Random Fruits on the Zielonka Tree

Stochastic games are a natural model for the synthesis of controllers confronted to adversarial and/or random actions. In particular, $\omega$-regular games of infinite length can represent reactive systems which are not expected to reach a correct state, but rather to handle a continuous stream of events. One critical resource in such applications is the memory used by the controller. In this paper, we study the amount of memory that can be saved through the use of randomisation in strategies, and present matching upper and lower bounds for stochastic Muller games

Random Fruits on the Zielonka Tree

Stochastic games are a natural model for the synthesis of controllers confronted to adversarial and/or random actions. In particular, $\omega$-regular games of infinite length can represent reactive systems which are not expected to reach a correct state, but rather to handle a continuous stream of events. One critical resource in such applications is the memory used by the controller. In this paper, we study the amount of memory that can be saved through the use of randomisation in strategies, and present matching upper and lower bounds for stochastic Muller games

How do we remember the past in randomised strategies?

Graph games of infinite length are a natural model for open reactive processes: one player represents the controller, trying to ensure a given specification, and the other represents a hostile environment. The evolution of the system depends on the decisions of both players, supplemented by chance. In this work, we focus on the notion of randomised strategy. More specifically, we show that three natural definitions may lead to very different results: in the most general cases, an almost-surely winning situation may become almost-surely losing if the player is only allowed to use a weaker notion of strategy. In more reasonable settings, translations exist, but they require infinite memory, even in simple cases. Finally, some traditional problems becomes undecidable for the strongest type of strategies

Declining fruit production before death in a widely distributed tree species, Sorbus aucuparia L.

International audienceAbstractKey messageTrees are commonly thought to increase their seed production before death. We tested this terminal investment hypothesis using long-term data on rowan trees (Sorbus aucuparia) and found no support. Rather, seed production declined significantly before death, which points to the potential detrimental effects of reproductive senescence on regeneration in stands of old trees.ContextAging poses a fundamental challenge for long-lived organisms. As mortality changes with with age due to actuarial senescence, reproductive senescence may also lead to declines in fertility. However, life history theory predicts that reproductive investment should increase before mortality to maximize lifetime reproductive success, a phenomenon termed terminal investment.AimsTo date, it is unclear whether long-lived, indeterminantly growing trees experience reproductive senescence or display terminal investment.MethodsWe investigated fruit production of rowan (Sorbus aucuparia L.), widely distributed trees that live up to 150 years, as they approached death.ResultsIn our study population in Poland’s Carpathian Mountains, 79 trees that died produced up to 20% fewer fruits in the years before their demise compared to 199 surviving trees of the same population.ConclusionThe pattern of reproductive investment in S. aucuparia is suggestive of age-independent reproductive senescence rather than terminal investment. These findings highlight that the understanding of the generality of life history strategies across diverse taxa of perennial plants is still in its infancy

Perfect poopers; passerine birds facilitate sexual reproduction in clonal keystone plants of the boreal forest through directed endozoochory towards dead wood

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Reproductive phenology determines the linkages between radial growth, fruit production and climate in four Mediterranean tree species

The influence of climate on seed or fruit production and tree growth is a central question in forest ecology, with a key role on forest dynamics. However, the mechanisms linking inter- and intra-annual climate variability, fruiting and growth remain poorly understood, although they seem to be largely species-specific. The resource-matching hypothesis posits that fruit production responds to annual environmental variability, predicting a positive relationship between reproduction and growth, whereas other hypotheses such as resource switching or resource budget imply allocation trade-offs between reproduction and growth. Climate control over fruiting is thought to be more likely in variable, limiting environments such as those existing in seasonally dry Mediterranean forests. However, the studies about these forests are much scarcer than on temperate ecosystems, probably due to the sparseness and shortness of fruit production records. Here we analysed the relationships between monthly climate variables, tree-ring widths and fruit production at stand level in four tree species under different climatic conditions in Spain (12-27 trees and 24-54 radii were sampled in each site). For the species from xeric sites such as Pinus pinea, Castanea sativa and Quercus ilex, tree-ring width positively correlated with fruit production, as envisaged by the resource-matching hypothesis. This association was driven by prior wet winter climate conditions, which enhanced growth. Some of the significant correlations between climate variables and fruit yield corresponded to key reproductive phenophases. On the other hand, for the species from mesic sites (Abies alba) no positive growth–fruit production correlation was found, likely due to the mismatch in the effect of climatic factors on growth and fruiting. These results support the hypothesis that climatic conditions, particularly a positive water balance, play a major role as a proximate cause of fruit production in seasonally dry Mediterranean forests and underscore the importance of species-specific reproductive phenology traitsPublishe

Tormikahjustuste käigus tekkinud mikroalade dünaamika ja häiringujärgne puurinde uuenemine hemiboreaalses segametsas

Windstorm, among the main natural disturbance agents in central and northern Europe, is found to have controversial effects on the direction of forest succession. Storm severity explains part of the contradiction, but the composition and distribution of biotic and abiotic storm legacies may also play a role. Storm-related microsites consist of the pit and mound complexes caused by uplifted root-plates, and of coarse woody debris (CWD). This thesis discusses availability and relevant conditions of microsites for tree regeneration development in storm-damaged areas at different successional stages. Furthermore, post-disturbance regeneration performance is evaluated based on a series of field inventories in two mature spruce-broadleaf mixed forests on humid, eutrophic soils in Estonia. Two major storms hit the areas in 2001 and 2002 respectively. Regeneration was investigated in patches with different disturbance severities. The focal species were Norway spruce (Picea abies (L.) Karst.), silver and downy birch (Betula pendula Roth. and B. pubescens Ehrh.), black alder (Alnus glutinosa (L.) J. Gaertn.) and European rowan (Sorbus aucuparia L.). Heterogeneity in microsite conditions is the result of variation in elevation, vegetation cover, surfacing bare mineral soil, litter and nutrients accumulation or runoff and substrate stability and penetrability. With time since disturbance, light incidence generally decreases while CWD gets more suitable as a germination substrate, which implies that timing of establishment and optimal growth conditions do not always match. Throughout inventories in the two storm areas in Estonia, pits in heavily damaged areas were dominated by birch, pits in moderately damaged areas by black alder and the mounds in heavily and moderately damaged areas by rowan and Norway spruce, respectively. Regeneration species composition turned out most diverse in moderately damaged areas. Harvested sites were significantly most densely populated, with birch as the dominant species, whereas control areas hosted least regeneration. Differences in regeneration density between heavily and moderately damaged areas were not significant. Disturbance severity exerted significant effect on height and height growth of shade-intolerant species throughout the inventories. Past performance, in its turn, was negatively linked to mortality probability. In contrast, more shade-tolerant species only showed impact of past competition on mortality at later stages since disturbance. Factors affecting regeneration mortality probability varied considerably among regeneration species, microsites and inventory moments since disturbance. Soil moisture explains occurrence and good performance of black alder in pits. The roles of CWD can range from sun-blocking in the first years after germination, to protection from ungulate browsing in later stages. Pre-disturbance management affects future species composition and stand structure. Salvage logging of wind disturbed areas increases the share of shade-intolerant species. Local post-disturbance retention of CWD and targeted pre-commercial thinning are tools to promote certain groups of tree species. The presence of storm-induced microsites is expected to increase long-term tree species diversity on the stand level, due to the species’ preferences. However, future research is necessary to judge whether the observed trends are persistent.Uurimistulemused tugevate tormide, mis on üheks olulisemaks häiringufaktoriks Põhja-Euroopa metsades, mõjust metsade suktsessioonilisele dünaamikale on küllaltki vastuolulised. Oluline tegur, mis aitab selgitada erinevaid arengustsenaariume on tormi tugevus, samas mängivad suurt rolli tormi käigus tekkinud biootilised ja abiootilised häiringuelemendid, mida nimetatakse mikroaladeks. Mikroalad moodustuvad juureaukudest ja -mätastest, mis tekkivad tormi poolt ümberpaisatud puude juurestike üleskerkimisega pinnasest ning suurte puude lamapuidust. Käesolevas uurimuses käsitletakse tormi poolt mõjutatud metsa mikroalade olemit ja tingimusi looduslikul uuenemisel häiringujärgse arengu eri etappides. Metsa uuenemise jälgimisel on kasutatud kordusmõõtmisi püsiproovialadel kahes erinevas Eesti piirkonnas küpsetes kuuse-lehtpuu segapuistutes. Häiringute näol on tegemist on 2001. ja 2002. aasta tugevate suvetormidega. Uuendust uuriti erineva kahjustusastmega metsaosades. Töö keskendus järgmiste puuliikide uuendusele: harilik kuusk (Picea abies (L.) Karst.), aru- ja sookask (Betula pendula Roth. and B. pubescens Ehrh.), sanglepp (Alnus glutinosa (L.) J. Gaertn.) ja harilik pihlakas (Sorbus aucuparia L.). Varieeruvus uuenduskohtade kõrguses maapinnast, taimkatte olemasolus, mulla mineraalse osa paljastumises ja surnud orgaanilise aine ja toitainete akumuleerumine või minemauhtumine ning idanemise ja kasvusubstraadi stabiilsus ja kättesaadavus taimejuurtele loovad mikroalade väga heterogeense mustri. Tavaliselt valgustatus häiringu järgselt tekkiva uuenduse all väheneb, samas muutub lagunev puit taimedele soodsamaks idanemis- ja kasvupinnaseks. Kasvu mõjutavate erinevate tegurite optimumid ei lange sageli kokku. Kahel tormikahjustatud alal läbi viidud kordusmõõtmised näitavad, et enamlevinud uuenduspuuliigid tugeva kahjustusastmega alade juureaukudes on aru- ja sookask , keskmise kahjustusastmega alade juureaukudes sanglepp ning juuremätastel vastavalt tugeva ja keskmise kahjustusastme korral harilik kuusk ja harilik pihlakas. Kõige liigirikkam oli uuendus keskmise kahjustusastmega aladel. Häiringujärgselt koristatud aladel oli uuendus kõige arvukam ja domineerisid aru- ja sookask. Kõige vähem oli uuendust kontrollaladel . Uuenduse arvukus keskmise ja tugeva kahjustusastmega aladel ei olnud usaldatavalt erinev. Kahjustusastme tugevus avaldas suurt positiivset mõju valgusnõudlike liikide kõrguskasvule ja kõrgusele, puutaimede kõrguskasv oli negatiivses korrelatsioonis suremise tõenäosusega. Varjutaluvatel liikidel hakkas konkurents suremust mõjutama alles koosluse arengu hilisemas faasis. Uuenduse suremust mõjutavad tegurid varieerusid suuresti sõltudes uuenduse liigist, mikroala omadustest ja vaatluse ajast. Sanglepa uuenemine ja hea kasv juureaukudes on põhjustatud paremate niiskustingimuste poolt. Lamapuidu mõju võib olla erinev, vahetult pärast häiringut vähendab suur kogus langenud puid uuenevate taimede valgustatust ning hilisemalt kaitseb see puutaimi metsloomade eest. See, milliselt majandati metsa häiringueelsel perioodil mõjutab häiringujärgse koosluse arengut. Häiringujärgsete alade koristamine (sanitaarraie) suurendab valgusnõudlike liikide osakaalu. Lamapuidu jätmine häiringualale ja harvendusraied häiringueelses metsas soosivad liikide uuendust vastavalt nende kohastumuslikele omadustele. Uuenevatel puuliikidel on erinev nõudlus mikroalade poolt loodavate tingimuste osas, seetõttu suurendab mikroalade olemasolu puurinde liigirikkust pikemas arenguperspektiivis. Edasiste uuringutega on vaja selgitada, kuivõrd püsivad ja millise suunaga on käesolevaks ajaks kirjeldatud arengutrendid

The effects of wild pigs (Sus scrofa) on woody understory vegetation in lowland rain forest of Malaysia

An increasingly urgent task in the field of conservation biology is to identify changes in abiotic and biotic interactions that result when large areas of forest are converted to small fragments surrounded by anthropogenic landscapes. My research, conducted in lowland dipterocarp rain forest at the 2,500-ha Pasoh Forest Reserve in Peninsular Malaysia, investigated a novel but strongly negative edge effect - namely, a tremendous increase in the density of wild pigs (Sus scrofa) and the resulting deleterious impacts on the understory plant community. The absence of feline predators due to the small size of the reserve and the presence of a year-round food supply in the agricultural areas surrounding the reserve are believed to be the main factors contributing to increased pig density. Line transects were conducted to determine pig density within Pasoh. Density in 1996 and 1998 was estimated to be 47 and 27 pigs/km2, respectively, or 10 – 100 times historical levels. Fences were constructed to exclude pigs from control plots to quantify the impact of soil rooting and seed predation on plants in the understory. After two years, plots inside exclosures had three times more recruits, greater species richness, and 53% more height growth among plants 1 – 7 m tall than did adjacent plots to which pigs had access. Surveys were conducted in 1995, 1996, and 1998 to determine the number of reproductive nests constructed by pigs. Pigs constructed an estimated 6.0 nests/ha/year during this time, with an average nest composed of 145 snapped saplings and 117 uprooted saplings. Nest building accounted for 28.9% of all mortality for trees 1 – 2 cm diameter at breast height. Uprooted stems died, but snapped stems produced a leafless stump that could resprout. Observations of \u3e1,800 stumps for 36 months revealed large differences in resprouting among species, families, and groups of plants with similar life history characteristics. Overall, the results of the different studies suggest that if elevated pig densities continue there could be a shift away from the currently dominant Dipterocarpaceae and Euphorbiaceae

Old-field recolonization by animal - dispersed keystoneplants: combining field work, genetics and apatially-explicit modelling

Doutoramento em Biologia / Instituto Superior de Agronomia. Universidade de LisboaThe transformation and destruction of natural areas for their exploitation as agricultural lands is a typical example of land-use change that generates high ecosystem degradation worldwide. However, during the last decades, many agricultural lands are being abandoned due to multiple socio-economic factors. Currently, there is a trend that sees the farmland abandonment as an opportunity to restore the functioning of these human-altered ecosystems. To take advantage of this opportunity, it is central to understand the ecological processes that drive and limit the natural regeneration of plant communities in these old-fields. This doctoral dissertation addresses the understanding of two central stages that determine the natural (re)colonization of old-fields where interspecific interactions are central: seed arrival and seedling recruitment and establishment. To do this, a multidisciplinary framework that combines intensive field work, molecular techniques and spatially explicit modelling was used in three old-fields located in southwestern Iberian Peninsula. Specifically, it was applied to the system comprised of the dwarf palm (Chamaerops humilis L.), a pioneer palm endemic to the Mediterranean, and its interspecific interactions with frugivorous mammals (ungulates and carnivores) and woody plant species. The results showed that carnivores promoted a considerable seed arrival. The fecal marking behavior of dispersers, especially badgers, impacted the spatial and genetic structure of dispersed seeds and seedlings. Several nurse shrub species were quantitatively compared, being identified the dwarf palm as a ‘keystone’ species. Indeed, for the first time, it was experimentally demonstrated the role of the dwarf palm as nurse plant of several woody species. However, the strong variation among individual palms confirmed the presence of the facilitation-competition continuum and therefore, that these plant-plant interactions are not always positive. Besides, spatial evidences of the role of the dwarf palm as ‘perche’ for frugivorous birds were found. In conclusion, using the system ‘dwarf palm-mammals-woody plants’ as a model, it was revealed how the natural (re)colonization of old-fields can promote their self-recovery, although active restoration interventions can be highly recommendable to increase the regeneration of some plant communities.N/

Natural and human-induced dynamics in plant–animal mutualistic networks

Species interactions are an integral part of ecological communities. Collectively, these interactions form complex and highly dynamic networks. The structure of these networks varies due to geographic and temporal variation in the abundance and co-occurrence of interacting species and due to species gains and losses after anthropogenic perturbation. In Europe’s last relict of old-growth lowland forest (Białowieża, Poland), I studied the outcomes of these natural and human-induced dynamics in highly diversified mutualistic networks of plants, pollinators and seed dispersers. These mutualistic interactions between plants and free-living animals are of great importance, as the flowers and fruits of many plant species are critical resources for a variety of animal species, which in turn contribute significantly to the regeneration of plant communities. As part of my studies, I was able to show that plant–animal mutualistic networks are highly dynamic systems that respond collectively to changing biotic context and human-induced perturbation. Observed shifts in facilitative and competitive interactions among plants sharing mutualistic partners show that biotic context is a strong determinant of the outcome of interspecific interactions. The use of network analyses, thereby, allowed me to identify some of the mechanisms that shape species interactions and their outcomes. For example, my studies show that a change in the population density of one species suffices to trigger cascading effects on the interactions and populations of other species. This finding highlights that species interactions may have a pervasive effect on the assembly and disassembly of ecological communities. Even more importantly, I could show that these community-wide dynamics were in all cases linked to consumer-resource relationships, which are key determinants of plant–animal mutualisms. Thus, changes in the foraging behaviour of animals in response to variation in the density of plant resources and competitors affected the structure of mutualistic communities. This underscores that despite the evolutionary conservatism in ecological interactions, biotic context determines to which extent these coevolved interactions are realized. The fact that the sharing of mutualistic partners among plant species was reflected in their co-occurrence demonstrates that the above-mentioned dynamics in ecological networks may also determine community assembly processes and species co-existence. Importantly, the comparison of several types of species interactions revealed how biotic context in its various forms can shape land-use effects on species interactions. I found that the mutualism between plants and seed dispersers was more susceptible to habitat degradation than the mutualism between plants and their pollinators. This finding highlights that a high degree of generalization, such as in the seed dispersal mutualism, does not necessarily buffer ecological communities against the loss of species. This becomes even more important if a few species have a disproportionate effect on a given target function and if species are particularly vulnerable to ecological perturbation, such as habitat specialists or large-bodied frugivores. Furthermore, I observed that shifts in the abundance of plant resources in degraded habitats can amplify land-use effects on plant–animal mutualistic interactions. Importantly, changes in the density of plant resources explained about 40 to 70 percent of the variation in land-use effects on interactions between plants and their pollinators and seed dispersers. This demonstrates that a consideration of biotic context (e.g. in the form of resources) may considerably improve predictions of the magnitude of land-use effects on species interactions. Thereby, the correlated responses of pollinators and seed dispersers to the shifts in plant population densities in degraded habitats highlight that these dynamics are not restricted to single types of interaction, but potentially operate at the level of ecosystems. Studies that only focus on subsets of species or interaction types may be unable to identify the consequences of human land-use that have been shown here. In principle, the results of the presented studies may also be valid for other types of mutualistic and antagonistic interactions that are based on consumer-resource relationships. Altogether, the results of my thesis suggest that natural and human-induced dynamics in plant–animal mutualistic networks follow similar principles. In the worst case these dynamics might have cascading effects on the functioning and integrity of ecosystems through a parallel loss of multiple animal-mediated ecosystem services after habitat degradation