Tropical Throughfall Displacement Experiments

The consequences of strong drought events on different ecosystems are hard to predict, but the particular ways in which these ecosystem respond to decreased water availability or

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increased occurrence of drought are considered a key issue in climate change scenarios (Wigley et al. 1984). Evaluating the reaction of an ecosystem based on a naturally occurring severe drought event would imply that the measurements had been carried out before, during and after its occurrence, which mostly happens accidentally due to long-term measurements.

Therefore, large-scale manipulative field experiments were found to provide a powerful tool in the identification of gradual and threshold ecosystem responses that might result from future precipitation changes (Hanson and O’Hara 2003). These field experiments should be of sufficient size and complexity to handle questions of individual plant response, interplant interactions, as well as stand-level carbon, water, and nutrient cycling responses (Hanson 2000). Meanwhile, several large-scale throughfall displacement experiments have been carried out over the last decades, mainly in temperate ecosystems, e.g. the ‘Walker Branch Throughfall Displacement Experiment’ in Tennessee, USA (Hanson and Wullschleger 2003), the Solling roof experiment in Germany (Bredemeier et al. 1998, Borken et al. 2003), a throughfall exclusion experiment in a Mediterranean Quercus ilex forest (Limousin et al.

2008, 2009) and several more. However, so far only two tropical large-scale in situ throughfall displacements experiments have been carried out, the ‘Tapajόs Throughfall Exclusion Experiment’ (TTEE, Santarém, Brazil, Nepstad et al. 2002) and the ‘Caxiuanã Throughfall Exclusion Experiment’ (CTEE, Pará, Brazil, Fisher et al. 2007, Costa et al.

2010). Both were located in the Eastern Amazon in a seasonal dry forest with a strong dry season of several months each year. The implications of these studies are that these investigated ecosystems have adapted appropriate mechanisms of drought tolerance or avoidance. Accordingly, these two experiments had a maximum rooting depth of coarse roots of 10 m and more (TTEE) or 5 m (CTEE) in support of the observation of Schenk and Jackson (2002), that rooting depth should increase with increasing dry season length.

Therefore, these forests were at first remarkably resistant. As expected, photosynthesis slowed down to conserve water, and the roots drew water from soil layers up to 13 m down. Trees in the experimental plots slowed their growth, and many of the smaller trees stopped growing entirely (Stockstad 2005).

Even though these two throughfall displacement experiments were geographically proximal, they showed different reactions within the first two years of the treatment. Asner et al. (2004) found no change in predawn leaf water potentials over the course of the first two years for the TTEE, suggesting that the treatment did not provoke substantial drought stress in the canopy, even though 1.2 years later the mortality rates had increased enormous. On the other hand, removing 50 % of the rainfall in the CTEE caused a decrease in total sap flow of 41 % with

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the most severe drought periods causing an 80 % reduction in sap flow compared with the control (Fisher et al. 2007).

One explanation for the different reactions between these two sites might be the differences in their soil properties. The TTEE was located on a clayey soil which is known to be at least 90 m deep. The vertical extent of the root system and the water holding capacity of the soil may therefore have contributed to the increased drought resistance of the TTEE forest compared to the CTEE stand. The CTEE on the other hand, is located on a sandy loam and has a stony laterite layer, which may prevent the development of substantial deep root systems, although roots were found below this layer.

Within these two studies, the nearly identical forest stands, which mainly differ in their soil type, reacted differently over the course of the first two years of experimental desiccation.

However, after approximately four years of throughfall displacement during the rain season, both forest stands showed remarkably consistent reaction in relation to tree mortality, wood production and above-ground biomass. The TTEE treatment resulted in 38 % increased mortality rates across all stems > 2 cm DBH. Mortality rates increased 4.5-fold among large trees and twofold among medium-sized trees in response to the treatment, whereas the smallest stems were less responsive. Overall, potential overstory tree species were more vulnerable than midcanopy and understory species. Additionally, lianas proved to be more susceptible to drought-induced mortality than trees or palms (Nepstad et al. 2007). Lianas are known to possess very large vessels and a long flow path (Ewers et al. 1997), and according to the relation between increasing cavitation risk with increasing conduit diameter, and an increase in hydraulic resistance with path length, this might be an explanation for this observation. After seven years of 50 % throughfall removal, the CTEE resulted in approximately a twice as high tree mortality for the experimental plot compared with the control plot, whereas differences in stem mortality between plots were, likewise with the TTEE, greatest in large trees (3-fold). In addition, wood production in the experimental plot was 30 % lower than in the control plot.

Summarizing the results from both experiments, tropical seasonal dry forest stands proved to be remarkably resistant within the first two years until a certain threshold in soil water content was reached after more than three years. However, to be able to adjust climate change scenarios on a regional and global scale, the two experiments proved that further data on tropical ecosystem responses to reduced rainfall regimes are urgently needed, especially for tropical ecosystems that are not assumed to possess drought-avoiding adaptations.

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