The temporal development of old-growth structural attributes in second-growth stands: a chronosequence study in the Coastal Western Hemlock zone in British Columbia
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Abstract:
One of the key issues facing forest resource planners is the conservation and recruitment of old-growth characteristics in managed forests. The paucity of long-term data sets in many regions has limited our ability to project the temporal patterns of structural development in second-growth forests. Age-based thresholds have been employed in some jurisdictions, but these lack flexibility and are arbitrary in nature. Here we conduct a chronosequence study consisting of second-growth and old-growth stands in the coastal forests of Vancouver Island, British Columbia, to identify structural attributes that are suitable for quantifying and monitoring the progressive development of old-growth characteristics. The following structural attributes were identified and evaluated in the chronosequence analysis: volume and density of large live stems, standard deviation of stem DBH, density of large-diameter snags, volume of woody debris, and understory vegetation cover. The rate at which old-growth structural characteristics developed in second-growth stands varied considerably, with the earliest reaching levels observed in old-growth stands within 112 years, while most requiring 200 to greater than 250 years. The use of quantifiable measures of old-growth structure will help forest managers plan for the continued protection and recruitment of old-growth structure within managed forest landscapes.Keywords:
Chronosequence
Understory
Coarse Woody Debris
Old-growth forest
Stand development
Silviculture
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THE INFLUENCE OF SUCCESSIONAL PROCESSES AND DISTURBANCE ON THE STRUCTURE OF TSUGA CANADENSIS FORESTS
Old‐growth forests are valuable sources of ecological, conservation, and management information, yet these ecosystems have received little study in New England, due in large part to their regional scarcity. To increase our understanding of the structures and processes common in these rare forests, we studied the abundance of downed coarse woody debris (CWD) and snags and live‐tree size‐class distributions in 16 old‐growth hemlock forests in western Massachusetts. Old‐growth stands were compared with eight adjacent second‐growth hemlock forests to gain a better understanding of the structural differences between these two classes of forests resulting from contrasting histories. In addition, we used stand‐level dendroecological reconstructions to investigate the linkages between disturbance history and old‐growth forest structure using an information–theoretic model selection framework. Old‐growth stands exhibit a much higher degree of structural complexity than second‐growth forests. In particular, old‐growth stands had larger overstory trees and greater volumes of downed coarse woody debris (135.2 vs. 33.2 m 3 /ha) and snags (21.2 vs. 10.7 m 3 /ha). Second‐growth stands were characterized by either skewed unimodal or reverse‐J shaped diameter distributions, while old‐growth forests contained bell‐shaped, skewed unimodal, rotated sigmoid, and reverse J‐shaped distributions. The variation in structural attributes among old‐growth stands, particularly the abundance of downed CWD, was closely related to disturbance history. In particular, old‐growth stands experiencing moderate levels of canopy disturbance during the last century (1930s and 1980s) had greater accumulations of CWD, highlighting the importance of gap‐scale disturbances in shaping the long‐term development and structural characteristics of old‐growth forests. These findings are important for the development of natural disturbance‐based silvicultural systems that may be used to restore important forest characteristics lacking in New England second‐growth stands by integrating structural legacies of disturbance (e.g., downed CWD) and resultant tree‐size distribution patterns. This silvicultural approach would emulate the often episodic nature of CWD recruitment within old‐growth forests.
Coarse Woody Debris
Tsuga
Old-growth forest
Understory
Stand development
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Coarse Woody Debris
Temperate rainforest
Old-growth forest
Basal area
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Coarse Woody Debris
Basal area
Temperate rainforest
Dead tree
Dead Wood
Old-growth forest
Temperate forest
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We examined the structural attributes of coarse woody debris (CWD) in balsam fir and mixed balsam fir-black spruce forests in western and central Newfoundland. The purpose of the study was to document CWD abundance and structure within various stages of stand development, and to generate simple models to predict CWD availability as habitat for forest wildlife. Our objectives were threefold: (1) determine the temporal patterns of CWD following clear-cut disturbance, and compare these second-growth stands to old sites (>80 yr) of natural origin; (2) relate the temporal patterns of CWD to stand development; and (3) demonstrate how factors such as site conditions and disturbance may influence these temporal patterns. Our chronosequence included 19 second-growth stands with a mean tree age ranging from 33 to 80 yr, and 7 old-growth stands ranging from 87 to 110 yr. The volume of coarse woody debris (CWD), here defined as downed wood, was relatively low to intermediate early in the chronosequence (e.g., 32.3 m3/ha in a 36-yr-old stand), lowest in a 58-yr-old stand (15.2 m3/ha), and highest in an 80-yr-old stand (78.1 m3/ha). Results indicated that CWD volume followed the general "U-shaped" temporal trend observed in other forest systems. The presence of CWD early in the sequence was strongly influenced by residual hardwoods (i.e., birch) left by the original logging operation. The highest observed volume of CWD corresponded with stand senescence and appeared to be maintained within old-growth stands. Snag (standing dead wood) density was low within the youngest stands (<200 snags/ha), and peaked within mature second growth. Defoliation disturbance increased both the volume and the structural diversity of CWD within silviculturally mature second-growth stands. Results from our study indicate that CWD in stands >50–60 yr of age is not residual but generated primarily from regenerating tree structure. Factors affecting the rate of stem growth, e.g., site quality and initial stocking levels, should influence the accumulation and overall abundance of CWD within later stages of forest development. We therefore applied yield-density relationships as a method of examining stand-level dynamics of CWD.
Coarse Woody Debris
Age structure
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Secondary forest
Old-growth forest
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Coarse Woody Debris
Old-growth forest
Secondary forest
Temperate rainforest
Clearcutting
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We characterized the structure of 25 old—growth hemlock—hardwood forests in northern Wisconsin and adjacent Michigan in order to examine our working hypotheses that differences in their structure are related to stand age (i.e., stage of development) and that changes in stand structure continue after old—growth status is achieved. Estimates of stand age, i.e., age of oldest tree cored, based on 10 cores taken from hemlocks of a range of diameters in each stand, ranged from 177 to 374. By investigating the patterns of live tree structure, coarse woody debris (CWD), tip—up mounds, and canopy gaps in relation to stand age, we were able to infer changes that occur during stand development. Along the gradient of old—growth stand development, some changes in structural features, including total volume of CWD, snag (Standing dead tree) basal area, and total area and average size of canopy gaps were continuous, linear increases over time. In contrast, changes in live tree and snag density, density of large trees, volume of well—decayed hemlock logs, and diameter—age relationships occurred after a threshold stand age of 275—300 yr was reached. Area and density of tip—up mounds and density of large seedlings and saplings were not correlated with stand age. Old—growth hemlock—hardwood stands at the upper end of the age continuum (>275—300 yr) have accumulated both gradual and threshold structural changes, acquiring most of the following characteristics: (1) a strong correlation between age and diameter of trees, (2) low densities of live trees distributed across all size classes, (3) trees >70 cm dbh (diameter at breast height), (4) dead wood >120—150 m 3 /ha, with logs >80 m 3 /ha, (5) hemlock logs present in all decay classes, and (6) canopy gaps occupying >10% of the stand, with the average gap size >50 m 2 , some gaps >200 m 2 , and no more than 30% of the gaps <10 m 2 . The two oldest stands may be at or near equilibrium, where maximum volumes of CWD are maintained through a balance of tree growth and mortality, and wood decay; and oldest tree age approaches 400+ yr, the maximum longevity typical for hemlock.
Coarse Woody Debris
Stand development
Basal area
Yellow birch
Age structure
Old-growth forest
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Citations (259)
We examined the structural attributes of coarse woody debris (CWD) in balsam fir and mixed balsam fir-black spruce forests in western and central Newfoundland. The purpose of the study was to document CWD abundance and structure within various stages of stand development, and to generate simple models to predict CWD availability as habitat for forest wildlife. Our objectives were threefold: (1) determine the temporal patterns of CWD following clear-cut disturbance, and compare these second-growth stands to old sites (>80 yr) of natural origin; (2) relate the temporal patterns of CWD to stand development; and (3) demonstrate how factors such as site conditions and disturbance may influence these temporal patterns. Our chronosequence included 19 second-growth stands with a mean tree age ranging from 33 to 80 yr, and 7 old-growth stands ranging from 87 to 110 yr. The volume of coarse woody debris (CWD), here defined as downed wood, was relatively low to intermediate early in the chronosequence (e.g., 32.3 m3/ha in a 36-yr-old stand), lowest in a 58-yr-old stand (15.2 m3/ha), and highest in an 80-yr-old stand (78.1 m3/ha). Results indicated that CWD volume followed the general “U-shaped” temporal trend observed in other forest systems. The presence of CWD early in the sequence was strongly influenced by residual hardwoods (i.e., birch) left by the original logging operation. The highest observed volume of CWD corresponded with stand senescence and appeared to be maintained within old-growth stands. Snag (standing dead wood) density was low within the youngest stands (<200 snags/ha), and peaked within mature second growth. Defoliation disturbance increased both the volume and the structural diversity of CWD within silviculturally mature second-growth stands. Results from our study indicate that CWD in stands >50–60 yr of age is not residual but generated primarily from regenerating tree structure. Factors affecting the rate of stem growth, e.g., site quality and initial stocking levels, should influence the accumulation and overall abundance of CWD within later stages of forest development. We therefore applied yield-density relationships as a method of examining stand-level dynamics of CWD.
Chronosequence
Coarse Woody Debris
Stand development
Black spruce
Silviculture
Old-growth forest
Salvage logging
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Citations (309)
We examined the effects of management on coarse woody debris, both standing and downed, in thinned and unthinned northern hardwood forests in upper Michigan. The unthinned conditions included old growth and second growth, while the thinned conditions included both even- and uneven-aged management. The structural features analyzed were stem diameter, density, basal area, and height of snags and live trees, as well as volume, diameter, and decay state of downed woody debris (DWD). As measured by these features, the relative structural complexity among the forest conditions was generally old growth > uneven-aged > second growth ~ even-aged. Although snag density was highest in second-growth forests, old growth had the highest snag basal area. Old growth also had the largest volume of DWD, second growth and even-aged had the least, and uneven-aged had an intermediate value. Unlike old growth, other treatments lacked large diameter (> 40 cm) snags and DWD. If prescriptions are changed to allow for the creation of larger snags and DWD, particularly those > 60 cm in diameter, stands can be managed to more closely resemble these structural aspects of old-growth forests.
Coarse Woody Debris
Basal area
Old-growth forest
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Chronosequence
Coarse Woody Debris
Black spruce
Fragmentation
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