Conclusion - E NVIRONMENT AND TREE SIZE CONTROLLING STEM SAP FLOW IN A PERHUMID

5 E NVIRONMENT AND TREE SIZE CONTROLLING STEM SAP FLOW IN A PERHUMID

5.5 Conclusion

This study in a rich tropical forest of Central Sulawesi, Indonesia shows that species-specific measurements of xylem sap flow can provide insights into the role of tree size and environmental factors controlling tree water consumption in this perhumid climate. All seven tree species studied revealed convergent patterns in the regulation of sap flow in that atmospheric saturation deficit was the principal environmental factor and DBH the dominant size attribute when the fluxes were analyzed on a daily basis. This convergence in sap flow regulation is associated with a considerable functional diversity in the tree hydraulic

Chapter 5 Environment and tree size controlling stem sap flow

architectures of the tree species present in this stand. The Fagaceae C. acuminatissima diverges from the other species with respect to the less tight coupling of daily xylem flux to the vapor pressure deficit increase in the course of the day. Moreover, this species, that is abundant in the study area with phylogenetic relationships to extra-tropical genera, showed significantly higher sap flow and daily water uptake rates than the other coexisting species which is partly a consequence of a very high hydraulic conductivity in the root-stem-twig flow path.

ACKNOWLEDGEMENTS

This study took place as part of the Collaborative Research Centre SFB 552 on the stability of rainforest margins in Indonesia (STORMA). We gratefully acknowledge the financial support granted by the German Science Foundation (DFG). We thank Heinz Coners and Hilmar Müller for technical support during the installation, Heike Culmsee for tree species identification, Wolfram Lorenz for organizing the logistics in Palu, and our Indonesian counterparts and field assistants for their support.

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6 SIXTH CHAPTER

CHANGE IN HYDRAULIC PROPERTIES AND LEAF TRAITS OF A TALL RAINFOREST TREE SPECIES SUBJECTED TO LONG-TERM THROUGHFALL EXCLUSION IN THE PERHUMID TROPICS

Chapter 6 Drought response of a tall-growing canopy tree species

Change in hydraulic properties and leaf traits of a tall rainforest tree species subjected to long-term throughfall exclusion in the perhumid tropics

Bernhard SCHULDT¹, Christoph LEUSCHNER¹,Viviana HORNA¹,Gerald MOSER² and Michael KÖHLER³

1 Department of Plant Ecology, Albrecht von Haller Institute for Plant Sciences, University of Göttingen, Germany

2 Department of Plant Ecology, University of Gießen, Germany

3 Tropical Silviculture and Forest Ecology, Burckhardt Institute, University of Göttingen, Germany

Abstract

In a throughfall displacement experiment on Sulawesi, Indonesia, three 0.16 ha stands of a premontane perhumid rainforest were exposed to a two-year soil desiccation period that reduced the soil moisture in the upper soil layers beyond the conventional wilting point.

About 25 variables, including leaf morphological and chemical traits, stem diameter growth and hydraulic properties of the xylem in the trunk and terminal twigs, were investigated in trees of the tall-growing tree species Castanopsis acuminatissima (Fagaceae) by comparing desiccated roof plots with nearby control plots. We tested the hypotheses that this tall and productive species is particularly sensitive to drought, and the exposed upper sun canopy is more affected than the shade canopy. Hydraulic conductivity in the xylem of terminal twigs normalised to vessel lumen area was reduced by 25 %, leaf area-specific conductivity by 10-33 % during the desiccation treatment. Surprisingly, the leaves present at the end of the drought treatment were significantly larger, but not smaller in the roof plots, though reduced in number (about 30 % less leaves per unit of twig sapwood area), which points to a drought effect on the leaf bud formation while the remaining leaves may have profited from a surplus of water. Mean vessel diameter and axial conductivity in the outermost xylem of the trunk were significantly reduced and wood density increased, while annual stem diameter increment decreased by 26 %. In contradiction to our hypotheses, (i) we found no signs of major damage to the C. acuminatissima trees nor to any other tree species in this stand, and (ii) the exposed upper canopy was not more drought susceptible than the shade canopy.

Key words: Tree height, vertical gradient, drought stress, leaf morphology, leaf nutrient content, isotope signature, δ¹³C, δ¹⁵N, hydraulic conductivity, stem increment, wood density, vessel anatomy.

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6.1 INTRODUCTION

The remaining tropical moist forests may be threatened by more frequent and more severe droughts in the future (e.g. Allen et al. 2010, Meir and Woodward 2010) that will come along with the predicted climate change in South-East Asia (Hulme and Viner 1998, Timmermann et al. 1999, Williams et al. 2007, Bates et al. 2008, Newbery and Lingenfelder 2008, Sheffield et al. 2008) and South America (Cox et al. 2008, Phillips et al. 2009, Phillips et al.

2010). Since the ecosystematic impact of strong drought events is hardly predictable, manipulative field experiments have been found to provide a powerful tool for identifying gradual ecosystem responses and threshold values of ecosystem functions that might result from these putative precipitation changes (Hanson and O’Hara 2003). Experiments with reduced rainfall have so far only been conducted in tropical forests with a distinct dry period in Amazonia (Nepstad et al. 2002, 2007, Fisher et al. 2007, da Costa et al. 2010), where the biota most likely possess specific adaptations (e.g. deep-reaching roots) to regular dry spells.

However, we expect that the results of these Amazonian experiments cannot simply be extrapolated to tropical forests with a perhumid climate where the trees should be less experienced in coping with drought because rainless periods occur only irregularly (Aldrian and Susanto 2003, Aldrian et al. 2004, Erasmi et al. 2009). Experiments on the drought response of perhumid tropical forests with continuously high soil moistures and air humidity do not yet exist. To fill this gap, a replicated throughfall displacement experiment (Sulawesi Throughfall Displacement Experiment, STDE) was carried out in a premontane perhumid rainforest in Central Sulawesi, Indonesia, to investigate the response of the trees and soil biological activity to a 24-months drought period. The study region is characterized by high amounts of rainfall throughout the year and air humidity at the canopy height that rarely drops below 80 %.

Both observational studies on natural drought events and the Amazonian throughfall displacement experiments showed that under prolonged drought especially large and tall canopy trees (and species) as well as lianas experienced higher mortalities than trees of smaller size (Slik et al. 2004, Van Nieuwstadt and Sheil 2005, Nepstad et al. 2007, da Costa et al. 2010, Phillips et al. 2010). Drought may harm trees through two pathways, exposure to increase of xylem embolism and reduced assimilate supply due to stomatal closure. Besides cell dehydration and a consequently reduced leaf expansion growth, carbon starvation could be one consequence of severe drought (Farooq et al. 2009), but this hypothesis has been questioned (Sala 2009).

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Here, we formulate the hypothesis that the higher mortality of tall rainforest trees and lianas observed in experimental or natural droughts is primarily caused by the long path length for water flow from root to leaf. The fact that conduits taper with increasing tree height (e.g.

Anfodillo et al. 2006, Petit et al. 2008, 2009) and tall trees are exposed to higher atmospheric saturation deficits than smaller ones, results in particularly wide vessels at the base of the trunk of tall trees (Zach et al. 2010). Tree size and vessel diameter at the stem base should be directly linked to each other, because tall trees are normally more productive than smaller trees, and a high hydraulic conductance in the soil-plant-atmosphere path seems to be an essential prerequisite for a high productivity of forests (Tyree 2003).

However, increasing vessel diameters for improving the water transport carry the burden of a higher vulnerability to cavitation (e.g. Zhu and Cao 2009, Awad et al. 2010, Cai and Tyree 2010, Hacke et al. 2010). In addition, plant communities growing under high precipitation rates typically have more shallow root systems (Schenk and Jackson 2002, Hertel et al. 2003, Jimenez et al. 2009), which may further increase their vulnerability to drought-induced cavitation.

For analyzing the effects of a two-year experimental desiccation period on tall tropical canopy trees, we selected one of the tallest and also most abundant upper canopy tree species in the premontane forest of Sulawesi, Castanopsis acuminatissima (Blume) Rheder. This species is a member of the Fagaceae family and has been found to be the most prominent species in terms of biomass in this forest stand (Culmsee et al. 2010). We assumed that cavitation caused by soil moisture deficits is a serious threat for large trees of this species. Furthermore, we expected the upper crown to be more susceptible to drought stress than the lower crown due to the growing cavitation risk with increasing path length (Ryan and Yoder 1997) and the exposure to higher vapor pressure deficits.

This study is part of the Sulawesi Throughfall Displacement Experiment and focussed on the drought response of one prominent tree species in a species-rich forest, the tall-growing and abundant Fagaceae Castanopsis acuminatissima. Study aims were to monitor a large number of morphological, anatomical, hydraulic and chemical traits at the leaf, branch and stem levels in drought-exposed (roof) and unaffected control trees over a desiccation period of 24 months and to identify traits that respond sensitively to soil water shortage. Among the investigated parameters were several variables related to xylem dysfunction and carbon starvation hypotheses proposed to explain drought damage to trees. We used tree climbing equipment in each of the seven mature C. acuminatissima in the roof and control plots in order to study the

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response of sun-lit upper canopy leaves and branches. Because leaf exposure and canopy position is known to exert a great influence on leaf morphology and physiology in trees, we investigated leaves and branches of both the sun and shade canopy and compared their response to the two-year desiccation. We further hypothesized that sun canopy leaves and branches are more susceptible to desiccation than are shade canopy organs.

6.2 MATERIAL AND METHODS

6.2.1 Site description

The Sulawesi Throughfall Displacement Experiment (STDE) was established in 2006 in a premontane rainforest in the Pono Valley on the western boundary of Lore Lindu National Park in Central Sulawesi, Indonesia (S 01°29.6’ E 120°03.4’, elevation 1050 m). The climate of the study area is perhumid with a mean annual precipitation of 2901 mm, a mean annual temperature of 20.6 °C and a mean relative air humidity of 88.7 % (data derived from measurements in 2008). The heavily weathered soils of this old-growth forest developed on metamorphic rocks. The clayey-loamy soil texture with dominant kaolinite and hematite has been classified as Acrisol (World Reference Base for Soil Resources, Leitner 2010). The forest has a canopy height of about 45 m with a few trees reaching 55 m, a high tree species diversity with about 130 species ha^-1 and an average stem density of 456 ha^-1 (>10 cm DBH, Culmsee and Pitopang 2009, Culmsee et al. 2010).

6.2.2 Experimental design

The STDE consisted of six floristically and structurally similar plots of 0.16 ha (40 m x 40 m) that were spread in a stratified random design over an area of approximately five ha. While three plots served as control, the remaining three plots were covered by sub-canopy roofs to displace a large fraction of the rainfall. The roofs were constructed with a large number of removable transparent plastic-lined bamboo-frames placed on a wooden gutter construction to collect the throughfall water. The desiccation period started in May 2007. At the beginning, approximately 70 % of the plot area was covered by the bamboo frames. In early 2008, the roof closure was further increased to approximately 90 % by building custom-sized panels to close gaps around the tree stems and odd-sized openings. To avoid lateral soil water movement or infiltration of surface runoff into the plots and to disable trees to take up water from the surroundings of the study plots, all plots were trenched along the perimeter to 0.4 m soil depth and lined with plastic foil. Since 74.3 % of the fine root and 91.1 % of the coarse

Im Dokument Effects of experimental drought on hydraulic properties and leaf traits of upper canopy and understory tree species in a perhumid tropical forest in Central Sulawesi, Indonesia (Seite 107-0)