Drought tolerance in faster- and slower-growing black spruce (Picea mariana) progenies: I. Stomatal and gas exchange responses to osmotic stress

Sixteen years growth in height and basal stem diameter of full-sib black spruce [ Picea mariana (Mill.) B. S. P.] progenies varied with soil moisture availability. The responses to water stress of two faster-growing progenies under drought were compared with two slower-growing progenies to determine the physiological basis of drought tolerance. Six-month-old seedlings were stressed using an osmoticum, polyethylene glycol-3350 (PEG). Seedlings were passed through a series of increasing concentration: 10, 18 and 25% PEG (w/v) each for 3 days to provide solution water potentials of -0.4, -1.0, and -2.0 MPa, respectively. The stress was then relieved by returning the seedlings to nutrient solution without PEG for 24 hours. Gas exchange and water relation parameters were similar in the 4 progenies prior to the imposition of the stress but varied during the stress and after stress relief. The two progenies which grew more vigorously on the driest site maintained significantly higher stomatal conductance, leaf transpiration rate, and net photosynthesis rate during mild (10% PEG) and moderate (18% PEG), but not severe (25% PEG) osmotic stress, and also recovered faster after release of the stress than the other two slower-growing progenies. Black spruce progenies did not differ in xylem water potential or water use efficiency. Progenies capable of faster growth under drought stress were thus characterized by a greater dehydration tolerance, rather than postponement, compared with slower-growing progenies.     

Nitrification and nitrate reductase activity along a secondary successional gradient

The control of nitrification was studied in a secondary successional gradient on Nantucket Island, MA. It was hypothesized that 1) variability in nitrification along the gradient is controlled by litter primary and secondary chemistry, and 2) differences in nitrate availability along the gradient are reflected in potential nitrate assimilation rates in plant tissue. Nitrification varied significantly (p<0.05) by successional stage in all study sites, generally increasing with successional age. The ratio of nitrification to total N mineralization did not vary significantly between successional stages, suggesting substrate limitation of nitrification. Litter terpenoid resin concentration was a significant predictor (p<0.05) of nitrification rate, but soil %C, %N, and water content also contributed significantly to a stepwise regression model predicting nitrification. Nitrate reductase activity (NRA), an index of potential nitrate assimilation, was measured in an assay species (Schizachyrium scoparium). Although there was no significant correlation with nitrification, NRA was significantly (p<0.05) negatively correlated with soil ammonium concentration along the successional gradient at one site, suggesting that plants preferentially utilized ammonium in this system.     

First evidence of somatic embryogenesis from needles of 1-year-old Picea abies plants

Summary Somatic embryogenesis was obtained from hypocotyls and cotyledons of one month old plantlets of Picea abies. Embryogenic yield was higher with expiants from somatic embryo-derived plantlets (80 %) than with plantlets issued from zygotic embryos (10 %). This report also describes production of embryogenic calli from needles of 14 month old somatic embryo-derived plants cultivated in greenhouse. The influence of the physiological status and genotype of the mother plant on somatic embryogenic potential is discussed.Abbreviations ABA abscisic acid - (±) ABA racemic ABA - BAP 6-benzylaminopurine - CI callus inducing culture medium - NAA 1-naphtaleneacetic acid     

Invertebrate colonization and processing of maple leaf litter in a forested and an agricultural reach of a stream

Processing of maple leaf (Acer saccharum Marsh) packs, their colonization by invertebrates and nutrient dynamics in leaves were investigated in a forested reach and agricultural reach of Canagagigue Creek, Ontario. Shredders, Pycnopsyche, and collectors, Ephemerella subvaria, Stenonema vicarium and Baetis were significantly more numerous in packs at the forest site than in packs at the agricultural site, whereas filter feeders, especially blackflies, were significantly more numerous in packs at the agricultural site. Weight loss of litter packs was nearly equivalent at the two sites. However, there were major differences in the mechanism of processing between the sites. Physical abrasion and microbial activity governed weight loss of maple leaf packs at the agricultural site, whereas processing was governed mainly by microbial and invertebrate activity and, to a much lesser extent, by physical abrasion at the forest site. Both shredders and collector species played an important role in the processing of leaf material at the forest site. Greater uptake of N and P (P<0.05 in spring) and higher C concentrations were observed in leaf packs at the forest site than the agricultural site. Therefore, the results support the concept of retention of nutrients in forested areas and their export in deforested (agricultural) areas. Findings also indicated that the processing of leaf litter is not an efficient means of monitoring changes in stream ecosystems since leaf processing is affected by many factors, particularly physical abrasion.     

Seasonal variation in the terpenes of the foliage of black spruce

The changes in the relative amounts of the mono- and sesquiterpenes of the volatile oil of the leaves, buds and twigs of black spruce were determined at various intervals throughout the year. Major changes take place in the young leaves just after bud burst until mid-summer. Upon maturing the same quantitative composition as that of the mature leaves is reached. The composition of the latter, as well as that of the twigs, shows only minor changes from bud burst to mid-summer and none during the rest of the year. In contrast, large relative quantitative changes take place in the bud oil throughout the year. The relative changes of santene, tricyclene, camphene, bornyl acetate and limonene in the new growth follow similar curves, but those of α-pinene, β-pinene, myrcene, car-3-ene and the sesquiterpenes differ considerably. The sesquiterpenes, and possibly myrcene, appear to be metabolized at least in part as the young leaves mature.     

Gibberellin A9 glucosyl ester in needles of Picea sitchensis

A polar gibberellin-like substance present in needles of Picea sitchensis was identified as GA₉-β-d-glucosyl ester on the basis of enzymatic hydrolysis and identification of the aglycone by GC-MS. The biological activity of the synthetic material was tested in two bioassays.     

Recovery of decomposition rates and decomposer invertebrates during rain forest restoration on disused pasture

Converting forest to pasture can alter the roles of biota in ecosystem functioning, while vegetation restoration should arguably assist functional recovery. Since tests of this are scarce, this study quantifies both litter decomposition rates and their association with decomposer invertebrates, across 25 sites representing different phases of deforestation and subsequent reforestation of rain forest. Open and closed (to exclude macro-invertebrates) mesh bags containing forest leaves were exposed in the field for up to eight months, and invertebrates were extracted from separate collections of ground surface litter. Sites spanned five vegetation categories (five sites in each): reference states of both old-growth forest and grazed pasture; unassisted woody regrowth aged 20–50 years on former pasture; and assisted regeneration aged 1–3 and 5–10 years after interventions were applied to similar regrowth. Decomposition rates in open bags were about 50% slower in pasture than old-growth forest, and abundances of macro- and meso-decomposer invertebrates were 95% and 77% lower, respectively. However, in all restoration site-types, decomposition rates had recovered to 83% of old-growth values, and abundances of invertebrate decomposers were similar in old-growth forest. Decomposer community composition at a broad taxonomic level differed strongly between pasture and all other vegetation types. Exclusion of macro-invertebrates decreased decomposition rates by only about 3.1%, but decomposition rates in open bags were significantly correlated (across sites) with abundances of both macro- and meso-decomposers, most strongly so for meso-decomposers. Drawing useful generalizations across studies is impeded by differing methodologies and because few include both agricultural and forest reference sites.     

Litter size and seasonality in reproduction for Guianan rodents and opossums

We studied the litter sizes of small rodents and opossums caught in the Guianan Region (Brazilian Amapá, French Guiana, Suriname, and Guyana) by pooling the data of animals collected during various field trips conducted primarily between 1990 and 2017. A series of 569 counts of embryos (or of pouch young for marsupials) in 40 species of Didelphidae (N = 12 species), Sigmodontinae (18), Murinae (2), and Echimyidae (8) allowed for a more detailed characterization of the reproductive condition of 14 species known each by more than 10 pregnant females. For eight species with at least 20 pregnant females, an examination of seasonality in breeding occurrence documented that the two months with the lowest percentage of pregnant females are July and August (16.0 and 17.3%, respectively) during the end of the long wet season and beginning of the dry season. By contrast, January and February showed the highest abundance of pregnancies (57.9% and 55.8%, respectively) during the beginning of the long wet season. This timing coincides with most juveniles foraging during the height of the wet season in May when food is presumably most prevalent.     

More individuals but fewer species: testing the ‘more individuals hypothesis’ in a diverse tropical fauna

A positive relationship between species richness and productivity is often observed in nature, but the causes remain contentious. One mechanism, the ‘more individuals hypothesis’ (MIH), predicts richness increases monotonically with density, as a function of resource flux. To test the MIH, we manipulated resource abundance in a community of tropical rainforest litter ants and measured richness and density responses. A unimodal relationship between richness and density most closely fitted the control and disturbance (resource removal) treatments in contrast to expectations of the MIH. Resource addition resulted in a monotonic increase in richness relative to density, a shift from the pattern in the control. In the disturbance treatment, richness was greater than in the control, opposite to expectations of the MIH. While large-scale correlations between ant diversity and net primary productivity or temperature are reconcilable with the MIH, key elements of the hypothesis are not supported.