P. L. Marks

Seed dispersal by white-tailed deer: implications for long-distance dispersal, invasion, and migration of plants in eastern North America

For many plant species in eastern North America, short observed seed dispersal distances (ranging up to a few tens of meters) fail to explain rapid rates of invasion and migration. This discrepancy points to a substantial gap in our knowledge of the mechanisms by which seeds are dispersed long distances. We investigated the potential for white-tailed deer (Odocoileus virginianus Zimm.), the dominant large herbivore in much of eastern North America, to disperse seeds via endozoochory. This is the first comprehensive study of seed dispersal by white-tailed deer, despite a vast body of research on other aspects of their ecology. More than 70 plant species germinated from deer feces collected over a 1-year period in central New York State, USA. Viable seeds included native and alien herbs, shrubs, and trees, including several invasive introduced species, from the full range of habitat types in the local flora. A mean of >30 seeds germinated per fecal pellet group, and seeds were dispersed during all months of the year. A wide variety of presumed dispersal modes were represented (endo- and exozoochory, wind, ballistic, ant, and unassisted). The majority were species with small-seeded fruits having no obvious adaptations for dispersal, underscoring the difficulty of inferring dispersal ability from diaspore morphology. Due to their broad diet, wide-ranging movements, and relatively long gut retention times, white-tailed deer have tremendous potential for effecting long-distance seed dispersal via ingestion and defecation. We conclude that white-tailed deer represent a significant and previously unappreciated vector of seed dispersal across the North American landscape, probably contributing an important long-distance component to the seed shadows of hundreds of plant species, and providing a mechanism to help explain rapid rates of plant migration.

AGRICULTURAL LEGACIES IN FOREST ENVIRONMENTS: TREE COMMUNITIES, SOIL PROPERTIES, AND LIGHT AVAILABILITY

Temperate deciduous forests across much of Europe and eastern North America reflect legacies of past land use, particularly in the diversity and composition of plant communities. Intense disturbances, such as clearing forests for agriculture, may cause persistent environmental changes that continue to shape vegetation patterns as landscapes recover. We assessed the long-term consequences of agriculture for environmental conditions in central New York forests, including tree community structure and composition, soil physical and chemical properties, and light availability. To isolate the effects of agriculture, we compared 20 adjacent pairs of forests that were never cleared for agriculture (primary forests) and forests that established 85-100 years ago on plowed fields (secondary forests). Tree communities in primary and secondary forests had similar stem density, though secondary forests had 14% greater basal area. Species composition differed dramatically between the two forest types, with primary forests dominated by Acer saccharum and Fagus grandifolia and secondary forests by Acer rubrum and Pinus strobus. Primary and secondary forests showed no consistent differences in soil physical properties or in the principal gradient of soil fertility associated with soil pH. Within stands, however, soil water content and pH were more variable in primary forests. Secondary forest soils had 15% less organic matter, 16% less total carbon, and 29% less extractable phosphorus in the top 10 cm than adjacent primary stands, though the ranges of the forest types mostly overlapped. Understory light availability in primary and secondary forests was similar. These results suggest that, within 100 years, post-agricultural stands have recovered conditions comparable to less disturbed forests in many attributes, including tree size and number, soil physical properties, soil chemical properties associated with pH, and understory light availability. The principal legacies of agriculture that remain in these forests are the reduced levels of soil organic matter, carbon, and phosphorus; the spatial homogenization of soil properties; and the altered species composition of the vegetation.

Five years of tree death in a Fagus-Magnolia forest, southeast Texas (USA)

SummaryDeath of trees (>4.5 cm dbh) and saplings (1.4 m tall–4.5 cm dbh) was monitored over 5 years in a Southern Mixed Hardwood Forest to determine causes of death, death rates, and to assess the nature and direction of forest change. Most trees died standing (77%), presumably the result of pathogen attack or adverse physiological condition; knockdown by other trees (10%) and wind breakage (11%) were other common causes of death. Frequency of wind break and knockdown varied with size, but standing death did not. Most trees (64%) and some saplings (26%) died outright (complete death of the genetic individual); the others had sprouts or living residual parts. For some species, survival of residual parts was high, and so resprouting or persistence may be important in population recruitment. For others, low survival of residual parts means that sprouting does not effectively prolong the life of an individual. Most populations were stable or declining slightly. Shade-intolerant species showed higher death rates in small size classes than did the more tolerant species, so in the absence of disturbance one might expect some change in species composition toward the putative climax dominants.

A CASE STUDY OF SUGAR MAPLE (ACER SACCHARUM) AS A FOREST SEEDLING BANK SPECIES

MARKS, P. L. AND S. GARDESCU (Section of Ecology & Systematics, Cornell University, Ithaca, NY 148532701, USA). A case study of sugar maple (Acer saccharum) as a forest seeding bank species. J. Torrey Bot. Soc. 125:287-296. 1998.-To examine the existence and behavior of a seedling bank, seedlings of sugar maple were censused for 24 years in a mixed-hardwood forest in central New York State. Each year we measured the heights of all sugar maple seedlings (individually-marked) in permanent plots. The population of seedlings persisted through time, with little net upward growth, in the shade. This seedling bank was maintained both by recruitment of new seedlings and by low annual mortality, with some sugar maple seedlings surviving > 30 years at < 1 m height. In an experimental canopy gap, long-suppressed sugar maple seedlings responded with substantial height growth. Thus, sugar maple exhibited the three defining characteristics for a seedling bank: a narrow range of heights in the shade, persistence in the shade, and retention of the capacity to grow rapidly upward once light availability increased.

Two hundred years of forest cover changes in Tompkins County, New York'

SMITH, B. E., P. L. MARKS AND S. GARDESCU (Section of Ecology & Systematics, Cornell University, Ithaca, NY 14853-2701). Two hundred years of forest cover changes in Tompkins County, New York. Bull. Torrey Bot. Club 120: 229-247. 1993.-The amount of land in forest within Tompkins County from the time of European settlement (1790) to the present was determined from land survey records, aerial photographs, and field reconnaissance. Forest cover in Tompkins County dropped from almost 100% in 1790 to 19% by 1900, then increased to 28% by 1938 and over 50% in 1980. Thus over half of the forest in Tompkins County today is post-agricultural. The number, size, shape, and distribution of forest stands within the landscape changed during a century of conversion of agricultural to forested lands. From 1900 to 1980, there was more forest in the southern part of the county, where the topography is hillier and soils are more acidic. For a portion of the county with extensive clearing for agriculture, the Ludlowville quadrangle, we mapped the outline of all forest stands present in 1900, 1938, and 1980. Post-agricultural forest developed predominantly on the steeper lakeside and streamside slopes rather than on the flatter uplands. Throughout the period 1900-1980 the majority of the forest stands were quite small, <10 ha in area. The distance from random points in forest to the nearest edge of the stand was often less than 50 m, except for some extensive stands on the slopes in 1980. The development of forest on former agricultural lands has resulted in the coalescence of stands, and the degree of fragmentation and isolation of forest stands in the Ludlowville quadrangle of Tompkins County is much reduced today compared to 1900.

Seed dispersal and seedling emergence in an old field community in central New York (USA)

SummarySeed dispersal and seedling emergence of common taxa growing in a Solidago-dominated old field in central New York (USA) were monitored from May 1982 to June 1984. Over 3.5x104 seeds per m2 were captured on seed traps in each of the two years, with peaks occuring in July (due to Hieracium) and in November (due to Solidago). About 4.0x103 seedling/m2 emerged beneath the intact community in each of the two years. Although seedlings emerged predominantly in the early spring, a secondary peak occurred in September and October when many seedlings of introduced grasses appeared. Two additional aspects of the reproductive biology of the major taxa were related to the seasonal timing of seed dispersal: As the date of peak seed dispersal (among taxa) became progressively later in the season, (1) the duration of dispersal increased from about one week to about one-half year, and (2) the delay between the peak of seed dispersal and the peak of seedling emergence increased from a few days to about one-half year.

Colonization of old fields by trees vs. shrubs: seed dispersal and seedling establishment1

A. rubrum, 0.7 for F. americana, vs. 0 to 0.02 for the shrubs). For the trees, but not the shrubs, both seed input and seedling emergence in the fields showed a strong relationship to seed source abundance along the field edges, using index values based on the distances from potential source plants. Results of seed augmentation experiments showed that seedling emergence was strongly limited by seed input, but secondarily by environment; more C. racemosa emerged in moister plots and more F. americana and V. dentatum under short vegetation. Once established, seedlings of the shrubs often survived better and grew faster than did the tree seedlings. As a result of net overall differences in seedling emergence and survival, the number of seeds required to get one surviving fifth-year seedling was lowest for the shrubs (20 to 35 seeds), somewhat higher for F. americana (55), and highest for A. rubrum (326).

WINDSTORM DAMAGE AND AGE STRUCTURE IN AN OLD GROWTH FOREST IN CENTRAL NEW YORK

A 1989 storm damaged 35% of the trees and 47% of the basal area in a 3 hectare plot in Smith Woods. Larger trees were more likely to be damaged. More hemlock and beech had broken stems than were uprooted; sugar maple and tulip trees were primarily uprooted. Hemlock ages ranged up to >300 years, whereas the sugar maples were <140 years. Two cohorts of tulip trees were apparent, indicating episodic recruitment following disturbance. Overstory dynamics in forests involve the replacement of existing trees with others. It is understood that recruitment of new stems into the canopy requires a gap, even for shade tolerant species (e.g., Canham and Marks 1985, Schupp et al. 1989). In many north temperate forests, in regions where fire is uncommon, wind is a major cause of large canopy

The vascular plant diversity of the Finger Lakes region of central New York State: changes in the 1800s and 1900s1

Abstract This study describes the principal changes in the flora of the Cayuga Region over the past 200 years. The regions flora has been thoroughly documented by botanists at different times but the changes have not before been synthesized. The region studied is approximately 10,000 km2 surrounding Ithaca, NY, USA. The plant diversity of the Cayuga Region of New York has increased substantially over the past two centuries, as the gains of non-native plant species have far outnumbered the losses of natives. Of 1,265 vascular plant species native to the region, only 43 have been lost, despite large changes in land use and habitat areas. None of the species losses represent global extinction. There are no clear correlates or predictors for the species that disappeared. They were not all rare, nor were they predominantly in one kind of habitat, or from particular plant families. The majority of the 777 non-native plant species in the extant flora are rare or scarce within the Cayuga Region, and only a small number are of concern as aggressive invaders. Most of the non-natives are Old World herbaceous species of open, unforested habitats.