Most forests are recovering from human land use, making it critical to understand the effect of disturbance on forest recovery. Forests of the eastern United States have a long history of land use, ...but it is unknown whether historical disturbances have contributed to their transition from ectomycorrhizal (ECM) to arbuscular mycorrhizal (AM) tree dominance. Disturbance may promote nitrogen (N)‐fixing trees in early succession, which can elevate soil N availability even after they die. Higher soil N availability may facilitate the competitive success of AM trees over ECM trees, but such ‘N fixer founder effects’ have not been empirically tested.
Here, we analysed data from three land‐use disturbances in a temperate forest historically dominated by ECM trees: selective‐cutting (ranging from 0 to 52 m2 ha−1), clear‐cutting and agricultural abandonment. These disturbances occurred at different times, but long‐term data capture 3–7 decades of forest recovery.
We found that the AM tree fraction in contemporary forests was 2, 4, and 6‐fold higher following selective‐cutting, clear‐cutting and agricultural abandonment, respectively, compared to forest composition in 1934. Across these disturbances we also observed an increasing abundance of the N fixer black locust immediately following disturbance. Using a simulation model parameterized by data from black locust, we estimated historical rates of symbiotic N fixation to understand the relationship between N fixation and AM dominance in individual plots. We found that N fixation was positively associated with the growth of ECM trees generally, and oak and hickory specifically, only following light selective‐cutting (<12 or <18 m2 ha−1 basal area extraction, respectively). Following higher levels of selective‐cutting and clear‐cutting, N fixation was positively associated with the growth of AM trees, particularly red maple and tulip poplar. Agricultural abandonment led to AM dominance regardless of N fixation rates.
Synthesis and applications. Our findings suggest that common land use practices and black locust, a native N fixer, can reduce the dominance of ECM trees. If N fixers are likely to proliferate following disturbance, we might maintain ECM dominance by cutting trees at low densities and by applying prescribed fire to remove N.
Our findings suggest that common land use practices and black locust, a native N fixer, can reduce the dominance of ECM trees. If N fixers are likely to proliferate following disturbance, we might maintain ECM dominance by cutting trees at low densities and by applying prescribed fire to remove N.
Carbon uptake by the terrestrial biosphere depends on supplies of new nitrogen (N) from symbiotic N fixation, but we lack a framework for scaling fixation accurately and for predicting its response ...to global change.
We scaled symbiotic N fixation from individual N fixers (i.e. plants that host N‐fixing bacteria), by quantifying three key parameters—the abundance of N fixers, whether they are fixing N and their N fixation rates. We apply this framework to black locust, a widespread N‐fixing tree in temperate forests of the eastern United States, and harness long‐term data from southern Appalachian forests to scale fixation from trees to the landscape and over succession.
Symbiotic N fixation at the landscape scale peaked in the first decade following forest disturbance, and then declined. This pattern was due to the declining density and declining fixation rates of individual black locust trees over succession. Independent of forest succession, and coincident with chronic atmospheric N deposition, we have evidence suggesting that nodule biomass produced by black locust trees has declined by 83% over the last three decades. This difference in nodule biomass translates to a maximum fixation rate of 11 kg N ha−1 year−1 and a landscape average of 1.5 kg N ha−1 year−1 in contemporary forests.
Synthesis. We find key controls on symbiotic N fixation by black locust over space and time, suggesting lower fixation rates in eastern deciduous forests than previous estimates. Our scaling framework can be applied to other N fixers to aid predictions of symbiotic N fixation and ecosystem response to global change.
We present a framework for capturing the dynamics of symbiotic N fixation at multiple scales. Our framework offers insight into how fixation is regulated, which is necessary for accurate predictions of ecosystem response to global change.
General circulation models project more intense and frequent droughts over the next century, but many questions remain about how terrestrial ecosystems will respond. Of particular importance, is to ...understand how drought will alter the species composition of regenerating temperate forests wherein symbiotic dinitrogen (N₂)-fixing plants play a critical role. In experimental mesocosms we manipulated soil moisture to study the effect of drought on the physiology, growth and competitive interactions of four co-occurring North American tree species, one of which (Robinia pseudoacacia) is a symbiotic N₂-fixer. We hypothesized that drought would reduce growth by decreasing stomatal conductance, hydraulic conductance and increasing the water use efficiency of species with larger diameter xylem vessel elements (Quercus rubra, R. pseudoacacia) relative to those with smaller elements (Acer rubrum and Liriodendron tulipifera). We further hypothesized that N₂ fixation by R. pseudoacacia would decline with drought, reducing its competitive ability. Under drought, growth declined across all species; but, growth and physiological responses did not correspond to species’ hydraulic architecture. Drought triggered an 80 % increase in nodule biomass and N accrual for R. pseudoacacia, improving its growth relative to other species. These results suggest that drought intensified soil N deficiency and that R. pseudoacacia’s ability to fix N₂ facilitated competition with non-fixing species when both water and N were limiting. Under scenarios of moderate drought, N₂ fixation may alleviate the N constraints resulting from low soil moisture and improve competitive ability of N₂-fixing species, and as a result, supply more new N to the ecosystem.
Linking quickflow response to subsurface state can improve our understanding of runoff processes that drive emergent catchment behaviour. We investigated the formation of non‐linear quickflows in ...three forested headwater catchments and also explored unsaturated and saturated storage dynamics, and likely runoff generation mechanisms that contributed to threshold formation. Our analyses focused on two reference watersheds at the Coweeta Hydrologic Laboratory (CHL) in western North Carolina, USA, and one reference watershed at the Susquehanna Shale Hills Critical Zone Observatory (SHW) in Central Pennsylvania, USA, with available hourly soil moisture, groundwater, streamflow, and precipitation time series over several years. Our study objectives were to characterise (a) non‐linear runoff response as a function of storm characteristics and antecedent conditions, (b) the critical levels of shallow unsaturated and saturated storage that lead to hourly flow response, and (c) runoff mechanisms contributing to rapidly increasing quickflow using measurements of soil moisture and groundwater. We found that maximum hourly rainfall did not significantly contribute to quickflow production in our sites, in contrast to prior studies, due to highly conductive forest soils. Soil moisture and groundwater dynamics measured in hydrologically representative areas of the hillslope showed that variable subsurface states could contribute to non‐linear runoff behaviour. Quickflow generation in watersheds at CHL were dominated by both saturated and unsaturated pathways, but the relative contributions of each pathway varied between catchments. In contrast, quickflow was almost entirely related to groundwater fluctuations at SHW. We showed that co‐located measurements of soil moisture and groundwater supplement threshold analyses providing stronger prediction and understanding of quickflow generation and indicate dominant runoff processes.
Non‐linear quickflow response formed a threshold with respect to the sum of gross precipitation and antecedent soil moisture but not rainfall intensity in three forested, headwater catchments. For storm events above the threshold, the relative response of soil moisture and groundwater was coupled in two catchments and decoupled in one catchment. High saturated hydraulic conductivities in undisturbed, forested soils allowed for rapid infiltration and transmission to the stream via preferential and matric flow or recharge into the groundwater, but specific runoff mechanisms were unique to each catchment.
•Evergreen shrub layer was removed to restore forest structure and composition.•Increased soil moisture and greater variation in light transmission were observed.•Water use of larger trees in the ...treatment plots increased in the drier year.•Total stand transpiration was similar in the reference and treatment plots.
Forest disturbance to overstory vegetation can result in increased abundance of understory vegetation, potentially affecting various ecosystem processes. Habitat expansion of a native evergreen woody shrub, Rhododendron maximum (rosebay rhododendron), in southern Appalachian Mountain riparian forests of the eastern United States has occurred following the mortality of Castanea dentata (American chesnut) in the early 20th century and more recently Tsuga canadensis (Eastern hemlock). Increased abundance of R. maximum has been associated with reduced tree seedling recruitment and survival, as well as shorter overstory tree height compared to where R. maximum understory was not present. As part of a larger investigation into the removal of R. maximum to restore vegetation structure and composition, this study examined the effects of removing R. maximum understory (by cutting) and soil O-horizon (by prescribed fire) on microclimate, whole tree and plot-level transpiration during 2016 and 2017 growing seasons. We found increased soil moisture in the wetter year (2017) and greater variation in light transmission to the forest floor after the treatment. Larger trees were able to increase their water use in the drier year (2016), resulting in similar plot-level transpiration of treatment and reference plots. Removal of R. maximum understory is not expected to significantly alter streamflow or stream chemistry during the growing season; however, it can help restore forest structure through improving tree seedling survival and recruitment as well as increasing growth of established trees.
Abstract
Forests provide the most stable and highest quality water supplies among all land uses. Quantitatively evaluating the benefits of forest water supply functions is important to effectively ...mitigate the impacts of land development, climate change, and population growth. Here, by integrating a water balance model and national drinking water data, we determined the amount of surface water yield originating on different forest ownership types at a fine resolution (88,000 watersheds) and tracked that water through the river network to drinking water intakes and the populations they serve. We found that forested lands comprised 36% of the total land area but contributed 50% of the total surface water yield. Of the 23,983 public surface drinking water intakes depending on surface water sources, 89% (serving around 150 million people) received some (>0.01%) surface water from forested lands, and 38% (serving about 60 million people) received more than 50% of their surface drinking water supply from forested lands. Privately-owned forests were the most important water source in the eastern U.S., benefiting 16 million people, followed by federal forests (14.4% of the total water supply). In contrast, federally-owned forested lands were the dominant water source (52% of the total water supply) in the West. Privately-owned forests are the most vulnerable to future land use change and associated water supply impacts. Continuing programs that support private forest landowners with financial and technical assistance through federal and state forest management agencies and potentially developing payment for ecosystem service schemes could maximize benefits for landowners so they may retain their land assets while minimizing forest loss and associated impacts on critical ecosystem services including the provisioning a clean and reliable water supply for the American public.
Tolerance or avoidance Minucci, Jeffrey M.; Miniat, Chelcy Ford; Teskey, Robert O. ...
The New phytologist,
07/2017, Volume:
215, Issue:
1
Journal Article
Peer reviewed
Climate change is increasing drought frequency, which may affect symbiotic N2 fixation (SNF), a process that facilitates ecosystem recovery from disturbance. Here, we assessed the effect of drought ...frequency on the ecophysiology and SNF rate of a common N2-fixing tree in eastern US forests.
We grew Robinia pseudoacacia seedlings under the same mean soil moisture, but with different drought frequency caused by wet–dry cycles of varying periodicity.
We found no effect of drought frequency on final biomass or mean SNF rate. However, seedlings responded differently to wet and dry phases depending on drought frequency. Under low-frequency droughts, plants fixed carbon (C) and nitrogen (N) at similar rates during wet and dry phases. Conversely, under high-frequency droughts, plants fixed C and N at low rates during dry phases and at high rates during wet phases.
Our findings suggest that R. pseudoacacia growth is resistant to increased drought frequency because it employs two strategies – drought tolerance or drought avoidance, followed by compensation. SNF may play a role in both by supplying N to leaf tissues for acclimation and by facilitating compensatory growth following drought. Our findings point to SNF as a mechanism for plants and ecosystems to cope with drought.
Clean water from forests is commonly used to supply drinking water to communities both within and outside basin boundaries through inter‐basin transfers (IBTs). Here, we modified the Water Supply ...Stress Index (WaSSI) model to provide estimates of mean water yield and the proportion of mean flow originating on forested lands at the 12‐Digit Hydrologic Unit Code scale across the conterminous United States (CONUS). We accounted for the benefits of forests for drinking water supply and receiving populations through IBTs by incorporating a new IBT database, surface intake location information for public drinking water systems, and modeled water yield from forests. We compiled the new database of 594 IBTs ranging from 0.01 million m3 yr−1 to 8,900 million m3 yr−1, for a total transferred volume of 116,894 million m3 yr−1. According to our results, forested lands comprised 28.7% of the total land area across CONUS, but contributed 46% of the total surface water yield. Approximately 125.5 million people derived more than 10% of their surface drinking water supply from forested lands, and 83.1 million people received more than 50% of their surface drinking water supply from forested lands. Of those 83.1 million people receiving more than 50% of their surface drinking water supply from forested lands, 19.4 million people obtained some (≥0.01%) of that water through IBTs. We conclude that accounting for IBTs is critical to accurately assess the contribution of forested watersheds for surface drinking water supply. Hydrologic models for assessment and decision making must include IBTs to fully account for the effects of climate change and human population dynamics on water resource availability at watershed to regional scales. Results from this study can aid water resource and forest managers in developing integrated watershed management plans at a time when climate change, population growth, and land use change threaten water supplies.
Key Points
National Forest System and other forested lands comprised 28.7% of the total land area but contributed 46.0% of the total water yield
Approximately 125.5 million and 83.1 million people received more than 10% and 50% of their surface drinking water supply from forested lands, respectively
Without considering inter‐basin transfer, benefits of forests to water supply would be underestimated
This open access book describes the serious threat of invasive species to native ecosystems. Invasive species have caused and will continue to cause enormous ecological and economic damage with ever ...increasing world trade. This multi-disciplinary book, written by over 100 national experts, presents the latest research on a wide range of natural science and social science fields that explore the ecology, impacts, and practical tools for management of invasive species. It covers species of all taxonomic groups from insects and pathogens, to plants, vertebrates, and aquatic organisms that impact a diversity of habitats in forests, rangelands and grasslands of the United States. It is well-illustrated, provides summaries of the most important invasive species and issues impacting all regions of the country, and includes a comprehensive primary reference list for each topic. This scientific synthesis provides the cultural, economic, scientific and social context for addressing environmental challenges posed by invasive species and will be a valuable resource for scholars, policy makers, natural resource managers and practitioners.
The Coweeta Hydrologic Laboratory (CHL) is a USDA Forest Service (FS) Experimental Forest, located in western North Carolina, in the southern Appalachian Mountains. Established in 1934, CHL has ...long‐term data records that include climate, streamflow, stream and atmospheric chemistry, and vegetation in several small, experimentally‐manipulated and reference watersheds. In addition to these long‐term data, additional data associated with specific projects have been collected and are available through publications and electronic archives. Notably, CHL was a member of the National Science Foundation‐funded Long‐Term Ecological Research (LTER) program from 1980–2020, which resulted in significant scientific advances and rich data sets on the five core LTER research areas: primary productivity, population studies, movement of organic matter, movement of inorganic matter, and disturbance patterns. Here we provide a brief site description and history of the CHL, including descriptions of gauged watersheds and data archives.
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