Upland Tree Research Articles

Global atmospheric methane uptake by upland tree woody surfaces

Methane is an important greenhouse gas1, but the role of trees in the methane budget remains uncertain2. Although it has been shown that wetland and some upland trees can emit soil-derived methane at the stem base3,4, it has also been suggested that upland trees can serve as a net sink for atmospheric methane5,6. Here we examine in situ woody surface methane exchange of upland tropical, temperate and boreal forest trees. We find that methane uptake on woody surfaces, in particular at and above about 2 m above the forest floor, can dominate the net ecosystem contribution of trees, resulting in a net tree methane sink. Stable carbon isotope measurement of methane in woody surface chamber air and process-level investigations on extracted wood cores are consistent with methanotrophy, suggesting a microbially mediated drawdown of methane on and in tree woody surfaces and tissues. By applying terrestrial laser scanning-derived allometry to quantify global forest tree woody surface area, a preliminary first estimate suggests that trees may contribute 24.6–49.9 Tg of atmospheric methane uptake globally. Our findings indicate that the climate benefits of tropical and temperate forest protection and reforestation may be greater than previously assumed.

Residents’ Perception of Household Wellbeing on Upland Trees Conservation in Mitigating Flood in Selected Flood-Prone Communities of Oyo State, Nigeria

Residents’ Perception of Household Wellbeing on Upland Trees Conservation in Mitigating Flood in Selected Flood-Prone Communities of Oyo State, Nigeria

Landscape position mediates drought vulnerability in California valley oak (Quercus lobata)

Future climate change will exacerbate drought stress in water-limited ecosystems. However, topography can alter the fine-scale climatic and hydrologic conditions that mediate plant response to meteorological drought. Here, with six new valley oak (Quercus lobata) tree-ring width chronologies, we assess how topography acts as a mediating factor on tree growth and drought sensitivity. Because valley oaks are known to be highly dependent on subsurface water, we predicted that trees growing in riparian sites would be less sensitive to precipitation variability due to greater access to groundwater. Trees were sampled in the Tehachapi Mountains of California across a landscape gradient of sites ranging from 375–1650 m elevation and across upland (55–69 m mean height above nearest drainage) and riparian (2–6 m mean height above the nearest drainage) hillslope positions. Interannual tree growth patterns and drought sensitivity varied substantially in association with hillslope position and elevation. Valley oak radial growth showed a consistently weaker response to precipitation at riparian sites. The influence of hillslope position on drought sensitivity varied with elevation, such that the riparian buffering effect was weakest at sites higher in the watershed and strengthened, progressively, toward the lower elevation sites with greater climatic water deficit. Upland tree growth exhibited a strong response to high-frequency interannual precipitation variability at the high elevation site, whereas trees at lower elevation upland sites responded more to low-frequency decadal trends in precipitation, possibly reflecting hydrogeological processes by which precipitation feeds groundwater lower in the watershed. Our results are consistent with groundwater-dependence of valley oak and indicate that riparian habitats are the most likely refugia for the species during prolonged drought.

Holocene vegetation dynamics in southern Ukraine under changing land use and climate

The Holocene vegetation dynamics of the Eurasian steppe are underinvestigated despite its vast extension, chiefly because of the scarcity of suitable sites for palaeoecological research. Here, we present a palaeoecological reinvestigation from Kardashynskyi mire (southern Ukraine), approximately 4 km from a previously cored site. Using pollen, spores and microscopic charcoal, we have reconstructed vegetation dynamics, fire history and land use for the past c. 8300 years. Regionally, steppe vegetation with Poaceae, Chenopodiaceae and Artemisia was always dominant. However, pollen assemblages also reflect the presence of floodplain and upland tree stands. At the beginning of the sequence, c. 8300 years ago, Pinus stands were growing on the sandy terraces of the Dnipro dry upland sites. Later, at c. 7950 cal yr BP, diverse stands with Quercus, Ulmus, Fraxinus, Tilia and Alnus established along the riverbanks, where moisture availability was sufficient. Around 6100 cal yr BP, those deciduous broadleaved stands experienced a severe decline, likely in response to changing water table levels. Cultural indicators suggest land use activities after c. 7900 cal yr BP. During the Bronze Age, human impact intensified. Overall, both climate and humans drove vegetation dynamics in the Pontic steppe for millennia. Nowadays, this once extensive biome has almost completely been converted to cropland. Similarly, the wetland vegetation, the riparian forests and, above all, the pine forests growing on the sandy terraces were strongly reduced by millennial-long land use. Under the current conditions, even the last remnants of these special vegetation types are severely threatened.

Effects of Wetland Degradation on Community Livelihoods: A Case of Mabengere Wetland in Kisiita Sub County, Kakumiro District, Uganda

Wetlands in Uganda play vital ecological, economic, socio-cultural, scientific and recreational purposes and yet they are currently threatened by anthropogenic and bio geophysical factors. The current study was initiated to determine the drivers of wetland degradation; and to assess the benefits and costs as a result of wetland degradation, to members of the local community in Kakumiro district, Uganda. A cross sectional research design that involved the collection of both quantitative and qualitative data from three hundred and eighty-seven respondents was used. Data were collected using a questionnaire, Key informant interview guide from the key informants and observation check list. Key findings revealed that sand and clay mining, agricultural activities, gathering building materials were the key activities carried out, which significantly affected the wetlands at P<0.05. In addition, the activities carried out in the wetlands significantly improved the livelihoods of the people living around them at P<0.05. The water levels in the wetlands were reduced that led to shortage of portable water and the water for watering animals, and also led to outbreak of fires which contributed to a reduction in the wetland’s biodiversity. This was confirmed by the Chi-square test which revealed that degradation of wetlands contributed to the costs incurred by members of the local community at P<0.05. It was therefore recommended that alternative sources of energy and building materials be availed like planting of upland trees. Also, there should be awareness intensification to members of the local community about the values and uses of wetlands aimed at scaling down their degradation.

Salinity exposure affects lower-canopy specific leaf area of upland trees in a coastal deciduous forest

Sea level rise and increasing storm surges are likely to affect the canopy physiology, ecology, and structure of coastal forests, even well in advance of tree mortality. Laboratory and greenhouse studies have documented that saltwater exposure can trigger changes in leaf-level physiology and morphology, but few in situ studies have examined how tree-specific leaf area (SLA), the ratio of leaf area to mass and a crucial trait and model parameter, is affected by saline soils. We conducted an observational study of SLA in a mid-Atlantic (USA) coastal deciduous forest, taking advantage of a natural gradient in salinity along a tidal creek. Measured SLA of the 239 trees and seven species sampled ranged from Carya glabra (N = 6 trees, mean SLA = 277.9 ± 36.3 cm2/g) to Fagus grandifolia (N = 60, 321.9 ± 62.9 cm2/g); as expected, trees species and canopy position (sun versus shade) significantly affected SLA. For trees (N = 100) directly exposed to the tidal creek, salinity was highly significant after accounting for species (P < 0.001), with trees in the lower reaches of the creek having lower SLA. Leaf area index (LAI), computed from SLA and litter traps, ranged from 4.8 to 15.8 and was inversely related to salinity exposure; the spatial variability in leaf litter production contributed much more to LAI uncertainty than did SLA variability. These in situ results are correlative but consistent with the hypothesis, based on previous greenhouse studies, that the stress of chronic salinity exposure changes species’ leaf morphology. Our findings are useful for understanding the growing effects of saltwater intrusion into upland forests, as well as parameterizing and testing ecosystem-scale models simulating forest stressors and disturbances at the terrestrial-aquatic interface.

Spatiotemporal patterns and environmental drivers of eastern redcedar (Juniperus virginiana) abundance along the Missouri River, USA

Changes in disturbance regimes, including reductions in flooding and geomorphic dynamism from dam construction and flow regulation, have facilitated invasion by eastern redcedar (Juniperus virginiana L.), an upland tree species, in the understory of floodplain forests along the Missouri National Recreational River (MNRR). Our aim was to determine the spatiotemporal patterns and environmental drivers of redcedar invasion along the MNRR. We used the Normalized Difference Vegetation Index (NDVI) calculated from winter Landsat imagery to construct a time series of maps showing spatial changes in redcedar abundance and distribution from 1982 to 2017 in both riparian and upland habitats along the MNRR. We determined how environmental factors (e.g., soil drainage ability, flood recurrence interval, 1980s LULC, lateral distance to the river, and channel incision) have influenced current (2017) redcedar occurrence and abundance in riparian habitats using random forest models (RFM). Time-series maps indicated that detectable redcedar cover occurred over less than 5% of the study area before 1985, increased steadily from 1985 to 2000, and more than tripled from 2000 to 2010. After 2010, redcedar abundance continued to increase in upland areas but declined following the 2011 Missouri River flood in the floodplain. RFMs indicated that river incision, distance to the river, soil drainage, 1984 LULC, and flood recurrence interval were important features influencing redcedar occurrence and abundance in the floodplain. Unless preventive measures are implemented, lack of floods and ongoing flow regulation will enable the continued spread of redcedar along the MNRR and other regulated rivers in the eastern Great Plains.

Coastal vegetation dynamics in response to climatic and relative sea level changes in Mahanadi River delta, NE coast of India

This research assesses the impacts of climate change and relative sea level (RSL) fluctuations on coastal vegetation during the past 2000 years along the Mahanadi River delta, northeast coast of India. Sediment samples of a 2.6 m trench from Barhatubi area located in the lower flood plain of Mahanadi River delta were studied for sedimentological and pollen analysis. Mangrove succession can be divided into three zones: (1) Freshwater marsh (∼1980–1420 cal yr BP); (2) Tidal flat (∼1420–770 cal yr BP) and (3) Landward mangrove margin (∼770 cal yr BP-Present). A transgressive shift from floodplain freshwater marsh to a tidal flat is recorded between ∼1980–1420 cal yr BP which is evidenced by upland tree taxa, Poaceae, pteridophyte and fungal spores along with low percentage of mangroves such as Avicennia, Sonneratia and Rhizophora representing the landward edge of the tidal region. Dominance of Sonneratia and a decline in terrestrial taxa reflects a rise in the relative sea level around ∼1420 cal yr BP with less freshwater input from land suggesting a weakened monsoon condition. After ∼770 cal yr BP, an overall regressive phase with small cycles of relative sea level rise/fall has been observed due to the dominance of salt tolerant Avicennia along with Rhizophora, Excoecaria agallocha and Aegialitis rotundifolia alternating with Sonneratia and other marine palynomorphs.

Changes in bark properties and hydrology following prescribed fire in Pinus taeda and Quercus montana

AbstractIn the eastern United States, the use of prescribed fire as a silvicultural technique to manage for desirable upland tree species is increasing in popularity. Bark physical properties such as thickness, density, and porosity have known associations with fire tolerance among species. These physical properties simultaneously influence rainfall interception and canopy storage and thus are of interest across a range of disciplines. Furthermore, while these characteristics are innate to a species, it is unknown whether repeated exposure to fire facilitates physical change in bark structure and whether these changes are consistent among species. To answer these questions, bark samples were collected from mature pine (Pinus taeda L.) and oak (Quercus montana Willd.) trees from sites across the Bankhead National Forest in Alabama, USA under three different burn regimes: 3‐year cycle, 9‐year cycle, and no fire. Samples were analysed in the laboratory for bulk density, porosity, water storage capacity, and hygroscopicity (the amount of atmospheric water vapour absorbed by bark during non‐rainfall conditions). Drying rates of saturated samples under simulated wetting conditions were also assessed. Oak bark had higher bulk density, lower porosity, and dried slower than pine bark. Interestingly, bark from both species had lower bulk density, higher porosity, greater water storage capacity, and dried faster in stands that were burned every 3 years compared to other fire regimes (p &lt; 0.001). In summary, this study demonstrates that prescribed fire regimes in an eastern US forest alter bark structure and thus influence individual tree control on hydrological processes. The increase in bark water storage capacity, coupled with faster bark evaporation times may lead to less water inputs to the forest floor and drier overall conditions. Further investigation of this fire‐bark‐water feedback loop is necessary to understand the extent of these mechanisms controlling landscape‐scale conditions.

‘Tree Mountaineers’: Arboreal Materiality on the Fells in the Lakeland Guides of William Wordsworth and Harriet Martineau

ABSTRACT This ascending line of enquiry will pay close attention to how, through their nineteenth-century Lakeland writings, William Wordsworth and Harriet Martineau attached meaning to the continued presence and perceived role of trees in the landscapes of the English Lake District. The authors wrote about the region when increased numbers of landowners were planting trees for aesthetic, agricultural, and financial purposes on their land, ranging from the villa garden to the fell-side plantation. In this context, this analysis will consider the authors’ perceptions of historical upland tree cover, their aesthetic evaluation of particular planted and self-seeded spaces, and how individual specimens are sites of natural and cultural convergence shaped by the ‘wildness’ of the fells. Focusing on literary Lakeland trees – as discussed by Wordsworth, Martineau, and their circle – this article illustrates an ecological and arbori-cultural understanding of the environment that shifts, in accordance with elevation, from the valley floor up to the mountain top.

Forest treatment effects on wood production in ponderosa pine

Forest treatments reduce wildfire risk and can promote the vigor and production of remaining trees, but they are also a disturbance. Understanding the type, timing, and longevity of tree response to treatment, as well as the potential for interactive effects of treatments and drought, could help managers plan and evaluate forest management practices. Environmental drivers, biological modifiers, and tree capacity to respond to prior disturbances were concurrently tested to predict ponderosa pine basal area increment (BAI) in a lowland and upland dry pine forest in south central Oregon, USA. Environmental drivers included current year and lags or running averages of a drought index, SPEI, and the sum or count of growing degree days >0°C or 10°C. Biological modifiers of environmental drivers considered pre-treatment response to disturbance, tree vigor, and tree-to-tree competition. A model was developed to predict BAI in both topographic positions for applicability to the landscape level, and then was used to test for specific differences in BAI between paired forest treatments differing by one treatment. Forest treatments tested included no management (NM), undercut and even spacing harvest (HE), prescribed fire (Rx), and their combinations. HE significantly increased BAI shortly after treatment. Post-harvest, one or two Rx did not provide additional BAI benefits, nor in the absence of HE, did 2Rx vs. 1Rx treatment. The 1Rx treatment was imposed between multi-year droughts; BAI significantly increased after the treatment and was resistant to droughts. Upland trees were affected by a single year of drought; lowland trees responded only after sequential drought years. A single treatment, HE or 1Rx appeared to be as effective as multiple or mixed treatments in improving BAI in dry pine forest stands. HE appeared to generate the largest effect. Timing of forest treatments relative to site water balance may affect short term (decadal) wood production.

Guiding Conservation for Mountain Tree Species in Lebanon

The objective of this study is to contribute to the conservation of upland tree species in the face of climate change. We used a conservation index to prioritize the areas and populations of three conifer species in the mountains of Lebanon. This conservation index integrates (1) mountain topography to identify areas that could provide a suitable microclimate, (2) genetic diversity to assess the adaptive capacity of populations in these mountain areas, and (3) a hypothetical climate change scenario that could affect this Mediterranean region. The idea of this index is to prioritize protected areas based on a match between the relevance of the area to be protected and the populations that need local and long-term protection. The stronger the match, the higher the priority of the area to be protected. We applied this conservation index to 36 populations of 15 fir, 15 cedar, and 6 juniper. These populations were genotyped by different authors whose published data we used. The results show that 10 populations of the 3 species have a very high index and 9 others have a lower but still high index, indicating a high conservation priority. These 19 populations occur in 5 different areas that we delineated and that form a network along the Lebanon Mountains. We hypothesize that the conservation of these 19 populations across the Lebanon Mountains could contribute to the long-term sustainability of the 3 species in the face of a 2 °C increase in mean seasonal temperature and a 20% decrease in seasonal precipitation compared to the current climate.

Strategic Application of Topoclimatic Niche Models in Managing Forest Change

Forest management traditionally has been based on the expectation of a steady climate. In the face of a changing climate, management requires projections of changes in the distribution of the climatic niche of the major species and strategies for applying the projections. We prepared climatic habitat models incorporating heatload as a topographic predictor for the 14 upland tree species of southwestern Colorado, USA, an area that has already seen substantial climate impacts. Models were trained with over 800,000 points of known presence and absence. Using 11 climate scenarios for the decade around 2060, we classified and mapped change for each species. Projected impacts are extensive. Except for the low-elevation woodland species, persistent habitat is rare. Most habitat is lost or threatened and is poorly compensated by emergent habitat. Three species may be locally extirpated. Nevertheless, strategies are described that can use the projections to apply management where it is likely to be most effective, to facilitate or assist migration, to favor species likely to be suited in the future, and to identify potential climate refugia.

Experimental Tree Mortality Does Not Induce Marsh Transgression in a Chesapeake Bay Low-Lying Coastal Forest

Transgression into adjacent uplands is an important global response of coastal wetlands to accelerated rates of sea level rise. “Ghost forests” mark a signature characteristic of marsh transgression on the landscape, as changes in tidal inundation and salinity cause bordering upland tree mortality, increase light availability, and the emergence of tidal marsh species due to reduced competition. To investigate these mechanisms of the marsh migration process, we conducted a field experiment to simulate a natural disturbance event (e.g., storm-induced flooding) by inducing the death of established trees (coastal loblolly pine, Pinus taeda) at the marsh-upland forest ecotone. After this simulated disturbance in 2014, we monitored changes in vegetation along an elevation gradient in control and treatment areas to determine if disturbance can lead to an ecosystem shift from forested upland to wetland vegetation. Light availability initially increased in the disturbed area, leading to an increase in biodiversity of vegetation with early successional grass and shrub species. However, over the course of this 5-year experiment, there was no increase in inundation in the disturbed areas relative to the control and pine trees recolonized becoming the dominant plant cover in the disturbed study areas. Thus, in the 5 years since the disturbance, there has been no overall shift in species composition toward more hydrophytic vegetation that would be indicative of marsh transgression with the removal of trees. These findings suggest that disturbance is necessary but not sufficient alone for transgression to occur. Unless hydrological characteristics suppress tree re-growth within a period of several years following disturbance, the regenerating trees will shade and outcompete any migrating wetland vegetation species. Our results suggest that complex interactions between disturbance, biotic resistance, and slope help determine the potential for marsh transgression.

Water isotope variation in an ecohydrologic context at a seasonally dry tropical forest in northwest Mexico

We conducted an isotopic characterization of precipitation, streamflow, groundwater and bulk soil moisture (<60 cm depth) to assess ecohydrologic separation and its seasonal variation in a tropical dry forest in northwestern Mexico using four representative tree species. We present a data set that allows analyzing the connection of vegetation to soil or movable water in a highly seasonal ecosystem throughout one year (2015) but relies on baseline data set of local precipitation. A local meteoric water line was created using 5 years (2012–2017) of isotopic data (δ2H = 7.64*δ18O + 7.34, r2= 0.97) at the study site. Ecohydrologic separation was clear throughout the dry season but during the wet monsoon season the upland trees L. divaricatum, A. cochliacanta and L. watsonii showed the opposite pattern since the isotopic composition of xylem water was similar to the meteoric water and shallow soil, stream and ground water. The riparian tree T. huegelii resulted in an isolated dynamic showing a similar isotopic composition to stream water. Results of this work suggest that ecohydrologic separation is not a universal pattern because it does not occur in all vegetation types and it may vary seasonally.

Tree growth, transpiration, and water-use efficiency between shoreline and upland red maple (Acer rubrum) trees in a coastal forest

Coastal shoreline forests are vulnerable to seawater exposure, the impacts of which will increase due to sea-level rise, but the long-term adaptation strategies and vulnerability of coastal forests are not well understood. We used whole-tree transpiration, leaf water potential, tree-ring width, and tree-ring δ13C (a proxy for intrinsic water use efficiency, iWUE) to examine the long-term adaptation strategies of red maple (Acer rubrum) trees at the coastal interface (i.e., shoreline) and nearby upland in Maryland, USA. Red maple trees that grew along the shoreline and were exposed to slightly saline water (up to two PSU) had higher transpiration rates than those growing in the nearby upland forest during a wet year, but these differences disappeared during a normal precipitation year. Shoreline trees grew more slowly than upland trees over the last four decades, but these growth differences have disappeared in the last six years. Shoreline and upland red maple trees had similar variation in iWUE, indicating that higher transpiration rates of the seawater-exposed trees did not translate into differences in water use efficiency. There were no differences in predawn and midday water potential between upland and shoreline trees, suggesting no additional water stress occurs in shoreline trees. These findings indicate that mature red maple in our coastal study site maintains gas exchange and growth at a consistent or homeostatic level under slight soil salinity.

Effect of Tree Size, Topography and Shoot age on Pest Infestation and Tree Growth of Palas (&lt;i&gt;Butea monosperma&lt;/i&gt;)

A bench mark survey was conducted in March, 2017 to generate information about health status of palas trees ( Butea monosperma ), the most important host tree of rangeeni lac insect ( Kerria lacca Kerr.). The principal objective of the study was to identify factors responsible for poor growth of this important tree. Infestation of termite and trunk borer is perennial problem for palas trees. Therefore, information was generated under three different conditions (i) tree size i.e. small and big (mean trunk girth 74.3 cm and 151 cm) (ii) land topography i.e. upland/ lowland and (iii) tree with old/ new shoots i.e. trees having shoots of &gt;2 year or one year old. Land topography was found to have effect on termite infestation. In upland condition, termite galleries covered 50 per cent of the tree length, while the same value was merely 5.8 per cent in case of lowland. Smaller trees were 43 per cent less infested in comparison to big ones. Similarly, upland trees were 2.5 times more infested than lowland. Number of live shoot and additive shoot diameter per prune point were also found to be higher in case of relatively smaller trees. Number of live shoot per prune point was recorded to be 3.6 and 3.1 in small and big sized trees, respectively and additive live shoot diameter was 52.4 and 43.2 mm per prune point on small and big trees. But, in contrast, average number of dead shoots were found higher in big sized trees. Only variation was observed in case of trees with old and new shoots, where 19.2 per cent increase in average diameter was observed in newly emerged shoots i.e. pruned one year back. Better performance of smaller sized trees in terms of growth and pest resistance could be attributed to better rejuvenation power and vitality for being younger in age. Similarly, higher attack of pest in upland soils could be attributed to maintenance of poor health on relatively less fertile uplands.

Natural channeling in riverine forests determines variations in their floristic composition, structure, and land use in southern Brazil

Hydrological processes in riparian forests affect their geomorphology and drainage, and define their structure. Interactions among many environmental variables and the tree community in continuous riverine forests affect their formation and add to their complexity, making our understanding of these habitats particularly challenging. We evaluated the tree species diversity and richness in relation to relief and flooding regimes in the watershed of a riparian forest in southern Brazil. The environments of the topographic gradients were classified according to relief and soil. Canonical correspondence analysis and indicator species analysis were used for the environmental classification. A detailed soil survey showed that the organic matter content and pH are quite distinct among the environmental categories. Frequent flooding of low intensity on the floodplain was associated with incipient and shallow channels, greater frequency of livestock encroachment, and lower frequency of shade-tolerant species dominance in the deep relief of the valleys and the headwaters. We conclude that there is a dynamic flow of species migration between the flooded and non-flooded environments in small channels and that the narrow deep channels in the floodplain with less frequent and intense flooding favor an expansion of the upland tree species into riparian areas, with channel shape and depth important variables in the impact of cattle encroachment on vegetation. The greatest tree diversity was found in the ravine habitats in the intermediate landscapes.

Investigating old-growth ponderosa pine physiology using tree-rings, δ13 C, δ18 O, and a process-based model.

In dealing with predicted changes in environmental conditions outside those experienced today, forest managers and researchers rely on process‐based models to inform physiological processes and predict future forest growth responses. The carbon and oxygen isotope ratios of tree‐ring cellulose (δ13Ccell, δ18Ocell) reveal long‐term, integrated physiological responses to environmental conditions. We incorporated a submodel of δ18Ocell into the widely used Physiological Principles in Predicting Growth (3‐PG) model for the first time, to complement a recently added δ13Ccell submodel. We parameterized the model using previously reported stand characteristics and long‐term trajectories of tree‐ring growth, δ13Ccell, and δ18Ocell collected from the Metolius AmeriFlux site in central Oregon (upland trees). We then applied the parameterized model to a nearby set of riparian trees to investigate the physiological drivers of differences in observed basal area increment (BAI) and δ13Ccell trajectories between upland and riparian trees. The model showed that greater available soil water and maximum canopy conductance likely explain the greater observed BAI and lower δ13Ccell of riparian trees. Unexpectedly, both observed and simulated δ18Ocell trajectories did not differ between the upland and riparian trees, likely due to similar δ18O of source water isotope composition. The δ18Ocell submodel with a Peclet effect improved model estimates of δ18Ocell because its calculation utilizes 3‐PG growth and allocation processes. Because simulated stand‐level transpiration (E) is used in the δ18O submodel, aspects of leaf‐level anatomy such as the effective path length for transport of water from the xylem to the sites of evaporation could be estimated.

Interspecific competition limits the realized niche ofFraxinus nigraalong a waterlogging gradient

Gradient studies of wetland forests have inferred that competition from upland tree species confines waterlogging-tolerant tree species to hydric environments. Little is known, however, about competition effects on individual-tree growth along stress gradients in wetland forests. We investigated tree growth and competition in mixed-species stands representing a waterlogging stress gradient in Fraxinus nigra Marsh. (black ash) forests in Minnesota, USA. Using competition indices, we examined how F. nigra basal area increment (BAI) responded to competition along the gradient and whether competition was size-asymmetric (as for light) or size-symmetric (as for soil resources). We modeled spatial distributions of F. nigra and associated tree species to assess how variation in species mixtures influenced competition. We found that although F. nigra BAI did not significantly differ with variations in site moisture, the importance of competition decreased as waterlogging stress increased. Competition across the gradient was primarily size-asymmetric (for light). Variation in species mixtures along the gradient was an important influence on competition. Some segregation of tree species occurred at all but the most upland site, where waterlogging stress was lowest and evidence of competition was greatest, confirming that competition from upland tree species confines F. nigra and potentially other waterlogging-tolerant species to hydric environments.