Abstract

Human-induced changes in climate and land cover are driving the coupled climatehydrological-ecological system (CHES) into unchartered territories with existential implications on natural resource availability and sustainability at both local and global scales. The Fifth Assessment Report by the Intergovernmental Panel on Climate Change noted that rising CO2 resulting from anthropogenic emissions has a first-order influence on ecosystem and hydrological responses (Settele et al., 2015). At the global scale, CO2 traps infrared radiation and reduces outgoing radiation from the top of the atmosphere, resulting in warming of the atmosphere. Warming temperature alters atmospheric humidity, precipitation pattern, and frequency and duration of extreme events such as storms and droughts (IPCC, 2013), which in turn changes surface and subsurface hydrologic regimes. These impacts are manifested in rivers and wetlands, aecting their ecological functions (Solomon, 2007; Miao et al., 2013; Rodriguez-Iturbe et al., 2007). Changes in hydrologic regimes also impact vegetation dynamics and distribution. Rising CO2 and altered hydroclimatic conditions affect vegetation growth, susceptibility and resilience through modulation of stomatal kinetics and root water uptake. As the dynamics of vegetation cover and stomatal kinetics control the exchange of water vapor and CO2 between the terrestrial biosphere and the atmosphere, response of vegetation to hydrometeorogloical change and variation provides feedbacks to the atmosphere through mediated water, carbon and energy budgets. As the physical processes controlling these interactions in the CHES remain far from being thoroughly understood (Council et al., 2012), uncertainties exist in predicting the concomitant dynamics of hydrological-ecological systems. Improved understanding and prediction of these dynamics, guided by synergistic use of diagnostic modeling and empirically based data analytics, are needed for managing natural resources and sustainability.

Among a range of CHES, freshwater wetlands are considered as one of the most vulnerable systems under changing climate (Dudgeon et al., 2006). As increasing evidence has shown, a multitude of ecological processes and functions of wetlands, including methane emission, nitrogen cycling, buffering and vegetation dynamics, are controlled and will be signicantly altered by climate-induced changes in hydrologic regimes (Settele et al., 2015). Thus it is imperative to understand the impact of inter-annual20 variability of climate conditions on hydrological regimes in wetlands. This improved understanding will serve as a key to predicting ecological functions under future climate.

Forested ecosystems are another CHES that are threatened by changing climate. With projected warming temperature and increasing variability of precipitation pattern, climate-induced water and heat stress to trees are expected to intensify in the future. A catastrophic impact of the stresses can be tree mortality, which is being widely observed throughout the globe in recent decades (IPCC, 2013; Allen et al., 2010). Given that forest covers 30% of the globe's land surface (Bonan, 2008) and assimilates around 2.4 Pg carbon per year (Pan et al., 2011), wide-spread tree mortality may offset the carbon sink provided by forests, impair a wide variety of ecological functions, and increase the risk of forest fires. However, prediction of wide-spread tree mortality and its corresponding impacts remains largely uncertain (Settele et al., 2015). Thus improved understanding of tree mortality in response to both long-term climate trends and near-term fluctuations of hydro-climatic stresses is required to provide critical insights into future dynamics of CHES. Furthermore, climate-induced stress on forested ecosystems also alters the biosphere-atmosphere interactions through dynamical response of stomatal kinetics. Plants remove CO2 from the atmosphere via photosynthesis while inevitably losing water into the dry atmosphere. Such gas exchange provides feedbacks to water-carbon budgets, energy partitioning, and cloud formation that could in turn modify convective precipitation (Konings et al., 2010; Manoli et al., 2016). However, how stomatal kinetics responds to hydro-climatic variation has long been an active research eld and remains as one major uncertainty source of future climate projections (IPCC, 2013). The challenge arises mainly from complex manifolds of driving forces, interactive mechanisms, and heterogeneous plant properties lacking extensive measurements. Therefore, a framework is needed to incorporate improved understanding in biophysical mechanisms and appropriate parameterization to further reduce uncertainties in estimating biosphere-atmosphere interactions under hydro-climatic stresses.

Citation

Liu, Yanlan (2019): Impacts of Climate Variation and Change on Hydrologic and Vegetation Dynamics. PhD Dissertation, Duke University.