‘Fire as a mechanism for allelopathy in a eucalyptus dominated riparian habitat in the Santa Cruz Mountains:’

ABSTRACT

Invasive species are a threat to both California ecosystems and the socio-economic systems dependent on them. According to the ‘novel weapons hypothesis,’ allelopathic strategies are the mechanisms and biochemical weapons used by invasive to inhibit the growth of native species. This is done, in part, by altering the chemical composition of soil.   Soil and their varied compositions have a unique ability, the power to shape an ecosystem's richness and diversity.Here we quantify the influence that the invasive species, Eucalyptus globulus, has on the pH Levels of soil of riparian habitat, when compared to a non invaded environment, 2 years after a major fire event.  Systematic soil sampling of both invaded and uninvaded riparian habitats has revealed statistically significant differences. The E. globulus invaded, riparian habitat analysis reveals invader-specific pH levels.  Understanding the post-fire succession strategies of E. globulus will enable evidence-based knowledge for the restoration, management and invasive expansion prevention efforts of E. globulus invaded environments. 

Key Words. Eucalyptus post-fire succession. Eucalyptus secondary succession, Invasive species, Allelopathic,  ‘novel weapons hypothesis’,  Invasive expansion prevention,   E. globulus

INTRODUCTION  

Soil is one of the essential building blocks of an ecosystem and the composition of this fundamental resource has the power to dictate the diversity and richness of a system. Healthy soils are an essential socioeconomic resource and eco-service provider. Large-scale disturbance events within an ecosystem can fuel biological invasions and reshape soil compositions (Gibbons et al., 2017.) Severe disturbances, such as a fire event, initiate a series of changes that allow for new species successions.  Recent studies have found that fires provide an opportunity for  accelerated growth by creating an abundance of resource availability.  (Chungu et al.,  2020)

Eucalyptus is an example of an invasive species that thrives in California's Mediterranean climate. (Boyd, David, 2022.)  Its unique adaptations have evolved in response to its native, fire-prone environment in Australia, and can withstand the intense heat and fire events common in California. Phenology and morphology of  E. globulus suggest not only the species’ fire retardancy, but also its dependency on fire for a competitive advantage over native species, and its active encouragement of fire event occurrences (Chungu et al., 2020.) Their seeds are fire and heat resistant and  E. globulus germination is dependent on both heat and smoke. Their serotinous cones are completely sealed with resin and only open to release their seeds after intense heat, or when the fire has physically melted the resin. The bark and litter of the  E. globulus tree are flammable and encourage explosive heat occurrences. (Boyd, David, 2022.)  

The aim of this field research was to advance our knowledge of E. globulus secondary succession strategies, with a focus on allelopathic mechanisms after a fire. Here we explore if a post fire riparian habitat has invader-specific pH levels capable of accelerating . We hypothesize that E.globulus was creating invader-specific soil chemistry following a fire event within the coastal riparian habitat.  (James et al., 2022.) 

The aim of this field research was to advance our knowledge of Eucalyptus’ secondary succession strategies, with a focus on allelopathic mechanisms post-fire. Soil composition is essential for species growth. Excess Eucalyptus bark and leaf-litter, covered in fresh layer of ash, has the potential to raise soil pH to conditions favorable to invasive species.  We wanted to know if this species of Eucalyptus was creating invader-specific soil chemistry following a fire event within the coastal riparian habitat. We investigated the influence of  E. globulus on pH levels within the soil of coastal riparian habitats by comparing an E.globulus invaded riparian habitat to a non invaded riparian habitat.

MATERIALS & METHODS 

Site description: Two adjacent riparian sites were chosen on the Pescadero coast side in Santa Cruz County. These adjacent riparian habitats are 2414 meters in distance from each other. Canopy cover analysis of the sites were performed and acted as a preliminary study establishing that Green Oaks Creek was invaded with E.globulus and Cascade Creek was uninvaded. Study site elevation levels spanned from 200-300m at both Cascade creek and Green Oaks  

Soil pH : We collected 10 soil samples by point intercept across a 500m transect at Cascade creek and Green Oaks Creek spanning from elevations of 200m to 300m. The samples were collected 1m from the edge of the creek bed in increments of 50m, starting at 0 m. Soil samples were taken from a depth of 2.5 cm (1 inch)  below the surface. Soil acidity levels were measured using ‘Rapitest’ soil testing kits and recorded. The kits contained test chambers for pH soil solutions as well as color comparator films for assessing pH levels. Soil solutions were comprised of soil, pH testing powder, and distilled water (20uL.)  Following soil solution preparation, samples were left to develop for one minute prior to analysis and record-keeping  

Canopy Cover vs Light We used the point intercept method with a 0.5 m by 0.5 m quadrat to measure light along 500 m transects of both Cascade and Green Oaks Creeks. Starting at the lowest elevation (200m,) we used transect tape to measure 50m increments of each creek. At each 50m point, we conducted canopy cover measurements 1 m from the creekside. Both riparian sites had walking paths or roads within close proximity to the creek. Quadrant measurements were conducted from the undisturbed creekside. 

Species Richness:We designated a 500-meter representative sampling to reach along both Cascade Creek and Green Oaks Creeks. Eucalyptus overstory and canopy data was collected along one side of each creek within a 1 m width from the water’s edge. This data acted as a preliminary study to establish that Green Oaks Creek was invaded with  E. globulus and Cascade Creek was not.

Data Analysis and statistical tests JMP® Pro 16.0.0 was used for statistical analysis of the research data. The chi-squared test was used in both to determine whether there is a statistically significant difference between the frequencies of acidic and alkaline soil samples. 

RESULTS : 

Invaded riparian habitat’s alkaline soil: The pH levels of the soil samples were placed into two categorical data fields, Low pH (for samples with a pH level of 5-6) and High pH (for samples with a pH level greater than 6.5.) A contingency table was developed based on the frequencies within the categorical data. Chi^2 statistical tests were performed using contingency table data and established between the variables. The results found (Figure 1)  demonstrate significantly higher pH levels  (X(0.05,1) = 20, p>0.0058) in the invaded riparian zones of Green Oaks than that of the non-invaded riparian habitat at Cascade creek 

Figure 1:  Mean percentage of cover was recorded and analyzed to compare level of invasion at Cascade creek (Noninvaded) vs Greenoak Creek (invaded) Error bars are constructed using 1 standard error from the mean.

Establishing  E. globulus Invasion:  Means for the percentage of Eucalyptus cover at Green Oaks Creek vs Cascade Creek where analyzed to establish invasion levels between the two habitats using a T-test statistical analysis. Statistically significant differences were found in the mean percentages differences of E. Globulus cover. (X2(0.05,1) = 20.9, p<0.0099).  (see figure 2)  

INVASION VS pH LEVEL FREQUENCIES. FIGURE 2:  Low and High pH level frequencies recorded in a contingency table Chi squared statistical analysis performed  to determine variances within the categorical pH level data at both creeks.

DISCUSSION 

Native species restorations are of vital importance both economically and culturally to those living on the California Coast. Understanding the mechanisms of invasive succession is an essential step to taking effective actions towards the restoration, management and protection of native species habitats. Studies have revealed that invaded soils can differ in composition from surrounding soil in ways that disrupt the restoration of native plant communities (Gibbons et al., 2017.) 

This study quantified the differences in pH scale ranges of invaded and non invaded riparian zones. We furthering the hypothesis created by Callaway and Ridenour’s study “Novel Weapons” (Callaway and Ridenour, 2004.) Here we test these ideas by proposing that invader-specific, elevated pH soils serve as part of an allelopathic mechanism benefiting the eucalyptus during post-fire succession. E. globulus invasion levels are supported by the statistical analysis of our  field research data . (X(0.05,1) = 32.76, p<0.05). Soil pH samples taken from the invaded creek have significantly higher  pH levels than samples from the  non-invaded riparian habitats (Figure 1 )

In testing the secondary hypothesis, that Green Oaks Creek was invaded with  E. globulus and Cascade was not invaded, the results were conducted by our statistical analysis work using a  T-test. We found statistically significant differences in the percentage of cover dominated by  E. globulus when measured in mean of overstory percentages  : A mean of 20.9% of E. globulus canopy cover was found at Green Oaks creek and a mean of 0% cover at Cascade Creek . (figure 2)

Studies have demonstrated the tolerance and successful growth response of Eucalyptus, within alkaline soils (Ames et al, 2004)  and Eucalyptus benefits from fire and post-fire succession (Qu et al.,)  The results of this preliminary study demonstrate that alkaline soils have been established in the eucalyptus dominated habitat but remain lower in non invaded habitat. This suggests the establishment of an invader- specific soil environment, with eucalyptus as the beneficiary of the post-fire soil composition.

Methodology improvements  Due to irregular heights of trees following a fire event, a layered approach is recommended in order to accurately estimate vegetative canopy cover that includes newly resprouting growth.  The singular layered vegetation count used in our method of canopy percentages underestimated early sprouting eucalyptus. By including layered height canopy measurements, one would record hits of vegetation that were encountered as the quadrant viewpoint is lowered. While the additional point intercepts at varying heights would be time-consuming, this additional layering would give a richer 3-dimensional view of varying heights of Eucalyptus growth. 

 Recommendations: we recommend further soil management analysis and management to combat the allelopathic effects of Eucalyptus on soil composition. This may include (but not limited to) Microbial analysis, bark and litter removal and chemical composition balancing of soils. Understanding a predictive model for eucalyptus invasion without restorative efforts will highlight the importance of timing in these restoration efforts. We also recommend further studies that entail a detailed impact report on vertebrate and invertebrate species likely to be impacted if eradication actions are pursued. Eradication of these groves will affect many species that now include eucalyptus groves in their migration habitats  

LITERATURE CITED  

Boyd, David. “IPCW Plant Report: Eucalyptus Globulus.” 2006-2022 California Invasive Plant Council, CalEPPC List A-1,CDFA nl, n.d. https://www.cal-ipc.org/resources/library/publications/ipcw/report48/.

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