Most landscape-scale fire severity research relies on correlations between field measures of fire effects and relatively simple spectral reflectance indices that are not direct measures of heat output or changes in plant physiology. Although many authors have highlighted limitations of this approach and called for improved assessments of severity, others have suggested that the operational utility of such a simple approach makes it acceptable. An alternative pathway to evaluate fire severity that bridges fire combustion dynamics and ecophysiology via dose–response experiments is presented. We provide an illustrative example from a controlled nursery combustion laboratory experiment. In this example, severity is defined through changes in the ability of the plant to assimilate carbon at the leaf level. We also explore changes in the Differenced Normalised Differenced Vegetation Index (dNDVI) and the Differenced Normalised Burn Ratio (dNBR) as intermediate spectral indices. We demonstrate the potential of this methodology and propose dose–response metrics for quantifying severity in terms of carbon cycle processes.

Climate change is projected to exacerbate the intensity of heat waves and drought, leading to a greater incidence of large and high-intensity wildfires in forested ecosystems. Predicting responses of seedlings to such fires requires a process-based understanding of how the energy released during fires affects plant physiology and mortality. Understanding what fire ‘doses’ cause seedling mortality is important for maintaining grasslands or promoting establishment of desirable plant species. We conducted controlled laboratory combustion experiments on replicates of well-watered nursery-grown seedlings. We evaluated the growth, mortality and physiological response of Larix occidentalis and Pinus contorta seedlings to increasing fire radiative energy density (FRED) doses created using natural fuels with known combustion properties. We observed a general decline in the size and physiological performance of both species that scaled with increasing FRED dose, including decreases in leaf-level photosynthesis, seedling leaf area and diameter at root collar. Greater FRED dose increased the recovery time of chlorophyll fluorescence in the remaining needles. This study provides preliminary data on what level of FRED causes mortality in these two species, which can aid land managers in identifying strategies to maintain (or eliminate) woody seedlings of interest.

Fire activity, in terms of intensity, frequency, and total area burned, is expected to increase with a changing climate. A challenge for landscape-level assessment of fire effects, often termed burn severity, is that current remote sensing assessments provide very little information regarding tree/vegetation physiological performance and recovery, limiting our understanding of fire effects on ecosystem services such as carbon storage/cycling. In this paper, we evaluated whether spectral indices common in vegetation stress and burn severity assessments could accurately quantify post-fire physiological performance (indicated by net photosynthesis and crown scorch) of two seedling species, Larix occidentalis and Pinus contorta. Seedlings were subjected to increasing fire radiative energy density (FRED) doses through a series of controlled laboratory surface fires. Mortality, physiology, and spectral reflectance were assessed for a month following the fires, and then again at one year post-fire. The differenced Normalized Difference Vegetation Index (dNDVI) spectral index outperformed other spectral indices used for vegetation stress and burn severity characterization in regard to leaf net photosynthesis quantification, indicating that landscape-level quantification of tree physiology may be possible. Additionally, the survival of the majority of seedlings in the low and moderate FRED doses indicates that fire-induced mortality is more complex than the currently accepted binary scenario, where trees survive with no impacts below a certain temperature and duration threshold, and mortality occurs above the threshold.

Light-emitting diode (LED) technology shows promise for supplementing photosynthetically active radiation (PAR) in forest nurseries because of the potential reduction in energy consumption and an ability to supply discrete wavelengths to optimize seedling growth. Our objective was to examine the effects of light spectra supplied by LED and traditional high-pressure sodium (HPS) lamps on growth and physiology of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and Engelmann spruce (Picea engelmannii Parry ex Engelm.) seedlings. We used three latitudinal sources for each species: British Columbia (BC), Idaho (ID), and New Mexico (NM). Container seedlings were grown for 17 weeks in the greenhouse under an 18 h photoperiod of ambient solar light supplemented with light delivered from HPS or LED. In general, seedlings grown under LED had significantly greater growth, gas exchange rates, and chlorophyll contents than those seedlings grown under HPS. The growth and physiological responses to supplemental lighting varied greatly among species and seed sources. Generally, LED-grown seedlings from BC had the greatest growth and tissue dry mass followed by ID and NM populations. Compared with HPS, the significant increase in seedling growth and concomitant energy savings with LED (29% energy consumption relative to HPS) demonstrates the promise of using LED as PAR supplemental lighting for container seedling production.

Abstract• Fall fertilization may increase plant nutrient reserves, yet associated impacts on seedling cold hardiness are relatively unexplored.• Bareroot red pine (Pinus resinosa Ait.) seedlings in north-central Minnesota, USA were fall fertilized at the end of the first growing season with ammonium nitrate (NH 4 NO 3 ) at 0, 11, 22, 44, or 89 kg N ha −1 . Seedling morphology and cold hardiness [assessed by freeze induced electrolyte leakage (FIEL)] were evaluated six weeks after fertilization and following the second growing season.• Seedling height and number of needle primordia increased with fertilizer rate for both sampling years. Seedlings fertilized with 44 and 89 kg N ha −1 attained target height (15 cm) after the second growing season. Shoot and root N concentration increased after the first growing season in fall fertilized seedlings compared to controls. Fall fertilized seedlings had lower FIEL (i.e., increased cold hardiness) compared to controls when tested at -40 • C after the first growing season, but no significant differences in FIEL of control and fertilized seedlings were observed after the second growing season.• Results suggest that fall fertilization of red pine seedlings can help render desired target height in the nursery, while maintaining or increasing cold hardiness levels. Mots-clés :rusticité au froid / croissance / fertilisation azotée / primordiums d'aiguilles Résumé -Fertilisation automnale des plants de Pinus resinosa : absorption des éléments nutritifs, rusticité au froid, et développement morphologique.• La fertilisation automnale peut augmenter les réserves en éléments nutritifs des plants, mais les répercussions sur la rusticité au froid des semis sont encore relativement inexplorées.• Des plants à racines nues de Pinus resinosa Ait.dans le centre-nord du Minnesota (USA) ont été fertilisés à l'automne à la fin de la première saison de croissance avec du nitrate d'ammonium (NH 4 NO 3 ) à 0, 11, 22, 44, ou 89 kg N ha −1 . La morphologie des plants et la rusticité au froid [estimée par la fuite d'électrolyte (FIEL) induite par le gel] ont été évaluées six semaines après la fertilisation et à la suite dans la deuxième saison de croissance.• La hauteur des plants et le nombre de primordiums d'aiguilles ont augmenté avec le taux de fertilisation pour les deux années d'échantillonnage. Les plants fertilisés avec 44 et 89 kg N ha −1 ont atteint l'objectif de hauteur (15 cm) après la deuxième saison de croissance. La concentration en N des tiges et des racines a augmenté après la première saison de croissance chez les plants fertilisés à l'automne par rapport aux témoins. Les plants fertilisés à l'automne ont eu un plus faible FIEL (c'est-à-dire, une augmentation de rusticité), comparativement aux témoins lors du test à -40 • C après la première saison de croissance, mais aucune différence significative de FIEL entre plants fertilisés et témoins n'a été observée après la deuxième saison de croissance.• Les résultats suggèrent que la fertilisation d'automne des plants de Pinus resinosa...

-We evaluated gas exchange, chlorophyll index (CI) using SPAD-502 chlorophyll meter, and leaf nutritional responses in one-year-old northern red oak (Quercus rubra L.) container seedlings transplanted into control (unfertilized) or fertilized (0.86 g N plant −1 ) sand culture and grown in a greenhouse for 90 days. We labeled current nitrogen (N) uptake with ( 15 NH 4 ) 2 SO 4 and directly quantified proportional contributions of N derived from fertilizer (NF) compared with retranslocation or N derived from plant (NP) in leaf growth of red oak seedlings. NF met a greater N demand in leaf growth of fertilized plants while unfertilized seedlings relied entirely on NP for their leaf growth. Fertilization increased leaf dry mass by 67% and new stem dry mass by 253% 90 days after transplanting compared to control seedlings. Specific leaf area (SLA) was not significantly altered but CI increased 90 days after transplanting. Higher leaf N concentration and content in fertilized compared with control seedlings was linked to greater chlorophyll concentrations in the former plants. The higher coefficient of determination (r 2 = 0.72) noted between leaf N concentrations and CI suggests that the SPAD meter could be a useful tool for assessing leaf N status in northern red oak seedlings. Fertilized seedlings exhibited higher net assimilation, stomatal conductance, and transpiration rates compared with controls. Increased seedling growth in response to fertilization was related to maintenance of higher gas exchange and greater nutrient uptake, which could improve outplanting success. fertilization / gas exchange / northern red oak / nitrogen / photosynthesis / retranslocation / SPAD meter Résumé -Croissance, échanges gazeux et réponses nutritionnelles de jeunes semis de Quercus rubra soumis à une fertilisation par ( 15 NH 4 ) 2 SO 4 . Nous avons estimé les échanges gazeux foliaires, un index de teneurs en chlorophylles (IC) avec un chlorophylle-mètre SPAD-502 et les teneurs en nutriments dans les feuilles de jeunes plants de chêne rouge d'Amérique (Quercus rubra L.) âgés d'un an. Les plants ont été transplantés dans du substrat sableux non fertilisé (témoins) ou fertilisé avec 0.86 g N par plante, et cultivés pendant 90 jours sous serre. L'azote apporté par la fertilisation était marqué avec ( 15 NH 4 ) 2 SO 4 et nous avons directement quantifié les contributions à la croissance foliaire de N apporté par la fertilisation (NF) par rapport à celle de N remobilisé depuis les pools de réserve de la plante (NP). NF constituait la fraction la plus importante d'azote foliaire de plants fertilisés, alors que l'azote foliaire des plants non fertilisés était exclusivement constitué de NP. La fertilisation s'est traduite par une augmentation, par rapport aux plantes témoins, de 67 % de la biomasse foliaire et de 253 % de la biomasse de tiges nouvellement formées 90 jours après la transplantation. La surface spécifique des feuilles n'était pas affectée par la fertilisation alors que CI avait significativement augmenté. Des teneur...

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