Alan J. Rebertus

Structure and tree-fall gap dynamics of old-growth Nothofagus forests in Tierra del Fuego, Argentina

Tree size and age structure, tree-fall and gap char- acteristics, and regeneration in gaps were studied in Nothofagus-dominated old-growth forests in Tierra del Fuego, Argentina. Gap-phase regeneration has resulted in all-aged populations for N. pumilio, N. betuloides, and Drimys winteri, and regeneration in gaps appears to be maintaining coexist- ence between species in mixed stands. N. betuloides fills many gaps via advance regeneration and some individuals persist for > 150 yr in the understory. Multiple periods of release and suppression indicate that N. betuloides may take advantage of several gap events to reach the main canopy. Likewise, Drimys grows well under closed canopy and can rapidly respond to gap formation, sometimes impeding the regeneration of N. betuloides. In contrast, N. pumilio regen- erates in gaps mainly from seed or from advance regenera- tion of small, ephemeral seedlings. Gap turnover times in Fuegian forests were estimated at 300 - 500 yr, although gap formation was highly episodic and possibly associated with regionally extensive windstorms, earthquakes, and stand-level dieback. 92 % of gaps involved multiple tree-falls, and at least 53 % involved secondary expansion. Gap and tree-fall characteristics in Tierra del Fuego were similar to results from northern Patagonia, Chile, and New Zealand; however, we emphasize that regeneration of Nothofagus spp. and Drimys winteri in gaps depends on associated vegetation and varies along both local and regional environmental gradients.

BLOWDOWN HISTORY AND LANDSCAPE PATTERNS IN THE ANDES OF TIERRA DEL FUEGO, ARGENTINA

The effects of periodic gales on Nothofagus (southern beech) stand devel- opment and landscape dynamics were studied in a 10.4-km2 study area in the Sierra de las Pinturas, part of the Andes in Argentine Tierra del Fuego. We reconstructed blowdown history (patch sizes, dates of origin, return intervals) since the late 1700s to assess how periodic large-scale wind disturbance influenced landscape pattern in a relatively simple system. Most previous studies have focused on single storms in more diverse forests and in landscapes influenced by several disturbance types and human activities. Boundaries of post-blowdown stands were digitized from aerial photographs and analyzed within a geo- graphic information system. Ages of blowdowns and return intervals were determined from scars, growth releases, and maximum tree ages. Discrete blowdown patches (0.1 to >100 ha) covered two-thirds of the study area and ranged from 19 to -200 yr of age, with 20- 30 yr between major events. The meteorology of these storms was unknown, but we suspect they were caused by intense low-pressure systems originating in Antarctica. The mean return interval for 34 sites was 145 yr, with a range of 103-218 yr. Based on treefall size distributions, most stands blown over in the past 100 yr were even-aged, with mean tree dbh (diameter at breast height) between 20 and 32 cm. Aerial photographs taken in 1970 were used for before-and-after comparisons of a 1972 blowdown. Seventy-one percent of the area blown over in 1972 was even-aged, and 35% of the boundaries from this storm exactly coincided with previous stand boundaries. Damage patterns from storms in 1924 and 1972 suggested that forests located on valleys parallel to the wind, windward side slopes, and possibly upper leeward slopes were most vulnerable to blowdown, but few landscape positions escaped being hit by repeated storms. Return intervals were not sig- nificantly related to slope, elevation, or aspect; but surprisingly, shorter return intervals were associated with deeper soils. The landscape pattern of blowdown and recovery shifted over time because of variation among individual storms and because a small proportion of old-growth stands were converted to blowdowns and vice versa. Browsing by guanacos (Lama guanaco), a large native camelid, has severely inhibited tree regeneration during the past 75 yr in small blowdowns and the perimeters of larger ones, converting some stands to open meadows and incipient alpine communities. In the relatively simple Noth- ofagus forests of Tierra del Fuego, periodic gales are the main determinants of forest structure and pattern across a range of scales from small patches to entire landscapes.

RATES AND MECHANISMS OF SUBALPINE FOREST SUCCESSION ALONG AN ENVIRONMENTAL GRADIENT

This paper examines how rates and mechanisms of succession vary spatially and temporally in xeric, subalpine forests in Colorado, United States. We reconstructed 300 years of succession from limber pine (Pinus flexilis) to Engelmann spruce (Picea engelmanii) and subalpine fir (Abies lasiocarpa) in two watersheds recovering from the same fire. More than 1850 live and dead trees were cored and mapped in 25 plots systematically spaced along a topographic gradient. We used tree ring analysis to reconstruct dates of tree establishment and death. Relative species abundances and basal areas were charted at 20-yr intervals, and three measures of ecological similarity were also used to capture different elements of change: simple Euclidean distance, Euclidean distance in ordination space, and Horn’s CH similarity index. Successional rates for mesic lower slopes and north aspects were roughly twice those on south-facing side slopes. Rates were positively correlated with soil P, N, Fe, organic matter, and Parker’s t...

ICE STORM DAMAGE TO AN OLD-GROWTH OAK-HICKORY FOREST IN MISSOURI

-On 6 December 1994 a severe ice storm hit several midwestern states. We inventoried damage and woody debris input from the storm in 30 0.1-ha permanent plots in an old-growth oak-hickory forest in northern Missouri. Twenty-seven percent of 1386 live trees 22 cm dbh were damaged but only 7% were severely damaged. Basswood (Tilia americana) and American elm (Ulmus americana) were the most susceptible species; white oak (Quercus alba), eastern hophornbeam (Ostrya virginiana) and shagbark hickory (Carya ovata) were among the least susceptible. Damage levels increased with stem diameter, and trees occupying dominant crown positions were more heavily damaged than suppressed trees. Damage severity was highly variable across the landscape, but trees on lower slopes and mesic aspects tended to have more damage, even for a given species. Coarse woody debris input from the storm averaged 5.1 m3/ha, 27% of the prestorm volume, with northern red oak (Q. rubra) and white oak contributing 79% of the total. This single event will probably have minimal impact on stand composition and structure, but repeated ice storms at 20-yr intervals likely influence forest dynamics in midwest oak-hickory forests.

Tornado damage and initial recovery in three adjacent, lowland temperate forests in Missouri

. We surveyed tornado damage 14 months after disturbance in three adjacent forest sites in southeastern Missouri, USA: upland, an occasionally-flooded lowland and a frequently flooded swamp. We analysed three 0.09-ha plots in the swamp, and three 0.04-ha plots in the other sites and recorded the herbaceous-layer in five 1-m2 quadrats per plot. Overall, 30 % of the individuals and 64 % of the basal area of these sites was blown down. However, mortality was less than structural damage: 20 % of the trees were dead 14 months post-disturbance, an intermediate level of mortality relative to other wind disturbances. Damage varied widely among species, sites and tree sizes, but large trees of all species were most likely to be damaged and least likely to resprout. While damage was greatest in the wettest site, we attribute the increase to larger tree sizes and lower density there, rather than to inherent rooting instability. This study suggests that different forest composition and structure may be sufficient to cause differential damage, without topographic effects. Herbaceous-layer response was more rapid than in a previously reported tornado windthrow in Pennsylvania, and was dominated by fast-growing, shade-intolerant forbs.

Stand dynamics of Pinus flexilis‐dominated subalpine forests in the Colorado Front Range

. Population age structure and succession were investigated in subalpine forests in the Colorado Front Range dominated by Pinus flexilis (limber pine). Age, size, and spatial data were collected from three recent burns ( 400 yr old). The sequence of colonization of now extant trees on these post-fire sites appeared to be consistent: first Pinus flexilis, then Picea engelmannii (Engelmann spruce), and later Abies lasiocarpa (subalpine fir) with a delay between the first Pinus flexilis and Abies of as long as 140 yr. The advantage of Pinus flexilis on post-burn sites can be attributed to avian seed dispersal and the exceptional drought tolerance of its seedlings. The three recent burns were not extensive, and the delay in establishment of Picea and Abies appeared to be limited by harsh site conditions rather than lack of seed dispersal. Spatial analysis indicated a consistent, although sometimes weak, attraction between Pinus flexilis and Picea and Pinus flexilis and Abies at a scale of 1–4 m, suggesting that Pinus flexilis may facilitate establishment of Picea and Abies seedlings by providing shade or protection from wind. On xeric to slightly xeric sites, Pinus flexilis appeared to form broadly even-aged, non-regenerating populations that were gradually being replaced by Picea and Abies. Replacement is proceeding at a faster rate on the least xeric sites (north aspects, valley bottoms) compared to the most xeric sites (south aspects). On the most extreme sites, Pinus flexilis formed all-aged, self-maintaining populations with no evidence of replacement by Picea and Abies. In these old-growth forests with occasional trees aged at > 1300 yr, recruitment is continuous or episodic.

THE IMPORTANCE OF GAP PROCESSES IN THE DEVELOPMENT AND MAINTENANCE OF OAK SAVANNAS AND DRY FORESTS

1 We examined the dynamics of a Quercus-dominated dry forest-savanna complex in the Ozark Highlands of the central United States, focusing on regeneration patterns within gaps. The study area includes one of very few temperate Quercus ecosystems not degraded by fire suppression, logging or excessive grazing. 2 We compared patterns of tree seedling establishment, tree height growth and postfire survival in gaps between savanna, found only on south and west aspects in our study area, and dry forest, found only on north and east aspects, to determine which factors contributed to maintaining structural differences. 3 Canopy gaps constituted 42% of savanna area compared with 17% for dry forest. Mean canopy gap size in savanna was 316 m2 compared with 185 m2 for dry forest, but gap sizes were highly variable and the difference was not significant. Remnants of canopy trees were found in 37 of 38 gaps, indicating that gaps in both savanna and dry forest can usually support trees. 4 Savanna and dry forest gaps were generally favourable for seedlings and growth of small trees. Seedlings of most woody species were widespread in the understoreys of both savanna and dry forest but were more abundant in gaps than under closed canopies. Species had different height growth patterns depending on site, with growth rates ordered as follows: Quercus velutina in savanna > Q. velutina in forest > Q. stellata in forest > Q. stellata in savanna. Site differences were significant for Q. stellata but not for Q. velutina. 5 Tree survival in two surface fires was significantly lower in gaps in savanna compared with gaps in dry forest. Survival within individual gaps was inversely related to grass cover. In savanna, survival of small trees (2 5-7 cm d.b.h.) ranged from >80% for gaps with 90% grass cover. 6 Canopy gaps in the Ozarks are created primarily by windfall and drought-related dieback, but fire influences the timing and rate of gap infilling. Understanding the interactions among these disturbances may help clarify the dynamics of many temperate woodlands once dominated by surface fires.

RAPID ESTABLISHMENT OF FISH IN ISOLATED PEATLAND BEAVER PONDS

Previous research has demonstrated that beavers (Castor canadensis) dramatically alter fish habitat in streams by their dam-building activities. Although less well-known, beavers also flood closed peatlands by damming seepage rather than streamflow. Our study focuses on the establishment of fish communities in isolated beaver ponds created in small, ombrogenous peatlands lacking any open water prior to beaver occupation. We selected 16 ponds that ranged in age from 4 to 42 years and three unaltered peatlands to determine whether beaver promote the use of peatlands by fish and if the patterns of individual species were related to macrophyte density. Fish were present in ponds as young as four years old, and five of the six species of fish were present in ponds≤16 years old. Submersed macrophyte abundance explained 40% of the variation in fish diversity. Rapid colonization of fish in beaver-altered peatlands provides further evidence of how beavers increase the complexity of biological food webs in boreal regions. Moreover, this work provides an even greater understanding of the historical role of beavers as ecosystem engineers in peatland dominated landscapes.