The phenology of growth in temperate deciduous forests, including the timing of leaf emergence and senescence, has strong control over ecosystem properties such as productivity and nutrient cycling, ...and has an important role in the carbon economy of understory plants. Extended leaf phenology, whereby understory species assimilate carbon in early spring before canopy closure or in late autumn after canopy fall, has been identified as a key feature of many forest species invasions, but it remains unclear whether there are systematic differences in the growth phenology of native and invasive forest species or whether invaders are more responsive to warming trends that have lengthened the duration of spring or autumn growth. Here, in a 3-year monitoring study of 43 native and 30 non-native shrub and liana species common to deciduous forests in the eastern United States, I show that extended autumn leaf phenology is a common attribute of eastern US forest invasions, where non-native species are extending the autumn growing season by an average of 4 weeks compared with natives. In contrast, there was no consistent evidence that non-natives as a group show earlier spring growth phenology, and non-natives were not better able to track interannual variation in spring temperatures. Seasonal leaf production and photosynthetic data suggest that most non-native species capture a significant proportion of their annual carbon assimilate after canopy leaf fall, a behaviour that was virtually absent in natives and consistent across five phylogenetic groups. Pronounced differences in how native and non-native understory species use pre- and post-canopy environments suggest eastern US invaders are driving a seasonal redistribution of forest productivity that may rival climate change in its impact on forest processes.
AIM: A major implication of natural selection is that species from different parts of the world will vary in their efficiency in converting resources into offspring for a given type of environment. ...This insight, articulated by Darwin, is usually overlooked in more recent studies of invasion biology that are often based on the more modern Eltonian perspective of imbalanced ecosystems. We formulate a renewed Darwinian framework for invasion biology, the evolutionary imbalance hypothesis (EIH), based only on the action of natural selection in historically isolated populations operating within a global network of repeated environments. This framework predicts that successful invaders are more likely to come from biotic regions of high genetic potential (with independent lineages of large population size), experiencing a given environment for many generations and under strong competition from other lineages. LOCATION: Global. METHODS: We test the predictive power of this framework by examining disparities in recent species exchanges between global biotic regions, including patterns of plant invasions across temperate regions and exchanges of aquatic fauna as a result of modern canal building. RESULTS: Our framework successfully predicts global invasion patterns using phylogenetic diversity of the world's biotic regions as a proxy that reflects their genetic potential, historical stability and competitive intensity, in line with the Darwinian expectation. Floristic regions of higher phylogenetic diversity are more likely to be source areas of invasive plants, and regions of lower phylogenetic diversity are more likely to be invaded. Similar patterns are evident for formerly isolated marine or freshwater assemblages that have been connected via canals. MAIN CONCLUSIONS: We advocate an approach to understanding modern species invasions that recognizes the potential significance of both the original Darwinian explanation and the more modern view that emphasizes novel ecological or evolutionary mechanisms arising in the introduced range. Moreover, if biological invasions are a natural outcome of Darwinian evolution in an increasingly connected world, then invasive species should continue to displace native species and drive widespread shifts in the functioning of ecosystems.
Despite increasing evidence of the importance of intraspecific trait variation in plant communities, its role in community trait responses to environmental variation, particularly along broad-scale ...climatic gradients, is poorly understood. We analyzed functional trait variation among early-successional herbaceous plant communities (old fields) across a 1200-km latitudinal extent in eastern North America, focusing on four traits: vegetative height, leaf area, specific leaf area (SLA), and leaf dry matter content (LDMC). We determined the contributions of species turnover and intraspecific variation to between-site functional dissimilarity at multiple spatial scales and community trait responses to edaphic and climatic factors. Among-site variation in community mean trait values and community trait responses to the environment were generated by a combination of species turnover and intraspecific variation, with species turnover making a greater contribution for all traits. The relative importance of intraspecific variation decreased with increasing geographic and environmental distance between sites for SLA and leaf area. Intraspecific variation was most important for responses of vegetative height and responses to edaphic compared to climatic factors. Individual species displayed strong trait responses to environmental factors in many cases, but these responses were highly variable among species and did not usually scale up to the community level. These findings provide new insights into the role of intraspecific trait variation in plant communities and the factors controlling its relative importance. The contribution of intraspecific variation to community trait responses was greatest at fine spatial scales and along edaphic gradients, while species turnover dominated at broad spatial scales and along climatic gradients.
Studies of plant resource‐use strategies along environmental gradients often assume that dry matter partitioning represents an individual's energy investment in foraging for above‐ versus ...below‐ground resources. However, ecosystem‐level studies of total below‐ground carbon allocation (TBCA) in forests do not support the equivalency of energy (carbon) and dry matter partitioning, in part because allocation of carbon to below‐ground pools and fluxes that are not accounted for by root biomass (e.g., mycorrhizal hyphae, rhizodeposition; root and soil respiration) can be substantial. Here, we apply this reasoning to individual plants in controlled environments and ask whether dry matter partitioning below‐ground (root mass fraction, RMF) accurately reflects TBCA in studies of optimal partitioning theory.
We quantified the relationship between RMF and TBCA in individual plants, using 311 observations from 51 studies that simultaneously measured both allocation variables. Our analysis included tests of whether the RMF‐TBCA relationship depended on mutualist soil microbes, plant growth form, age and study methodology including isotopic pulse–chase duration.
We found that RMF was a poor proxy for below‐ground energy investment. This disconnect of RMF and TBCA was driven in part by plants of low RMF (<0.4) exhibiting significantly higher rates of root and soil respiration per unit root mass than plants of high RMF. Root colonization by mutualist microbes, including arbuscular mycorrhizal fungi and nitrogen‐fixing bacteria, increased TBCA by 5%–7%, and TBCA was lower in grasses than other species by 9%–16%. These patterns were evident for relationships assessed both within and between species.
We conclude that optimal partitioning studies of plants along environmental gradients are likely to underestimate plant energy allocation below‐ground if the C costs of root and soil respiration are ignored, especially under conditions favouring low RMF. Because energy rather than biomass better reflects how assimilated C supports fitness, this omission of respired C suggests existing studies misrepresent the significance of below‐ground processes to plant function.
A free Plain Language Summary can be found within the Supporting Information of this article.
A free Plain Language Summary can be found within the Supporting Information of this article.
Landscape-driven microclimates in mountainous terrain pose significant obstacles to predicting the response of organisms to atmospheric warming, but few if any studies have documented the extent of ...such finescale variation over large regions. This paper demonstrates that ground-level temperature regimes in Great Smoky Mountains National Park (Tennessee and North Carolina) vary considerably over fine spatial scales and are only partially linked to synoptic weather patterns and environmental lapse rates. A 120-sensor network deployed across two watersheds in 2005–06 exhibited finescale (<1000-m extent) temperature differences of over 2°C for daily minima and over 4°C for daily maxima. Landscape controls over minimum temperatures were associated with finescale patterns of soil moisture content, and maximum temperatures were associated with finescale insolation differences caused by topographic exposure and vegetation cover. By linking the sensor array data to 10 regional weather stations and topographic variables describing site radiation load and moisture content, multilevel spatial models of 30-m resolution were constructed to map daily temperatures across the 2090-km² park, validated with an independent 50-sensor network. Maps reveal that different landscape positions do not maintain relative differences in temperature regimes across seasons. Near-stream locations are warmer in the winter and cooler in the summer, and sites of low elevation more closely track synoptic weather patterns than do wetter high-elevation sites. This study suggests a strong interplay between near-ground heat and water balances and indicates that the influence of past and future shifts in regional temperatures on the park’s biota may be buffered by soil moisture surfeits from high regional rainfall.
1. In temperate deciduous forests, understorey light environments vary dramatically throughout spring, summer and autumn due to tree canopy leaf display. This variability in light level is a ...physiological challenge for understorey species that produce sun-adapted leaves in the spring before being shaded by the tree canopy. Similarly, some understorey species display leaves late into autumn after the tree canopy senesces. 2. Many species in North American deciduous forests with extended leaf display are not native to North America. Since many non-native species have been shown to have greater plasticity than natives, we hypothesized that leaves of non-native species may be more plastic with respect to seasonal light changes than natives, and that this plasticity may allow them to assimilate more carbon in the same environment. 3. We measured leaf traits and photosynthetic capacity of 17 native and 13 nonnative understorey shrub species in a common garden in Syracuse, New York, during spring, summer and autumn. We tested for the contribution of seasonal mean and variance (plasticity) of leaf traits to a species' average photosynthetic rate and total leaf production. We also analysed the extent to which leaf adjustments depended on whether plants continued to produce new leaves over the growing season. 4. Leaf traits of both native and non-native species varied seasonally, but plasticity varied in extent and contribution to overall carbon gain. Non-native species had the highest seasonal plasticity, but, contrary to our hypothesis, such plasticity did not contribute to their overall carbon gain. However, leaf trait plasticity was adaptive for native species that continued to produce leaves throughout the year, primarily due to increases in quantum efficiency and electron transport rate in leaves produced mid-year compared to leaves produced early in the year. 5. Synthesis. Despite large adjustments in leaf traits across seasonal light environments for both native species and non-native species, we found little evidence that leaf-level plasticity drives non-native invasion or contributes strongly to annual carbon gain or productivity in understorey species. Instead, differences in mean leaf traits across seasons are sufficient to explain carbon gain advantages of non-native woody species in deciduous forests.
Studies of whether plant community structure and ecosystem properties depend on the genetic diversity of component populations have been largely restricted to species monocultures and have involved ...levels of genetic differentiation that do not necessarily correspond to that exhibited by neighboring mature individuals in the field. We established experimental communities of varying intraspecific genetic diversity, using genotypes of eight species propagated from clonal material of individuals derived from a small (100-m
2
) limestone grassland community, and tested whether genetic diversity (one, four, and eight genotypes per species) influenced community composition and annual aboveground productivity across communities of one, four, and eight species. Eight-species communities were represented by common grass, sedge, and forb species, and four- and one-species communities were represented by four graminoids and the dominant grass
Festuca ovina
, respectively. After three years of community development, there was a marginal increase of species diversity with increased genetic diversity in four- and eight-species communities, and genetic diversity altered the performance of genotypes in monospecific communities of
F. ovina
. However, shifts in composition from genetic diversity were not sufficient to alter patterns of community productivity. Neighborhood models describing pairwise interactions between species indicated that genetic diversity decreased the intensity of competition between species in four-species mixtures, thereby promoting competitive equivalency and enhancing species equitability. In
F. ovina
monocultures, neighborhood models revealed both synergistic and antagonistic interactions between genotypes that were reduced in intensity on more stressful shallow soils. Although the dependence of
F. ovina
genotype performance on neighborhood genetic composition did not influence total productivity, such dependence was sufficient to uncouple genotype performance in genetic mixtures and monocultures. Our results point to an important connection between local genetic diversity and species diversity in this species-rich ecosystem but suggest that such community-level dependence on genetic diversity may not feedback to ecosystem productivity.
Aim
Species’ climatic niches may be poorly predicted by regional climate estimates used in species distribution models (SDMs) due to microclimatic buffering of local conditions. Here, we compare SDMs ...generated using a locally validated below‐canopy microclimate model to those based on interpolated weather station data at two spatial scales to determine the effects of scale and topography on potential future below‐canopy warming and species distributions.
Location
Great Smoky Mountains National Park (2,090 km2; NC, TN, USA).
Time period
1970–2006, late‐century warming.
Major taxa studied
Vascular plant species of Southern Appalachian forests.
Methods
We compared the fit and spatio‐temporal predictions of SDMs generated using occurrence records of 154 plant species and three climate models: macroclimate (1 km, WorldClim), downscaled climate (based on a 30‐m digital elevation model), and fine‐scale microclimate (30 m) from a below‐canopy sensor network.
Results
We found that, although SDM fit was similar across models, microclimate‐derived SDMs predicted substantially greater species persistence with 4°C of regional warming, with a difference of 50% of the species pool in some areas. Microclimate models predicted that warming trends will be buffered in high‐elevation and near‐stream habitats. Microclimate SDMs predicted higher stability of mid‐elevation species, particularly in thermally buffered areas near streams, and critically, less change in species composition at high elevation. In contrast, predictions of macroclimate and downscaled climate models were similar despite improved resolution.
Main conclusions
Our results demonstrate that careful selection of climate drivers, including local near‐ground validation rather than downscaling solely with elevation, is critical for projecting distributions. They also suggest that some species at risk from climate change might persist, even with 4°C of macroclimate warming, in cryptic refugia buffered by microclimate, pointing to the roles of forest cover and topography in explaining slower‐than‐expected changes in understorey communities. However, certain species, such as those currently occurring on low‐elevation ridges that are sensitive to atmospheric changes, may be at more risk than macroclimate or downscaled climate SDMs suggest.
Biogeographic patterns of species invasions hold important clues to solving the recalcitrant 'who', 'where', and 'why' questions of invasion biology, but the few existing studies make no attempt to ...distinguish alien floras (all non-native occurrences) from invasive floras (rapidly spreading species of significant management concern), nor have invasion biologists asked whether particular habitats are consistently invaded by species from particular regions.
Here I describe the native floristic provenances of the 2629 alien plant taxa of the Eastern Deciduous Forest of the Eastern U.S. (EUS), and contrast these to the subset of 449 taxa that EUS management agencies have labeled 'invasive'. Although EUS alien plants come from all global floristic regions, nearly half (45%) have native ranges that include central and northern Europe or the Mediterranean (39%). In contrast, EUS invasive species are most likely to come from East Asia (29%), a pattern that is magnified when the invasive pool is restricted to species that are native to a single floristic region (25% from East Asia, compared to only 11% from northern/central Europe and 2% from the Mediterranean). Moreover, East Asian invaders are mostly woody (56%, compared to just 23% of the total alien flora) and are significantly more likely to invade intact forests and riparian areas than European species, which dominate managed or disturbed ecosystems.
These patterns suggest that the often-invoked 'imperialist dogma' view of global invasions equating invasion events with the spread of European colonialism is at best a restricted framework for invasion in disturbed ecosystems. This view must be superseded by a biogeographic invasion theory that is explicitly habitat-specific and can explain why particular world biotas tend to dominate particular environments.
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