Abstract
We investigate cambial growth periodicity in Brachystegia spiciformis, a dominant tree species in the seasonally dry miombo woodland of southern Africa. To better understand how the brevi-deciduous (experiencing a short, drought-induced leaf fall period) leaf phenology of this species can be linked to a distinct period of cambial activity, we applied a bi-weekly pinning to six trees in western Zambia over the course of one year. Our results show that the onset and end of cambial growth was synchronous between trees, but was not concurrent with the onset and end of the rainy season. The relatively short (three to four months maximum) cambial growth season corresponded to the core of the rainy season, when 75% of the annual precipitation fell, and to the period when the trees were at full photosynthetic capacity. Tree-ring studies of this species have found a significant relationship between annual tree growth and precipitation, but we did not observe such a correlation at intra-annual resolution in this study. Furthermore, a substantial rainfall event occurring after the end of the cambial growth season did not induce xylem initiation or false ring formation. Low sample replication should be taken into account when interpreting the results of this study, but our findings can be used to refine the carbon allocation component of process-based terrestrial ecosystem models and can thus contribute to a more detailed estimation of the role of the miombo woodland in the terrestrial carbon cycle. Furthermore, we provide a physiological foundation for the use of Brachystegia spiciformis tree-ring records in paleoclimate research.
Highlights
The periodicity of tree growth is controlled by an interaction of genetic and environmental factors [1] and is expressed on intraannual as well as inter-annual and longer time-scales
The development of secondary tree growth is regulated by photoperiod [5] and temperature [6,7,8], with threshold temperatures controlling the length of the cambial growth season [7,9] and seasonal mean temperatures determining the onset of wood formation [10] and shoot growth [11] in spring
seasonally dry tropical forests (SDTFs) consist of a mosaic of four main leaf phenological functional types which are adapted to drought and respond to seasonal variations in tree water status (TWS) in different ways [18,20]
Summary
The periodicity of tree growth is controlled by an interaction of genetic and environmental factors [1] and is expressed on intraannual (seasonal) as well as inter-annual and longer time-scales. Temperate phenological cycles, including the timing and the length of the growth season, are closely related to environmental factors (temperature, photoperiod) and respond to inter-annual temperature variations [2], as well as to 20th Century warming [3,4]. In seasonally dry tropical forests (SDTFs), tree water status (TWS), a function of both endogenous and seasonal environmental factors, is the primary driver of leaf phenology and cambial activity [18,19,20]. The seasonality of cambial activity in SDTFs and its relation to climatic seasonality has primarily been studied using band and electric point dendrometer measurements [18,25,26,27,28]. A comparative study of wood formation monitoring methodologies [31] has revealed that pinning and microcoring techniques are more reliable techniques than dendrometer measurements
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