr BP) and for a period of the early Holocene
(7000-8000 cal. yr BP). Satisfactory matches were obtained overall between
predicted biomass and pollen for the recent dataset (r = 0.73, P < 0.001),
in spite of expected poor correspondences for some pollen taxa known to be
under-represented in the modem pollen rain. Nothofagus species tended to be
over-represented by the simulation model, due most likely to dispersal lim
iting to their spread under actual conditions. Raising mean annual temperat
ures by 1<degrees>C and lowering precipitation by up to 60% for the forest
simulation produced a satisfactory match to the early Holocene site data (r
= 0.69, P < 0.001). To test for consistency between recent and past period
s, regressions for each period of modelled relative biomass against pollen
percentages were compared, using all tree taxa from all sites. No discernib
le bias was found between the different climate regimes modelled. However,
an examination of each site showed the dominant early-Holocene hardwood for
ests of Stewart Island were not reproduced by a simulation under the hypoth
esized past climate. These forests required a different set of conditions f
rom those for the South Island sites. suggesting they grew under a differen
t climatic regime. The low variation in climate among several of the sites
tested the forest model's ability to reproduce the distinct forest communit
ies identified from the pollen data. Comparisons with the pollen record imp
roved confidence in the species attribute data used by the model, the compl
eteness of the ecosystem processes explicitly modelled, and the disturbance
regimes employed. A forest reconstruction of the region, under current cli
mate conditions, indicated extensive areas of grassland and grassland-scrub
vegetation could potentially be replaced by a range of podocarp, broadleaf
, and beech forest types. Overall, the exercise suggested such approaches c
an improve our understanding of the processes required to restore forest in
depleted landscapes and to model forest dynamics under changed climates.