DOI: 10.5061/dryad.0gb5mkm02
title: Level and spatial pattern of overstory retention impose tradeoffs for regenerating and retained trees

description: 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
 (&gt;0.1 m tall, &lt;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
 (&lt;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.
description: <strong>Experimental design and treatment
 implementation</strong> 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,
 A<sub>p</sub>). 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 (&gt;18 cm at
 diameter breast height, dbh) were felled in the adjacent harvested area
 (15A<sub>h</sub> or 40A<sub>h</sub>) 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 (&lt;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 <i>Pseudotsuga menziesii</i>—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.
 <strong>Timing and methods of
 sampling</strong> 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 (&gt;0.1 m
 tall and &lt;5 cm dbh, including advance regeneration and post-harvest
 establishment) and overstory trees (≥5 cm dbh). Regeneration plots were
 distributed at alternate (<i>n</i> = 32) grid points in
 dispersed treatments and at a subset of grid points in the harvested areas
 of aggregated treatments (<i>n</i> = 22 in
 15A<sub>h</sub>; <i>n</i> =12 in
 40A<sub>h</sub>). 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 (<i>n</i> = 32),
 at all grid points in the patches of aggregated treatments
 (<i>n</i> = 10 in 15A<sub>p</sub>;
 <i>n</i> = 24-25 in 40A<sub>p</sub>), and at the
 same grid points used to sample regeneration in the harvested areas of
 aggregated treatments (<i>n</i> = 22 in
 15A<sub>h</sub>; <i>n</i> =12 in
 40A<sub>h</sub>). 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).
 <strong>Data aggregation and
 processing</strong> <i>Seral status and
 size class of trees</i>. — 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’ (&lt;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. <i>Response variables</i>. — For
 each plot within a harvested area (D or A<sub>h</sub>), 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;
 cm<sup>2</sup>/yr) of each seral group and for trees in total.
 For each plot with retained trees (D or A<sub>p</sub>), 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
 (m<sup>2</sup>/ha/yr) to quantify (1) ingrowth productivity in
 the <i>harvested area</i> (D or A<sub>h</sub>),
 (2) retained-tree productivity in <i>areas with retention</i>
 (D or A<sub>p</sub>), and (3) ingrowth and retained-tree
 productivity for the <i>harvest unit as a whole</i> (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
 <i>betadisper</i> function in the <i>vegan</i> package in R (Oksanen et al. 2018). <i>Post-harvest covariates</i>. — We also computed a set of structural metrics to capture initial post-harvest (year 1) variation unaccounted for by the nominal retention treatments. <i>Post-harvest advance regeneration density</i>—which varied widely among sites, harvest units, and plots—was used in models of final regeneration and ingrowth density. <i>Post-harvest CVs and multivariate dispersion</i>, measures of spatial variability in advance regeneration density and composition within harvest units, were used in models of final variability. <i>Density and basal area of early- and late-seral species</i>, 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, <i>total density and basal area</i>, plot-scale proxies of local tree influence, were used in models of retained-tree growth and survival. <strong>Literature cited</strong> Anderson, M. J., K. E. Ellingsen, and B. H. McArdle. 2006. Multivariate dispersion as a measure of beta diversity. Ecology Letters 9:683– 693. Aubry, K. B., M. P. Amaranthus, C. B. Halpern, J. D. White, B. L. Woodard, C. E. Peterson, C. A. Lagoudakis, and A. J. Horton. 1999. Evaluating the effects of varying levels and patterns of green-tree retention: Experimental design of the DEMO Study. Northwest Science 73 (Special Issue):12–26. Aubry, K. B., and C. B. Halpern. 2020. The Demonstration of Ecosystem Management Options (DEMO) Study, a long term-experiment in variable-retention harvests: rationale, experimental and sampling designs, treatment implementation, response variables, and data accessibility. General Technical Report PNW-GTR-978. USDA Forest Service, Portland, Oregon, USA. Burns, R. M., and B. H. Honkala, technical coordinators. 1990. Silvics of North America: 1. Conifers. Agriculture Handbook 654. USDA Forest Service, Washington, D.C., USA. Halpern, C. B., and D. McKenzie. 2001. Disturbance and post-harvest ground conditions in a structural retention experiment. Forest Ecology and Management 154:215–225. Minore, D. 1979. Comparative autecological characteristics of northwestern tree species - A literature review. USDA Forest Service General Technical Report PNW-GTR-087. Oksanen, J., F. G. Blanchet, M. Friendly, R. Kindt, P. Legendre, D. McGlinn, P. R. Minchin, R. B. O’Hara, G. L. Simpson, P. Solymos, M. H. H. Stevens, E. Szoecs, and H. Wagner. 2018. Vegan: Community ecology package. R package version 2.5-2. https://cran.r‑project.org/web/packages/vegan/vegan.pdf Tuchmann, E. T., K. P. Connaughton, L. E. Freedman, and C. B. Moriwaki. 1996. The Northwest Forest Plan: A Report to the President and Congress. USDA Forest Service, Portland, Oregon, USA. Urgenson, L. S., C. B. Halpern, and P. D. Anderson. 2013a. Level and pattern of overstory retention influence rates and forms of tree mortality in mature, coniferous forests of the Pacific Northwest, USA. Forest Ecology and Management 308:116–127. Urgenson, L. S., C. B. Halpern, and P. D. Anderson. 2013b. Twelve-year responses of planted and naturally regenerating conifers to variable-retention harvest in the Pacific Northwest, USA. Canadian Journal of Forest Research 43:46–55. USDA, and USDI. 1994. Record of Decision for Amendments to U.S. Forest Service and Bureau of Land Management Planning Documents Within the Range of the Northern Spotted Owl. USDA Forest Service, Portland, Oregon, USA.
description: See Readme.pdf for meta-data including variable definitions,
 units, and codes.