Climate-Driven Synchronized Growth of Alpine Trees in the Southeast Tibetan Plateau

<div><p>Knowledge about the spatiotemporal tree growth variability and its associations with climate provides key insights into forest dynamics under future scenarios of climate change. We synthesized 17 tree-ring width chronologies from four tree species at the high-elevation sites in the southeast Tibetan Plateau (SETP) to study the regional tree growth variability and climate-growth relationships. Despite of diverse habitats and different physiological characteristics of these species, these tree-ring chronologies shared a significant common variance in SETP. An unprecedented increase in the shared variance is found along the latter half of the 20th century, coinciding with the enhancement of the frequency of extreme rings among chronologies. It is found that minimum winter temperature tends to be the dominant climate for trees in this region. The site-specific responses in cold (1965–1980) and warm (1990–2005) intervals by means of Fuzzy Cmeans (FCM) clustering reveal that the remarkable enhancement of growth synchrony among trees mainly occur in warm conditions. This is different from previous findings indicating that increased consistence among temperature sensitive tree rings in cold periods. This may be related to the reduced temperature sensitivity of regional tree growth as winter minimum temperature is lower than a certain threshold, which is in agreement with the “principle of ecological amplitude”. In addition, it is worth noting that precipitation in June have started to restrain the tree growth since the beginning of the 1980s, which is possibly an important contributor for synchronized growth among trees in SETP.</p></div

Correlations performed between the regional chronology PC1 and monthly a) mean temperature and b) total precipitation from previous May to current September during the period 1951–2005.

<p>The 95% and 99% significance levels are indicated by dash and bold lines, respectively.</p

Correlations between tree-ring chronologies derived from traditional method (white bar), signal-free method (grey bar) and the ensemble weighting method (black bar) and the (a) monthly precipitation and annual precipitation from previous August to current July (A–J) for the Songmingyan Mountain, the (b) monthly temperature and annual temperature for the Songmingyan Mountain, the (c) monthly precipitation and annual precipitation from previous August to current July (A–J) for the Shenge site and the (d) monthly temperature and annual temperature for the Shenge site.

<p>The significance level of 0.1 is indicated by horizontal line.</p

Partitions for all the indexed tree-ring widths occurring in relatively a) cold and b) warm winter years computed by the FCM clustering.

Partitions for all the indexed tree-ring widths occurring in relatively a) cold and b) warm winter years computed by the FCM clustering.</p

Descriptions of the geographic features and characteristics of the 17 tree-ring chronologies across SETP.

<p>Descriptions of the geographic features and characteristics of the 17 tree-ring chronologies across SETP.</p

Temporal changes of the growth-climate relationships as shown by moving correlations between tree growth and a) November-January average temperature and b) June precipitation totals using a sliding window of 30 years assigned to the center year of the window.

<p>The shaded areas are the 95% confidence limits.</p

The instrumental record (black line) and EEMD-based nonlinear trend (red line) of a) November-January temperature, b) June temperature, c) June precipitation and d) March-May temperature over SETP during the period from 1951–2005.

<p>The instrumental record (black line) and EEMD-based nonlinear trend (red line) of a) November-January temperature, b) June temperature, c) June precipitation and d) March-May temperature over SETP during the period from 1951–2005.</p

Map showing the sites of selected tree-ring chronologies, gridded climate records and major cities over SETP <i>Picea likiangensis (Franchet) Pritzel</i> chronologies are represented by green stars, <i>Abies forestii Rogers</i> chronologies by blue squares, <i>Juniperus tibetica Kom</i> chronologies by purple circles and <i>Tsuga dumosa Eichler</i> chronologies by red triangles.

<p>Map showing the sites of selected tree-ring chronologies, gridded climate records and major cities over SETP <i>Picea likiangensis (Franchet) Pritzel</i> chronologies are represented by green stars, <i>Abies forestii Rogers</i> chronologies by blue squares, <i>Juniperus tibetica Kom</i> chronologies by purple circles and <i>Tsuga dumosa Eichler</i> chronologies by red triangles.</p