Initial hydraulic failure followed by late-stage carbon starvation leads to drought-induced death in the tree Trema orientalis.

Drought-induced tree death has become a serious problem in global forest ecosystems. Two nonexclusive hypotheses, hydraulic failure and carbon starvation, have been proposed to explain tree die-offs. To clarify the mechanisms, we investigated the physiological processes of drought-induced tree death in saplings with contrasting Huber values (sapwood area/total leaf area). First, hydraulic failure and reduced respiration were found in the initial process of tree decline, and in the last stage carbon starvation led to tree death. The carbohydrate reserves at the stem bases, low in healthy trees, accumulated at the beginning of the declining process due to phloem transport failure, and then decreased just before dying. The concentrations of non-structural carbohydrates at the stem bases are a good indicator of tree damage. The physiological processes and carbon sink-source dynamics that occur during lethal drought provide important insights into the adaptive measures underlying forest die-offs under global warming conditions.

Enhanced drought tolerance in seedlings of Neotropical tree species inoculated with plant growth-promoting bacteria.

The inoculation of tree species with plant growth-promoting bacteria (PGPB) has emerged as an important strategy for the acclimation of seedlings by improving plant tolerance to biotic and abiotic stresses. This study aimed to evaluate the effects of inoculation with bacterial species (Azospirillum brasilense - Ab-V5, Bacillus sp., Azomonas sp. and Azorhizophillus sp.) on the growth and physiology of the Neotropical tree species Trema micrantha and Cariniana estrellensis under drought conditions. When associated with Ab-V5 and Azomonas sp., T. micrantha showed increased protein in the leaves, starch in the leaves and roots, photosynthesis, instantaneous carboxylation efficiency and root and shoot dry mass. Moreover, there were reductions in hydrogen peroxide, lipid peroxidation, water potential and proline. In C. estrellensis associated with Ab-V5, higher values of photosynthesis and instantaneous carboxylation efficiency were observed, in addition to higher starch content in the leaves and roots and higher protein content in the leaves; lower hydrogen peroxide and lipid peroxidation contents were also observed. The associations of T. micrantha with Ab-V5 and Azomonas sp. and C. estrellensis with Ab-V5 favored the activation of metabolic processes under drought, leading to greater drought tolerance. This work demonstrates the effects of compatible associations of Neotropical tree and PGPB species and suggests that the identification of compatible PGPB strains can result in tree seedlings with increased tolerance to abiotic stresses, such as drought.

Responses to nitrogen pulses and growth under low nitrogen availability in invasive and native tree species with differing successional status.

Invasive species are frequently found in recently disturbed sites. To examine how these disturbance-dependent invasive species exploit resource pulses resulting from disturbance, twelve physiological and morphological traits, including age-dependent responsiveness in leaf traits to nitrogen pulse, were compared between Bischofia javanica, an invasive tree species in Ogasawara islands, and three native Ogasawara species, each having a different successional status. When exposed to a nitrogen pulse, invasive B. javanica showed higher increases in photosynthetic capacity, leaf area, epidermal cell number and cell size in leaves of broad age classes, and root nitrogen absorption ability than two native mid-/late or late-successional species, but showed no particular superiority to a native pioneer species in these responses. Under low nitrogen, however, it showed the largest relative growth rate among the four species, while the native pioneer showed the lowest growth. From these results, we concluded that the combination of moderately high responsiveness to resource pulses and the ability to maintain steady growth under resource limitations may give B. javanica a competitive advantage over a series of native species with different successional status from early to late-successional stages.

Response to dehydration and irrigation in invasive and native saplings: osmotic adjustment versus leaf shedding.

To clarify the mechanism underlying successful invasion by tree species into xeric sites on Japan's Bonin Islands, we compared the water use of an alien species, Psidium cattleianum, which is rapidly expanding on ridge sites with shallow soil, with that of a native species, Trema orientalis. We hypothesized that there is a trade-off between leaf shedding with low cavitation resistance (frequent xylem cavitation plus refilling ability) and leaf osmotic adjustment with high cavitation resistance (cessation of xylem cavitation plus canopy leaf retention), indicating contrasting strategies for drought tolerance and water use in semi-arid regions. We examined leaf turnover, leaf gas exchange, leaf water potential and water distribution in stem xylem conduits using cryo-scanning electron microscopy for the saplings of both species under three cycles of artificial drought and sudden pulse irrigation. Invasive P. cattleianum saplings were highly resistant to cavitation in stem xylem conduits, retained their leaves and exhibited effective leaf osmotic adjustment under the drought treatment. In contrast, native T. orientalis saplings exhibited xylem cavitation, conspicuous leaf shedding and less effective leaf osmotic adjustment under the drought treatment. Leaf gas exchange rate recovered more rapidly in P. cattleianum saplings than in T. orientalis saplings immediately following pulse irrigation after a period without irrigation, especially in the first drought cycle. Embolized conduits in T. orientalis were refilled by pulse irrigation, and leaf gas exchange rate recovered following refilling. The two tree species showed contrasting strategies for drought tolerance and water use along a trade-off axis. Cavitation avoidance and effective leaf osmotic adjustment in P. cattleianum saplings under drought conditions partially support their survival at the xeric ridge sites on the Bonin Islands. Our results help to explain the success of P. cattleianum in its invasion of a sub-arid environment.

Long-term persistence of pioneer species in tropical rain forest soil seed banks.

In tropical forests, pioneer tree species regenerate from seeds dispersed directly into canopy gaps and from seeds that persisted in soil seed banks before gap formation. Life-history models have suggested that selection for the long-term persistence of tree seeds in the soil should be weak because persistence potentially reduces population growth rate by extending generation time and because adult life spans may exceed the return interval of favorable recruitment sites. Here we use accelerator mass spectrometry to carbon-date seeds of three pioneer tree species extracted from undisturbed seed banks in seasonally moist lowland Neotropical forest. We show that seeds of Croton billbergianus, Trema micrantha, and Zanthoxylum ekmannii germinate successfully from surface soil microsites after 38, 31, and 18 years, respectively. Decades-long persistence may be common in large-seeded tropical pioneers and appears to be unrelated to specific regeneration requirements.

Differences on post-fire regeneration of the pioneer trees Cecropia glazioui and Trema micrantha in a lowland Atlantic Forest

A study of natural post-fire succession was carried out in a disturbed vegetation around fragments of the Atlantic Rain Forest (National Biological Reserve of Poço das Antas (22° 30’- 22° 33’S, 42° 15’- 42° 19’ W), Rio de Janeiro State). All the pre-fire individuals of Cecropia and Trema in the area were numbered with plastic labels.In order to check for the presence of new sprouts and mortality, two other censuses were carried out, at 3 and 12 months after the fire.The dominant species were:Pteridium aquilinum,Panicum maximum,Trema micrantha and Cecropia glazioui. Few days after the passage of fire, grasses and ferns spread their area,while the stands of Trema and Cecropia were completely burned. Most of individuals of Cecropia produced some sprouts while most of individuals of Trema died.However,a great number of seedlings of Trema were recruited while only one single seedling of Cecropia were observed during a period of one year.Most of these seedlings died through the year while the sprouts were already reproducing.The uses of Cecropia in places where fire is recurrent could be more appropriate because of its higher chance of survival and faster recovering ability after fire

Se realizó un estudio sobre la sucesión natural después del fuego en una vegetación alrededor de fragmentos de la selva lluviosa atlántica (Reserva Biológica Nacional de Poço das Antas (22° 30’-22° 33’ S, 42° 15’- 42° 19’ W), Estado de Río de Janeiro).Todos los individuos de Cecropia y Trema previos del fuego en el área fueron numerados con marcas plásticas. Se realizaron otros dos censos, 3 y 12 meses después del fuego con el objetivo de detectar mortalidad y buscar la presencia de nuevos brotes. Las especies dominantes fueron: Pteridium aquilinum, Panicum maximum, Trema micrantha y Cecropia glazioui. Pocos días después del paso del fuego,los pastos y helechos se dispersaron por el área, mientras que los troncos de Trema y Cecropia estaban completamente quemados. La mayoría de los individuos de Cecropia produjeron algunos rebrotes mientras que la mayoría de los individuos de Trema murieron. Sin embargo, un gran número de plántulas de Trema fueron reclutadas mientras que únicamente una plántula de Cecropia fue observada durante el período de un año. La mayoría de estos plántulas murieron a lo largo del año, mientras que los brotes se estan ya reproduciendo. Los usos de Cecropia en lugares donde el fuego es recurrente puede ser más apropiado debido a su mayor chance de sobrevivir y a su habilidad de recuperarse más rápido después del fuego

Differences on post-fire regeneration of the pioneer trees Cecropia glazioui and Trema micrantha in a lowland Brazilian Atlantic forest.

A study of natural post-fire succession was carried out in a disturbed vegetation around fragments of the Atlantic Rain Forest (National Biological Reserve of Poço das Antas (22 degrees 30'-22 degrees 33'S. 42 degrees 15'-42 degrees 19'W), Rio de Janeiro State). All the pre-fire individuals of Cecropia and Trema in the area were numbered with plastic labels. In order to check for the presence of new sprouts and mortality, two other censuses were carried out, at 3 and 12 months after the fire. The dominant species were: Pteridium aquilinum, Panicum maximum. Trema micrantha and Cecropia glazioui. Few days after the passage of fire, grasses and ferns spread their area, while the stands of Trema and Cecropia were completely burned. Most of individuals of Cecropia produced some sprouts while most of individuals of Trema died. However, a great number of seedlings of Trema were recruited while only one single seedling of Cecropia were observed during a period of one year. Most of these seedlings died through the year while the sprouts were already reproducing. The uses of Cecropia in places where fire is recurrent could be more appropriate because of its higher chance of survival and faster recovering ability after fire.

Blockade of adenosine A2A receptors antagonizes parkinsonian tremor in the rat tacrine model by an action on specific striatal regions.

Acute administration of the acetylcholinesterase inhibitor tacrine to rats induces tremulous jaw movements which can be used as a valuable model of parkinsonian tremor. In the present study, the number of tremor episodes and jaw movements were evaluated to assess the effects of the selective A2A antagonists SCH 58261 and SCH BT2 on tremorgenesis. SCH 58261 dose-dependently, and maximally at 5 mg/kg, reduced the number of both tremor episodes (-35%) and jaw movements (-50%), induced in rats by tacrine (2.5 mg/kg ip). Since adenosine A2A receptors are largely expressed throughout the striatum, chronic cannulae were implanted in the rat dorsomedial (DMS) and ventrolateral striatum (VLS) to investigate whether A2A antagonists could act at this level. Infusion of SCH BT2 (5 microg/microl), a water-soluble analogue of SCH 58261, in VLS antagonized both tremor episodes (-68%) and jaw movements (-76%) elicited by tacrine (2.5 mg/kg ip), whereas SCH BT2 infusion in DMS was less effective in blocking jaw movements (-50%) and did not significantly affect the number of tremor episodes. Taken together, the results of this study indicate that A2A antagonists effectively reduce the magnitude of tremulous jaw movements induced in rats by acute tacrine, mainly by an action in VLS and suggest that A2A antagonists might be used as specific agents against parkinsonian tremor.