Agriculture and development have made the temperate forests the most heavily fragmented forest biome in the world. Landscapes with fragmented forests are typically characterized by abrupt transitions between forest and non-forest land covers (e.g., meadows, agricultural land, development). These abrupt transitions between forest and non-forest land covers induce large gradients in environmental conditions (e.g., light, temperature, humidity, and soil moisture) between the forest edge and interior. For example, forest edges are typically hotter, drier, and have greater exposure to wind and light than the forest interior (Smith et al. 2018). Gradients in environmental conditions across the non-forest-to-forest interior ecotone can be large and create gradients in carbon and water cycling and forest structure (e.g., above and belowground biomass, rooting behavior, and canopy architecture) (Reinmann and Hutyra 2017; Harper et al. 2005; Herbst et al. 2007). At the same time, forest edge effects can exacerbate the negative impacts of climate stress (e.g., excessive heat, drought) on ecosystem processes (Reinmann et al. 2020; Reinmann and Hutyra 2017). This summer, we will use the Climate Interactions with Forest Fragmentation (CLIFF) experiment–a new forest fragmentation x precipitation manipulation experiment–as a model system to study how carbon and water cycling, tree growth, leaf ecophysiology, and rooting behavior respond to changes in important resources and stressors related to temperature and availability of light and water. Through this new experiment we are also manipulating precipitation to better understand how edge effects, water availability, and air temperatures all interact to mediate ecosystem processes. This project is part of the larger 'Fragmentation & Climate Change' group, which will meet weekly for discussions and collaborations
Subproject 1
Living on the edge: Tree growth in fragmented forests
Forest fragmentation creates abrupt transitions (i.e., forest edges) which induce large gradients in key environmental drivers of tree growth (e.g., light, temperature, and soil moisture) between the forest edge and interior. However, we are only beginning to learn how these changes in environmental conditions alter tree architecture and wood production dynamics. The higher light conditions at the edge likely trigger a cascade of ecophysiological responses that, over time, result in a vegetative “walling off” of the forest edge. During this walling off period, tree canopy architecture likely becomes more complex from epicormic branching–lateral growth of tree limbs–and a proliferation of new leaves taking advantage of higher light conditions at the edge. As the walling off process matures, the influence of excess light and elevated temperatures on the forest interior likely subsides. We know that trees along mature forest edges can grow twice as fast or faster than trees in the interior, but these trees are also more adversely impacted by heat stress. This REU project will use measurements at CLIFF to address three key remaining areas of uncertainty. First, how do tree growth dynamics evolve over time as forest canopy structure changes following the creation a new forest edge? Second, how does water availability modulate the magnitude of heat sensitivity of trees near a forest edge versus the forest interior? Third, does the response of tree growth respond before or after changes in leaf area (e.g., walling off)? Field work will center around using dendrometer bands and automated point dendrometers to quantify the magnitude and temporal patterns of tree stem wood production along transects between the forest edge and interior. Additional measurements will include quantifying leaf area index and possibly tree sap flow. A typical week will entail: field work to measure dendrometer bands and download data from point dendrometers and microenvironment sensors, compile/review literature on forest edge effects, curating and analyzing datasets, and preparation for a final presentation. We expect this project will fill important knowledge gaps on environmental controls tree growth that support broader research goals of linking tree ecophysiology to ecosystem carbon sequestration across human-altered landscapes.
Subproject 2
Thirsty roots in wide-open spaces: investigating the impact of forest fragmentation on root system dynamics
The large environmental gradients present between forest edges and interior forests allows us opportunities to investigate the impacts that forest fragmentation has on the belowground carbon allocation of forest trees. Forest trees at the edge of new forest fragments are introduced to a suite of new environmental conditions that could alter their overall growth and resource acquisition strategies. More is known about the aboveground response of trees to new edge conditions, such as increased heat stress, along with an increased abundance of lateral branching and leaf biomass to take advantage of the newly abundant light available at the edge, creating the “walling off effect”. However, we know little about the changes that occur belowground at forest edges. Plant root systems are the primary organ responsible for nutrient and water acquisition, and forest edges greatly impact belowground dynamics. For example, new edges lead to reduced competition as there is open space available for trees at the edge to now occupy. Additionally, the decomposition of necromass from plants previously occupying the new open areas at the edge along with the additional thermal energy in open areas that could speed-up microbial activity may lead to an increase in nutrient availability. Therefore, we seek to understand how belowground resources, water and nutrients, are altered along the transition from open-edge-interior forests, and how these dynamics alter the plant root systems of dominant trees in this altered environment. At the CLIFF experiment, we will collect a transect of soil cores from the clearing adjacent to the forest edge and through to the interior forest to measure the vertical distribution of root biomass for dominant tree species. Additionally, we plan to measure fine root traits to see how root morphology may differ across these environmental gradients. We will compare the root distribution and belowground dynamics to soil and environmental characteristics to see what drives the patterns across the treatments. The project will begin by taking a transect of soil cores and installing root ingrowth cores in the same locations during the first few weeks. Following this, the majority of the laboratory analyses will be conducted to measure the root and soil traits during the summer. We expect this project will fill important knowledge gaps on environmental controls belowground tree growth that support broader research goals of linking tree ecophysiology to ecosystem carbon sequestration across human-altered landscapes.
Subproject 3
Impacts of forest fragmentation and climate on CO2 and CH4 fluxes from trees and soils
Forest fluxes of CO2 and CH4 are largely driven by biological metabolic processes of roots and microbes living in the soil and tree stems. These metabolic processes are heavily mediated by microenvironmental conditions such as temperature, soil moisture, and substrate availability, all of which are altered by forest edge effects. Prevailing climate conditions can also influence biological fluxes of CO2 and CH4. Recent research has indicated that forest edge effects can stimulate rates of soil respiration in rural landscapes but suppress soil respiration in urban landscapes (Smith et al. 2019; Garvey et al. 2022). In addition, there is growing evidence that tree stems can be an important source of CH4 in forest ecosystems, likely by venting CH4 produced in the soil and through local production from anaerobic decomposition of wood inside the tree stem (Barba et al., 2019; Covey & Megonigal, 2019). In this subproject, we will conduct weekly or bi-weekly gas flux measurements along transects from the cleared area adjacent to the forest to the forest edge and through the forest interior across precipitation treatments at CLIFF to better understand the environmental controls and influences of forest fragmentation on forest CO2 and CH4. Through measurements of soils with and without roots, we will also quantify the separate contributions of roots + rhizosphere and heterotrophs (i.e., microbes) to soil gaseous carbon fluxes. Through measurements of stem gas efflux, we will quantify the role of tree stems as a pathway for the release of gases into the atmosphere. A typical week will entail: field work to measure CO2 and CH4 fluxes from PVC collars we have placed in the soil and on tree stems, download data from microenvironment sensors, compile/review literature on forest edges and forest carbon fluxes, curating and analyzing datasets, and preparation for a final presentation. We expect this project will fill important knowledge gaps on environmental controls on carbon fluxes in temperate forests that support broader research goals of linking above and belowground processes to ecosystem carbon sequestration across human-altered landscapes.
Subproject 4
Response of tree leaf ecophysiological process to forest edge effects and climate
Forest fragmentation creates abrupt transitions (i.e., forest edges) which induce large gradients in key environmental drivers of tree ecophysiology (e.g., light, temperature, and soil moisture) between the forest edge and interior. However, we are only beginning to learn how these changes in environmental conditions alter tree carbon and water exchange with the atmosphere. We know that trees along a forest edge can grow more than twice as fast as trees in the forest interior (Reinmann and Hutyra 2017) and there is evidence that trees near a forest edge also use more water (Herbst et al. 2007), which in turn accelerates soil drying during the growing season (Reinmann et al. 2020). Our previous work has also demonstrated that while conditions near a forest edge can greatly enhance rates of tree productivity, they can also make trees more negatively impacted by climate stressors such as excessive heat (Reinmann and Hutyra 2017). Despite these advances in understanding of how forest edge effects alter tree productivity and water use, we know very little about the underlying ecophysiological mechanisms. In this subproject we will conduct leaf-level measurements of gas exchange on canopy leaves from trees along forest edge-to-interior transects and across our precipitation treatments at CLIFF. In addition, we will conduct mini experiments on these leaves that will help us understand temperature sensitivities of gas exchange and to separate stomatal versus biochemical limitations to photosynthesis. This subproject may also incorporate measurements of tree sap flow (i.e., transpiration). A typical week may entail some combination of field work to measure leaf-level ecophysiological processes, maintenance and data downloads from sap flow sensors, compile/review literature on forest edges and tree ecophysiology, curating and analyzing datasets, and preparation for a final presentation. Collectively, we expect these measurements will provide data that advances our understanding of how forest edge effects and water availability alter tree ecophysiology modulators of ecosystem carbon and water exchange. We also expect this work to provide new insight into how water availability mediates the response of tree ecophysiological processes to heat stress.
General requirements for all overall project, regardless of sub-project:
1. Students can expect to spend ~50% of their time in the field. The ideal candidate has a positive attitude in group environments and is comfortable working under a range of conditions (e.g., hot and humid, cool and rainy, buggy, etc.)
2. An inquisitive nature and comfortable asking questions
3. Experience using R or the motivation to learn. This will be the primary software used for data analysis and visualization
4. Capable of walking 2+ miles (in a day) on and off trails to visit field sites
1. Reinmann AB and Hutyra LR. 2017. Edge effects enhance carbon uptake and its vulnerability to climate change in temperate broadleaf forests. Proceedings of the National Academy of Sciences 114(1): 107-112. DOI: 10.1073/pnas.1612369114
2. Smith et al. 2018. Piecing together the fragments: elucidating edge effects on forest carbon dynamics. Frontiers in Ecology and the Environment 16: 213-221.
3. Reinmann, A. B., Smith, I. A., Thompson, J. R., & Hutyra, L. R. (2020). Urbanization and fragmentation mediate temperate forest carbon cycle response to climate. Environmental Research Letters, 15, 114036–114036.
4. Mourelle, C., Kellman, M., & Kwon, L. (2001). Light occlusion at forest edges: An analysis of tree architectural characteristics. Forest Ecology and Management, 154(1–2), 179–192.
5. Herbst, M., Roberts, J. M., Rosier, P. T. W., Taylor, M. E., & Gowing, D. J. (2007). Edge effects and forest water use: A field study in a mixed deciduous woodland. Forest Ecology and Management, 250(3), 176–186. https://doi.org/10.1016/j.foreco.2007.05.013
6. Smith, I. A., Hutyra, L. R., Reinmann, A. B., & Thompson, J. R. (2019). Evidence for Edge Enhancements of Soil Respiration in Temperate Forests Geophysical Research Letters. Geophysical Research Letters, 46, 1–10. https://doi.org/10.1029/2019GL082459
7. Garvey SM, Templer PH, Pierce EA, Reinmann AB, and Hutyra LR. 2022. Diverging patterns at the forest edge: soil respiration dynamics of fragmented forests in urban and rural areas. Global Change Biology. 28:3094-3109. DOI: 10.1111/gcb.16099
8. Barba, J., Bradford, M. A., Brewer, P. E., Bruhn, D., Covey, K., Van Haren, J., Megonigal, J. P., Mikkelsen, T. N., Pangala, S. R., Pihlatie, M., Poulter, B., Rivas?Ubach, A., Schadt, C. W., Terazawa, K., Warner, D. L., Zhang, Z., & Vargas, R. (2019). Methane emissions from tree stems: A new frontier in the global carbon cycle. New Phytologist, 222(1), 18–28. https://doi.org/10.1111/nph.15582
9. Covey, K. R., & Megonigal, J. P. (2019). Methane production and emissions in trees and forests. New Phytologist, 222(1), 35–51. https://doi.org/10.1111/nph.15624