4) Forests on Limestone - Key Concepts

Forests on Limestone

Limestone Formations

  • Limestone formations in the wet tropics of Asia are generally of mid-Cenozoic age or older.
  • Cenozoic Period: 66 MYA - today
  • Limestone is primarily calcium carbonate (CaCO_3) rock.
  • CaCO3(s) + H2O(l) + CO2(aq) \rightarrow Ca(HCO3)_2(aq)

Limestone Formation Process

  • Limestone is a sedimentary rock primarily composed of calcite (CaCO_3).
  • Forms from calm, clear, warm, shallow marine waters.
  • Biological sedimentary rock: Accumulation of shell, coral, algal, fecal, and other organic debris.
  • Chemical sedimentary rocks: Formed by direct precipitation of calcium carbonate from marine or fresh water (less abundant than biological limestones).

Karst Morphology

  • Youthful Karst (kII): pavement, integrated relict cave cave system, rock scars, suffosion sinkholes, dry valley, stream, sink, buried, sinkhole, rockhead fissures, sinks, more dissolution at karst margin
  • Mature Karst (kIII): integrated caves, buried sinkhole collapse sinkhole, buried cave, dissolution doline, subsidence sinkholes, fissured rockhead, fissured floor, fissured outcrop
  • Complex Karst (kIV): stoped cave roof, remanent hill, collapse sinkhole, subsidence sinkholes, irregular rockhead, foot cave, buried sinkhole, dissolution sinkhole
  • Extreme Karst (kV): large stoped cave, stone teeth, remanent tower, re-activated buried sinkhole, pinnacled rockhead, old foot cave, dropout undercut sinkhole, cliff

Limestone and Engineering Construction

  • Karst landscapes can pose serious dangers in engineering construction, especially when underlain by clay and lubricant sediments.
  • Cavities in limestone bedrock can cause construction problems, varying in size and depth based on fractures, faults, and groundwater levels.
  • Shallow cavities (less than 1m diameter) may not be geotechnical problems.
  • Deep cavities (more than 2m diameter) may require concert injection.

Forests Around Limestone Base

  • Base: Soil rich in Calcium (alluvium), supports tall forests with species from flooded areas and fertile clay loams.
  • Towers: Average slope of 30°, red-brown loams with thin soil layer that easily loses humidity, causing stress for plants.
  • Common tree species: Shorea guiso, Chukrasia tabularis, Furmniana, Pterocymbium, Toona sureni (Peninsula). Some species are endemic and adapted to dry conditions and high Calcium concentrations.
  • Many herbaceous species are endemic to karst areas: Gesneriaceae, Rubiaceae, Acanthaceae, Cannabiaceae, Orchidaceae.
  • Cliffs: Exceeding 500m vertical, largely bare. Fissures support rich herbaceous flora adapted to extreme conditions.
  • Karst Summits: Level summit surface, limestone eroded by rain forming pinnacles. Thin soil layer makes forests prone to fire, degrading the soil further.

Plant Traits in Limestone Forests

  • Common traits:
    • Deciduous
    • Succulent
    • Sclerophyll

Lecanopteris

  • 13 species
  • Rhizome creeping, thick (1–4 cm diam.), hollow and ant-inhabited.
  • Stipes 3–8 cm long, winged on uppermost portion.
  • Laminae simple, oblong to linear, lobed.
  • Midrib raised on both surfaces, veins visible.
  • Sori medial or submarginal, in one row at each side of midrib, round to oblong.

Rhodomyrtus tomentosa (Family Myrtaceae)

  • Leaves are small and hairy on the lower side.
  • Sclerophyll adaptation to long periods of dryness
  • Hard leaves, short internodes, leaf orientation parallel or oblique to direct sunlight