Noxious invasives like this European beachgrass could spread along the California coast.

As global temperatures rise, the exotic grasses that currently dominate California’s hot interior are likely to expand into its cooler ecosystems, according to a new study. “Warming temperatures should favor exotics everywhere, but a given warming should matter more in a cold area”, explains lead author Brody Sandel of Aarhus University (Denmark). The state’s cool areas, notably the north coast and mid-elevation mountains, are richest in native grasses and lowest in exotics.

Sandel and his coauthor Emily Dangremond (University of California, Berkeley) began by asking if native and exotic grasses differed for a variety of traits. “We already knew most exotics were annuals”, Dangremond points out, “but we also found differences in many other traits”. For example, exotic grasses are generally taller, with longer, wider leaves, making them more efficient than native grasses at capturing light. “We were surprised to find such clear differences”, Sandel comments on their study, which was published online in July in Global Change Biology (2011; doi:10.1111/j.1365-2486.2011. 02480.x). “Finding consistent trait differences between native and exotic species has been a difficult challenge”, he continues.

The researchers then asked whether these traits are linked to climate, and found that most of those characteristic of exotics are favored by higher temperatures. Grasses in warmer parts of California are taller, have bigger leaves and seeds, and flower earlier and longer. “The higher proportion of exotics in warm regions is a major contributor to this”, Sandel explains, adding that warming should likewise enhance the spread of exotics into other parts of the state. Noxious invasives stand to receive the biggest boost because they are the most extreme for the traits studied.

Another surprise was that, in contrast to previous findings, precipitation had little effect on the trait composition of California grasslands. “We thought this was strange, but it could be that temperature is such a strong filter that it washes out the impact of precipitation”, Dangremond suggests. Exotic grasses do far more damage than merely crowding out native grass species. For example, one of the most pervasive Pacific coast invaders is European beachgrass (Ammophila arenaria), which, as Dangremond has previously demonstrated, provides shelter for deer mice (Peromyscus maniculatus) that eat the seeds of an endangered coastal lupine (Lupinus tidestromii).

Follow-up work to the current study includes comparing exotic grass growth in the laboratory under today’s climatic conditions to those predicted for California by 2100, when temperatures are expected to be 2.2–5.8°C higher. “Do traits change when grasses are grown under warmer temperatures?”, asks Dangremond. “If so, some of today’s exotic species could become noxious invaders.”

Frontiers in Ecology and the Environment, 2011