D. Scott Green

Foliar morphology and canopy nitrogen as predictors of light-use efficiency in terrestrial vegetation

The net primary productivity (NPP) of a plant community is often positively and linearly related to the amount of photosynthetically active radiation absorbed by its canopy (APAR). The slope of this relationship is governed by the efficiency ( e )o f APAR use in biomass production (NPP = APAR×e). This intuitive model offers a promising means of generating large-scale NPP estimates, but its utility is compromised by our inability to explain considerable differences in e across species, functional groups, and environments. Using data from the literature, we examined the possibility that variation in e was governed largely by two chemical and morphological characteristics of the vegetation, canopy nitrogen content ( Ncanopy) and the canopy average for leaf mass per unit area (Marea). Specifically, we hypothesized that e was positively related to the quotient of Ncanopy (adjusted for the fraction of incident PAR absorbed by the canopy, fPAR ) and Marea. This e index accounts for the dependence of light utilization on the quantity of photosynthetic “machinery” ( Ncanopy) and its inherent efficiency, which is inversely related to Marea. Across a wide array of C3 species, functional groups and environments, e (based on aboveground NPP) was strongly and positively related to [Ncanopy/fPAR ]/Marea (r 2 = 0.85, P< 0.0001). Adoption of the index as a basis for estimating e could improve APAR-based predictions of terrestrial NPP, agricultural crop yield and vegetation responses to global change.

Effects of polyethylene mulch in a short-rotation, poplar plantation vary with weed-control strategies, site quality and clone

Abstract The utility of mulching in various forestry applications remains unclear due to mixed results in field trials. Additionally, few studies have attempted to assess the utility of mulching across a range of conditions, to determine the circumstances that maximize the degree and longevity of mulching-related enhancements. The objective of this study was to gauge the effect of black polyethylene mulch (poly mulch) across a range of site conditions, weed-control treatments and genotypes in a short-rotation, poplar plantation, to describe the circumstances that maximize the benefits of mulching on early growth and survival. Poplar plantations composed of one native clone ( Populus deltoides Bartr. [D105]) and one hybrid clone ( P. nigra L .×P. maximowiczii A. Henry [NM6]) were established in a randomized complete-block design ( 2.4 m ×3 m spacing) during May, 1999, under intensive and minimal weed-control strategies at two sites of different soil quality. Nested within each weed-control treatment was an assessment of tree performance using poly mulch vs. no mulch. Stem volume increments (SVI, dm 3 ) were measured on trees in all treatments during the 1999 and 2000 growing seasons. During establishment (1999), poly mulch enhanced SVI in all treatment combinations. In the second year, the benefit of mulching was restricted to conditions of higher vegetative competition and lower site quality. The relative benefit of poly mulch (i.e., the ratio of SVI under mulched vs. non-mulched conditions) increased in the second year in conditions of high vegetative competition at both sites, while it decreased in the intensively managed plots at both the sites. Tree survival exceeded 90% for both the clones in all treatment combinations, except under the greatest vegetative competition (i.e., high-quality site with minimal weed control), where survival for D105 fell below 40% in both mulched and non-mulched conditions. While poly mulch may improve early growth in short-rotation, woody crop plantations under a wide range of conditions, its practical utility appears to be restricted to specific applications. At the high-quality site in this study, mulching showed little potential as an economically feasible tool under either intensive weed control (due to the rapid attrition of mulching benefits) or minimal weed control (due to low survival and slow growth). Conversely, on marginal sites poly mulch may provide a more attractive management option in both intensive and minimal weed-control applications, particularly for certain clones.

Soil microbial communities from an elevational cline differ in their effect on conifer seedling growth

Sub-alpine environments consist of altitudinal gradients associated with dramatic changes in plant growth and community composition, but the role of soil feedbacks and microbe interactions is largely unknown. Here, we examine the influence of the overall soil microbial community, with a focus on ectomycorrhizal and dark septate endophytic root colonizing fungi, from low, mid, and high elevations on the growth of Pinus contorta and Picea glauca × engelmannii. The influence of the soil microbial community was tested on seedlings from the same three elevations in order to determine ‘home’ versus ‘away’ effects on conspecifics of differing elevations. The low elevation soil was the most fertile and harbored a soil microbial community with an overall negative effect on seedling growth. In contrast, the high elevation soil was the least fertile and had a microbial community that enhanced seedling growth. However, only the soil microbial community in the highest elevation soil resulted in a stronger influence on the native P. contorta seedlings than seedlings originating from lower elevations. Despite the overall influence of the soil microbial community, ectomycorrhizal colonization was significantly correlated with P. glauca × engelmannii growth rates, but colonization by dark septate endophytes showed no relationship with seedling growth. The results provide evidence that plant—soil microbial community relationships are dependent on soil environment. Moreover, our results provide further support for the importance of soil microbes in facilitating seedling growth toward the edge of their elevational range.

Regional and ecological patterns in interior Douglas-fir climate–growth relationships in British Columbia, Canada

How climate change will affect tree growth across species’ geographic and climatic ranges remains a critical knowledge gap. Tree-ring data were analyzed from 33 interior Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) stands spanning wide geographic and climatic conditions in the interior of British Columbia to gain insights into how within-species growth responses to climate can vary based on local environmental conditions over a broad climatic and geographic range, including populations growing at the species’ range and climatic margins. Populations growing in relatively warm and dry climates had growth patterns correlated mostly with annual precipitation, whereas populations growing in high-elevation wet and cold climates had growth patterns correlated with snowfall, winter and annual temperatures, and ocean–atmosphere climate systems. Populations growing at climatic extremes (e.g., coldest, driest, warmest) in each study region had the strongest responses to climate. Projected climate...

Host and habitat filtering in seedling root-associated fungal communities: taxonomic and functional diversity are altered in ‘novel’ soils

Climatic and land use changes have significant consequences for the distribution of tree species, both through natural dispersal processes and following management prescriptions. Responses to these changes will be expressed most strongly in seedlings near current species range boundaries. In northern temperate forest ecosystems, where changes are already being observed, ectomycorrhizal fungi contribute significantly to successful tree establishment. We hypothesised that communities of fungal symbionts might therefore play a role in facilitating, or limiting, host seedling range expansion. To test this hypothesis, ectomycorrhizal communities of interior Douglas-fir and interior lodgepole pine seedlings were analysed in a common greenhouse environment following growth in five soils collected along an ecosystem gradient. Currently, Douglas-fir’s natural distribution encompasses three of the five soils, whereas lodgepole pine’s extends much further north. Host filtering was evident amongst the 29 fungal species encountered: 7 were shared, 9 exclusive to Douglas-fir and 13 exclusive to lodgepole pine. Seedlings of both host species formed symbioses with each soil fungal community, thus Douglas-fir did so even where those soils came from outside its current distribution. However, these latter communities displayed significant taxonomic and functional differences to those found within the host distribution, indicative of habitat filtering. In contrast, lodgepole pine fungal communities displayed high functional similarity across the soil gradient. Taxonomic and/or functional shifts in Douglas-fir fungal communities may prove ecologically significant during the predicted northward migration of this species; especially in combination with changes in climate and management operations, such as seed transfer across geographical regions for forestry purposes.

Site properties have a stronger influence than fire severity on ectomycorrhizal fungi and associated N-cycling bacteria in regenerating post-beetle-killed lodgepole pine forests

Following a pine beetle epidemic in British Columbia, Canada, we investigated the effect of fire severity on rhizosphere soil chemistry and ectomycorrhizal fungi (ECM) and associated denitrifying and nitrogen (N)-fixing bacteria in the root systems of regenerating lodgepole pine seedlings at two site types (wet and dry) and three fire severities (low, moderate, and high). The site type was found to have a much larger impact on all measurements than fire severity. Wet and dry sites differed significantly for almost all soil properties measured, with higher values identified from wet types, except for pH and percent sand that were greater on dry sites. Fire severity caused few changes in soil chemical status. Generally, bacterial communities differed little, whereas ECM morphotype analysis revealed ectomycorrhizal diversity was lower on dry sites, with a corresponding division in community structure between wet and dry sites. Molecular profiling of the fungal ITS region confirmed these results, with a clear difference in community structure seen between wet and dry sites. The ability of ECM fungi to colonize seedlings growing in both wet and dry soils may positively contribute to subsequent regeneration. We conclude that despite consecutive landscape disturbances (mountain pine beetle infestation followed by wildfire), the “signature” of moisture on chemistry and ECM community structure remained pronounced.