Contrasting physiological responsiveness of establishing trees and a C4 grass to rainfall events,intensified summer drought,and warming in oak savanna

Climate warming and drought may alter tree establishment in savannas through differential responses of tree seedlings and grass to intermittent rainfall events. We investigated leaf gas exchange responses of dominant post oak savanna tree (Quercus stellata and Juniperus virginiana) and grass (Schizachyrium scoparium, C₄ grass) species to summer rainfall events under an ambient and intensified summer drought scenario in factorial combination with warming (ambient, +1.5 °C) in both monoculture and tree‐grass mixtures. The three species differed in drought resistance and response of leaf gas exchange to rainfall events throughout the summer. S. scoparium experienced the greatest decrease in A_area (?56% and ?66% under normal and intensified drought, respectively) over the summer, followed by Q. stellata (?44%, ?64%), while J. virginiana showed increased A_area under normal drought (+13%) and a small decrease in A_area when exposed to intensified summer drought (?10%). Following individual rainfall events, mean increases in A_area were 90% for S. scoparium, 26% for J. virginiana and 22% for Q. stellata. The responsiveness of A_area of S. scoparium to rainfall events initially increased with the onset of drought, but decreased dramatically as summer drought progressed. For Q. stellata, A_area recovery decreased as drought progressed and with warming. In contrast, J. virginiana showed minimal fluctuations in A_area following rainfall events, in spite of declining water potential, and warming enhanced recovery. J. virginiana will likely gain an advantage over Q. stellata during establishment under future climatic scenarios. Additionally, the competitive advantage of C₄ grasses may be reduced relative to trees, because grasses will likely exist below a critical water stress threshold more often in a warmer, drier climate. Recognition of unique species responses to critical global change drivers in the presence of competition will improve predictions of grass–tree interactions and tree establishment in savannas in response to climate change.     

Distinct roles of savanna and forest tree species in regeneration under fire suppression in a Brazilian savanna

Questions: Has fire suppression relaxed barriers to the exchange of species between savanna and forest? Do all species or a subset of species participate in this exchange? Would current vegetation structure persist if fire suppression were to cease? Location: A gallery forest edge in the Cerrado region of central Brazil that burned only once in the past 35 years. Methods: Density of tree seedlings, saplings and adults, leaf area index (LAI), tree basal area and diameter were surveyed in 12, 10 m × 70 m transects centred on and perpendicular to the forest–savanna boundary. Community composition was assessed using non‐metric multi‐dimensional scaling (NMDS). Results: Basal area and LAI declined substantially from forest to savanna, with an associated shift in species composition. Savanna tree species were nearly absent in the forest, but accounted for the majority of stems in the savanna. In contrast, forest species comprised 14% of adults and more than one‐third of juveniles in the savanna. Despite the high diversity of trees (85 species) in the forest, five species play a particularly large role in this initial phase of forest expansion. Reintroduction of fire, however, would result in widespread topkill of juveniles and the majority of adult forest trees, thereby interrupting the succession towards forest. Conclusions: After 35 years during which the site burned only once, the savanna still remains dominated by savanna species. Nevertheless, the dominance of forest juveniles in border and savanna tree communities suggests that with a continued policy of fire suppression, the forest will continue to expand.     

Vegetation state changes in the course of shrub encroachment in an African savanna since about 1850 CE and their potential drivers

1. Shrub encroachment has far‐reaching ecological and economic consequences in many ecosystems worldwide. Yet, compositional changes associated with shrub encroachment are often overlooked despite having important effects on ecosystem functioning.
2. We document the compositional change and potential drivers for a northern Namibian Combretum woodland transitioning into a Terminalia shrubland. We use a multiproxy record (pollen, sedimentary ancient DNA, biomarkers, compound‐specific carbon (δ¹³C) and deuterium (δD) isotopes, bulk carbon isotopes (δ¹³Corg), grain size, geochemical properties) from Lake Otjikoto at high taxonomical and temporal resolution.
3. We provide evidence that state changes in semiarid environments may occur on a scale of one century and that transitions between stable states can span around 80 years and are characterized by a unique vegetation composition. We demonstrate that the current grass/woody ratio is exceptional for the last 170 years, as supported by n‐alkane distributions and the δ¹³C and δ¹³Corg records. Comparing vegetation records to environmental proxy data and census data, we infer a complex network of global and local drivers of vegetation change. While our δD record suggests physiological adaptations of woody species to higher atmospheric pCO₂ concentration and drought, our vegetation records reflect the impact of broad‐scale logging for the mining industry, and the macrocharcoal record suggests a decrease in fire activity associated with the intensification of farming. Impact of selective grazing is reflected by changes in abundance and taxonomical composition of grasses and by an increase of nonpalatable and trampling‐resistant taxa. In addition, grain‐size and spore records suggest changes in the erodibility of soils because of reduced grass cover.
4. Synthesis. We conclude that transitions to an encroached savanna state are supported by gradual environmental changes induced by management strategies, which affected the resilience of savanna ecosystems. In addition, feedback mechanisms that reflect the interplay between management legacies and climate change maintain the encroached state.

     

A grass–fire cycle eliminates an obligate‐seeding tree in a tropical savanna

A grass–fire cycle in Australian tropical savannas has been postulated as driving the regional decline of the obligate-seeding conifer Callitris intratropica and other fire-sensitive components of the regional flora and fauna, due to proliferation of flammable native grasses. We tested the hypothesis that a high-biomass invasive savanna grass drives a positive feedback process where intense fires destroy fire-sensitive trees, and the reduction in canopy cover facilitates further invasion by grass. We undertook an observational and experimental study using, as a model system, a plantation of C. intratropica that has been invaded by an African grass, gamba (Andropogon gayanus) in the Northern Territory, Australia. We found that high grass biomass was associated with reduced canopy cover and restriction of foliage to the upper canopy of surviving stems, and mortality of adult trees was very high (>50%) even in areas with low fuel loads (1 t·ha⁻¹). Experimental fires, with fuel loads >10 t·ha⁻¹, typical of the grass-invasion front, caused significant mortality due to complete crown scorch. Lower fuel loads cause reduced canopy cover through defoliation of the lower canopy. These results help explain how increases in grass biomass are coupled with the decline of C. intratropica throughout northern Australia by causing a switch from litter and sparse perennial grass fuels, and hence low-intensity surface fires, to heavy annual grass fuel loads that sustain fires that burn into the midstorey. This study demonstrates that changes in fuel type can alter fire regimes with substantial knock-on effects on the biota.     

Temperature‐related increases in grass growth and greater competition for food drive earlier migrational departure of wintering Whooper Swans

Understanding the departure decisions of migratory birds is critical for determining how changing climatic conditions will influence subsequent arrival times on the breeding grounds. A long‐term dataset (1972–2008) of Whooper Swan Cygnus cygnus departure dates from a wintering site in Ireland was used to assess the factors determining the timing of migration. Early and late migrating swans showed different departure patterns. Earlier wintering ground departure was more pronounced for the first 50% of the population than the last 10% of departing individuals. Earlier departure was associated with an increase in February temperatures at the wintering site for all departure phases except the date when the last individual departed. The date by which the first 50% of Swans had departed was earlier with increasing numbers of wintering Swans, suggesting that competition on the wintering grounds may further influence the timing of departure. The results also suggested that departure is mediated by the influence of spring temperature on food resources, with increased February grass growth in warmer years enabling earlier departure of migrating Swans. To determine why arrival dates in the breeding ground have altered, environmental conditions in the wintering grounds must be taken into account.     

Climate warming increases biological control agent impact on a non‐target species

Climate change may shift interactions of invasive plants, herbivorous insects and native plants, potentially affecting biological control efficacy and non‐target effects on native species. Here, we show how climate warming affects impacts of a multivoltine introduced biocontrol beetle on the non‐target native plant Alternanthera sessilis in China. In field surveys across a latitudinal gradient covering their full distributions, we found beetle damage on A. sessilis increased with rising temperature and plant life history changed from perennial to annual. Experiments showed that elevated temperature changed plant life history and increased insect overwintering, damage and impacts on seedling recruitment. These results suggest that warming can shift phenologies, increase non‐target effect magnitude and increase non‐target effect occurrence by beetle range expansion to additional areas where A. sessilis occurs. This study highlights the importance of understanding how climate change affects species interactions for future biological control of invasive species and conservation of native species.     

Net primary productivity and rain‐use efficiency as affected by warming,altered precipitation,and clipping in a mixed‐grass prairie

Grassland productivity in response to climate change and land use is a global concern. In order to explore the effects of climate change and land use on net primary productivity (NPP), NPP partitioning [f_BNPP, defined as the fraction of belowground NPP (BNPP) to NPP], and rain‐use efficiency (RUE) of NPP, we conducted a field experiment with warming (+3 °C), altered precipitation (double and half), and annual clipping in a mixed‐grass prairie in Oklahoma, USA since July, 2009. Across the years, warming significantly increased BNPP, f_BNPP, and RUE_BNPP by an average of 11.6%, 2.8%, and 6.6%, respectively. This indicates that BNPP was more sensitive to warming than aboveground NPP (ANPP) since warming did not change ANPP and RUE_ANPP much. Double precipitation stimulated ANPP, BNPP, and NPP but suppressed RUE_ANPP, RUE_BNPP, and RUE_NPP while half precipitation decreased ANPP, BNPP, and NPP but increased RUE_ANPP, RUE_BNPP, and RUE_NPP. Clipping interacted with altered precipitation in impacting RUE_ANPP, RUE_BNPP, and RUE_NPP, suggesting land use could confound the effects of precipitation changes on ecosystem processes. Soil moisture was found to be a main factor in regulating variation in ANPP, BNPP, and NPP while soil temperature was the dominant factor influencing f_BNPP. These findings suggest that BNPP is critical point to future research. Additionally, results from single‐factor manipulative experiments should be treated with caution due to the non‐additive interactive effects of warming with altered precipitation and land use (clipping).     

Fire‐sensitive species dominate seed rain after fire suppression: Implications for plant community diversity and woody encroachment in the Cerrado

Woody encroachment is becoming common in tropical savannas. We studied natural seed rain and performed seed addition experiments in a Brazilian savanna that had not been burned for several decades. We found greater abundance of fire‐sensitive species in the seed rain, likely contributing to woody encroachment. Flexible fire management policies that allow for natural and prescribed fires may be required to maintain savanna diversity.     

Climate change‐driven extinctions of tree species affect forest functioning more than random extinctions

Aim

Climate change affects forest functioning not only through direct physiological effects such as modifying photosynthesis and growing season lengths, but also through indirect effects on community composition related to species extinctions and colonizations. Such indirect effects remain poorly explored in comparison with the direct ones. Biodiversity–ecosystem functioning (BEF) studies commonly examine the effects of species loss by eliminating species randomly. However, species extinctions caused by climate change will depend on the species’ vulnerability to the new environmental conditions, thus occurring in a specific, non‐random order. Here, we evaluated whether successive tree species extinctions, according to their vulnerability to climate change, impact forest functions differently than random species losses.

Location

Eleven temperate forests across a gradient of climatic conditions in central Europe.

Methods

We simulated tree community dynamics with a forest succession model to study the impact of species loss on the communities’ aboveground biomass, productivity and temporal stability. Tree species were removed from the local pool (1) randomly, and according to (2) their inability to be recruited under a warmer climate or (3) their increased mortality under drier conditions.

Results

Results showed that non‐random species loss (i.e., based on their vulnerability to warmer or drier conditions) changed forest functioning at a different rate, and sometimes direction, than random species loss. Furthermore, directed extinctions, unlike random, triggered tipping points along the species loss process where forest functions were strongly impacted. These tipping points occurred after fewer extinctions in forests located in the coldest areas, where ecosystem functioning relies on fewer species.

Main conclusions

We showed that the extinction of species in a deterministic and mechanistically motivated order, in this case the species vulnerability to climate change, strengthens the selection effect of diversity on ecosystem functioning. BEF studies exploring the impact of species loss on ecosystem functioning using random extinctions thus possibly underestimate the potential effect of biodiversity loss when driven by a directional force, such as climate change.

     

Increased heat requirement for leaf flushing in temperate woody species over 1980–2012: effects of chilling,precipitation and insolation

Recent studies have revealed large unexplained variation in heat requirement‐based phenology models, resulting in large uncertainty when predicting ecosystem carbon and water balance responses to climate variability. Improving our understanding of the heat requirement for spring phenology is thus urgently needed. In this study, we estimated the species‐specific heat requirement for leaf flushing of 13 temperate woody species using long‐term phenological observations from Europe and North America. The species were defined as early and late flushing species according to the mean date of leaf flushing across all sites. Partial correlation analyses were applied to determine the temporal correlations between heat requirement and chilling accumulation, precipitation and insolation sum during dormancy. We found that the heat requirement for leaf flushing increased by almost 50% over the study period 1980–2012, with an average of 30 heat units per decade. This temporal increase in heat requirement was observed in all species, but was much larger for late than for early flushing species. Consistent with previous studies, we found that the heat requirement negatively correlates with chilling accumulation. Interestingly, after removing the variation induced by chilling accumulation, a predominantly positive partial correlation exists between heat requirement and precipitation sum, and a predominantly negative correlation between heat requirement and insolation sum. This suggests that besides the well‐known effect of chilling, the heat requirement for leaf flushing is also influenced by precipitation and insolation sum during dormancy. However, we hypothesize that the observed precipitation and insolation effects might be artefacts attributable to the inappropriate use of air temperature in the heat requirement quantification. Rather than air temperature, meristem temperature is probably the prominent driver of the leaf flushing process, but these data are not available. Further experimental research is thus needed to verify whether insolation and precipitation sums directly affect the heat requirement for leaf flushing.     

Thermal adaptation affects interactions between a range‐expanding and a native odonate species

1. Increasing temperature and invading species may interact in their effects on communities. In this study, we investigated how rising temperatures alter larval interactions between a naturally range‐expanding dragonfly, Crocothemis erythraea, and a native northern European species, Leucorrhinia dubia. Initial studies revealed that C. erythraea grow up to 3.5 times faster than L. dubia at temperatures above 16 °C. As a result, we hypothesised that divergent temperature responses would lead to rapid size differences between coexisting larvae and, consequently, to asymmetric intraguild predation at higher ambient temperatures. 2. Mortality and growth rates were measured in interaction treatments (with both species present) and non‐interaction controls (one species present) at four different temperature regimes: at an ambient temperature representative of central Germany, where both species overlap in distribution, and at temperatures increased by 2, 4 and 6 °C. 3. The mortality of C. erythraea did not differ between treatment and control. In contrast, mortality of L. dubia remained similar over all temperatures in the controls, but increased with temperature in the presence of the other species and was significantly higher there than in the controls. We concluded that L. dubia suffered asymmetric intraguild predation, particularly at increased temperature. Reduced growth rate of L. dubia in the interaction treatment at higher temperatures also suggested asymmetric competition for prey in the first phase of the experiment. 4. The results imply that the range expansion of C. erythraea may cause reduction in population size of syntopic L. dubia when temperature rises by more than 2 °C. The consequences for future range patterns, as well as other factors that may influence the interaction in nature, are discussed.     

Diversity in thermal affinity among key piscivores buffers impacts of ocean warming on predator–prey interactions

Asymmetries in responses to climate change have the potential to alter important predator–prey interactions, in part by altering the location and size of spatial refugia for prey. We evaluated the effect of ocean warming on interactions between four important piscivores and four of their prey in the U.S. Northeast Shelf by examining species overlap under historical conditions (1968–2014) and with a doubling in CO₂. Because both predator and prey shift their distributions in response to changing ocean conditions, the net impact of warming or cooling on predator–prey interactions was not determined a priori from the range extent of either predator or prey alone. For Atlantic cod, an historically dominant piscivore in the region, we found that both historical and future warming led to a decline in the proportion of prey species’ range it occupied and caused a potential reduction in its ability to exert top‐down control on these prey. In contrast, the potential for overlap of spiny dogfish with prey species was enhanced by warming, expanding their importance as predators in this system. In sum, the decline in the ecological role for cod that began with overfishing in this ecosystem will likely be exacerbated by warming, but this loss may be counteracted by the rise in dominance of other piscivores with contrasting thermal preferences. Functional diversity in thermal affinity within the piscivore guild may therefore buffer against the impact of warming on marine ecosystems, suggesting a novel mechanism by which diversity confers resilience.     

Among‐tree variability and feedback effects result in different growth responses to climate change at the upper treeline in the Swiss Alps

Upper treeline ecotones are important life form boundaries and particularly sensitive to a warming climate. Changes in growth conditions at these ecotones have wide‐ranging implications for the provision of ecosystem services in densely populated mountain regions like the European Alps. We quantify climate effects on short‐ and long‐term tree growth responses, focusing on among‐tree variability and potential feedback effects. Although among‐tree variability is thought to be substantial, it has not been considered systematically yet in studies on growth–climate relationships. We compiled tree‐ring data including almost 600 trees of major treeline species (Larix decidua, Picea abies, Pinus cembra, and Pinus mugo) from three climate regions of the Swiss Alps. We further acquired tree size distribution data using unmanned aerial vehicles. To account for among‐tree variability, we employed information‐theoretic model selections based on linear mixed‐effects models (LMMs) with flexible choice of monthly temperature effects on growth. We isolated long‐term trends in ring‐width indices (RWI) in interaction with elevation. The LMMs revealed substantial amounts of previously unquantified among‐tree variability, indicating different strategies of single trees regarding when and to what extent to invest assimilates into growth. Furthermore, the LMMs indicated strongly positive temperature effects on growth during short summer periods across all species, and significant contributions of fall (L. decidua) and current year's spring (L. decidua, P. abies). In the longer term, all species showed consistently positive RWI trends at highest elevations, but different patterns with decreasing elevation. L. decidua exhibited even negative RWI trends compared to the highest treeline sites, whereas P. abies, P. cembra, and P. mugo showed steeper or flatter trends with decreasing elevation. This does not only reflect effects of ameliorated climate conditions on tree growth over time, but also reveals first signs of long‐suspected negative and positive feedback of climate change on stand dynamics at treeline.     

Vapor–pressure deficit and extreme climatic variables limit tree growth

Assessing the effect of global warming on forest growth requires a better understanding of species‐specific responses to climate change conditions. Norway spruce and European beech are among the dominant tree species in Europe and are largely used by the timber industry. Their sensitivity to changes in climate and extreme climatic events, however, endangers their future sustainability. Identifying the key climatic factors limiting their growth and survival is therefore crucial for assessing the responses of these two species to ongoing climate change. We studied the vulnerability of beech and spruce to warmer and drier conditions by transplanting saplings from the top to the bottom of an elevational gradient in the Jura Mountains in Switzerland. We (1) demonstrated that a longer growing season due to warming could not fully account for the positive growth responses, and the positive effect on sapling productivity was species‐dependent, (2) demonstrated that the contrasting growth responses of beech and spruce were mainly due to different sensitivities to elevated vapor–pressure deficits (VPD), (3) determined the species‐specific limits to VPD above which growth rate began to decline, and (4) demonstrated that models incorporating extreme climatic events could account for the response of growth to warming better than models using only average values. These results support that the sustainability of forest trees in the coming decades will depend on how extreme climatic events will change, irrespective of the overall warming trend.     

Impacts of warming and elevated CO2 on a semi‐arid grassland are non‐additive,shift with precipitation,and reverse over time

It is unclear how elevated CO₂ (eCO₂) and the corresponding shifts in temperature and precipitation will interact to impact ecosystems over time. During a 7‐year experiment in a semi‐arid grassland, the response of plant biomass to eCO₂ and warming was largely regulated by interannual precipitation, while the response of plant community composition was more sensitive to experiment duration. The combined effects of eCO₂ and warming on aboveground plant biomass were less positive in ‘wet’ growing seasons, but total plant biomass was consistently stimulated by ~ 25% due to unique, supra‐additive responses of roots. Independent of precipitation, the combined effects of eCO₂ and warming on C₃ graminoids became increasingly positive and supra‐additive over time, reversing an initial shift toward C₄ grasses. Soil resources also responded dynamically and non‐additively to eCO₂ and warming, shaping the plant responses. Our results suggest grasslands are poised for drastic changes in function and highlight the need for long‐term, factorial experiments.