feeding dinosaurs

["James Farlow" <farlow@ipfw.edu>]

For those of you with a paleoecological bent, I've been skimming through a book 
that should interest you:

Beerling and Woodward, 2001, _Vegetation and the terrestrial carbon cycle: 
modelling the first 400 million years_, Cambridge Univ. Press.

The authors employ linked General Circulation Models (GCMs) of global climate 
and Dynamic Global Vegetation Models to "retrodict" the plant cover, biome 
type, and net primary productivity (NPP-the amount of organic matter left over 
after plants use what they need for themselves via respiration) for certain 
intervals of the geologic past.  Now, these are predictions, not observations, 
but the authors check their retrodictions against geological and paleobotanical 
proxies for the parameters of interest wherever possible.  I was particularly 
impressed with the way estimates of NPP can be made from growth ring data in 
tree trunks--very clever).  Anyway, the retrodictions generally match the 
geologic and paleontological data fairly well.

So, here are the predictions of global terrestrial NPP in Gt C yr**-1 (I didn't 
see these units made explicit in the book, but I imagine they are gigatonnes of 
organic Carbon fixed per year):

Late Carboniferous (300 Ma): 38.2

Late Jurassic (150 Ma): 108.3

Mid-Cretaceous (100 Ma): 100.7

Late Cretaceous (66 Ma): 78.9

Eocene (50 Ma): 90.0

Recent (estimate for 1988): 52 (nice match with 56.4, the latter estimated from 
satellite remote sensing data)

It's interesting that global NPP seems to have jumped dramatically upward 
between the late Paleozoic and the Mesozoic, just in time to feed populations 
of really big herbivores.  This is consistent with speculation made by various 
authors (including myself) that dinosaurian gigantism may have been fueled in 
part by higher plant productivity in a greenhouse earth.  NPP in the late 
Jurassic thru the mid-K is estimated to have been twice modern levels, but it's 
interesting that predictions for the Eocene (the last time pre-human greenhouse 
world) are greater than for the late Maastrichtian, just before God turned off 
the lights and started dropping big rocks on the heads of our poor dinosaurs.

Another interesting thing, if I interpret the authors' global maps of NPP for  
the Mesozoic, is that in some places with faunas of big dinosaurs, regional 
values of NPP (in terms of NPP/hectare/year) don't seem to have been much 
higher than modern values in productive terrestrial ecosystems.  That is, if I 
am interpreting the data correctly, higher global values of NPP for the 
Mesozoic than the present are not due to huge increases in NPP/area, but 
instead more modest per area increases summed over the entire land surface of 
the planet.

I love this arm-waving stuff....