Effects of prescribed fire on a drought-stressed Trembling Aspen (<i>Populus tremuloides</i>) stand

Influence of drought on growing season carbon and water cycling with changing land cover

The replacement of grasslands by woody plants represents one of the main threats to the biodiversity conservation in open landscapes around the world. This transformation occurs when ecological processes that are essential to avoid the canopy closure are prevented, such as fire. We sampled trees and shrubs and evaluated the effects of fire in an old-growth grassland remnant released from fire for 18 years in Southern Brazil. Our main objective was to analyze whether woody encroachment promotes a biome shift of the grassland ecosystem. We hypothesized if woody encroachment is deterministically structured towards a dominance of forest species and a significant decrease in fire intensity and severity along the encroachment gradient occur. Then, the woody encroachment does not represent a simple process of woody densification, but a biome shift towards an alternative forest state, that change the ecology of the system in a positive feedback loop and get stability as the woody community expands. We found clear evidences that woody encroachment promotes a biome shift from an open grassland state to an alternative forest state. In this process, our results indicated that woody encroachment was structured deterministically in the multivariate space with the occurrence of indicator species in the early and late stages. Throughout the entire encroachment gradient analyzed, a significant dominance of forest species was found, indicating that Southern Brazilian grasslands have the potential to be converted into forests more rapidly than savannas in the absence of fire. A significant decrease in fire intensity was found as the process of woody encroachment increased, indicating a change in the system behaviour that favoured the occurrence of a positive feedback loop between the canopy cover and the recruitment of shade-tolerant forest species. Simultaneously to fire intensity decrease, we observed that the reversion of the woody encroachment process was significantly reduced as the woody dominance increased. Thus, the woody encroachment mediated by fire suppression can be considered a degradation factor in our study area, since it can shift the system balance to another alternative stable state. By elucidating the ecological consequences behind woody encroachment, we recommend the use of prescribed fires to the conservation of the grasslands landscapes in Southern Brazil.

Increases in mortality of trembling aspen (Populus tremuloides Michx.) have been recorded across large areas of western North America following recent periods of exceptionally severe drought. The resultant increase in standing, dead tree biomass represents a significant potential source of carbon emissions to the atmosphere, but the timing of emissions is partially driven by dead-wood dynamics which include the fall down and breakage of dead aspen stems. The rate at which dead trees fall to the ground also strongly influences the period over which forest dieback episodes can be detected by aerial surveys or satellite remote sensing observations. Over a 12-year period (2000-2012), we monitored the annual status of 1010 aspen trees that died during and following a severe regional drought within 25 study areas across west-central Canada. Observations of stem fall down and breakage (snapping) were used to estimate woody biomass transfer from standing to downed dead wood as a function of years since tree death. For the region as a whole, we estimated that >80% of standing dead aspen biomass had fallen after 10 years. Overall, the rate of fall down was minimal during the year following stem death, but thereafter fall rates followed a negative exponential equation with k = 0.20 per year. However, there was high between-site variation in the rate of fall down (k = 0.08-0.37 per year). The analysis showed that fall down rates were positively correlated with stand age, site windiness, and the incidence of decay fungi (Phellinus tremulae (Bond.) Bond. and Boris.) and wood-boring insects. These factors are thus likely to influence the rate of carbon emissions from dead trees following periods of climate-related forest die-off episodes.

Understanding the pathways through which drought stress kills woody vegetation can improve projections of the impacts of climate change on ecosystems and carbon-cycle feedbacks. Continuous in situ measurements of whole trees during drought and as trees die hold promise to illuminate physiological pathways but are relatively rare. We monitored leaf characteristics, water use efficiency, water potentials, branch hydraulic conductivity, soil moisture, meteorological variables, and sap flux on mature healthy and sudden aspen decline-affected (SAD) trembling aspen (Populus tremuloides) ramets over two growing seasons, including a severe summer drought. We calculated daily estimates of whole-ramet hydraulic conductance and modeled whole-ramet assimilation. Healthy ramets experienced rapid declines of whole-ramet conductance during the severe drought, providing an analog for what likely occurred during the previous drought that induced SAD. Even in wetter periods, SAD-affected ramets exhibited fivefold lower whole-ramet hydraulic conductance and sevenfold lower assimilation than counterpart healthy ramets, mediated by changes in leaf area, water use efficiency, and embolism. Extant differences between healthy and SAD ramets reveal that ongoing multi-year forest die-off is primarily driven by loss of whole-ramet hydraulic capability, which in turn limits assimilation capacity. Branch-level measurements largely captured whole-plant hydraulic limitations during drought and mortality, but whole-plant measurements revealed a potential role of other losses in the hydraulic continuum. Our results highlight the importance of a whole-tree perspective in assessing physiological pathways to tree mortality and indicate that the effects of mortality on these forests' assimilation and productivity are larger than expected based on canopy leaf area differences.

Droughts are expected to become more frequent and severe, due to climate change, with uncertain consequences for savanna vegetation. Drought-driven tree mortality has been observed in some savannas, but little is known about how tolerant savanna trees are to drought, or what determines differences among species in drought vulnerability. Here, we examine which characteristics best distinguished tree species that were most negatively affected by a recent drought in Kruger National Park, South Africa. Woody-encroaching, root-suckering species, which tended to have relatively low investments in non-structural carbohydrates, had the highest mortality rates. Of particular interest was Dichrostachys cinerea, a notable contributor to widespread woody encroachment across southern African savannas, which suffered high drought mortality. Using rainfall maps and vegetation survey data, we show that D. cinerea distributions in Kruger National Park are limited to areas with higher mean annual rainfall and lower past drought frequency, and stem densities are lower where droughts have occurred more frequently, suggesting that past droughts have had lasting impacts on the distribution and abundance of this species. Woody encroachers, such as D. cinerea, may face a trade off between traits promoting proliferation vs. drought tolerance, and more frequent and severe droughts could increasingly limit their spread.

In contrast with other native Populus species in North America, Populus tremuloides (aspen) can successfully establish itself in drought-prone areas, yet no comprehensive analysis has been performed on the ability of seedlings to withstand and recover from a severe drought resulting in complete leaf mortality. Here, we subjected 4-month-old aspen seedlings grown in two contrasting soil media to a progressive drought until total leaf mortality, followed by a rewatering cycle. Stomatal conductance (g(s) ), photosynthesis and transpiration followed a sigmoid decline with declining fraction of extractable soil water values. Cessation of leaf expansion occurred close to the end of the linear-decrease phase, when g(s) was reduced by 95%. Leaf mortality started after g(s) reached the lowest values, which corresponded to a stem-xylem pressure potential (Ψ(xp)) of -2.0 MPa and a percent loss of stem hydraulic conductivity (PLC) of 50%. In plants with 50% leaf mortality, PLC values remained around 50%. Complete leaf mortality occurred at an average stem PLC of 90%, but all seedlings were able to resprout after 6-10 days of being rewatered. Plants decapitated at soil level before rewatering developed root suckers, while those left with a 4-cm stump or with their stems intact resprouted exclusively from axillary buds. Resprouting was accompanied by recovery of stem hydraulic conductivity, with PLC values around 30%. The percentage of resprouted buds was negatively correlated with the stem %PLC. Thus, the recovery of stem hydraulic conductivity appears as an important factor in the resprouting capacity of aspen seedlings following a severe drought.

Defoliating insects are a major factor impacting tree growth in temperate and boreal forests, but the effects of climate change on the severity and frequency of outbreaks of these insects are not well understood. Dendrochronological reconstruction of forest tent caterpillar (Malacosoma disstria) outbreaks on trembling aspen (Populus tremuloides) in southwestern Manitoba, Canada, from 1851 to 2010 was used to examine the effects of changes in temperature, incident surface‐level solar radiation, and drought severity on the cyclicity, frequency, and severity of outbreaks. Outbreak severity was determined by comparing the tree‐ring widths of trembling aspen with those of white spruce (Picea glauca), which is not a forest tent caterpillar host. Wavelet analysis suggested the cyclicity of outbreaks increased after 1930. From 1930 to 2010, the dominant period length of outbreak cycles steadily increased over time from three to seven years, coinciding with long‐term declines in drought severity. The strength of forest tent caterpillar suppression of trembling aspen growth in a given year was inversely related to incident solar radiation and drought severity during late spring (the larval feeding period) of the same year. The effect of late spring drought on the impact of forest tent caterpillar on ring growth differed from that of summer drought. There was a nonlinear relationship between growth suppression in a given year and summer drought severity in the previous year; slightly above‐average drought severity was associated with reduced impacts of the defoliator, whereas severe drought was associated with intense growth suppression. Strong suppression of aspen growth observed during outbreaks from 1983 to 2010 may be partly explained by observed long‐term trends of decreasing atmospheric transmittance of solar radiation (a phenomenon termed “global dimming”) and increasing soil moisture availability during late spring. However, as the severity and frequency of summer droughts are expected to increase over the 21st century, the nonlinear relationship between summer drought severity and growth suppression the following year suggests a future in which summer droughts precede severe outbreaks that threaten the health of boreal forest hardwood trees.

North American grasslands have experienced increased relative abundance of shrubs and trees over the last 150yr. Alterations in herbivore composition, abundance, and grazing pressure along with changes in fire frequency are drivers that can regulate the transition from grassland to shrubland or woodland (a process known as woody encroachment). Historically, North American grasslands had a suite of large herbivores that grazed and/or browsed (i.e., bison, elk, pronghorn, deer), as well as frequent and intense fires. In the tallgrass prairie, many large native ungulates were extirpated by the 1860s, corresponding with increased homesteading (which led to decreased fire frequencies and intensities). Changes in the frequency and intensity of these two drivers (browsing and fire) have coincided with woody encroachment in tallgrass prairie. Within tallgrass prairie, woody encroachment can be categorized in to two groups: non-resprouting species that can be killed with fire and resprouting species that cannot be killed with fire. Resprouting species require additional active management strategies to decrease abundance and eventually be removed from the ecosystem. In this study, we investigated plant cover, ramet density, and physiological effects of continuous simulated browsing and prescribed fire on Cornus drummondii C.A. Mey, a resprouting clonal native shrub species. Browsing reduced C.drummondii canopy cover and increased grass cover. We also observed decreased ramet density, which allowed for more infilling of grasses. Photosynthetic rates between browsed and unbrowsed control shrubs did not increase in 2015 or 2016. In 2017, photosynthetic rates for browsed shrubs were higher in the unburned site than the unbrowsed control shrubs at the end of the growing season. Additionally, after the prescribed fire, browsed shrubs had ~90% decreased cover, ~50% reduced ramet density, and grass cover increased by ~80%. In the roots of browsed shrubs after the prescribed fire, nonstructural carbohydrates (NSC) experienced a twofold reduction in glucose and a threefold reduction in both sucrose and starch. The combined effects of browsing and fire show strong potential as a successful management tool to decrease the abundance of clonal-resprouting woody plants in mesic grasslands and illustrate the potential significance of browsers as a key driver in this ecosystem.

به‏منظور بررسی تأثیر سطوح فیلترکیک بر برخی صفات مرفوفیزیولوژیک و عملکرد ذرت شیرین (Zea mays var. Saccharata) بهاره تحت شرایط تنش خشکی آخر فصل، آزمایشی در مزرعه دانشگاه کشاورزی و منابع طبیعی رامین خوزستان به صورت کرت‏های خرد شده در قالب بلوک‏های کامل تصادفی در بهار سال 1391 اجرا گردید. سطوح تنش خشکی (آبیاری پس از تخلیه 25، 50 و 75 درصد رطوبت قابل استفاده خاک) در کرت‏های اصلی و فیلترکیک (صفر، 10، 20 و 30 تن در هکتار) در کرت‏های فرعی جای داده شدند. نتایج نشان داد که اعمال تنش خشکی بر غلظت یون سدیم در اندام هوایی گیاه اثر معنی‏داری نداشت، اما بر صفات مرفوفیزیولوژیک و عملکرد ذرت اثر معنی‏دار کاهشی بوده است. تنش شدید وزن تر بلال و عملکرد بیولوژیک را به ترتیب 7/21 % و 3/27 % کاهش داد. کاربرد فیلترکیک در شرایط بدون تنش باعث افزایش ارتفاع گیاه، وزن تر بلال و عملکرد بیولوژیک گیاه شد، اما در شرایط تنش شدید، سطوح بالای فیلترکیک به دلیل شوری زیاد باعث افزایش آسیب به غشاء سلول، کاهش پایداری غشاء، کاهش شاخص سطح برگ و کاهش وزن تر بلال و عملکرد بیولوژیک به ترتیب به میزان 7/18 و 3/23 درصد نسبت به تیمار بدون فیلترکیک شد و چنین نتیجه‏گیری شد که تأثیر مثبت فیلترکیک تنها در شرایط بدون تنش می‏باشد و در شرایط تنش خشکی حتی باعث کاهش عملکرد اقتصادی هم می‏گردد. به‏منظور بررسی تأثیر سطوح فیلترکیک بر برخی صفات مرفوفیزیولوژیک و عملکرد ذرت شیرین بهاره تحت شرایط تنش خشکی آخر فصل، آزمایشی در بهار سال 1391 در مزرعه دانشگاه کشاورزی و منابع طبیعی رامین خوزستان به صورت کرت‏های خرد شده در قالب بلوک کامل تصادفی اجرا گردید. سطوح تنش خشکی (آبیاری پس از تخلیه 25 ، 50 و 75 درصد رطوبت قابل استفاده خاک) در کرت‏های اصلی و فیلترکیک (صفر، 10، 20 و 30 تن در هکتار) در کرت‏های فرعی جای داده شدند. نتایج نشان داد که اعمال تنش خشکی بر غلظت یون سدیم در اندام هوایی گیاه اثر معنی‏داری نداشته اما بر صفات مرفوفیزیولوژیک و عملکرد ذرت دارای اثر معنی‏دار کاهشی بوده است. همچنین تنش شدید باعث کاهش 7/21 و 3/27 درصد به ترتیب وزن تر بلال و عملکرد بیولوژیک شده است. از طرفی، کاربرد فیلترکیک در شرایط بدون تنش باعث افزایش ارتفاع گیاه، وزن تر بلال و عملکرد بیولوژیک گیاه شد. اما در شرایط تنش شدید، سطوح بالای فیلترکیک به دلیل شوری زیاد باعث افزایش آسیب به غشاء سلول و کاهش پایداری غشاء و شاخص سطح برگ و در نتیجه کاهش عملکرد اقتصادی گردید. به طوری که در تنش شدید کاربرد 30 تن فیلترکیک در هکتار باعث کاهش 7/18 و 3/23 درصد به ترتیب وزن تر بلال و عملکرد بیولوژیک نسبت به تیمار بدون فیلترکیک شده است.

Within Southern African biomes, droughts are recurrent with devastating impacts on ecological, economic, and human wellbeing. In this context, understanding the drought impact on vegetation is of extreme importance. However, information on drought impact on natural vegetation at the biome level is scanty and remains poorly understood. Most studies of drought impact on vegetation have largely focussed on crops. The few existing studies on natural vegetation are based on experiments and field measurements at individual tree level which are not representative of biomes. In this study, we mapped the spatial extent and severity of drought using the Standardized Precipitation Evapotranspiration Index (SPEI) and then quantified the drought impact on Southern African biomes using the Vegetation Condition Index (VCI) for the period 1998 to 2017. To compare drought impact across the biomes, we computed the percentage area of the biome with seasonal VCI <30. The drought trend for each biome was computed for each pixel using a linear regression model in R software using the seasonal VCI images from 1998 to 2017. Our result showed that extreme drought impact on vegetation was mainly confined to the southwestern biomes (i.e. the Nama karoo and desert biomes) with most drought occurring during the first half of the season. We also observed an increasing trend of VCI (1998 to 2017) across all biomes and this increasing VCI trend might be explained by woody encroachment which is prevalent in the Savannah and Grassland biomes. The results of this study provide baseline information on drought hotspots.

Physiological mechanisms behind plant-herbivore interactions are commonly approached as input-output systems where the role of plant physiology is viewed as a black box. Studies evaluating impacts of defoliation on plant physiology have mostly focused on changes in photosynthesis while the overall impact on plant water relations is largely unknown. Stem hydraulic conductivity (k(h)), stem specific conductivity (k(s)), percent loss of hydraulic conductivity (PLC), CO(2) assimilation (A) and stomatal conductance (g(s)) were measured on well-irrigated 1-month-old Populus tremuloides (Michx.) defoliated and control seedlings until complete refoliation. PLC values of defoliated seedlings gradually increased during the refoliation process despite them being kept well irrigated. k(s) of defoliated seedlings gradually decreased during refoliation. PLC and k(s) values of control seedlings remained constant during refoliation. k(s) of new stems, leaf specific conductivity and A of leaves grown from new stems in defoliated and control seedlings were not significantly different, but g(s) was higher in defoliated than in control seedlings. The gradual increase of PLC and decrease of k(s) values in old stems after defoliation was unexpected under well-irrigated conditions, but appeared to have little impact on new stems formed after defoliation. The gradual loss of conductivity measured during the refoliation process under well-irrigated conditions suggests that young seedlings of P. tremuloides may be more susceptible to cavitation after herbivore damage under drought conditions.

Fire plays an important role in maintaining the savanna tree-grass balance by limiting the recruitment of heat-sensitive tree seedlings. However, fire behaviour may change under increasing CO2 concentrations, due to altered flammability of the grassy layer. Here, we determined the effect of predicted future CO2 concentrations, and how it interacts with water-availability, on grass flammability and traits influential to flammability, and uncovered the physiological mechanisms underpinning these responses. Using the widespread C4 savanna grass, Themeda triandra, as a model, we found that improved water-use efficiency under elevated CO2 (800 ppm) resulted in a larger (greater aboveground biomass), but wetter (higher moisture content) grass fuel load, that cured at a slower rate under drought conditions. These changes were associated with increased time to ignition, reduced flaming times and reduced predicted rate of spread. We modelled the effect of altered grass flammability on fire behaviour at a national level (South Africa), finding large-scale reductions in fire spread under elevated CO2, mitigating the converse effects of predicted increases in aridity, and marginal increases in fireline intensity. CO2-induced reductions in fire frequency, spread or intensity could have serious implications for savanna vegetation dynamics, possibly exacerbating the woody encroachment already seen in these ecosystems across the world.

AimClimate warming and increased wildfire activity are hypothesized to catalyse biogeographical shifts, reducing the resilience of fire‐prone forests world‐wide. Two key mechanisms underpinning hypotheses are: (1) reduced seed availability in large stand‐replacing burn patches, and (2) reduced seedling establishment/survival after post‐fire drought. We tested for regional evidence consistent with these mechanisms in an extensive fire‐prone forest biome by assessing post‐fire tree seedling establishment, a key indicator of forest resilience.LocationSubalpine forests, US Rocky Mountains.MethodsWe analysed post‐fire tree seedling establishment from 184 field plots where stand‐replacing forest fires were followed by varying post‐fire climate conditions. Generalized linear mixed models tested how establishment rates varied with post‐fire drought severity and distance to seed source (among other relevant factors) for tree species with contrasting post‐fire regeneration adaptations.ResultsTotal post‐fire tree seedling establishment (all species combined) declined sharply with greater post‐fire drought severity and with greater distance to seed sources (i.e. the interior of burn patches). Effects varied among key species groups. For conifers that dominate present‐day subalpine forests (Picea engelmannii,Abies lasiocarpa), post‐fire seedling establishment declined sharply with both factors. One exception was serotinousPinus contorta, which did not vary with either factor. For montane species expected to move upslope under future climate change (Larix occidentalis, Pseudotsuga menziesii,Populus tremuloides) and upper treeline species (Pinus albicaulis), establishment was unrelated to either factor. Greater post‐fire tree seedling establishment on cooler/wetter aspects suggested local topographic refugia during post‐fire droughts.Main conclusionsIf future drought and wildfire patterns manifest as expected, post‐fire tree seedling establishment of species that currently characterize subalpine forests could be substantially reduced. Compensatory increases from lower montane and upper treeline species may partially offset these reductions, but our data suggest important near‐ to mid‐term shifts in the composition and structure of high‐elevation forests under continued climate warming and increased wildfire activity.

Future climate changes are expected to increase risks of drought, which already represent the most common stress factor for stable barley (Hordeum vulgare L.) production in Iran. Up to now, extensive research projects have been done to study effects of drought stress on the antioxidant enzyme activity. While there is a few works of such studies on the field condition. In order to study of water deficit effects on the antioxidant enzymes activities as a secondary stress, we evaluate the effects of mild and severe drought stress on activities of antioxidative enzymes including superoxide dismutases, ascorbate peroxidase, catalase and peroxidase, among four barley genotypes, differing in the capacity to maintain the grain yield under drought condition during beginning on anthesis, kernel watery ripe and late milk stages under field condition. Results showed that drought increased the activity of antioxidant enzymes in all genotypes. At beginning of anthesis, POX activity of Q22 was higher than it in other genotypes ( P<0.01). Accordingly grain yield under drought condition Q22 is more tolerant than others. Therefore it seems POX has major role in reactive oxygen species (ROS) detoxifications under drought condition and can be use as biological marker for selection of tolerant barley genotypes at breeding programs.

AimOur understanding of how climate and fire have impacted quaking aspen (Populus tremuloidesMichx.) communities prior to the 20th century is fairly limited. This study analysed the period between 4500 and 2000 cal. yrbpto assess the pre‐historic role of climate and fire on an aspen community during an aspen‐dominated period.LocationLong Lake, south‐eastern Wyoming, central Rocky Mountains,USA.MethodsSedimentary pollen and charcoal were analysed to reconstruct the vegetation and fire history for a subalpine catchment currently dominated by lodgepole pine. Modern pollen‐climate relationships were applied to the fossil pollen spectra to interpret past climate variability. NonparametricANOVAand TukeyHSDtests were used to determine whether the reconstructed climate and fire parameters were different throughout the study period.ResultsThe modern pollen‐climate data suggest ac. 150‐year long drought centred on 4200 cal. yrbp, which caused the aspen ecotone to shift upslope. Between 3950 and 3450 cal. yrbp, an anomalous period of abundant quaking aspen pollen(Populus) occurred at the study site. Optimal climatic conditions coupled with frequent fires facilitated local quaking aspen dominance for roughly 500 years. After 3450 cal. yrbp,Populuspollen declined coincident with a return to less frequent fires and conifer dominance. Reconstructed climate variables from 550 cal. yrbpto present suggest conditions were not favourable for quaking aspen establishment at Long Lake. The TukeyHSDtest confirms that the period of abundantPopuluspollen was significantly different than any other period during this study.Main conclusionsQuaking aspen shifted upslope in response to warmer temperatures, and persisted for roughly 500 years as a result of optimal climatic conditions and frequent fire events.