Biological Field Study Results: Oak Regeneration, Leaf Morphology, and Herbivory Defense Theories

Field Trip 2: Comparative Analysis of Blue Oak and Live Oak

Starting with the data collected from the second field trip, the analysis combined data from the Blue Oak and Live Oak species to compare their growth and fire response patterns.

Tree Size and Fire Damage Relationship

This analysis investigated whether tree size influences the level of damage sustained during a fire.

• Proxies Used:

  • Tree Size: Measured via Diameter at Breast Height ($\text{DBH}$).

  • Fire Damage: Measured via the percentage of green canopy remaining (Percentage Green Canopy).

• Statistical Method: Linear regression was used to fit a line to the data using the formula:     $Y = MX + B$

  • $Y$ is the Percent Green Canopy.

  • $M$ is the coefficient ($\text{DBH}$ slope).

  • $X$ is the $\text{DBH}$.

  • $B$ is the Intercept.

Blue Oak Results

• Intercept: Estimated at approximately $61.0$. This implies a theoretical tree with a $\text{DBH}$ of $0$ would have $61\%$ of its canopy green, though this is not biologically possible.

• $\text{DBH}$ Coefficient (Slope): $0.35$. This indicates a positive relationship where larger trees have more green canopy.

• Scale: The x-axis is scaled over $150\,cm$, making the slope appear visually significant.

• Statistical Significance: The $P \text{-value}$ is $0.16$. Because this is greater than the traditional threshold of $0.05$, the slope is not statistically significant and could be due to chance.

Live Oak Results

• Intercept: $72.8$.

• $\text{DBH}$ Coefficient (Slope): $-0.06$. This shows a slightly negative relationship.

• Statistical Significance: The $P \text{-value}$ is $0.74$, indicating no significant relationship between size and damage.

Fire Intensity and Canopy Damage

This analysis asked if fire intensity, measured by how high up the tree burned, affects the percentage of green canopy remaining.

• Variable: Scorch height (proxy for fire intensity).

• Blue Oak Statistics:

  • Intercept: $77.83$.

  • Slope: $-0.85$.

  • $P \text{-value}$: $0.5$. Not significant.

• Live Oak Statistics:

  • Intercept: $70.07$.

  • Slope: $0.002$ (near zero/flat line).

  • $P \text{-value}$: $0.99$. Not significant.

Oak Tree Regeneration and Seedling Comparisons

Tree Size and Seedling Counts

• Blue Oak: Slope is $0.02$ with a $P \text{-value}$ of $0.16$. The trend is slightly positive but not significant.

• Live Oak: Slope is $-0.01$ with a $P \text{-value}$ of $0.08$. The speaker notes this is "marginally significant" as it is close to the threshold.

Post-Fire Regeneration Strategies

Live Resprouts (Paired Samples T-Test)

• Method: Used to compare the mean value of live resprouts between the two species.

• T-Value: Represents the standard deviation/variance comparison.

• Sample Size: There were $27$ samples.

• Degrees of Freedom ($DF$): Defined as the number of samples minus one ($n - 1$).

• Findings: There are fewer Blue Oak resprouts than Live Oak resprouts. The $P \text{-value}$ is much less than $0.05$, indicating a statistically significant difference in resprouting strategies.

Seedling Counts (Poisson Regression / GLM)

• Method: A Generalized Linear Model (GLM) using a Poisson distribution. This was chosen because the data were count data and were not normally distributed.

• Results:

  • Intercept: $0.86$.

  • Slope (Difference between species): $-0.4$.

  • $P \text{-value}$: $0.047$.

• Conclusion: There is a significant difference between species, suggesting opposite or distinct regeneration strategies post-fire.

Non-Native Species Abundance and Environmental Factors

Species Descriptions and Visual Trends

• Wild Oat (Venus or Avena): Represented by red plots.

• Yellow Star Thistle (Centauria): Represented by green plots.

• Hedgehog Dogtail (Cyanoceros or Cyanosaurus): Represented by blue plots.

Distance from Trail Analysis

• Wild Oat: Slope of $-0.12$; $P \text{-value}$ of $0.6$ (identified by speaker as $0.06$ eventually, noting it as marginally significant). Trend: abundance decreases further from the trail ($10\,m$ to $15\,m$).

• Yellow Star Thistle: Slope of $1.95$; P \text{-value} < 0.05. Significant positive trend indicating more abundance further from the trail.

• Hedgehog Dogtail: Slope of $18.98$; $P \text{-value}$ of $0.99$. Low observations, no significance.

Slope Position and Abundance

• Labels: D (downhill), L (level), U (upslope).

• Wild Oat: Slope $-0.033$; $P \text{-value}$ $0.51$. Not significant.

• Yellow Star Thistle: Slope $2.1$; Significant $P \text{-value}$. Abundance increases as position moves more uphill.

• Hedgehog Dogtail: Slope $-18.8$; $P \text{-value}$ not significant.

Leaf Morphology in Toyons: Sun vs. Shade Leaves

This study compared leaves at the top of the plant (exposed to sun) versus the bottom (shaded).

Light Level

• Statistical Test: Two-sample t-test comparing means.

• Result: Light levels at the bottom are significantly less than at the top (P < 0.05).

Morphology Statistics

Conclusion: Bottom leaves have a larger surface area (longer and wider) compared to top leaves.

Herbivory and Plant Defense: Yerba Santa

Reproduction and Defense Trade-offs

• Herbivory vs. Flowers: Linear regression ($X = \text{flower count}$, $Y = \% \text{tissue missing}$).

  • Intercept: $6.0$. Plants with zero flowers have $~6\%$ missing tissue.

  • Slope: $-0.01$ (flat).

  • $P \text{-value}$: $0.7$ (Not significant).

Plant Size and Herbivory

• Height: Slope is slightly negative; $P \text{-value}$ is $0.98$.

• Branch Number: Slope slightly negative; $P \text{-value}$ is $0.97$.

• Conclusion: No relationship between plant size and herbivory damage.

Leaf Position

• Comparison: Top 5 leaves vs. Bottom 5 leaves.

• Test: Two-sample t-test.

• T-Value: $-2.5$.

• $P \text{-value}$: $0.01$ (Significant).

• Finding: Top leaves tend to have significantly more herbivory than bottom leaves.

Density of Surrounding Plants

• Variable: Number of other Yerba Santa plants within $1\,m$.

• Slope: $0.19$.

• $P \text{-value}$: $0.08$ (Marginally significant).

• Trend: As surrounding density increases, herbivory might slightly increase.

Diversity of Mosses and Lichens on Boulders

Analysis utilized GLM with a Poisson distribution for count data.

Environmental Gradients

• Distance from Creek:

  • Lichens: Slope $0.002$, $P = 0.9$.

  • Mosses: Slope $-0.002$, $P = 0.92$.

  • No change in diversity based on water source proximity.

• Light Levels (High, Low, Medium):

  • Lichens: Slope $0.19$, $P = 3.2$ (not significant).

  • Mosses: Slope $0.02$, $P = 0.94$ (not significant).

• Aspect (North vs. South Facing):

  • Lichens: Slope $0.02$, $P = 0.02$. Significant relationship: More lichen diversity on South-facing aspects.

  • Mosses: Slope $-0.22$, $P = 0.21$. No significant trend.

• Boulder Volume ($m^3$):

  • Lichens: Slope $0.37$, $P = 0.13$. Positive trend but not significant.

  • Mosses: Slope $0.47$, $P = 0.11$. Positive trend but not significant.

Questions & Interaction

Q: What do the degrees of freedom mean for the t-test? A: They are the number of samples minus one ($n - 1$).

Discussion on iClicker Quiz results:

• The most "convincing" results according to students were the Oak regeneration and Toyon leaf morphology analyses.

• Student (Boris): Mentioned interest in the oaks because his group's sapling readings were zero across the board, noting variability among groups.

Interactive Game Activity - Herbivory Defense Theories:

• Nitrogen (N) vs. Carbon (C) Defense: Strategy depends on nutrient availability.

• Qualitative vs. Quantitative: Thorns vs. chemical compounds (e.g., cyanogenic compounds or tannins).

• Apparency Theory: High apparency plants (large/long-lived) invest in quantitative defenses; low apparency (hard to find) invest in qualitative defenses.

• Inducible vs. Constitutive: Defenses that are always present vs. those triggered by damage.

• Trade-offs: Growth rate vs. defense intensity.

Named Examples from Activity:

• Mini Death Thorn: Tiny cactus-rose plant. Uses thorns (quantitative) and cyanogenic compounds (qualitative/Nitrogen intensive). Invests in roots over foliage and nitrogen-fixing symbionts.

• Elephant: Generalist herbivore, 100% digestion efficiency, thick skin.

• Belladonna (Deadly Nightshade): Prioritizes proteins in high N environments and toxins in low N environments.

• Pink Acorn Weevil: Specialist herbivore. Larvae grow in acorns. Adapted to high tannin content. Uses aposematic coloration (pink).