Ecosystem processes are comparable in native and non-native forests in Hawaii
Joseph Mascaro is one of the scientists Emma Marris interviewed for her ground-breaking book, Rambunctious Garden. (1) Marris visited Mascaro on the Big Island of Hawaii, where he was studying the forests, comparing native with “novel” forests, the name given to ecosystems composed of both native and introduced species of plants.
According to Marris, Mascaro considers Ariel Lugo his mentor. Lugo is a US Forest Service scientist living and working in Puerto Rico. He is one of the first scientists to observe and report that non-native forests in Puerto Rico are performing important ecological functions and benefiting native forests by restoring depleted agricultural soils and providing shelter to native seedlings.
Lugo, like many native plant advocates, received his education in ecology at a time when there was deep suspicion of introduced species. The conventional wisdom was that introduced species were competitors of preferred native species, that they were inferior members of an ecosystem and that they would eventually dominate and replace their native predecessors.
When Lugo’s team reported that the understory in the non-native forest was so dense that it made the forest impenetrable, he was incredulous. Slowly, the reality of the non-native forest penetrated the prejudices of his training. He submitted his findings for publication repeatedly. After a lengthy debate, his study was finally published in 1992. He still considers himself an outlier amongst his colleagues in the Forest Service.
Joseph Mascaro is now a Postdoctoral Associate at the Carnegie Institution for Science in Stanford, California. His study of the novel forests of Hawaiian lowland rain forests has recently been published: “Novel forests maintain ecosystem processes after the decline of native tree species.” (2) We will do our layman’s best to report his main findings.
Judging the forest by the functions it performs
The scientific community seems to agree that introduced plant species have resulted in a net increase in species on the Hawaiian Islands: “Seventy-one vascular plant species are known to have become extinct in Hawaii over the past ~1700 years, while at least 1,090 introduced plant species have become naturalized during this period: an approximate doubling of its pre-human contact flora.” (2)
Mascaro’s study asks and answers the question, what are the functional implications of increased diversity due to invasion? He proposes and tests three hypotheses:
- Species richness and diversity are greater in novel forests than native forests in lowland Hawaii.
- Basic measures of ecological functions of novel forests meet or exceed measures in native forests. He used these basic measures:
- Aboveground and belowground production of biomass, called productivity
- Aboveground and belowground storage of carbon
- Cycling (or turnover) of nitrogen and phosphorous between soil, trees, and leaf litter.
- Because forest establishment in Hawaii begins on barren lava flows on which there is no available nitrogen and it takes several centuries to accumulate the nitrogen needed by native trees, the disparity between the functioning of novel and native forests are greatest on younger lava flows where novel forests are composed of nitrogen-fixing tree species.
He reports his findings: “At local scales, we found that novel forests had significantly higher tree species richness and higher diversity of dominant tree species. We further found that aboveground biomass, productivity, nutrient turnover (as measured by soil-available and litter-cycled nitrogen and phosphorus) and belowground carbon storage either did not differ significantly or were significantly greater in novel relative to native forests.” (2) Our interpretation of this study is that the novel forests of the lowland rain forests of Hawaii maintain basic functioning where native forests are now absent and that the novel forest facilitates the revegetation of barren lava flows by creating fertile soil.
He also speculated that “Because large portions of the Earth’s surface are undergoing similar transitions from native to novel ecosystems, our results are likely to be broadly applicable.” (2) It is this conclusion that his findings can be generalized to other locations that brought Joseph Mascaro’s study to our attention.
Mascaro recently wrote to the Webmaster of the Save Mount Sutro Forest website and sent his study. He lives in San Francisco and drives over Mt. Sutro daily, on his way to work. Mascaro told the Save Sutro Webmaster, “I wanted to let you know that your website and effort are much appreciated. As a practicing ecologist, I find it bewildering that efforts to restore native plant communities (some of which I find very important) would be directed at a diverse, old-growth, functioning ecosystem smack in the middle of one of the largest cities in the country….Cases like Sutro are often emotional and controversial, and while I don’t disparage anyone’s view, I tend to think that great pause must be taken before destroying something that is centuries old. I hope you will continue your effort.” (quoted with permission)
We are grateful to Joseph Mascaro for his research on the novel forests of Hawaii and for expressing his opinion of the value of the forest on Mt. Sutro. We are also grateful to the Webmaster of Save Sutro Forest for her insightful and articulate defense of the Sutro Forest.
The evolution of ecology
We began this post with the observation that scientists have found it difficult to report their findings about novel ecosystems that are not consistent with their educational training. When we expect to see something, it is often difficult for us to see something that contradicts those expectations. We commend scientists such as Ariel Lugo for reporting his observations, although they weren’t consistent with his training.
We are also pleased to report that we have observed, first-hand, a change in the training of university students in ecology. We attended two sessions of an undergraduate seminar in a local, major university. That seminar is reading and evaluating Rambunctious Garden. The students were entirely receptive to the revision of traditionally negative judgments of introduced species. That revision is the main theme of Rambunctious Garden. These students are the next generation of ecologists. They are the beginning of a new conventional wisdom about the role of introduced species in our ecosystems.
(1) Emma Marris, Rambunctious Garden, Bloomsbury, 2011
(2) Joseph Mascaro, R. Flint Hughes, Stefan A. Schnitzer, “Novel forests maintain ecosystem processes after the decline of native tree species,” Ecological Monographs, 82(2), 2012, pp. 221-228 by Ecological Society of America