More Like this

1. Demography and dispersal at a grass‐shrub ecotone: A spatial integral projection model for woody plant encroachment

   https://doi.org/10.1002/ecm.1574  

   Drees, Trevor; Ochocki, Brad M.; Collins, Scott L.; Miller, Tom E. X. (May 2023, Ecological Monographs)

   Abstract The encroachment of woody plants into grasslands is a global phenomenon with implications for biodiversity and ecosystem function. Understanding and predicting the pace of expansion and the underlying processes that control it are key challenges in the study and management of woody encroachment. Theory from spatial population biology predicts that the occurrence and speed of expansion should depend sensitively on the nature of conspecific density dependence. If fitness is maximized at the low‐density encroachment edge, then shrub expansion should be “pulled” forward. However, encroaching shrubs have been shown to exhibit positive feedbacks, whereby shrub establishment modifies the environment in ways that facilitate further shrub recruitment and survival. In this case there may be a fitness cost to shrubs at low density causing expansion to be “pushed” from behind the leading edge. We studied the spatial dynamics of creosotebush (Larrea tridentata), which has a history of encroachment into Chihuahuan Desert grasslands over the past century. We used demographic data from observational censuses and seedling transplant experiments to test the strength and direction of density dependence in shrub fitness along a gradient of shrub density at the grass–shrub ecotone. We also used seed‐drop experiments and wind data to construct a mechanistic seed‐dispersal kernel, then connected demography and dispersal data within a spatial integral projection model (SIPM) to predict the dynamics of shrub expansion. Contrary to expectations based on potential for positive feedbacks, the shrub encroachment wave is “pulled” by maximum fitness at the low‐density front. However, the predicted pace of expansion was strikingly slow (ca. 8 cm/year), and this prediction was supported by independent resurveys of the ecotone showing little to no change in the spatial extent of shrub cover over 12 years. Encroachment speed was acutely sensitive to seedling recruitment, suggesting that this population may be primed for pulses of expansion under conditions that are favorable for recruitment. Our integration of observations, experiments, and modeling reveals not only that this ecotone is effectively stalled under current conditions but also why that is so and how that may change as the environment changes. 

   more » « less
2. Density-dependent demography of creosote bush (Larrea tridentata) along grass-shrub ecotones.

   https://doi.org/10.6073/pasta/ca53c16f16dcf9fb11f3ee99ea5445ac  

   Ochocki, Brad M.; Drees, Trevor H.; Miller, Tom E.X. (January 2023, Environmental Data Initiative)

   {"Abstract":["The encroachment of woody plants into grasslands is a global\n phenomenon with implications for biodiversity and ecosystem\n function. Understanding and predicting the pace of expansion and the\n underlying processes that control it are key challenges in the study\n and management of woody encroachment. Theory from spatial population\n biology predicts that the occurrence and speed of population\n expansion should depend sensitively on the nature of conspecific\n density dependence. If fitness is maximized at the low-density\n encroachment edge then shrub expansion should be "pulled"\n forward. However, encroaching shrubs have been shown to exhibit\n positive feedbacks, whereby shrub establishment modifies the\n environment in ways that facilitate further shrub recruitment and\n survival. In this case there may be a fitness cost to shrubs at low\n density causing expansion to be "pushed" from behind the\n leading edge. We studied the spatial dynamics of creosotebush\n (Larrea tridentata), which has a history of\n encroachment into Chihuahuan Desert grasslands over the past\n century. We used demographic data from observational censuses and\n seedling transplant experiments to test the strength and direction\n of density dependence in shrub fitness along a gradient of shrub\n density at the grass-shrub ecotone. We also used seed-drop\n experiments and wind data to construct a mechanistic seed dispersal\n kernel, then connected demography and dispersal data within a\n spatial integral projection model (SIPM) to predict the dynamics of\n shrub expansion. The SIPM predicted that, contrary to expectations\n based on potential for positive feedbacks, the shrub encroachment\n wave is "pulled" by maximum fitness at the low-density\n front. However, the predicted pace of expansion was strikingly slow\n (ca. 8 cm/yr), and this prediction was supported by independent\n re-surveys of the ecotone showing little to no change in spatial\n extent of shrub cover over 12 years. Encroachment speed was acutely\n sensitive to seedling recruitment, suggesting that this population\n may be primed for pulses of expansion under conditions that are\n favorable for recruitment. Our integration of observations,\n experiments, and modeling reveals not only that this ecotone is\n effectively stalled under current conditions, but also\n why that is so and how that may change as the\n environment changes."]} 

   more » « less
3. Shrub Age and Water Dynamics Influence Primary Production, Carbon, and Nitrogen Stocks in a Coastal Environment

   https://doi.org/10.1007/s13157-024-01786-8  

   Wood, Lauren K.; Young, Donald R.; Zinnert, Julie C. (February 2024, Wetlands)

   Abstract Drivers of shrub primary production and associated landscape impacts of encroachment are well known in drylands but have not been thoroughly studied in mesic and coastal habitats. The native, nitrogen-fixing shrub,Morella cerifera,has expanded into coastal grassland along the US Atlantic coast due to warming temperatures, but impacts on ecosystem function are not well known. Annual net primary production (ANPP) ofMorella ceriferaand key environmental drivers were measured long-term (1990 – 2007) across a chronosequence of shrub age on a mid-Atlantic barrier island. Soil and groundwater nutrients were compared with un-encroached grassland soil to evaluate impacts of vegetation on nutrient dynamics. Shrub ANPP declined with age at the same rate among all thickets, but there was variability from year to year. When climate variables were included in models, shrub age, precipitation, and freshwater table depth were consistent predictors of ANPP. Water table depth decreased over time, reducing ANPP. This may be due to rising sea-level, as well as to feedbacks with shrub age and evapotranspiration. Soil N and C increased with shrub age and were higher than adjacent grassland sites; however, there was a significant loss of N and C to groundwater. Our results demonstrate that drivers influencing the encroachment of shrubs in this coastal system (i.e., warming temperature) are not as important in predicting shrub primary production. Rather, interactions between shrub age and hydrological properties impact ANPP, contributing to coastal carbon storage. 

   more » « less
4. Shrub encroachment of coastal ecosystems depends on dune elevation

   https://doi.org/10.1007/s11258-024-01453-2  

   Woods, Natasha N; Zinnert, Julie C (August 2024, Plant Ecology)

   Woody plant encroachment is infuenced by interactions between the physical environment and vegetation, which create heterogenous microenvironments some of which favor shrub recruitment through mitigation of the abiotic environment. Encroachment of native shrub, Morella cerifera into grasslands on Hog Island, Virginia has been attributed to warmer winter temperature; however, recruitment of seedlings in grasslands may be impacted by multiple factors at the level of the microhabitat. Our study focuses on a critical gap in understanding factors specifcally infuencing M. cerifera seedling recruitment and survival. By experimentally planting M. cerifera seedlings at varying dune elevations and grass densities, we tested hypotheses that dune elevation infuences the microclimate, soil characteristics and vegetation cover and that grass cover/density is related to shrub establishment. We tested these hypotheses through gathering data from temperature data loggers, conducting soil water content and chloride analyses, and determining percent cover of grasses relative to dune elevation. Results indicate that dune elevation was positively related to moderated temperatures with reduced temperature extremes and vegetation cover/composition that led to favorable locations for M. cerifera establishment and growth. Where dune elevation is>2 m, we document an upper limit of grass cover on natural seedling establishment, suggesting a switch from facilitative to competitive efects with grass density. Overall, our work demonstrates interactions between dune elevation and medium grass density has a facilitative infuence on M. cerifera establishment and can be used for future predictions of shrub growth with rising sea-levels. 

   more » « less
5. Long-Term Community Dynamics Reveal Different Trajectories for Two Mid-Atlantic Maritime Forests

   https://doi.org/10.3390/f12081063  

   Woods, Natasha N.; Tuley, Philip A.; Zinnert, Julie C. (August 2021, Forests)

   null (Ed.)

   Maritime forests are threatened by sea-level rise, storm surge and encroachment of salt-tolerant species. On barrier islands, these forested communities must withstand the full force of tropical storms, hurricanes and nor’easters while the impact is reduced for mainland forests protected by barrier islands. Geographic position may account for differences in maritime forest resilience to disturbance. In this study, we quantify two geographically distinct maritime forests protected by dunes on Virginia’s Eastern Shore (i.e., mainland and barrier island) at two time points (15 and 21 years apart, respectively) to determine whether the trajectory is successional or presenting evidence of disassembly with sea-level rise and storm exposure. We hypothesize that due to position on the landscape, forest disassembly will be higher on the barrier island than mainland as evidenced by reduction in tree basal area and decreased species richness. Rate of relative sea-level rise in the region was 5.9 ± 0.7 mm yr−1 based on monthly mean sea-level data from 1975 to 2017. Savage Neck Dunes Natural Area Preserve maritime forest was surveyed using the point quarter method in 2003 and 2018. Parramore Island maritime forest was surveyed in 1997 using 32 m diameter circular plots. As the island has been eroding over the past two decades, 2016 Landsat imagery was used to identify remaining forested plots prior to resurveying. In 2018, only plots that remained forested were resurveyed. Lidar was used to quantify elevation of each point/plot surveyed in 2018. Plot elevation at Savage Neck was 1.93 ± 0.02 m above sea level, whereas at Parramore Island, elevation was lower at 1.04 ± 0.08 m. Mainland dominant species, Acer rubrum, Pinus taeda, and Liquidambar styraciflua, remained dominant over the study period, with a 14% reduction in the total number of individuals recorded. Basal area increased by 11%. Conversely, on Parramore Island, 33% of the former forested plots converted to grassland and 33% were lost to erosion and occur as ghost forest on the shore or were lost to the ocean. Of the remaining forested plots surveyed in 2018, dominance switched from Persea palustris and Juniperus virginiana to the shrub Morella cerifera. Only 46% of trees/shrubs remained and basal area was reduced by 84%. Shrub basal area accounted for 66% of the total recorded in 2018. There are alternative paths to maritime forest trajectory which differ for barrier island and mainland. Geographic position relative to disturbance and elevation likely explain the changes in forest community composition over the timeframes studied. Protected mainland forest at Savage Neck occurs at higher mean elevation and indicates natural succession to larger and fewer individuals, with little change in mixed hardwood-pine dominance. The fronting barrier island maritime forest on Parramore Island has undergone rapid change in 21 years, with complete loss of forested communities to ocean or conversion to mesic grassland. Of the forests remaining, dominant evergreen trees are now being replaced with the expanding evergreen shrub, Morella cerifera. Loss of biomass and basal area has been documented in other low elevation coastal forests. Our results indicate that an intermediate shrub state may precede complete loss of woody communities in some coastal communities, providing an alternative mechanism of resilience. 

   more » « less