Terrestrial Biomes, Soil, and Deforestation: Key Concepts and Impacts

Course logistics and recap

• Previous two presentations (last week and week four) uploaded to Brightspace in the content section under Presentations; a page for presentations was created this time.
• The instructor asked students to confirm access during the lecture if possible and to report any access issues.
• An announcement was posted with the previous presentation; today a dedicated Brightspace page for presentations was added.

Terrestrial biomes: intro and connections to soil

• Today's lecture continues on terrestrial ecosystems with a focus on geographic distribution and climate and how these shape biome characteristics.
• Climate and soils are primary drivers of plant distribution; environmental conditions influence biome biology and distribution.
• Patterns in climate are changing with global warming, which has spurred renewed research on how climate shifts affect wildlife distributions.
• Example: bumblebees in Colorado studied via historic museum specimens (50–60 years old) compared to present-day populations; distribution shifts observed as warmer temperatures move plants and bees upslope.
• Biomes are distributed along climate gradients into bands around the equator and toward the poles; precipitation and temperature govern biome types.

Soil: structure, horizons, and processes

• Soil is a complex mix of living and nonliving material; generally, the lower you go, the less living material and more abiotic matter.
• Soils are organized into horizons (layers) with distinct properties; can be in constant flux due to processes like root growth, tree fall, erosion, and dust deposition (e.g., Saharan dust).
• Factors shaping soils include:
  • Topography (mountains, slopes -> faster water movement and erosion; wind effects around mountains).
  • Parent material (bedrock type) affecting persistence and weathering.
  • Climate interactions with soil development and living organisms.
• Major soil horizons (layers):
  • O horizon: organic matter at the surface; freshly fallen material; increasingly fragmented with depth.
  • A horizon: mineral materials (clay, silt, sand) with organic inputs from the O horizon; partial mixing with decomposed material.
• Soil structure and layers are dynamic due to biological and physical processes (roots growing, root pruning by trees, frost, erosion).

Soil biology and habitat engineering

• Organisms beyond plants contribute to soil structure:
  • Animals burrow, fungi are abundant in soils, and many organisms rely on soil for habitat.
  • Forest roots form interconnected networks; fungi create mycorrhizae that extend root systems and exchange nutrients with plants.
• The soil ecosystem is tightly linked to plant community composition and stability; non-native monocultures tend to degrade soil structure and function.
• Human disturbance (logging, heavy machinery, foot traffic) compacts soil, reducing root penetration and water infiltration, altering habitat and drainage.
• Example from local fieldwork: long-term salamander density showed a negative (though not always statistically significant) trend in site-transects with higher soil compaction, illustrating wildlife responses to soil disturbance.

Groundwater, land subsidence, and regional hydrology (West Coast example)

• Central Valley of California receives substantial precipitation due to mountain-fed snowpack that melts annually, feeding rivers and recharging groundwater.
• Groundwater extraction for wells and agriculture often exceeds natural recharge, leading to land subsidence (subsidence can be feet to meters over time).
• This subsidence demonstrates the direct coupling between climate, hydrology, and land stability.

Terrestrial biomes: climate, distribution, and diagnosis with examples

• Terrestrial environmental conditions shape each biome's biology; climate and soils govern plant distribution and adaptation.
• Climate change reopens questions about biome distributions and wildlife ranges (e.g., shifts in species’ ranges and seasonality patterns).
• Conceptual map: biomes exist on a spectrum driven by temperature and precipitation; colder, drier conditions tend toward tundra and desert; higher precipitation at warm temperatures shifts from savanna to dry forest to rainforest.
• A common classroom tool is to use a climate-based graph to predict biome type from temperature and precipitation data.

Tropical biomes: rainforest and rainforest-dry forest transitions

• Major tropical region bands are within about ±10° latitude of the equator; tropical biomes dominate these zones due to stable temperatures and high rainfall.
• Tropical rainforest (months with little seasonality):
  • Annual rainfall: $2000\text{--}4000\ \mathrm{mm}$ (roughly $79\text{--}158\ \mathrm{in}$).
  • Rainfall is relatively evenly distributed year-round; some locations have wet and dry seasons, but many remain consistently wet.
  • High biodiversity with complex canopy structure and abundant epiphytes.
  • Mutualistic networks with fungi (mycorrhizae) extend resource uptake; epiphytic diversity is extremely high.
• Ecological significance of rainforest structures:
  • Canopy can reach hundreds of feet; epiphytes and bromeliads create microhabitats for amphibians and invertebrates.
  • Numerous plant–fungus–animal interactions contribute to nutrient cycling and forest resilience.
• Deforestation pressures: persistent and historically accelerating loss of rainforest area.
  • Estimates from 2019 suggest the world lost a football-field-sized area of rainforest every six seconds; roughly 31,000,000 to 32,000,000 football fields of forest lost in that period, illustrating enormous land-use change.
  • Epiphyte estimates in such cleared areas: up to 96 epiphyte types per football-field footprint; 33 amphibian species represented in that same footprint; and 300+ tree species recorded per football-field area.
• Rainforest degradation has broad implications for medicines, foods, and cultural resources used by human populations,
  and exploitation is increasing, often linked to logging, mining, and agriculture.

Tropical dry forest and savannah (tropical grasslands) dynamics

• Tropical dry forest occurs at latitudes still warm but with lower precipitation than rainforest; climate bands shift with latitude.
• Tropical savannah and tropical grasslands are characterized by a pronounced wet season and a dry season; vegetation is adapted to fire and drought.
• Key distinguishing features:
  • Tropical savannah: low soil permeability; water tends to accumulate near the surface rather than infiltrate deeply, supporting scattered trees and open grassland.
  • Seasonal fires are common and help recycle nutrients and maintain grass dominance.
  • Many herbivores migrate with rainfall patterns, following moisture and forage availability; predators and other trophic levels follow.
• Longleaf pine savanna (Southeast United States) is an example of a savanna ecosystem in the region historically spanning from Texas to Virginia and into Florida.
  • Modern distribution shows major reductions; restoration efforts exist in some areas.
  • Fire regime: historically, fires occurred roughly every other year, burning grasses and understory but typically not the canopy; pines and other woody plants adapt to these fire cycles.
  • Open habitat with sunlit, sparse canopy compared to dense tropical forests.
• Deforestation drivers in tropical regions overlap with rainforest but also broaden to agricultural expansion and cattle ranching, contributing to rainforest decline through edge effects and replacement with pasture.
• Amazon rainforest deforestation (case studies):
  • Between 2018 and 2019, rapid changes in forest cover were observed; maps show peripheral deforestation adjacent to major rivers and along roads.
  • NASA time-lapse data show annual clearing events on the scale of 5–20 km² (or 5,000–20,000 km² in some reports) per year in affected regions.
  • Causes include cattle pasture expansion, mining, palm plantations, road construction, and logging; deforested land is often burned to facilitate pasture conversion, but soils in former rainforest are nutrient-poor, limiting pasture longevity and prompting repeated cycles of clearing.
• Impacts of rainforest clearing on biodiversity, soil, and regional climate are substantial, with knock-on effects for local livelihoods and global carbon dynamics.

Deforestation drivers and patterns in practice

• Palm oil expansion, mining, and cattle ranching are major drivers in tropical regions; road networks create edge effects and encourage further clearing in a fishbone pattern (roads radiating outward with deforestation on both sides).
• The pattern of deforestation compounds the climate and hydrological impacts by reducing evapotranspiration, altering rainfall patterns, and increasing surface runoff.
• In addition to ecological impacts, deforestation has socio-economic dimensions (livelihoods, energy, and food supply chains) that intersect with policy and governance.

Global and regional governance, projections, and management responses

• Projections for 2050 under different scenarios:
  • Governance scenario (policy- and law-driven protections) vs. business-as-usual scenario (less stringent protections, higher development pressure).
  • In some regions, both governance and business-as-usual projections indicate more intense warming and drier conditions in tropical zones, accentuating land-use pressures.
• In the United States, there have been contemporary debates about expanding logging in national forests to reduce wildfire risk; recent announcements indicate broad logging targets in major national forests.
  • Specific US forest examples referenced include Nantahala and Pisgah National Forests in North Carolina, with mentions of Uwharrie and Coechan (and references to other named forests such as Hizga and Antihala); local opposition by communities and environmental groups has occurred in the past through legal action and advocacy.
  • The claim: 95% of all national forest land has been identified for some level of logging—though this is a complex regulatory and planning issue requiring careful interpretation of agency plans and environmental analyses.
• The underlying tension: does timber harvesting genuinely mitigate wildfire risk, or does it amplify ecological degradation and undermine forest resilience? The lecture notes the debate and encourages critical consideration of ecosystems-based management.

Connections to broader concepts and real-world relevance

• The material connects groundwater depletion, soil health, and land subsidence to broader climate and hydrological cycles, showing how human water use interacts with land stability.
• The discussion of mycorrhizal networks highlights the importance of below-ground interactions for plant communities and ecosystem services (water uptake, nutrient cycling, soil structure).
• Biodiversity metrics from tropical forests (epiphyte diversity, tree species richness, amphibian diversity) illustrate the extreme complexity of these systems and the potential consequences of habitat loss.
• The examples of deforestation patterns (road networks, cattle pasture conversion) emphasize how infrastructure and land-use decisions shape ecological outcomes and climate feedbacks.
• The regional focus on Southeast US savannas (longleaf pine) and North American forest management provides concrete case studies of fire regimes, habitat restoration, and policy debates that have broad implications for biodiversity conservation and carbon management.

Key numerical references (for quick recall)

• Tropical rainforest annual rainfall: $2000\text{--}4000\ \mathrm{mm}$ (approx. 79\text{--}158 inches)
• Deforestation rate (rainforest footprint): a football-field-sized area cleared every 6 seconds; roughly $31{,}000{,}000\text{ to }32{,}000{,}000$ football fields in the cited period
• Global land-clearing area (NASA time-lapse): approximately $5\text{ to }20\ \mathrm{km^2}$ per year in some contexts, or $5{,}000\text{ to }20{,}000\ \mathrm{km^2}$ per year depending on reporting scale
• Biodiversity per football-field footprint (rainforest): ≈ 96 epiphyte types and ≈ 300 tree species; ≈ 33 amphibian species
• Fire regimes in longleaf pine savannas: historically ~every other year; canopy typically not burned by fire, but understory and grasses are
• 2050 projections: governance vs. business-as-usual scenarios for tropical forest cover and climate, illustrating divergent outcomes depending on policy and market forces

Concluding takeaways

• Terrestrial biomes are shaped by a dynamic interplay of climate, soils, topology, and biological interactions; understanding soil horizons and biotic networks is essential to predicting biome responses to disturbance and climate change.
• Deforestation and land-use change in tropical regions have profound ecological and socio-economic consequences, including biodiversity loss, soil degradation, and regional climate feedbacks.
• Governance, policy, and land-management decisions will play critical roles in determining the future trajectory of tropical forests and associated ecosystem services.
• Local case studies (e.g., longleaf pine savannas in the Southeast US and regional logging in national forests) illustrate the trade-offs between resource extraction and ecological resilience, providing concrete contexts for applying ecological principles to policy and planning.