Overstory influences on light attenuation patterns and understory plant community diversity and composition in southern boreal forests of Quebec

We have characterized overstory light transmission, understory light levels, and plant communities in mixedwood boreal forests of northwestern Quebec with the objective of understanding how overstory light transmission interacts with composition and time since disturbance to influence the diversity and composition of understory vegetation, and, in turn, the further attenuation of light to the forest floor by the understory. Overstory light transmission differed among three forest types (aspen, mixed deciduous-conifer, and old cedar-dominated), with old forests having higher proportions of high light levels than aspen and mixed forests, which were characterized by intermediate light levels. The composition of the understory plant communities in old forests showed the weakest correlation to overstory light transmission, although those forests had the largest range of light transmission. The strongest correlation between characteristics of overstory light transmission and understory communities was found in aspen forests. Species diversity indices were consistently higher in aspen forests but showed weak relationships with overstory light transmission. Light attenuation by the understory vegetation and total height of the understory vegetation were strongly and positively related to overstory light transmission but not forest type. Therefore, light transmission through the overstory influenced the structure and function of understory plants more than their diversity and composition. This is likely due to the strong effect of the upper understory layers, which tend to homogenize light levels at the forest floor regardless of forest type. The understory plant community acts as a filter, thereby reducing light levels at the forest floor to uniformly low levels

Planning considerations related to the organic contamination of martian samples and implications for the Mars 2020 rover

© Copyright 2014, Mary Ann Liebert, Inc. Data gathered during recent NASA missions to Mars, particularly by the Rovers Spirit, Opportunity, and Curiosity, have provided important insights into the past history and habitability of the Red Planet. The Mars science community, via input through the National Research Council (NRC) Planetary Science Decadal Survey Committee, also identified the prime importance of a Mars sample return (MSR) mission to further exploration of the Red Planet. In response, the Mars 2020 Mission (Mars 2020) Science Definition Team (SDT) (Mustard et al., 2013) was chartered by the NASA Mars Exploration Program to formulate a new rover mission that would take concrete steps toward an eventual sample return. The SDT recommended that the 2020 rover should select and cache scientifically compelling samples for possible return to Earth. They also noted that organic contamination of the samples was a significant and complex issue that should be independently investigated by a future committee. Accordingly, NASA chartered the Mars 2020 Organic Contamination Panel (OCP). The OCP was charged with evaluating and recommending sample contamination requirements for the proposed Mars 2020. A further task was to assess implementation approaches in support of the investigation of broad scientific questions concerning the history and habitability of Mars. Central to these objectives would be the ability to reliably differentiate indigenous martian organic molecules from terrestrial contamination in any future samples returned from Mars. Early on during its deliberations, the OCP recognized that the scientific and planetary protection (PP) objectives of MSR are intimately linked, in that both rely heavily on measurements of organic molecules in the returned samples. In each case, a key aspect of the problem is being able to recognize and interpret organic molecules as indigenous to Mars against a potential background of Earthsourced contamination. It was within this context that the OCP committee considered the structure for a set of measurement goals related to organic molecules in the returned samples that would be of common interest to science and PP. The following is a summary of the most significant findings of the OCP regarding organic geochemical measurements that would be shared for both science and PP in relation to potential future MSR