Mesic Trees Research Articles

Developing plant functional groups to identify changes in functional composition and diversity in a dryland river experiencing artificially sustained flows

Land use and climate changes are driving significant shifts in the magnitude and persistence of dryland stream surface flows. The impact of these shifts on ecological functioning is largely unknown, particularly where streams have become wetter rather than drier. This study investigated relationships between hydrologic regime (including surface water persistence, differences in groundwater depth and altered flooding dynamics) with plant traits and riverine vegetation functional composition. Our study system was a previously ephemeral creek in semi-arid northwest Australia that has received groundwater discharge from nearby mining operations for >15 years; surface flows are now persistent for ∼27 km downstream of the discharge point. We aimed to (i) identify plant functional groups (FGs) associated with the creek and adjacent floodplain; and (ii) assess their distribution across hydrological gradients to predict shifts in ecological functioning in response to changing flow regimes. Seven FGs were identified using hierarchical clustering of 40 woody perennial plant species based on morphometric, phenological and physiologic traits. We then investigated how FG abundance (projective foliar cover), functional composition, and functional and taxonomic richness varied along a 14 km gradient from persistent to ephemeral flows, varying groundwater depths, and distances from the stream channel. Dominant FGs were (i) drought avoidant mesic trees that are fluvial stress tolerant, or (ii) drought tolerant xeric tall shrubs that are fluvial stress intolerant. The drought avoidant mesic tree FG was associated with shallow groundwater but exhibited lower cover in riparian areas closer to the discharge (persistent surface flows). However, there were more FGs and higher species richness closer to the discharge point, particularly on the floodplain. Our findings demonstrate that quantifying FG distribution and diversity is a significant step in both assessing the impacts of mine water discharge on riverine ecosystems and for planning for post-mining restoration.

Evidence of mesophication following selective cutting and shelterwood in Virginia’s eastern deciduous forest, USA

The replacement of oaks (Quercus) with mesic tree species in the eastern deciduous forests of North America involves a positive feedback loop being driven by fire suppression and climate change. In this study, we explored the impact of timber harvesting as a potential facilitator of mesophication. Young stands resulting from shelterwoods (on federal lands) and selective cutting (on private lands) were sampled to compare the composition and density of seedlings, saplings, and trees in western Virginia, USA. Oak overstory trees had lower importance values and densities at selective cutting sites compared to shelterwood sites. Among saplings, mesic species had higher importance values and densities than oaks at both selective cutting and shelterwood sites. Evidence of mesophication was found at both selective cutting and shelterwood sites; but a higher level of mesophication appeared to be occurring at the selective cutting sites than at shelterwood sites. At selective cutting sites scarlet oak (Quercus coccinea) was absent from all sampled plots and a dense midstory of subcanopy trees, especially eastern redbud (Cercis canadensis), appeared to be hindering successful oak regeneration. Our results indicate timber harvesting, without regard for the composition of the regeneration layer, may facilitate mesophication in the eastern deciduous forest, with selective cutting resulting in more successful establishment of mesic species compared to shelterwoods. These findings have implications for mesophication on private forest lands, where selective cutting is a very common method of timber harvesting.

Palaeoenvironmental features of the early Middle Pleistocene site of Wadi Sarrat in NW Tunisia (Northern Africa)

This paper focuses on pollen and non-pollen palynomorphs (NPPs) analysis in a sequence from the archaeo-palaeontological site of Wadi Sarrat (Tunisia). The series comprises a lapse time of Early-Middle Pleistocene vegetation succession and the evolution of the past lacustrine system of Wadi Sarrat. Some changes in the pollinic taxa are observed throughout the sequence in the region: fluctuations between Mediterranean (evergreen Quercus-type, Olea/Phillyrea, Pistacia, CISTACEAE, LAMIACEAE, CUPRESSACEAE, Pinus), mesic trees (deciduous Quercus-type, Myrica/Corylus, Alnus, Ulmus, Betula, Salix,Acer), hygro-hydrophytes (CYPERACEAE,Typha/Sparganium,Myriophyllum, PTERIDOPHYTA), and desert taxa (AMARANTHACEAE, APIACEAE, LACTUCEAE - ASTERACEAE, Ephedra, Asphodelus, CISTACEAE, Cerealia-type, BORAGINACEAE). The assemblages contained many desert elements (mostly herbaceous taxa) that occupy the high plateau and some trees that must come from the regional mountains or the banks of the lake. The NPPs and pollen analysis have allowed us to determine a cyclicity of nine events or pluvial phases, with tendencies to increase or decrease step by step. Taxa such as algae, Pseudoschizaea and Arcella have been of great help in the determination of the pluvial cycles.

Stand and environmental conditions drive functional shifts associated with mesophication in eastern US forests

There is a growing body of evidence that mesic tree species are increasing in importance across much of the eastern US. This increase is often observed in tandem with a decrease in the abundance and importance of species considered to be better adapted to disturbance and drier conditions (e.g.,Quercusspecies). Concern over this transition is related to several factors, including the potential that this transition is self-reinforcing (termed “mesophication”), will result in decreased resiliency of forests to a variety of disturbances, and may negatively impact ecosystem functioning, timber value, and wildlife habitat. Evidence for shifts in composition provide broad-scale support for mesophication, but we lack information on the fine-scale factors that drive the associated functional changes. Understanding this variability is particularly important as managers work to develop site-and condition-specific management practices to target stands or portions of the landscape where this transition is occurring or is likely to occur in the future. To address this knowledge gap and identify forests that are most susceptible to mesophication (which we evaluate as a functional shift to less drought or fire tolerant, or more shade tolerant, forests), we used data from the USDA Forest Service Forest Inventory and Analysis program to determine what fine-scale factors impact the rate (change through time) and degree (difference between the overstory and midstory) of change in eastern US forests. We found that mesophication varies along stand and environmental gradients, but this relationship depended on the functional trait examined. For example, shade and drought tolerance suggest mesophication is greatest at sites with more acidic soils, while fire tolerance suggests mesophication increases with soil pH. Mesophication was also generally more pronounced in older stands, stands with more variable diameters, and in wetter sites, but plots categorized as “hydric” were often highly variable. Our results provide evidence that stand-scale conditions impact current and potential future changes in trait conditions and composition across eastern US forests. We provide a starting point for managers looking to prioritize portions of the landscape most at risk and developing treatments to address the compositional and functional changes associated with mesophication.

Holocene water levels of Silver Lake, Montana, and the hydroclimate history of the Inland Northwest

AbstractThe wettest portion of the interior of western North America centers on the mountainous region spanning western Montana, Idaho, British Columbia, and Alberta. Inland ranges there capture the remnants of Pacific storms. Steep east–west hydroclimate gradients make the region sensitive to changes in inland-penetrating moisture that may have varied greatly during the Holocene. To investigate potential hydroclimate change, we produced a 7600-yr lake-level reconstruction from Silver Lake, located on the Montana–Idaho border. Ground-penetrating radar profiles and a transect of four shallow-water sediment cores that were dated using radiocarbon dating and tephrachronology revealed substantial changes in moisture through time. An organic-rich mud unit indicating wet and similar to modern conditions prior to 7000 cal yr BP is overlain by an erosional surface signifying drier than modern conditions from 7000–2800 cal yr BP. A subsequent time-transgressive increase in water levels from 2800–2300 cal yr BP is indicated by a layer of late Holocene muds, and is consistent with glacier expansion and increases in the abundance of mesic tree taxa in the region. Millennial-scale trends were likely driven by variations in orbital-scale forcing during the Holocene, but the regional outcomes probably depended upon factors such as the strength of the Aleutian Low, Pacific sea-surface temperature variability, and the frequency of atmospheric rivers over western North America.

Pre-Emptive Detection of Mature Pine Drought Stress Using Multispectral Aerial Imagery

Drought, ozone (O3), and nitrogen deposition (N) alter foliar pigments and tree crown structure that may be remotely detectable. Remote sensing tools are needed that pre-emptively identify trees susceptible to environmental stresses could inform forest managers in advance of tree mortality risk. Jeffrey pine, a component of the economically important and widespread western yellow pine in North America was investigated in the southern Sierra Nevada. Transpiration of mature trees differed by 20% between microsites with adequate (mesic (M)) vs. limited (xeric (X)) water availability as described in a previous study. In this study, in-the-crown morphological traits (needle chlorosis, branchlet diameter, and frequency of needle defoliators and dwarf mistletoe) were significantly correlated with aerially detected, sub-crown spectral traits (upper crown NDVI, high resolution (R), near-infrared (NIR) Scalar (inverse of NDVI) and THERM Δ, and the difference between upper and mid crown temperature). A classification tree model sorted trees into X and M microsites with THERM Δ alone (20% error), which was partially validated at a second site with only mesic trees (2% error). Random forest separated M and X site trees with additional spectra (17% error). Imagery taken once, from an aerial platform with sub-crown resolution, under the challenge of drought stress, was effective in identifying droughted trees within the context of other environmental stresses.

Humans, fire, and ecology in the southern Missouri Ozarks, USA

A multiproxy study from Sweeton Pond, Ozark County, Missouri, USA, provides a high-resolution 1900-year-long history of vegetation and fire in the southern Missouri Ozarks, where the modern vegetation is oak-hickory ( Quercus-Carya) forest. Pollen and charcoal data are compared with dendroecological data to assess how climate and fire shaped local vegetation history. Land use, particularly by the Osage tribe of Native Americans, is assessed from historical and archaeological records. Three cultural periods are superimposed on the paleoenvironmental history: (1) The Pre-Osage period, ending ~1500 CE, was characterized by open oak-hickory forest and frequent low-severity fires, suggesting interannual climate variability as a driver of vegetation and fire occurrence. At ~1360 CE, mesic tree species began to expand, while fire frequency remained low. (2) The Osage period (~1500–1820 CE) was characterized by the continued expansion of mesic, fire-sensitive species, especially elm ( Ulmus), in conjunction with cool, effectively wet conditions in the southern Missouri Ozarks. Despite climate conditions less favorable for fire, Osage expansion in the region was accompanied by increased fire and fire-dependent shortleaf pine ( Pinus echinata). The expansion of both fire-sensitive and fire-dependent taxa coincident with Osage occupation suggests that anthropogenic fire and land use was local in nature and increased landscape heterogeneity prior to Euro-American settlement. (3) The Euro-American period (since ~1820 CE) was characterized by increased disturbance pollen types (e.g. Ambrosia-type) at the expense of shortleaf pine pollen, resulting from increased settlement size and extensive agricultural and logging activities. During this period, forest clearance led to fuel fragmentation, reducing fire activity; after 1920 CE, fire was actively suppressed.

Tree water use strategies and soil type determine growth responses to biochar and compost organic amendments

Background and aimsOrganic matter is often used as an amendment to attempt restoration of degraded soils to improve tree establishment and growth. One key aim is to increase plant available water in the soil profile. The texture of the soil, the type of organic amendment (e.g. compost or biochar), and the native environment of the tree (mesic or xeric) will impact how successful restoration efforts are. We aimed to determine whether compost and biochar amendments, either individually or in combination, would improve plant available water (PAW) in both clay and sand soils. We then aimed to measure whether changes in PAW would translate into increased water use and plant growth of both mesic (Corymbia maculata) or xeric (Eucalyptus torquata) tree species under well-watered (WW) or water deficit (WD) conditions. MethodsClay and sand soils were amended with compost, biochar or a compost and biochar mix, whilst unamended soils acted as controls. Soil moisture characteristics of the soil mixes were determined with samples in the laboratory. Fifteen replicate pots (6 l) of each soil treatment were then planted with either mesic and xeric tree species. All pots and trees were subjected to either a WW or WD irrigation regime for 7 weeks. WD irrigation was a set percentage of the daily WW evapotranspiration (ET). ET was calculated as total pot mass one hour after irrigation (to allow for drainage) minus the total pre-dawn pot mass of the subsequent day. The tree biomass, biomass partitioning, ET, and tree water status were measured throughout the experiment to understand growth and stress responses. Key resultsThe xeric tree species (E. torquata) grown in sand soil had a significant increase in growth with all three OM amendments but did not when grown in clay soil. In contrast, we found no significant growth response for the mesic tree species (C. maculata) when grown in either clay or sand amended with any OM treatment. The ET of the xeric tree species was greater for all the OM amended soils for both WW and WD plants in the sand soil. ConclusionsThis study shows that OM amendments may improve the soil water properties of sand-based soils which in turn can increase the growth of xeric tree species. However, more expensive organic amendments may not be necessary, nor mixing of OM types, as we found no tree growth differences amongst the three different OM treatments.

Linking conservation implications of modified disturbance regimes, plant communities, plant associations, and arthropod communities.

Modifications to disturbance regimes have landscape-level effects on plant communities and have the potential to influence organisms at multiple trophic levels. We examined differences in the arthropod community across a gradient of oak/hickory dominance, a plant community maintained by disturbance such as periodic fires and extensive land clearing. In southern Illinois, we used patches of forest that varied in tree dominance ranging from 94 to 0% oak/hickory composition dependent on prior land usage that occurred > 50years ago at minimum, to test two predictions: (1) oak (Quercus) and hickory (Carya) species contain more arthropod biomass and diversity than mesic tree species [e.g., American beech (Fagus grandifolia) and maples (Acer spp.)] and (2) due to plant associations, arthropod communities are more diverse and abundant on host trees within oak/hickory stands than non-oak/hickory stands. Our results were consistent with the prediction that arthropod biomass, guild Shannon diversity, and guild richness are higher on oaks, hickories and tulip tree (Liriodendron tulipifera) than beech and maples. We also found support for the prediction that due to plant associations, % non-oak/hickory stand composition negatively influenced arthropod guild Shannon diversity and guild richness on host trees, including maples and beech. These results are the first to demonstrate that modified disturbance regimes can influence multiple trophic levels both directly due to species-specific variation in susceptibility of plants to herbivory and indirectly through effects of plant associations. This result is concerning as modified disturbance regimes are influencing large-scale plant community composition among biomes worldwide.

Late Holocene environmental dynamics, vegetation history, human impact, and climate change in the ancient Literna Palus (Lago Patria; Campania, Italy)

A new late-Holocene high-resolution pollen record is presented from Lago Patria with the aim of investigating past vegetation and environmental dynamics in a still under-investigated coastal sector of the Campania region (Italy). Our results show the evolution of a mixed deciduous and evergreen oak-dominated lowland forest, rich in both xeric and mesic woody taxa, under the influence of climate, human impact, geomorphic processes and their interplay. Between 4800 and 2800 cal BP, the pollen record highlights only slight vegetation changes, featured by a modest forest decline around 4200 cal BP, consistent with the deforestation pattern produced by the 4.2 ka arid climate event in southern and central Italy. This event was followed, between 3900 and 3300 cal BP, by a forest recovery with a remarkable development of mesic trees, notably Fagus, influenced by wet climate conditions. While Bronze Age and Iron Age settlements are missing from this coastal area, which suggests a marginal role of human activity in the environmental changes of this period, the continuous presence of anthropogenic pollen indicators in the pollen record stimulates new archeological investigations. Between 2800 and 2200 cal BP, the area experienced successive domination by Greek, Etruscan, Italic and Roman populations, culminating with the foundation and expansion of cities like Cumae, Capua, and Dicearchia/Puteoli, which determined major impact on the natural forest through intense agricultural practices. After a sedimentation hiatus between 2200 and 180 BP, the pollen record documents a few tens of years, during the Bourbon domination, when the vegetational landscape, characterized by cultivations and pasturelands, appears almost completely treeless. Pollen and other palynomorphs of aquatic taxa evidence a change from freshwater to brackish conditions, consistent with the transition from cat's eye freshwater ponds to open lagoon.

The potential of paleoecology for functional forest restoration planning: lessons from Late Holocene Italian pollen records

We describe forest landscape transformations during the last two millennia in the Italian peninsula by analyzing local (Rieti basin – Lago Lungo) and regional (RF93-30 Adriatic Sea) sediment cores. We identify a dynamic forest ecosystem through paleoecologic reconstruction and consider potential interventions for effective restoration of the most ancient, least disturbed forest ecosystem. The most degraded ecosystems in consequence of human activities were hygrophilous (wet) and mesic forests. In the Rieti Basin, degraded forest ecosystems on mountain slopes are undergoing some degree of forest succession and have less need of restoration. However, management plans for biodiversity, ecosystem services and resources conservation are needed to achieve more sustainable development. In Rieti, the paleoecological investigation revealed a dramatic decrease of deciduous wet and mesic tree taxa through time due to human landscape transformation. The starting point for restoration of a Mediterranean forest ecosystem that preserves natural biodiversity and associated ecosystem services requires recreating some portion of the floodplain wetland ecological niche. Once floodplain forest ecological niche has been recreated, the original ecosystem composed of Alnus, Fraxinus excelsior, Tilia spp., Carpinus betulus and Acer spp., all species which today are rare, should be planted on the basis of microsite characteristics and tree autoecology.

Pronounced variations in Fagus grandifolia abundances in the Great Lakes region during the Holocene

Mesic tree species such as Fagus grandifolia and Tsuga canadensis experienced multiple abundance declines in eastern North America during the last 8000 years, but the causes remain unclear. This paper presents a new sub-centennial record of Holocene vegetation, fire and sedimentological changes at Spicer Lake, IN, to test hypotheses about the role of fire and hydrological variations on shifts in vegetation composition. Four pollen zones are reported: Abies–Picea forests (15–11.8 ka BP), Pinus-dominated mixed forest (11.8–10.6 ka BP), transitional mixed forest (10.6–6.8 ka BP), and deciduous forest characterized by the expansion and high variability of F. grandifolia (after 6.8 ka BP). Macroscopic charcoal indicates five to seven fires between 6.1 and 4.4 ka BP and no fires between 4.4 and 2 ka BP, despite several large declines in F. grandifolia, and more fires after 1.8 ka BP likely linked to declining F. grandifolia abundances after 1.1 ka BP. Six peaks in mineralogenic sediments are suggestive of hydroclimate variability, but do not consistently correspond to shifts in F. grandifolia abundances. A Bayesian change-point analysis of 15 regional F. grandifolia pollen records identifies peak probabilities of events at 4.8 and 1.1 ka BP, similar in timing to variations in T. canadensis at other sites. Hence, fire can be ruled out as a driver of the mid-Holocene declines of F. grandifolia, but more work is needed to confidently establish the regional timing of F. grandifolia declines and to link them to past droughts and T. canadensis declines in eastern North America.

Which tree species and biome types are most vulnerable to climate change in the US Northern Rocky Mountains?

The goal of this study was to assess components of vulnerability of tree species and biome types to projected future climate within the Great Northern Landscape Conservation Cooperative (GNLCC) in the US Northern Rockies and the ecosystems surrounding Glacier and Yellowstone/Grand Teton National Parks. We drew on the results of five published studies and analyzed current and projected future climate suitability for 11 tree species and 8 biome types under two IPCC emissions scenarios. We assessed components of vulnerability based on four metrics of current and projected future climate suitability. Results for biome types indicated largely a shift from climates suitable for alpine and subalpine conifer to climates suitable for desert scrub and grassland types. Results from the four studies of tree species indicated substantial loss of area of climate suitability for the four subalpine species by 2100. This was especially true for Whitebark pine (Pinus albicaulis). Suitable climate for this species dropped from just over 20% of the study area in the reference period to 0.5–7.0% by 2070–2100 under the A2 scenario. The studies agreed in projecting expansion of climate suitability for some montane tree species but disagreed on expansion of climate suitability of west-side mesic tree species to eastside locations such as Yellowstone National park. Importantly, the rankings of tree species vulnerability were similar among studies, scenarios, and geographic areas and indicated highest vulnerability for Whitebark pine and Mountain hemlock (Tsuga mertensiana). The results should help federal managers in the GNLCC prioritize tree species for climate adaptation strategies. Moreover, our methods for using published data as a basis for climate vulnerability assessment can be applied within other LCCs across the US and other management units internationally.

Charcoal and pollen analysis: Examples of Holocene fire dynamics in Mediterranean Iberian Peninsula

The present study focuses on charcoal particles (a component of vegetal ashes, Bodí et al., 2014) counted during palynological analysis. The absolute frequencies of this microcharcoal have been correlated with pollen percentages of resprouts, mesic trees, Pinus or AP (arboreal pollen), in order to observe patterns in the spread of wildfires, dynamic changes in plant succession, and possible cyclicities in Holocene microclimatic changes. The sites selected for this work are located within the Mediterranean area of the Iberian Peninsula and in the Eivissa Balearic Islands, between 42°16′ N and 36°47′ N. Results show two brief changes in vegetation dynamics during the Holocene. The first coincides with the 8.2calkyr event and marks both the beginning of Holocene dynamic changes in plant succession and the Climatic Optimum. During this first phase, charcoal peaks coincide with minimum percentages of mesic taxa (low rainfall) and occur just before an increase of resprout taxa (taxa which grow after fires). The second change happens at the ~4.2calBPkyr event, when fires are most frequent and resprout taxa increase their values compared to mesic taxa and forest cover (AP). We have also observed some differences in vegetal dynamics studied at different latitudes, which appear to be punctuated by climate or insularity. In short, (i) actual succession of Mediterranean vegetation can be recognized in the past; (ii) resilience in vegetation is more common during the first half of the Holocene than in the recent Holocene: A trend to aridity, reduced biomass and anthropization has caused vegetation resilience to decline; (iii) synchronicity between fire peaks of different sequences studied is very rare; and (iv) coincidence of many peaks of charcoal with Bond events inclines us to think that fires are more conditioned by the climate than by anthropogenic impact.

Competitive effect and response of savanna tree seedlings: Comparison of survival, growth and associated functional traits

Questions What is the effect of neighbour competition on the survival, growth and biomass of mesic and humid savanna tree species? Can competitive effect and response be linked to plant functional traits? Location Neil Tainton Arboretum, University of KwaZulu-Natal, Pietermaritzburg, ZA. Methods Using a target–neighbour design, all combinations of four humid and four mesic savanna tree seedlings were tested in a greenhouse experiment, to establish the effects of neighbourhood competition on target plant performance. The competitive response and effect of each species was quantified, and regressed against several functional traits to determine which traits are predictive of competitive ability. Results We found that neighbour density negatively affected the survival of mesic tree seedlings but not humid tree seedlings. Mesic plants were able to maintain their relative growth rates (RGRs) despite increasing neighbourhood competition. The RGRs of humid species significantly decreased as the density of neighbours increased, indicating that competition may be a factor affecting plants in savannas receiving more than 650 mm mean annual rainfall. We used the quantile regression method to show that the biomass of humid tree seedlings decreased with increased neighbour biomass, when considering competitive response only. We also found that resource-acquiring traits (such as maximum RGR, plant height and root biomass) were positively correlated with the competitive effect of mesic savanna seedlings, while specific leaf area captured the competitive response of humid savanna seedlings to neighbour competition. Conclusions Competitive interactions between savanna trees negatively affect survival and growth rate, although the competitive effect or response of a particular species depends on its adaptation to either high- or low-resource environments. Our results show significant relationships between plant traits and competitive ability, which may be used to predict competitive interactions between tree seedlings from humid and mesic savannas.

Competitive effect and response of savanna tree seedlings: comparison of survival, growth and associated functional traits

AbstractQuestionsWhat is the effect of neighbour competition on the survival, growth and biomass of mesic and humid savanna tree species? Can competitive effect and response be linked to plant functional traits?LocationNeil Tainton Arboretum, University of KwaZulu‐Natal, Pietermaritzburg, ZA.MethodsUsing a target–neighbour design, all combinations of four humid and four mesic savanna tree seedlings were tested in a greenhouse experiment, to establish the effects of neighbourhood competition on target plant performance. The competitive response and effect of each species was quantified, and regressed against several functional traits to determine which traits are predictive of competitive ability.ResultsWe found that neighbour density negatively affected the survival of mesic tree seedlings but not humid tree seedlings. Mesic plants were able to maintain their relative growth rates (RGRs) despite increasing neighbourhood competition. The RGRs of humid species significantly decreased as the density of neighbours increased, indicating that competition may be a factor affecting plants in savannas receiving more than 650 mm mean annual rainfall. We used the quantile regression method to show that the biomass of humid tree seedlings decreased with increased neighbour biomass, when considering competitive response only. We also found that resource‐acquiring traits (such as maximum RGR, plant height and root biomass) were positively correlated with the competitive effect of mesic savanna seedlings, while specific leaf area captured the competitive response of humid savanna seedlings to neighbour competition.ConclusionsCompetitive interactions between savanna trees negatively affect survival and growth rate, although the competitive effect or response of a particular species depends on its adaptation to either high‐ or low‐resource environments. Our results show significant relationships between plant traits and competitive ability, which may be used to predict competitive interactions between tree seedlings from humid and mesic savannas.

Measured and modelled leaf and stand‐scale productivity across a soil moisture gradient and a severe drought

Environmental controls on carbon dynamics operate at a range of interacting scales from the leaf to landscape. The key questions of this study addressed the influence of water and nitrogen (N) availability on Pinus palustris (Mill.) physiology and primary productivity across leaf and canopy scales, linking the soil-plant-atmosphere (SPA) model to leaf and stand-scale flux and leaf trait/canopy data. We present previously unreported ecophysiological parameters (e.g. V(cmax) and J(max)) for P. palustris and the first modelled estimates of its annual gross primary productivity (GPP) across xeric and mesic sites and under extreme drought. Annual mesic site P. palustris GPP was ∼23% greater than at the xeric site. However, at the leaf level, xeric trees had higher net photosynthetic rates, and water and light use efficiency. At the canopy scale, GPP was limited by light interception (canopy level), but co-limited by nitrogen and water at the leaf level. Contrary to expectations, the impacts of an intense growing season drought were greater at the mesic site. Modelling indicated a 10% greater decrease in mesic GPP compared with the xeric site. Xeric P. palustris trees exhibited drought-tolerant behaviour that contrasted with mesic trees' drought-avoidance behaviour.

Commentary: Ecological complexity at the forest–grassland transition revealed by lake sediment records

news and update ISSN 1948-6596 commentary Ecological complexity at the forest–grassland transition revealed by lake sediment records Understanding how ecological processes play out over time scales relevant to long-lived species such as trees, and through periods of significant climate change, remains a major hurdle in trans- lating climate-change projections into biological impacts. Over the past few decades, paleoecolo- gists have embraced the prospect of using sepa- rate lines of evidence of climate, vegetation, and disturbance from sediment records to assess bio- geographic responses to climate change. Well- known complexities in responses to climate change include positive feedbacks within ecosys- tems, lag times, and the possibility of multiple po- tential states. This requires paleoecologists to have a robust understanding of past climate, and thus they have been thrust into the field of paleo- climatology. As there are multiple approaches to reconstructing past climate, researchers are often faced with the task of reconciling their climatic reconstructions with a myriad of existing paleocli- mate studies. In a new paper (Shuman et al. 2009), a cli- mate reconstruction based on fluctuating lake lev- els found evidence of a severe drought concurrent with a non-intuitive vegetation response. Shuman’s group studied the paleoclimatic context for the development of the Big Woods, Minessota, a large patch of mesic-adapted forest trees that occurred at the forest–grassland transition at the time of pre-European settlement but is now largely converted to agriculture and suburbia. Earlier paleoecological studies had determined that open woodland and grassland began being replaced by Big Woods vegetation at ca. 1300 AD, concurrent with a decrease in fire and, assumed but never addressed explicitly, increased moisture that favored tree establishment. The new study, however, claims that lake levels, and thus regional moisture, were lower during the Big Woods devel- opment than at any other time during the past 1800 years. The authors also show, using age dis- tributions of trees, that a comparable drought during the 1930’s was not severe enough to pre- clude establishment of mesic tree species such as sugar maple. The “ecological surprise” from these findings is that the local-scale controls of the for- est–grassland transition may operate, at times, in the opposite direction from the regional moisture gradient that explains the broad-scale pattern of the forest-to-grassland transition. The mechanisms underlying the Big Woods establishment, if indeed it happened during drought, require rethinking. Shuman et al. (2009) invoke a previously published hypothesis (Umbanhowar 2004) that fire may have declined because of drought-caused fuel discontinuity, and thus increasing fire-free intervals permitting re- cruitment of Big Woods tree species. This hy- pothesis fits broadly with the assertion that over large expanses of the planet where productivity is marginally sufficient for forest, the observed vege- tation is largely determined by fire and herbivores (Bond & Keeley 2005). The primary role of distur- bance over climate in these areas leaves wiggle- room for complexities including processes that reverse the climate–vegetation relationship at small spatial scales. One of these complexities not discussed with respect to the Big Woods is the effect of fluctuating populations of grazers, such as bison. The influence of grazer populations on the forest–grassland transition may be as great or greater than other factors, and because pa- leoecologists rarely discuss grazing impacts, they represent an “elephant in the room” in the inter- pretation of these records (Craine & McLauchlan 2004). For example, increased grazing pressure during drought may have been required to reduce fuel continuity and fire extent. Of course, devel- oping sediment proxies of grazer populations, such as from spores of the dung-fungus Sporormiella, would further the discussion of these dynamics (e.g., Gill et al. 2009; see also the update by Koch in this issue). The search for concordance among paleocli- mate records is a difficult task because of uncer- tainties in sediment chronologies, in the climate © 2010 the authors; journal compilation © 2010 The International Biogeography Society — frontiers of biogeography 2.1, 2010

Late Pleistocene and Holocene environments of the Southeastern United States from the stratigraphy and pollen content of a peat deposit on the Georgia Coastal Plain

A 5.4 m peat core from the Sandy Run Creek (SRC) valley in the upper Coastal Plain of Georgia shows that before 30 ka the valley had an aggradational sandy floor with shallow swales and low ridges of 1–2 m amplitude suggesting a braided stream pattern and a low water table. The climate from 30–25 ka was cool and dry and the vegetation open grassland with stands of pine and spruce that produced few fires. At ca. 16 ka a warmer, wetter climate caused SRC to meander and incise the valley fill removing some previously deposited sediment at the site of our peat core so that sediments of 13 ka rest directly on sediments deposited at 25 ka. After ca. 13 ka higher groundwater levels initiated vertical accretion of floodplains allowing peat accumulation in shallow depressions across the valley. Pollen from the Younger Dryas period (ca. 13–11 ka) indicates a cool, moist environment of open oak woodland, mesic trees, riparian populations of alder, and reduced levels of pine . By the early to mid Holocene, tupelo and oak dominated over pine indicating wetter conditions, while sand stringers in peat record periodic heavy rains, an elevated water table, and intervals of substantial runoff. Cooling after ca. 4.5 ka brought drier and more variable conditions. Fires increased and southern pine replaced tupelo and oak. The absence of sand stringers in peat younger than 4.5 ka indicates fewer intense rainfall events.

Tree mortality in the African Sahel indicates an anthropogenic ecosystem displaced by climate change

AbstractAim Widespread reports of disappearing tree species and senescing savanna parklands in the Sahel have generated a vigorous debate over whether climate change or severe human and livestock pressure is principally responsible. Many of the tree taxa in decline are closely associated with human settlement and farming, suggesting that the parkland ecosystem may not be a natural vegetation assemblage. The aim of this study is to assess the possibility that human activities promoted the spread of taxa with edible fruit into dry Sudano‐Sahelian areas during high‐rainfall periods in the climate cycle.Location West African savannas (Mali, Burkina Faso, Ghana, Togo, Benin).Methods Cultivated savanna parklands and adjacent forests and transitional landscapes were inventoried at 27 sites in five countries. All trees with basal diameters > 10 cm were counted within 500‐m2 belt transects. Species composition and abundance were contrasted between three landscape classes to assess the degree of influence exerted by traditional human management. Twentieth century rainfall data were averaged for two sets of weather stations encompassing the north–south range of typical parkland tree species. Rainfall trends were used to evaluate the putative impact of climate change on edible and/or succulent fruit species at the northern limit of the parkland savanna zone.Results Species composition and spatial distribution data indicate that the parkland ecosystem is significantly shaped by human activities. Indigenous land management favours edible‐fruit‐yielding taxa from the wetter Sudanian and Guinean vegetation zones over Sahelian species. Rainfall isohyets at the northern range limits of parkland species shifted southwards in the late 20th century, crossing the critical 600‐mm mean annual rainfall threshold for Sudanian flora. Relict vegetation and historical records indicate that the Sudanian parkland system extended in the past to near 15° N latitude in middle West Africa, compared with 13.5° N today.Main conclusions The current loss of mesic trees in the Sudano‐Sahel zone appears to be driven by the sharp drop in rainfall since the 1960s, which has effectively stranded anthropogenically distributed species beyond their rainfall tolerance limits.