| dc.description.abstracteng | Warming climate associated with increasing drought spells are threatening plants from both natural and transformed systems of Indonesia. On one side, Indonesian natural forests are home of an incredible plant diversity but their ability to cope with drought events is still poorly known. The fate of the large trees growing in these forests is particularly concerning since they are re- ported particularly vulnerable to drought. On the other side, important land-use conversions have led to the cultivation of highly productive species such as rubber (Hevea brasiliensis) and oil palm (Elaeis guineensis). These usually mono-specific plantations have rapidly expanded in the extensive lowlands of Indonesia, reaching areas that may not be suitable for their growth. Thus, understanding properties involved to mitigate the negative impact of water stress is of primary importance in the context of a changing climate.
In the present thesis we investigated mechanisms associated with plant mortality during heat- induced droughts in lowland rainforests, and in oil palm and rubber plantations from the region of Jambi in the island of Sumatra, Indonesia. To achieve this, we studied interrelationships across plant height, growth, and structural and functional wood and leaf properties related to hydraulic safety and efficiency. We especially focused on the anatomy of the xylem and its resistance to drought-induced embolism, investigating changes associated with variations of plant structure in the forest and fluctuations of the local environment in the plantations.
The 7 to 10 investigated rainforest species presented a wide range of vulnerability to drought induced embolism as they had a water potential at 50% loss of hydraulic conductivity (P50) stretching from –1.71 to –3.11 MPa. Their leaf turgor loss point (ptlp) was particularly high, ranging from –1.37 to –2.09 MPa, indicating leaves with limited functions during mild droughts. Further investigations showed that tall and light-wooded trees were more vulnerable to drought- induced embolism than short and dense-wooded trees. This was associated with vascular ad- justments since vessel diameter (D) and lumen-to-sapwood area ratio (F) were higher both in the stem and in distant twigs of tall trees compared to trees with smaller height at maturity. Likewise, trees with a lower wood density presented larger D in the stem and in the branches compared to denser-wooded trees. Beside embolism resistance, ptlp tended to be positively re- lated to height but not to wood density. Despite a missing relationship between P50 and annual aboveground biomass increment (ABI), fast-growing trees were more efficient, as indicated by their low sapwood-to-leaf-area ratio (Huber value), and had a high ptlp, indicating a potential trade-off between growth performance and safety. Our findings contribute to show that tall and
light-wooded trees may be more vulnerable to drought probably because of their large vessels and lumen area. We conclude that D and F associated with tree height are promising as easily accessible traits giving insights on plant vulnerability to drought.
In mono-specific plantations, we focused on trait plasticity in response to the environment. We found that oil palm had fronds rather vulnerable to embolism with a P50 of –1.86 MPa. How- ever, diurnal courses of stomatal conductance evidence a sensitive water loss regulation which may provide a higher safety during mild drought events. Moreover, palms growing in plots with higher soil water availability (i.e. riparian sites) had on average by 0.4 MPa higher P50 values than genetically similar palms growing in sites with a well-drained soil. Like in the forest, P50 of oil palm fronds was highly dependent of palm height. However, P50 was not associated with vascu- lar changes although D decreased with height. The high plasticity of frond embolism resistance may be part of oil palm strategy to cope with heat-induced droughts.
In contrast, rubber trees were more resistant to drought than oil palms with respect of their ptlp (mean = –2.12 MPa) and their P50. Indeed, across eight plantation plots, we found that P50 values of genetically similar rubber trees ranged from –2.14 to –2.58 MPa, with no dependence on soil moisture unlike for oil palm. Instead, we found that embolism resistance and hydraulic safety margin (ptlp – P50) increased with increasing vapour pressure deficit. This was also associated with vascular changes but in this species, vessel density, and not diameter, was closely related to P50. These results demonstrate a high degree of plasticity of traits related to hydraulic safety but only in response to changes in evaporative demand and not soil moisture.
Our results highlight the diversity of mechanisms involved by plants to mitigate water stress. The association of structural, functional and hydraulic traits allowed us to draw general species hydraulic strategies, helping to understand how increasing drought events may affect them and the systems they belong. Notably, forest dynamic would probably change if large trees were to die more from hydraulic failure, shifting the community towards short-statured stands which may harm forest function as a carbon sink. In addition, oil palm plantation sites with tall palms at the end of their life cycle may be particularly vulnerable, especially if grown on riparian sites. If these sites were to get dry, oil palms may be unable to cope even with their plastic fronds. Finally, it emerged that rubber trees are more resistant than previously described and could probably be grown in a drier habitat, although this may be at the cost of their productivity. Overall, our results give knowledge on the resistance to drought of the understudied lowland Indonesian systems and should be taken into account in future studies. | de |