{"id":885,"date":"2014-08-28T23:35:58","date_gmt":"2014-08-28T23:35:58","guid":{"rendered":"http:\/\/localhost:8888\/EH\/?p=885"},"modified":"2014-08-28T23:35:58","modified_gmt":"2014-08-28T23:35:58","slug":"climate-change-forces-new-understanding-of-plant-invasions-in-hawaii-ecosystems","status":"publish","type":"post","link":"https:\/\/environment-hawaii.org\/?p=885","title":{"rendered":"Climate Change Forces New Understanding of Plant Invasions in Hawai`i Ecosystems"},"content":{"rendered":"
Carla D\u2019Antonio has been in the weeds \u2013 literally \u2013 for the better part of 25 years. The professor of environmental studies at the University of California, Santa Barbara, has been investigating invasive plants in Hawai`i Volcanoes National Park since 1990, and she has come up with an innovative way at looking at their impact.<\/div>\n
\n\u201cWe see pictures of landscapes where the invader is dominant,\u201d she said at a recent seminar she gave at the Institute of Pacific Islands Forestry in Hilo. \u201cSnapshots that suggest these landscapes will always be like this once the invader takes over. But we don\u2019t really know if the invaders will remain on the landscape without management.\u201d<\/p>\n

The issue is important, she went on to say, \u201csince managers have so little money with which to manage vast landscapes dominated by invasives. They have to prioritize [and understand] which landscapes might undergo succession naturally.\u201d<\/p>\n

D\u2019Antonio said this was brought home to her clearly five years ago, when she spent time in New Zealand. \u201cI was shown sites where gorse had dominated after the forest was cleared,\u201d she said. \u201cManagers were claiming that without further disturbance, the gorse had been replaced by native species.\u201d<\/p>\n

As then-and-now photos of once-gorse-infested New Zealand hills went up on the screen behind her, gasps of disbelief could be heard in the room from those resource managers who have watched helplessly the relentless march across the slopes of Mauna Kea of gorse, one of the most intractable invaders in the islands.<\/p>\n

D\u2019Antonio had their full attention.<\/p>\n

Transformational Grasses<\/i><\/b>
\nThe challenge for resource managers, she said, was to understand the persistence of invaders in a given ecosystem. On the one hand, there\u2019s \u201cclassic succession,\u201d in which the \u201cinvaders are part of a series of changes over time and may also facilitate other invaders,\u201d setting off an \u201cinvasional meltdown,\u201d D\u2019Antonio said.<\/p>\n

On the other, invaders can facilitate \u201calternative stable states,\u201d in which the system is fixed in a persistent alternative state. This approach \u201cdescribes communities that were changed but are now relatively stable,\u201d she said. After prominent ecologists began to use this concept about 10 years ago, she added, \u201cit\u2019s come to dominate our thinking about how native species persist on the landscape.\u201d<\/p>\n

How are resource managers to know which system is the appropriate one for a given landscape?<\/p>\n

\u201cWatch the systems over a long time period,\u201d she said. If possible, \u201cexperimentally tweak systems and see what happens. And maybe you can predict based on an information blitz \u2013 such as the potential species pool, species traits, their interactions, and expert knowledge.\u201d<\/p>\n

She drew on her work at submontane woodlands in the Volcanoes National Park as a case study. \u201cWe were looking at invasive perennial grases in seasonal dry forests that were dominated by `ohi`a,\u201d she said. These areas were invaded by drought-tolerant grasses, especially tufted beardgrass (Schizachyrium condensatum<\/i>). Between 1988 and 1998, this represented 79 percent of groundcover in the understory of the study area, she said.<\/p>\n

\u201cThe grass was a transformer,\u201d she said, causing changes in the ecosystem through promotion of fire, in this case. \u201cThere was a four-fold increase in the frequency of fires, and a large increase in their size as well.\u201d<\/p>\n

D\u2019Antonio and her co-workers set up 20×20-meter-square plots in which the grasses were removed to see what the system would be like without the grasses. They found that \u201cgrasses reduced the growth of native woody species by 40 percent,\u201d she said. \u201cThey are suppressing the understory community, and also suppressed recruitment.\u201d<\/p>\n

However, \u201cthey also reduced the recruitment of two even more aggressive invaders: molasses grass and faya,\u201d\u00a0Melinis minutiflora<\/i>\u00a0andMorella faya,<\/i>\u00a0respectively. \u201cThese are much more potent invaders, in terms of their impact on natives. And they were recruiting heavily in the removal plots.\u201d<\/p>\n

In the early 1990s, the beardgrass seemed to be in a steady-state relationship with the ecosystem, she said. \u201cWe believed that the system had had a little tweak, due to goat grazing, that allowed\u00a0Schizachyrium<\/i>\u00a0to become dominant, and it was able to maintain itself. We predicted that this was an unchanging alternative stable state. And in 1998, it looked identical to the way it had looked in 1988.\u201d<\/p>\n

\u201cIt was a good thing, in a way. The system seemed resistant\u201d to further invasion.<\/p>\n

D\u2019Antonio predicted that in the absence of fire, the beardgrass would resist invasion by molasses grass and faya. There would be a slow accumulation of dead natives, and also an accumulation of standing grass fuel \u2013 \u201ca slow creep towards the precipice of fire.\u201d<\/p>\n

\u201cSooner or later, there would be an ignition, and fire is definitely a precipice\u201d that can convert woodlands and shrublands to a savannah of molasses grass, she said.<\/p>\n

\u2018Over the Precipice\u2019<\/b><\/i><\/b>
\nBut the fire didn\u2019t come. In 2012, D\u2019Antonio found, the beardgrass had declined by more than 75 percent, with 62 percent of all the beardgrass plants dead. Live biomass, she said, had decreased by 80 percent.<\/p>\n

In the meantime, the two invaders that beardgrass had fended off \u2013 molasses grass and faya \u2013 had increased, while `ohi`a had declined by 50 percent and pukiawe, the dominant shrub in the understory, had declined by 25 percent.<\/p>\n

\u201cWhat\u2019s driving the change in the system? There\u2019s been no fire in the entire study period,\u201d D\u2019Antonio said.<\/p>\n

The answer: \u201ca change in precipitation.\u201d<\/p>\n

Annual rainfall has trended down since the late 1980s, she pointed out. \u201cAnd probably what\u2019s more important is the dry-season rainfall, from May to September.\u201d By comparing yearly dry-season rainfall against a 30-year average, D\u2019Antonio found that before 1998, wet summers were more common. \u201cSince then, we\u2019re seeing much drier summers. Plants in the ecosystems are having to undergo much longer drought stress in summer than they experienced before.\u201d<\/p>\n

\u201cWe noticed a couple of summers back that there was just no rain,\u201d she said. \u201cThen we saw that that has been typical for the last 10 to 15 years.\u201d<\/p>\n

Along with the decline in rainfall has been a rise in temperatures. \u201cWe\u2019re talking about a 5 degree Fahrenheit average increase in temperature over the same time period,\u201d she said. \u201cSo not only is it drier, but it is hotter as well. Both spring and summer are warmer, and plants are facing more stress than before.\u201d<\/p>\n

The changes, she noted, are consistent with data generated by Tom Giambelluca, Hawai`i\u2019s climate-change guru. \u201cOver time, at high elevation stations, above 800 meters \u2026 we see increases in temperatures,\u201d she said, adding that her study site was at an elevation of 900 meters. \u201cNo question, these habitats are getting warmer, and also drier.\u201d<\/p>\n

\u201cNative species are also showing signs of stress\u2026 The canopy has decreased in live cover quite considerably,\u201d she said.<\/p>\n

D\u2019Antonio looked for other possible reasons for the decline of the invasive beardgrass, but found no evidence of changes in the soil or damage from insects or other organisms.<\/p>\n

\u201cSo,\u201d she said posing the question for the crowd: \u201clooking at the framework for evaluating the persistence of the invader \u2013 is it succession or an alternative stable state?\u201d<\/p>\n

Unanticipated Changes<\/i><\/b>
\nIn the case of the changing landscape at the park, she said, \u201cit\u2019s succession: from an uninvaded to an invaded initial state. Then drought, climate change, and eventually a changed ecosystem.\u201d<\/p>\n

\u201cWe did not predict the strong decline of\u00a0Schizachyrium<\/i>\u00a0or of the native woody species,\u201d she noted. \u201cWhen we started in 1990, climate change was not on anybody\u2019s radar screen.\u201d<\/p>\n

But once the decline began, \u201cwe could predict the trajectory \u2013 invasion by other species. We\u2019ve now done experimental work that could have helped us predict that if something knocked out\u00a0Schizachryium,<\/i>\u00a0it would go the wrong way.\u201d<\/p>\n

Looking back, she said, \u201cin 1990, we should have recommended controlling the seed supply of future invaders. The park had been working to control faya until the 1980s, when they gave up. It\u2019s too bad. Now we\u2019re at a point where we\u2019re over the precipice.\u201d<\/p>\n

Now, she said, \u201cclimate change is important to consider in looking at the persistence of invaders. Some decline, others benefit. We just don\u2019t know the relative strengths of individual invaders to withstand climate change.\u201d<\/p><\/div>\n

\u00a0<\/span><\/p>\n

Patricia Tummons<\/b><\/div>\n

\u00a0<\/span><\/p>\n

Volume 24, Number 11 — May 2014<\/div>\n

 <\/p>\n","protected":false},"excerpt":{"rendered":"

Carla D’Antonio has been in the weeds – literally – for the better part of 25 years. The professor of environmental studies at the University of California, Santa Barbara, has been investigating invasive plants in Hawai`i Volcanoes National Park since … Continued<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[68],"tags":[],"class_list":["post-885","post","type-post","status-publish","format-standard","hentry","category-may-2014"],"_links":{"self":[{"href":"https:\/\/environment-hawaii.org\/index.php?rest_route=\/wp\/v2\/posts\/885","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/environment-hawaii.org\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/environment-hawaii.org\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/environment-hawaii.org\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/environment-hawaii.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=885"}],"version-history":[{"count":0,"href":"https:\/\/environment-hawaii.org\/index.php?rest_route=\/wp\/v2\/posts\/885\/revisions"}],"wp:attachment":[{"href":"https:\/\/environment-hawaii.org\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=885"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/environment-hawaii.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=885"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/environment-hawaii.org\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=885"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}