Do vegetation-microclimate feedbacks promote shrub encroachment in the Southwestern United States?

 

Research Team:

Paolo D'Odorico (UVA)

Yufei He (UVA)

Marcy Litvak (UNM)

Jose Fuentes (PennState)

Stephan de Wekker (UVA)

Scott Collins (UNM)

William Pockman (UNM)

 

    Woody plant encroachment into grasslands is a global phenomenon that results from a variety of global change drivers. Over the last 150 years the southwestern United States has undergone dramatic changes in the composition and structure of vegetation due to invasion by Larrea spp. and Prosopis spp in southwestern deserts. The relatively abrupt character of grassland-to-shrubland transitions suggests that arid and semiarid rangelands may be bistable systems, with stable states characterized by either grass or shrub dominance. Due to the presence of alternative stable states, aridland ecosystems may have limited resilience and undergo abrupt state transitions from grassland to shrubland. Bistable dynamics are induced by positive feedbacks between external drivers and the current system state. What remains unclear is whether feedbacks between land cover change and atmospheric boundary layer dynamics may contribute to shrub encroachment in the southwestern US.

 

Research Team:

Greg Okin (UCLA)

Kelly Caylor (Princeton)

Paolo D'Odorico (UVA)

Frances O’Donnell (Princeton)

Thoralf Meyer (UVA)

Abi Bhattachan (UVA)

Danielle Perrot (UCLA)

Kebonyethata Dintwe (UCLA)

Moe Tatlhego (University of Botswana)

Natalie Mladenov (CU-Boulder)

Sue Ringrose (University of Botswana)

 

    Recent developments in the study of savanna vegetation suggests that root structure and function are crucial to processes controlling the co-existence and relative abundance of different vegetation life forms. Vegetation composition and structure, in turn, determines both total ecosystem carbon storage and short- and long-term response of these ecosystems to climate variability. Savannas cover about 20% of the global land surface, including about one-half of Africa, Australia and South America. Understanding the dynamics of savanna vegetation and associated soil carbon pools is vital to the study of the global biosphere and the global carbon budget. The proposed research will address a major issue in savanna ecology: how belowground plant processes in water-limited ecosystems control plant spacing and thus structure the entire pedosphere under different climate conditions. To address these central issues, this project will define the distribution and dynamics of belowground carbon through extensive field data collection and modeling at a suite of sites across a regional climate gradient in the savannas of the Kalahari, on homogenous sandy soils. The following specific goals will be pursued:

Goal #1: Understanding landscape- and species-level root morphology and dynamics

Goal #2: Understanding  the direction and rate at which belowground carbon stocks can change with climate change at multiple timescales

 

    Semi-arid landscapes, which comprise about 20% of the Earth’s surface, are readily susceptible to disturbance by fire and wind. Despite important impacts on human health, soil productivity, ecosystem dynamics, and climate over large areas of the United States and the world, the interactions between fire and wind in semi-arid areas remain poorly understood. This project will assess the ability of fires to enhance the susceptibility of soils to wind erosion, and will seek to explain this effect as a result of the impact of fires on soil surface chemistry and water retention properties, which, in turn, depend on soil texture, fire temperature, and vegetation type. To quantitatively assess and quantify the enhancement of soil erodibility by fire, this project will use a set of controlled burns at two semi-arid rangeland sites and wind tunnel tests of soil erodibility before and after the fires. To test the hypothesis that the changes in soil erodibility are caused by soil surface chemistry and water retention properties, laboratory tests of these properties will be carried out on pre- and post-fire soil samples. Finally, the experimental insights will be used to develop a process-based theoretical framework to explain the ability of fires to increase soil erodibility. With successful completion of this research, better understanding of the interactions between land use practices (fires) and wind erosion susceptibility will result, which will provide information needed for better prediction of wind erosion and management of lands susceptible to wind erosion.

 

Research Team

Paolo D'Odorico (UVA)

Sujith Ravi (University of Arizona)

Ted Zobeck (ARS-USDA, Lubbock TX)

Abi Bhattachan (UVA)

Scott Collins (UNM)

Tom Over (Eastern Illinois Univ.)

Jonathan Blitz (Eastern Illinois Univ.)

 

 

 

 

Sponsor: National Park Service - Everglades National Park

Research Team:

Paolo D'Odorico, Joel Carr (UVA)

Vic Engel (South Florida Natural Resources Center, Everglades National Park )

Michael Ross (Florida International University)

Jay Sah (Florida International University)

    Environmental systems are typically forced by a number of drivers such as climate, and natural or anthropogenic disturbances, which are not constant in time but fluctuate. With the exception of processes dominated by deterministic oscillations (e.g., daily and seasonal cycles), a significant part of environmental variability is  random, due to the uncertainty inherent to weather patterns, climate fluctuations, and episodic disturbances such as hurricanes, landslides, fires, insect outbreaks, and epidemics. The recurrence of random drivers in biogeophysical processes motivates the study of how a stochastic environment may affect and determine the dynamics of natural systems.

    Recent climate change studies indicate that, in addition to trends in the mean values of climate variables, the interannual variability is also increasing. How will this increase in the variance of environmental parameters affect the dynamics of earth system? We are finding that noise could cause the emergence of new stable states, induce pattern formation, determine new bifurcations, or destabilize the stable states existing in the underlying deterministic dynamics. We use minimalist stochastic models to study to study this organizing effect of environmental fluctuations on ecosystem dynamics.

 

Luca Ridolfi (Politecnico di Torino)

Francesco Laio (Politecnico di Torino)

Fabio Borgogno (Politecnico di Torino)

    "Wetlands are humid environments which exhibit either shallow water tables or standing water for an extended period of time. Despite the lack of general consensus on the distinctive features defining wetland environments [e.g., Mitsch and Goesselink, 2000], the presence of saturated soils/shallow water tables or the recurrence of flooding conditions remain key elements of wetland hydrology. These elements, in turn, determine other commonly-recognized attributes of wetland ecosystems, such as the presence of undrained hydric soils and of species adapted to water logging conditions (hydrophytes and phreatophytes). Thus, the definition of wetland ecosystems would require that shallow water tables or flooding persist for sufficiently long periods of time to determine the dominance of phreatophytic or hydrophytic plant communities. Crucially important to this definition are both the quantitative understanding of the hydrologic regime determining  the persistence of hydrophyte vegetation, and the development of quantitative frameworks capable of relating climate, soil, and vegetation to water table fluctuations or the occurrence of flooding."

(Rodriguez-Iturbe, I., P. D’Odorico, F. Laio. L. Ridolfi, and S. Tamea, “Challenges in wetland ecohydrology: interactions of water table and unsaturated zone with climate, soil, and vegetation”, Water Resour Res., 43, W09301, doi:10.1029/2007WR006073, 2007).

Research in Humidland Ecohydrology

 

Collaborators

Ignacio Rodriguez-Iturbe (Princeton University)

Francesco Laio (Politecnico di Torino)

Luca Ridolfi (Politecnico di Torino)

Stefania Tamea (Politecnico di Torino)

Hydrological feedbacks between phosphorus deposition and canopy cover in dry seasonal forests

 

Paolo D’Odorico, Deborah Lawrence, Marcia DeLonge, Rishiraj Das, Christiane Runyan (UVA)

 

 

    Dry forests represent a large percentage of tropical forests and are vulnerable to both anthropogenic and natural disturbances, yet important aspects of their sensitivity to disruption remain poorly understood. It is particularly unclear how changes in land-use or tropical storm patterns may affect the resiliency of phosphorus (P)-limited Neotropical forests. In these systems, vegetation is sustained in the long-term by atmospheric P-inputs through rainfall, dust, or fog. Past research supports the idea that dust and fog deposition are dependent on canopy density (e.g., Leaf Area). Thus, the canopy may function as a “trap” for P, enabling a positive feedback between vegetation and P-deposition. The proposed research will investigate the ecohydrological mechanisms controlling the occurrence and strength of these feedbacks, and their susceptibility to changes in climate and land cover. In particular, the following research objectives will be addressed:

a)    To quantify the total P input (from deposition) to a dry neotropical forest and investigate its fluctuations associated with seasonality (dry vs wet season), fire occurrences, and proximity to areas affected by frequent fires (e.g., “slash and burn”).

b)    To quantify the strength of the canopy-P deposition feedback.

Impact of stochastic soil moisture dynamics on vegetation water stress and nutrient cycling (with H. Epstein, University of Virginia), NSF, 2003-2006

Advances in Stochastic Wind Modeling for Wind Erosion Estimation (with T.M. Over, Eastern Illinois University), USDA, 2001-2003

The effect of atmospheric humidity on the susceptibility of dry soils to wind erosion (with T.M. Over, Eastern Illinois University, and Ted M. Zobeck, Agricultural Research Service, Lubbock (TX)), NSF, 2004-2007

Hydrological and nutrient controls on the structure and function of southern African savannas: A multi-scale approach (P.I.: H.H. Shugart. Also with K.K. Caylor, G.S. Okin, S. Macko, T.M. Scanlon, and R. Swap, University of Virginia), NASA, 2004-2007

Transferring alterations in climatic extremes to plant productivity and soil biogeochemistry in grasslands: the soil water balance as a mediator of the dynamics (P.I.: A. Porporato, Duke University), NIGEC-DOE, 2004-2005