Sobrado M A
Departmento de Biología de Organismos, Universidad Simón Bolívar, Aptodo. 89.000, Caracas, Venezuela.
Oecologia. 1993 Oct;96(1):19-23. doi: 10.1007/BF00318025.
Drought-deciduous and evergreen species coexist in tropical dry forests. Drought-deciduous species must cope with greater seasonal leaf water-potential fluctuations than evergreen species and this may increase their susceptibility to drought-induced xylem embolism. The relationship between water transport efficiency and leaf life-span were determined for both groups. They differed in seasonal changes of both, wood water content (W ) and wood specific gravity (G). During the dry season, the W in drought-deciduous species declined and the minimum value was recorded when leaf fall was complete. At this time, the volumetric fraction of gas (V ) increased indicating air entry into xylem vessels. In contrast, W , G and V changed only slightly throughout the year for evergreen species. Maximum hydraulic conductivity of drought-deciduous species was 2-6 times that of the evergreen species. but was severely reduced at leaf fall. In the evergreen species, similar water conductivities were measured during wet and dry seasons. The trade-off between xylem water transport capacity and leaf lifespan found in species coexisting in this forest reveals the existence of contrasting but successful adaptations to this environment. Drought-deciduous species maximize production in the short term with higher water transport efficiency which leads to the seasonal occurrence of embolisms. Conversely, the behaviour of evergreen species with reduced maximum efficiency is conservative but safe in relation to xylem embolism.
旱季落叶树种和常绿树种共存于热带干旱森林中。旱季落叶树种必须应对比常绿树种更大的季节性叶片水势波动，这可能会增加它们对干旱诱导的木质部栓塞的易感性。研究了这两组树种水分运输效率与叶片寿命之间的关系。它们在木材含水量（W）和木材比重（G）的季节性变化方面存在差异。在旱季，旱季落叶树种的W下降，在叶片完全脱落时记录到最小值。此时，气体体积分数（V）增加，表明空气进入木质部导管。相比之下，常绿树种的W、G和V全年变化较小。旱季落叶树种的最大水力传导率是常绿树种的2至6倍，但在落叶时会大幅降低。在常绿树种中，湿季和旱季测得的水分传导率相似。在这片森林中共存的树种中发现的木质部水分运输能力与叶片寿命之间的权衡，揭示了对这种环境存在截然不同但却成功的适应性。旱季落叶树种通过更高的水分运输效率在短期内实现产量最大化，这导致了栓塞的季节性发生。相反，最大效率降低的常绿树种的行为较为保守，但在木质部栓塞方面是安全的。
