Forest Resources Dissertations Collection

Fire Regime Dynamics Following the Mid-Holocene Hemlock Decline in Eastern North America

Clark, Kennedy Helm — Thu, 23 Sep 2010 11:02:41 PDT

Approximately 5,000 years ago, eastern hemlock (Tsuga canadensis) experienced a sudden, rapid, range-wide decline most probably due to pest, disease, or climate change. An aphid-like defoliating insect, the hemlock woolly adelgid (Adelges tsugae), recently (1950’s) introduced to eastern North America has been spreading across the eastern United States. The adelgid attacks all size and age classes of hemlocks causing up to 95% mortality in affected stands. The potential for another range-wide hemlock decline has raised a number of concerns including the increased threat of wildfire. Altered fuel loadings in modern adelgid-affected stands and the effects of presumably similar changes in fuels and subsequently altered fire regimes following the prehistoric decline are examined. Fuels data from an adelgid-infested stand in Connecticut and an uninfested stand in Massachusetts were used to generate custom fuel models and predict fire behavior in each stand. Sediment cores were extracted from three sites in western Massachusetts and analyzed for fossil pollen and charcoal around the period of the prehistoric decline. Fossil data from two previously studied sediment cores from coastal Maine are included in the analysis. Results demonstrate a clear and highly significant increase in both fuel loadings and predicted fire behavior in the modern, adelgid-affected stand. Three of the coring sites reflect distinct, significant, short-lived increases in charcoal associated with the prehistoric decline; two do not. Results from the first three sites suggesting increased fire activity also were associated with changes in vegetation which indicate disturbance. Increased fire activity after the decline seems most pronounced in areas where fire was common before the decline. Results indicate that fire was not universally a significant factor driving post-decline succession. Research across a broader geographic area is needed to clarify the relationship between fire and hemlock following the mid-Holocene decline, but the results presented here suggest that managers of modern stands affected by the adelgid should include the possibility of intense fires as a threat to landscapes heavily affected by hemlock decline.

Fire regime dynamics following the mid-Holocene hemlock decline in eastern North America

Clark, Kennedy H — Wed, 25 Aug 2010 11:52:23 PDT

Approximately 5,000 years ago, eastern hemlock ( Tsuga canadensis) experienced a sudden, rapid, range-wide decline most probably due to pest, disease, or climate change. An aphid-like defoliating insect, the hemlock woolly adelgid (Adelges tsugae), recently (1950’s) introduced to eastern North America has been spreading across the eastern United States. The adelgid attacks all size and age classes of hemlocks causing up to 95% mortality in affected stands. The potential for another range-wide hemlock decline has raised a number of concerns including the increased threat of wildfire.^ Altered fuel loadings in modern adelgid-affected stands and the effects of presumably similar changes in fuels and subsequently altered fire regimes following the prehistoric decline are examined. Fuels data from an adelgid-infested stand in Connecticut and an uninfested stand in Massachusetts were used to generate custom fuel models and predict fire behavior in each stand. Sediment cores were extracted from three sites in western Massachusetts and analyzed for fossil pollen and charcoal around the period of the prehistoric decline. Fossil data from two previously studied sediment cores from coastal Maine are included in the analysis.^ Results demonstrate a clear and highly significant increase in both fuel loadings and predicted fire behavior in the modern, adelgid-affected stand. Three of the coring sites reflect distinct, significant, short-lived increases in charcoal associated with the prehistoric decline; two do not. Results from the first three sites suggesting increased fire activity also were associated with changes in vegetation which indicate disturbance. Increased fire activity after the decline seems most pronounced in areas where fire was common before the decline. Results indicate that fire was not universally a significant factor driving post-decline succession. Research across a broader geographic area is needed to clarify the relationship between fire and hemlock following the mid-Holocene decline, but the results presented here suggest that managers of modern stands affected by the adelgid should include the possibility of intense fires as a threat to landscapes heavily affected by hemlock decline. ^

Politics and policy processes in federal urban forest policy formation and change

Ricard, Robert Marshall — Tue, 18 Aug 2009 14:52:40 PDT

This study examined the politics and policy processes that led to the formation and change of federal urban forest policy in the United States. Using the Three Streams Framework and Punctuated Equilibrium Model of policy processes for the analysis, findings suggest that federal urban forest policy began as a “top-down” policy process originating from presidential attention to New Conservation issues. In the 1960s a “wave of enthusiasm” formed for government solutions to environmental problems. Policy entrepreneurs in the federal bureaucracy recognized this opportunity and acted to use it for organizational advantage. ^ Federal urban forest policy forms due to bureaucratic response to presidential attention and policy changes due to the United States Forest Service quest for legislator attention, new resources, and to cultivate an urban constituency. The agency accomplished this in cooperation with two principal and historically allied forestry interest groups, the American Forestry Association and state foresters. A key element was the rise of skillful policy entrepreneurs advancing policy. Political advocates were also present in Congress. Important too was that several urban forestry policy entrepreneurs cross back and forth between these groups. Another key element was not so much the absence of organized opposition, but the control of it. Opposition to federal urban forest policy existed among members of the policy monopoly due to distribution politics. However, opposition ceased when federal cooperative forestry law changed in 1978 permitting distribution of federal urban forestry funds to and by state foresters. ^ Policy stabilizes due to the formation of a policy monopoly resulting from an advocacy coalition who controlled membership. Policy becomes unstable due to the introduction of new, competing actors into the policy monopoly and due to issue ambiguity affecting the urban forest policy image. By 1989 presidential attention to the causal story of urban reforestation (tree planting) emerges again as it did in the 1960s. This causes the coupling of the three policy process streams resulting in a punctuated moment in federal urban forest policy. This results in increased political attention and the acquisition of scarce resources for the Forest Service and distribution of funds to partners in the advocacy coalition. ^ Keywords: Advocacy coalition; American governance; cooperative forestry; distributive politics; Forest Service; interorganizational relationships; policy entrepreneurs; policy monopoly; policy stability; political advocates; political behavior; urban forest history; urban forest policy.^

Landscape Ecology of Large Fires in Southwestern Forests, USA

Haire, Sandra L. — Tue, 21 Jul 2009 07:50:58 PDT

The recent increase in large fires in southwestern forests has prompted concern regarding their ecological consequences. Recognizing the importance of spatial patterns in influencing successional processes, I asked: (1) How do large fires change plant communities?; (2) What are the implications of these changes for ponderosa pine forests?; and (3) What is the relationship of fire severity to gradients of climate, fuels, and topography? To address the first two questions, I studied succession in the woody plant community at two sites that burned in high-severity fire: La Mesa fire in northern New Mexico (1977) and Saddle Mountain in northern Arizona (1960). After large fires, abiotic conditions, associated prefire plant distributions, and spatial patterns of burning interacted to result in particular successional outcomes. Variation in abundance and diversity of species that spread from a refuge of seed sources remaining after the fire followed the model of wave-form succession. I investigated the implications of large fires for ponderosa pine by examining the influence of spatial patterns of burning on regeneration. Tree density corresponded most closely with particular scales of seed dispersal kernel and neighborhood severity metrics. Spatial patterns of burning remained influential even after consideration of variables describing subsequent burning and the physical and biotic environment. Age structure of young forests indicated that populations spread in a moving front and by long-distance dispersal. To explore the relationship between fire severity and climate, I investigated how the spatial heterogeneity of high-severity patches varied among 20 fires across gradients in fire size and climate. The largest fires generally occurred during cool dry La Niña climates, however, several fires deviated from this trend. Some spatial properties of severity did not correspond to fire size or to changes in climate. Characteristics of fuels and topography altered spatial patterns of severity, but interactions with extreme burning conditions may have disrupted these local influences in both La Niña and El Niño fires. Spatial patterns of fire severity are central to understanding ecological dynamics following large fires in southwestern forests. Moreover, simplistic assumptions regarding the relation of fire severity to fire size and climate should be viewed with caution.

The effects of multiple resources on forest regeneration: Microsite variation and seedling response

McKenna, John — Thu, 19 Jul 2007 17:32:28 PDT

Forest gaps play a major role in forest dynamics because increased resource availability favors plant species that differ from those in undisturbed forest. This dissertation investigates the spatial heterogeneity of resource availability in gaps, how it varies by site, and how it affects regeneration. Gap effects on resource availability and seedling growth were studied at three sites, which differed in drainage, using large gaps (108 m × 30 m) where light ranged from understory shade to full sun. Light levels in plots were measured using hemispherical photos; soil moisture, with a capacitance probe; soil temperature, with a portable thermocouple; and nitrogen, with buried-bag incubations. ^ The strong asymmetric, light-induced temperature gradient drove nitrogen mineralization rates. Patterns of mineralization were modified by variation in soil moisture within sites and increased with the average moisture levels among sites. Nitrogen availability was highest in the center of gaps, while root uptake reduced nitrogen availability at the north edge. Contrary to predictions, relative height growth of Betula lenta growing in the north center of gaps was greater than that of B. papyrifera. For both species, the difference between understory and gap growth increased from least to most fertile sites. ^ To separate the effects of resources on plant- and leaf-level responses in the two species, a greenhouse experiment was conducted with high and low levels of light, nitrogen, and water. High nitrogen increased height growth even in shade. Nitrogen also increased leaf area through greater leaf initiation and retention. High rates of growth in gaps is caused by nitrogen effects on leaf display and light effects on realized carbon gain. ^ This research shows that increased nitrogen availability only occurs if gaps are large enough to increase soil temperatures that drive mineralization and to overcome the effects of root uptake near gap edges. It also highlights how the effects of canopy openings varies by site and how this can influence the regeneration of gap-dependent species.^

Structural attributes, disturbance dynamics, and ecosystem properties of old-growth forests in western Massachusetts

D'Amato, Anthony William — Thu, 19 Jul 2007 17:32:08 PDT

Natural disturbance patterns, forest structural attributes, patterns of nitrogen availability, and the abundance and composition of understory vegetation were studied in eighteen old-growth stands in western Massachusetts. Dendroecological analyses indicated that disturbance regimes for these systems were dominated by relatively frequent, low intensity disturbances (average of 5.0% canopy area disturbed per decade) operating randomly on the landscape. Comparisons of dendroecological patterns with model simulations of past hurricane events and historical documents suggested that broad-scale disturbances, such as hurricanes and ice storms, resulted in common disturbance peaks in the 1790s, 1820s, and 1920s at several study areas separated by over 50 km. No stand-replacing disturbances were detected at any old-growth area during the period examined (1700-1989). ^ Comparisons of structural characteristics in old-growth and second-growth hemlock forests illustrated that old-growth stands exhibit a higher degree of structural complexity compared to second-growth hemlock forests. In particular, old-growth stands had larger overstory trees and a greater abundance of downed coarse woody debris (135.2 versus 33.2 m³/ha) and snags (21.2 versus 10.7 m³/ha) compared to second-growth systems. The range in variation of structural attributes within my old-growth study areas was similar to those in other old-growth eastern hemlock forests located on more moderate terrain in the Upper Midwest and New York. This range in variation was related to differences in disturbance history and site productivity among old-growth stands. ^ Soil measurements indicated that there were no detectable differences in soil characteristics, such as total C and N, between old-growth and second-growth hemlock stands; however, inorganic N (NO₃-N and NH₄-N) availability was much greater in old-growth stands. In contrast, differences existed in soil characteristics and N availability between old-growth hemlock and old-growth northern hardwood forests, with hardwood dominated systems exhibiting lower forest floor C:N ratios and greater amounts of inorganic N. ^ Old-growth hemlock stands had higher species richness and diversity, as well as a greater abundance of understory herbs and shrubs, and tree seedlings and saplings compared to second-growth forests. In addition, several common understory plants, including Aralia nudicaulis, Dryopteris intermedia , and Viburnum alnifolia, were more abundant in old-growth stands.^