The grassmapr package has two related objectives: (i) to predict the
spatial distribution of terrestrial C3 and C4
grass cover – using input climate layers and crossover temperature -
and (ii) to model plant stable carbon ((\delta)13C)
isoscapes by applying isotopic endmembers to plant functional type
cover layers. The user may optionally include other (i.e., non-grass)
vegetation layers (e.g., % woody cover, % crop, etc.).
The primary driver of (\delta)13C spatial variation in terrestrial plant tissue is the greater isotopic fractionation in C3 plants compared to C4 plants. The physiologically based crossover temperature model explains the turnover from C3 to C4 plants along gradients of temperature (Ehleringer et al. 1997, Collatz et al. 1998, Still et al. 2003).
The grassmapr package combines the crossover temperature model with
gridded climate and land-cover data to predict the relative abundance of
C3 and C4 vegetation distribution. Isotopic
endmember values are then applied to map plant (\delta)13C,
resulting in a spatially continuous representation, or isoscape. These
layers are useful for understanding grass biogeography (e.g., Powell et
al. 2012, Griffith et al. 2015) and for studies seeking to identify the
movement of animals (e.g., Hobson 1999, Bowen & West 2008).
Note that (\delta)13C also varies with photosynthetic
subtype in C4 plants and with rainfall and water availability
in woody C3 plants (Cerling & Harris 1999, Diefendorf et
al. 2010, Kohn 2010). Currently, grassmapr functions neglect these
secondary sources of spatial variation.
The grassmapr package is hosted on GitHub. You can install the latest
released version using devtools1:
install.packages("devtools")
devtools::install_github(repo = "rebeccalpowell/grassmapr")For a detailed guide to using grassmapr, see the vignette included
with package installation. Examples of commented scripts are provided in
the main-level folder. Data included in these examples are installed
with the grassmapr package.
The grassmapr package is free and open source software; you may
redistribute and/or modify it under the terms of the GNU General Public
License, version 3, as published by the Free Software Foundation.
This package is distributed without any warranty, without even the implied warranty of merchantability or fitness for a particular purpose. See the GNU General Public License for more details.
A copy of the GNU General Public License, version 3, is available at https://www.r-project.org/Licenses/GPL-3
To cite package grassmapr in publications, use:
Powell, R. L. et al. 2019. grassmapr, an R package to predict C3/C4 grass distributions and model terrestrial (\delta)13C isoscapes. https://github.com/rebeccalpowell/grassmapr
Bowen, G. J. and West, J. B. 2008. Isotope landscapes for terrestrial migration research. – In: Hobson, K. A. and Wassenaar, L. I. (eds.), Tracking Animal Migration with Stable Isotopes. Academic, pp. 79-105.
Cerling, T. E. and Harris, J. M. 1999. Carbon isotope fractionation between diet and bioapatite in ungulate mammals and implications for ecological and paleoecological studies. – Oecologia 120: 347-363.
Collatz, G. J. et al. 1998. Effects of climate and atmospheric CO2 partial pressure on the global distribution of C4 grasses: present, past, and future. – Oecologia 114: 441-454.
Diefendorf, A. F. et al. 2010. Global patterns in leaf (\delta)13C discrimination and implications for studies of past and future climate. – Proc. Natl. Acad. Sci. 107: 5738–43.
Ehleringer, J. R. et al. 1997. C4 photosynthesis, atmospheric CO2, and climate. – Oecologia 112: 285-299.
Griffith, D. M. et al. 2015. Biogeographically distinct controls on C3 and C4 grass distributions: merging community and physiological ecology. – Global Ecol. Biogeogr. 24: 304-313.
Hobson, K. A. 1999. Tracing origins and migration of wildlife using stable isotopes: a review. – Oecologia 120: 314-326.
Kohn, Matthew J. 2010. Carbon isotope compositions of terrestrial C3 plants as indicators of (paleo) ecology and (paleo) climate. – Proc. Natl. Acad. Sci. 107: 19691–19695.
Powell, R. L. et al. 2012. Vegetation and soil carbon-13 isoscapes for South America: integrating remote sensing and ecosystem isotope measurements. – Ecosphere 3: 109.
Still, C. J. et al. 2003. Global distribution of C3 and C4 vegetation: carbon cycle implications. – Global Biogeochem. Cycles 17: 1006.
1 Package devtools is available from GitHub:
https://github.com/r-lib/devtools, or from CRAN:
https://cran.r-project.org/web/packages/devtools/index.html