Our research explores how plant biochemical and physiological traits drive ecological processes such as invasion, community assembly, and nutrient cycling. 

Invasion.  Much of our recent work has focused on how functional traits influence the establishment and persistence of invasive plant species.  The idea that native species will outperform invasive species under conditions of low resource (e.g., light, nutrients, water) availability is a crucial component of invasive species control and ecosystem restoration strategies.  However, most mechanistic studies that support this idea have focused on invaders in disturbed or resource-rich environments.  Sampling across a broad range of native and invasive species in Hawaii, we found that species invading low resource systems were similarly or more efficient at using limiting resources relative to native species adapted to those systems (Funk & Vitousek 2007), which contradicted the general paradigm that invasive species invest resources in growth or reproduction at the expense of resource conservation.  A more recent analysis of native and invasive species across the five Mediterranean climate ecosystems (California, Western Australia, South Africa, Chile and Spain) confirms this finding: native and invasive species within annual and perennial groups had similar patterns of carbon assimilation and resource-use (Funk et al., 2016).  Collectively, these results suggest that trait differences between native and invasive species are context dependent and management strategies based on plant resource use (e.g., soil manipulations, timing of herbicide) will vary across vegetation communities (e.g., Steers et al. 2011, Funk et al. 2015).

Ecological Restoration.  A second major theme of our research focuses on using plant functional traits to direct ecological restoration.  The concept of ‘limiting similarity’, which posits that species co-exist only if they use different resource pools, has been used to understand community invasibility, but is rarely used to guide ecological restoration.  One of our current research goals is to examine whether community resistance to invasion can be increased by assembling communities with native plants that are physiologically similar to invaders (Funk et al. 2008).  Results from two recent experiments refute this idea.  First, we found that functional traits and phylogenetic relatedness were poor predictors of competition outcomes among native species and invasive species in serpentine grassland (Funk & Wolf 2016).  Second, in collaboration with researchers at UC Irvine and practitioners at the Irvine Ranch Conservancy, we found that native species characterized by a fast-growing “weedy” suite of traits were more resistant to invasion by exotic forbs than were functionally similar native species.  The extent to which optimizing for invasion resistance conflicts with other goals of restoration (e.g., establishing diversity or functional redundancy) remains to be studied.

Climate change.  Demonstrating links between functional traits and long-term plant performance, reproductive fitness, and population growth rates is essential if traits are to predict the response of species and communities in response to environmental change.  Our current work in this area links traits to plant fitness and community dynamics in response to drought (Funk et al. 2020).  Our results demonstrate that the functional significance of traits can vary across species, even within life history groups (e.g., drought-deciduous perennial species).  For example, traits associated with resource acquisition (e.g., low leaf mass per area, high photosynthetic rate) are positively correlated with fitness in annual species across a water gradient but this does not hold for perennial species.  We have also found that invasive species can adapt to drought, via shifts in physiological and phenological traits, in just five generations (Nguyen et al. 2016).  We are also working to identify traits that predict susceptibility to drought. 

Root traits.  Although roots play a critical role in water uptake, our recent work suggests that root traits are highly variable within growth forms (Nguyen et al. 2017) and inconsistently correlated with plant fitness under a range of environmental conditions.  Our work has identified a single spectrum of belowground trade-offs related to resource acquisition and plant growth (Larson and Funk 2016); however, the ability of root traits to predict fitness and demographic responses may be limited by a decoupling between above and belowground traits.  Future work in our lab will identify the mechanisms by which root trait variation can influence fitness and demography in semi-arid grassland systems. 

Nitrogen physiology.  Recent collaborative work in our lab has uncovered a diverse range of nitrogen fixation strategies, from obligate to facultative, in leguminous herbs (Menge et al. 2015, Wolf et al. 2017).  This work led to two projects that explore how variation in symbiotic nitrogen fixation influences leaf and root physiology and whether the proximate driver of latitudinal nitrogen fixation patterns is environmental (e.g., climate) or taxonomic (actinorhizal vs. rhizobial).  Legumes are an important component of grassland ecosystems and understanding how fixation impacts physiology will inform a range of processes, including community assembly, competition, and nitrogen cycling.