Canopy Dieback Research Articles

Remnant woodland biodiversity gains under 10 years of revealed‐price incentive payments

Abstract Evaluation of conservation incentive programme outcomes is needed to direct future investment, however, monitoring of large‐scale programmes is relatively rare. Research in this area has frequently relied on space‐for‐time substitution or similar designs, rarely controlling for counterfactual trends or potential learning or leakage effects using repeated measures taken before and after intervention at impact and independent control sites. Using a Before‐After‐Control‐Impact (BACI) monitoring design, we investigate the ecological impact of a conservation stewardship programme, where landholders set their own contract price through a reverse auction. In order to maintain or increase species and structural diversity of remnant native vegetation, contracted landholders agreed to manage grazing pressure (from stock and feral animals), control weeds and retain fallen logs. We test whether impact sites (private land under contract) changed over a 9–10 year period relative to control sites (independently managed, non‐contracted private land) and reference sites (public conservation areas). Modest improvements were detected in native plant species richness, log abundance and grazing pressure at impact sites compared with background changes observed at control sites. While log abundance and grazing pressure were directly managed at impact sites, the improvement in native plant species richness was assumed to be influenced by reduction in multiple pressures such as grazing pressure and weed abundance. No significant intervention effects were detected in other variables including regeneration, plant litter cover, weed cover and canopy dieback. However, these variables did show changes over time which were likely due to weather, particularly an extended period of drought followed by unusually high rainfall. Policy implications. This study shows that revealed‐price incentive contracts can produce biodiversity improvement compared with the business‐as‐usual scenario of native vegetation management on private land. The use of a Before‐After‐Control‐Impact monitoring design enabled the separation of treatment effects from background changes such as those linked to climate and weather, demonstrating the importance of this approach for programme evaluation. We recommend the allocation of appropriate resources for monitoring and also highlight the need for the collection of baseline data prior to contract establishment at both impact and independently managed control sites.

White ash (Fraxinus americana) survival in the core of the emerald ash borer (Agrilus planipennis) invasion

Despite catastrophic ash (Fraxinus spp.) mortality observed by the mid-2000s in the epicenter of the emerald ash borer (EAB) (Agrilus planipennis Fairmaire) invasion in southeast Michigan, we noticed numerous live white ash (Fraxinus americana L.) in some forests in this region. In 2015, we inventoried overstory trees and regeneration in 28 white ash sites spanning 11 counties. White ash survival ranged from 0% to 100% of stems. Overall, 75% of 821 white ash trees, ranging from 10.0 to 44.0 cm diameter at breast height, and 66% of the white ash basal area, ranging from 0.3 to 3.5 m2·ha–1, were alive. Nearly all live white ash had signs of previous EAB colonization, but 83% had healthy canopies (≤10% canopy dieback). Green ash trees were recorded in 27 sites, but 92% were killed by EAB. Model selection indicated that variation in white ash survival was related to white ash abundance and distance of sites from the original EAB epicenter but not to green ash related variables or to the distance of sites from the Asian parasitoid release or recovery locations. Regeneration strata were dominated by white ash, suggesting that some white ash populations may persist in post-invasion areas.

Regeneration of Metrosideros polymorpha forests in Hawaii after landscape‐level canopy dieback

AbstractQuestions(a) Have Metrosideros polymorpha trees become re‐established in Hawaiian forests previously impacted by canopy dieback in the 1970s? (b) Has canopy dieback expanded since the 1970s? (c) Can spatial patterns from this dieback be correlated with habitat factors to model future dieback in this area?Study SiteAn 83,603 ha study area on the eastern slopes of Mauna Loa and Mauna Kea volcanoes on the island of Hawaii, USA.MethodsWe analyzed very‐high‐resolution imagery to assess status of Metrosideros polymorpha forests across the eastern side of the island of Hawaii. We generated 1,170 virtual vegetation plots with a 100‐m radius; 541 plots in areas mapped in 1977 with trees dead or mostly defoliated (dieback), and 629 plots in adjacent wet forest habitat, previously mapped as non‐dieback condition. In each plot we estimated the frequency of M. polymorpha trees that were dead or mostly defoliated, and the frequency of trees with healthy crowns. These results were combined with habitat data to produce a spatial model depicting probability of canopy dieback within the study area.ResultsSeventy‐nine percent of plots mapped in 1977 in dieback condition recovered their canopy and were now considered in non‐dieback condition. Ninety‐one percent of plots in previous non‐dieback areas were found to still have a healthy M. polymorpha canopy in 2015. A spatial model allowed us to identify areas within the study area with high, medium, and low probability of experiencing this same type of canopy dieback in the future.ConclusionsMost former dieback areas mapped within the study area in 1977 now show recovery of the tree canopy through growth of new cohorts of young M. polymorpha trees. This suggests these forest communities are resilient to this type of canopy loss and tree death so long as other factors do not disrupt the natural regeneration process.

Drought-induced dieback of Pinus nigra: a tale of hydraulic failure and carbon starvation.

Ongoing climate change is apparently increasing tree mortality rates, and understanding mechanisms of drought-induced tree decline can improve mortality projections. Differential drought impact on conspecific individuals within a population has been reported, but no clear mechanistic explanation for this pattern has emerged. Following a severe drought (summer 2012), we monitored over a 3-year period healthy (H) and declining (D) Pinus nigra trees co-occurring in a karstic woodland to highlight eventual individual-specific physiological differences underlying differential canopy dieback. We investigated differences in water and carbon metabolism, and xylem anatomy as a function of crown health status, as well as eventual genotypic basis of contrasting drought responses. H and D trees exploited the same water pools and relied on similar hydraulic strategies to cope with drought stress. Genetic analyses did not highlight differences between groups in terms of geographical provenance. Hydraulic and anatomical analyses showed conflicting results. The hydraulic tracheid diameter and theoretical hydraulic conductivity were similar, but D trees were characterized by lower water transport efficiency, greater vulnerability to xylem conduit implosion and reduced carbohydrate stores. Our results suggest that extreme drought events can have different impacts on conspecific individuals, with differential vulnerability to xylem embolism likely playing a major role in setting the fate of trees under climate change.

Fine-scale stand structure mediates drought-induced tree mortality in pinyon-juniper woodlands.

Severe drought has resulted in widespread tree die-off events in forests and woodlands globally and is forecast to become more frequent in coming decades. Tree mortality is a complex process influenced by climate, soils, characteristics of individual trees, interactions between trees, and the dynamics of pests and pathogens. The role of stand structure and stand density in mediating the resistance of trees to drought remains poorly understood, especially in semiarid woodlands, which are expected to be highly susceptible to future severe drought. We sampled permanent plots in central Nevada woodlands dominated by single-leaf pinyon pine and Utah juniper before and after a severe multi-year drought (2013-2015) to investigate the importance of climate, tree attributes, and local-neighborhood stand structure on tree mortality and canopy dieback at the level of individual trees and 0.1-ha plots. We observed widespread tree mortality of pinyon at approximately eight times the reported background mortality rate, and substantial canopy dieback in both pinyon and juniper. Both species were more prone to mortality and dieback in hotter, drier sites. Canopy dieback was associated with both long-term average climate and the severity of recent drought, with elevated mortality on sites with higher water deficits, average summer temperatures, and vapor pressure deficits. Soils also played a role in tree dieback, with greater mortality on deeper soils. While mortality was driven largely by climate at coarse scales, fine-scale stand structure interacted with climate to mediate mortality and dieback. Neighborhood statistics showed that trees were susceptible to competitive influence, and pinyon trees were especially sensitive to neighborhood density on drier sites. Mortality and dieback were associated with diverse, co-occurring insect and parasitic plant mortality agents. Canopy dieback prior to the drought was strongly associated with tree mortality during the drought, implying that current widespread defoliation caused by these agents may foreshadow future elevated woodland decline. Fine-scale influences such as stand structure and soil characteristics play a key role in the long-term dynamics of semiarid woodlands, and these factors should be considered in predictive models of forest and woodland susceptibility to drought.

Impacts of recent climate change on terrestrial flora and fauna: Some emerging Australian examples

AbstractThe effects of anthropogenic climate change on biodiversity are well known for some high‐profile Australian marine systems, including coral bleaching and kelp forest devastation. Less well‐published are the impacts of climate change being observed in terrestrial ecosystems, although ecological models have predicted substantial changes are likely. Detecting and attributing terrestrial changes to anthropogenic factors is difficult due to the ecological importance of extreme conditions, the noisy nature of short‐term data collected with limited resources, and complexities introduced by biotic interactions. Here, we provide a suite of case studies that have considered possible impacts of anthropogenic climate change on Australian terrestrial systems. Our intention is to provide a diverse collection of stories illustrating how Australian flora and fauna are likely responding to direct and indirect effects of anthropogenic climate change. We aim to raise awareness rather than be comprehensive. We include case studies covering canopy dieback in forests, compositional shifts in vegetation, positive feedbacks between climate, vegetation and disturbance regimes, local extinctions in plants, size changes in birds, phenological shifts in reproduction and shifting biotic interactions that threaten communities and endangered species. Some of these changes are direct and clear cut, others are indirect and less clearly connected to climate change; however, all are important in providing insights into the future state of terrestrial ecosystems. We also highlight some of the management issues relevant to conserving terrestrial communities and ecosystems in the face of anthropogenic climate change.

Microbat responses to forest decline

AbstractTree declines have been recorded across forests and woodlands on most continents, causing tree mortality over thousands of square kilometres, yet the impact of tree declines upon mammals have only rarely been quantified. Once the dominant tree over the western parts of the Swan Coastal Plain in Western Australia, tuart (Eucalyptus gomphocephala) forest has been reduced to less than a third of its former range through clearing for agriculture and urban development. Additionally, over the last 30 years, the remnant population has been heavily impacted by a decline that has an unknown cause, but is likely related to a root pathogen coupled with abiotic factors (reduced rainfall, increased salinity and elevated temperatures). Tuart decline is evident as marked canopy dieback, replacement with epicormic growth and increasing bare branches, while leaf litter is lost from the tree surrounds. We examined the effect of tuart decline and other vegetation measures upon bat activity using auditory monitoring. Vegetation structure was correlated with Vespertilionidae bat activity. Falsistrellus mackenziei were more likely to forage around healthy canopies (activity positively correlated with tuart crown density and negatively correlated with tuart crown dieback). By contrast, the other three taxa were more often encountered in declining rather than healthy tuart sites. Chalinolobus gouldii was positively associated with tuart crown dieback. Activity of Vespadelus regulus and Nyctophilus spp. (species not distinguishable from their calls) were significantly positively correlated with an open tall canopy (positively with cover of plants >10 m tall and negatively with overall canopy cover density). There were no vegetation measurements that were strong predictors of activity of two Molossidae species (Ozimops kitcheneri and Austronomus australis), which intercept insects above the forest canopy. This study clearly reveals different factors influencing the activity of bat taxa, which are likely related to where they feed and their manoeuvrability around tree canopies.

Linking morphological and ecophysiological leaf traits to canopy dieback in Persian oak trees from central Zagros

Intraspecific variability in morphological and ecophysiological leaf traits might be theorized to be present in declining populations, since they seem to be exposed to stress and plasticity could be advantageous. Here we focused on declining Persian oaks (Quercus brantii Lindl. var. persica (Jaub and Spach) Zohary) in the Zagros Mountains of western Iran, representing the most important tree species of this region. We selected trees with contrasting crown dieback, from healthy to severely defoliated, to investigate the relationships between canopy dieback and leaf morphology, water content and pigments. We also measured esterase and peroxidase, as enzymatic antioxidants and indicators of contrasting genotypes. Trees showing moderate to severe defoliation showed higher leaf mass area (LMA), reduced relative water content (RWC), and lower stomatal density (SD). Increasing LMA indicates a more sclerophyllic structure, according to drier conditions. We did not find significant differences in leaf pigments (chlorophyll a and b, and carotenoids) among crown dieback classes, suggesting that Persian oak trees are able to maintain accurate photochemical efficiency, while reduced RWC and SD suggest hydraulic limitations. Our results do not provide a consistent pattern as regards enzymatic antioxidant defense in Persian oak. Morphological leaf traits would be important drivers of future adaptive evolution in Persian oak, leading to smaller and thicker leaves, which have fitness benefits in dry environments. Nonetheless, drought responses may be critically affecting carbon uptake, as photosynthetic compounds are less effectively used in leaves with higher sclerophylly.

Beneath the canopy: Linking drought-induced forest die off and changes in soil properties

Climate warming and the occurrence of more severe dry spells are causing widespread drought-induced forest die-off events. Despite research on drought-triggered die-off processes is rapidly increasing, little is known on how soil conditions and rhizosphere features are affected by canopy dieback and tree death. We studied the soils in the rhizosphere of three coniferous forests where die-off was induced by a severe drought in 2012. We selected three forest types subjected to contrasting climatic and edaphic conditions dominated by three different tree species: silver fir (Abies alba; temperate conditions), Scots pine (Pinus sylvestris; continental and Mediterranean conditions) and Aleppo pine (Pinus halepensis; semi-arid and Mediterranean conditions). In each forest, we analyzed soil physical characteristics such as water retention capacity and soil texture, nutrient availability and microbial community structure (Phospholipid fatty acids, PLFA) below non-declining and declining or dying trees. We did not observe differences in nutrient availability between the two vigor classes. Conversely, we found strong differences in soil microbial community structure below non-declining and declining trees in the Silver fir and Aleppo pine stands. Soils in the Scots pine stand presented extremely low values of soil saturated sorptivity indicating a reduction of soil water infiltration after prolonged dry periods which could exacerbate drought stress. We conclude that forest dieback impacts the soil microbial community structure in the short term. Further research is required to understand the linkages between a reduced capacity of soil water infiltration after prolonged droughts, short-term changes in the soil microbiota, long-term nutrient imbalances and tree death. Soil conditions shall be considered as an important part of forest management strategies after drought-induced die off.

A synoptic review of Tamarix biocontrol in North America: tracking success in the midst of controversy

Woody shrubs in the genus Tamarix L. (Tamaricaceae) were introduced into western North America in the nineteenth century and have invaded riparian areas, acting as drivers of ecosystem change by altering fire cycles, soil chemistry, hydrology and native plant composition. The scope and severity of the invasions provided impetus for a classical weed biological control program using Diorhabda spp. (Coleoptera: Chrysomelidae). Since the first releases in 2001 Diorhabda spp. have moved into many of the areas dominated by Tamarix resulting in defoliations, canopy dieback, and in some locations substantial Tamarix mortality. Success of the program has been overshadowed by concern that Tamarix is used by a federally-listed bird sub-species, the southwestern willow flycatcher. The controversy has led to lawsuits, cancelled biological control research and release permits and to a negative perception of Tamarix biocontrol by some. Long term success is likely, but only with continued monitoring and riparian restoration will the program reach its full potential.

Site factors and stand conditions associated with Persian oak decline in Zagros mountain forests

Aim of study: Drought and stand structure are major and interconnected drivers of forest dynamics. Water shortage and tree-to-tree competition may interact under the current climate change scenario, increasing tree mortality. In this study, we aimed to investigate climate trends, site and stand structure effects on tree mortality, with the main hypothesis that drought-induced mortality is higher as competition increases.Area of study: Persian oak forests from Zagros Range, western Iran.Material and Methods: We split the study area into 20 topographical units (TUs), based on aspect, slope and elevation. In each TU, three 0.1 ha plots were established to quantify site and stand characteristics, namely the diameter of all trees and shrubs, stand density and basal area, canopy dieback and mortality. In addition, soil profiles were analyzed to obtain physical and chemical soil properties. Six transects 100 m length were established per TU to measure tree-to-tree competition for alive and dead trees.Main Results: The highest mortality rates and crown dieback were found at higher elevations and southern and western aspects. Our findings confirm increasing rates of tree mortality in stands with higher tree density and shallow soils. As regard links between climate change and forest decline, our results suggest that changing forest structure may have a significant impact on dust emission.Research highlights: Despite severe dry years occurred recently the study area, they are not significantly different than those recorded in the past. Stand structure appears as a modulating factor of climate change effects, linked to competition-related tree vulnerability to drought.

Assessment of Six Indicators of Street Tree Establishment in Melbourne, Australia

A number of factors can influence successful street tree establishment, including inappropriate selection of stock, poorquality stock, poor planting technique, insufficient irrigation, poor weed control, and inadequate maintenance. Urban tree managers need reliable, accurate, and practical indicators of whether street trees have successfully established to inform decisions about removing supporting stakes or continuing irrigation regimes. The establishment of street trees planted as large format stock was investigated using a survey of 510 trees from 21 species managed by three municipalities in Greater Melbourne, Australia. A planted tree can only be considered established when the partial root system in the root ball or the confined root system in a container has developed into a spreading root system, utilizing moisture and available nutrients throughout a larger soil volume. Thus, an indicator of root expansion into soil of a greater volume is necessary, as are measures of canopy growth, such as shoot-tip growth or canopy dieback. The survey assessed six factors indicative of successful or poor establishment: whether trees had been planted too deeply, whether trees exhibited trunk movement at or below the soil surface, damage to the trunk, trunk sunscald injury, presence of epicormic shoots on the trunk, and whether there were co-dominant stems. The results demonstrated that trunk damage (8%) and codominant stems (5%) occurred at low rates in the trees, but the presence of epicormic shoots (12%) and sunscald damage (12.5%) was more common, and 12% of trees had been planted too deeply and 42% of trees were unstable. Three criteria—shoot tip extension, trunk movement in the ground, and percentage of canopy dieback—provided useful measures of whether street trees were assessed as being well- or poorly-established. This is the first study to record a high incidence of sunscald injury to urban trees in Australia.

Evaluation of Ornamental Tropical Plants for Resistance to White Mold Caused by Sclerotinia sclerotiorum

Sclerotinia sclerotiorum (Lib.) de Bary is a fungal pathogen that causes stem rot, crown rot, wilt, and death of many common annual flowering plants. Infested flower beds often suffer significant plant loss each year, and the identification of disease resistant plants would be a useful management tool. Caladium (Caladium ×hortulanum Birdsey), canna (Canna ×generalis L.H. Bailey), and elephant ear [Colocasia esculenta (L.) Schott] were evaluated for potential resistance to S. sclerotiorum. Plants grown in field conditions in Minnesota in 2012 and 2013 were inoculated through the application of sorghum grains colonized by S. sclerotiorum. The number of plants infected and percent of canopy dieback were recorded weekly for 3 months. The susceptibility of leaves, flowers, and below ground storage organs was also examined through direct inoculation of plant tissue with a mycelial plug of the pathogen in controlled environmental conditions favorable for disease development. Symptoms and progression of the infection were recorded after 24 days. Symptoms of infection on all three species were similar in field and controlled environments. Caladium plants were susceptible to S. sclerotiorum. Petioles, leaves, and corms developed a watery soft rot. Elephant ear was highly resistant to infection. Sclerotinia sclerotiorum infected only wounded or senescent tissue and did not result in significant symptoms under any conditions. Canna had partial resistance to the pathogen. Although canna petals were readily infected, infection of petioles was restricted to small necrotic lesions. Neither infection progressed to the main stem or resulted in plant death. This study indicates that canna and elephant ear have resistance to S. sclerotiorum and could be used in an integrated disease management program for infested landscape beds.

Changes in the nature of environmental limitation in two forest herbs during two decades of forest succession

AbstractAimsThe environmental legacies of past land use generally have an important impact on the establishment and persistence of forest understorey plant species in post‐agricultural forests. Here we studied how the nature of such environmental limitation changes during the first decades of forest development.LocationDeciduous forest ‘Muizenbos’, northern Belgium.MethodsTwo forest herbs with comparable life‐history traits, but contrasting rates of post‐agricultural forest colonization (Primula elatior, Geum urbanum) were introduced 18 yr ago. Introduction sites were ancient or post‐agricultural forest; seeds or adults were added to permanent plots where the understorey vegetation was initially cleared or left undisturbed. We measured chemical soil conditions and recorded the understorey plant community to characterize the sites. The effects of the treatments (land use, vegetation clearing) were tested on the present‐day occurrence and abundance of the introduced species across the plots. We also quantified whether the temporal changes in the total number of individuals per site was different between the two land‐use types and consistent within land‐use types.ResultsLand use had an important effect on soil and vegetation. In post‐agricultural forest, the P availabilities were many times higher, the plant community diversity was lower and the competitive species were significantly more abundant compared with ancient forest. The present‐day abundance of P. elatior was significantly higher in the ancient forest sites, both for planted and seeded plots, while for G. urbanum land use had no effect. The initial vegetation clearing had a persistent effect on the establishment success of both species. The 18‐yr‐long time series showed consistent changes in the total population sizes in ancient forest: P. elatior has been expanding and G. urbanum initially increased and then declined over the past decade. In post‐agricultural forest, temporal patterns were more divergent and partly related to differences in dominant tree species: differences in acidification rates and changes in insolation caused by canopy dieback have put the populations on new and unforeseen trajectories.ConclusionsThe longer the experiment is running, the more the individual sites show separate dynamics. In general, we showed that environmental limitation affects plant colonization at least during the first two decades of secondary forest succession, but also that as the post‐agricultural forests develop, the nature of environmental limitation changes.

Effects of rainfall exclusion on leaf gas exchange traits and osmotic adjustment in mature canopy trees of Dryobalanops aromatica (Dipterocarpaceae) in a Malaysian tropical rain forest.

Climate change exposes vegetation to unusual levels of drought, risking a decline in productivity and an increase in mortality. It still remains unclear how trees and forests respond to such unusual drought, particularly Southeast Asian tropical rain forests. To understand leaf ecophysiological responses of tropical rain forest trees to soil drying, a rainfall exclusion experiment was conducted on mature canopy trees of Dryobalanops aromatica Gaertn.f. (Dipterocarpaceae) for 4 months in an aseasonal tropical rain forest in Sarawak, Malaysia. The rainfall was intercepted by using a soft vinyl chloride sheet. We compared the three control and three treatment trees with respect to leaf water use at the top of the crown, including stomatal conductance (gsmax), photosynthesis (Amax), leaf water potential (predawn: Ψpre; midday: Ψmid), leaf water potential at turgor loss point (πtlp), osmotic potential at full turgor (π100) and a bulk modulus of elasticity (ε). Measurements were taken using tree-tower and canopy-crane systems. During the experiment, the treatment trees suffered drought stress without evidence of canopy dieback in comparison with the control trees; e.g., Ψpre and Ψmid decreased with soil drying. Minimum values of Ψmid in the treatment trees decreased during the experiment, and were lower than πtlp in the control trees. However, the treatment trees also decreased their πtlp by osmotic adjustment, and the values were lower than the minimum values of their Ψmid. In addition, the treatment trees maintained gs and Amax especially in the morning, though at midday, values decreased to half those of the control trees. Decreasing leaf water potential by osmotic adjustment to maintain gs and Amax under soil drying in treatment trees was considered to represent anisohydric behavior. These results suggest that D. aromatica may have high leaf adaptability to drought by regulating leaf water consumption and maintaining turgor pressure to improve its leaf water relations.

Old trees as a key source of epiphytic lichen persistence and spatial distribution in mountain Norway spruce forests

Habitat loss and fragmentation can negatively impact the persistence of dispersal-limited lichen species with narrow niches. Rapid change in microclimate due to canopy dieback exposes species to additional stressors that may limit their capacity to survive and colonize. We studied the importance of old trees as micro-refuges and microclimate stability in maintaining lichen survival and diversity. The study was situated in mountain Norway spruce (Picea abies) forests of the Gorgany Mountains of the Ukrainian Carpathian mountain belt. Lichens were collected on 13 circular study plots (1000 m2). Dendrochronological methods were used to reconstruct age structure and maximum disturbance event history. A linear mixed effects model and general additive models were used to explain patterns and variability of lichens based on stand age and disturbance history for each plot. Tree age was the strongest variable influencing lichen diversity and composition. Recent ( 200 years old) that survived the disturbances served as microrefuges for the habitat-specialized and/or dispersal limited species, thus epiphytic lichen biodiversity was markedly higher on those plots in comparison to plots without any old trees. Most species were able to survive microclimatic change after disturbances, or recolonize disturbed patches from surrounding old-growth forests. We concluded that the survival of old trees after disturbances could maintain and/or recover large portions of epiphytic lichen biodiversity even in altered microclimates.

Chronic N enrichment and drought alter plant cover and community composition in a Mediterranean-type semi-arid shrubland.

Anthropogenic nitrogen (N) deposition has caused a decline in native plant species and an increase in exotic plant species in many terrestrial ecosystems; however, vegetation change depends on the rate and/or duration of N input, individual species responses, interactions with other resources, and ecosystem properties such as species richness and canopy cover, soil texture, pH, and/or disturbance regime. Native shrub and exotic forb responses to N enrichment were evaluated over a 13-year field experiment in a mature coastal sage scrub (CSS) shrubland of southern California to test the hypothesis that dry-season N input will cause a decline in native shrubs and an increase in exotic annuals. Nitrogen enrichment caused the dominant native shrubs, Artemisia californica and Salvia mellifera, to respond differently, with A. californica initially increasing with N input but declining thereafter and S. mellifera declining consistently over the 13-year-period. Both species exhibited higher canopy dieback during drought conditions, especially in N plots. Brassica nigra, an exotic annual, invaded N plots significantly more than control plots, but only after 10years of N addition and a prolonged drought, which increased native shrub canopy dieback. These results indicate a possible synergism between N enrichment and drought on native shrub and exotic forb abundance, which would have important implications for plant diversity in semi-arid shrublands of southwest US that are anticipated to experience an increase in anthropogenic N enrichment and the frequency and duration of drought.

Structural overshoot of tree growth with climate variability and the global spectrum of drought-induced forest dieback.

Ongoing climate change poses significant threats to plant function and distribution. Increased temperatures and altered precipitation regimes amplify drought frequency and intensity, elevating plant stress and mortality. Large-scale forest mortality events will have far-reaching impacts on carbon and hydrological cycling, biodiversity, and ecosystem services. However, biogeographical theory and global vegetation models poorly represent recent forest die-off patterns. Furthermore, as trees are sessile and long-lived, their responses to climate extremes are substantially dependent on historical factors. We show that periods of favourable climatic and management conditions that facilitate abundant tree growth can lead to structural overshoot of aboveground tree biomass due to a subsequent temporal mismatch between water demand and availability. When environmental favourability declines, increases in water and temperature stress that are protracted, rapid, or both, drive a gradient of tree structural responses that can modify forest self-thinning relationships. Responses ranging from premature leaf senescence and partial canopy dieback to whole-tree mortality reduce canopy leaf area during the stress period and for a lagged recovery window thereafter. Such temporal mismatches of water requirements from availability can occur at local to regional scales throughout a species geographical range. As climate change projections predict large future fluctuations in both wet and dry conditions, we expect forests to become increasingly structurally mismatched to water availability and thus overbuilt during more stressful episodes. By accounting for the historical context of biomass development, our approach can explain previously problematic aspects of large-scale forest mortality, such as why it can occur throughout the range of a species and yet still be locally highly variable, and why some events seem readily attributable to an ongoing drought while others do not. This refined understanding can facilitate better projections of structural overshoot responses, enabling improved prediction of changes in forest distribution and function from regional to global scales.

Effects of groundwater abstraction on two keystone tree species in an arid savanna national park.

BackgroundIn arid systems with no surface water, deep boreholes in ephemeral river beds provide for humans and animals. With continually increasing infrastructure development for tourism in arid wildlife parks such as the Kgalagadi Transfrontier Park in southern Africa, we ask what effects increased abstraction may have on large trees. Large trees in arid savannas perform essential ecosystem services by providing food, shade, nesting sites and increased nutrients for many other plant and animal species and for this are regarded as keystone species.MethodsWe determine seasonal fluctuations in the water table while also determining the water source for the dominant large tree species in the Auob and Nossob rivers in the Park. We also determine the extent to which these trees are physiologically stressed using leaf δ13C, xylem pressure potentials, specific leaf area and an estimate of canopy death. We do this both upstream and downstream of a low water use borehole in the Auob River and a high water use borehole in the Nossob River.ResultsOur results show that the trees are indeed using deep groundwater in the wet season and that this is the same water used by people. In the dry season, trees in the Auob downstream of the active borehole become detached from the aquifer and use more isotopically enriched soil water. In the Nossob in the dry season, all trees use isotopically enriched soil water, and downstream of the active borehole use stomatal regulation to maintain leaf water potentials. These results suggest that trees in the more heavily utilised Nossob are under more water stress than those trees in the Auob but that trees in both rivers demonstrate physiological adaptation to the changes in available water with smaller heavier leaves, no significant canopy dieback and in the dry season in the Nossob stomatal regulation of leaf water potentials.DiscussionAn increase in abstraction of groundwater particularly at the Nossob borehole may cause an additional draw down of the water table adding to the physiological stress demonstrated in our study. The managers of the Kgalagadi Transfrontier Park have a mandate that includes biodiversity conservation. To fulfil this mandate, upper and lower thresholds for groundwater abstraction that allow for an adequate ecological reserve have to be determined.

Life and death of Picea abies after bark-beetle outbreak: ecological processes driving seedling recruitment.

The severity and spatial extent of bark-beetle outbreaks substantially increased in recent decades worldwide. The ongoing controversy about natural forest recovery after these outbreaks highlights the need for individual-based long-term studies, which disentangle processes driving forest regeneration. However, such studies have been lacking. To fill this gap, we followed the fates of 2,552 individual seedlings for 12years after a large-scale bark-beetle outbreak that caused complete canopy dieback in mountain Norway spruce (Picea abies) forests in southeast Germany. We explore the contribution of advance, disturbance-related, and post-disturbance regeneration to forest recovery. Most seedlings originated directly within the three-year dieback of canopy trees induced by bark-beetle outbreak. After complete canopy dieback, the establishment of new seedlings was minimal. Surprisingly, advance regeneration formed only a minor part of all regeneration. However, because it had the highest survival rate, its importance increased over time. The most important factor influencing the survival of seedlings after disturbance was their height. Survival was further modified by microsite: seedlings established on dead wood survived best, whereas almost all seedlings surrounded by graminoids died. For 5cm tall seedlings, annual mortality ranged from 20 to 50% according to the rooting microsite. However, for seedlings taller than 50cm, annual mortality was below 5% at all microsites. While microsite modified seedling mortality, it did not affect seedling height growth. A model of regeneration dynamics based on short-term observations accurately predicts regeneration height growth, but substantially underestimates mortality rate, thus predicting more surviving seedlings than were observed. We found that P.abies forests were able to regenerate naturally even after severe bark-beetle outbreaks owing to advance and particularly disturbance-related regeneration. This, together with microsite-specific mortality, yields structurally and spatially diverse forests. Our study thus highlights the so far unrecognized importance of disturbance-related regeneration for stand recovery after bark-beetle outbreaks.