Many forests of eastern North American are undergoing a species composition shift in which maples (Acer spp.) are increasingly important while oak (Quercus spp.) regeneration and recruitment has ...become increasingly scarce. This dynamic in species composition occurs across a large and geographically complex region. The elimination of fire has been postulated as the driver of this dynamic; however, some assumptions underlying this postulate have not been completely examined, and alternative hypotheses remain underexplored. Through literature review, and a series of new analyses, we examined underlying assumptions of the "oak and fire" hypothesis and explored a series of alternative hypotheses based on well-known ecosystem drivers: climate change, land-use change, the loss of foundation and keystone species, and dynamics in herbivore populations. We found that the oak-maple dynamic began during a shift in climate regime -from a time of frequent, severe, multi-year droughts to a period of increased moisture availability. Anthropogenic disturbance on the landscape changed markedly during this same time, from an era of Native American utilization, to a time characterized by low population densities, to Euro-American settlement and subsequent land transmogrification. During the initiation of the oak-maple dynamic, a foundation species, the American chestnut, was lost as a canopy tree across a broad range. Several important browsers and acorn predators had substantial population dynamics during this period, e.g. white-tailed deer populations grew substantially concurrent with increasing oak recruitment failure. In conclusion, our analyses suggest that oak forests are reacting to marked changes in a suite of interlocking factors. We propose a "multiple interacting ecosystem drivers hypothesis", which provides a more encompassing framework for understanding oak forest dynamics.
Ecological communities are structured in response to spatial and temporal variation of numerous factors, including edaphic conditions, biotic interactions, climatic patterns and disturbance regimes. ...Widespread anthropogenic factors such as timber harvesting can create long-lasting impacts, obscuring the relationship between community structure and environmental conditions. Minimally impacted systems such as old-growth forests can serve as a useful ecological baseline for predicting long-term compositional shifts. We utilized decadal tree species sampling data (1979-2010) divided into three strata (understory, midstory, overstory) to examine temporal changes in relative abundances and spatial distributions of dominant taxa, as well as overall shifts in community composition, in a relatively pristine Appalachian old-growth forest in eastern Kentucky, USA. Quercus and Carya species persisted mainly as mature canopy trees with decreasing juvenile recruitment, especially in mesic areas. In contrast, Acer, Fagus, and other mesophytic species were abundant and spatially widespread in subcanopy layers suggesting these species are more likely to recruit in gap-scale canopy openings. In the overstory, mesophytic species were spatially restricted to lower and mid-slope mesic habitats. Temporal changes in community composition were most evident in the understory and tended to be greater in mesic areas, a trend seemingly driven by recruitment failure among xerophytic species. In subcanopy vegetation we discovered a loss of distinction through time among the ecological community designations established following the 1979 survey (Chestnut oak, Mixed mesophytic, and Beech). The overstory was more stable through time, suggesting a storage effect where long-lived trees have maintained a particular community composition through time in areas where regeneration opportunities are minimal under current environmental conditions. Overall, sitewide canopy succession is occurring slowly in the absence of major disturbance, and topography-driven environmental variation appears to have an important local-scale filtering effect on communities.
In humid, broadleaf-dominated forests where gap dynamics and partial canopy mortality appears to dominate the disturbance regime at local scales, paleoecological evidence shows alteration at ...regional-scales associated with climatic change. Yet, little evidence of these broad-scale events exists in extant forests. To evaluate the potential for the occurrence of large-scale disturbance, we used 76 tree-ring collections spanning ∼840 000 km
2
and 5327 tree recruitment dates spanning ∼1.4 million km
2
across the humid eastern United States. Rotated principal component analysis indicated a common growth pattern of a simultaneous reduction in competition in 22 populations across 61 000 km
2
. Growth-release analysis of these populations reveals an intense and coherent canopy disturbance from 1775 to 1780, peaking in 1776. The resulting time series of canopy disturbance is so poorly described by a Gaussian distribution that it can be described as "heavy tailed," with most of the years from 1775 to 1780 comprising the heavy-tail portion of the distribution. Historical documents provide no evidence that hurricanes or ice storms triggered the 1775-1780 event. Instead, we identify a significant relationship between prior drought and years with elevated rates of disturbance with an intense drought occurring from 1772 to 1775. We further find that years with high rates of canopy disturbance have a propensity to create larger canopy gaps indicating repeated opportunities for rapid change in species composition beyond the landscape scale. Evidence of elevated, regional-scale disturbance reveals how rare events can potentially alter system trajectory: a substantial portion of old-growth forests examined here originated or were substantially altered more than two centuries ago following events lasting just a few years. Our recruitment data, comprised of at least 21 species and several shade-intolerant species, document a pulse of tree recruitment at the subcontinental scale during the late-1600s suggesting that this event was severe enough to open large canopy gaps. These disturbances and their climatic drivers support the hypothesis that punctuated, episodic, climatic events impart a legacy in broadleaf-dominated forests centuries after their occurrence. Given projections of future drought, these results also reveal the potential for abrupt, meso- to large-scale forest change in broadleaf-dominated forests over future decades.
Understanding the role of biodiversity (B) in maintaining ecosystem function (EF) is a foundational scientific goal with applications for resource management and conservation. Two main hypotheses ...have emerged that address B–EF relationships: niche complementarity (NC) and the mass-ratio (MR) effect. We tested the relative importance of these hypotheses in a subtropical oldgrowth forest on the island nation of Taiwan for two EFs: aboveground biomass (ABG) and coarse woody productivity (CWP). Functional dispersion (FDis) of eight plant functional traits was used to evaluate complementarity of resource use. Under the NC hypothesis, EF will be positively correlated with FDis. Under the MR hypothesis, EF will be negatively correlated with FDis and will be significantly influenced by community-weighted mean (CWM) trait values. We used path analysis to assess how these two processes (NC and MR) directly influence EF and may contribute indirectly to EF via their influence on canopy packing (stem density). Our results indicate that decreasing functional diversity and a significant influence of CWM traits were linked to increasing AGB for all eight traits in this forest supporting the MR hypothesis. Interestingly, CWP was primarily influenced by NC and MR indirectly via their influence on canopy packing. Maximum height explained more of the variation in both AGB and CWP than any of the other plant functional traits. Together, our results suggest that multiple mechanisms operate simultaneously to influence EF, and understanding their relative importance will help to elucidate the role of biodiversity in maintaining ecosystem function.
In the American Midwest, grasslands are highly diverse, endangered ecosystems that require conservation and restoration because they face the threat of plant species invasions.
Pyrus calleryana
...(Decne.; Rosaceae) is a particularly problematic invasive tree in these ecosystems and relatively little is known about the factors that dictate its success. Landscape features regulate the spread of some invasive plants and may play an important role in dictating the vulnerability of grasslands to
P. calleryana
invasion. Towards developing a predictive framework for grassland invasion, we performed a fine-scale spatial assessment of invaded and uninvaded prairies to determine if landscape characteristics including proximity to forest edge, roads or residences, plant communities or soil characteristics could be used to predict
P. calleryana
invasion. We found increased invasion near forest edges, likely due to bird dispersal and optimal foraging habitats. Within invaded prairies,
P. calleryana
was prevalent where there was lower ground cover by grasses and forbs and where there was limited organic matter but increased soil phosphorus, nitrate and micronutrients. This study could not determine if
P. calleryana
caused these changes or if it established because these parameters were favorable. Our data also indicated an intriguing relationship of increased species richness in
P. calleryana
invaded sites, potentially because uninvaded sites were characterized by dominant grasses that may reduce available niche space. Taken together, this work suggests that
P. calleryana
establishment depends on the relative propagule pressure and niche availability and indicates potential for developing a predictive framework for landscape invasion.
Invasive species are of global importance because of their impacts on ecological communities, habitat structure, native community dynamics, and ecosystem processes and function. Scientists and ...conservation managers are increasingly focusing on the biological impacts of invasive species and on devising management practices that emphasize the health of ecosystems based on measured biological processes. Amur honeysuckle (Lonicera maackii (Rupr.) Herder) is a highly successful invasive shrub in forests of eastern North America. The scientific literature surrounding this species has grown in the past several decades as researchers have investigated L. maackii impacts across multiple ecological scales. In this review we synthesized literature on (a) the key traits related to this species' invasion success, (b) the impacts this invasive species has at various ecological scales, (c) the outcomes of restoration efforts for this species, and (d) the connections of this weed to invasion ecology theories. Lonicera maackii impacts are complex and vary across ecosystems and spatial scales; we report findings from studies demonstrating a wide range of effects on species composition, community structure, ecosystem function, and successional trajectories. We end by providing a working ecological framework that may help guide future research and conservation efforts.
Biological invasion of woody plants into grasslands is a widespread phenomenon that threatens the cultural value, biodiversity, and ecosystem function of these unique systems. In the American ...Midwest, grasslands are increasingly threatened by invasion of the tree Pyrus calleryana (Callery pear) which is particularly challenging to manage and has strong potential to alter ecosystem function. Mowing is a standard practice for maintaining Midwestern grasslands; however, P. calleryana exhibits an aggressive sprout response to cutting and the ecological implications of this behavior are not well understood. We measured the response of soil moisture and pH, and soil enzyme activities representing labile carbon cycling (β-glucosidase), recalcitrant carbon cycling (peroxidase and phenol oxidase), nitrogen cycling (leucine aminopeptidase) and phosphorus cycling (phosphatase) to determine how P. calleryana trees that are untreated and single stemmed alter nutrient cycling compared to their cut and resprouting counterparts. We found lower β-glucosidase activity and higher peroxidase activity underneath single stemmed trees than underneath those which had resprouted, indicating that there may be lower nutrient availability underneath untreated trees accounting for differences in enzyme activity. Generally, invasive species leaf litter has faster decay rates than native species, which results in higher activities of enzymes that degrade labile materials in soil underneath the plants. Because soils underneath P. calleryana do not follow this pattern, it is possible that its leaf material is not as labile as other common invaders. We also found that increasing P. calleryana basal diameter was associated with reductions in soil pH, which indirectly increased peroxidase and phenol oxidase activities, enzymes which are indicative of recalcitrant C sources such as lignin. This demonstrates that P. calleryana may alter carbon cycling by altering the C inputs to the soil system from its leaf litter. Taken together, P. calleryana may reduce plant richness and promote further invasion by lowering the availability of labile carbon and lowering soil pH, indicating that this invasion may be soil-mediated and self-reinforcing. Further, we recommend continued mowing as a treatment for P. calleryana invasion to ameliorate the impacts of its invasion even though there will likely be a sprout response to treatment.
Identifying the drivers of community assembly has long been a central goal in ecology, and the development of functional diversity indices has provided a new way of detecting the influence of ...environmental gradients on biotic communities. For an old-growth Appalachian forest, we used path analysis to understand how patterns of tree functional diversity relate to topography and soil gradients and to determine whether topographic effects are mediated through soil chemistry. All of our path models supported the idea of environmental filtering: stressful areas (high elevation, low soil moisture, low soil nutrients) were occupied by communities of low functional diversity, which suggests a selective effect for species with traits adapted to such harsh conditions. The effects of topography (slope, aspect, elevation) on functional diversity were often indirect and moderated through soil moisture and fertility. Soil moisture was a key component of our models and was featured consistently in each one, having either strong direct effects on functional diversity or indirect effects via soil fertility. Our results provide a comprehensive view of the interplay among functional trait assemblages, topography, and edaphic conditions and contribute to the baseline understanding of the role of environmental filtering in temperate forest community assembly.
Invasive plant species' success may be a result of allelopathy, or the release of secondary metabolites that are harmful for surrounding plant species. Allelopathy can be mediated through the abiotic ...environment by chemical sorption or transformation, so the substrate on which interactions occur can lead to differential outcomes in allelopathic potential. One aggressive invader,
, has become dominant in many ecosystems throughout Eastern US, and has reduced the abundance of native species where it invades. Thus, our goal was to identify if
had allelopathic potential by testing the impact of leaf and flower leachate on gemination of six common grassland species (three grasses and three forbs) in either sterilized sand or field collected soils. Germination of five out of six tested species was reduced by
.
leaf litter, with weaker impacts from flower leachate. This suggests that allelopathy is one mechanism driving the success of
and that allelopathic effects may change with plant phenology. For instance,
has late leaf senescence in the fall and copious blooming in the spring that may elongate the timeframe that allelopathic inhibition can occur. Further, germination was higher in sand than in soil, suggesting that the context of the abiotic environment can mediate this relationship. In our study, two grass species that could be overabundant in restored grasslands had higher germination rates in soil than sand and one was not altered by
suggesting that this relationship could further promote the overabundance of grass species. Taken together,
likely inhibits the germination of native species where it invades, but there is context dependency of this relationship with both soil chemistry and seasonality.
Larix cajanderi forests, which occupy vast regions of Siberia, grow atop and protect carbon‐rich permafrost. Regeneration of these forests has important implications for long‐term feedbacks into the ...climate system and their regeneration is strongest following stand‐replacing fires. The goal of this project was to assess sources of regeneration limitation in L. cajanderi forests in northeastern Siberia. We focused on (1) regeneration potential of stands varying in tree density and (2) analyzing seedling establishment patterns in relationship to microsite conditions (safe sites) in the landscape. Seed sources were assessed through cone counts and stand surveys in the summers of 2017 and 2018 in 17 mature L. cajanderi stands. L. cajanderi recruitment patterns in relationship to safe site availability were assessed in 15 areas, spanning approximately 800 km2 along the northern portion of the Kolyma River (69.5477° N, 161.3641° E). Density of trees in a stand was negatively related to the number of cones that the average tree produced and stands of moderate density produced more cones per area than either high‐ or low‐density stands. L. cajanderi seedling establishment was facilitated by safe sites in the landscape. We discovered strong evidence that safe sites are considerably more important for seedling establishment in lowland sites than upland areas. The biological explanation for this pattern is presently unknown; however, we hypothesize this pattern is driven by persistently wet (marshy) soils in some lowland sites as a limiter of seedling establishment. Overall, these data suggest the potential for complex linkages between forest density, propagule availability, fire, safe sight colonization, and seedling establishment that may regulate long‐term dynamics in the understudied L. cajanderi forests of the Siberian Arctic.
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