Level and spatial pattern of overstory retention impose tradeoffs for regenerating and retained trees
Halpern, Charles; Urgenson, Lauren (2020), Level and spatial pattern of overstory retention impose tradeoffs for regenerating and retained trees, Dryad, Dataset, https://doi.org/10.5061/dryad.0gb5mkm02
Variable retention (VR) has been adopted globally as an alternative to more intensive forms of regeneration harvest. By retaining live trees within harvest units, VR seeks balance among the commodity, ecological, and aesthetic values of managed forests. Achieving these multiple, often competing objectives requires an understanding of how level and spatial pattern of retention shape the abundance, growth, and mortality of regenerating and retained trees. Using long-term (18-19 yr) data from a regional-scale VR experiment, we explore the individual and interactive effects of retention level (15% vs. 40% of initial basal area) and pattern (dispersed vs. aggregated) on the post-harvest dynamics of forests of differing structure and seral composition.
Level and pattern of retention imposed tradeoffs for the density and growth of regenerating trees (>0.1 m tall, <5 cm dbh) and ingrowth (trees attaining 5 cm during the study). Greater retention led to greater density of late-seral regeneration, but lower density of early-seral ingrowth, and slower growth of late-seral ingrowth. Dispersed retention enhanced the density of early- and late-seral regeneration (compared to aggregated treatments), but reduced the growth of early-seral ingrowth. We also observed tradeoffs for retained trees. Lower retention enhanced the growth of smaller trees (<25 cm dbh)—particularly in dispersed settings—but reduced the survival of larger trees, which were more susceptible to windthrow. Greater retention reduced the growth, but enhanced the survival of smaller trees. Pattern imposed similar tradeoffs, with dispersed retention enhancing growth, but reducing survival of small trees. Finally, level and pattern resulted in tradeoffs for productivity of regenerating vs. retained-tree cohorts. Ingrowth productivity was greater at lower retention and in aggregated treatments; retained-tree productivity was greater at higher retention and in dispersed treatments.
Our results provide a unique, long-term perspective on the sensitivity of tree regeneration, growth, and mortality to key structural elements of VR systems. Strong responses to level and pattern of retention produce tradeoffs for different ecological or resource objectives. Balancing these objectives may require the combined use of aggregates, dispersed retention, and clearings, to mimic the spatial heterogeneity of habitats, physical structures, and resource conditions that are produced by natural disturbances.
Experimental design and treatment implementation
The experiment is a randomized complete-block design (Aubry et al. 1999, Aubry and Halpern 2020). Here, we consider four of the treatment combinations representing one of two levels of overstory retention (15% vs. 40% of initial basal area) distributed in one of two spatial patterns (trees uniformly dispersed, D, or in intact circular patches or aggregates, Ap). We do not consider the unharvested controls. Harvest units are 13 ha in area and square or slightly rectangular (edge lengths of 320–400 m). In the dispersed treatments (15D and 40D), a portion of the dominant and co-dominant trees was retained in a uniformly dispersed pattern using, as the retention target, the cumulative basal area of the corresponding aggregated treatment (15A or 40A). The aggregated treatments contained either two or five 1-ha (56 m radius) patches. In 15A, patches were spaced ~115 m apart on the diagonal of the harvest unit; in 40A, the patches were spaced ~30 m apart. All merchantable stems (>18 cm at diameter breast height, dbh) were felled in the adjacent harvested area (15Ah or 40Ah) except those converted to snags (described below).
Felling and yarding occurred over a period of 3-7 months in 1997 or 1998. Methods of yarding varied: helicopters were used on steep slopes (DP, B, and LW) and tracked shovel-loaders or rubber-tired skidders, on gentler terrain (WF and PH). Tree limbs were left attached to the bole to reduce slash accumulation. However, woody residues were deemed excessive at WF and were partially reduced by piling and burning (Halpern and McKenzie 2001). Treatment of non-merchantable stems (<18 cm dbh) varied among sites. Stems were retained at DP, B, and LW. In contrast, they were cut at PH and cut if damaged at WF, reducing the density of advance regeneration.
To meet requirements of the Northwest Forest Plan (USDA and USDI 1994, Tuchmann et al. 1996), snags felled during harvest were replaced, in part, by artificially created snags. A total of 6.5 live trees/ha—typically large, decadent or broken-topped Pseudotsuga menziesii—were retained in the harvested portion of each unit, then were topped (B and PH) or girdled (WF, DP, and LW).
To meet federal stocking requirements (312 stems/ha), 1- and 2-year-old bare-root seedlings (a mix of conifer species) were planted in the harvested portion of each unit in the spring or early summer after harvest. Planting densities were kept low (416–591 seedlings/ha) to minimize interactions with natural regeneration. We report on the growth and survival of planted trees in an earlier paper (Urgenson et al. 2013b), but not here.
Timing and methods of sampling
Pre-harvest measurements (1994-1996), were used to characterize initial forest structure and to establish the basal-area targets for dispersed-retention units. The first post-harvest measurement (year 1) was made in 1998 (B and PH) or 1999 (WF, DP, and LW); subsequent measurements were made in 2003 (year 5 or 6), 2009 (year 11 or 12), and 2016 (year 18 or 19).
Prior to harvest, we established a systematic grid of 63 or 64 points (40-m spacing) in each experimental unit. Grid points serve as the centers of nested plots used to sample regeneration (>0.1 m tall and <5 cm dbh, including advance regeneration and post-harvest establishment) and overstory trees (≥5 cm dbh). Regeneration plots were distributed at alternate (n = 32) grid points in dispersed treatments and at a subset of grid points in the harvested areas of aggregated treatments (n = 22 in 15Ah; n =12 in 40Ah). In each regeneration plot we tallied all conifer and hardwood species in four 1 × 6 m belt transects oriented along perpendicular radii, 4 to 10 m from the plot center.
Overstory plots (0.04 ha, 11.28 m radius) were distributed at alternate grid points in dispersed treatments (n = 32), at all grid points in the patches of aggregated treatments (n = 10 in 15Ap; n = 24-25 in 40Ap), and at the same grid points used to sample regeneration in the harvested areas of aggregated treatments (n = 22 in 15Ah; n =12 in 40Ah). In the first post-harvest year, all retained trees were identified to species, tagged, and measured for dbh (0.1 cm resolution). At each re-measurement we recorded dbh and ‘status’ (live or dead). For dead trees, we also recorded ‘position’ (i.e., standing intact, standing but broken, or down), allowing us to distinguish between wind and other causes of mortality (Urgenson et al. 2013a). At each re-measurement we tagged and recorded dbh of all ingrowth stems (i.e., trees attaining 5 cm dbh since the previous measurement).
Data aggregation and processing
Seral status and size class of trees. — We classified all conifers as early seral (ES) or late seral (LS) based on their shade tolerance or ability to regenerate under a closed canopy (Minore 1979, Burns and Honkala 1990). Hardwoods (uncommon at most sites) were not classified by seral status, but were included in any analyses that combined the responses of seral groups. In addition to seral status, retained trees (conifers and hardwoods) were grouped by size class: ‘small’ (<25 cm dbh; typically subordinate or subcanopy stems) and ‘large’ (≥25 cm dbh; typically dominants or co-dominants ranging to 171 cm dbh). This allowed us to compare growth and survival of trees of similar size and canopy position.
Response variables. — For each plot within a harvested area (D or Ah), we computed final regeneration and ingrowth density by seral group, and for trees in total. For each plot with surviving ingrowth, we also computed the mean individual growth rate (annualized basal area increment; cm2/yr) of each seral group and for trees in total. For each plot with retained trees (D or Ap), we computed the mean individual growth rate of surviving trees of each size class (small and large) and seral group. For each harvest unit, individual growth rates were summed and expressed on area basis (m2/ha/yr) to quantify (1) ingrowth productivity in the harvested area (D or Ah), (2) retained-tree productivity in areas with retention (D or Ap), and (3) ingrowth and retained-tree productivity for the harvest unit as a whole (thus accounting for the differences in harvested and patch area between 15A and 40A). Finally, for each harvest unit we computed survival of retained trees of each size class and seral group. Ingrowth survival was consistently high (92%) and was not analyzed further.
We computed two measures of variability among plots within harvest units: the coefficient of variation (CV) in density of regeneration and ingrowth (by seral group and total), and the ‘multivariate dispersion’ of plots (Anderson et al. 2006). Multivariate dispersion was computed as the mean Euclidean distance of plots to the harvest-unit centroid based on a principal coordinates analysis (PCoA) of a Bray-Curtis dissimilarity matrix of early- and late-seral regeneration and ingrowth density. Density values were natural-log transformed prior to generating the matrix. To facilitate inclusion of plots lacking regeneration and ingrowth, a pseudo-species with a minimum density was added to each sample. Multivariate dispersion was computed using the betadisper function in the vegan package in R (Oksanen et al. 2018).
Post-harvest covariates. — We also computed a set of structural metrics to capture initial post-harvest (year 1) variation unaccounted for by the nominal retention treatments. Post-harvest advance regeneration density—which varied widely among sites, harvest units, and plots—was used in models of final regeneration and ingrowth density. Post-harvest CVs and multivariate dispersion, measures of spatial variability in advance regeneration density and composition within harvest units, were used in models of final variability. Density and basal area of early- and late-seral species, which varied widely among replicates of the same retention treatment, were used as treatment-scales proxies of seed availability in models of regeneration density. Finally, total density and basal area, plot-scale proxies of local tree influence, were used in models of retained-tree growth and survival.
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See Readme.pdf for meta-data including variable definitions, units, and codes.
Pacific Northwest Research Station