Multi-scale spatial controls of understory vegetation in Douglas-fir-western hemlock forests of western Oregon, USA

Forest understory vegetation is influenced by broad-scale variation in climate, intermediate-scale variation in topography, disturbance and neighborhood interactions. However, little is known about how these multi-scale controls interact to influence observed spatial patterns. We examined relationships between the aggregated cover of understory plant species (%C-U) and multi-scale controls using a large-scale experiment including treatments of low (LS), moderate (MS) and variable (VS) disturbance severity replicated in second-growth Douglas-fir (Pseudotsuga meziesii)-western hemlock (Tsuga heterophylla) forests spanning climate and topographic gradients. We developed hierarchical models using a multi-step selection process, assessing changes residual spatial autocorrelation associated with progressively broader spatial scales of influence and interaction. To examine the role of plant traits in mediating multi-scale controls, we contrasted effects for early-(%C-ES) and late-seral(%C-LS) species cover. At neighborhood scales, decreases in %C-U with overstory density were accelerated with increases in the relative importance of hemlock in the overstory in the in all but the LS treatment. At intermediate scales, %C-U was lower in areas with higher potential radiation in undisturbed control treatments but that trend was reversed in the harvested, disturbed areas. When separated, effects of multi-scale controls differed between %C-ES and %C-LS. Rates of increases in %C-ES with reductions in density increased with disturbance severity and decreased with increases in %C-LS. At broader scales, %C-ES increased with climatic moisture deficit where potential radiation was high, and %C-LS low. Similarly to %C-U, %C-LS was related to a three-way interaction between overstory density, disturbance and hemlock abundance. %C-LS declined with increases in climatic moisture deficit where overstory density and the relative abundance of hemlock was high, and decreased with local increases in %C-ES. Multi-scale controls explained a portion of the observed spatial autocorrelation for %C-ES but not %C-LS, suggesting the spatial patterning of %C-LS is related primarily to unmeasured processes. Results show how understory responses to overstory density and disturbance severity vary across the landscape with moisture and potential radiation, at fine scales with neighborhood structure, and with species traits. Hence, understory responses to climate change likely depend on overstory composition and structure, disturbance and species traits.