U.S. Geological Survey, Pacific Island Ecosystems Research Center, Hawai'i National Park, Hawai'i, 96718, USA.
Department of Ecology, Evolution and Marine Biology, University of California at Santa Barbara, Santa Barbara, California, 93106, USA.
Ecol Appl. 2022 Jan;32(1):e02477. doi: 10.1002/eap.2477. Epub 2021 Nov 8.
Trees can have large effects on soil nutrients in ways that alter succession, particularly in the case of nitrogen-(N)-fixing trees. In Hawai'i, forest restoration relies heavily on use of a native N-fixing tree, Acacia koa (koa), but this species increases soil-available N and likely facilitates competitive dominance of exotic pasture grasses. In contrast, Metrosideros polymorpha ('ōhi'a), the dominant native tree in Hawai'i, is less often planted because it is slow growing; yet it is typically associated with lower soil N and grass biomass, and greater native understory recruitment. We experimentally tested whether it is possible to reverse high soil N under koa by adding 'ōhi'a litter, using additions of koa litter or no litter as controls, over 2.5 yr. We then quantified natural litterfall and decomposition rates of 'ōhi'a and koa litter to place litter additions in perspective. Finally, we quantified whether litter additions altered grass biomass and if this had effects on native outplants. Adding 'ōhi'a litter increased soil carbon, but increased rather than decreased inorganic soil N pools. Contrary to expectations, koa litter decomposed more slowly than 'ōhi'a, although it released more N per unit of litter. We saw no reduction in grass biomass due to 'ōhi'a litter addition, and no change in native outplanted understory survival or growth. We conclude that the high N soil conditions under koa are difficult to reverse. However, we also found that outplanted native woody species were able to decrease exotic grass biomass over time, regardless of the litter environment, making this a better strategy for lowering exotic species impacts.
树木可以通过改变演替的方式对土壤养分产生重大影响，尤其是在固氮树种的情况下。在夏威夷，森林恢复严重依赖于使用本地固氮树种——相思树（koa），但这种树种会增加土壤中可用的氮，并可能促进外来牧草的竞争优势。相比之下，夏威夷本土树种多态杨梅（'ōhi'a）由于生长缓慢而不太常被种植；然而，它通常与较低的土壤氮和草生物质以及更多的本土林下植物有关。我们通过在 2.5 年内添加或不添加 koa 凋落物作为对照，实验测试了是否有可能通过添加 'ōhi'a 凋落物来逆转 koa 下的高土壤氮。然后，我们量化了 'ōhi'a 和 koa 凋落物的自然凋落物和分解率，以便从更广阔的视角看待凋落物添加。最后，我们量化了凋落物添加是否改变了草生物质，以及这是否对本土幼苗产生了影响。添加 'ōhi'a 凋落物增加了土壤碳，但增加了而不是减少了无机土壤氮库。与预期相反，koa 凋落物的分解速度比 'ōhi'a 慢，尽管它每单位凋落物释放的氮更多。我们没有看到 'ōhi'a 凋落物添加导致草生物质减少，也没有看到本土幼苗林下植物的存活率或生长有任何变化。我们得出的结论是，koa 下高氮土壤条件难以逆转。然而，我们也发现，无论凋落物环境如何，种植的本土木本物种都能够随着时间的推移减少外来草种的生物量，这是降低外来物种影响的更好策略。
