My research is motivated by my fascination with the startling amount of biodiversity in the world - I'm not just talking about species diversity, but also the diversity seen in the various and extraordinary morphological, behavioral, and physiological forms and functions that different organisms encapsulate. As a plant ecologist, I take a trait-based approach to uncover how this trait diversity can help explain the persistence of plant populations across different environments, and how diverse plant communities are maintained in the face of abiotic and biotic homogenizing factors. In this work, I have been extremely fortunate to work in incredibly beautiful places, such as tropical forests in Costa Rica, old-temperate forests in New Zealand, and the Cape Floristic Region in South Africa.
Current Research Directions
The role of intraspecific trait variation for plant persistence in South African Protea species. There are many commonly measured plant functional traits that are assumed to associate with some aspect of plant performance within an environment, and reflect fundamental trade-offs assumed to associate with various plant strategies. Rarely, however, do we measure trait-performance relationships at the level of an individual plant, where we'd predict selection to be operating. I measured a suite of structural traits (e.g., LMA and wood density), physiological traits (e.g., photosynthetic rate and stem hydraulic conductance), size, and reproductive effort (a fitness proxy) across several populations for five Protea species. In general, I found that variation in structural traits, physiological traits, and size traits were associated with variation in reproductive effort (using a Bayesian hierarchical path model; Nolting et al. 2020). Variation in these fitness-related traits seemed to reflect individual variation along a "spendy-thrifty" continuum, and while combinations of structural traits predicted physiological traits well, there was limited evidence of the expected trait-physiology relationships (e.g., photosyntheses ~ LMA) in these populations. This highlights the importance of evaluating trait relationships at the individual plant level, and accounting for the non-independence of traits measured (Nolting et al. 2020).
Trait-mediated species coexistence in the Cape Floristic Region, a 'Biodiversity Hotspot'.
While variation in plant functional traits is often considered with respect to adaptation to the abiotic environment, trait variation can also reflect differential resource-use and niche partitioning, and thus be important in predicting species-coexistence. Fynbos habitats in the Cape Floristic Region, South Africa, are fire-adapted, meaning plant communities are subject to recurring fire (~ every 12-15 years). Many Protea species in these habitats have a life history strategy such that individuals grow up in large stands, holding onto their seeds in serotinous seedheads, until fire returns at which time the adults release their seeds and are killed in fire. As a result, these Protea communities often naturally occur in single-species stands (monospecific), or occur as an individual in a stand surrounded by another Protea species (heterospecific stands). I took advantage of this 'natural experiment' to test whether trait differences between a focal individual and its neighbors influence the fitness of the focal individual, as would be expected if these traits are contributing to niche partitioning. Further, I was able to evaluate this in monospecific versus heterospecific stands, to see if this trait-difference effect is bigger in monospecific stands (which would be consistent under a scenario of species coexistence). The results show: (1) That focal individuals surrounded by neighbors with more dissimilar trait values had higher reproductive output, which may be indicative of a stabilizing effect (i.e., niche partitioning). (2) In the majority of sampled populations, density effects were greater in monospecific than in heterospecific stands, which is a key component for predicting species coexistence. (3) Trait‐mediated effects were greater in magnitude in monospecific stands. That is, conspecifics do better if they are different in traits from their neighbors (Nolting and Holsinger, in prep). Together, these results indicate that trait-mediated niche partitioning is in part contributing to species coexistence in these diverse plant communities.
Resolving a trait-performance paradox: When traits vary across species but performance doesn't.
In evaluating the multivariate trait relationships described above (Nolting et al. 2020), we noted an interesting observation: species differed dramatically in structural traits, but were largely convergent in physiological performance. Given that together structural traits predicted variation in physiological traits quite well, we might have expected much more differentiation among species in physiological performance. Using a simple mathematical model, we show that variation in performance is minimized when the major axes of structural trait variation DO NOT align with the functional axis describing the relationship between traits and performance (Nolting and Holsinger, in prep). This is consistent under a scenario in which stabilizing selection is operating to minimize variation within populations (or among co-existing species within a community) in traits related to fitness. We demonstrate that this model can explain the mismatch in variation in the empirical Protea dataset, and we are working to extend this framework to a broader Protea trait dataset.
Quantifying phenotypic integration and measuring its change in response to abiotic stress in cultivated sunflower.
A new direction in my research is developing tools to evaluate the integration of whole plant phenotypes, and to investigate the degree to which the covariation of traits within these phenotypes is constrained or labile, at different ecological and evolutionary scales. We take advantage of an existing dataset that reports variation in phenotypic traits across the sunflower association mapping (SAM) population, grown under different salinity concentrations. This dataset represents a large-scale greenhouse experiment in which 239 genotypes – planted in replicates of four – were grown in benign (0mM NaCl) and stressful (100mM NaCl) environments. At the end of the experiment, plants were harvested at the late-vegetative stage and a suite of traits related to plant size and allocation were recorded. We quantify the phenotypic integration of the SAM population within each environment by estimating the trait correlation matrix using Bayesian multiple response models, and summarizing with eigenvalue and entropy metrics. We find that integration is higher in more stressful conditions, which is largely the result of stronger estimated trait associations in the the stressful environment. We need more formal investigations of integration across different plant systems, and across different ecological and evolutionary scales, to determine the utility of considering phenotypic integration as an emergent trait that coordinates changes in the variances of its component traits.
Previous Research
Hanging out at the Dunedin Botanical Gardens, in search of Pittosporum!
The Role of Evolutionary History and Niche Differentiation in Structuring Species Co-Occurrence in New Zealand Pittosporum (Pittosporaceae)
My MSc research at Michigan State University in the Swenson Lab involved the investigation of the mechanisms that contributed to species diversity in the New Zealand Pittosporum (~23 endemics to NZ), and those that continue to structure species distributions and species co-occurrence of this diverse genus of plants today. We found that Pittosporum diversified into different environmental niches, with respect to temperature and precipitation regimes, and that there was coordinated evolution of wood and leaf traits, and those corresponding to water use efficiency.
Functional Trait Differentiation of Saplings in the Genus Inga (Fabaceae) at La Selva Biological Station, Costa Rica
Typical day at La Selva: Anolis lizard hanging out on an Inga sapling
As part of a summer REU (NSF Research Experience for Undergraduates) I got the amazing opportunity to conduct a research project at La Selva Biological Station in the tropical lowland wet forest in the Sarapiqui region in Costa Rica. Working with my mentor, Dr. Danielle Palow, I quantified and evaluated differences in several leaf functional traits of saplings for 8 species of Inga that occur throughout the La Selva forest plot. Previous work by Dr. Palow shows that Inga species in this region exhibit different soil preferences, with some species occurring on fertile alluvial soils and others occurring on older residual soils. Through my summer project, we found that the Inga species that differed in their soil type preference are also different in several key leaf functional traits. This suggests that co-existence in this diverse genus in this forest is likely facilitated by niche partitioning along a soil gradient, and that the differentiation along this environmental axis is coupled with differentiation in functional traits.
Not only did this project solidify my goals of continuing on with research at the graduate level, but it gave me my first taste of field work in a tropical forest and convinced me that I wanted to pursue my research in these incredibly diverse regions. Additionally, this project really started to pique my interest in investigating how evolution influences present-day niche partitioning, species co-existence, and community assembly as a whole.