Near-ground solar radiation along the grassland–forest continuum: Tall-tree canopy architecture

Solar radiation directly and indirectly drives a variety of ecosystem processes. Our aim was to evaluate how tree canopy architecture affects near-ground, incoming solar radiation along gradients of increasing tree cover, referred to as the grassland–forest continuum. We evaluated a common type of canopy architecture: tall trees that generally have their lowest level of foliage high above, rather than close to the ground as is often the case for shorter trees. We used hemispherical photographs to estimate near-ground solar radiation using the metric of Direct Site Factor (DSF) on four sites in north Queensland, Australia that formed a grassland–forest continuum with tree canopy cover ranging from 0% to 71%. Three of the four sites had tall Eucalyptus trees with foliage several metres above the ground. We found that: (i) mean DSF exceeded >70% of the potential maximum for all sites, including the site with highest canopy cover; (ii) DSF variance was not highly sensitive to canopy coverage; and (iii) mean DSF for canopy locations beneath trees was not significantly lower than for adjacent intercanopy locations. Simulations that hypothetically placed Australian sites with tall tree canopies at other latitude–longitude locations demonstrated that differences in DSF were mostly due to canopy architecture, not specific site location effects. Our findings suggest that tall trees that have their lowest foliage many metres above the ground and have lower foliar density only weakly affect patterns of near-ground solar radiation along the grassland–forest continuum. This markedly contrasts with the strong effect that shorter trees with foliage near the ground have on near-ground solar radiation patterns along the continuum. This consequence of differential tree canopy architecture will fundamentally affect other ecosystem properties and may explain differential emphases that have been placed on canopy– intercanopy heterogeneity in diverse global ecosystem types that lie within the grassland–forest continuum.