Browsing by Subject "Ecotone"
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Item Climate and interrelated tree regeneration drivers in mixed temperate–boreal forests(Springer, 2013) Fisichelli, Nicholas A; Frelich, Lee E; Reich, Peter BForest compositional shifts in response to climate change are likely to be initially detectable in the understory tree regeneration layer near species range limits. Because many factors in addition to climate, such as seedbed and soil characteristics, overstory composition, and interactions with other understory biota, drive tree regeneration trends, a thorough understanding of the relative importance of all variables as well as their interrelationships is needed. The range limits of several widespread temperate and boreal tree species overlap in the upper Great Lakes region, USA, thus facilitating an observational study over relatively short regional climate gradients. We used redundancy analysis and variation partitioning to quantify the unique, shared, and total explanatory power of four sets of explanatory variables. The results showed that all four variable sets (climate 9.5 %, understory environment 13.7 %, overstory composition 26.3 %, and understory biota 13.8 %) were significantly associated with tree regeneration compositional variation in mixed temperate–boreal forests. Partitioning also revealed high confounded or shared explanatory power, but also that each set contributed significant unique explanatory power not shared with other sets. Spatial patterning in regeneration composition was strongly related to broad scale environmental patterns, while the large majority of unexplained variation did not have a detectable spatial structure, suggesting factors with local scale variability. Future forest shifts across the landscape will depend not only on the rate and direction of climate change but also on how the strengths and interrelationships among other explanatory variables, such as overstory composition and understory biota, shift with a changing climate.Item Climate Change and Future Forests of the Boundary Waters Canoe Area Wilderness: An assessment of Temperate Tree Abundance, Earthworm Invasion and Understory Regeneration Trends(2019-07) Chaffin, DavidThe forests of the Boundary Waters Canoe Area Wilderness (BWCAW) are dominated by boreal tree species at their southern range limit, making them particularly sensitive to climate change. Numerous studies have modeled potential climate change induced tree species range shifts across northern Minnesota and the BWCAW, projecting declines for boreal tree species and increases for northern temperate tree species currently at low abundances in the wilderness. The coarse resolution of these models, their lack of some biological interactions and the unprecedented velocity of projected future climate change could lead to over and/or under estimates of tree species range shifts at the scale of the BWCAW. To better understand potential forest successional shifts within this ecologically important wilderness we assessed 1) the abundance and spatial distribution of northern temperate tree species, 2) the stage, pattern, and extent of earthworm invasion in the wilderness, and 3) the impact of seasonal temperature, overstory composition, earthworm invasion and soil moisture potential on the understory relative density of ten boreal and northern temperate tree species. We found that red maple (Acer rubrum) is the most abundant temperate tree species in the BWCAW by orders of magnitude across all size classes, with its spatial distribution strongly correlated with an existing west to east summer temperature gradient. Conversely, our results raise questions about whether adequate seed sources of other temperate species are currently present in the wilderness to support a successful boreal to temperate transition. Earthworm invasion in the BWCAW is widespread, yet incomplete. Modeling results indicate the spatial pattern of earthworm invasion in the wilderness is driven by anglers dumping unused earthworm fishing bait at campsites. We predict that 33% of total land area in the BWCAW is currently invaded by earthworms. Lastly, our results indicate that summer temperature is a key driver of differences in understory relative density of boreal and northern temperate tree species across the mixed-boreal forests of the BWCAW, but that earthworm invasion and areas with increased soil moisture availability may support the short-term resistance of boreal tree species under future warmer and drier conditions.Item Climatic and Vegetative Drivers of Soil Genesis, Soil Properties, and Soil Distribution in Sankuru Province, Central Democratic Republic of Congo (DRC)(2021-08) Lohese, EsakakondoAbstractMuch of what is known regarding soil properties in the Democratic Republic of Congo (DRC) is based on broad estimates from studies covering only 15% of the vast country’s territory, about a 1⁄4 of the USA. Understanding how soil types and soil properties vary across the landscape and change with major vegetation types across climatic zones is a critical foundation that must be established in order to ensure the lasting success of future effort to improve land use, soil health, and food security. In this study, soil investigations were undertaken on a transect across the forest-savanna ecotone in Sankuru province, central DRC. Soils across the transect were generally sandy, acidic, kaolinitic, and low carbon soils, but significant differences in soil properties and soil classification existed between climate zones and vegetation types. Soils formed under savanna vegetation were characterized by thick, dark colored topsoils (umbric epipedons) and classified as Inceptisols regardless of climate zone, while soils formed under forest vegetation had thin, light-colored topsoils (ochric epipedons) and were more diverse, classifying as Entisols, Inceptisols, Ultisols, and Oxisols. Analysis of soil magnetic properties and the 13C ratio of soil organic matter revealed that while climatic differences are recorded in iron oxide assemblages, it is likely that forest was much more expansive in the mid-late Holocene and covered modern-day savanna sites across the transect. Finally, soil phosphorus stocks vary significantly across the transect and is highest under savanna vegetation in wetter climate zones. These phosphorus (P) stocks exist largely in association with iron and aluminum oxy-hydroxides, but are non-occluded, which may allow P to become available to crops under innovative agronomic management. These results suggest that by focusing crop production in savanna areas in the Am and Af climate zones, and applying locally available amendments that may boost soil pH and CEC could increase crop productivity and food security in central DRC.Item Tree regeneration dynamics and drivers across the temperate-boreal transition zone.(2012-03) Fisichelli, Nicholas AnthonyThe upper Great Lakes region in central North America contains a forest transition zone where temperate and boreal tree species reach their northern and southern range limits, respectively. It is only within this narrow latitudinal band (~3 degrees), that relatively warm-adapted temperate and cold-adapted boreal tree species are found growing together in upland mesic sites. If climate is a main driver of forest dynamics within this region, recent and predicted climate change should result in major forest shifts, including the expansion of temperate species and range contraction of boreal species. Such changes should first be manifest in growth and abundance trends of tree species in the understory regeneration layers. In addition to climate, numerous other factors such as overstory composition, understory abiotic environment, competition with shrub and herbaceous layers, and browse pressure drive tree regeneration trends. Interrelationships and interactions among these drivers will ultimately determine the direction and rate of forest change. We explored these research topics through field studies of naturally established seedlings and saplings at 124 upland mesic forest sites across a three state (Minnesota, Wisconsin, and Michigan U.S.A) 170,000 km2 area of the temperate-boreal transition zone. Chapter 1 examined relative abundance shifts of temperate and boreal tree regeneration at two spatial scales: local ecotonal boundaries between temperate and boreal dominated stands and across the regional temperate-boreal transition zone. Because we compared understory performance across locally changing overstory composition, we calculated species regeneration success as the difference in relative abundance between the understory and overstory layers. At the local scale, both shade-tolerant temperate and boreal species exhibited positive tree regeneration success across ecotonal boundaries. However, across the region, regeneration performance varied with mean summer temperature and to a lesser extent mean annual precipitation. Changes in regeneration success were generally greatest at the warm end of the transition zone, with temperate broadleaf Acer saccharum, Fraxinus nigra, and Ostrya virginiana responding positively and boreal Abies balsamea showing significantly reduced performance. For the most frequent temperate species, Acer rubrum, regeneration success was greatest in boreal neighborhoods and at cool and dry sites. Other species did not exhibit detectable shifts in regeneration success, potentially due to traits such as shade-tolerance, palatability, and mode of reproduction. Overall we found that numerous tree species growing across the temperate-boreal transition zone are likely sensitive to climate at early stages of development, with observed shifts in regeneration success concomitant with the direction predicted in response to climate change. Chapter 2 assessed the relative importance of and interrelationships among explanatory variable sets in explaining the composition of the tree regeneration layer. We used redundancy analysis (RDA) and variation partitioning to quantify the unique, shared, and total explanatory power of four sets of drivers: climate, understory abiotic environment, overstory composition, and understory biota. The results showed that all four driver sets individually explained a significant portion of tree regeneration compositional variation and additionally that there were strong relationships among explanatory variables. Overstory composition, which directly influences seed availability and also was found to be closely associated with understory environmental conditions and biota, had approximately twice the explanatory power of any of the other three driver sets. Some of the strongest individual drivers were overstory Acer saccharum and Populus tremuloides, soil pH, mean summer temperature, and mean annual precipitation. Suites of associated drivers included cool, moist, sandy, and acidic conditions; overstory boreal broadleaf species, light availability, shrub abundance, and forb cover; and warm temperatures and graminoid cover. Due to the strong interrelationships among drivers, the direction and rate of forest change will likely depend on how the importance of drivers shifts with climate and, for the biotic drivers, on the rate and magnitude of their own responses to climate change. Chapter 3 investigated sapling height and radial growth rates of five temperate and boreal species. This study included over 1700 stems of naturally established, competing saplings growing at 14 sites across the temperate-boreal transition zone. Top performing linear mixed-effects models typically included two-way interactions among mean summer temperature, browse pressure, understory light levels, and initial sapling size. As hypothesized, temperate sapling growth increased and boreal growth decreased with increasing temperatures. However, the relative performance of competing species shifted depending on the level of browse pressure. Positive temperate growth responses to temperature were eliminated by heavy browse pressure, tilting growth rates in favor of less palatable boreal conifers at all but the warmest sites. Spatial variations in browse pressure levels across the region suggest that temperate expansion may proceed most rapidly in areas where browsing is least intense. Growth responses to temperature also varied with sapling size and, for the least shade-tolerant species in the study, Quercus rubra, light availability. Enhanced growth by temperate species in response to warmer temperatures was most detectable under favorable conditions including low browse pressure and high understory light availability, suggesting that any efforts to facilitate forest compositional changes will need to take into account these trends.