Elsevier

Forest Ecology and Management

Volume 458, 15 February 2020, 117759
Forest Ecology and Management

Diverse responses of radial growth to climate across the southern part of the Asian boreal forests in northeast China

https://doi.org/10.1016/j.foreco.2019.117759Get rights and content

Highlights

  • Diverse growth-climate responses driven by local climate exist across the study area.

  • Limitations to radial growth varied in relation to the latitudinal climate gradient.

  • Warming negatively affects tree growth in the southern part of Asian boreal forests.

  • The percentage of trees threatened by warming will increase by 46%–64% by 2100.

  • Regionally-specific measures are urgent to take for sustaining forest productivity.

Abstract

In the context of rapid warming, a better understanding of the spatially varying growth-climate responses of widely-distributed forests is essential for predicting the warming effects on trees, so as to guide the forest management. Here, we characterized the growth-climate relationships of the southern part of Asian boreal forests in northeast China at multiple levels to deduce how warming impacts tree growth. Specially, we aimed to distinguish climatic limitations to growth and clarify how local climate determines the response pattern. We established a tree-ring network, which includes 516 trees from 20 populations across the study area. We then correlated regional, population, and tree chronologies with climate data. In the north of the study area, radial growth responded positively to temperature before the growing season and precipitation during the growing season. In contrast, growth in the south responded positively to precipitation and negatively to temperature during the growing season. Thus, diverse growth patterns exist in response to climate. The extent to which growth was limited by climate varied along the latitudinal climate gradient. The north parts were limited by both frost and moisture, whereas the central-south parts were limited by moisture deficit induced by high temperature. The growth-climate response was quantified as the percentage of significant tree-level correlations. Through associating this quantitative index of each population with local climate, we confirmed the decisive role of local temperature in forming the spatial heterogeneity of response. We also confirmed that warming negatively affected tree growth across the study area. Our models predict that as warming continues, the trees threatened by high temperature in summer will increase by 45.5%–63.6% by the end of this century. All these findings warn of growth declines in Asian boreal forests, and suggest the need for urgent efficient regionally-specific measures to sustain the forest productivity in this region.

Introduction

Profound global warming over recent decades has challenged and reshaped global forest ecosystems (Allen et al., 2010, Franklin et al., 2016, Nolan et al., 2018). As one of the largest forest biomes, boreal forests account for 20% of the global forest carbon sink (Pan et al., 2011), and encompass 30% of global forest land cover (Gauthier et al., 2015). As such, they have attracted extensive research focus, yielding a consensus that the responses of tree growth to climate change are heterogeneous across the distribution area of boreal forests (Babst et al., 2013, D'Orangeville et al., 2016, Hellmann et al., 2016). This spatial heterogeneity has been attributed to a variety of factors, including population structure (D'Amato et al., 2013), tree species (Bosela et al., 2019), and habitat topography (Saulnier et al., 2019). However, at a large scale, local climate patterns tend to represent the main driver of this heterogeneity (D'Orangeville et al., 2016, Klesse et al., 2018, Martin-Benito and Pederson, 2015). Warming-intensified droughts have triggered declines in productivity or increased mortality in the boreal forests of western Canada (Peng et al., 2011), western Europe (Allen et al., 2010), southwestern Russia (Schaphoff et al., 2015), and northern Mongolia (Liu et al., 2013). By contrast, forests limited by energy deficit have benefited from the warmer climate, as indicated by greening and increases in productivity at higher latitudes (Zhao and Running, 2010, Zhu et al., 2016) and the northward expansion of tree lines (Dyderski et al., 2018, Esper and Schweingruber, 2004, Miller et al., 2017). Furthermore, in some transition regions, the main climatic factor limiting tree growth is likely to shift from energy to moisture as a result of continuous warming (Babst et al., 2019, D’Orangeville et al., 2018, Schurman et al., 2019). Diverse growth responses along the latitudinal or altitudinal climate gradient might be translated to gradient growth trends. Such trends might drive the horizontal or vertical migration of forest distributions, as observed in Europe and North America (Beck et al., 2011, Vitasse et al., 2012).

The Asian boreal forests are experiencing warming at one of the fastest rates worldwide (IPCC, 2013). In comparison to the energy-limited northern margins of this forest biome, growth responses to climate change along the southern margins are more complex and variable (Babst et al., 2019, Hellmann et al., 2016). The southernmost distribution of the Asian boreal forests extends to the Greater Khingan Range, northeast China. This marginal forest community is dominated by two widely-distributed coniferous species, Larix gmelinii and Pinus sylvestris var. mongolica, and it intersects with temperate deciduous broadleaf forests in the southeast and with semi-arid steppes to the west. The latitudinal range of this boreal forest extends 12°, from 42°N to 54°N, with mean annual temperatures ranging from –4.5 °C to 5.5 °C. When considering the susceptibility and vulnerability of forests living in transitional climate conditions between two biomes (Allen et al., 2016), efforts have been made to identify the climate conditions limiting the radial growth of the dominant tree species in this region. Some researchers have suggested low temperature as the main climatic limitation to tree growth, wherein warming could promote growth by alleviating energy deficits (Bai et al., 2019), accelerating snowmelt (Zhang et al., 2019b), and thawing permafrost (Zhang et al., 2019a). In contrast, others have suggested moisture deficit as the limiting factor, based on evidence of trees exhibiting negative responses to increasing temperature and positive responses to increasing precipitation (Chen et al., 2012, Yu et al., 2018, Zhang et al., 2018a). Under this framework, warming-intensified droughts would represent the most immediate threat to tree growth, pushing this forest community into a predicament comparable to semi-arid forests in central Asia (Liu et al., 2013). However, other evidence indicate that climatic signals recorded in radial growth in this region are relatively unclear (Wang et al., 2009, Zhang et al., 2018b). This observed variability in growth responses might be attributed to a number of factors, including species (Zhang et al., 2018b), tree age (Wang et al., 2009), slope exposure (Liu and Liu, 2014), and geomorphological form (Bai et al., 2019), as well as other factors. Considering the contiguous latitudinal distribution and broad climate niches of the study area, we suggest that growth responses in this forest community are aligned with a temperature gradient, as demonstrated for the boreal coniferous forests of northeastern North America and western Europe (D'Orangeville et al., 2016, Klesse et al., 2018).

As such, we hypothesize that local climate patterns strongly regulate the growth responses of boreal forest species to climate change in our study area. To investigate this hypothesis, we complied a widely- and contiguously-distributed tree-ring network to examine the diverse growth-climate relationships across the study area, using both qualitative and quantitative indices. A comprehensive understanding of how the growth-climate responses of trees vary in spatial could help improve estimations of potential forest growth responses in the Asian boreal forests under future climate regimes.

Section snippets

Study area

Our study focused on the southern part of the Asian boreal forests in the Greater Khingan Range, which is also the main distribution of China’s cold-temperate forest. The ecosystem is a climatic climax forest community dominated by Larix gmelinii and Pinus sylvestris var. mongolica. It extends over 1200 km, from 42°N to 54°N, narrowing from a width of over 300 km in the north to <100 km in the south. The elevation of the study area ranges from 400 to 1500 m above sea level (a.s.l.) (Fig. 1).

Geographical distributions and growth patterns of population clusters

Both the hierarchical cluster analysis and the correlation matrix of population chronologies produced four clusters for our tree-ring network, roughly along a latitudinal gradient. The four clusters displayed diverse regional growth patterns during 1960–2014 (Fig. 3a–c). The cluster that contained three northern populations (1, 2, and 5) and one outlier population (18) was labeled north–north (NN). Its regional chronology showed a significant increasing trend of 0.024 dec–1 (p < 0.05, n = 55;

Geographic pattern of growth-climate responses and climatic drivers

To clarify how local climate contributes towards determining the diversity of growth-temperature response, the impacts of climate on forest growth were usually translated to certain measureable indices (Alkama and Cescatti, 2016, Peng et al., 2013, Yang et al., 2017). Our study demonstrated that indicators of the diversity in growth-climate response include pairwise correlations among population tree-ring chronologies, spatial growth trends, and the percentage of negatively responding trees to

Conclusions

A tree-ring network consisting of spatially continuous sampling populations and abundant samples provides a viable basis for investigating spatial heterogeneity of growth-climate responses in a widely-distributed forest. Our findings at regional and population level demonstrated a clear and contiguous geographic pattern in growth-climate response across the southern part of Asian boreal forests in northeast China. The results indicated that the varying responses were aligned with the

Declaration of competing interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China [Grant No. 41630750 and 41771051], the National Key Research and Development Program of China [Grant No. 2018YFA0606101-02], and the China Scholarship Council. We thank Mingchang Wang, Biao Wang, and Kun Liu for assistance with fieldwork. We thank Fang Wu and Yan Wen for providing valuable comments on this work. We also are grateful to the anonymous reviewers for their contributions to improving this paper.

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