Stem Mortality Research Articles

Compositional shifts in Costa Rican forests due to climate‐driven species migrations

Species are predicted to shift their distributions upslope or poleward in response to global warming. This prediction is supported by a growing number of studies documenting species migrations in temperate systems but remains poorly tested for tropical species, and especially for tropical plant species. We analyzed changes in tree species composition in a network of 10 annually censused 1-ha plots spanning an altitudinal gradient of 70-2800m elevation in Costa Rica. Specifically, we combined plot data with herbarium records (accessed through GBIF) to test if the plots' community temperature scores (CTS, average thermal mean of constituent species weighted by basal area) have increased over the past decade as is predicted by climate-driven species migrations. In addition, we quantified the contributions of stem growth, recruitment, and mortality to the observed patterns. Supporting our a priori hypothesis of upward species migrations, we found that there have been consistent directional shifts in the composition of the plots, such that the relative abundance of lowland species, and hence CTS, increased in 90% of plots. The rate of the observed compositional shifts corresponds to a mean thermal migration rate (TMR) of 0.0065°Cyr(-1) (95% CI=0.0005-0.0132°Cyr(-1) ). While the overall TMR is slower than predicted based on concurrent regional warming of 0.0167°Cyr(-1) , migrations were on pace with warming in 4 of the 10 plots. The observed shifts in composition were driven primarily by mortality events (i.e., the disproportionate death of highland vs. lowland species), suggesting that individuals of many tropical tree species will not be able to tolerate future warming and thus their persistence in the face of climate change will depend on successful migrations. Unfortunately, in Costa Rica and elsewhere, land area inevitably decreases at higher elevations; hence, even species that are able to migrate successfully will face heightened risks of extinction.

The Effect of Injection Needle Gauge Size on the Viability of Equine Mesenchymal Stem Cells

AimsPerformance horses are at high risk of injury to the superficial digital flexor tendon (SDFT). Studies have shown that autologous mesenchymal stem cells (MSCs) injected into SDFT lesions subsequently result in a significant reduction of re‐injury in National Hunt racehorses; however, recent studies show that only 25% of implanted MSCs survive 24 h post injection. The reason for this loss is unclear but may relate to cell quiescence or injection‐related mortality. We hypothesised that cell viability and mortality is increased with needle gauge.MethodsEquine MSCs cultured in vitro were resuspended to a final suspension density of 5 x 105 cells/ml to mimic that used for implantation in clinic. The cell suspension was injected through a 19 gauge, 21 gauge (current practice in clinic) or 23 gauge needle. Mesenchymal stem cells viability and mortality was analysed over a 24 h period post injection using alamarBlue® and Annexin V (apoptosis) assays, respectively.ResultsThere was a 25% reduction in viability (P<0.01) and mortality (P<0.01) compared with noninjected MSCs over the 24 h period post injection. All needle gauges also induced a decrease in cell metabolic activity immediately post injection but with recovery by 2 h post injection. Furthermore, 21 gauge and 23 gauge needles increased early apoptotic cells immediately post injection, whereas the 19 gauge needle showed a delayed increase in apoptosis until 2 h post injection.ConclusionsThe delayed apoptosis may correlate with a subpopulation of quiescent cells subsequently becoming apoptotic. The proportion of early and late apoptotic MSCs, while significant, does not account for the total cell loss reported after intra‐lesional injection.Practical significanceNeedle gauge selection has significant implications for the survival of MSC intra‐lesional injections but it may not be the sole factor implicated in post injection cell mortality. We therefore recommend a minimum diameter of 19 gauge should be used for implantation.Ethical animal researchStem cell lines were obtained from bone marrow with approval of the Institutional Ethical Committee and under UK Home Office licence. Sources of funding: Funded by the Medical Research Council and The Royal Veterinary College. Competing interests: None.

Dampening effects of long‐term experimental drought on growth and mortality rates of a Holm oak forest

Forests respond to increasing intensities and frequencies of drought by reducing growth and with higher tree mortality rates. Little is known, however, about the long-term consequences of generally drier conditions and more frequent extreme droughts. A Holm oak forest was exposed to experimental rainfall manipulation for 13 years to study the effect of increasing drought on growth and mortality of the dominant species Quercus ilex, Phillyrea latifolia, and Arbutus unedo. The drought treatment reduced stem growth of A. unedo (-66.5%) and Q. ilex (-17.5%), whereas P. latifolia remained unaffected. Higher stem mortality rates were noticeable in Q. ilex (+42.3%), but not in the other two species. Stem growth was a function of the drought index of early spring in the three species. Stem mortality rates depended on the drought index of winter and spring for Q. ilex and in spring and summer for P. latifolia, but showed no relation to climate in A. unedo. Following a long and intense drought (2005-2006), stem growth of Q. ilex and P. latifolia increased, whereas it decreased in A. unedo. Q. ilex also enhanced its survival after this period. Furthermore, the effect of drought treatment on stem growth in Q. ilex and A. unedo was attenuated as the study progressed. These results highlight the different vulnerabilities of Mediterranean species to more frequent and intense droughts, which may lead to partial species substitution and changes in forest structure and thus in carbon uptake. The response to drought, however, changed over time. Decreased intra- and interspecific competition after extreme events with high mortality, together with probable morphological and physiological acclimation to drought during the study period, may, at least in the short term, buffer forests against drier conditions. The long-term effects of drought consequently deserve more attention, because the ecosystemic responses are unlikely to be stable over time.Nontechnical summaryIn this study, we evaluate the effect of long-term (13 years) experimental drought on growth and mortality rates of three forest Mediterranean species, and their response to the different intensities and durations of natural drought. We provide evidence for species-specific responses to drought, what may eventually lead to a partial community shift favoring the more drought-resistant species. However, we also report a dampening of the treatment effect on the two drought-sensitive species, which may indicate a potential adaptation to drier conditions at the ecosystem or population level. These results are thus relevant to account for the stabilizing processes that would alter the initial response of ecosystem to drought through changes in plant physiology, morphology, and demography compensation.

Effects of high-frequency understorey fires on woody plant regeneration in southeastern Amazonian forests

Anthropogenic understorey fires affect large areas of tropical forest, yet their effects on woody plant regeneration post-fire remain poorly understood. We examined the effects of repeated experimental fires on woody stem (less than 1 cm at base) mortality, recruitment, species diversity, community similarity and regeneration mode (seed versus sprout) in Mato Grosso, Brazil. From 2004 to 2010, forest plots (50 ha) were burned twice (B2) or five times (B5), and compared with an unburned control (B0). Stem density recovered within a year after the first burn (initial density: 12.4-13.2 stems m(-2)), but after 6 years, increased mortality and decreased regeneration--primarily of seedlings--led to a 63 per cent and 85 per cent reduction in stem density in B2 and B5, respectively. Seedlings and sprouts across plots in 2010 displayed remarkable community similarity owing to shared abundant species. Although the dominant surviving species were similar across plots, a major increase in sprouting occurred--almost three- and fourfold greater in B2 and B5 than in B0. In B5, 29 species disappeared and were replaced by 11 new species often present along fragmented forest edges. By 2010, the annual burn regime created substantial divergence between the seedling community and the initial adult tree community (greater than or equal to 20 cm dbh). Increased droughts and continued anthropogenic ignitions associated with frontier land uses may promote high-frequency fire regimes that may substantially alter regeneration and therefore successional processes.

Sudden forest canopy collapse corresponding with extreme drought and heat in a mediterranean-type eucalypt forest in southwestern Australia

Drought and heat-induced forest dieback and mortality are emerging global concerns. Although Mediterranean-type forest (MTF) ecosystems are considered to be resilient to drought and other disturbances, we observed a sudden and unprecedented forest collapse in a MTF in Western Australia corresponding with record dry and heat conditions in 2010/2011. An aerial survey and subsequent field investigation were undertaken to examine: the incidence and severity of canopy dieback and stem mortality, associations between canopy health and stand-related factors as well as tree species response. Canopy mortality was found to be concentrated in distinct patches, representing 1.5 % of the aerial sample (1,350 ha). Within these patches, 74 % of all measured stems (>1 cm DBHOB) had dying or recently killed crowns, leading to 26 % stem mortality six months following the collapse. Patches of canopy collapse were more densely stocked with the dominant species, Eucalyptus marginata, and lacked the prominent midstorey species Banksia grandis, compared to the surrounding forest. A differential response to the disturbance was observed among co-occurring tree species, which suggests contrasting strategies for coping with extreme water stress. These results suggest that MTFs, once thought to be resilient to climate change, are susceptible to sudden and severe forest collapse when key thresholds have been reached.

Drought's legacy: multiyear hydraulic deterioration underlies widespread aspen forest die‐off and portends increased future risk

Forest mortality constitutes a major uncertainty in projections of climate impacts on terrestrial ecosystems and carbon-cycle feedbacks. Recent drought-induced, widespread forest die-offs highlight that climate change could accelerate forest mortality with its diverse and potentially severe consequences for the global carbon cycle, ecosystem services, and biodiversity. How trees die during drought over multiple years remains largely unknown and precludes mechanistic modeling and prediction of forest die-off with climate change. Here, we examine the physiological basis of a recent multiyear widespread die-off of trembling aspen (Populus tremuloides) across much of western North America. Using observations from both native trees while they are dying and a rainfall exclusion experiment on mature trees, we measure hydraulic performance over multiple seasons and years and assess pathways of accumulated hydraulic damage. We test whether accumulated hydraulic damage can predict the probability of tree survival over 2 years. We find that hydraulic damage persisted and increased in dying trees over multiple years and exhibited few signs of repair. This accumulated hydraulic deterioration is largely mediated by increased vulnerability to cavitation, a process known as cavitation fatigue. Furthermore, this hydraulic damage predicts the probability of interyear stem mortality. Contrary to the expectation that surviving trees have weathered severe drought, the hydraulic deterioration demonstrated here reveals that surviving regions of these forests are actually more vulnerable to future droughts due to accumulated xylem damage. As the most widespread tree species in North America, increasing vulnerability to drought in these forests has important ramifications for ecosystem stability, biodiversity, and ecosystem carbon balance. Our results provide a foundation for incorporating accumulated drought impacts into climate-vegetation models. Finally, our findings highlight the critical role of drought stress accumulation and repair of stress-induced damage for avoiding plant mortality, presenting a dynamic and contingent framework for drought impacts on forest ecosystems.

Post-fire response variability in Mediterranean Basin tree species in Portugal

Fire is the most important natural disturbance driving vegetation dynamics in the Mediterranean Basin. However, studies relating fire-induced tree responses to both fire severity and plant traits are still scarce in this region. We aimed to investigate such relationships further and to develop simple models that could help improve forest management in these fire-prone ecosystems. We compiled data from 16 fire sites in different regions and used models to relate post-fire responses of 4155 trees from 14 species with fire severity indicators and tree characteristics. The influence of several spatiotemporal factors at the site level was also considered. Results showed that pine mortality was usually high and mainly determined by fire severity, whereas plant traits played a minor role. In contrast, mortality of broadleaved trees was usually low, even for high-severity fire, but most trees were top-killed. Stem mortality increased with fire severity and decreased with bark thickness and tree size. The models for predicting individual mortality of pines and stem mortality of broadleaves showed very good performance, including when validated against independent datasets. Our results suggest that it is possible to accurately predict the most common post-fire responses of Mediterranean species based on simple fire and tree characteristics.

Efeito do volume do recipiente na produção de mudas e no crescimento inicial de Jatropha curcas L. no Oeste Paranaense

This study compares the influence of the volume of containers in seedling production and growth during the first year of Jatropha in western Paraná. The production of seedlings in the nursery of 1,178 cm3 used plastic bags and containers of 120 cm3 and 180 cm3 filled with commercial substrate. Measurements included increments in height, stem diameter and leaf area. After planting, measurements every three months included the increase in number of leaves, branches, and the survival percentage. At the end of the nursery production phase the largest increase in height (1.56 cm) and stem diameter (1.3 mm) were measured in seedlings of J. curcas conducted in 1,178 cm3 plastic bags, whereas the values calculated for seedlings from other containers resulted in values of 1.24 cm and 1.2 mm from 180 cm3 plugs, and 1.48 cm and 1.1 mm from 120 cm3 plugs. After planting, there were no differences in the increments in height, stem diameter, leaf number, number of branches (primary and secondary) and mortality rate due to container volume. The increments obtained in the first quarter after planting showed higher values for stem diameter, leaf number, and mortality. J.curcas seedlings sown in late August and conducted in the nursery from September to November in containers of 120 cm3, and planted in December in a dystrophic red alfisol in western Paraná resulted in growth equivalent to that of seedlings grown in containers with greater volume with saving of substrate, space and efforts in the nursery.

Aspen Restoration in the Eastern Sierra Nevada: Effectiveness of Prescribed Fire and Conifer Removal

AbstractAspen (Populus tremuloidesMichx.) comprises only a small fraction (1 %) of the Sierra Nevada landscape, yet contributes significant biological diversity to this range. In an effort to rejuvenate declining aspen stands, the Bureau of Land Management conducted conifer removal in three sites (2004 to 2006) and prescribed fire in two sites (2007). The goal of this study was to evaluate the efficacy of these treatments. In each site, aspen densities in three regeneration size classes were measured in treated and untreated transects before and up to five years post-treatment. Five years after treatment, two of the three conifer removal sites showed significant improvement over controls in the density of total stems and two of three regeneration size classes. The third site did not show significant gains over controls in any size class and experienced significant aspen overstory mortality three years after treatment, which was attributed to sunscald and advanced age at the time of treatment. Three years after treatment, the two prescribed fire sites showed significant increases in total stem density and two regeneration size classes, but also exhibited significant stem mortality, which was likely due to a combination of herbivory and drought. Overall, both treatments can be effective, but future treatments should incorporate methods to reduce post-treatment mortality of residual aspen and new sprouts.

Prescribed burning and browsing to control tree encroachment in southern European heathlands

Land abandonment and the cessation of traditional management practices is favouring scrubland and forest expansion throughout Europe, reducing the extent of many semi-natural open habitats of a high ecological value. Tree encroachment is threatening several Calluna vulgaris (L.) Hull heathlands. The objective of this study was to analyze vegetation dynamics resulting from an experiment designed to test management options aiming at controlling the encroachment of Populus tremula (aspen) and favouring heathland recovery. The research was conducted at Vauda Heath, North-western Italy, from 2005 till 2009. We studied prescribed burning and goat browsing, both alone or in combination, assessing their effects on structure and density of encroaching trees, as well as on heathland main species, C. vulgaris and Molinia arundinacea. Treatments were applied to 42 experimental units (300–650m2 each), with each treatment combination replicated from six to twelve times (including untreated controls).Prescribed burning and browsing showed complementary effects on post-fire vegetation development. Both back and head fire behaviors were effective in inducing stem mortality of aspen and rejuvenating the heathland, but in the absence of measures to control post-fire suckering, aspen tended to recover the pre-treatment stand structure. A higher fire frequency (annual winter fire) controlled tree encroachment, but hindered Calluna recovery, whilst favouring competing grass species. Goat browsing alone was initially able to reduce tree growth and slow down woodland expansion, until aspen individuals reached an escape size of 1.5m in height. Prescribed burning followed by yearly goat browsing effectively contained tree encroachment while maximizing post-fire heathland regeneration. This integrated approach thus proved its efficacy also at the southern limits of Calluna distributional area.

Shifting dynamics of climate-functional groups in old-growth Amazonian forests

Background: Climate change is driving ecosystem shifts, which has implications for tropical forest system function and productivity. Aim: To investigate Amazon forest dynamics and test for compositional changes between 1985 and 2005 across different plant groups. Methods: Tree census data from 46 long-term RAINFOR forest plots in Amazonia for three climate-functional groups were used: dry-affiliate, climate-generalist and wet affiliate. Membership of each group was ascribed at genus level from the distribution of individuals across a wet–dry gradient in Amazonia, and then used to determine whether the proportions of these functional groups have changed over time, and the direction of any change. Results: In total, 91 genera, representing 59% of the stems and 18% of genera in the plots, were analysed. Wet-affiliates tended to move from a state of net basal area gain towards dynamic equilibrium, defined as where gain ≈ loss, governed by an increase in loss rather than a decrease in growth and mainly driven by plots in north-west Amazonia, the wettest part of the region. Dry-affiliates remained in a state of strong net basal area gain across western Amazonia and showed a strong increase in stem recruitment. Wet-affiliates and climate-generalists showed increases in stem mortality, and climate-generalists showed increased stem recruitment, resulting in overall equilibrium of stem numbers. Conclusions: While there were no significant shifts in most genera, the results suggest an overall shift in climate-functional forest composition in western Amazonia away from wet-affiliates, and potential for increased forest persistence under projected drier conditions in the future.

Causes of tree line stability: stem growth, recruitment and mortality rates over 15 years at New Zealand Nothofagus tree lines

AbstractAimAbrupt tree lines are relatively rare world‐wide and are poorly understood. We studied demographic processes at abrupt tree lines to better understand the factors governing their dynamics.LocationFive Nothofagus‐dominated tree line sites across the South Island, New Zealand.MethodsAll Nothofagus stems at or above the tree line were tagged and their spatial locations and heights recorded on three occasions over 15 years at each site. We estimated rates of stem height growth, recruitment and mortality, and used these data to develop a demographic model to project stem numbers into the future.ResultsStem numbers had increased above the tree line at most sites over the last 15 years, but with little evidence of tree line advance. Growth, mortality and recruitment rates, modelled as a function of stem size, showed variation through time and among sites. Using a demographic model to project the numbers of stems above tree line 15 years into the future suggests that stem number will continue to increase, but that these tree lines are unlikely to advance at the rate predicted by recent climate warming. Across sites, variation in the rate of change in the number of stems above the tree line was most strongly related to variation in recruitment rates.Main conclusionsForest expansion at the abrupt Nothofagus tree line in New Zealand appears to be limited primarily by a lack of recruitment of new stems, which may be due to a paucity of suitable sheltered microsites for seedling establishment. Based on past demographic rates, our results suggest that these tree lines are relatively unresponsive to recent climate warming and are unlikely to show substantial upslope movement if past demographic trends continue.

Heparin-Binding Epidermal Growth Factor-Like Growth Factor and Mesenchymal Stem Cells Act Synergistically to Prevent Experimental Necrotizing Enterocolitis

We have shown that administration of heparin-binding EGF (epidermal growth factor)-like growth factor (HB-EGF) protects the intestines from experimental necrotizing enterocolitis (NEC). We have also demonstrated that systemically administered mesenchymal stem cells (MSC) can engraft into injured intestines. This study investigated the effects of HB-EGF on MSC in vitro, and whether MSC and HB-EGF can act synergistically to prevent NEC in vivo. In vitro, the effect of HB-EGF on MSC proliferation, migration, and apoptosis was determined. In vivo, rat pups received MSC either intraperitoneally (IP) or intravenously (IV). Pups were assigned to 1 of 7 groups: Group 1, breast-fed; Group 2, experimental NEC; Group 3, NEC+HB-EGF; Group 4, NEC+MSC IP; Group 5, NEC+HB-EGF+MSC IP; Group 6, NEC+MSC IV; or Group 7, NEC+HB-EGF+MSC IV. Mesechymal stem cell engraftment, histologic injury, intestinal permeability, and mortality were determined. Heparin-binding EGF-like growth factor promoted MSC proliferation and migration, and decreased MSC apoptosis in vitro. In vivo, MSC administered IV had increased engraftment into NEC-injured intestine compared with MSC administered IP (p < 0.05). Heparin binding EGF-like growth factor increased engraftment of IP-administered MSC (p < 0.01) and IV-administered MSC (p < 0.05). Pups in Groups 3 to 7 had a decreased incidence of NEC compared with nontreated pups (Group 2), with the lowest incidence in pups treated with HB-EGF+MSC IV (p < 0.01). Pups in Group 7 had a significantly decreased incidence of intestinal dilation and perforation, and had the lowest intestinal permeability, compared with other treatment groups (p < 0.01). Pups in all experimental groups had significantly improved survival compared with pups exposed to NEC, with the best survival in Group 7 (p < 0.05). Heparin-binding EGF-like growth factor and MSC act synergistically to reduce injury and improve survival in experimental NEC.

Cork oak vulnerability to fire: the role of bark harvesting, tree characteristics and abiotic factors.

Forest ecosystems where periodical tree bark harvesting is a major economic activity may be particularly vulnerable to disturbances such as fire, since debarking usually reduces tree vigour and protection against external agents. In this paper we asked how cork oak Quercus suber trees respond after wildfires and, in particular, how bark harvesting affects post-fire tree survival and resprouting. We gathered data from 22 wildfires (4585 trees) that occurred in three southern European countries (Portugal, Spain and France), covering a wide range of conditions characteristic of Q. suber ecosystems. Post-fire tree responses (tree mortality, stem mortality and crown resprouting) were examined in relation to management and ecological factors using generalized linear mixed-effects models. Results showed that bark thickness and bark harvesting are major factors affecting resistance of Q. suber to fire. Fire vulnerability was higher for trees with thin bark (young or recently debarked individuals) and decreased with increasing bark thickness until cork was 3–4 cm thick. This bark thickness corresponds to the moment when exploited trees are debarked again, meaning that exploited trees are vulnerable to fire during a longer period. Exploited trees were also more likely to be top-killed than unexploited trees, even for the same bark thickness. Additionally, vulnerability to fire increased with burn severity and with tree diameter, and was higher in trees burned in early summer or located in drier south-facing aspects. We provided tree response models useful to help estimating the impact of fire and to support management decisions. The results suggested that an appropriate management of surface fuels and changes in the bark harvesting regime (e.g. debarking coexisting trees in different years or increasing the harvesting cycle) would decrease vulnerability to fire and contribute to the conservation of cork oak ecosystems.

Stem exclusion and mortality in unmanaged subalpine forests of the Swiss Alps

Understanding the causes and consequences of spatiotemporal structural development in forest ecosystems is an important goal of basic and applied ecological research. Most existing knowledge about the sequence and timing of distinct structural stages following stand origin in unmanaged (not actively managed in >50 years) forests has been derived from forests in North America, which are characterized by particular topographic, climatic, biotic and other environmental factors. Thus, the effects on structural development remain poorly understood for many other forest systems, such as the dense, unmanaged, subalpine Norway spruce forests of the Swiss Alps. Over the past century, land abandonment and reductions in active forest management have led to a substantial increase in the density of these forests types. Consequently, many stands are entering the stem exclusion stage and are currently characterized by associated self-thinning mortality. However, the environmental influences on the rate of this structural development as well as this structural stage itself have not yet been examined. We studied stem exclusion processes based on forest inventory data (National Swiss Forest Inventory; NFI) over three survey periods (1983–1985, 1993–1995 and 2004–2006) using repeated measures statistics. To complement these analyses, we also collected and analysed 3,700 increment cores from 20 field plots within dense subalpine Norway spruce forests dispersed across the Swiss Alps. Over the past decades, basal area (BA) has generally increased, particularly on N-facing and steeper slopes, and within 300 m of potential treeline. The number of dead trees was higher on N-facing compared with S-facing slopes, but the BA of dead wood was higher on S-facing slopes. Tree ring analysis confirmed important differences in growth patterns between N- and S-facing slopes and verified the results of the NFI analysis. This study provides a detailed example of how environmental heterogeneity and management history can influence the spatiotemporal structural development of forest ecosystems.

Altered growth, population structure and realised niche of the weed Campuloclinium macrocephalum (Asteraceae) after exposure to the naturalised rust Puccinia eupatorii (Pucciniaceae)

The alien forb Campuloclinium macrocephalum (Less.) DC. (Asteraceae, pompom weed) invades disturbed rangelands in South Africa. After >40 years of naturalisation, infestations around Pretoria were severely damaged in 2006 by the biotrophic rust Puccinia eupatorii Dietel (Pucciniaceae). The disease spread rapidly and has now established at most pompom weed infestations in the country. This study reports on the efficacy of the pathogen as a biocontrol agent of pompom weed, its effect on the retention of the weed’s realised niche and tolerance to the disease. Eighty rangeland infestations originally assessed over two years in 2003 and 2004 were re-surveyed in 2009. Data from untreated plots at two pompom weed experimental sites where the rust was first detected were also used. Puccinia eupatorii induced premature senescence in late summer and stimulated the production of compensatory growth in autumn. Plants without disease symptoms senesced in late autumn without compensatory growth and remained dormant throughout winter. Pompom weed density did not change significantly over time in both the rangeland study and untreated plots. Disease incidence in rangelands in 2009 was most severe in dense infestations, with sparse infestations having the highest incidence of natural senescence and fewer pustules. The rust did not reduce the realised niche; infestations in grasslands, savanna and wetlands persisted despite significant damage by the disease. Persistence, despite high annual stem mortality, suggests that pompom weed remains adapted to the fungus after prolonged separation from the disease. This study supports earlier findings that disturbance remains key to pompom weed invasion even under biological control. The rust may enhance the biological control of pompom weed with augmentation of additional insect agents, but is unlikely to inflict a significant reduction in the weed’s population on its own. This may be an important consideration for the biological control of weeds involving biotrophic fungi.

California black oak responses to fire severity and native conifer encroachment in the Klamath Mountains

Fire exclusion in forests across the western United States has resulted in widespread vegetation change, often with notable increases in density and cover of fire-sensitive species. In California oak woodlands, encroachment by the more shade-tolerant native Douglas-fir (Pseudotsuga menziesii) is particularly common, and results in the piercing and eventual overtopping of crowns of shade-intolerant trees. We investigated canopy competition, paired tree ages, and post-fire effects in a recently burned, encroached California black oak (Quercus kelloggii) woodland. Pre-fire woodland overstory was heavily dominated by Douglas-fir, which commonly pierced and overtopped California black oak crowns. Trees that pierced overstory oak crowns were younger (mean difference=43.6years, p<0.001) than their paired oaks. Fire effects were variable, leaving some areas unchanged and others with complete above-ground stem mortality. Although above-ground stem-kill was often high for California black oak, the majority (82%) re-sprouted from the root collar. Logistic modeling revealed a significant relationship between California black oak mortality and neighboring Douglas-fir height and plot heat load index. The probability of California black oak mortality increased as neighboring Douglas-fir height increased and decreased with increasing heat load index. Probability of mortality for Douglas-fir decreased with increasing tree size, while the probability of California black oak top-kill was much higher than that for 20–30cmdbh Douglas-fir across a continuous measure of char height. Results indicate that competitive pressure from encroaching trees may compromise California black oak’s ability to survive fire while resilience of encroaching Douglas-fir improves with greater size. Caution should be taken when planning and implementing restoration activities in California black oak woodlands to minimize loss of compromised, remnant oaks while still achieving adequate removal of encroaching conifers.

Absolute Lymphocyte Count As Predictor of Survival Following Hematopoietic Stem Cell Transplants

Abstract 4538Following hematopoietic stem cell transplantation (HSCT), morbidity and mortality are often related to opportunistic infection early post HSCT and it is accepted that immune reconstitution has a protective effect. Absolute lymphocyte counts (ALC) act as a surrogate for immune reconstitution. Prior studies suggest that an ALC greater than 0.5×10^9/L on day 15 post autologous HSCTs and greater than 0.3×10^9/L on day 30 post allogeneic HSCTs can prognosticate overall survival (OS) and progression-free survival (PFS). However, most of these studies have been single disease small studies. We designed this study to evaluate whether patients with a wide variety of diseases undergoing either autologous or allogeneic HSCT at Thomas Jefferson University Hospital would fit this model or if other cut-points were more appropriate.Data was abstracted from the charts of 585 HSCTs (265 autologous, 320 allogeneic (228 HLA identical and 92 haplo-identical) preformed consecutively between January 1999 and April 2009. Prior treatments, disease status at time of HSCT, conditioning regimens, and progression-free and overall survival were collected. Survival methods (Kaplan-Meier and logrank test, and Cox proportional hazards regression) were used to analyze PFS and OS as a function of ALC and other clinical variables. Separate models were fit for ALC counts on day 15, day 30, and day 90 post-transplantation, as predictors of PFS (or OS). ALC was categorized as less than or equal to 0.5 or greater than 0.5 ×10^9/L. The possibility of differential association between ALC counts and PFS (or OS) among autografts versus allografts was tested by including interaction terms between ALC counts at each time point and graft type.For these analyses, we excluded third and fourth HSCTs (n = 4), as well as the first of tandem autologous/allogeneic HSCTs (n = 26) and the first of two HSCTs that were performed during the same admission (n = 8). We also excluded HSCTs for which ALC counts on admission or on day 15 post-HSCT were not available (n = 14). Therefore, these analyses were based on data for a total of 532 HSCTs, including 443 first HSCTs and 89 second HSCTs.Relapse was seen in 52% (229/443) first HSCTs and 45% (40/89) second HSCTs. A total of 212 (48%) first HSCTs and 63 (71%) of second HSCTs were followed up until the patient's death. In all first HSCTs (figure 1), the median PFS correlated with ALC significantly on day 30 and day 90 (p=0.007, 0.027 respectively), while the OS correlated significantly at all three time points (p=0.003, 0.001, 0.001 respectively). For PFS, the hazard ratio at day 15 was not significant for any type of transplant, but was significant at day 30 for haplo-identical transplants (HR 0.48, p=0.032) and day 90 for HLA identical transplants (HR 0.42, p=0.001). For OS, the hazard ratio was significant at day 15 (HR 0.58, p=0.022) and day 30 (HR 0.51, p=0.021) for autologous HSCTs, day 30 (HR 0.62, p=0.022) and 90 (HR 0.27, p=0.001) for HLA identical transplants, and day 30 (HR 0.017, p=0.017) and day 90 (HR 0.24, p=0.002) for haplo-identical HSCTs (Figure 1). Similarly, for all second HSCTs, the median PFS correlated with ALC significantly only day 30 (p=0.004), while the OS correlated significantly at all three time points (p=0.006, 0.001, 0.002 respectively). For PFS, the hazard ratio for day 15 was significant for HLA identical allogeneic transplants (HR 0.44, p=0.013), while for OS the hazard ratio was significant at day 15, 30, and 90 for HLA identical allogeneic transplants (HR 0.47, p=0.021; HR 0.50, p=0.034; HR 0.46, p=0.039 respectively). There was not enough data to calculate HR for second autologous transplants. An analysis was conducted to determine if the historical cut point of 0.50 × 10^9/L was the most predictive of survival. For first transplants, the best cut points differ for autologous and for allogeneic/haplo-identical grafts (0.33 ×10^9/L and 0.08 ×10^9/L, respectively) at day 15. While for day 30 and day 90, the cut point most predictive of survival is the same for autologous and for allogeneic/haplo-identical grafts (0.34 × 10^9/L for day 30 and 0.50 × 10^9/L for day 90). [Display omitted] Disclosures:No relevant conflicts of interest to declare.

How do small savanna trees avoid stem mortality by fire? The roles of stem diameter, height and bark thickness

To recruit to reproductive size in fire-prone savannas, juvenile trees must avoid stem mortality (topkill) by fire. Theory suggests they either grow tall, raising apical buds above the flames, or wide, buffering the stem from fire. However, growing tall or wide is of no advantage without stem protection from fire. In Litchfield National Park, northern Australia, we explored the importance of bark thickness to stem survival following fire in a eucalypt-dominated tropical savanna. We measured bark thickness, prefire height, stem diameter and resprouting responses of small stems under conditions of low to moderate fire intensity. Fire induced mortality was low (<10%), topkill was uncommon (<11% of 5 m to 37% of 1 m tall stems) and epicormic resprouting was common. Topkill was correlated only with absolute bark thickness and not with stem height or width. Thus, observed height and diameter growth responses of small stems are likely different pathways to achieving bark thick enough to protect buds and the vascular cambium. Juvenile height was traded off against the cost of thick bark, so that wide stems were short with thicker bark for a given height. The fire resilience threshold for bark thickness differed between tall (4–5 mm) and wide individuals (8–9 mm), yet tall stems had lower PTopkill for a given bark thickness. Trends in PTopkill reflected eucalypt versus non-eucalypt differences. Eucalypts had thinner bark than non-eucalypts but lower PTopkill. With deeply embedded epicormic buds eucalypts do not need thick bark to protect buds and can allocate resources to height growth. Our data suggest the only ‘strategy' for avoiding topkill in fire-prone systems is to optimise bark thickness to maximise stem bud and cambium protection. Thus, escape height is the height at which bark protects the stem and a wide stem per se is insufficient protection from fire without thick bark. Consequently, absolute bark thickness is crucial to explanations of species differences in topkill, resprouting response and tree community composition in fire-prone savannas. Bark thickness and the associated mechanism of bud protection offer a proximate explanation for the dominance of eucalypts in Australian tropical savannas.

Is selective thinning an adequate practice for adapting Quercus ilex coppices to climate change?

Future climatic scenarios demand an increasing involvement of management for forest preservation, but little is known on how forestry practices will benefit stands in facing variation of climatic components. We investigated how selective thinning affected the response of an old Quercus ilex coppice to 6 years of throughfall reduction. Plots thinned from below (≈30% basal area removal) and unthinned plots were subject to either throughfall exclusion (TE; ≈33% throughfall reduction) or normal rainfall. Stem diameter growth, stem survival, and seed and sprout production were measured. TE did not have a significant effect on stem growth but it reduced the production of viable acorns. Also, in the absence of thinning, TE accelerated the mortality of small stems and stimulated the emergence of new sprouts. Thinning reduced stem mortality, enhanced growth of residual stems, and caused a profuse emission of resprouts. Thinning also increased total seed production, but the crop had a large proportion of aborted seeds, especially in those areas subject to TE. The mere elimination of suppressed and diseased stems in abandoned Q. ilex coppice stands helps remaining trees to cope with current and future (probably longer and more intense) droughts. Potentially drier conditions might attenuate the success of thinning in producing a viable seed crop.