LB 9-88 - Xylem vessel traits of common old-growth and pioneer species from Malaysian Borneo

Background/Question/MethodsUnderstanding the tolerance and susceptibility to drought in tropical trees is fundamental for predicting the forest stability in both human-modified and old-growth tropical forests under global climate change. Functional water conducting systems are essential for both growth and survival of plants especially when they are challenged by drought. Vessels in the tree stem play a key role in ensuring sufficient supply of water from roots to leaves. Variation in xylem vessel anatomy is of major functional significance in trees. While large vessels transport water more efficiently than smaller ones, they are also prone to hydraulic failure. Trees can adjust the efficiency and safety of their hydraulic systems by modifying vessel dimensions or the fraction of xylem occupied by vessel lumens, and hence, altering their water-use strategies. We hypothesized that the anatomical traits of the pioneer species would reflect the need to capture and transport resources to support its fast growth rate while the species of the old-growth forest would display more conservative anatomical traits.Samples collected from eight 1ha plots comprising of old-growth forests and selectively logged forests which are part of the Global Ecosystem Monitoring Network in Malaysian Borneo. Wood anatomical sections were obtained from tree branches and cross-section images were captured and analyzed using imaging software. Results/ConclusionsOur results revealed significant differences in wood anatomical traits between the common species of old-growth and selectively logged Bornean forests. The late-successional species exhibited more conservative traits, for instance, smaller vessel size but higher vessel density than the resource-acquisitive pioneer species from selectively logged forest. Small vessels are associated with high resistance to xylem cavitation and are an adaptation to prevent reduction in hydraulic conductivity during drought. Dense vessels lower the risk of detrimental damage during stressful environmental conditions by enabling alternative pathways for water conductivity when cavitation occurs. In contrast, pioneer species possessed larger water-conducting tissue and lower density which related to efficient water transport system but prone to embolism and lack of alternative pathways if many water-conducting tissues are embolised during cavitation. Therefore, pioneer-dominated logged forests may be much more susceptible to low soil moisture than old-growth forests, because pioneers lack the safety of their hydraulic systems. While both old-growth and selectively logged Bornean forests experience supra-annual extreme droughts under El Niño Southern Oscillation (ENSO), these impacts of drought may be further worsened in selectively logged forests because they suffer additional microclimate extremes resulting from their open canopies.