Divergent Growth Responses to Warming between Stand-Grown and Open-Grown Trees in a Dryland Montane Forest in Northwestern China

Abstract

Dryland montane forests conserve water for people living in the fluvial plains. The fate of these forests under climate warming is strongly affected by local environmental factors. The question remains of how internal factors contribute to climate change impacts on forest growth in these regions. Here, we investigated tree ring records for similar-aged stand-grown trees and their neighboring open-grown trees at elevation in a dryland montane forest (Picea crassifolia Kom.) in northwestern China. The growth rate of open-grown trees is much higher than their neighboring stand-grown trees across the entire elevation gradient, and the lower the altitude, the greater the difference. Open-grown trees at different elevations showed similar growth patterns, as tree growth at all sites was accelerated over time. In contrast, growth patterns of stand-grown trees were divergent at different altitudes, as growth at high elevations (3100-3300 m a.s.l.) was accelerated, whereas growth at low elevations (2700-2900 m a.s.l.) became stable after the year 1990. Analysis of growth-climate relationships indicated that warming promoted open-grown tree growth across the entire altitude gradient, and also stand-grown tree growth at high elevations, but negatively affected the growth of stand-grown trees at low elevations. Water scarcity can be exacerbated by competition within forests, inhibiting the warming-induced benefits on tree growth. Moving window correlation analysis suggested the negative effect of warming on tree growth at low elevations was diminished after the late 1990s, as the drought stress was alleviated. Our research shows the divergent growth responses to warming of stand-grown and open-grown trees along elevation. It reveals effects of internal factors in determining tree growth response to warming and holds the potential to aid forest management and ecosystem models in responding to climate change.

Divergent Seasonal Patterns of Qinghai Spruce Growth with Elevation in Northwestern China

“…The soil is thicker and the sand content lower at elevations from 2500 to 2900 m a.s.l., but the soil layer is relatively thin and unevenly distributed at elevations from 3000 to 3300 m a.s.l. [9]. The main shrub community include Salix Gilashanica C. Wang et P. Y. Fu, Caragana jubata (Pall.)…”

“…All five plots were located on semi-shaded hillsides with similar slope gradients of approximately 27-41°. The soil type of the five plots was a mountainous gray cinnamon soil; this soil texture was considered a loam, with a sand content of 34-58% [9], and the soil thickness was 50-80 cm. The forest in each plot consisted of Qinghai spruce trees.…”

“…Therefore, to assess the impacts of future climate change on forest growth, it is crucial to study the climatic forcing of tree growth at annual and seasonal time scales. However, a number of studies about growth-climate relationships were mostly based on tree-ring-based investigations [7][8][9][10]. Such studies do not fully explain the effects of climate conditions on tree growth and do Forests 2022, 13, 388 2 of 14 not capture seasonal growth changes and relevant growth-limiting factors.…”

“…tree growing in this region is a native evergreen conifer growing at an elevation of 2500-3300 m a.s.l. [7,9]. Earlier studies showed that daily and seasonal variation in stem growth of Qinghai spruce trees at the single scale and among tree classes, and that stem growth initiation was governed by air temperature and/or soil temperature [20,[23][24][25], and that growth cessation was affected by soil moisture under drought conditions [20].…”

Relationships between Regenerating Qinghai Spruce Seedlings and Soil Stoichiometry across Elevations in a Forest in North-Western China

“…(Figure 1). The soil type in each plot was a mountainous gray cinnamon soil; the texture was considered loam, with a sand content of 34-58% [22]. The soil thickness was 50-80 cm.…”