Structurally diverse and species rich canopy tree assemblages support native herbs in grassy woodlands

Abstract

The world's dwindling woodlands face ongoing pressures from agriculture and urban expansion, with most having been disturbed by human activities, resulting in degradation and biodiversity losses. To better manage and promote resilience in threatened woodlands, we need to know more about the relationships between structure and diversity in these ecosystems. The influences of tree assemblage attributes on herb species have been investigated in forests, but such effects are poorly understood in more open woodland systems. This study aimed to determine how tree species richness, size class diversity and canopy cover influence native herb species richness and cover in coastal grassy woodlands of south-eastern Australia. Tree and herb composition and structure data were captured in 93 plots situated across the distribution of critically endangered Cumberland Plain Woodland. Bayesian models were used to elicit effects of trees on herbs. Where tree species richness was high, there were 28 % more native herb species on average, with both grasses and forbs contributing to the increase, and herb cover was 16 % higher. Tree size diversity increased native grass cover (by 19 %) but had no effect on forb cover or herb species richness. Native herb cover was 10 % lower under high tree canopy cover. In more recently disturbed woodlands, native grass cover increased under simulated thinning of dense, small tree regrowth, highlighting its potential as a conservation management tool. We found that species rich and structurally diverse canopy tree assemblages supported components of native herb species richness and cover in grassy woodlands.