Impact of extreme pre-monsoon drought on xylogenesis and intra-annual radial increments of two tree species in a tropical montane evergreen broad-leaved forest, southwest China.

Tropical montane evergreen broad-leaved forests cover the majority of forest areas and have high carbon storage in Xishuangbanna, southwest China. However, stem radial growth dynamics and their correlations with climate factors have never been analyzed in this forest type. By combining bi-weekly microcoring and high-resolution dendrometer measurements, we monitored xylogenesis and stem radius variations of the deciduous species Betula alnoides and the evergreen species Schima wallichii. We analyzed the relationships between weekly climate variables prior to sampling and the enlarging zone width or wall thickening zone width, as well as weekly radial increments and climate factors during two consecutive years (2020-2021) showing contrasting hydrothermal conditions in the pre-monsoon season. In the year 2020, which was characterized by a warmer and drier pre-monsoon season, the onset of xylogenesis and radial increments of B. alnoides and S. wallichi were delayed by three months and one month, respectively, compared to the year 2021. In 2020, xylem formation and radial increments were significantly reduced for B. alnoides, but not for S. wallichill. The thickness of enlarging zone and wall thickening zone in S. wallichill were positively correlated with relative humidity, minimum and mean air temperature, but were negatively correlated with vapor pressure deficit during 2020-2021. The radial increments of both species showed significant positive correlations with precipitation and relative humidity, and negative correlations with vapor pressure deficit and maximum air temperature during two years. Our findings reveal that drier pre-monsoon conditions strongly delay growth initiation and reduce stem radial growth, providing deep insights to understand tree growth and carbon sequestration potential in tropical forests under a predicted increase in frequent drought events.

High vapour pressure deficit enhances turgor limitation of stem growth in an Asian tropical rainforest tree.

Tropical forests are experiencing increases in vapour pressure deficit (D), with possible negative impacts on tree growth. Tree-growth reduction due to rising D is commonly attributed to carbon limitation, thus overlooking the potentially important mechanism of D-induced impairment of wood formation due to an increase in turgor limitation. Here we calibrate a mechanistic tree-growth model to simulate turgor limitation of radial stem growth in mature Toona cilitata trees in an Asian tropical forest. Hourly sap flow and dendrometer measurements were collected to simulate turgor-driven growth during the growing season. Simulated seasonal patterns of radial stem growth matched well with growth observations. Growth mainly occurred at night and its pre-dawn build-up appeared to be limited under higher D. Across seasons, the night-time turgor pressure required for growth was negatively related to previous midday D, possibly due to a relatively high canopy conductance at high D, relative to stem rehydration. These findings provide the first evidence that tropical trees grow at night and that turgor pressure limits tree growth. We suggest including turgor limitation of tree stem growth in models also for tropical forest carbon dynamics, in particular, if these models simulate effects of warming and increased frequency of droughts.

[Intra-annual radial growth of Garuga floribunda in tropical seasonal rain forest and its response to environmental factors in Xishuangbanna, Southwest China]. / çƒ­å¸¦å­£èŠ‚é›¨æž—å¤šèŠ±ç™½å¤´æ ‘å¹´å† å¾„å‘ç”Ÿé•¿åŠ¨æ€åŠå ¶å¯¹çŽ¯å¢ƒå› å­çš„å“åº”.

We examined the seasonal growth dynamics of a deciduous tree species Garuga floribunda in the tropical seasonal rain forest in Xishuangbanna and monitored the stem radial growth with both high resolution dendrometer and microcoring methods. Combining with the monitoring of non-structural carbohydrates (NSCs) in stem and environmental factors, we analyzed the eco-physiological drivers underlying the seasonal cambial activity and radial growth dynamics. The results of high reso-lution dendrometer method showed that the growth of G. floribunda began at the end of May (day of year, DOY: 149.3±7.2) and ended at the end of August (DOY: 241.0±14.7) in 2020, the annual total radial growth was 3.12 mm, and the maximum growth rate was 0.04 mm·d-1. Based on the microcoring methods, we found that xylem cell enlarging started from March 9th (69.2±6.2) and cell wall thickening ended on September 19th (DOY: 262.8±2.8). The cumulative xylem radial growth was 1.76 mm, and the maximum growth rate was 0.009 mm·d-1. The daily radial growth rate of G. floribunda was significantly and positively correlated with precipitation, relative humidity, daily minimum air temperature, soil moisture and temperature at the depth of 20 cm, and was negatively correlated with daily maximum air temperature, vapor pressure deficit, maximum wind speed, and water vapor pressure. The starch and soluble sugar contents in the sapwood of G. floribunda were relatively higher before the growing season. The starch content was lowest in the end of March, while the content of soluble sugars was lowest in middle of May. At the end of the growing season, the contents of starch and soluble sugar in G. floribunda peaked in the middle of October and the end of December, respectively.