Invasion of an Old-Growth Forest in New York by Ailanthus altissima: Sapling Growth and Recruitment in Canopy Gaps

1. Invasion of an Old-Growth Forest in New York by Ailanthus altissima: Sapling Growth and Recruitment in Canopy Gaps Jason Taylor

2. The Invasion of Exotics Some 4000 exotic plants are known to grow wild in North America (Stein and Flack 1996). The majority of U.S. National Park managers polled in 1996 identified invasive non-indigenous plant species as a problem within their parks. The United States economy has lost more than $97 billion either from species control or the cost of their existence.

3. Not All Exotics are Bad Only a small fraction of imported species become established and lesser still become pests (Crawley 1987; Vitousek 1990; Lodge 1993; Bright 1995). No reliable predictability���..

4. Ailanthus altissima (Tree of Heaven, Weed Tree, Tree from HELL!!!!!!!!!!!!!!!) Native of China Introduced from England to Philadelphia in 1784 by William Hamilton Very successful invasive. Rapid growth rates in disturbed, mobile environments. Produces huge clusters of wind-borne seeds (samaras) sexually, but can also reproduce asexually through root sprouts (Hu 1979). Tolerates stress very well (Graves et al. 1989).

5. Distribution Identified as a possible threat in urban environments, along roadways, and forest edges. Seldom identified as a threat to native forests ( Cronk and Fuller 1995). WHY?????

6. Ailanthus is a shade intolerant species. Ailanthus is thought to be unable to compete with native species in a forest environment. Ailanthus does form part of the canopy and shrub layers of intact, secondary-growth forests and will often fill in open canopy gaps.

7. How do trees reach the canopy? Shade Tolerant Species Continuous slow growth in shade, (e.g. Tsuga canadensis). Alternating periods of suppression and release due to small forest gaps, �Gap Facultative�. (e.g. Acer saccharum). Shade Intolerant Species, (independent of shade) Rapid growth to canopy height, (e.g. Liriodendron tulipifera). Ailanthus altissima may be the most rapidly growing tree species in the north eastern United States (Petrides 1978). Allows A. altissima to reach the canopy within a single period of release, thus out-competing native species.

8. This Study Looks at two sets of questions. Can natural foest gaps provide suitable conditions for growth of A. altissima saplings? Can A. altissima saplings grow rapidly enough to compete with antive species within these gaps? Can A. altissima established in forest gaps survive to reach the canopy? How do A. altissima saplings respond to differing light levels in the field? How does this response compare to those of native species? Specifically, does A. altissima fit the Michaelis Menten function used to model growth and succession of native saplings?

9. The ability of a species to invade a particular forest depends on both the growth characteristics of the species and the scale of disturbance relative to those characteristics (Runkle 1985). With maturation of secondary forests, canopy gaps may become more prevalent, thus increasing the potential for invasion by gap obligate species. With increasing logging operations and urbanization, environmental disturbance and canopy gaps or entire openings are also increasing

10. Materials and Methods August 1-23, 1995. Site Location Montgomery Place, Dutchess County, New York 176 hectares = 470 acres Historic estate bordering the Hudson River. Bordered by areas of old-growth hemlock hardwoods forest (South Woods and North Woods). Other species include red and white oak, sugar maple and tulip poplar. These stands haven�t been logged since the early 1700�s and have sparse undergrowth (little regeneration of tree species). Only disturbance has been the removal of deadfall.

11. Growth of Ailanthus in Old-Growth Gaps. Sample quadrats were placed within six canopy gaps in South Woods. All open canopy gaps in South Woods that contained young Ailanthus saplings were included, unless they were formed or bordered by footpaths. Area was estimated as an ellipse, with the gaps greatest length and perpendicular width as diameters. Every Ailanthus within South Woods with a stem diameter greater than 10 cm dbh were cored and growth ring width was measured.

12. Growth of Ailanthus in Old-Growth Gaps. Five quadrats were placed in each canopy gap of North Woods. Varying light levels were sampled within the gap by placing quadrats (1 m by 4 m measuring east to west) in the center of the gap, at the northern and southern edges of the gap opening and in the canopy 2 m north and 5 m south of the gap. Vertical height was measured, along with diameter at 10 cm above the ground, and extension growth (from terminal bud scars) of each Ailanthus sapling was measured as well as each native sapling taller than 30 cm. All Ailanthus and their roots were removed without harm to any native plants (assumably) and fisheye photography was used to find the GLI or Gap Light Index for each quadrant. GLI is the percent of seasonal total photosynthetically active radiation transmitted by the canopy. Light availability was then compared across quadrant and gap size, using two-way analysis of variance. Density, height, diameter and extension growth was then compared through a one-way analysis of variance using the tallest individual in each quadrant. (similar was done for those native plants taller than 30 cm).

13. Growth of Ailanthus Saplings Under Varying Light. Forty saplings were harvested to represent varying light conditions ranging from full open sky to forest gap to intact canopy. Each harvested was the tallest representative from a 5 m radius location (never harvesting two individuals closer than 10 m together to avoid spatial correlation). Relative radial growth rates were calculated for each sapling harvested. GLI was tested using three models (linear function, logarithmic function and Michaelis Menten function (a * GLI)/[(a/b) + GLI]

14. What did they find? GLI was greatest at the North Edge quadrats, and least at the South Edge quadrats. DUH!!!!! In spite of the variation in light availability across quadrats, position had no significant effect on the density of saplings, nor the vertical height, diameter at 10 cm or extension growth of the tallest Ailanthus. (Same applies for native tree saplings). Note: No Ailanthus were ever present in Canopy quadrats except as single first year seedling

15. Among saplings taller than 30 cm, Ailanthus was as abundant as all other native species combined. Several species were found to be represented by more than one individual at a height greater than 30 cm suggesting they may compete to a greater degree with Ailanthus than other native species. Prunus serotina, Betula lenta, and Liriodendron tulipifera. Dominant Ailanthus saplings were significantly taller than the dominant native saplings, as well as exhibiting a significantly greater diameter and extension growth than the tallest competitors.

16. Age and Growth of Saplings Oldest Ailanthus ranged from seven to sixteen years in North Woods. Saplings were nearly all first year Ailanthus seedlings. Twelve pole-sized Ailanthus in South Woods were aged between 14-23 years old. At any given site, the estimated age of trees varied by only two to three years, suggesting a limited recruitment window following disturbance (e.g. tree fall). Trees showed high overall mean annual growth rates, ranging from 1.96 to 3.7 mm/yr. The minimum annual growth rate for individual trees ranged from .85 to 2.39 mm/yr, while maximum ranged from 3.53 to 6.51 mm/yr.

17. Growth of Ailanthus Saplings under Varying Light. Radial growth of saplings was not strongly affected by light availability. All three models of relative radial growth provided a poor statistical fit ( R^2 = .131 to .167. Michaelis Menten function explained less than 17% of the variation in relative radial growth.

18. Other Findings Forgione found a mortality rate of over 90% for Ailanthus seeds experimentally planted in a closed-canopy forest (1993). Grime also found high mortality of Ailanthus seedlings grown in shade (1965). Kowarik studied a population of forest Ailanthus in West Virginia and found few first-year seedling and none from previous seasons = 100% mortality (1995).

19. Conclusions A tree�s success in reaching the canopy may be determined by it�s early performance as a sapling. Within the six gaps surveyed, Ailanthus stands a good chance of canopy recruitment. Ailanthus saplings out-performed native species in all measures examined, most with a significant difference. To reach canopy heights, Ailanthus must maintain its competitiveness, (e.g. growth rates) for many years. Within the sites sampled, Ailanthus did show to maintain it�s competitiveness in the oldest individual (23 yrs old). Forests that lack advance regeneration rates, may provide a more suitable environment for the establishment of a rapid-growing, shade-intolerant species.