@article{KahmenSachseArndtetal.2011,
  author    = {Kahmen, Ansgar and Sachse, Dirk and Arndt, Stefan K. and Tu, Kevin P. and Farrington, Heraldo and Vitousek, Peter M. and Dawson, Todd E.},
  title     = {Cellulose delta O-18 is an index of leaf-to-air vapor pressure difference (VPD) in tropical plants},
  series = {Proceedings of the National Academy of Sciences of the United States of America},
  volume    = {108},
  journal   = {Proceedings of the National Academy of Sciences of the United States of America},
  number    = {5},
  publisher = {National Acad. of Sciences},
  address   = {Washington},
  issn      = {0027-8424},
  doi       = {10.1073/pnas.1018906108},
  pages     = {1981 -- 1986},
  year      = {2011},
  abstract  = {Cellulose in plants contains oxygen that derives in most cases from precipitation. Because the stable oxygen isotope composition, delta O-18, of precipitation is associated with environmental conditions, cellulose delta O-18 should be as well. However, plant physiological models using delta O-18 suggest that cellulose delta O-18 is influenced by a complex mix of both climatic and physiological drivers. This influence complicates the interpretation of cellulose delta O-18 values in a paleo-context. Here, we combined empirical data analyses with mechanistic model simulations to i) quantify the impacts that the primary climatic drivers humidity (e(a)) and air temperature (T-air) have on cellulose delta O-18 values in different tropical ecosystems and ii) determine which environmental signal is dominating cellulose delta O-18 values. Our results revealed that e(a) and T-air equally influence cellulose delta O-18 values and that distinguishing which of these factors dominates the delta O-18 values of cellulose cannot be accomplished in the absence of additional environmental information. However, the individual impacts of e(a) and T-air on the delta O-18 values of cellulose can be integrated into a single index of plant-experienced atmospheric vapor demand: the leaf-to-air vapor pressure difference (VPD). We found a robust relationship between VPD and cellulose delta O-18 values in both empirical and modeled data in all ecosystems that we investigated. Our analysis revealed therefore that delta O-18 values in plant cellulose can be used as a proxy for VPD in tropical ecosystems. As VPD is an essential variable that determines the biogeochemical dynamics of ecosystems, our study has applications in ecological-, climate-, or forensic-sciences.},
  language  = {en}
}
@article{HoffmannKahmenCernusaketal.2013,
  author    = {Hoffmann, Bernd and Kahmen, Ansgar and Cernusak, Lucas A. and Arndt, Stefan K. and Sachse, Dirk},
  title     = {Abundance and distribution of leaf wax n-alkanes in leaves of Acacia and Eucalyptus trees along a strong humidity gradient in northern Australia},
  series = {Organic geochemistry : the international journal for rapid publication of current research in organic geochemistry and biochemistry},
  volume    = {62},
  journal   = {Organic geochemistry : the international journal for rapid publication of current research in organic geochemistry and biochemistry},
  number    = {9},
  publisher = {Elsevier},
  address   = {Oxford},
  issn      = {0146-6380},
  doi       = {10.1016/j.orggeochem.2013.07.003},
  pages     = {62 -- 67},
  year      = {2013},
  abstract  = {Environmental parameters such as rainfall, temperature and relative humidity can affect the composition of higher plant leaf wax. The abundance and distribution of leaf wax biomarkers, such as long chain n-alkanes, in sedimentary archives have therefore been proposed as proxies reflecting climate change. However, a robust palaeoclimatic interpretation requires a thorough understanding of how environmental changes affect leaf wax n-alkane distributions in living plants. We have analysed the concentration and chain length distribution of leaf wax n-alkanes in Acacia and Eucalyptus species along a 1500 km climatic gradient in northern Australia that ranges from subtropical to arid. We show that aridity affected the concentration and distribution of n-alkanes for plants in both genera. For both Acacia and Eucalyptus n-alkane concentration increased by a factor of ten to the dry centre of Australia, reflecting the purpose of the wax in preventing water loss from the leaf. Furthermore, Acacian-alkanes decreased in average chain length (ACL) towards the arid centre of Australia, whereas Eucalyptus ACL increased under arid conditions. Our observations demonstrate that n-alkane concentration and distribution in leaf wax are sensitive to hydroclimatic conditions. These parameters could therefore potentially be employed in palaeorecords to estimate past environmental change. However, our finding of a distinct response of n-alkane ACL values to hydrological changes in different taxa also implies that the often assumed increase in ACL under drier conditions is not a robust feature for all plant species and genera and as such additional information about the prevalent vegetation are required when ACL values are used as a palaeoclimate proxy.},
  language  = {en}
}
@article{CrausbayGenderjahnHotchkissetal.2014,
  author    = {Crausbay, Shelley and Genderjahn, Steffi and Hotchkiss, Sara and Sachse, Dirk and Kahmen, Ansgar and Arndt, Stefan K.},
  title     = {Vegetation dynamics at the upper reaches of a tropical montane forest are driven by disturbance over the past 7300 years},
  series = {Arctic, antarctic, and alpine research : an interdisciplinary journal},
  volume    = {46},
  journal   = {Arctic, antarctic, and alpine research : an interdisciplinary journal},
  number    = {4},
  publisher = {Institute of Arctic and Alpine Research, University of Colorado},
  address   = {Boulder},
  issn      = {1523-0430},
  doi       = {10.1657/1938-4246-46.4.787},
  pages     = {787 -- 799},
  year      = {2014},
  abstract  = {We assessed tropical montane cloud forest (TMCF) sensitivity to natural disturbance by drought, fire, and dieback with a 7300-year-long paleorecord. We analyzed pollen assemblages, charcoal accumulation rates, and higher plant biomarker compounds (average chain length [ACL] of n-alkanes) in sediments from Wai 'anapanapa, a small lake near the upper forest limit and the mean trade wind inversion ('IWI) in Hawai`i. The paleorecord of ACL suggests increased drought frequency and a lower awl elevation from 2555-1323 cal yr B.P. and 606-334 cal yr B.P. Charcoal began to accumulate and a novel fire regime was initiated ca. 880 cal yr B.P., followed by a decreased fire return interval at ca. 550 cal yr B.P. Diebacks occurred at 2931, 2161, 1162, and 306 cal yr B.P., and two of these were independent of drought or fire. Pollen assemblages indicate that on average species composition changed only 2.8\% per decade. These dynamics, though slight, were significantly associated with disturbance. The direction of species composition change varied with disturbance type. Drought was associated with significantly more vines and lianas; fire was associated with an increase in the tree fern Sadleria and indicators of open, disturbed landscapes at the expense of epiphytic ferns; whereas stand-scale dieback was associated with an increase in the tree fern Cibotium. Though this cloud forest was dynamic in response to past disturbance, it has recovered, suggesting a resilient TMCF with no evidence of state change in vegetation type (e.g., grassland or shrubland).},
  language  = {en}
}