‘Ōhi’a abundance on the 1855 Waiakea lava flow

ABSTRACT

‘Ōhi’a (Metrosideros polymorpha) trees are a pioneer species of lava substrates on the island of Hawai’i. The construction of the Saddle Road highway increased ‘Ōhi’a exposure to roadway pollutants, caused habitat fragmentation, and decreased ecosystem resistance of the 3000-5000 year old kīpuka system in Waiakea. The current experiment aimed to examine the effects of highway proximity on the abundance of ‘Ōhi’a trees. Results indicated that highway proximity did not deter ‘Ōhi’a abundance, but a significant relationship with kīpuka distance was discovered.

‘Ōhi’a dispersal methods and growing conditions near kīpukas are discussed, as well as implications of highway pollutants on resistance to the kīpuka ecosystem.

INTRODUCTION

Observed natural phenomena were recorded in ancient Hawaiian chants and texts well before Westernized science was introduced to the archipelago. They predicted the effects of invasive species, as demonstrated in the following Hawaiian ‘ōlelo no‘eau:

“O ka lā‘au o ke kula e noho ana i ka ‘āina, o ka lā‘au o ka ‘āina e nalowale aku ana” (Pukui 2413).

This ‘ōlelo no‘eau predicts that native species will be replaced by invasive species that take over the land. The ‘ōlelo no‘eau coincides with recent Western findings that Hawaii has become an extinction hotspot mainly due to the introduction of invasive species (Lockwood 2006; Gurevitch & Padilla 2004). Invasive species are more likely to establish near roadways because roads act as dispersal corridors for non-native plants (Hansen & Clevenger 2005).

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This also results in decreased ecosystem resistance as a result of fragmentation, because the effects of roadway construction extend far beyond the roadway itself and create barriers for native plant dispersal (Goosem 2007; Hansen & Clevenger 2005). Roadway proximity also increases exposure to highway pollutants, and changes soil composition, density, and moisture (Benefenati et al. 1992). Concentrations of heavy metals in plants adjacent to roadways have a significant positive correlation with traffic density, which inhibits plant metabolic activity and decreases photosynthetic rates (Deepalakshmi et al. 2014; Clijsters & VanAssche 1985). Roadways are thus detrimental to native ecosystems as they decrease ecosystem resistance and increase the likelihood of invasions.

Hawaiian ‘ōlelo no‘eau describe effects of primary succession after lava flows as:

“Mōhala i ka wai ka maka o ka pua” (Pukui 2178)

The ‘ōlelo no‘eau can be interpreted through the lens of this experiment to predict that life will grow where the conditions are favorable. ‘Ōhi’a (Metrosideros polymorpha) is the dominant pioneer species of ecosystems undergoing primary succession after lava flows on the island of Hawai’i (Eggler 1971). ‘Ōhi’a seeds are wind dispersed and typically germinate soon after dispersal or are rendered unviable (Drake 1993; Drake 1992). They experience the fastest relative growth rates in moist, nitrogen rich soil but can grow in a variety of soil types (Treseder & Vitousek 2000). ‘Ōhi’are commonly found in the 3000-5000 year old forest kīpuka system that was created after the 1855 lava flow in Waiakea. The kīpuka now act as tree “islands” and can be studied using the principles of island biogeography theory (MacArthur & Wilson 1967).

Observations made in the kīpuka system distinguished the roadway as a major anthropogenic disturbance to the historic ecosystem. Most roadway research has focused on grassland ecosystems, and the effects of Saddle Road construction on kīpuka systems in general are largely unknown, presenting a gap in knowledge. The current study aimed to determine whether highway proximity deterred the abundance of ‘Ōhi’a trees on the 1855 lava flow.

METHODS

Data was collected in Waiakea along the two mile stretch of Saddle Road that runs relatively parallel to Kaumana Trail. Google Earth was used to place fifteen evenly spaced transects, perpendicular to Saddle Road and avoiding kīpuka intersections, between the two Kaumana Trail trailheads. Transect coordinates were located using the Google Earth app (2018) on the same Samsung smartphone to maximize precision. Each transect was 50m in length and began at the apparent vegetation line (indicated by the presence of ferns or ‘Ōhi’a) on the lava flow adjacent to the highway. Three plots on each transect were placed between stratified randomized distances of 0-16.6m (a), 16.7-33.3m (b), and 33.4-50.0m (c). Plots had a diameter of 5m and distances were randomized using a random number generator.

The abundance of ‘Ōhi’a was operationalized as the number of ‘Ōhi’a trees or shrubs counted within each plot. Only ‘Ōhi’a greater than 1m in height were used in abundance counts for this experiment. Plot boundaries were determined using a premeasured rope and field markers. The same group members carried the transect tape and measured ‘Ōhi’a abundances at each stratified plot distance to maximize precision of abundance counts.

Results were analyzed using the RStudio software, version 3.5.1 (2018). Nonparametric statistical tests were conducted because the data did not display normal distributions. Homogeneity of variance was tested using the Flinger-Killen test. Group homogeneity was determined using the Kruskal-Wallace test. A Spearman’s correlation was used to evaluate relationships between the abundance of ‘Ōhi’a and distances to the highway or nearest kīpuka.

RESULTS

Mean ‘Ōhi’a abundance of overall was 1.89 trees, with a median of 1 and standard deviation of 2.27. Descriptive statistics of the stratified distance groups are displayed in Table 1. Medians for groups a and b are equal (M=1), while group c had a slightly higher median (M= 2). Flinger-Killeen test results yielded a p value of 0.16 resulting in a failure to reject the null hypothesis that groups display homogeneity of variance. Kruskal-Wallace test results yielded a p value of 0.27 resulting in a failure to reject the null hypothesis that group medians are equal.

Spearman correlations tests yielded insignificant results in highway proximity comparisons (p= 0.078; ρ =0.265), resulting in a failure to reject the null hypothesis that there is no relationship between ‘Ōhi’a abundance and highway proximity. However, Spearman correlation tests yielded significant results in kīpuka proximity comparisons (p= 0.011; ρ= -0.372), rejecting the null hypothesis that there is no relationship between ‘Ōhi’a abundance and kīpuka proximity. This relationship indicated a slightly negative correlation between ‘Ōhi’a abundance and kīpuka proximity with high certainty.

DISCUSSION

The current experiment found no significant relationship between ‘Ōhi’a abundance and highway proximity. A statistically significant relationship was documented between ‘Ōhi’a abundance and kīpuka proximity, indicating a slight negative correlation. This is likely due to ‘Ōhi’a dispersal patterns and kīpukas’ providing the most favorable conditions for ‘Ōhi’a growth.

Drake (1992) found the highest ‘Ōhi’a seed rain concentration within 0-25m of a kīpuka’s edge. It also documented that most ‘Ōhi’a seeds are embryo lacking, suggesting that the majority of dispersed ‘Ōhi’a seeds will not germinate (Drake 1992). This suggests that the highest number of ‘Ōhi’a seeds are found on the kīpuka’s border and thus that the highest rate of new ‘Ōhi’a growth will occur within 25m of this perimeter. This growth is also facilitated by the favorable soil conditions that are a result of kīpuka proximity.

The Waiakea kīpuka system receives a mean annual precipitation of 5500mm per year (Austin & Vitousek 2000). Soil surrounding ‘Ōhi’a trees holds nutrients for longer in wet environments, and because forest ecosystems hold more moisture than lava flow ecosystems, soil surrounding the kīpuka perimeters holds a higher nutrient content as proximity increases (Austin & Vitousek 2000).

Although this experiment did not use kīpuka size as a variable, future studies should investigate the relationship between ‘Ōhi’a abundance and kīpuka size. Birds aid in ‘Ōhi’a seed dispersal by landing in the trees and dislodging seeds. The size of a kīpuka may influence bird behavior, as birds are documented to visit and spend more time at larger tree islands than small islands (Zahawi & Auspurger 2006).

The current data does not indicate a significant relationship between ‘Ōhi’a abundance and highway proximity although a p=0.078 could be improved with increased sample size. However, the highway introduces a number of stressors to the ecosystem and may results in decreased ecosystem resistance (Deepalakshmi et al. 2014; Hansen & Clevenger 2005; Benefenati et al. 1992; Clijsters & VanAssche 1985). This decreased resistance may make ‘Ōhi’a trees more susceptible to Rapid ‘Ōhi’a Death (ROD), a recently introduced disease that kills large numbers of ‘Ōhi’a trees. ROD is caused by the Ceratocystis fimbriata fungus which enters its host through open wounds and blocks the flow of water through the xylem (Asner et al. 2018; Keith et al. 2015).

Ancient Hawaiian texts indicate that this knowledge was documented before Western influences, as indicated by the passage below:

“O Waiakea ka ‘aina

He lau ke akua, he ha’a nei la

Ha’a wale ana ka ohia I ka lehua o Mokaulele

Kuhi paha I ku’u opio

He opio i luna ke alo e”

This passage describes the growth and flowering patterns of ‘Ōhi’a trees among the lands of Waiakea. It suggests that young ‘Ōhi’a are expected to stand on the “face” or “edge”, which can be interpreted as the edge of a kīpuka. This observation is consistent with the results of this experiment which suggest ‘Ōhi’a abundance is greatest near the kīpuka edges. It is the result of precise observations by ancient Hawaiians who were able to recognize this relationship without the use of empirical tools and analyses.

One potential error in the experimental design could arise because of only using ‘Ōhi’a that were greater than 1m tall in abundance counts. Because ‘Ōhi’a occur in both shrub-like and tree phenotypes, this methodology could have introduced bias against ‘Ōhi’a shrub phenotypes. Future research in this area should aim to conduct abundance counts without height discrimination and potentially document the different phenotypes present to account for these biases.

Overall, this experiment aimed to measure the effects of Saddle Road construction on further fragmentation of the kīpuka ecosystem. ‘Ōhi’a abundance was not significantly deterred by highway proximity, but favorable conditions near kīpuka increased ‘Ōhi’a abundance. The roadway introduces a number of stressors to the kīpuka system, including the potential for ‘Ōhi’a trees to be less resistant to ROD. It also increases the likelihood that invasive species will be introduced to the ecosystem. The findings coincide with ancient Hawaiian knowledge that predicted these effects through effective observation and documentation methods. The construction of roadways should aim to minimize habitat fragmentation, and future research should continue to investigate how to mitigate decreased ecosystem resistance as a result of such anthropogenic effects.

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