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Tree stems from wetland, floodplain and upland forests can emit CH4. This emerging field of research has revealed a high spatial and temporal variability on CH4 stem emissions between trees and species, and within and across ecosystems, which is not completely understood. Additionally, there is no consensus on the biophysical mechanisms that could support stem CH4 emissions, including the origin of these emissions. This hinders our understanding of spatial and temporal patterns and hamper the identification of biophysical drivers. Here, we summarize up to 30 opportunities and challenges on stem CH4 emissions research in order to improve estimates of magnitudes, patterns, drivers and trace the potential origin of CH4 emissions. We propose two main challenges: the need for long-term high frequency measurements of stem CH4 emissions, and the need for a mechanistic model including passive and active transport of CH4 from the soil-tree-atmosphere continuum. The first challenge would allow to constrain magnitudes and patterns of CH4 emissions at different temporal scales, and the second would require discovery and integration of pathways and mechanisms of CH4 production and emissions to be integrated into process-base models. Addressing these challenges might improve upscaling of CH4 emissions from trees to the ecosystem scale and the quantification of the role of stem CH4 emissions for the local-to-global CH4 budget.

It is well known that bird richness in the Amazon is greater in upland forests and that seasonally flooded forest is particularly species poor. However, the misleading pattern of greater bird richness in seasonally flooded forest has emerged seemingly unnoticed numerous times in richness maps in the literature. We hypothesize that commission errors in digital distribution maps (DDMs) are the cause behind the misleading richness pattern. In the Amazon, commission errors are a consequence of the different methodological treatment given to large-ranged versus small-ranged habitat specialists when mapping distributions. DDMs of 1007 Amazonian birds were examined, and maps that had commission errors were corrected. We generated two richness maps, one from the overlay of original DDMs and another from the overlay of the corrected ones. We identified 291 species whose distribution maps had errors. In the original data, seasonally flooded forests showed higher species richness than upland forest, but this pattern was reverted in the corrected richness map. Commission errors were 35 times more likely in the seasonally flooded forest. We conclude that DDMs accurately portray the distribution of single species in the Amazon. Commission errors in individual maps, however, accumulate when they are overlaid, explaining the misleading pattern for birds in the Amazon. DDMs can continue to be used mapping richness, as long as, at a regional scale: (1) basic map refinements are carried, or (2) only small-range species are used for mapping species richness.

Mercury (Hg) is a potent neurotoxin that biomagnifies within both aquatic and terrestrial food webs resulting in adverse physiological and reproductive effects on impacted wildlife populations, including songbird communities. Due to reducing conditions, wetland ecosystems promote the formation of methylmercury. Regional studies have documented elevated blood mercury concentrations in songbird species within these habitat types. The overall goal of this research was to examine spatial and seasonal patterns of Hg exposure for targeted songbird species within Sphagnum bog wetland systems and compare these patterns with adjacent upland forests in the Adirondack Park of New York State. Project sampling was conducted at study plots within four Sphagnum bog and associated upland forest sites from May - August during the 2008, 2009, and 2011 field seasons. The overall results documented: (1) blood Hg concentrations were elevated in songbird species inhabiting Sphagnum bog habitats as compared to nearby upland forest species; (2) target species within each habitat type exhibited consistent species-level patterns in blood Hg concentrations at each study site; and (3) no seasonal change in blood Hg concentrations within Sphagnum bog habitats was documented, but an increasing, followed by a decreasing seasonal pattern in mercury exposure was detected for upland forest species. Habitat type was demonstrated to influence avian Hg exposure levels. Moreover, Sphagnum bog ecosystems may be contributing to elevated Hg concentrations in biota within the surrounding environment. Seasonal patterns for blood Hg concentrations were found to vary between habitat type and are likely related to a combination of variables including habitat-driven Hg concentrations in prey items, seasonal dietary shifts, and annual molting cycles. This project emphasizes the importance of prioritizing future research efforts within identified high Hg habitat types, specifically wetland systems, to better characterize associated avian exposure levels, estimate the spatial extent of wetland systems on the surrounding environment, and identify locations of potential biological hotspots across the Adirondack Park.

Tree stems from wetland, floodplain and upland forests can produce and emit methane (CH₄). Tree CH₄ stem emissions have high spatial and temporal variability, but there is no consensus on the biophysical mechanisms that drive stem CH₄ production and emissions. Here, we summarize up to 30 opportunities and challenges for stem CH₄ emissions research, which, when addressed, will improve estimates of the magnitudes, patterns and drivers of CH₄ emissions and trace their potential origin.We identified the need: (1) for both long-term, high-frequency measurements of stem CH₄ emissions to understand the fine-scale processes, alongside rapid large-scale measurements designed to understand the variability across individuals, species and ecosystems; (2) to identify microorganisms and biogeochemical pathways associated with CH₄ production; and (3) to develop a mechanistic model including passive and active transport of CH₄ from the soil–tree–atmosphere continuum. Addressing these challenges will help to constrain the magnitudes and patterns of CH₄ emissions, and allow for the integration of pathways and mechanisms of CH₄ production and emissions into process-based models. These advances will facilitate the upscaling of stem CH₄ emissions to the ecosystem level and quantify the role of stem CH₄ emissions for the local to global CH₄ budget.

Global budgets ascribe 4–10% of atmospheric methane (CH4) sinks to upland soils and have assumed until recently that soils are the sole surface for CH4 exchange in upland forests. Here we report that CH4 is emitted from the stems of dominant tree species in a temperate upland forest, measured using both the traditional static-chamber method and a new highfrequency, automated system. Tree emissions averaged across 68 observations on 17 trees from May to September were 1.59 ± 0.88 μmol CH4m−2 stem h−1 (mean ± 95% confidence interval), while soils adjacent to the trees consumed atmospheric CH4 at a rate of −4.52 ± 0.64 μmol CH4m−2 soil h−1 (P < 0.0001). High-frequency measurements revealed diurnal patterns in the rate of tree-stem CH4 emissions. A simple scaling exercise suggested that tree emissions offset 1–6%of the growing season soil CH4 sink and may have briefly changed the forest to a net CH4 source.

Upland forests are traditionally thought to be net sinks for atmospheric methane (CH4). In such forests, in situ CH4 fluxes on tree trunks have been neglected relative to soil and canopy fluxes. We measured in situ CH4 fluxes from the trunks of living trees and other surfaces, such as twigs and soils, using a static closed-chamber method, and estimated the CH4 budget in a temperate upland forest in Beijing. We found that the trunks of Populus davidiana emitted large quantities of CH4 during July 2014–July 2015, amounting to mean annual emissions of 85.3 and 103.1 μg m−2 h−1 on a trunk surface area basis on two replicate plots. The emission rates were similar in magnitude to those from tree trunks in wetland forests. The emitted CH4 was derived from the heartwood of trunks. On a plot or ecosystem scale, trunk CH4 emissions were equivalent to c. 30–90% of the amount of CH4 consumed by soils throughout the year, with an annual average of 63%. Our findings suggest that wet heartwoods, regardless of rot or not, occur widely in living trees on various habitats, where CH4 can be produced.

To investigate how source and processing control the composition of “terrestrial” dissolved organic matter (DOM), we combine soil and tree leachates, tree DOM, laboratory bioincubations, and ultrahigh resolution Fourier-transform ion cyclotron resonance mass spectrometry in three common landscape types (upland forest, forested wetland, and poor fen) of Southeast Alaska’s temperate rainforest. Tree (Tsuga heterophylla and Picea sitchensis) needles and bark and soil layers from each site were leached, and tree stemflow and throughfall collected to examine DOM sources. Dissolved organic carbon concentrations were as high as 167 mg CL−1 for tree DOM, suggesting tree DOM fluxes may be substantial given the hypermaritime climate of the region. Condensed aromatics contributed as much as 38% relative abundance of spruce and hemlock bark leachates suggesting coniferous trees are potential sources of condensed aromatics to surface waters. Soil leachates showed soil wetness dictates DOM composition and processing, with wetland soils producing more aromatic formulae and allowing the preservation of traditionally biolabile, aliphatic formulae. Biodegradation impacted soil and tree DOM differently, and though the majority of source-specific marker formulae were consumed for all sources, some marker formulae persisted. Tree DOM was highly biolabile (> 50%) and showed compositional convergence where processing homogenized DOM from different tree sources. In contrast, wetland and upland soil leachate DOM composition diverged and processing diversified DOM from different soil sources during bioincubations. Increasing precipitation intensity predicted with climate change in Southeast Alaska will increase tree leaching and soil DOM flushing, tightening linkages between terrestrial sources and DOM export to the coastal ocean.

Forest soils and canopies are major components of ecosystem CO₂ and CH₄ fluxes. In contrast, less is known about coarse woody debris and living tree stems, both of which function as active surfaces for CO₂ and CH₄ fluxes. We measured CO₂ and CH₄ fluxes from soils, coarse woody debris, and tree stems over the growing season in an upland temperate forest. Soils were CO₂ sources (4.58 ± 2.46 lmol m-2 s-1, mean ± 1 SD) and net sinks of CH₄ (-2.17 ± 1.60 nmol m⁻² s⁻¹). Coarse woody debris was a CO₂ source (4.23 ± 3.42 lmol m⁻² s⁻¹) and net CH₄ sink, but with large uncertainty (-0.27 ± 1.04 nmol m⁻² s⁻¹) and with substantial differences depending on wood decay status. Stems were CO₂ sources (1.93 ± 1.63 lmol m⁻² s⁻¹), but also net CH₄ sources (up to 0.98 nmol m⁻² s⁻¹), with a mean of 0.11 ± 0.21 nmol m⁻² s⁻¹ and significant differences depending on tree species. Stems of N. sylvatica, F. grandifolia, andL. tulipifera consistently emitted CH₄, whereas stems of A. rubrum, B. lenta, and Q. spp. were intermittent sources. Coarse woody debris and stems accounted for 35% of total measured CO₂ fluxes, whereas CH₄ emissions from living stems offset net soil and CWD CH₄ uptake by 3.5%. Our results demonstrate the importance of CH₄ emissions from living stems in upland forests and the need to consider multiple forest components to understand and interpret ecosystem CO₂ and CH₄ dynamics.

The Upper Mississippi River is thought to provide important stopover habitat for migrating landbirds because of its north-south orientation and floodplain forests. The river flows through the Driftless Area of southwestern Wisconsin and southeastern Minnesota where forests are plentiful, yet forests of the floodplain and Driftless Area uplands differ greatly in landscape setting, tree species composition, and topography. We compared landbird assemblages in these upland and floodplain forests over three springs, 2005–2007, using line-transect surveys at randomly selected areas in and within 16 km of the floodplain. We found more species of both transient and locally breeding migrants per survey in floodplain than in upland forest. Detections of transient neotropical migrants did not differ statistically by habitat. Detections of locally breeding neotropical and temperate-zone migrants and transient temperate-zone migrants were greater in floodplain than in upland forest. Between floodplain and upland forest, assemblages of locally breeding species, including neotropical and temperate-zone migrants (of which some individuals were in transit), differed substantially, but assemblages of transients (including both neotropical and temperate-zone migrants) did not differ as much. Only two species of transient migrants had clear affinities for floodplain forest, and none had an affinity for upland forest, whereas most locally breeding migrants had an affinity for either upland or floodplain forest. Within each spring, however, detections of transient neotropical migrants shifted from being greater in floodplain to greater in upland forests. This intraseasonal shift may be related to the phenology of certain tree species.

Long-term fire exclusion has altered ecological function in many forested ecosystems in North America. The invasion of fire-sensitive tree species into formerly pyrogenic upland forests in the southeastern United States has resulted in dramatic shifts in surface fuels that have been hypothesized to cause reductions in plant community flammability. The mechanism for the reduced flammability or \"mesophication\" has lacked empirical study. Here we evaluate a potential mechanism of reduced flammability by quantifying moisture retention (response time and initial moisture capacity) of foliar litter beds from 17 southeastern tree species spanning a wide range of fire tolerance. A k -means cluster analysis resulted in four species groups: a rapidly drying cluster of eight species; a five-species group that absorbed little water but desorbed slowly; a two-species group that absorbed substantial moisture but desorbed rapidly; and a two-species cluster that absorbed substantial moisture and dried slowly. Fire-sensitive species were segregated into the slow moisture loss clusters while fire-tolerant species tended to cluster in the rapid drying groups. Principal-components analysis indicated that several leaf characteristics correlated with absorption capacity and drying rates. Thin-leaved species with high surface area : volume absorbed the greatest moisture content, while those with large, curling leaves had the fastest drying rates. The dramatic shifts in litter fuels as a result of invasion by fire-sensitive species generate a positive feedback that reduce the windows of ignition, thereby facilitating the survival, persistence, and continued invasion of fire-sensitive species in the uplands of the southeastern United States.