Functional Diversity in the Hyper-diverse Mangrove Communities in Papua New Guinea

Variation in plant functional traits reflect the differences in an environment a species occupies and the variation in the functional traits across an environmental gradient and growth form reflects the functional performance of the coexisting species. Therefore, detecting the differences in functional traits of species is important to our understanding of the performance of species. The leaf functional traits; Leaf mass per area (LMA) and vein density (DV) are key traits in the resource use strategies and photosynthetic gas exchange capacity (Amax, gsmax) of all plants. Mangroves occupy a hypersaline narrow ecological range and therefore are thought to have uniform functional performance. This view makes sense for low diversity mangrove communities, but what about the hyperdiversity communities such as those in New Guinea? A comprehensive review of the present understanding on the ecology and socio-ecological values of mangrove was undertaken to establish a good foundational understanding of my study on New Guinea mangroves (Chapter 1). It is widely held that mangroves thriving in a hypersaline environment experience high water constraints and as a result have high water use efficiency, therefore have narrow functional performance. I investigated the leaf and photosynthetic functional traits of 31 mangrove species among different zonation bands and growth from six mangrove communities in New Guinea to test the view that mangroves have a narrow functional performance (Chapter 2). I then investigated the differences in wood and stem hydraulic traits among different zonation bands and growth forms and the relationship between wood hydraulics and leaf photosynthetic gas assimilation functional performance to further test the long standing view that mangroves have a narrow functional performance (Chapter 3). I then summarized the major findings of my two studies and their implications on mangrove restoration and rehabilitation with particular reference to the recent mangrove rehabilitation initiatives in Papua New Guinea (Chapter 4). My studies on leaf and wood functional traits across different zonation bands, growth forms and root system types consistently revealed that mangroves have a wide functional performance, and different species exhibited different resource use strategies

Seasonal variability in leaf and whole-tree responses of Populus tremula L. to elevated CO2 and drought

Trees are exposed to unprecedented climate change, characterized by rapidly rising atmospheric CO2 concentration and longer and more intense drought episodes. Despite research efforts to predict effects of elevated CO2 (eCO2) on tree functioning, its temporal and spatial variability and the interaction between eCO2 and drought remain intensely debated. To address these knowledge gaps, this PhD dissertation investigates the effects of eCO2 on one-year-old European aspen trees at the beginning and the end of the growing season, and under well-watered and drought conditions. For this, leaf and whole-tree water use, carbon gain and carbon loss were monitored during two consecutive growing seasons under ambient or elevated atmospheric CO2 concentration. The conducted literature review and experiments highlight three important insights. First, the magnitude of the effects of eCO2 is highly variable over time, even within a single growing season, as a likely result of the different seasonal carbon requirements throughout plant development. Second, tree responses to eCO2 should not be derived from observations made at the leaf level. Finally, the alleviating effects of eCO2 when facing drought were limited to the leaf level and the late season in European aspen, suggesting a negligible role of CO2 fertilization in mitigating detrimental effects of drought

Earlier Phenology Associated With Spring Warming Will Help Offset Some of the Negative Impacts of Climate Change on Temperate Tree Seedling Recruitment

The warming temperatures and increased drought predicted to occur over the course of the next century have the potential to profoundly impact the composition and structure of global plant communities. Because of the relevance of forest ecosystems in storing a large amount of the planet’s carbon and thus in regulating the earth’s climate, there is a major effort to forecast forest composition, structure, and functioning. Accurate predictions will require the application of studies that identify how climate drivers (and interactions between multiple drivers) affect physiological processes that underlie patterns of demography and assembly. In forest systems, community composition is strongly shaped by bottleneck effects that occur during recruitment at small size classes, size classes that are highly vulnerable to climate change. In this dissertation, I investigated how climate change will affect the seedling demography of two temperate tree species that commonly co-occur across eastern North America: Acer saccharum (sugar maple) and Quercus rubra (northern red oak). In chapter 2 I investigated how potential climate-driven shifts in seedling foliar phenology (in relation to shifts in canopy phenology) could affect the ability of seedlings to maintain positive net carbon assimilation over the growing season, a dynamic that is commonly referred to as phenological escape. I also modeled how environmental conditions drive photosynthetic rates and used that information to estimate the relative proportion of carbon that is assimilated in different seasons. In my third chapter I used the same photosynthesis models to estimate annual carbon assimilation for individual tree seedlings and then modeled the relationship between carbon assimilation and demographic performance (growth and survival). I used results from both chapters to project how climate change in my study region could affect seedling demography directly (e.g., via changes in respiration rates associated with higher temperatures) and indirectly (e.g., via changes in access to light caused by different phenology shifts between seedlings and the canopy). In my last chapter I used a greenhouse study to investigate how seedlings of these two species respond to drought, specifically looking for differences in stomatal regulation of leaf water potential, reductions in photosynthetic capacity, reduction of non-structural carbohydrates, and loss of hydraulic conductivity. My results suggest that climate change will primarily affect seedling recruitment via changes in annual carbon assimilation. Although I found evidence that seedlings are likely to gain access to light with warming spring temperatures (thereby increasing net carbon assimilation in spring), elevated leaf respiration rates in hotter and drier summers would outweigh these gains and lead to net reductions in annual assimilation. In turn, these reductions would reduce seedling demographic performance and lead to less growth and higher mortality rates. Access to water could affect plant performance via reductions in photosynthetic rates, but seedlings of both species are also highly vulnerable to hydraulic failure during severe drought events. In sum, my results indicate that seedlings of both species may experience steep reductions in performance under extreme climate change, but that phenological escape dynamics may be enough to compensate for these reductions under more conservative climate change scenarios.PHDNatural Resources & EnvironmentUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/163249/1/benrlee_1.pd

Functional Attributes of Post-Disturbance Regeneration in Puerto Rican Tropical Dry Forest

The global extent of tropical dry forests has rapidly diminished in recent decades due to a variety of threats, largely from human activity. Efforts to restore these forests require understanding of the various modes of regeneration and how they are impacted across a range of disturbance-types. Studies of tropical forest recovery have traditionally neglected the concept of ‘persistence’ in favor of ‘recruitment’ and seedling dynamics. Increasingly, the role of resprouting as a form of persistence in stressful environments is recognized as an important factor that has implications for population turnover, minimizing disturbance impacts and reducing reliance on seeds. Using a functional-trait approach, this research investigated the functional basis of resprouting and persistence within tropical dry forest from the individual to community scale. The study area was a threatened Puerto Rican tropical dry forest where resprouting is a dominant form of recovery and thought to be an adaptation to drought and occasional windthrow. Firstly, I sought to determine the range of functional types within the community by asking what water-use strategies characterize dominant tree species? A broad range of water-use behaviors were observed but most species converged on a high degree of drought tolerance maintaining dry season resource uptake. Secondly, I considered the life-history consequences associated with resprouting. Conservative, drought tolerant strategies were associated with low adult growth, which unexpectedly also translated to weaker resprouting. The occurrence of Hurricane Maria presented the opportunity to study the short-term physiological responses of trees following defoliation. Interestingly, dry forest species were found to exhibit highly plastic responses suggesting a common ability to exploit high resource windows possibly to fuel recovery. Finally, I asked whether functional recovery and assembly mechanisms were predictable across clearcut and fire chronosequences where resprouting was the dominant form of regeneration. Both types of chronosequences were characterized by functional shifts from conservative to more acquisitive resource-use, but recovery trajectories in clearcut sites were more stable as the effects of lower disturbance severity promoted successful regeneration of resprouts similar to ‘natural’ patterns of recovery. Fire legacy effects by contrast inhibit functional diversity and create species poor communities. Overall, my results suggest that successful persistence through resprouting in tropical dry forest is strongly dependent on species identity, life-history strategy and the type of disturbance. These forests have a diversity of mechanisms available to drive recovery but severe disturbances such as fire will reduce that diversity and ultimately reduce forest resilience

Forest ecosystem water use : does species identity and ecosystem composition matter?

Transpirational water use by trees has been long known to be regulated by evaporative demand and temperature, solar radiation, stomatal conductance and tree leaf area. More recently control of transpiration by plant hydraulic traits has been highlighted, and these as well as stomatal conductance and its response to air saturation vapour pressure deficit remain unstudied for the majority of Australian native tree species. To predict how forested ecosystem water use may change under future climates and enable better estimates of catchment water losses, we must understand stomatal and hydraulic behaviour of trees in the field under a range of conditions. In this study, I quantified traits describing stomatal and hydraulic behaviour for five Eucalyptus species from differing climates. Patterns in whole tree water use, stomatal sensitivity and responses to low water availability of these species in a common garden were correlated with species identity and with their characteristic climate of origin. I found that different Eucalyptus species employed different strategies to deal with water deficits which were linked to hydraulic, anatomical and leaf tissue water relations characteristics, and also with the original climatic range of the species. Tree water use, growth and tolerance of low water availability were enhanced in species mixtures compared to monocultures, an effect ascribed to asymmetric competition of component species in these mixtures. A basis for incorporating species stomatal and hydraulic parameters into forest stand-level water use models is provided. Ultimately, doing so will enhance predictions of water use, and enable estimates of stand water-use efficiency and productivity under current and future climate conditions. The findings are key to inform plantation and land management decisions, and can assist in the identification of vulnerable species or ecosystems, and the conservation of catchment water supplies in a changing climate

Leaf physiological and morphological constraints of water-use efficiency in C3_3 plants

The increasing evaporative demand due to climate change will significantly affect the balance of carbon assimilation and water losses of plants worldwide. The development of crop varieties with improved water-use efficiency (WUE) will be critical for adapting agricultural strategies under predicted future climates. This review aims to summarize the most important leaf morpho-physiological constraints of WUE in C3 plants and identify gaps in knowledge. From the carbon gain side of the WUE, the discussed parameters are mesophyll conductance, carboxylation efficiency and respiratory losses. The traits and parameters affecting the waterside of WUE balance discussed in this review are stomatal size and density, stomatal control and residual water losses (cuticular and bark conductance), nocturnal conductance and leaf hydraulic conductance. In addition, we discussed the impact of leaf anatomy and crown architecture on both the carbon gain and water loss components of WUE. There are multiple possible targets for future development in understanding sources of WUE variability in plants. We identified residual water losses and respiratory carbon losses as the greatest knowledge gaps of whole-plant WUE assessments. Moreover, the impact of trichomes, leaf hydraulic conductance and canopy structure on plants’ WUE is still not well understood. The development of a multi-trait approach is urgently needed for a better understanding of WUE dynamics and optimization

Gas exchange and hydraulic strategies of pasture species under climate change

Climate change-induced increases in global air temperature and concurrent modulations in the hydrological cycle have led to higher levels of drought stress globally. Grassland ecosystems such as pastures are particularly sensitive to climate change as they lack the deep roots and carbohydrate reserves that partially dampen the negative impact of abiotic stress in woody plant systems. Pastures have significant economic and ecological value, and it is important to assess the current and future impact of climate change on these systems. While many studies have investigated the response of pasture species to warming and drought separately, few studies have focused on their interactions. Furthermore, the mechanistic basis for the response of pasture species to abiotic stress, especially drought and heat stress, is not well understood. While the mechanisms underlying drought-induced impacts on hydraulic processes are well studied in woody species, there are currently few assessments in pasture grasses. In this thesis, I examined leaf gas exchange and hydraulic traits across a range of widely cultivated pasture species from different plant functional groups and investigated the response of these traits to warming and drought stress. My research sought to understand how these traits determine plant function under normal and abiotic stress conditions and how that will determine the response of pasture systems in a warmer, drier future. The primary objective was to identify more resilient pasture species and investigate the mechanistic basis for their resilience, aiding the development of species selection and management strategies that may mitigate the effects of climatic change. Findings show that increases in global air temperature may not have a positive impact on the productivity of pasture species in eastern Australia, even during the cool season and warming will have a more negative impact on C3 pasture species than C4 species during warmer periods. We found that the ability to resist xylem embolism and hydraulic dysfunction, rather than recovery, underpins pasture species resilience to drought, with early stomatal closure being crucial to species’ survival under water limiting conditions. Collectively, climate change induced increases in air temperature and drought are likely to have negative impacts on growth and productivity of temperate C3 pasture species and physiological adjustment may not be sufficient to alleviate the impacts of rapid global change. Therefore, an overall decline in temperate C3 species performance and an increase in C4 dominance are expected in pastures in a warmer, drier future

Differences in xylem and leaf hydraulic traits explain differences in drought tolerance among mature Amazon rainforest trees

Considerable uncertainty surrounds the impacts of anthropogenic climate change on the composition and structure of Amazon forests. Building upon results from two large-scale ecosystem drought experiments in the eastern Brazilian Amazon that observed increases in mortality rates among some tree species but not others, in this study we investigate the physiological traits underpinning these differential demographic responses. Xylem pressure at 50% conductivity (xylem-P50 ), leaf turgor loss point (TLP), cellular osmotic potential (πo ), and cellular bulk modulus of elasticity (ε), all traits mechanistically linked to drought tolerance, were measured on upper canopy branches and leaves of mature trees from selected species growing at the two drought experiment sites. Each species was placed a priori into one of four plant functional type (PFT) categories: drought-tolerant versus drought-intolerant based on observed mortality rates, and subdivided into early- versus late-successional based on wood density. We tested the hypotheses that the measured traits would be significantly different between the four PFTs and that they would be spatially conserved across the two experimental sites. Xylem-P50 , TLP, and πo , but not ε, occurred at significantly higher water potentials for the drought-intolerant PFT compared to the drought-tolerant PFT; however, there were no significant differences between the early- and late-successional PFTs. These results suggest that these three traits are important for determining drought tolerance, and are largely independent of wood density-a trait commonly associated with successional status. Differences in these physiological traits that occurred between the drought-tolerant and drought-intolerant PFTs were conserved between the two research sites, even though they had different soil types and dry-season lengths. This more detailed understanding of how xylem and leaf hydraulic traits vary between co-occuring drought-tolerant and drought-intolerant tropical tree species promises to facilitate a much-needed improvement in the representation of plant hydraulics within terrestrial ecosystem and biosphere models, which will enhance our ability to make robust predictions of how future changes in climate will affect tropical forests

Drought and the interannual variability of stem growth in an aseasonal, everwet forest

Linking drought to the timing of physiological processes governing tree growth remains one limitation in forecasting climate change effects on tropical trees. Using dendrometers, we measured fine-scale growth for 96 trees of 25 species from 2013 to 2016 in an everwet forest in Puerto Rico. Rainfall over this time span varied, including an unusual, severe El Niño drought in 2015. We assessed how growing season onset, median day, conclusion, and length varied with absolute growth rate and tree size over time. Stem growth was seasonal, beginning in February, peaking in July, and ending in November. Species growth rates varied between 0 and 8 mm/year and correlated weakly with specific leaf area, leaf phosphorus, and leaf nitrogen, and to a lesser degree with wood specific gravity and plant height. Drought and tree growth were decoupled, and drought lengthened and increased variation in growing season length. During the 2015 drought, many trees terminated growth early but did not necessarily grow less. In the year following drought, trees grew more over a shorter growing season, with many smaller trees showing a post-drought increase in growth. We attribute the increased growth of smaller trees to release from light limitation as the canopy thinned because of the drought, and less inferred hydraulic stress than larger trees during drought. Soil type accounted for interannual and interspecific differences, with the finest Zarzal clays reducing tree growth. We conclude that drought affects the phenological timing of tree growth and favors the post-drought growth of smaller, sub-canopy trees in this everwet forest