No. 134 

1962 

SOME EFFECTS OF COMPETITION 

ON THE SURVIVAL OF YELLOW-POPLAR SEEDLINGS 

In the upper Coastal Plain and northward in New Jersey, yellowpoplar is an important component of the hardwood forests because of 

its rapid growth, straight and clear bole, and desirable wood characteristics. But attempts to reproduce this species naturally after harvest cuttings have often been unsuccessful. Though poor seed crops and unfavorable seedbed conditions may at times restrict germination, observations 

indicate that mortality of seedlings after germination is of greater 

significance. 

In this area, yellow-poplar is intolerant of competition for light and 

moisture: it requires more light than that normally reaching the ground 

through an intact tree canopy. Thus there is never much yellow-poplar 

reproduction present before cutting. Whenever seedlings do germinate 

under the canopy, they usually die early in life; some degree of cutting 

of the overstory is necessary for their survival. 

But removal of the overstory also stimulates the growth of pre-existing 

understory vegetation. Often a dense, tall shrub thicket develops, and 

many years may elapse before trees penetrate it. And those that do 

penetrate it usually are of the most tolerant, less desirable species. In the 

absence of shrubs, herbaceous plants may dominate the understory; or in 

rare instances there may be little understory vegetation. 

To determine the effects of these understory vegetation types on yellowpoplar seedling survival, and to compare the effects of two different 

intensities of canopy removal, a small study was started in the fall of 

1958 in southern New Jersey. 
Study Methods 

A number of plots were located under (I) uncut stands, with nearly 

complete canopies; (2) shelterwood-cut stands, with about 50-percent 

canopies; and (3) clearcut stands, with hardly any canopy. For all samples 

in the last two classes, cutting had been done during the preceding year. 

Within cutover areas the plots sampled three understory types: (1) 60- 

to 90-percent cover of herbs, or (2) of shrubs,' and (3) little or no cover 

of herbs and shrubs. Under uncut stands, plots were located only where 

the herb and shrub cover was sparse or nonexistent. 

Each plot consisted of 10 seedlings that had started from seed the 

previous spring. Because of variations in seedling distribution, the plots 

were irregular in size and shape. Stakes were used to mark plot corners. 

The location of each seedling was marked by a wire pin. Two sizes of 

seedlings were recognized: 2 or more inches tall, and less than 2 inches 

tall. The two sizes were differentiated by use of pins bent in two distinctive 

shapes. 

Originally, many such plots were established; but because of vandalism 

and losses from other sources, only 72 plots remained at the end of the 

study. Forty-four plots remained in the clearcut areas, 20 in the shelterwood-cut areas, and 8 in the undisturbed areas. This distribution does not 

allow effective use of statistical analysis, so only the means for the various 

treatments are presented. Within the two types of cutting, however, the 

plots were about equally distributed among the three understory conditions. 

Distribution of seedlings in the two size classes varied with overstory 

conditions, too, but apparently was not affected by the understory classes. 

Under uncut stands only a few seedlings were taller than 2 inches. The 

proportion of such seedlings was greatest in the clearcut areas, but only 

slightly more than in the shelterwood-cut stands. 

Plots were tallied when established in the fall of 1958, and again in the 

falls of 1959 and 1960. Unmarked seedlings found in 1959 and 1960 also 

were tallied; most of these had started after 1958. 

'The common shrubs included sweet pepperbush (Clethra alnifolia), blackberries (Rubus spp.), 

spicebush (Lindera benzoin), black-haw (Viburnum pmnifoliolium), maple-leaved viburnum (V. acerifolium), southern arrowwood (V. dentaturn); flowering dogwood (Cornus porida), azaleas (Rhododendron 

rpp.), and low-bush blueberries (Vaccinium spp.), with some scattered vines of Japanese honevsucklr 

\Lonzcera japonica). Common herbs were jewelweed (Impatiens capensis), pokeweed (Phytolacca 

amepicana), Solomon's-seal (Polygonatm biporum), false Solomon's-seal (Smilacina racemosa), trillium 

(Trillium spp.), May-apple (Podophyllum peltatm), and jack-in-the-pulpit (Arisa~ma spp. ) 
Results 

Undisturbed stands provided the poorest conditions for survival of the 

yellow-poplar seedlings. All marked seedlings in those stands died within 

2 years. 

Surprisingly, survival differed little between the shelterwood and clearcut areas. In shrubby understories, but not elsewhere, survival in the fall 

of 1959 was somewhat better on clearcut areas; this difference faded to 

insignificance by the fall of 1960 (table 1). 

Seedling mortality was greater in shrub understories than in herb understories. The difference was slower to develop in clearcut areas than in 

shelterwood-cut stands (table I), but was apparent in both when the 

seedlings were 3 years old. 

Herb understories had little effect on seedling survival during the first 

2 years, but their effect became more marked in the third year: by the fall 

of 1960 about a third fewer seedlings were still living in herbaceous 

cover than where the cover was sparse. 

Table 1-Szcrvival of 1958 yellow-poplar seedlings as affected 

by overstory and understory conditions, in percent 

Undisturbed Shelterwood-cut Clearcut 

overs tory overstory areas 

Understory 

October October October October October October 

1959 1960 1959 1960 1959 1960 -- - 

Little or none. ......... 17 0 30 24 35 25 

Herb. .................. - - 3 7 16 31 16 

Shrub. ................. - - 16 7 30 9 

Nearly all seedlings that were less than 2 inches tall at the end of their 

first growing season soon died. Only 19 percent survived their first winter, 

and most of these were dead by the following fall. 

In 1959, sixty new seedlings started in the plots, 44 of them in clearcut 

areas. By the fall of 1960, only 18 percent of these seedlings still were 

alive. At that time nearly all the surviving seedlings in the plots were of 

1958 origin; that is, they had started just after the logging was done. 

Only 1.7 percent were of 1959 or 1960 origin. 

Damage by wildlife, especially rabbits, was observed each year on about 

10 percent of the living seedlings. However, its effect on seedling mortality 

was not determined. 
Summary and Conclusions 

Yellow-poplar seedlings starting under three overstory and three understory conditions were marked in the fall of their first year and were 

observed through the next two growing seasons. Survival data were 

obtained on 72 plots of 10 trees each. 

All marked seedlings under uncut stands died during the 2-year observation period. Survivals did not differ significantly between clearcut and 

shelterwood-cut areas. Under both degrees of cutting about 25 percent of 

the marked seedlings survived where the understory was sparse. In herbaceous understories, 16 percent of the seedlings still were alive after 2 

years, and in shrubby understories about 8 percent. Regardless of overstory and understory conditions, most of the original marked seedlings 

that were less than 2 inches tall, and most later seedlings-those starting 

a year or more after the cutting-soon died. 

Thus, to generalize, our observations indicate that: 

At least through their first 3 years, yellow-poplar seedlings survive 

about as well under a 50-percent overstory as in clearcut areas, but will 

not survive at all under uncut stands. 

Understory vegetation is detrimental to survival, and shrubs are more 

detrimental than herbs. 

To survive in any appreciable numbers, seedlings must start during the 

first year after cutting and must attain heights greater than 2 inches 

during their first growing season. 

-J. J. PHILLIPS 

Soil Scientist 

Northeastern Forest Experiment Station 

Forest Service, U. S. Dept . Agriculture 

New Lisbon, N. J .