Ecology of C3 and C4 Grasses (Poaceae) in the United States: a Phylogenetically Controlled Analysis
Patrick J. Alexander, Department of Biology, New Mexico State University, Las Cruces, NM, paalexan@nmsu.edu
However, although it is common knowledge that C3 and C4 grasses segregate
Introduction:
along climatic gradients, with C4 grasses in hotter, drier habitats, this is based
Grasses (Poaceae) are the fifth-largest plant family
on phylogenetically-uncorrected analyses. Phylogenetic constraints on
world-wide, and yet far more important than their
photosynthesis evolution reduce the number of independent data points, since
diversity indicates both in structuring ecosystems
closely-related taxa will tend to be similar in all traits rather than only
and in human nutrition, through direct and indirect photosynthetic pathway. Unless comparisons are limited to independent
consumption. As a result, grass ecology is of
derivations of C4 photosynthesis with their sister groups, confounding similarities
fundamental importrance. A major factor affecting among clade members may be responsible for the observed pattern. A
grass distribution across climates is photosynthesis preliminary analysis (results not shown) using six sites in Arizona showed no
type. The family includes members with both C3
significant relationship between photosynthesis type and either maximum mean
and C4 photosynthesis, with C4 photosynthesis
annual temperature or total annual precipitation, so a nationwide analysis
arising independently multiple times.
including a wider variety of habitats and representing more independent
derivations of C4 photosynthesis was conducted.
Process:
1 Choose sites across the US to represent a
wide array of climates within the constraints of
availability of climatological and distributional
data:
4 Calculate a diversity-weighted annual
mean value of each climatological
variable for each genus ( =( across sites
(# species at each site * climate value for
site)) / total # of occurrences of the
genus ) and a phenologically corrected
mean (since climatic separation can
occur at a single site through different
phenology; p.c. mean is calculated as for
annual mean except that only climate
data from estimated growth period (each
of the 2 months of greatest collection
minus one) are included)
2 Compile a list of grass species
present at each site from online
herbarium databases (1, 2, 3, 4,
5, 6, 7, 8, 9, 10); check names
against the online Manual of
Grasses of North America (11)
and USDA PLANTS Database
(12) for nomenclatural
consistency; score number of
species in each genus at each
site; find the 2 months of
greatest collection for each site.
3 Compile climatological data-annual mean maximum temperatue,
annual mean temperature, and total
precipitation
--for all sites from Regional Climate
Center data (13, 14, 15).
In addition to the CAIC analysis results shown at left, highly significant (p<.0001)
relationships were found in all phylogenetically-uncorrected Student’s T-Test analyses.
Results from CAIC analysis are shown below:
These results suggest that C4 photosynthesis does indeed significantly affect the
Annual Mean Values
p-value r2
distribution of grasses along heat and moisture gradients. Failure to find a relationship
Annual maximum mean temperature
0.0530 0.4906
in the preliminary Arizona analysis may be due to either insufficient climatic variation
Annual mean temperature
0.0208 0.6174
among sample areas or to poor representation of the multiple origins of C4 photosynthesis
Precipitation
0.0490 0.5025
in Panicoideae.
2
Phenologically-Corrected Values
p-value r
The effect of phenological correction is puzzling. In the Arizona analysis,
Annual maximum mean temperature
0.0136 0.6650
phenological correction increased the strength of all tested relationships (data not
Annual mean temperature
0.1079 0.3728
Precipitation
0.4463 0.0996
shown), but in the nationwide analysis it rendered associations with both annual
mean temperature and precipitation nonsignifcant. Although phenological separation of C3 and C4
plants is common in warm climates, the tendency for all plants to grow together during brief warm
growing seasons in colder climates may tend to erode differences between these predominantly C3
areas and the warmer sites of the southern US. This may explain the pattern with precipitation and
annual mean temperature, although the contrary increase in significance of association of
photosynthesis type with annual maximum mean temperature remains anomalous.
Results & Discussion:
Citations:
1.
2.
3.
6 Test for a relationship between C3/C4
photosynthesis and each of the three climatic
variables using Comparative Analysis via
Independent Contrasts (CAIC, 22) and by a
Student’s T-Test in Microsoft Excel (23).
Pooideae
Bambusoideae
Oryzoideae
Aristidoideae
Arundinoideae
Danthonioideae
Chloridoideae
Centothecoideae
Andropogoneae
Steinchisma
Axonopus, Paspalum, Panicum § Obtusa
Phanopyrum
Panicum §s Agrostoidea and Tenera
Digitaria
5 Compile a phylogeny of the included genera from published
Dichanthelium
phylogenies (16, 17, 18, 19); at right is a simplified phylogeny,
Panicum § Verrucosa
generated in NONA (20) with Winclada (21), showing major lineages
Echinochloa
Oplismenus
and all independent derivations of C4 photosynthesis. Branches
Panicum §s Dichotomiflora, Panicum, Bulbosa,
leading to C4 taxa are in red. Genera not included in published
and Virgata; Urochloa, Setaria, Stenotaphrum,
phylogenies were entered as basal polytomies in the least-inclusive
Cenchrus, and Pennisetum
higher taxon (usually the tribe) available.
{
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
Panicoideae
16.
17.
18.
19.
20.
21.
22.
23.
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