Lessons learned from a two-year throughfall displacement experiment in

The Sulawesi Throughfall Displacement Experiment was the first experimental study about the effects of an extended soil desiccation event on the trees of a perhumid tropical rainforest where natural droughts occur only exceptionally. The very shallow depth distribution patterns of fine and coarse roots reported in this forest (Hertel et al. 2009) are interpreted as resulting from the continuously high rainfall and permanently low atmospheric saturation deficit. After 24 months of throughfall displacement, soil moisture content in the upper soil layers was reduced beyond the conventional wilting point, whereas relative air humidity stayed unaltered throughout the experiment. According to the biweekly conducted volumetric water content

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(VWC) measurements, we can exclude a strong edge effect by surface water entering the experimental plots from outside (Fig. 7.8). Because the soil moisture maps do not reveal major horizontal gradients, we assume that the trenching to a depth of 40 cm was sufficient for isolating the plots.

To test the effects of this two-year-continuous desiccation experiment on tall tropical canopy trees, one of the most abundant upper canopy tree species in this tropical perhumid forest was selected. Castanopsis acuminatissima (Blume) Rheder is a member of the family Fagaceae and has been reported to produce the highest above ground biomass in this forest stand (Culmsee et al. 2010). We assumed that cavitations caused by soil moisture deficits are a serious threat for large trees of this species (Nepstad et al. 2007).

Fig. 7.8: Spatial distribution of volumetric soil water content (VWC) measured within the top 30 cm of the soil layer for the three experimental roof (red) and control plots (blue). The data presented here were averaged over 100 measurements per plot from January, 2009 until roof opening in May, 2009 taken biweekly. For further explanation see chapter 2.3.1.

The long and severe desiccation of the upper soil caused a marked reduction in the xylem hydraulic conductivity of terminal twigs normalised to vessel lumen area (by 25 %). Leaf area-specific conductivity was reduced by 10-33 % as well. Surprisingly, at the end of the

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drought treatment, individual leaf area was significantly larger in the roof plots, but the number of leaves was reduced by about 30 % per unit of twig sapwood area. This result points to a drought effect on leaf bud formation. In parallel, the stand fine litter production (see chapter 2.3.5) was reduced by 30 % during the driest phase from March until June 2009. A similar finding was reported from a tropical throughfall exclusion experiment in the Amazon (Nepstad et al. 2002). Reduced soil moisture content for a prolonged period appears to inhibit the formation of new leaves rather than causing a pulse of leaf shedding.

Further drought effects after the two-year experimental desiccation were observed in the stem xylem with significant reductions in mean vessel diameter and axial conductivity and a parallel increase in wood density in the outermost xylem of the trunk of tall C.

acuminatissima individuals located under the experimental roofs. Similarly, annual stem diameter increment decreased by 26 %, though not significant.

Even though we found no signs of major damage (e.g. strong leaf shedding, increased mortality rates etc.), there was evidence that tall C. acuminatissima individuals were to a higher degree more susceptible to drought than smaller ones. During drought, one of the first reactions expected is a decrease in sap flow either due to stomatal regulation responding to elevated evaporative demand, or due to decreasing soil moisture content. Four tree individuals of this species with different size were continuously monitored for xylem sap flux density (XFD, Fig. 7.9) from January, 2008 until September, 2010.

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03/2008 06/2008 09/2008 12/2008 03/2009 06/2009 09/2009

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Fig. 7.9: Xylem sap flux density (XFD) for a tall and a small individual of the species C. acuminatissima from the control (blue) and the experimental roof (orange) plots, respectively. Tree height is given as well. The data presented were recorded from January, 2008 until September, 2010. For further explanation see chapter 2.3.3.

Notice different scaling for XFD for the two panels.

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After December, 2008, during the driest phase of the throughfall exclusion experiment (see Fig. 6.1), the two tall tree individuals differed significantly (by 40 %) in XFD between control and experimental roof plots until the end of the experiment. The two small tree individuals had more similar flux densities (only 20 % difference between treatments for December, 2008 and January, 2009). The results on xylem flux density are in agreement with other findings on drought effects on the tall C. acuminatissima individuals. Even though mean vessel diameter in the outermost xylem of the trunk was significantly reduced by only 3 % (788-1033 measured vessels per treatment), the resulting reduction in hydraulic conductivity was 12 % for the individual vessel according to the Hagen-Poiseuille equation. Such an adaptive response of the hydraulic system to reduced soil water contents has also been described for temperate dicotyledonous trees (Sass and Eckstein 1995) and conifers (Eilmann et al. 2006).

According to García-Gonzáles and Eckstein (2003), the effect of water availability at the time of cell differentiation is reflected in the vessel size.

All throughfall displacement experiments have the disadvantage that they can reduce soil moisture to a critical level, but they leave the air humidity at canopy height unchanged. It is likely that soil desiccation in a natural dry spell will have much stronger effects than was simulated in our experiment. Moreover, two similar throughfall exclusion experiments in Eastern Amazonia (Nepstad et al. 2007, da Costa et al. 2010) revealed that tropical seasonal dry forest stands were remarkably resistant against drought within the first two years, until a certain threshold in soil water content was reached approximately after more than three years.

Even though our experiment was carried out in a perhumid climate and the occurring tree species should not possess adaptations to severe drought events, two years of throughfall displacement might still have been insufficient to provoke symptoms of critical damage.

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