Abstract
Intra-annual density fluctuation (IADF) is a structural modification of the tree ring in response to fluctuations in the weather. The expected changes in monsoon flow would lead to heterogeneous moisture conditions during the growing season and increase the occurrence of IADF in trees of the arid ecosystems of continental Asia. To reveal the timings and physiological mechanisms behind IADF formation, we monitored cambial activity and wood formation in Chinese pine (Pinus tabuliformis) during 2017–2019 at three sites in semi-arid China. We compared the dynamics of xylem formation under a drought event, testing the hypothesis that drought affects the process of cell enlargement and thus induces the production of IADF. Wood microcores collected weekly from April to October were used for anatomical analyses to estimate the timings of cambial activity, and the phases of enlargement, wall thickening, and lignification of the xylem. The first cells started enlargement from late April to early May. The last latewood cells completed differentiation in mid-September. Trees produced IADF in 2018. During that year, a drought in June limited cell production in the cambium, only 36% of the xylem cells being formed in IADF trees, compared to 68% in normal tree rings. IADF cells enlarged under drought in early July and started wall thickening during the rainfall events of late July. The drought restricted cell enlargement and affected wall thickening, resulting in narrow cells with wide walls. Cambium and cell enlargement recovered from the abundant rainfall, producing a new layer with large earlywood tracheids. IADF is a specific adaptation of trees to cope with water deficit events occurring during xylem formation. Our findings confirmed the hypothesis that the June-July drought induces latewood-like IADFs by limiting the process of cell enlargement in the xylem. Our finding suggests a higher occurrence of IADF in trees of arid and semi-arid climates of continental Asia if the changes to monsoon flows result in more frequent drought events during the earlywood formation in June.
Highlights
Intra-annual density fluctuation (IADF) is a structural modification of the tree ring involving an abrupt change in wood density (Battipaglia et al, 2016), producing a cell layer similar to the boundary of a true tree ring
We investigated the dynamics of xylogenesis and IADF formation in Chinese pine (Pinus tabuliformis) in a semi-arid region of northwest China and found that IADF cells were produced under a prolonged summer drought and differentiate during the precipitation deficit in June and abundant rainfall in July and August
Our results indicate that climate drivers are uniform for cell enlarging and wall thickening of IADF cells: large IADF cells with thick walls were mainly induced by summer drought
Summary
Intra-annual density fluctuation (IADF) is a structural modification of the tree ring involving an abrupt change in wood density (Battipaglia et al, 2016), producing a cell layer similar to the boundary of a true tree ring. Thick cell walls contribute to the increase of wood density (Beeckman, 2016). IADF represents the ability of a species to adapt to the substantial changes in the growing conditions according to changes in xylogenesis, i.e., cambial activity or cell differentiation or both (Eilmann et al, 2011; Deslauriers et al, 2016; De Micco et al, 2016a). The morphology of IADF cells affects the hydraulic properties of wood and tree survival. An abrupt modification of the treering structure due to IADF affects the physical or mechanical properties of the xylem and reduces wood quality and its potential use (Olivar et al, 2015)
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