Allen James A, Krauss Ken W, Hauff Robert D
USDA Forest Service, Pacific Southwest Research Station, Institute of Pacific Islands Forestry, 1151 Punchbowl St, Rm 323, Honolulu, HI 96813, USA.
Oecologia. 2003 Mar;135(1):110-21. doi: 10.1007/s00442-002-1167-2. Epub 2003 Feb 1.
The tree species Xylocarpus granatum is commonly described as occurring in the upper intertidal zone of mangrove forests, but mature trees are occasionally found at lower elevations. In the Utwe River basin, on the Pacific island of Kosrae, we investigated the relative importance of several biotic and abiotic factors that may control the intertidal distribution of X. granatum. Factors we evaluated included differential seed predation across the lower, mid, and upper intertidal zones and seedling responses to salinity, tidal flooding, and shade. Seed predation was 22.4% over the first 34 days and varied little among zones or in gaps versus under the forest canopy. By day 161, there were still no differences in seed mortality, but a significant difference was found in seedling establishment, with much greater establishment in the upper intertidal plots. X. granatum seedlings in a greenhouse experiment exhibited greater growth in freshwater than seedlings in 23 ppt salinity, which is typical of salinity levels found in the mid intertidal zone in our field study sites in Micronesia, where mature X. granatum trees are generally absent. Seedlings grown in 23 ppt salinity, however, exhibited few visible signs of stress associated with patterns in growth. Seedlings grown in a simulated tidal flooding treatment (with 23 ppt salinity) also showed few signs of stress. Growth declined dramatically under 80% shade cloths, but there were few interactions of shading with either 23 ppt salinity or simulated tidal flooding. Differential seed predation is not likely to be the primary factor responsible for the intertidal distribution of X. granatum on Kosrae. However, seedling tolerance of flooding or salinity may be more important, especially relative to a potential contribution to secondary stress mortality. Other factors may ultimately prove to be more critical, such as physiological effects of salinity on seed germination, effects of tides on seed dispersal and rooting, or differential herbivory on seedlings.
树种木果楝通常被描述为生长在红树林的高潮间带，但偶尔也能在较低海拔处发现成熟树木。在太平洋科斯雷岛上的乌特韦河流域，我们调查了几种可能控制木果楝潮间带分布的生物和非生物因素的相对重要性。我们评估的因素包括低潮间带、中潮间带和高潮间带不同的种子捕食情况以及幼苗对盐度、潮汐淹没和遮荫的反应。在前34天内，种子捕食率为22.4%，不同区域之间或林中空地与林冠下相比差异不大。到第161天，种子死亡率仍无差异，但在幼苗定植方面发现了显著差异，高潮间带地块的定植情况要好得多。在温室实验中，木果楝幼苗在淡水中的生长比在盐度为23 ppt（这是我们在密克罗尼西亚实地研究地点中潮间带典型的盐度水平，那里通常没有成熟的木果楝树）中的幼苗更好。然而，在23 ppt盐度下生长的幼苗几乎没有与生长模式相关的明显胁迫迹象。在模拟潮汐淹没处理（盐度为23 ppt）中生长的幼苗也几乎没有胁迫迹象。在80%的遮阳网下生长时，生长显著下降，但遮荫与23 ppt盐度或模拟潮汐淹没之间几乎没有相互作用。不同的种子捕食不太可能是科斯雷岛上木果楝潮间带分布的主要因素。然而，幼苗对洪水或盐度的耐受性可能更重要，特别是相对于对次生胁迫死亡率的潜在影响而言。最终可能证明其他因素更为关键，例如盐度对种子萌发的生理影响、潮汐对种子传播和生根的影响，或对幼苗的不同食草作用。
