Physiological performance of three pine species provides evidence for gap partitioning

Forest Ecology and Management (Impact Factor: 2.66). 12/2008; 256(12). DOI: 10.1016/j.foreco.2008.08.003
Source: OAI


Gradients of light and moisture availability peak at different positions within canopy gaps in northern latitudes providing the opportunity for niche partitioning in and around gaps based on differences in individual species' life history attributes. This gap partitioning offers potential for increasing diversity in forests impacted by gap-creating disturbances. We examined resource availability and the physiological performance of three Pinus species with varying tolerances for shade and moisture stress across large (0.3ha) canopy gaps to investigate relationships between gap position and species performance. Light availability was lowest in southern gap edges, while water availability was lowest in northern edges, and higher at gap interior positions than edges. Pinus banksiana seedlings had higher light-saturated CO₂ assimilation rates than P. resinosa or P. strobus seedlings at interior gap positions, and outperformed P. strobus at northern gap edges, but there were no differences between species at southern edges. Both transpiration and stomatal conductance were greatest for P. banksiana in gap centers, but showed few differences between species at edges. Foliar nitrogen concentrations were highest for P. banksiana, suggesting the dominance of this species in central gap locations may be due to a combination of high photosynthetic capacity and tight stomatal control to regulate moisture stress at drier gap positions. Our results suggest P. banksiana seedlings may be competitively superior in gap positions with high light and moisture availability, but P. resinosa and P. strobus become competitive under the drier conditions and moderate shade near gap edges. These findings support the concept of gap partitioning, and suggest silvicultural systems that incorporate patch cuttings could be used to promote diverse regeneration in northern pine forests.

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Available from: Matt Powers, Jul 07, 2014
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    • "This treatment is omitted from the current study because the residual basal area in this treatment was somewhat higher after harvest than treatments 2 and 3, which added another influencing variable beyond spatial pattern, making interpretation of response in the current study difficult. There is a growing portfolio of research from this experimental setting, including work on songbird communities (Atwell et al. 2008), seedling disease and mortality (Ostry et al. 2012), tree physiological processes (Powers et al. 2008, 2009a, b, 2010, 2011), early survival and growth of seedlings in gaps (Peck et al. 2012), individual seedling mortality and diameter and height growth (Montgomery et al. 2013), and, importantly for our purpose, resource availability (Montgomery et al. 2010, Boyden et al. 2012). Stands were cut in winter 2002–2003 to a residual basal area of 17 m 2 /ha. "
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    ABSTRACT: Variable-retention harvesting (VRH) is an approach for sustaining complex structure in managed forests. A criticism of VRH is that ecological benefits may come at a cost of reduced growth of regeneration, due to competition with residual trees. However, the spatial pattern of retention, i.e., dispersed or aggregated, in VRH systems can be manipulated to minimize suppression of regeneration, and resource limitation to regeneration might be mitigated by reduction of woody shrubs. Continued growth of the residual cohort will compensate for growth reduction of regeneration, although this may differ with retention pattern. We examined aboveground whole-stand biomass growth of trees in a VRH experiment in Pinus resinosa forest in Minnesota, USA. Treatments included dispersed retention, aggregated retention, and an uncut control, as well as a shrub treatment (reduced density or ambient). We addressed the following hypotheses: (1) biomass growth of a cohort of planted pine seedlings will be highest with aggregated rather than dispersed retention, (2) biomass growth of the planted seedlings will increase with shrub reduction, and (3) biomass growth of the residual overstory will be higher with dispersed rather than aggregated retention. Aboveground biomass growth of the planted pines ranged from 0.4 kg·ha-1·yr-1 in the overstory-control-ambient-shrub treatment to 23 kg·ha-1·yr-1 in the aggregated-retention-shrub-reduction treatment. The difference between the control and the retention treatments was significant ( P < 0.0001), but not between dispersed and aggregated retention (P = 0.97). Thus, our first hypothesis was not supported. In all treatments, biomass growth was significantly higher (>100% increase) with shrub reduction ( P = 0.001), supporting our second hypothesis. Biomass growth of residual trees ranged from 2404 kg·ha-1·yr-1 in the uncut-control-ambient-shrub treatment to 1043 kg·ha-1·yr-1 in the aggregated-retention-shrub reduction treatment. Differences were significant between the control and retention treatments ( P = 0.003), and marginally higher with dispersed vs. aggregated retention (P = 0.09), lending support to our third hypothesis. Our results suggest that managers have flexibility in application of VRH and can expect similar stand-level biomass growth of planted regeneration regardless of retention pattern, but somewhat higher stand-level biomass growth of retained trees with dispersed retention.
    Full-text · Article · Dec 2014 · Ecological Applications
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    • "Thus, the positive relationship between d 18 O and basal area found in this study is most likely caused by a decline in E across the overstory basal area gradient associated with decreased g s . A previous study of niche partitioning within the gap environments at the same sites found light and soil moisture availability were lower in the partial canopy environment of gap edges than in gap interiors (Powers et al., 2008b), which would lead to decreased g s and E. While the declines in seedling gas exchange parameters observed from gap centers to gap edges in this previous study were not always statistically significant, they suggest the potential for declines in E across the larger range of overstory environments encompassed within our current study. "
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    ABSTRACT: Overstory conditions influence understory microclimate and resource availability, leading to gradients in evaporative demand and moisture availability that influence seedling water relations. Partial canopies may either reduce seedling moisture stress by ameliorating environmental conditions, or increase moisture stress by reducing soil moisture availability. This study used stable isotope ratios of oxygen (δ¹⁸O) and carbon (δ¹³C) and mass-based foliar nitrogen concentrations to investigate changes in transpiration (E), stomatal conductance (g s) and intrinsic water use efficiency (iWUE) of pine seedlings across an overstory gradient from open canopy gap environments to closed canopy forest. Foliar δ¹⁸O increased sharply from basal areas of 0-10m² ha⁻¹ in Pinus banksiana, Pinus resinosa, and Pinus strobus seedlings, followed by a more gradual increase with further increases in basal area. Foliar δ¹³C followed a similar, but less pronounced pattern in P. banksiana and P. strobus seedlings, and had no apparent relationship with overstory basal area in P. resinosa seedlings. The slope of the δ¹⁸O:δ¹³C relationship was positive for every species. Foliar nitrogen concentrations were not correlated with overstory basal area. These results suggest seedling E declined as overstory basal area increased due to reductions in g s, while iWUE increased slightly from open gaps to partial canopy environments. Open gap environments appear to provide sufficient moisture to sustain high leaf-level gas exchange rates in the species we studied, while relatively small increases in overstory basal area apparently promote rapid declines in g s, leading to greatly reduced seedling water loss and small increases in iWUE.
    Full-text · Article · Sep 2009 · Forest Ecology and Management
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    No preview · Article · Apr 2009 · Natural Areas Journal
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