Sara Hotchkiss

Background/Question/Methods Understanding the cumulative impacts of multiple stressors on north-temperate lake ecosystems is increasingly important as resource managers work to develop regional mitigation and adaptation plans. Our study investigated changes in phytoplankton communities in two northern Wisconsin lakes subjected to multiple stressors over a ten-year period that included both a four-year drought and a dramatic increase in rainbow smelt populations. We tested whether drought or rainbow smelt had the strongest relationship with resultant changes in phytoplankton communities, and whether community response differed depending on lake landscape position. Phytoplankton were collected fortnightly over a ten-year period (1984-1993), enumerated, and identified to species. We used ordination, MANOVA, and classification and regression trees to identify the response of phytoplankton communities to multiple stressors. Results/Conclusions Results showed that changes in phytoplankton...

Background/Question/Methods Ecological theory, models, and limited empirical data predict that forest communities will vary in their responsiveness to climate change, depending on factors such as site quality, disturbance regimes, and competitive interactions. Paleoecological records from a dense network of sites in northwestern Wisconsin documenting changes in plant communities and fire regimes during the past 3000 years allowed us to test predictions about factors that stabilize vegetation during periods of centennial-scale climate change. In particular, we tested whether on poor quality sites jack pine (P. banksiana) communities would be more stable because of strong feedbacks between fire and jack-pine and lack of competition from other tree species. At sites on finer-grained sand we expected vegetation would change more because changes in climate (and possibly fire regimes) would alter the outcome of competitive interactions among species and provide the opportunity for a new c...

Background/Question/Methods To understand how the dynamics of variation in plant communities have responded to past climate changes paleoecologists must devise ways to translate changes in pollen assemblages to changes in plant communities. Fossil pollen data can be used to interpret vegetation in both typological and continuous ways. A typological interpretation of pollen records can be used to test models predicting changes in transition probabilities. We developed a method for vegetation interpretations of pollen assemblages for northern Wisconsin from a library of 77 modern and 65 pre-European analogs. Each potential analog was matched with vegetation data: LANDSAT classifications for the modern vegetation, and General Land Office (GLO) Survey data for the pre-European settlement period. Species relative abundance (modern) or relative basal area (pre-European) were summarized within 5km of each site. Cluster analysis was used to classify vegetation types. A decision-analysis fra...

Background/Question/Methods The vegetation of the northwest Wisconsin sand plain is dominated by oak and pine communities maintained by periodic forest fires. Regional climate records suggest extreme, spatially extensive droughts in the western Great Lakes Region before ~3300, 1900-1600, and 1100 to 600 cal yr BP. The response of vegetation and fire regimes to these events is poorly understood and has management implications for responses to future climate change. We used pollen and charcoal in cores from three small, deep lakes located < 20 km from one another to test the hypothesis that forests on different textures of sandy soil would respond differently to climatic changes. Hell Hole (HH) and Lily (LY) lakes are in areas with soils of relatively fine sand; soils around Lone Star (LS) are coarser sand. We measured the degree of change of vegetation with ordinations and squared chord distance (SCD) between each fossil sample and the sample that corresponds to pre-European settl...

Background/Question/Methods A former fishering village of Nu‘alolo Kai, located on the Na Pali Coast on the north shore of Kaua‘i, was occupied for at least 500 years between AD 1300 and 1900. It preserves one of the richest assemblages of organic materials on the island of Kaua‘i. We examined fragments of wood charcoal, recovered from eleven stratigraphic layers of the habitation feature K3; they correspond to prehistoric (AD 1410–1570), late prehistoric (AD 1570–1800) and historic (>AD 1800) periods respectively. Our objectives were to learn about environmental changes associated with early settlement and about past ethnobotanical uses of woody plants. In particular, we were interested in the role of native Hawaiians in the modification of vegetation and landscape. The archaeological charcoal was also compared with observations of recent flora, to examine the persistency of native species as opposed to invasions of introduced taxa until modern times. Results/Conclusions Thirty-...

Background/Question/Methods Significant issues facing land managers of natural areas include landscape fragmentation and its influence on effective preserve size. This study quantifies the influence of adjacent land use on a wilderness area, the Sylvania Wilderness (Sylvania), an old-growth hemlock-hardwood temperate forest in Michigan’s Upper Peninsula. Old-growth forests are rare, discontinuous, unlogged stands that are significant reservoirs of biodiversity. Thus, changes along old-growth borders can negatively impact stand development, habitat, and native species. This research estimates the magnitude and pacing of edge effects on the Sylvania border as a result of changes in adjacent land use. To quantify edge effects, I conducted forest surveys in transects crossing the Sylvania border. Collected data include vegetation variables (diversity, canopy height, importance, density, and cover), stand structure (bare ground, coarse woody debris, graminoids, windfalls, and snags), and...

Landscape-scale vulnerability assessment from multiple sources, including paleoecological site histories, can inform climate change adaptation. We used an array of lake sediment pollen and charcoal records to determine how soils and landscape factors influenced the variability of forest composition change over the past 2000 years. The forests in this study are located in northwestern Wisconsin on a sandy glacial outwash plain. Soils and local climate vary across the study area. We used the Natural Resource Conservation Service's Soil Survey Geographic soil database and published fire histories to characterize differences in soils and fire history around each lake site. Individual site histories differed in two metrics of past vegetation dynamics: the extent to which white pine (Pinus strobus) increased during the Little Ice Age (LIA) climate period and the volatility in the rate of change between samples at 50-120 yr intervals. Greater increases of white pine during the LIA occurred on sites with less sandy soils (R² = 0.45, P < 0.0163) and on sites with relatively warmer and drier local climate (R² = 0.55, P < 0.0056). Volatility in the rate of change between samples was positively associated with LIA fire frequency (R² = 0.41, P < 0.0256). Over multi-decadal to centennial timescales, forest compositional change and rate-of-change volatility were associated with higher fire frequency. Over longer (multi-centennial) time frames, forest composition change, especially increased white pine, shifted most in sites with more soil moisture. Our results show that responsiveness of forest composition to climate change was influenced by soils, local climate, and fire. The anticipated climatic changes in the next century will not produce the same community dynamics on the same soil types as in the past, but understanding past dynamics and relationships can help us assess how novel factors and combinations of factors in the future may influence various site types. Our results support climate change adaptation efforts to monitor and conserve the landscape's full range of geophysical features.

The goal of this study was to determine the effects of atmospheric demand on both plant water relations and daily whole-tree water balance across the upper limit of a cloud forest at the mean base height of the trade wind inversion in the tropical trade wind belt. We measured the microclimate and water relations (sap flow, water potential, stomatal conductance, pressure-volume relations) of Metrosideros polymorpha Gaudich. var. polymorpha in three habitats bracketing the cloud forest's upper limit in Hawai'i to understand the role of water relations in determining ecotone position. The subalpine shrubland site, located 100 m above the cloud forest boundary, had the highest vapor pressure deficit, the least amount of rainfall and the highest levels of nighttime transpiration (EN) of all three sites. In the shrubland site, on average, 29% of daily whole-tree transpiration occurred at night, while on the driest day of the study 50% of total daily transpiration occurred at night...