Embolism vulnerability of an evergreen tree

Leaf bearing stems ofCuratella americana L. were very susceptible to induced cavitation: embolisms began at a pressure of 0.5 MPa (15 %) and at 2.0 MPa most of the conductivity was lost (85 %). Nevertheless, in nature similar leaf specific conductivities, of about 90 x 10^-5 kg m^-2 s^-1 MPa^-1 during both wet and dry seasons indicated absence of drought induced embolisms. Leaf water potentials were neither very negative or considerably different between seasons but stomatal conductance decreased from 236 mmol m^-2 s^-1 measured during wet period to 100 mmol m^-2 s^-1 during drought season. Therefore, it was concluded thatCuratella had an accurate homeostatic balance of leaf water status to keep up xylem integrity. Acknowledgements: Financial support was provided by Decanato de Investigaciones - USB (S1-CB-811). Dr John Sperry (University of Utah) allowed me to learn how to use and to build the equipment used in this study. Mr. M. Edreida and Mr. T. Perez helped me in the field and in the laboratory, respectively. Dr D. HenrIquez corrected the English grammar.     

Modelling the soil-plant-atmosphere continuum in a Quercus–Acer stand at Harvard Forest: the regulation of stomatal conductance by light, nitrogen and soil/plant hydraulic properties

Our objective is to describe a multi-layer model of C₃-canopy processes that effectively simulates hourly CO₂ and latent energy (LE) fluxes in a mixed deciduous Quercus-Acer (oak–maple) stand in central Massachusetts, USA. The key hypothesis governing the biological component of the model is that stomatal conductance (g_s) is varied so that daily carbon uptake per unit of foliar nitrogen is maximized within the limitations of canopy water availability. The hydraulic system is modelled as an analogue to simple electrical circuits in parallel, including a separate soil hydraulic resistance, plant resistance and plant capacitance for each canopy layer. Stomatal opening is initially controlled to conserve plant water stores and delay the onset of water stress. Stomatal closure at a threshold minimum leaf water potential prevents xylem cavitation and controls the maximum rate of water flux through the hydraulic system. We show a strong correlation between predicted hourly CO₂ exchange rate (r²= 0.86) and LE (r²= 0.87) with independent whole-forest measurements made by the eddy correlation method during the summer of 1992. Our theoretical derivation shows that observed relationships between CO₂ assimilation and LE flux can be explained on the basis of stomatal behaviour optimizing carbon gain, and provides an explicit link between canopy structure, soil properties, atmospheric conditions and stomatal conductance.     

Comparative water relations of co-occurring trees in a mixed podocarp-broadleaf forest

Aims As extreme climatic events including droughts and heat waves become more common in a changing climate, tree mortality has increased across the globe. In order to determine whether certain species have a competitive advantage over others, we explored the water-relations and leaf-gas exchange of four co-occurring species in a forest in northern Aotearoa-New Zealand. We studied the ecologically and culturally significant foundation species,Agathis australis(a conifer), two additional conifers,Phyllocladus trichomanoidesandPodocarpus totaraand the angiospermKnightia excelsa.     

The response of invertebrates to a gradient of flow reduction – an instream channel study in a New Zealand lowland river

1. The impacts of low flows on invertebrates were assessed in six experimental channels placed in a lowland stream. Each channel consisted of replicate stacked gravel‐filled baskets (buried 0–7, 8–14 and 15–21 cm deep) from which invertebrates were sampled. Samples were collected before, and 1 and 2 months after flows were reduced by varying amounts. We predicted that invertebrate communities would change most with the lowest flows that persisted longest. The effects of reduced flow on channel and intragravel‐velocities, temperature, dissolved oxygen and algal biomass were also monitored. 2. Mean velocity (17 cm s⁻¹) and depth (20 cm) were similar in all channels prior to flow reduction, but were reduced in five channels by 25% to 98%, a sixth channel serving as a control. Mean intragavel velocity prior to flow reduction (1.7 mm s⁻¹) was also reduced from 17% to 51%. Flow reduction had no effect on temperature, which ranged from 11.2 °C (night) to 19.6 °C (day). Dissolved oxygen was generally high (mean = 94% saturation) but variable (range from 50% to 145%), and did not differ between channels after flow reduction. Periphyton biomass (chlorophyll a and AFDM_algae) increased over time and was positively related to velocity after 1 month of flow reduction. After 2 months, however, this relationship reversed, with higher chlorophyll and AFDM_algae in lower velocity channels. Organic matter biomass (AFDM_total) was significantly higher in channels with lower velocities, and did not change with time. There was no relationship between AFDM_total and velocity in baskets from different depths. 3. Greater than 85% of animals were found in the upper baskets (0–7 cm deep), and flow reduction had no influence on the vertical distribution of invertebrates. After 1 month, invertebrate density had declined roughly in proportion to the magnitude of flow reduction in all channels where flows were reduced more than 25%. Densities recovered to pre‐reduction levels within 2 months. The amphipod, Paracalliope fluviatilis dominated all channels numerically prior to flow reduction, but its density declined markedly during the study. Densities of Ostracoda, Oxyethira albiceps and Cladocera increased dramatically after 2 months of flow reduction, especially in the lowest flow channels. Of the physical variables measured, chlorophyll a biomass and discharge best explained the temporal changes in the invertebrate community. 4. The effects of increased duration and magnitude of flow reduction on invertebrate communities were restricted to changes in the relative abundances of just a few taxa. Our results suggest that invertebrates in this lowland stream were resistant to the experimental flow reduction, presumably because of their broad ecological tolerances. We also found that more prolonged low flows did not result in predictable changes. This finding may have implications in terms of using hydraulic‐habitat models to set minimum flows in lowland streams if invertebrates can persist at flows much lower than their supposed optima.     

Primary cutaneous melanoma in Black patients: An analysis of 2464 cases from the National Cancer Database 2004–2018

Melanoma in Black patients carries a poor prognosis. Due to its rarity, melanoma in this population has not been well characterized. This study evaluates survival predictors in Black patients with melanoma. This was a retrospective cohort study of Black patients with cutaneous melanoma from the National Cancer Database 2004–2018. Of the 2464 cases, melanoma was more common among females than males (57.1% vs. 42.9%, p < .001). Median Breslow depth was 1.8 mm (interquartile range 0.4–4.4). Lower extremities were the most common location (52.8%), followed by upper extremities (13.1%) along with otherwise specified/overlapping/other (13.1%), then by trunk (11.8%), and lastly head and neck (9.2%). Stage at diagnosis was I (30.7%), II (27.5%), III (24.1%), and IV (17.7%). Ulceration was observed in 41.4% of lesions. Acral lentiginous melanoma (ALM) was the most common specific histologic subtype (20.3%), followed by superficial spreading melanoma (9.4%). After adjusting for confounders, higher stages and primary site on the head and neck were the strongest independent predictors of worse overall survival. Melanoma in Black patients is most likely to appear on the lower extremities. A large portion (41.8%) presented with stage III or IV disease. ALM was the most common specific histologic subtype.     

Drought-induced increase in tree mortality and corresponding decrease in the carbon sink capacity of Canada's boreal forests from 1970 to 2020

Canada's boreal forests, which occupy approximately 30% of boreal forests worldwide, play an important role in the global carbon budget. However, there is little quantitative information available regarding the spatiotemporal changes in the drought-induced tree mortality of Canada's boreal forests overall and their associated impacts on biomass carbon dynamics. Here, we develop spatiotemporally explicit estimates of drought-induced tree mortality and corresponding biomass carbon sink capacity changes in Canada's boreal forests from 1970 to 2020. We show that the average annual tree mortality rate is approximately 2.7%. Approximately 43% of Canada's boreal forests have experienced significantly increasing tree mortality trends (71% of which are located in the western region of the country), and these trends have accelerated since 2002. This increase in tree mortality has resulted in significant biomass carbon losses at an approximate rate of 1.51 ± 0.29 MgC ha⁻¹ year⁻¹ (95% confidence interval) with an approximate total loss of 0.46 ± 0.09 PgC year⁻¹ (95% confidence interval). Under the drought condition increases predicted for this century, the capacity of Canada's boreal forests to act as a carbon sink will be further reduced, potentially leading to a significant positive climate feedback effect.     

Post-injury hydraulic fracturing drives fissure formation in the zebrafish basal epidermal cell layer

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