Determinants of tree assemblage composition at the mesoscale within a subtropical eucalypt forest

Area of Science:

• Ecology
• Forest Science
• Environmental Science

Background:

• Environmental factors like topography, soil, and disturbances shape vegetation composition.
• Influence of these factors varies with scale; mesoscale drivers are less studied.
• Subtropical eucalypt forests in urbanizing landscapes present unique ecological challenges.

Purpose of the Study:

• To investigate the relative importance of topography, catchment flow, and soil in influencing tree assemblages at the mesoscale.
• To determine if environmental drivers of tree composition differ across size cohorts (small, intermediate, large).

Main Methods:

• Surveyed 33 Long-Term Ecological Research (LTER) plots at the mesoscale (909 ha) in Karawatha Forest Park.
• Sampled all woody stems (≥1.3 m height) and divided them into three diameter at breast height (DBH) cohorts.
• Measured topographic variables (slope, aspect, elevation, catchment area), soil chemistry, and structure; used ordinations and smooth surface modeling.

Main Results:

• Elevation strongly correlated with woody vegetation composition across all size cohorts (R2: 0.69-0.75).
• Catchment area (R2: 0.43-0.45) and slope (R2: 0.23-0.61) also significantly explained variation in composition.
• Soil chemistry showed association with composition (R2: 0.09-0.75); its influence was modulated by topography (e.g., drier ridges vs. riparian areas).

Conclusions:

• Topography, particularly elevation, is the primary determinant of tree assemblage patterns at the mesoscale, irrespective of tree size.
• Environmental drivers of tree composition remained consistent across small, intermediate, and large tree cohorts.
• Topographic position influences soil characteristics, further mediating tree community structure in subtropical forests.