Growth performance as a function of wood hydraulic architecture

Trees in moist tropical regions are characterized by a high growth and biomass accumulation rate, as in a climate with high precipitation and stable temperatures plant growth is supported throughout the year. To maintain a permanent water supply from roots to leaves, efficient water transportation is essential. For this highly productive systems not only nutrient, but also water supply may thus become a challenge.

Adaptation mechanisms in wood anatomy features as well as hydraulic strategies may differ compared to trees from temperate and subtropical regions. However, strategies of

tropical trees in hydraulic properties and their adaptations to the non-seasonal wet climate remain mostly unstudied (Zach et al., 2010).

In the study on hydraulic architecture of the root, stem and branch wood in Theobroma cacao and five common shade tree species in agroforestry systems on Sulawesi, we found wood anatomical and derived hydraulic properties to be a good predictor for tree stem growth performance (chapter 5). Theoretical sapwood area-specific hydraulic conductivity (KStheo

) of all tree organs was strongly positively correlated with stem basal area increment (BAI) on a tree and species level as hypothesized. In contrast, neither wood density (WD), nor empirically measured branch and root hydraulic conductivity, foliar ¹³C or foliar nitrogen content were good predictors for aboveground growth performance. Less negative ¹³C values are expected to be linked to reductions in stomatal aperture, a decreased photosynthetic carbon gain and consequentially slower growth (Ryan & Yoder, 1997), while high foliar nitrogen contents should theoretically be connected with high aboveground productivity (Smith et al., 2002). Hypothetically, KStheo

should correlate negatively with WD (Meinzer et al., 2008) when a relatively large fraction of vessels shows values close to the samples mean vessel diameter. As a relatively easy to measure functional wood property, WD has been linked to various ecological and functional traits. While light wood is associated with fast-growing at low carbon and nutrient cost, high wood density is characteristic for slow-growing trees assuring high biomechanical strength and hydraulic safety e.g. (Hacke et al., 2001). Our results contradict former results on a close relation between WD and growth for tropical trees (King et al., 2006, Poorter et al., 2009, Hietz et al., 2013). Variation of frequency and size of fibers may account for the decoupling of WD from hydraulic conductivity (Zanne et al., 2010). However, our sample size and species number was small, so the relationship might change with increasing sample number. Therefore, we tested the hypothesis also on stem wood of 92 sampled trees in natural forest, jungle rubber and rubber monocultures on Sumatra. A similar relationship was found between BAI and KStheo

(Fig.6.3), while WD did not correlate with growth performance.

149 Figure 6. 3 :Relationship between stem basal area increment (BAI) and theoretically calculated hydraulic conductivity (KS

theo) in the stem wood of 92 trees in natural forest, jungle rubber and rubber monocultures. Where several samples per genus were available, mean values per genus were used (n = 42).

Furthermore, our results are in contrast to the common assumption of continuous vessel tapering (Baas, 1982, Tyree & Zimmermann, 2002) as we found the largest vessels along the flow path in the stem xylem and not in the roots. Vessels with larger diameters are likely to embolize fastest, following the commonly observed trade-off between conduit size and vulnerability to cavitation (Wheeler et al., 2005, Cai et al., 2010, Domec et al., 2010) Therefore, at the root level a hydraulic segmentation with roots embolizing fastest might protect the below-ground system and prevent a reverse water flow from main to lateral roots and back to the dryer soil. In contrast, a deviating pattern between species from seasonally dry and more humid regions was observed with larger vessels in stem wood e.g. (Machado et al., 2007). In regions with high precipitation and continuous soil water supply, adaptations to water shortage seem to be less economic. Under conditions of prevailing high atmospheric humidity, it seems more advantageous for trees to develop a high plant hydraulic conductance in the trunk, rather than to minimize the drought-induced risk of xylem embolism as suggested by Schuldt et al. (2013).

In this context, the study in cacao agroforests revealed divergent patterns of hydraulic conductivity, vessel density and relative vessel lumen area between root, stem, and branch wood of drought-adapted species compared to subhumid forest species. We have observed that specific conductivity in the three seasonal species, i.e.

G. sepium, E. subumbrans and L. leucocephala, was higher in roots than in stems, even though the largest vessels were observed in the stem wood. On the other hand,

hydraulically weighted vessel diameter (d_h) was not significantly higher in stem than in root wood for the three seasonal tree species i.e. T. cacao, G. gnemon and D. zibethinus, and vessel density (VD) was comparable between root and branch wood. Apparently due to varying adaptation strategies to periodical water limitation, a separation between the three strictly wet tropical species and the three seasonal tree species was enabled.

However, many factors such as tree age, small-scale site specific conditions, and management history may co-influence the relationships among wood anatomy and plant functional traits making them more complex and non-linear. Nonetheless, we expect patterns in vessel traits along the flow path from roots to branches to be dependent on the long-term precipitation regime at the biogeographical origin of the investigated tree species. Therefore, the study results suggest that even though vessel traits, growth performance and wood density relations follow distinct conceptually determined trade-offs, some of these long-established paradigms might not be uniformly applicable to all tree species. Future research on conceptual trade-offs of tree hydraulic architecture should thus include a systematic approach to biogeographic regions and cover principally underrepresented biomes.