Greening, an increase in photosynthetically active plant biomass, has been widely reported as period-related and region-specific. We hypothesized that vegetation trends were highly density-dependent with intensified browning in dense canopies and increased greening in sparse canopies. We exploited this insight by estimating vegetation trends in peak growth from dense to sparse canopies graded from 1 to 20 using the non-parametric Mann-Kendall trend test based on the 500 m 8-day composite MODIS Near Infrared Reflectance of terrestrial vegetation (NIRv) time series datasets in the past two decades (2001-2019) at the global scale. We found that global greening increased by 1.42% per grade with strong fit before grade 15 (R-2 = 0.95): net browning (11% browning vs 9% greening) exhibited in high-density canopies (NIRv > 0.39) in contrast to 32% greening in low-density canopies (NIRv asymptotic to 0.15). While the density-dependent greening was evidenced across different biomes and ecosystems, the steepest gradient (changes per grade) in cropland highlighted the increasingly intensified agricultural activities globally. Greening gradients declined in the dryland, but enhanced in the High-latitude ecosystems driven by warming, especially in the shrubland. Density-dependent vegetation trends were accounted for by the disproportionately impacts from climate changes and the un-equal contributions of Land Cover Changes (LCC) among dense and sparse canopies. Vegetation trends and greening gradients could be extensively facilitated by Wetting or Decreasing solar Radiation (WDR), especially in sparse grass -land and shrubland. Browning was dominant in dense canopies, which was further aggravated by Drying and Increasing solar Radiation (DIR), especially woody vegetation. This study implied the widespread degradation or mortality of highly productive vegetation hidden among global greening dominant in open ecosystems, which might be further exacerbated by the predicted increasing drought under global warming.

Solar-induced chlorophyll fluorescence (SIF) shows great potential to assess plants physiological state and response to environmental changes. Recently the near-infrared reflectance of vegetation (NIRv) provides a promising way to quantify the confounding effect of canopy structure in SIF, while the difference between SIF and NIRv under varying environmental conditions has not been well explored. Here we developed a simple approach to extract the fluorescence yield (Phi(F)) by the combined use of SIF and the near-infrared radiance of vegetation (NIRvR). The proposed NIRvR approach was evaluated in multiple ways, including with the seasonal leaf-level steady-state fluorescence yield. Results indicate that NIRvR-derived Phi(F) well captured the seasonal variation of the fluorescence yield changes, and achieved similar results with the existing approach. Both SIF and NIRvR were derived from the airborne imaging fluorescence spectrometer HyPlant for three case studies to evaluate the impacts of light adaptation, heat stress and water limitation on Phi(F). For the light adaptation case study, Phi(F) over the low-light adapted sugar beet field was about 1.3 times larger compared to an unaffected reference area while the difference in NIRvR was minimal, which clearly shows the short-term photosynthetic light induction effect and the ability of SIF to detect plant physiological responses. For the heat stress experiment, OF decreased during a natural heatwave in 2015 in the fields of rapeseed from 0.0150 to 0.0130, barley from 0.0152 to 0.0144, and wheat from 0.0146 to 0.0142 which showed signs of senescence, while slightly increased from 0.0125 to 0.0130 in the corn field which was still in growing. At the water-limited sugar beet field, Phi(F) first increased towards solar noon and then slightly decreased during the afternoon over the water-limited areas from 0.017 to 0.021 and 0.020, with high temperature and high light at noon. The advantages to use SIF/NIRvR as a proxy of Phi(F) to detect stress-induced limitations in photosynthesis include that the impacts of canopy structure and sun-sensor geometry on the Phi(F) estimation are explicitly cancelled, and photosynthetically active radiation (PAR) is not required as input. Finally, our approach is directly applicable to satellite-derived estimates of SIF, enabling the study of variations in Phi(F) to detect the effects of abiotic changes and stresses at large scale.

We have obtained Washington CCD photometry with the CTIO 4m and 1.5m for approximately 50 intermediate-to-old age star clusters in the Clouds. The data extend to near or below the main sequence and provide excellent photometry for the giants, from which precise (internal errors < 0.l dex) mean cluster abundances can be determined. We present data for several of the clusters and discuss the results. Intermediate resolution spectra have also been obtained for some 16 clusters with the CTIO 4m ARGUS multiple-object fibre-fed spectrograph. Finally, we have also obtained high dispersion (R approximately 20,000) echelle spectra for several of the brighter giants in a small sample of Large Magellanic Cloud (LMC) clusters. Detailed elemental abundances derived from these spectra will be presented. These data will help refine our knowledge of the age-metallicity relation in the Clouds.

We have obtained high-resolution, high-signal-to-noise-ratio echelle CCD spectra of three red giants in the metal-poor globular cluster NGC 2298. A detailed analysis of their chemical composition yields [Fe/H] = - 1.91 +/- 0.1 (using a solar Fe abundance of 7.52), where the error includes the dispersion in the observed values and uncertainties in the model-atmosphere parameters. From spectrum synthesis of the [O I] line at 6300 angstrom, we find [O/Fe] = + 0.2 +/- 0.2, with all three giants having the same ratio within the errors. These stars fall somewhat below the mean trend between [O/Fe] and M(V) exhibited by the O-rich sample in Sneden et al. (1991). The alpha elements Mg, Si, and Ca are enhanced more than oxygen, with an average of [alpha/Fe] = - + 0.47, suggesting an oxygen depletion as might be expected from proton burning by the CNO tri-cycle. Al is strongly overabundant in all three stars relative to typical values found in field halo giants of similar metallicity. This Al overabundance could be due to proton burning of the envelope material by the Mg-Al cycle. The apparent dispersion in Al abundance would suggest that the Al overabundance is due to self-enrichment by the individual stars, rather than Al-rich primordial gas, but this conclusion requires verification. Based on our metallicity and composition we apply corrections to the estimated age of NGC 2298 found by previous studies, giving an age range of 14-18 Gyr. Our O and alpha-element abundances for NGC 2298, combined with those of other recent investigations of the most metal-poor globular clusters, indicate substantially lower values than measured by Abia and Rebolo (1989) or assumed O enhancements in the isochrones of McClure et al. (1987), leading to somewhat larger ages for these most ancient Galactic systems.

We report analysis of echelle spectra (R = 17,000, S/N almost-equal-to 50) of 12 Galactic bulge K giants in Baade's window from the sample of Rich ( 1988). We perform an abundance analysis for 11 stars ranging from [Fe/H] = -1 to [Fe/H] = 0.45. The accuracy of our abundance scale is confirmed relative to the disk super-metal-rich stars via the metal-rich giant mu Leonis, and to the Galactic globular clusters using a star in NGC 5927.

We analyze the neutron-capture element (Z > 30) abundance distribution of the ultra-metal-poor (but neutron-capture element rich) halo star CS 22892-052. The observed stellar elemental distribution is compared with those produced by the slow and rapid neutron capture processes (i.e., the s- and r-process) in solar system material. This comparison indicates that the elemental abundances, from barium to erbium, in this Galactic halo star, have the same relative proportions as the solar system r-process distribution. Within the uncertainties of the abundance determinations, the elements strontium and zirconium, but not yttrium, also fall on the same scaled solar r-process curve. The main component of the s-process cannot reproduce the observed neutron-capture abundances in this star. The weak component of the s-process, expected to occur during core helium burning in massive stars, can fit the relative abundance distribution of Sr and Y, but not Zr, suggesting that for the currently observed abundances in CS 22892-052, an admixture of the weak s- and the r-process might be required for production of the elements Sr to Zr. These results give evidence of the occurrence of heavy element nucleosynthesis, particularly the r-process, early in the history of the Galaxy, and further suggest a generation of massive stars (the astrophysical site for the r-process), preceding the formation of this very metal poor halo star, that was responsible for producing the observed heavy elements.

The metallicity of stars in the Galaxy ranges from [Fe/H] = -4 to +0.5 dex, and the solar iron abundance is epsilon(Fe) = 7.51 +/- 0.01 dex. The average values of [Fe/H] in the solar neighborhood, the halo, and Galactic bulge are -0.2, -1.6, and -0.2 dex respectively.

We analyze the recent thorium detection in the metal-poor halo star CS 22892-052. Sneden et al. have demonstrated that all of the stable elements with Z greater than or equal to 56, including those near the thorium nuclear region, are consistent with the solar r-process abundances. This result strongly suggests that thorium (formed in the r-process) was also produced in solar proportions in the progenitor of CS 22892-052. Theoretical calculations, presented here, that reproduce the observed stable solar system r-process abundances predict a thorium/europium (Th/Eu) ratio in close agreement with the extrapolated, corrected r-process-only ratio (0.463) at the time of the formation of the solar system, Sneden et al, found that Th/Eu = 0.219 in CS 22892-052, substantially below the current solar ratio, indicating a much greater age for this star. Ignoring additional production of thorium over time, and thus any chemical evolution effects, comparison between the observed and the solar system corrected Th/Eu ratios gives a simple radioactive-decay age for CS 22892-052 of 15.2 +/- 3.7 Gyr. Additional age estimates, based upon theoretically determined Th/Eu ratios, also suggest a range of 11.5 less than or similar to t(CS 22892-052) less than or similar to 18.8 Gyr and are thus consistent with that simple radioactive-decay age determination for this star. Since additional Galactic production of thorium leads to an increase in this decay time, this is a lower limit on the stellar age, and hence the age of the Galaxy. We evaluate the magnitude of this effect on the chronometric age determinations by employing several simple chemical evolution models, including a closed-box model and one which allows for Galactic infall. These Galactic chemical evolution models suggest an age of 17 +/- 4 Gyr for CS 22892-052. We discuss the implications and possible sources of errors and uncertainties in these age estimates.