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Harvard Forest Data Archive

HF265

Age and Size of Smith Firs at Treeline in Tibet 1700-2013

Related Publications

Data

Overview

  • Lead: Yafeng Wang, Eryuan Liang
  • Investigators: Hannah Buckley, Bradley Case, J. Julio Camarero, Aaron Ellison, Neil Pederson
  • Contact: Aaron Ellison
  • Start date: 1700
  • End date: 2013
  • Status: completed
  • Location: Sygera Mountains (China)
  • Latitude: +29.17 to +30.25
  • Longitude: -95.58 to -93.20
  • Elevation: 4370 to 4388 meter
  • Taxa: Abies georgi (Smith fir)
  • Release date: 2016
  • Revisions:
  • EML file: knb-lter-hfr.265.2
  • DOI: digital object identifier
  • Related links:
  • Study type: historical, paleological
  • Research topic: historical and retrospective studies; physiological ecology, population dynamics and species interactions
  • LTER core area: primary production, populations, disturbance
  • Keywords: climate change, competition, dendrochronology, distribution, trees
  • Abstract:

    The most widespread response to global warming among alpine treeline ecotones is not an upward shift, but an increase in tree density. However, the impact of increasing density on interactions among trees at treeline is not well understood. Here, we test if treeline densification induced by climatic warming leads to increasing intraspecific competition. We mapped and measured the size and age of Smith fir trees growing in two treelines located in the southeastern Tibetan Plateau. We used spatial point-pattern and codispersion analyses to describe the spatial association and covariation among seedlings, juveniles, and adults grouped in 30-year age classes from the 1860s to the present. Effects of competition on tree height and regeneration were inferred from bivariate mark-correlations. Since the 1950s, a rapid densification occurred at both sites in response to climatic warming. Competition between adults and juveniles or seedlings at small scales intensified as density increased. Encroachment negatively affected height growth and further reduced recruitment around mature trees. We infer that tree recruitment at the study treelines was more cold-limited prior to 1950 and shifted to a less temperature-constrained regime in response to climatic warming. Therefore, the ongoing densification and encroachment of alpine treelines could alter the way climate drives their transitions towards subalpine forests.

  • Methods:

    We characterized the structure and reconstructed the dynamics of Smith fir at two treeline sites, designated as “N1” and “N2”. Both sites were located on north-facing slopes, and encompassed the treeline ecotone, including the upper treeline (uppermost 2-m tall trees) and the forest limit (elevation at which tree cover ≥ 30%). The current altitudinal positions of N1 and N2 were 4388 and 4370 m a.s.l., with mean slopes of 10º and 15º, respectively. These treelines were not locally disturbed by yak (Bos grunniens) grazing or logging, and defoliation due to insect outbreaks or wild herbivores were not observed during field sampling. Above the current treeline, dense 2-3 m tall individual Rhododendron shrubs dominated the vegetation, but no stumps or remains of old dead trees were found there.

    The age structure of the Smith fir trees (DBH greater than 5 cm) within two large rectangular plots (150 × 150-m) was characterized using standard methods. Age was estimated from basal cores taken from the main stem of each tree and collected with an increment borer. We processed the core samples using standard dendrochronological techniques, including air drying, sanding with successively finer sandpapers, and visually cross-dating the cores under a stereomicroscope. Mean series intercorrelation for plots N1 and N2 were 0.61 and 0.60, respectively, indicating reliable cross-dating. If the pith was not present in the core, a pith geometric locator was used to estimate the innermost distance missing up to the theoretical center of the stem. Ages of hollow trees were estimated using DBH-age regressions (N1: r2 = 0.88, P less than 0.001, n = 385; N2: r2 = 0.83, P less than 0.001, n = 406. In our collection, missed piths typically occurred only in trees greater than 200-years old, so any errors in estimated ages using this method would have little effect on recruitment that occurred within the last 150 years. The germination age of young individuals (DBH less than 5 cm and height less than 1.3 m, age greater than 3 years) was estimated by counting the successive bud scars or internodes observed along their main stem. Since uncertainties related to age determination were always present, we binned tree ages by decades for analysis.

    To identify temporal changes in the variation in spatial patterns of Smith fir in these two plots, we examined tree size distributions in five consecutive 30-yr intervals beginning with the 1862−1891 period and ending either with 1982−2011 (for N1) or 1982−2013 (N2). For each interval, individual Smith fir trees were grouped into three age classes: seedlings (age ≤ 30 years), juveniles (31 ≤ age ≤ 100 years), and adults (age greater than 100 years). These age classes produced groups similar to those found when grouping trees by their height or DBH. For instance, a tree aged 30 years was often approximately 50-cm tall, a threshold which is often used as a criterion to define the maximum height of seedlings growing in treeline ecotones. Likewise, trees between 100 and 150 years in age corresponded to heights greater than 6 m with DBH greater than 17.5 cm, which are categories usually employed to consider a tree as a mature, dominant, and reproductive individual (Camarero and Gutiérrez 2004, Wang et al. 2012). Given the lack of long-term forest survey data near treelines, grouping trees into age or stage classes provides a reliable method to investigate the variations of competition through time.

  • Use:

    This dataset is released to the public under Creative Commons license CC BY (Attribution). Please keep the designated contact person informed of any plans to use the dataset. Consultation or collaboration with the original investigators is strongly encouraged. Publications and data products that make use of the dataset must include proper acknowledgement.

  • Citation:

    Wang Y, Liang E. 2016. Age and Size of Smith Firs at Treeline in Tibet 1700-2013. Harvard Forest Data Archive: HF265.

Detailed Metadata

hf265-01: plot n1 data

  1. number: plant number
  2. x: location (unit: meter / missing value: NA)
  3. y: location (unit: meter / missing value: NA)
  4. dbh: diameter at breast height (unit: meter / missing value: NA)
  5. height: height (unit: meter / missing value: NA)
  6. age: age in years (unit: number / missing value: NA)
  7. living: whether alive, dying or dead
    • 1: alive
    • 0.5: dying
    • 0: dead

hf265-02: plot n2 data

  1. number: plant number
  2. x: location (unit: meter / missing value: NA)
  3. y: location (unit: meter / missing value: NA)
  4. dbh: diameter at breast height (unit: meter / missing value: NA)
  5. height: height (unit: meter / missing value: NA)
  6. age: age in years (unit: number / missing value: NA)
  7. living: whether living, dying or dead
    • 1: alive
    • 0.5: dying
    • 0: dead

hf265-03: R code for codispersion analysis of the data

  • Compression: none
  • Format: R code
  • Type: R code

hf265-04: additional R code called (sourced) by hf265-03-treeline-codisp-temporal.r

  • Compression: none
  • Format: R code
  • Type: R code