Czech University of Life Sciences Prague, Department of Forest Ecology, Czech Republic, Malaysia
Tropical forests are threatened by increasing incidence of drought and altered environmental conditions due to anthropogenic disturbances. Understanding the tolerance/susceptibility to drought in trees is fundamental for predicting the forest stability in both human-modified and old-growth tropical forests under climate change. Functional water-conducting systems are essential for tree growth and survival. Vessels in the 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. While large vessels transport water more efficiently than smaller ones, they are also prone to hydraulic failure. Trees can adjust the efficiency/safety of their hydraulic systems by modifying vessel dimensions or the fraction of xylem occupied by vessels, and hence, altering their water-use strategies. We hypothesized that the anatomical traits of the logged forest pioneer species would reflect the need to capture and transport resources to support the fast growth rate while the old-growth forest species would display more conservative anatomical traits.
Samples were collected from common pioneer and old-growth tree species in eight 1ha plots within the GEM Network in Malaysian Borneo. Wood anatomical sections were obtained from tree branches and cross-section images were captured and analyzed using imaging software.
The study unveiled 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 the 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 which supported their efficient water transport but made them also prone to embolism, while they had lower vessel density which may lead to the 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.