Climate Change and the Invasion of California by Grasses

Abiotic conditions—such as temperature and precipitation—determine local plant community membership by favoring groups with specific functional traits. However, climate change will alter abiotic factors, causing the composition of plant communities to shift by selecting for different functional traits. In some ecosystems, exotic, invasive species may possess functional traits favored by the new climate regime. Therefore, climate change may exacerbate native biodiversity loss by facilitating the spread of invasive species. To determine if climate change will alter the course of invasion of California’s already heavily invaded grass flora community, Sandel and Dangremond (2011) evaluated the differences in trait composition of native and exotic species groups and evaluated the contemporary trait-climate relationships across the state. The authors mapped the distributions of all grass species within California and then calculated the mean trait characteristics, mean climate values, and human influence indexes across 800 discrete zones within the state. They found that exotic species were more likely to be annual, taller, with larger leaves, larger seeds, a higher specific leaf area, and a higher leaf nitrogen percentage than native species. These traits were associated with higher temperatures across the entire state, indicating that increasing temperatures caused by climate change will favor traits possessed by exotic species. Ultimately, this may lead to the dominance of exotic species within California’s grassland communities.—Megan Smith
Sandel, B., and Dangremond, E.M., 2011. Climate change and the invasion of California by grasses. Global Change Biology, doi: 10.1111/j.1365-2486.2011.02480.x

Sandel and Dangremond mapped the distributions of all grass species within California. The study’s maps were based on a map of California that divided the state into 35 floristically defined sub-regions. These sub-regions were divided into 100 m elevation bands using a digital elevation map of California. These divisions resulted in 800 discrete zones across the state. The authors used a flora of California, the Jepson Manual, to determine where grass species occur in each zone. The Jepson Manual was also used to determine whether each species was native or exotic. Exotic species were defined as those that were least naturalized and could be invasive. Particular attention was paid to the species listed by the USDA as invasive and noxious weeds.
The authors collected trait information on the grass species in California. These traits included maximum height, plant lifespan, leaf lifespan, seed mass, month of first flowering, length of flowering period, specific leaf area, leaf length and width, leaf N concentration per mass and per area, and photosynthetic pathway. The data were collected from the Jepson Manual species accounts, published sources such as the Glopnet database, genus-level information, and garden seed information databases. When multiple trait values were available for a species, the authors used the mean of all values. Trait information for species varied from complete to very incomplete. The means for each trait were calculated across all species present in each of the 800 zones. A figure demonstrating trait-based filtering on community membership imposed by climate was constructed, as was a table comparing trait geometric means of exotic and native grass species of California.
The time of introduction for each exotic species was obtained from the California Consortium of Herbaria records, which recorded plant species introductions based on the species first date of collection. The number of exotic species known in a particular year was divided by the percentage of native species that were known for that year to estimate the number of exotic grass species in the state through time.
The authors combined PRISM and Daymet climate data for California to calculate climate variable means for each of the 800 zones. The final set of climate variables obtained were mean annual temperature, seasonality of temperature (annual maximum minus annual minimum temperatures), annual precipitation, potential evapotranspiration, water balance (total precipitation minus PET), months of water deficit (the number of months of the year with PET > precipitation), and cumulative water deficit (summed water deficit in all months of deficit, expressed as negative numbers).
Human impacts on California’s ecosystems included increasing the rate of species introductions or producing disturbances that favor exotic species. These possibilities were examined using the Human Influence Index (HII), which measures human impacts by incorporating population density, land cover changes, accessibility, and electrical power infrastructure. The mean HII value was calculated within each of the 800 zones. Both HII and climate variables were treated equally within the study’s analysis.
After collecting data, Sandel and Dangremond statistically assessed whether native and exotic species differed in their trait states. The richness of native and exotic species were calculated and then the species richness of each group, as well as the proportion of species in each zone that were exotic, were compared to mean annual temperature and mean annual precipitation. Next, the authors determined how the traits of the grass flora as a whole related to climate by plotting climate variables against zone mean trait values across all 800 zones.
A quantitative prediction for the prevalence of exotic species per zone was calculated based on the relationship between temperature and zone mean trait values for native species, as well as the relationship between species’ trait value and the probability that species was native. The authors only used zone mean trait values of native species to avoid predicting the proportion of exotic species from trait means that included exotic species. A loess regression was used to fit a curve to the temperature-trait relationship for native species. Then, a logistic regression was used to estimate the probability that a species with a given trait value was native. When combined, these two regressions allowed the authors to start with a temperature, obtain the predicted zone mean trait value of a zone at that temperature, and to convert this into a prediction of the fraction of the community in that zone that was native. This approach was demonstrated using leaf width in Figure 2. 
Since mean zone traits were less variable than individual species traits, the range of predictions for proportion of natives was smaller than the observed range. Therefore, the predicted proportion provided an index of relative susceptibility to exotic species, rather than a 1:1 prediction for the proportion of native species. This index was easily rescaled by using information of the actual proportion of native species within just a few sites. The authors then used further statistical methods to relate the predicted proportion and the actual proportion of native species for five randomly selected sites. They rescaled all predicted proportions according to these results to obtain a properly scaled prediction of the proportion of native species.
Finally, the authors assessed spatial structure in climate and species-level data by separating two sources of climate variation: moving up an elevational gradient within a sub-region, and moving across sub-regions at a constant elevation. Relationships were calculated across all 800 zones, along elevation gradients within each zone, and across sub-regions at a constant elevation.
 Sandel and Dangremond found that grass species richness varied across the state, with a maximum of 163 and minimum of 3 species in a zone. The proportion of exotic grass species within a zone varied between 0% and 66% within zones. Native species richness showed a hump-shaped relationship with temperature while the proportion of species that were exotic increased strongly with temperature. Mean annual precipitation was not strongly related to the richness or the proportion of exotic species. A map displaying the patterns of species richness of grass in California was constructed, as was a map displaying the proportion of species within a zone that were exotic. Additionally, figures displaying the relationship of species richness and climate were constructed.
There was a total of 258 native and 177 exotic grass species in California. These two groups differed significantly in their traits. Exotic species were more likely to be annual, taller, have longer and wider leaves, a higher specific leaf area, a higher leaf N percentage, and a higher seed mass. Noxious invasive weeds had the most extreme trait values, while most exotic species were intermediate between weeds and native species. Many of these traits were strongly related to mean annual temperature. At warmer sites, species were larger (taller and larger-leaved), with a higher specific leaf area, a greater leaf N percentage and mass, shorter-lived leaves, larger seeds, earlier flowering times, and longer flowering seasons. The proportion of grass perennial species decreased with increasing temperatures as well. Figures displaying the relationship of temperature and grass traits were constructed, as was a table displaying the statistical results of comparisons of climate variables and zone mean traits.
The authors also found that with increasing cumulative water deficits, decreasing water balances, and increasing months at water deficit, grasses became longer-leaved with a higher specific leaf area, a higher N leaf percentage and mass, and with larger leaves. As human impacts increased, zones became exotic-like in their trait composition, revealing the increased richness of exotic species in heavily used areas.
Additionally, the results showed that an increase in elevational gradients within sub-regions led to reductions in grass mean height. However, there was little relationship between height and temperature when elevation was constant. Seed mass showed both positive and negative relationships to temperature within sub-regions. At low elevations (across sub-regions) seed mass increased with temperature, while at high elevation, it decreased.  Figures displaying the relationships between mean annual temperature and these two traits were constructed.
Sandel and Dangremond predicted the proportion of species in each zone that were native using only zone mean annual temperature, trait-temperature relationships for native species, and trait differences between native and exotic species groups. Using leaf width patterns, they found that the proportion of native species was strongly correlated with the observed proportion. However, the relationship was nonlinear and the predicted proportions covered a smaller range of values than the observed proportions, causing a poor fit to the 1:1 line. The authors rescaled the predicated proportions based on five randomly sampled sites where the proportion of native species was known. This led to quantitative and accurate predictions of the proportion of native species. A figure displaying the corrected prediction for the proportion of species that were native was constructed.
Finally, the authors found that prior to 1860, there were 20–30 exotic species established in California. This number increased sharply through the 1900s. The continued arrival of exotic species into California significantly changed the composition of the exotic flora. California’s exotic flora became more perennial, more C4, and larger-seeded over time. Figures displaying the changes in the exotic grass flora of California over time were constructed.
Overall, two conditions must be met for climate change to favor one species group (exotic grass) over another (native grasses). The changing climate (an increase in California’s mean annual temperature) must alter filters that act on plant functional traits, leading to communities with altered trait compositions. The two groups must also differ along trait axes. Both these conditions were met in California.
The present distribution of grass species richness within California already show that the proportion of species within a zone that were exotic and the proportion that were noxious weeds were strongly and positively related to mean annual temperature. However, since exotic species were taller and had more light-capturing ability than native species, they may outcompete natives for light. The larger seeds of exotic species also could give them a competitive advantage at the seedling stage. Additionally, increasing temperatures favored traits for which exotics had higher mean values than natives. Therefore, exotic and invasive species may come to dominate California’s grassland community since current noninvasive exotics could become invasive as temperatures increase within the state over time.  

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