Principal indicators of past climates - Climate Change: Biological and Human Aspects - page 45

to lose it through transpiration over a large surface area (which also benefits sunlight
capture, but which is not restricted by sunlight).
More recently still there has been a further twist in leaf morphology as an envir-
onmental indicator. At the turn of the millennium (2001) the UK team of Beerling,
Osborne and Chaloner developed a biophysical model that seemed to explain why
leaves evolved in the first place. For the first 40 million years of their existence land
plants were leafless or had only small, spine-like appendages. It would appear that
megaphylls - leaves with broad lamina (leaf blades) - evolved from simple, leafless,
photosynthetic branching systems in early land plants, to dissected and eventually
laminate leaves. This took some 40 million years. It seems that these transformations
might have been governed by falling concentrations of atmospheric carbon dioxide
(some 90%) during the Devonian period 410-363 mya (see Figure 3.1). As noted
earlier, atmospheric carbon dioxide has a bearing on climate so that the Devonian did
represent a period of global climate change. (Although if we are to draw climatic ana-
logies with the more recent [Quaternary] glacial- interglacial climatic cycles of the
past 2 million years, we need to bear in mind that the Sun's energy output was slightly
less at that time than it is today; see Chapter 3.) This episode again demonstrates that
climate change can influence evolution.
Another clue as to why leaf shape helps us elucidate climate is to turn this question
on its head: climatic conditions favour certain leaf shapes. This came in 2008 when
Brent Helliker and Suzanna Richter discovered that the mean temperature around
leaves around midday during the middle of the thermal growing season (TGS) was
remarkably constant across a range of latitudes. They found that mean leaf temperature
for 39 tree species during the TGS across 50 ◦ latitude - from subtropical to boreal

2.2 ◦ C. Clearly there is evolutionary pressure for a species
to have optimal photosynthesis and leaf shape, as well as leaf albedo, and leaf
transpiration (both affected by leaf composition and structure), will affect the leaf
temperature. Furthermore, leaf spacing and canopy further affect leaf microclimate.
Leaf microclimate itself is a modification of the general climate and the way leaves
help modify the general climate is central to their being used as a climatic indicator.
Of course, leaf morphology is not the only way different leaf types make the most of
different climates. This brings us to leaf physiology.

biomes - was 21.4 ±

2.1.4 Leafphysiology

Whereas leaf morphology shows a general relationship with climate, one aspect of
leaf physiology in particular has a close relationship with one of the key factors
forcing global climate, that of atmospheric carbon dioxide concentration. A leaf 's
primary function is to enable photosynthesis, a case of form driven by function.
The theory underpinning leaf physiology as an indicator of atmospheric carbon
dioxide, and hence global climate, is that the higher the carbon dioxide concentration
the fewer stomata - the small openings in plant leaves that facilitate gaseous ex-
change - are needed to transfer the required amount of carbon dioxide that the plant
needs. So, there is an inverse relationship between stomatal densities and atmospheric
carbon dioxide concentrations. This relationship was mainly deduced and established
in the late 1980s.