Nā Pōhaku o Hauwahine


Seed Bank


What is the seed bank?

In the present context, "seed bank" is not a place where we protect our extra seeds (although this is another definition); it is where nature stores dispersed seeds. The seed bank is essentially the soil where seeds shed from parent plants eventually end up, or should end up if the plants have any say in the matter. In fact, plants do have lots to say, most species producing large numbers of seeds so that at least some—hopefully many viable ones—will end up in the soil seed bank. Here the seeds wait until a set of circumstances (unique in subtle ways for each species) initiates the process of germination. The mechanisms built into seeds that allow some to remain dormant for years, even decades, are fascinating in themselves, but a topic divergent from our immediate interest in the ways of the seed bank.<1>

The seed bank is an important component of the plant community. Through the seed bank, the composition of the assemblage is maintained season after season. The plants we observe at any point in time are but the mortal and therefore temporary representatives of each species population, growing for the primary purpose of gathering the necessary energy and nutrients to produce seeds. Not that seeds are immortal; but they are designed to withstand a wider range of adverse conditions than the plant body. In the case of annuals, such as pōpolo, the seed will be the only part that survives through the dry season. And yet, year after year, we see pōpolo as a common constituent of our more open areas during approximately half of the year.

For most plants, their copious production of seeds ends up in the soil not far away. On the face of it, this is an excellent choice, since the soil and other conditions there have a proven record of suitability in the growth and survival of the parent. Of course, many plant species are specialized at widely dispersing their seeds and use a variety of mechanisms to accomplish this; another fascinating aspect of seeds outside our immediate interest.

Milo seedlings (Thespesia populnea) germinating in the ripened fruit

Why are we concerned about the seed bank?

Each place has its specific seed bank, which naturally changes over time. The origin of the seeds is largely the plants growing in the immediate area, although some have traveled greater distances to get there. While the age of the seeds is variable, most are likely to be from something less than a year old to perhaps two or three years old. Seeds lose their viability over time, then becoming contributions to the compost. Some seeds can germinate after a longer span of time in or on the soil, an important adaptation to an environment having an uncertain wet season (a charactertistic of many mesic and most xeric locations in Hawai‘i).

Seedlings that germinate close together enter a competition between "siblings" as well as with seedlings of other species in the seed bank (a process at the heart of natural selection). For example, milo grows to be a moderately large, spreading tree at Nā Pōhaku o Hauwahine and produces large numbers of round fruit throughout the year. These disperse by rolling away on slopes but mostly end up under the parent. Early in the wet season, hundreds of seedlings appear under the parent tree. Because of the deep shade there, these seedlings fail to thrive and gradually disappear during the following dry season. It is also the case that few plant species are able to survive in this shaded place, a fact we can use to our advantage.

The battle we wage to establish native plants and eliminate non-natives at Nā Pōhako o Hauwahine is essentially a battle for control of the seed bank. Removing alien plants through weeding is only a temporary "victory" as long as seeds of non-natives remain in the soil. With the next rainfall or certainly by the next wet season, the battle is rejoined—a fact often discouraging to hopes for success<2>. Consequently, we cannot measure progress by our clearing out weeds, but must observe and understand the patterns of seed distribution, seed longevity, and seed germination as these relate to the soil seed bank.

Measauring Success Reflected in the Seed Bank

In recent years we have seen a distinct shift within those areas that have been maintained "weed free" for a number of years. New germinations are mostly of seeds from the native plants present in these areas. Weed seedlings have all but disappeared. These signs of success reflect a clear shift in the proportion of weed versus native plant seeds in the localized seed bank. Important, as well, may be the fact that because hand weeding is no longer required in areas with no or few weeds, disturbances to the soil destructive of native seedlings are minimized.




Note <1> – A Wikipedia article provides additional information on the soil seed bank.

Note <2a> – While participating in a number of community planting efforts around O‘ahu, I was struck by the impermanence of these altruistic efforts. Yes, volunteers learned something about native plants; and yes, we got out into nature—two clearly beneficial outcomes. But what of the stated or implied purpose of establishing native plants in a particular place? This benefit was not achieved if one returned a year or two later to view the changes. Instead, the same previously-cleared, non-native plants dominated, with perhaps a few native shrubs or small trees remaining from the clearing and planting effort. The work of the Nā Pōhaku Ecology Group (NPEG) at Nā Pōhaku represents an experiment to test the question: can planting natives succeed in a place where the disturbance of a previously native ecosystem was near total?

Note <2b> –If planned properly, these projects incude application of pre-emergence herbicides between weeding and planting stages. This step alters the seed bank in favor of the plantings, but is not practical for one-day community efforts.

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