Julia I. Burton

Management trade‐off between aboveground carbon storage and understory plant species richness in temperate forests

Because forest ecosystems have the capacity to store large quantities of carbon (C), there is interest in managing forests to mitigate elevated CO2 concentrations and associated effects on the global climate. However, some mitigation techniques may contrast with management strategies for other goals, such as maintaining and restoring biodiversity. Forest thinning reduces C storage in the overstory and recruitment of detrital C. These C stores can affect environmental conditions and resource availability in the understory, driving patterns in the distribution of early and late-seral species. We examined the effects of replicated (N = 7) thinning experiments on aboveground C and understory vascular plant species richness, and we contrasted relationships between aboveground C and early- vs. late-seral species richness. Finally, we used structural equation modeling (SEM) to examine relationships among early- and late-seral species richness and live and detrital aboveground C stores. Six years following thinning, aboveground C was greater in the high-density treatment and untreated control than in moderate- (MD) and variable-density (VD) treatments as a result of reductions in live overstory C. In contrast, all thinning treatments increased species richness relative to controls. Between the growing seasons of years 6 and 11 following treatments, the live overstory C increment tended to increase with residual density, while richness decreased in MD and VD treatments. The richness of early-seral species was negatively related to aboveground C in MD and VD, while late-seral species richness was positively (albeit weakly) related to aboveground C. Structural equation modeling analysis revealed strong negative effects of live overstory C on early-seral species richness balanced against weaker positive effects on late-seral species richness, as well as positive effects of detrital C stocks. A trade-off between carbon and plant species richness thus emerges as a net result of these relationships among species traits, thinning treatments, and live and detrital C storage. Integrating C storage with traditional conservation objectives may require managing this trade-off within stands and landscapes (e.g., maintain early-seral habitat and species within dense, C-rich forests and, conversely, live and detrital C stores in early-seral habitats) or separating these goals across scales and species groupings.

Evidence for a recent increase in forest growth is questionable

In a recent article, McMahon et al. (1) examined forest-plot biomass accumulation across a range of stands in the mid-Atlantic United States and suggest that climate change and trends in atmospheric CO2 explain an increase in forest growth. To show this increase, they fit a simple model to live above-ground forest biomass (AGB) as a function of stand age, and then propose that the derivative of this model is the expected rate of ensemble biomass change (). They conclude that biomass changes within census plots that exceed the ensemble expectation constitute recent increases in growth rates.

Experimentally linking disturbance, resources and productivity to diversity in forest ground-layer plant communities

Summary 1. Disturbance can function to maintain diversity within forest communities; however, specific mechanisms and the relationship to productivity are not well understood. 2. We examined these linkages in forest ground-layer plant communities using a replicated, manipulative field experiment. Treatments included a range of gap sizes and untreated controls. We assessed spatial and temporal responses over the first three years following gap creation. 3. Light transmittance and soil water content increased with gap size, while rates of colonization and species richness increased after a critical threshold. Subsequent increases in productivity were associated with declines in species richness, increased rates of local extirpation and a unimodal relationship between species richness and productivity at the individual quadrat scale (4 m 2 ). 4. The richness and productivity of vines, shrubs and especially graminoids, increased within 200– 380 m 2 gaps treatments. However, the productivity of forbs and tree seedlings did not, showing possible drought sensitivity overriding treatments. Spatial and temporal partitioning of gaps occurred as a result of interactions between species traits and environmental conditions. Significantly, productivity and richness showed complex relationships with canopy structure. 5. Synthesis. Our results show that richness increases to an asymptote after a critical threshold in disturbance severity initially. Decreases in species richness over time associated with increases in productivity may eventually result in the unimodal relationship predicted by the intermediate disturbance hypothesis. However, species composition continues to differ with canopy gap size, suggesting a range of canopy gap sizes is required to maintain the greatest diversity of plant species over broader spatial and temporal scales.

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 (%CU) 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- (%CES) and late-seral (%CLS) species cover. At neighborhood scales, decreases in %CU 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, %CU 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 %CES and %CLS. Rates of increases in %CES with reductions in density increased with disturbance severity and decreased with increases in %CLS. At broader scales, %CES increased with climatic moisture deficit where potential radiation was high, and %CLS low. Similarly to %CU, %CLS was related to a three-way interaction between overstory density, disturbance and hemlock abundance. %CLS declined with increases in climatic moisture deficit where overstory density and the relative abundance of hemlock was high, and decreased with local increases in %CES. Multi-scale controls explained a portion of the observed spatial autocorrelation for %CES but not %CLS, suggesting the spatial patterning of %CLS 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.

Intraspecific variability and reaction norms of forest understorey plant species traits

Summary 1.Trait-based models of ecological communities typically assume intraspecific variation in functional traits is not important, though such variation can change species trait rankings along gradients in resources and environmental conditions, and thus influence community structure and function. 2.We examined the degree of intraspecific relative to interspecific variation, and reaction norms of 11 functional traits for 57 forest understory plant species, including: intrinsic water-use efficiency (iWUE), Δ15N, 5 leaf traits, 2 stem traits and 2 root traits along gradients in light, nitrogen, moisture and understory cover. 3.Our results indicate that interspecific trait variation exceeded intraspecific variation by at least 50% for most, but not all traits. Intraspecific variation in Δ15N, iWUE, leaf nitrogen content and root traits was high (47-70%) compared with most leaf traits and stem traits (13-38%). 4.Δ15N varied primarily along gradients in abiotic conditions, while light and understory cover were relatively less important. iWUE was related primarily to light transmission, reflecting increases in photosynthesis relative to stomatal conductance. Leaf traits varied mainly as a function of light availability, with some reaction norms depending on understory cover. Plant height increased with understory cover, while stem specific density was related primarily to light. Resources, environmental conditions and understory cover did not contribute strongly to the observed variation in root traits. 5.Gradients in resources, environmental conditions and competition all appear to control intraspecific variability in most traits to some extent. However, our results suggest that species cross-over (i.e., trait rank reversals) along the gradients measured here are generally not a concern. 6.Intraspecific variability in understory plant species traits can be considerable. However, trait data collected under a narrow range of environmental conditions appears sufficient to establish species rankings and scale between community and ecosystem levels using trait-based models. Investigators may therefore focus on obtaining a sufficient sample size within a single set of conditions rather than characterizing trait variation across entire gradients in order to optimize sampling efforts. This article is protected by copyright. All rights reserved.

Enhancing Public Trust in Federal Forest Management

The connections between social and biophysical sciences are being forged in new ways as researchers and practitioners of natural resources seek to understand how lands can be managed for the benefit of human societies and the broader biotic community. Increasingly, we recognize that social and physical systems are tightly integrated, with human actions and decisions both shaping and shaped by the ecological systems in which they are embedded (e.g., Carpenter et al. 2009). In this context, a variety of social actors, including scientists, managers, policy makers, and the public, are collectively playing a larger role in decisions about environmental governance (e.g., collaboratives, chap. 9), drawing upon an accumulating body of knowledge describing the dynamics of complex socioecological systems. Learning-based approaches using adaptive-management experiments (chap. 8) represent one particular type of formal tool that can be appropriated to this process of adaptive environmental governance.