Doctoral research

Introduction      Here is a brief description of the research that I carried out during my PhD at UPenn. Although most of the chapters are published, I still continue doing long-term (>14 years) demographic census at the fieldsite in the Great Basin desert (Utah, US).      Because size plays an crucial role in the determination of individual plant fitness, we will not achieve a full understanding of plant demography  until the overlooked possibility for individual plants to decrease in size, plant shrinkage, is integrated in the field. My work investigates the anatomical structures and physiological mechanisms that result in internally regulated size declines, as well as the frequency, contribution to the demographic dynamics and potential evolutionary adaptations of plant shrinkage across taxa, growth forms and habitats. In order to address this question, I am undertaking a multi-disciplinary approach that includes histological work, physiological experiments, long-term field demographic censuses involving the effects of climate change, and a phylogenetic-based literature survey of demographic data.      My doctoral research is composed of five areas of ecological knowledge: 1- Acquisition of belowground resources:      I have established that the acquisition of belowground resources in my study species, the aridland chamaephyte Cryptantha flava L. (Boraginaceae), occurs in a modular manner (Salguero-Gomez & Casper, in prep.); the lateral roots of this species are activated  independently upon spatially heterogeneous watering (Fig. 1). I suggest that the ability of the plant to activate some foraging structures, but not others, allows it to maximize resource uptake in such arid, patchy environment. Seven days after an intense (e.g. 4.5 cm) pulse of water (Fig. 2), the node-like structures (N) found on the lateral roots (LT) develop ephemeral white fine roots (WFR). The white fine roots develop always from the center of the node, and at any time during the growing season, the node contains fine ephemeral roots of different morphology and coloration (and possibly different degree of functioning) that range from turgent, white, to fragile, thin and black ones.      The fine roots and nodes are associated with arbuscular mycorrhizae (See Side Projects). Fig. 2: watering a section of an individual's belowground zone of influence   Fig. 1: the entire/part of the belowground zone of influence of individuals of Cryptantha flava L. (Boraginaceae) received 4.5 cm of water, and white fine root standing crop was measured a week later in 2007 and 2008. Fig. 3: longitudinal and cross sections of a lateral root containing three nodes and their corresponding fine roots. 2- Distribution of belowground resources:      Because higher plants are extremely modular organisms, shrinkage in them may be the result of the reduction in the number of modules. Consequently, a mechanism that may explain plant shrinkage is the internal regulation of these modules in a way that could facilitate their elimination without compromising whole plant survival. Up until recently, it had been commonly assumed that resources taken up by individual roots were redistributed to the whole plant, but this integrative view has been juxtaposed in the last decades with strong evidence of hydraulic sectoriality, i.e. preferential transport of resources from specific roots to specific shoots.      My work with Cryptantha flava reveals for the first time functional, developmental hydraulic sectoriality in a desert chamaephyte (Salguero-Gomez & Casper, in review). Additional work with other aridland chamaephyte (Salguero-Gomez, unpublished) suggests that hydraulic sectoriality may be adaptive and I suggest that the ability to become internally sectored, a phenomenon that is very frequent among aridland woody species, may have allowed herbaceous perennials to find their niche in cold deserts. Fig. 4: Adult individual of C. flava that shrunk from >200 rosettes in 2007 (bottom right side of the plant shows remainings of dead rosettes) to 80 rosettes in 2008. Fig. 5: Size changes for individuals of C. flava that survived from 2007 to 2008. Note that shrinkage is a size-specific phenomenon, where large individuals are more likely to decrease in size while small (arguably juvenile) individuals are more prone to growth. Fig. 6: Dye-tracking experiment with acid fuchsin on a lateral root of C. flava. Fig. 7: Adult individuals -but not juveniles- of C. flava are hydraulically sectored because when one side of the plant is watered, only one module within the plant experiences increases in leaf water potential. Such module in the plant is the same group of rosettes to which dyes (Fig. 6) will travel to from the lateral root that was foraging in the watered soil section. 3- Within-plant dynamics: rosette demography      The hypothesized advantages of being hydraulically sectored (see above) are (i) to be able to lower mortality risk of the entire plant by localizing stress and avoiding disease spread, (ii) consequentially lengthening longevity, and (iii) slowing down/escaping senescence , as well as (iv) to increase the overall plant efficiency at resource uptake by sacrificing modules whose roots are not obtaining enough resources, thus (v) allowing for costly functions with a clear impact on the population, such as reproduction. All this advantages are of tremendous importance for the demography of a sectored species, but to date they are only hypotheses. This section of my doctoral work is set to address whether being sectored is truly advantageous in individuals of C. flava.      Since adults of C. flava are hydraulically sectored for their lateral roots, but hydraulically integrated for their only tap root (Salguero-Gomez & Casper, in review), the theory described above would predict that the dead of a module (Fig. 9) within an individual should result in an increase in survival probability and reproductive output (thus fitness) of the remaining modules. To test this hypotheses I have been following all the rosettes (~50 on average per individual) for over 200 adults of C. flava since 2006.      I am currently analyzing the data and will soon be able to give you an answer!! Fig. 8: close-up to and adult of C. flava where every single rosette has been marked with plastic-covered wire with color beaded IDs. Fig. 9: In most of the cases, shrinkage in C. flava occurs through the dead of an entire module of rosettes, as the one that I am holding above. 4- Among-plant dynamics: individual demography      Demography is based at the interface of conservation biology and evolution because it allows for the estimation of invasiveness rates and extinction risks, for the determination of the life cycle stages that most affect demographic dynamics (elasticity analyses), and for the exploration of processes that are most strongly under selection (sensitivity analyses). Scientists have paid much attention to the structural and hydraulic factors that control and limit maximum plant size (e.g. work by Enquist, Niklas, Reicht, etc). However, the ability of plants to decrease in size, a frequent yet overlooked phenomenon has passed by under the radar of plant ecologists and evolutionists. The impacts of shrinkage, however, are predictably important because size is the strongest predictor of population- and individual-based fitness.      I am studying the demographic dynamics of a natural population of C. flava in the Great Basin Desert, in Utah. The goal of this project is to understand how increases in precipitation at the end of the growing season, as predicted by regional climatic models, will affect the viability of this population, and what role plant shrinkage will play in tracking environmental stochasticity. To this end, I have been following +3000 individuals from 2005 to 2010, in May and August, and supplying 4.5 cm of rainfall to some plots every August. I am currently constructing integral projection models (IPMs)... and thus will be able to give you an answer very soon!!   Fig. 10: Watering an experimental plot in the Redfleet State Park, Utah   Fig. 11: View of the fieldsite with the BLM vehicle in which I transport the water. Fig. 12: IPM for the control plots of 2008-2009. Green surface represent size changes and pink surface represents sexual recruitment. Fig. 13: Elasticity analysis for IPM of figure 12, showing that recruitment and fast growth of juveniles is crucial to the population. Unfortunately, recruitment has been very low in the last 4 years of research. 5- Comparative demography:      Projection matrix models are powerful tools to study the effects of size on the dynamic of species populations. A size-based matrix model classifies individuals in a population according to discrete ranges of size, and describes for each class the average probability of survival, probabilities of change in size (growth or shrinkage), and the reproductive contributions to the population. Due to the wide range of ecological and evolutionary questions that the matrix approach has allowed to address (e.g. determination of exploitation regimes that are sustainable for natural populations, or whether or not plants age) matrix data for over 600 plants species has been published (Salguero-Gomez, unpublished). This vast amount of information has no precedent in demography, and is now allowing for further exploration of even broader ecological questions. An example is the comparison of biological processes such as growth, stasis and reproduction both intra- and inter-specifically. Surprisingly, shrinkage, one of the processes that can be described by these matrices when it occurs, has never been examined in comparative demographic studies.      Questions such as which species are able to shrink, and in what habitat (desert, tropical forest, temperate forest, etc) is shrinkage more frequent and/or demographically important can, in principle, be addressed with this large amount of matrix demographic information. However, because evolution is a branching process, all biological traits – demographic ones included – are related by common ancestry, and therefore the use of statistical analyses that assume data independence are not suitable here. An appropriate way to analyze these data is the use of PICs (Phylogenetic Independent Contrast analyses) because they account for phylogenetic relationships.      I have shown that previous assumptions used in comparative demography (lumping shrinkage with stasis, or shrinkage with growth) are not mathematically/biologically sound (Salguero-Gomez & Casper J Ecol 2010). Drawing from a subset of my database, in this case with 80 herbaceous perennial species, I have pointed out that species whose individuals show shrinkage (=retrogression probabilities in projection matrix >0) have greater resilience, and through loop analyses, I have also demonstrated that the ability of plants to grow and shrink, an important plastic trait, is positively correlated with lifespan.