Certified Arborist Test Urban Forestry and Tree Selection 1 — Questions and Answers
Question 1: The '10-20-30 rule' is a widely cited guideline in urban forestry species diversity. What does it prescribe?
- No more than 10% of trees from one species, 20% from one genus, and 30% from one family (Correct answer)
- At least 10 trees per acre, 20 per block, and 30 per neighborhood
- A 10-year planting cycle, 20-year pruning cycle, and 30-year replacement cycle
- Tree canopy should cover 10% of residential zones, 20% of commercial zones, and 30% of parks
Correct answer: No more than 10% of trees from one species, 20% from one genus, and 30% from one family
The 10-20-30 rule, popularized by Frank Santamour, recommends limiting any single species to 10% of the urban tree population, any single genus to 20%, and any single family to 30%. This biodiversity threshold reduces the risk of catastrophic loss from a single pest, disease, or climate event affecting taxonomically related trees.
Question 2: Which soil condition is most commonly cited as the primary limiting factor for urban tree establishment and long-term survival?
- Low soil pH caused by acid rain deposits
- Soil compaction reducing pore space, aeration, and root penetration (Correct answer)
- Excess nitrogen from urban runoff stimulating overgrowth
- High clay content retaining too much moisture around the root zone
Correct answer: Soil compaction reducing pore space, aeration, and root penetration
Soil compaction from foot traffic, construction, and vehicle loads collapses macropores, reducing oxygen and water infiltration. Compacted soils have bulk densities that physically prevent root elongation, making it the single most pervasive limiting factor in urban tree establishment even when other conditions are adequate.
Question 3: When selecting street trees for a roadway that receives winter deicing salt applications, which characteristic is most critical to evaluate in candidate species?
- Tolerance to soil sodium and chloride ion accumulation (Correct answer)
- Deep taproot architecture to escape surface salt deposits
- High transpiration rate to dilute salt concentrations in tissue
- Bark thickness as a physical barrier to salt splash damage
Correct answer: Tolerance to soil sodium and chloride ion accumulation
Deicing salts elevate soil sodium and chloride levels, causing osmotic stress and ion toxicity in root tissue. Species selection must prioritize documented tolerance to soil salt accumulation, since absorbed chloride ions are transported to foliage causing marginal scorch and dieback regardless of root depth or transpiration rate.
Question 4: A minimum rooting soil volume is often cited as essential for urban tree longevity. What is the generally accepted minimum soil volume required to sustain a mature canopy street tree?
- 5–10 cubic meters (approximately 175–350 cubic feet) (Correct answer)
- 1–2 cubic meters (approximately 35–70 cubic feet)
- 25–30 cubic meters (approximately 880–1,060 cubic feet)
- 0.5 cubic meters (approximately 18 cubic feet)
Correct answer: 5–10 cubic meters (approximately 175–350 cubic feet)
Research, including work from the Urban Tree Foundation and international studies, consistently identifies roughly 30 cubic meters (about 1,000 cubic feet) as an ideal target, with 5–10 cubic meters representing a practical minimum below which trees rarely achieve healthy mature size or longevity exceeding 10–15 years in constrained urban pits.
Question 5: What is a 'urban forest master plan' and what distinguishes it from a standard municipal tree inventory?
- A master plan is a strategic, long-term document setting canopy goals, policies, and management priorities; an inventory is a data-collection record of existing trees and their condition (Correct answer)
- A master plan lists approved tree species for planting; an inventory tracks only trees requiring removal
- A master plan is required by federal law; an inventory is optional and locally developed
- A master plan covers all vegetation including shrubs and turf; an inventory covers street trees only
Correct answer: A master plan is a strategic, long-term document setting canopy goals, policies, and management priorities; an inventory is a data-collection record of existing trees and their condition
A tree inventory is a systematic database of individual trees—species, DBH, condition, location—while an urban forest master plan is a policy and strategic framework that uses inventory data to establish canopy cover targets, prioritize planting and removal, allocate budgets, guide species diversity, and align urban forestry with climate resilience and equity goals.
Question 6: Artificial light at night (ALAN) from urban streetlights and buildings has been documented to affect urban trees in which specific way?
- It delays autumn leaf senescence and prolongs the growing season, increasing frost damage risk (Correct answer)
- It accelerates photosynthesis year-round, causing excessive growth and structural weakness
- It suppresses root development by disrupting circadian carbon allocation cycles
- It bleaches chlorophyll in sun-exposed foliage, reducing photosynthetic efficiency
Correct answer: It delays autumn leaf senescence and prolongs the growing season, increasing frost damage risk
Studies show that ALAN disrupts photoperiodic cues that trigger dormancy, delaying bud set and leaf senescence in autumn. Trees exposed to artificial light remain physiologically active later into the season, increasing vulnerability to early frost events before they have hardened off, a documented cause of branch dieback in urban specimens near streetlights.
