Seasonal Dynamics of Stem Radial Increment of Pinus taiwanensis Hayata and Its Response to Environmental Factors in the Lushan Mountains, Southeastern China

Abstract

Seasonal radial-increment records can help to elucidate how tree growth responds to climate seasonality. Such knowledge is critical for understanding the complex growth-climate relationships in subtropical China. We hypothesize that under subtropical monsoon climate characterized by mild winters and hot summers, summer drought constrains stem radial increment, which generally results in growth-limiting factors switching from temperatures in spring and early summer to precipitation in summer and autumn. Here, we monitored intra-annual dynamics of stem radial increment with band dendrometers in a montane stand of Taiwan pine (Pinus taiwanensis Hayata) from Lushan Mountains for two consecutive years (2016-2017). A pronounced bimodal seasonal pattern of stem radial increment was observed in 2016. However, it was less clear in 2017 when late-summer rainfall events occurred in early August. Changing growth-climate relationships were detected throughout the two growing seasons. Stem increments were consistently positively correlated with temperatures before early July, whereas the growth-temperature dependency was gradually weakened and more variable after early July. Conversely, stem increments were significantly correlated with precipitation and soil moisture since early July, indicating that moisture variables were the main factor limiting stem increments in dry period. More precipitation was received in the dry period (July-November) of 2017 as compared with the year 2016, which favoured a wider annual increment in 2017, although growing-season temperature and precipitation was similar between years. Our study suggests a seasonal shift in growth-limiting factors in subtropical forests, which should be explicitly considered in forecasting responses of tree growth to climatic warming.

Monthly Radial Growth Model of Chinese Fir (Cunninghamia lanceolata (Lamb.) Hook.), and the Relationships between Radial Increment and Climate Factors

“…On a growth period time resolution, we found the variation of growth limiting factors from temperature in the spring and early summer to precipitation in summer and autumn. This seasonal variation of growth limiting factors of coniferous and deciduous species has been widely observed in temperate and subtropical forests in the summer [47,49]. Climate factors mainly influence moisture availability to trees, thereby affecting their stem radial growth [50].…”

“…The climate of China's subtropical region is characterized by East Asian monsoon, which is mild in winter and hot in summer. Suppression of summer growth seems to be a strategy for coping with harsh environmental conditions during summer drought [47]. The growth rate of Chinese fir decreased in the summer (June and July), showing a bimodal growth pattern (Figure 3).…”

Seasonal Drought Effects on Intra-Annual Stem Growth of Taiwan Pine along an Elevational Gradient in Subtropical China

“…The mean annual temperature and precipitation are 11.9 • C and 2009 mm, respectively (1980-2011, meteorological station at Lushan Mountain, 1165 m a.s.l.) [21]. We selected three forest plots located at elevations between 921 and 1402 m, coving an elevation range of circa 500 m for Taiwan pine (Table 1).…”

“…Importantly, dendrometer-derived diurnal stem radial variations consist not only of irreversible radial growth of new xylem cells but also of reversible stem shrinking and swelling (reflecting water storage dynamics in tree stems), thereby providing continuous information on tree water status and further facilitating the physiological interpretation of stem growth under drought stress [14,15,19,20]. Despite this, intra-annual stem growth dynamics and its response to regional climate are still very scarce in subtropical China, among which most studies focused on a single site with limited observation years [21][22][23]. On the other hand, it has been documented that studying seasonal stem growth dynamics over an elevational gradient, in which warm and dry conditions at the lower elevation served as a natural analog to simulate future climate, could shed more light on the mechanisms of tree growth under future climate change scenarios [24,25].…”

“…Pinus taiwanensis Hayata (Taiwan pine), a most important foundation conifer tree species in southeastern China, is distributed over a wide range of elevations between 800 to 1700 m. This conifer is moderately cold-tolerant and well adapted to cool and wet habitats in mountainous areas, thus, providing crucial ecological values in soil and water conservation and biodiversity for regional ecosystems [33]. Our preliminary studies indicated that intra-annual stem growth of high-elevation Taiwan pine trees was constrained by seasonal drought, and growth-limiting factors shifted seasonally from temperatures in spring and early summer to water availability in summer and autumn [21]. Given a high sensitivity to water stress, the ongoing increase in seasonal drought intensity and severity is expected to reduce tree growth and suitable habitats for Taiwan pine, especially at the lower edges.…”