My research is motivated by the striking differences between plants and animals. The sessile nature, modular construction, and evolutionary flexibility that plants exhibit lead to intriguing challenges in ‘thinking like a plant’. Such challenges include how over 600 species of tree can coexist within 1-ha of rain forest, how plants allocate limited resources to growth, defense and reproduction, and how populations respond to biotic and abiotic factors. Answering these fundamental biological questions is essential for predicting how anthropogenic impacts, such as climate change and land-use, will alter forests in the future, and how we can better manage, conserve, and restore forests to mitigate these impacts.
Comparative macroecological studies offer insight into the diversity of ways that organisms ‘make a living’. Understanding how the abiotic and biotic environments shape trait evolution and organismal function is vital to inform principles of biogeography and determine the limits of species distributions. In particular, processes that are essential to a plant’s survival and fitness, such as transpiration, nutrient uptake, and photosynthesis, are all affected to a greater or lesser degree by variation in leaf shape, size, color, and form. My students and I are developing databases of plant leaf traits that we integrate with existing data on plant distributions (often networks of tree plots), to ask how plant form and function respond to climate. For example, the abundance of drip-tips—long, acuminate leaf tips thought to aid the removal of water from the leaf surface—in tropical wet forests is a key ecological pattern among plant communities. The evidence for this pattern comes from early comparative studies that identified high frequencies of drip-tips in tropical lowland rain forest and montane cloud forest sites, whereas temperate rain forest and drier forest sites lacked drip-tips. The increasing availability of digitized herbarium specimens and online floras and taxonomic information allowed us to categorize species leaf tips and quantitativly test whether the macroecological patterns in leaf form found in wet forests such as Amazonia are also found in the drier forests of the cerrado savannas of Brazil (Sullivan and Queenborough, in review), across the strong rainfall gradient in the isthmus of Panama (Almonte, 2019), and in Brazilian Atlantic forest (Caglioni and Queenborough, in prep).
Leaves vary not only in size and shape, but also in many other traits. For example, many tropical tree species have specialized glands that secrete nectar in order to attract ants that defend the leaf from herbivores. Tree and seedling communities vary in the incidence and abundance of these extra-floral nectaries (Boudouris and Queenborough, 2013; Muehleisen et al., 2016). Similarly, many tropical tree species delay the greening of their new leaves, to avoid the loss of expensive N-rich chlorophyll to herbivores. Some species even add red anthocyanins to their leaves so that they appear brown and dead to herbivorous insects and mammals. I have documented that leaf color can also significantly affect seedling and tree growth and survival (Queenborough et al., 2013; Anderson et al., in review).
With collaborators, I am also analysing patterns of phenology in various regional floras, such as the UK and China, and asking how climate change will likely impact species distributions and phenology (Amano et al., 2014; Du et al., 2015, in review).
The mechanisms underlying the coexistence of hundreds of sympatric species are still much debated. Using spatially-explicit data on >3,500 seedlings and >3,000 trees in the Myristicaceae (nutmeg) family, I am investigating the influence of negative density- dependence, abiotic and biotic neighborhoods, and species traits on mechanisms of species coexistence at the individual and community levels in a hyper-diverse Neotropical forest. Coexisting adult populations show evidence of niche partitioning of light and soil nutrients (Queenborough et al., 2007a), but dynamics are most rapid in seedlings. I quantified the scale of spatial autocorrelation in mortality of tree seedlings for the first time (Queenborough et al., 2007c), critical to understanding the scale of density- dependence. I demonstrated a negative relationship between adult abundance and seedling survival that was not driven by species’ traits, suggesting that negative density-dependence could promote species coexistence in this community, and found stronger inter-specific competition between more closely-related species (Queenborough et al., 2009a). Recently, I showed that over a period of 10 years, the presence of lianas significantly decreased the growth, survival, and reproduction of these trees, despite the lianas themselves not increasing in line with the generally-accepted global trend (Smith et al., 2017).
I have since expanded this intensive individual-based approach to several other groups, including the ant myrmecophyte Duroia (Báez et al., 2016; Hudson, 2019), palms (Ninazunta et al., 2016; Queenborough et al., 2012), and the herbaceous genus Heliconia (Tokarz et al., 2019).
With collaborators, we are conducting community-wide analyses assessing the strength of density-dependence (Zhu et al., 2018), and the factors that determine community structure and function in a local temperate forest (Krishnadas et al., 2018; Fotis et al., 2018; Murphy et al., 2016).
The resources available to an organism are finite and are usually allocated to one function: e.g. reproduction, growth, or defense. Trade-offs among these functions are central to life history theory, but the ease of quantifying allocation to each varies among taxa. For example, comparing males and females of dioecious species (Queenborough et al., 2007b) isolates the resource axis 2 . I have begun detailed studies of the fast- growing pioneer genus Cecropia, which may live only 30–40 years, thus uniquely among most tree species allowing a single researcher to document the entire life of an individual tree. This summer (2019), an MFS student will assess variation in the growth of co-existing Cecropia species as a function of climate, using a newly-developed dendrochronology technique that allows us to use leaf and inflorescence scars to recreate the history of each individual, something that is impossible for many tropical tree species (Gerhke, 2020).
Other traits, such as extra-floral nectaries, have evolved independently many times and are almost certainly strictly defensive. Many tropical tree species have red-colored juvenile leaves, thought to be cryptic to herbivores that cannot see this wavelength radation. In two tropical tree communities, I found lower mortality rates in seedlings and trees for species with red-colored leaves, supporting this theory (Queenborough et al., 2013). Extra-floral nectaries in seedlings and trees also appear to have a positive demo- graphic effect (Muehleisen et al., 2016). This summer (2019), a Yale College student will document the incidence of extra-floral nectaries, juvenile leaf color, and associated leaf damage in the tree seedling community at Yasuni (Liu, 2019).
One of the most “curious and obscure” problems in evolutionary biology is how dioecy evolved. The ‘seed shadow handicap’ of dioecious populations requires at least a twofold greater relative fitness than hermaphrodite competitors for coexistence. Thus, successful invasion into hermaphrodite populations must offset this handicap. Specifically, more resources must be invested per seed, producing offspring with higher fitness; but apart from seed size, other traits have received little attention. I am addressing this deficit using demographic data on thousands of species and their reproductive traits, within a phylogenetic statistical framework. To date, I have shown that breeding system actually explains very little of the variation in seed size for over 1,000 species in two tropical floras (Queenborough et al., 2009b).
I am also investigating factors that drive the incidence and abundance of dioecy in communities. Up an elevational gradient in Costa Rica, dioecy did not correlate with the likely decrease of pollinators, but instead with the mid-elevation peak in species diversity (Vamosi and Queenborough, 2010). These investigations of the ecological and evolutionary significance of dioecy will be expanded in the near future.
I am applying my expertise in quantitative field ecology to examine the population dy- namics of herbaceous annual plants at multiple temporal and spatial scales. Studies of annual plants are usually conducted at very small spatial scales (0.1-10 m 2 ) and involve counting every individual. I am developing work initiated during my post-doc to mod- ify existing well-known matrix models to use plant density as the state variable rather than life-history stage or plant size. We have shown that using only a few categorical density states, rather than individual counts, accurately captures the population dynam- ics (Freckleton et al., 2011), enabling us to carry out field surveys very rapidly over large areas. These methods are being used to model the hierarchical dynamics of arable weed species for several hectares in each of 500 fields, over 50 farms and three UK counties, to determine how ecological, socio-economic and spatial factors influence distribution, abundance and dynamics (Queenborough et al., 2011). An extremely novel element of this work is that we can account for both inter- and intra-observer error, something that is rarely done in ecology.
Through this project, I am collaborating with sociologists and economists to de- velop models for integrating ecological and socio-economic data. We published a re- view of the interface between ecological and socio-economic models(Cooke et al., 2009), and also discussed the issue in a more informal context (Queenborough and Cooke, 2010). We have since developed a socio-economic model for farm management and farmer behavior (Cooke et al., 2013) and plan to integrate it with models of weed and bird populations.
I am also contributing phylogenetic analyses to a number of studies, including ex- amining how climate change is shifting plant phenology (Callinger et al., 2013; Amano et al., 2014), and a recent meta-analysis of plant-soil feedback (Crawford et al., in press), as well as the analyses for a recent highly-cited meta-analysis (Comita et al., 2014).