How extreme summer weather may limit control of Festuca paniculata by mowing in subalpine grasslands

Background: The tussock grass Festuca paniculata can become strongly dominant in subalpine grasslands after cessation of mowing. The depletion of water-soluble carbohydrate (WSC) reserves has been suggested as a mechanism by which mowing can contain this species. By affecting plant physiology and especially by favouring WSC accumulation, extreme summer weather (i.e. exceptionally hot and dry) could however counterbalance the effects of mowing on WSC reserves in F. paniculata. The relevance of this hypothesis needs to be tested in the current context of climate and land-use changes.

Aims: We investigated (1) the physiological mechanisms that control the growth of F. paniculata, (2) how they are affected by mowing and (3) whether extreme summer heat and drought could influence physiological mechanisms and thereby the ecological response of F. paniculata to mowing.

Methods: In a field experiment we manipulated weather and mowing during two summers. For current summer weather (W0), ambient temperature was unchanged and precipitation was adjusted on the past 30-year average. Extreme summer weather (W+) corresponded to a seasonal change (+1 °C, –80% in precipitation compared to W0) and a three-week heatwave (+4.3 °C) in the first year. In addition, vegetation was either mown at 5 cm in late summer (M) or left unmown (U). Concentrations and absolute contents of WSC contained in tiller bases, leaf nitrogen concentration (LNC), vegetative multiplication, plant growth and leaf senescence were measured from one to four times, depending on the variable considered, throughout the summer of the second year of the experiment.

Results: As compared to the unmown treatment, late-summer mowing decreased tillering, tussock size and LNC, regardless of the summer weather treatment. However, it depleted WSC pools, including fructans, only under current summer weather (W0).

Conclusions: These results suggest that extreme summer heat and drought could alleviate the sensitivity of F. paniculata to mowing. They raise the question of the consequences of recurrent summer extremes for conservation management in subalpine grasslands.     

Carbohydrate and nitrogen stores in Festuca paniculata under mowing explain dominance in subalpine grasslands

Cessation of traditional management threatens semi‐natural grassland diversity through the colonisation or increase of competitive species adapted to nutrient‐poor conditions. Regular mowing is one practice that controls their abundance. This study evaluated the ecophysiological mechanisms limiting short‐ and long‐term recovery after mowing for Festuca paniculata, a competitive grass that takes over subalpine grasslands in the Alps following cessation of mowing. We quantified temporal variations in carbon (C) and nitrogen (N) content, starch, fructan and total soluble sugars in leaves, stem bases and roots of F. paniculata during one growth cycle in mown and unmown fields and related them to the dynamics of soil mineral N concentration and soil moisture. Short‐term results suggest that the regrowth of F. paniculata following mowing might be N‐limited, first because of N dilution by C increments in the plant tissue, and second, due to low soil mineral N and soil moisture at this time of year. However, despite short‐term effects of mowing on plant growth, C and N content and concentration at the beginning of the following growing season were not affected. Nevertheless, total biomass accumulation at peak standing biomass was largely reduced compared to unmown fields. Moreover, lower C storage capacity at the end of the growing season impacted C allocation to vegetative reproduction during winter, thereby dramatically limiting the horizontal growth of F. paniculata tussocks in the long term. We conclude that mowing reduces the growth of F. paniculata tussocks through both C and N limitation. Such results will help understanding how plant responses to defoliation regulate competitive interactions within plant communities.     

Effects of mowing on fungal endophytes and arbuscular mycorrhizal fungi in subalpine grasslands

In French subalpine grasslands, cessation of mowing promotes dominance of Festuca paniculata, which alters plant diversity and ecosystem functioning. One of the mechanisms underpinning such effects may be linked to simultaneous changes in the abundance of fungal symbionts such as endophytes and arbuscular mycorrhizal fungi. In field conditions, mowing reduced the abundance of the endophyte Neotyphodium sp. in leaves of F. paniculata by a factor of 6, and increased mycorrhizal densities by a factor of 15 in the soil. In greenhouse experiments, the mycorrhizal colonization of Trifolium pratense and Allium porrum increased 3- fold and 3.8- fold respectively in mown vs unmown grassland soil. Significantly reduced growth of the two host plants was also observed on soil from the unmown grassland. Such opposite effects of mowing on the two functional groups of fungal symbionts could suggest interactions between these two groups, which in turn could contribute to structuring plant communities in subalpine grasslands.     

Effects of extreme weather events and legume presence on mycorrhization of Plantago lanceolata and Holcus lanatus in the field

Little is known about direct and indirect effects of extreme weather events on arbuscular mycorrhizal fungi (AMF) under field conditions. In a field experiment, we investigated the response of mycorrhization to drought and heavy rain in grassland communities. We quantified AMF biomass in soil, mycorrhization of roots of the grass Holcus lanatus and the forb Plantago lanceolata, as well as plant performance. Plants were grown in four‐species communities with or without a legume. We hypothesised that drought increases and heavy rain decreases mycorrhization, and that higher mycorrhization will be linked to improved stress resistance and higher biomass production. Soil AMF biomass increased under both weather extremes. Heavy rain generally benefitted plants and increased arbuscules in P. lanceolata. Drought neither reduced plant performance nor root mycorrhization. Arbuscules increased in H. lanatus several weeks after drought, and in P. lanceolata several weeks after heavy rain spells. These long‐lasting effects of weather events on mycorrhization highlight the indirect influence of climate on AMF via their host plant. Legume presence increased plant community biomass, but had only minor effects on mycorrhization. Arbuscule colonisation was negatively correlated with senescence during the dry summer. Mycorrhization and biomass production in P. lanceolata were positively related. However, increased mycorrhization was related to less biomass in the grass. AMF mycelium in soil might generally increase under extreme events, root colonisation, however, is host species specific. This might amplify community shifts in grassland under climate change by further increasing stress resistance of species that already benefit from changed precipitation.     

Reinstatement of traditional mowing regimes counteracts population senescence in the rare perennial Primula vulgaris

Question: Traditional management of grassland verges or ditch banks included mowing as a way to provide additional harvesting of hay. Nowadays, such sites are often left unmanaged, as mowing verges is no longer profitable in modern agricultural systems. Are vulnerable plant species able to withstand competition with the surrounding vegetation and maintain viable populations under these circumstances? How do they respond to reinstatement of traditional mowing regimes? Location: Oedelem, northwestern Belgium. Methods: To investigate the effect of reinstatement of the rare perennial Primula vulgaris, demography and adult plant performance were monitored in a grassland verge between 1999 and 2003 under different mowing regimes. Year transitions between life stages were analysed with matrix population models. To disentangle the contributions of the deviations in different life stage transitions to the variation in overall population growth rate, life table response experiments were used. Results: Both management and year had a strong impact on demographic traits of P. vulgaris. If plots were left unmanaged, lower plant performance and declining population growth rates were observed. While population growth rates differed significantly between mowing regimes, mowing of plots only in July did not differ from mowing in July and October in terms of vegetative and reproductive output of adults. Mowing twice a year appeared to be most efficient in increasing population growth rate both by raising recruitment and growth of individuals into large reproductive adults. Conclusions: Large P. vulgaris populations show a good ability to recover from recent abandonment of traditional management regimes. By mowing twice a year, managers are able to target vital rates that are most influential: growth and flowering of adult individuals.     

Sensitivity and responses of diatoms to climate warming in lakes heavily influenced by humans

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Land use intensity,rather than plant species richness,affects the leaching risk of multiple nutrients from permanent grasslands

The intensification of land use constitutes one of the main drivers of global change and alters nutrient fluxes on all spatial scales, causing landscape‐level eutrophication and contamination of natural resources. Changes in soil nutrient concentrations are thus indicative for crucial environmental issues associated with intensive land use. We measured concentrations of NO₃–N, NH₄–N, P, K, Mg, and Ca using 1,326 ion‐exchange resin bags buried in 20 cm depth beneath the main root zone in 150 temperate grasslands. Nutrient concentrations were related to land use intensity, that is, fertilization, mowing, grazing intensities, and plant diversity by structural equation modeling. Furthermore, we assessed the response of soil nutrients to mechanical sward disturbance and subsequent reseeding, a common practice for grassland renewal. Land use intensity, especially fertilization, significantly increased the concentrations of NO₃–N, NH₄–N, K, P, and also Mg. Besides fertilization (and tightly correlated mowing) intensity, grazing strongly increased NO₃–N and K concentrations. Plant species richness decreased P and NO₃–N concentrations in soil when grassland productivity of the actual year was statistically taken into account, but not when long‐term averages of productivity were used. Thus, we assume that, in the actual study year, a distinct drought period might have caused the observed decoupling of productivity from fertilization and soil nutrients. Breaking up the grassland sward drastically increased NO₃–N concentrations (+146%) but reduced NH₄–N, P, and K concentrations, unbalancing soil nutrient stoichiometry and boosting the risk of N leaching. Reseeding the sward after disturbance did not have a short‐term effect on nutrient concentrations. We conclude that renewal of permanent grassland should be avoided as far as possible and future grassland management has to strongly rise the effectiveness of fertilization. Additionally, grassland management might have to increasingly taking care of periods of drought, in which nutrient additions might not increase plant growth but potentially only facilitate leaching.     

Does an extreme drought event alter the response of grassland communities to a changing climate?

Global temperatures and atmospheric CO₂ concentrations are expected to both increase, but their combined effect on plant communities has been far less investigated than the single factors of global change. Moreover, drought events are expected to become more frequent and intense in the near future what might alter plant responses to the changing climate.In this study synthesised grassland communities in a current or future climate were subjected to several drought levels (0, 15, 22 and 35 days of drought). The grassland communities were grown in six sunlit, climate-controlled chambers. Three of the chambers were exposed to ambient temperature and CO₂ (current climate), while the other three were continuously warmed 3 °C above ambient temperature at 620 ppm of CO₂ (future climate).The aim of this study was to investigate the effect of drought on the response of grassland communities to a future climate. Therefore, the response to future climate was observed (1) in the absence of drought and (2) in the occurrence of an extreme drought event, both early and late in the growing season.

• (1)In the absence of drought, plant productivity was positively affected by future climate early in the growing season. Later in the growing season this effect tended to turn negative, resulting in a disappearance of the overall effect of climate at the end of the growing season.
• (2)During drought there was a stronger decrease in net CO₂ assimilation rate (A_sat) in future than in current climate due to stronger stomatal closure. Consistently, the beneficial biomass response to future climate stagnated during drought. At the end of the season, after a period of recovery, there was no effect of climate on plant productivity. As in the absence of drought, plant productivity was not affected by climate at the end of the growing season. Hence, the occurrence of an extreme drought event during the growing season did not alter the overall response of plant productivity to a future climate.

     

Invasion windows for a global legume invader are revealed after joint examination of abiotic and biotic filters

• Successful alien plant invasion is influenced by both climate change and plant–plant interactions. We estimate the single and interactive effects of competition and extreme weather events on the performance of the global legume invader Lupinus polyphyllus (Lindl.).
• In three experimental studies we assessed (i) the stress tolerance of seedling and adult L. polyphyllus plants against extreme weather events (drought, fluctuating precipitation, late frost), (ii) the competitive effects of L. polyphyllus on native grassland species and vice versa, and (iii) the interactive effects of extreme weather events and competition on the performance of L. polyphyllus.
• Drought reduced growth and led to early senescence of L. polyphyllus but did not reduce adult survival. Fluctuating precipitation events and late frost reduced the length of inflorescences. Under control conditions, interspecific competition reduced photosynthetic activity and growth of L. polyphyllus. When subjected to competition during drought, L. polyphyllus conserved water while simultaneously maintaining high assimilation rates, demonstrating increased water use efficiency. Meanwhile, native species had reduced performance under drought.
• In summary, the invader gained an advantage under drought conditions through a smaller reduction in performance relative to its native competitors but was competitively inferior under control conditions. This provides evidence for a possible invasion window for this species. While regions of high elevation or latitude with regular severe late frost events might remain inaccessible for L. polyphyllus, further spread across Europe seems probable as the predicted increase in drought events may favour this non‐native legume over native species.

     

Restoration of Species‐Rich,Nutrient‐Limited Mountain Grassland by Mowing and Fertilization

Mowing and management to reduce nutrient levels have often been successfully used to restore species‐rich grasslands in various parts of Europe. However, such treatments have failed to restore the species‐rich Central European mountain grasslands dominated by Polygonum bistorta. P. bistorta builds an extensive underground rhizome system that monopolizes available nutrients in these nutrient‐poor grasslands, enabling this species to persist at high densities even in the presence of mowing. Therefore, we tested a restoration approach using a factorial combination of fertilization and mowing, as well as a litter removal treatment. The experiment was run over 5 years and species composition response to these treatments was recorded at two spatial scales. Mowing suppressed flowering and cover of P. bistorta and promoted target grassland species and richness. Fertilization prevented nutrient impoverishment and increased height and dominance of the broad‐leaved grasses with which many species‐rich grassland herbs could coexist. The additive effect of the mowing/fertilization treatments was strong enough to act as a driver of P. bistorta suppression and associated community change. The litter removal treatment, however, had little effect on plant composition. The experiment demonstrates that in nutrient‐limited grasslands, increasing nutrient levels in addition to mowing to manage competition for light can be used to control dominants. This contrasts with restoration of systems where after abandonment increased nutrient levels lead to the establishment of tall dominants that suppress other species by competition for light.     

Drought reduces floral resources for pollinators

Climate change is predicted to result in increased occurrence and intensity of drought in many regions worldwide. By increasing plant physiological stress, drought is likely to affect the floral resources (flowers, nectar and pollen) that are available to pollinators. However, little is known about impacts of drought at the community level, nor whether plant community functional composition influences these impacts. To address these knowledge gaps, we investigated the impacts of drought on floral resources in calcareous grassland. Drought was simulated using rain shelters and the impacts were explored at multiple scales and on four different experimental plant communities varying in functional trait composition. First, we investigated the effects of drought on nectar production of three common wildflower species (Lathyrus pratensis, Onobrychis viciifolia and Prunella vulgaris). In the drought treatment, L. pratensis and P. vulgaris had a lower proportion of flowers containing nectar and O. viciifolia had fewer flowers per raceme. Second, we measured the effects of drought on the diversity and abundance of floral resources across plant communities. Drought reduced the abundance of floral units for all plant communities, irrespective of functional composition, and reduced floral species richness for two of the communities. Functional diversity did not confer greater resistance to drought in terms of maintaining floral resources, probably because the effects of drought were ubiquitous across component plant communities. The findings indicate that drought has a substantial impact on the availability of floral resources in calcareous grassland, which will have consequences for pollinator behaviour and populations.     

Fungal communities differentially respond to warming and drought in tropical grassland soil

Climate change is predicted to cause more extreme events, such as heatwaves, and different precipitation patterns. The effects of warming and short‐term drought on soil microbial communities, in particular fungal communities, remain largely unexplored under field conditions. Here, we evaluated how the fungal community of a tropical grassland soil responds to these changes. A field experiment was carried out in a temperature free‐air controlled enhancement (T‐FACE) facility in Ribeirão Preto, Brazil. The isolated and combined effects of drought and a 2°C increase in temperature were investigated. Based on metabarcoding of the ITS2 region, a total of 771 operational taxonomic units were observed. While warming affected the community structure, drought affected the alpha diversity, and the interaction between warming and drought affected both diversity and structure. The change in community composition driven by warming affected only the less abundant species (>1% of the total sequences). The aspect of the fungal communities that was most affected was diversity, which was increased by drought (p < .05), mostly by reducing the dominance of a single species, as observed in the watered plots. In a phylogenetic context, some fungal taxa were favoured by changes in temperature (Hypocreales) and drought (Sordariales) or disadvantaged by both (Pleosporales). It was of note that a water deficit increased the abundance of phytopathogenic fungi, such as Curvularia, Thielavia and Fusarium species. Overall, our results provide evidence that fungal communities in tropical grassland soils have greater sensitivity to drought than to temperature, which might increase the incidence of certain soil‐borne diseases.     

施肥与刈割对内蒙古草地生态化学计量特征的影响

为明确施肥和刈割对蒙古半干旱草地四种植物化学计量特征的影响,该文于2014年8月以内蒙古半干旱草地的四种植物冷蒿(Artimesia frigida)、菊叶委陵菜(Potentilla tanacetifolia)、羊草(Aneurotepidimu chinense)、扁蓿豆(Melissitus ruthenica)为研究对象,利用方差分析及多重比较分析对照、施氮肥、割草、割草和施氮肥处理下的植物碳、氮、磷含量的差异以及不同处理下碳氮比和氮磷比的变化。结果表明:施氮肥与割草均未影响植物碳含量。施氮肥显著提高了植物氮含量(P<0.05),进而降低了碳氮比(P<0.05),提高了氮磷比(P<0.05)。割草处理后扁蓿豆的氮含量显著提高了0.18%(P<0.05),羊草的氮含量显著降低了0.13%(P<0.05),冷蒿与菊叶委陵菜的氮含量无显著变化。割草显著提高了冷蒿和菊叶委陵菜的磷含量(P<0.05),但是未影响羊草和扁蓿豆的磷含量。该研究证实了氮添加或割草会影响部分草地物种的化学计量特征,但是氮添加或割草对植物化学计量特征的影响存在种间差异。     

Effects of nitrogen addition and mowing on rodent damage in an Inner Mongolian steppe

Rodent damage is a serious threat to sustainable management of grassland. The effects of nitrogen (N) deposition and grassland management on rodent damage have been scarcely studied. Here, we reported the effects of 2 years of N addition and mowing on burrow density and damage area of Citellus dauricus in a semiarid steppe in Inner Mongolia. N addition significantly aggravated, while mowing alleviated rodent damage in the grassland under study. Burrow density and damage area increased 2.8‐fold and 4.7‐fold, in N addition plots compared to the ambient N addition treatment, respectively. Conversely, mowing decreased burrow density and damage area by 75.9% and 14.5%, respectively, compared to no mowing plots. Observed changes in rodent damage were mainly due to variations in plant community cover, height, and aboveground net primary productivity. Our findings demonstrate that N addition and mowing can affect the rodent density and activity in grassland, suggesting that the effects of a changing atmospheric composition and land use on rodent damage must be considered in order to achieve better grassland management.     

Plant sizes mediate mowing‐induced changes in nutrient stoichiometry and allocation of a perennial grass in semi‐arid grassland

While mowing‐induced changes in plant traits and their effects on ecosystem functioning in semi‐arid grassland are well studied, the relations between plant size and nutrient strategies are largely unknown. Mowing may drive the shifts of plant nutrient limitation and allocation. Here, we evaluated the changes in nutrient stoichiometry and allocation with variations in sizes of Leymus chinensis, the dominant plant species in Inner Mongolia grassland, to various mowing frequencies in a 17‐yr controlled experiment. Affected by mowing, the concentrations of nitrogen (N), phosphorus (P), and carbon (C) in leaves and stems were significantly increased, negatively correlating with plant sizes. Moreover, we found significant trade‐offs between the concentrations and accumulation of N, P, and C in plant tissues. The N:P ratios of L. chinensis aboveground biomass, linearly correlating with plant size, significantly decreased with increased mowing frequencies. The ratios of C:N and C:P of L. chinensis individuals were positively correlated with plant size, showing an exponential pattern. With increased mowing frequencies, L. chinensis size was correlated with the allocation ratios of leaves to stems of N, P, and C by the tendencies of negative parabola, positive, and negative linear. The results of structure equation modeling showed that the N, P, and C allocations were co‐regulated by biomass allocation and nutrient concentration ratios of leaves to stems. In summary, we found a significant decoupling effect between plant traits and nutrient strategies along mowing frequencies. Our results reveal a mechanism for how long‐term mowing‐induced changes in concentrations, accumulations, ecological stoichiometry, and allocations of key elements are mediated by the variations in plant sizes of perennial rhizome grass.     

Constraining invader dominance: Effects of repeated herbicidal management and environmental factors on curlyleaf pondweed dynamics in 50 Minnesota lakes

1. Curlyleaf pondweed (Potamogeton crispus) is one of the most widespread and widely managed aquatic invasive plants in North America. Despite decades of management, the efficacy of long-term management strategies and the effects of environmental drivers on curlyleaf pondweed populations remain uncertain.
2. To evaluate the effects of management and environmental factors on within-lake distribution and local density of curlyleaf pondweed, we collated monitoring data from point–intercept surveys collected by a variety of lake managers across Minnesota, U.S.A. Using this dataset, comprising 177 lake-years of plant data, we examined the influence of herbicide treatment, water clarity, snow depth, and ice cover duration on curlyleaf pondweed distribution and density between 2006 and 2015.
3. We evaluated the effects of herbicides on curlyleaf pondweed at three time points relative to treatment: within year, carryover effects the following year, and cumulative effects over multiple years of treatment. All three temporal measures were associated with significant reductions of curlyleaf pondweed. Additionally, herbicidal management reduced both the density and distribution of curlyleaf pondweed. Given that herbicide management led to reductions that carried over into future years, managers may be able to design multi-year treatments to reduce total management effort over time.
4. We also found strong effects of environmental conditions on curlyleaf pondweed. Elevated lake productivity and decreased winter snow cover were associated with increased springtime distributions of curlyleaf pondweed, whereas duration of winter ice cover had no influence. The influence of productivity suggests that reductions of this invasive species may be an ancillary benefit of water-quality improvements that lower lakes’ trophic status. Our results also show that decreased winter snow cover, as predicted under climate change, could exacerbate problematic growth of curlyleaf pondweed.
5. Harnessing monitoring data from multiple projects, as this study does, allows for robust inference about environmental and management constraints on macrophytes. Because of environmental and management variability, we suggest that treatment regimens follow an adaptive management cycle, with outcomes of management monitored and evaluated, and strategies updated accordingly. It is also vital to continue monitoring both managed and unmanaged lakes to enable stronger inferences about treatment effectiveness.

     

Invader presence disrupts the stabilizing effect of species richness in plant community recovery after drought

Higher biodiversity can stabilize the productivity and functioning of grassland communities when subjected to extreme climatic events. The positive biodiversity–stability relationship emerges via increased resistance and/or recovery to these events. However, invader presence might disrupt this diversity–stability relationship by altering biotic interactions. Investigating such disruptions is important given that invasion by non‐native species and extreme climatic events are expected to increase in the future due to anthropogenic pressure. Here we present one of the first multisite invader × biodiversity × drought manipulation experiment to examine combined effects of biodiversity and invasion on drought resistance and recovery at three semi‐natural grassland sites across Europe. The stability of biomass production to an extreme drought manipulation (100% rainfall reduction; BE: 88 days, BG: 85 days, DE: 76 days) was quantified in field mesocosms with a richness gradient of 1, 3, and 6 species and three invasion treatments (no invader, Lupinus polyphyllus, Senecio inaequidens). Our results suggest that biodiversity stabilized community productivity by increasing the ability of native species to recover from extreme drought events. However, invader presence turned the positive and stabilizing effects of diversity on native species recovery into a neutral relationship. This effect was independent of the two invader's own capacity to recover from an extreme drought event. In summary, we found that invader presence may disrupt how native community interactions lead to stability of ecosystems in response to extreme climatic events. Consequently, the interaction of three global change drivers, climate extremes, diversity decline, and invasive species, may exacerbate their effects on ecosystem functioning.     

Fire frequency regulates tussock grass composition,structure and resilience in endangered temperate woodlands

Abstract The importance of disturbance for regulating the structure and diversity of grassy ecosystems is widely recognized, but disturbance‐mediated interactions between grassland composition and grassland resilience, and consequent implications for conservation management, are less well documented. We established replicated burning, mowing and (non‐livestock) grazing regimes in two contrasting grassy woodland remnants in south‐eastern Australia, and monitored the dynamics and resilience of the matrix‐forming tussock grasses, Poa sieberiana (Poa) and Themeda australis (Themeda), over 12 years. Introduction of frequent burning to a Poa‐dominated understorey in a rarely burnt woodland enhanced dominance by Themeda, and conversely, reduced fire frequency in a frequently burnt Themeda grassland substantially increased Poa abundance. Burning was potentially detrimental in the Poa‐dominated woodland, but sward resilience (recovery after the 2002 burn) increased as Themeda increased with repeated burning. By contrast, the Themeda grassland was resilient to 4‐ and 8‐yearly burning, but biennial burning led to poor resilience and high tussock mortality under drought conditions. Contrary to other mesic grasslands, cessation of burning had not caused sward collapse by 14 years post‐fire despite high litter accumulation, potentially due to compensatory growth of Poa, lower site productivity and drought. Biennial mowing without slash removal was similar to 4‐yearly burning in effects, while exclusion from kangaroo and rabbit grazing significantly increased sward biomass and contributed to increased Poa cover and inflorescence production. We conclude that functional complementarity associated with mixed dominants enhances resilience to variable disturbance regimes, and that below certain thresholds of abundance of each dominant, this resilience declines. Conservation management of Themeda–Poa ecosystems should thus aim to maintain an effective balance of these dominants.     

Alpine plant functional group responses to fertiliser addition depend on abiotic regime and community composition

Background and aims

We ask how productivity responses of alpine plant communities to increased nutrient availability can be predicted from abiotic regime and initial functional type composition.

Methods

We compared four Caucasian alpine plant communities (lichen heath, Festuca varia grassland, Geranium-Hedysarum meadow, snow bed community) forming a toposequence and contrasting in productivity and dominance structure for biomass responses to experimental fertilization (N, P, NP, Ca) and irrigation for 4–5?years.

Results

The dominant plants in more productive communities monopolized added N and P, at the expense of their neighbors. In three out of four communities, N and P fertilizations gave greater aboveground biomass increase than N or P fertilization alone, indicating overall co-limitation of N and P, with N being most limiting. Relative biomass increase in NP treatment was negatively related to biomass in control plots across the four communities. Grasses often responded more vigorously to P, but sedges to N alone. Finally, we present one of the rare examples of a forb showing a strong N or NP response.

Conclusion

Our findings will help improve our ability to predict community composition and biomass dynamics in cool ecosystems subject to changing nutrient availability as induced by climate or land-use changes.     

Extreme climatic events affect populations of Asian chestnut gall wasps,Dryocosmus kuriphilus,but do not stop the spread

1. Global climate change affects the frequency of extreme weather events that can influence plant–insect interactions.
2. We evaluated how the late-spring frost and severe drought that occurred in Spain in 2017 affected interactions between the invasive gall insect, Dryocosmus kuriphilus, and the native tree, Castanea sativa. We assessed effects on insect survival, fertility, population growth, and effects through changes in tree palatability and in other pests and pathogens.
3. Late-spring frost reduced D. kuriphilus to 25–40% of previous abundance. Wasp populations recovered rapidly (>7-fold in 3 years), consistent with density-dependence in population dynamics.
4. Larvae affected by freeze or drought were smaller. Female fecundity was affected by the freeze 1 year later.
5. Late-spring frosts and severe drought affected leaf size and physiology. Water content was higher within galls, but nitrogen was higher within galls in non-freeze plots after weather conditions improved.
6. Freezing also influenced the secondary chemistry of leaves. Phenol concentrations were lower, and terpenes higher, in frozen plots, while condensed tannins remained the same. Condensed tannins were reduced to half in the drought year.
7. Freezing had limited effects on damage from other pests and pathogens.
8. Our work expands understanding of how climate and weather affects forest pests.