Mechanisms of Native Shrub Encroachment on a Virginia Barrier Island

 

Recent shrub encroachment is generally recognized as a response to anthropogenic disturbance and often a threat to ecosystems, although historically, shrubs represent a shift in successional states after a natural disturbance.  Negative effects associated with recent shrub expansion include decreased species diversity, extreme alteration in community structure, the creation of irreversible alternate stable states, nutrient cycling shifts, and increased susceptibility of shrubland compared to previous ecosystems.  On the Virginia barrier islands, Morella cerifera thickets clearly represent a different community structure compared to grasslands, and decreased plant diversity as well as increased soil nitrogen has been observed with the shift to shrubland. Joseph Thompson's master's research evaluated the effects of Morella cerifera on fine-scale abiotic and biotic factors upon encroachment into grassland. Species composition, temperature, soil nutrients, and leaf area index (LAI) were recorded across three encroaching M. cerifera thicket edges and three free-standing shrubs on Hog Island, Virginia to characterize the effect of shrub thickets on the plant community and microclimate. Electron transport rate (ETR) was taken on shrub leaves to determine if microclimate benefits M. cerifera physiology. Species richness was lowest inside shrub thickets. Soil water content and LAI were higher in shrub thickets compared to grassland. Soil organic matter, N, and C were higher inside shrub thickets. Summer and fall maximum temperatures were more moderate in shrub thickets and at free-standing shrubs. Fall and winter minimum temperatures were warmer inside shrub thickets. ETR was higher at the free-standing shrubs compared to the thicket edge. Morella cerifera significantly changes the microenvironment including temperature, edaphic factors, and plant species composition. These results show that expansion of M. cerifera in coastal systems has an immediate and significant impact on the surrounding environment.

 

 

Emergent interactions influence functional traits and success of dune building ecosystem engineers

Stability of coastal systems are threatened by oceanic and atmospheric drivers of climate change.  Sea-level rise compounded with increased frequency and intensity of storms emphasizes need for protection of inner island systems by dune formations.  Dune building processes are affected by interactions between growth of ecosystem engineering dune grasses and environmental factors associated with disturbance such as sand burial and salt spray.  Climate change may also cause latitudinal expansion of some species, resulting in emergence of competitive interactions that were previously absent.  Topographic structure of coastlines, traditionally influenced by sand burial, could change as a result of competition emergence.  The master's work of Joseph Brown was to determine if species functional trait responses to common abiotic factors are altered by novel and current biotic interactions.  He performed a multi-factorial greenhouse experiment by planting three common dune grasses (Ammophila breviligulata, Uniola paniculata, and Spartina patens) in different biotic combinations, using sand burial and salt spray as abiotic stressors.  He hypothesized that biotic interactions would cause these dune grasses to shift functional trait responses to abiotic factors that are associated with dune building.  The results of this study found that plants consistently decreased in biomass when buried.  When grown together, competition between A. breviligulata and U. paniculata negatively affected dune building function traits of A. breviligulata.  This indicates that competition, if future northward expansion of U. paniculata continues, could lead to dune engineering alterations, especially in the Virginia barrier islands.  In comparison A. breviligulata had a positive interaction with S. patens which increased functional trait responses to abiotic stress. Coexistence between these three species is possible via competitive intransitivity.  In intransitive competition, varying species-to-species interactions create a rock-paper-scissors scenario in which competitive hierarchy no longer exists. Current models suggest that within plant communities, intransitive interactions are most commonly found between dominant species, and dependent on short disturbance intervals and abiotic stress. Overall, these results can be used to make implications on cross-scale consequences of novel competitive events. This experiment also provides evidence that consideration of biotic interactions is important in substantiating connections between plant level dynamics and large-scale landscape patterns in high stress environments.

Joe will defend his thesis on March 29, 2016!

Conceptual model by Joe Brown on how plant functional responses ultimately affect island level processes

High habitat complexity co-occurs with high dune ridge formation

Low habitat complexity co-occurs with overwash and small hummock dunes

Species interactions affect dune grasses

 

Barrier islands are on the forefront of sea level rise and climate change. High disturbance regimes and sediment mobility make these systems sensitive, dynamic, and sentinels of climate change. Island foredunes act as protective structures against storm included overwash. Dune grasses are integral to the biogeomorphic feedback that create and alter foredunes. Shifts in dune grass dominance on foredunes could have implications on dune morphology and susceptibility to overwash, altering island stability. In a recent study by MS student April Harris, two dune grasses Ammophila breviligulata and U. paniculata were planted together and subjected to a 20 cm burial to quantify morphological and physiological response. These species form different types of dunes based on vegetative propogation strategy. Ammophila breviligulata uses a network of horizontal rhizomes to stabilize substrates and U. paniculata exhibits a dense compact growth form that binds substrates. The effect of these different strategies is seen in the types of dunes formed (ridge vs. hummock). In the glasshouse experiment the most significant finding was a decline in physiological and morphological performace in A. breviligulata when planted with U. paniculata while U. paniculata was not affected when planted with A. breviligulata. As U. paniculata migrates into A. breviligulata dominated habitat, changes in dune formation could result in altered island stability via increased overwash due to reduced growth in A. breviligulata. Burial had a positive effect on both species as indicated by increased electron transport rate and total biomass. This is typical of dune grasses as the need to rebound after burial is necessary to survive in foredune environments which frequently experience sediment deposition. The decline in growth of A. breviligulata when interacting with U. paniculata can alter foredune community structure by shifting dominant species. Overall, due to the biogeomorphic feedbacks that couple dune morphology to vegetation type on barrier islands, any shifts in dominance could alter island stability and structure over time leading islands to experience new, previously unachieved states.

April Harris will defend her Master's thesis next Thursday!

 

 

 

 

Ammophila breviligulata

Uniola paniculata

 

 

The importance of species and functional groups to barrier islands

We are continuing to understand the importance of woody vegetation to barrier islands, especially in stabilizing sediments and reducing overwash/limiting rollover. In a forthcoming paper in Ecosphere "Woody expansion facilitates liana expansion and affects physical structure in temperate coastal communities", we show the prevalence of lianas on barrier islands and the strong dependency with woody vegetation. The harsh abiotic conditions (e.g. high light, salinity, varying water availability) of these communities are most influential in structuring woody vegetation while liana species are primarily generalists associated with any woody plants. Interacting and possibly synergistic effects of woody expansion and liana proliferation appear to alter the traditional successional processes in coastal environments. Using a space-for-time geographic substitution (i.e. chronosequence) on two stable, unmanaged barrier islands, we show the lack of maritime forest regeneration due to the high leaf area index of woody shrubs and associated liana tangles. Historical photos from the Virginia barrier islands shows significant pine-oak forests that have been washed away from several islands due to erosion. New maritime forest does not appear to be establishing on the islands but replaced by new functional groups. We are continuing to investigate the ecological and ecosystem service consequences of this shift in succession and community structure.

Historical photo from Hog Island shows extensive maritime forest

 

Ghost trees due to erosion of maritime forest from Parramore Island

 

Present day woody vegetation - shrub thicket and lianas on Hog Island

 

A moment of reflection and remembrance

As a scientist, I often feel encouraged to remain objective and shy away from personal and philosophical matters. As a human, I can't do this. What began as a series on the ongoing and broad research in our lab has been temporarily halted. Sometimes life gets in the way. This time death demanded a shift in perspective on the importance in life. My mom passed away February 18, 2016 after months of illness from cancer. I originally wanted to pursue cancer research, but my love of outdoors made indoor research difficult. My mom always encouraged my love of science as a child and it is in her memory that I pause and share my thoughts and tribute to her.

 

To mom -

My sister and I have had the great honor to be with mom as she transitioned from her earthly life unto the next. While sadness and deep loss accompanied these moments, mom gave us the privilege to prepare and accept a new reality.  But while we tried to protect our hearts against the pain of losing our mother, it still hurts terribly.  Even though we grieve these days now that mom is not physically here with us, we also celebrate her life and spirit as she has moved on to a new journey. 

Someone recently described mom as a rare bird. I like this image of mom as a bird with showy feathers – where she has flight over this earth and without the limitations of a body that battled illness and major medical issues for over twenty five years. Through this time I came to see a community of love around her, around all of us. I stand amazed at the reach of one person  - the impact someone can have on so many people. There are many lessons to learn from these moments – lessons that are learned and repeated over and over by the 7 billion people here on earth and all those that came before us. There is this thread that connects all of humanity. We all experience the pain of suffering, the thrill of living. And mom soars above it all – free from the limits imposed on us now.

Mom has always been a fighter with a will stronger than anyone I know. She is a believer and has an amazing and admirable faith. She always provided an undercurrent of love and strength to us, even when we failed to see it. She experienced a lot of difficulty in her lifetime here, yet she always kept her faith to guide her through. She always remained positive even when dealing with cancer, treatments, surgeries, and unknown outcomes. She never once asked, “why is the happening to me”. By watching her deal with the challenges before her, mom gave me a strength I did not know I had. At times, as all of us do, we had a complicated relationship. I am fortunate that we were able to find the simplicity in love and being together. I know mom is proud of both Jill and myself and the women that we became. She loves dearly her three grandchildren Cody, Aksel, and Alice Rose. They were the light in her eyes and the joy in her heart.

Mom always had a great sense of humor and was a very social person. We would always find something to laugh about at family or group gatherings. She loved to tell other people all the childhood stories about me and Jill and her grandkids. She had an amazing memory to never forget the really good ones (like when Jill poured molasses in her brand new Sunday shoes – I can’t seem to remember any of the stories about me). Mom never drank. But she kept a bottle of vodka in the closet with her art supplies. We always joked that mom was a closet drinker. Mom had such amazing talents that she shared with all who knew her.  She had a glorious voice and loved to sing. When I was a small child, I ran up to the pulpit in church one time when mom was singing. I always loved it when she sang or played the piano – it was soothing to me. She was a true artist who kept pushing her own boundaries with the wonderful creations she made. When she was teaching at Montessori school, she would spend hours late into the night making a small quilted gift for each of the children in her class. She loved to create and to see the beauty in her world. She always carried her camera and would show me photos of the way light hit some object, flowers, trees, almost always photos of the natural world. Then she would use these images in her art.  

A friend said these words, that are so very true. A mother is our place of solace, our ass-kicker, the person we so desperately want to be nothing like, and later appreciate the traits we find of hers in ourselves. We don’t want to say goodbye to her now. As strong as we try to be, as much as I know we can keep moving forward, there is a hole in my heart that will take time to heal. By hearing her stories, learning more about the woman who was more than just my mother, connecting with family and friends that meant so much to her, we can honor her memory and carry on her legacy by offering love to those who touch our lives.