Gap Light Regimes Influence Canopy Tree Diversity

Gap Light Regimes Influence Canopy Tree DiversityAuthor(s): Thomas L. Poulson and William J. PlattSource: Ecology, Vol. 70, No. 3 (Jun., 1989), pp. 553-555Published by: Ecological Society of AmericaStable URL: http://www.jstor.org/stable/1940202 .

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June 1989 SPECIAL FEATURE-TREEFALL GAPS AND FOREST DYNAMICS 553

Ecology, 70(3), 1989, pp. 553-555 1989 by the Ecological Society of America

GAP LIGHT REGIMES INFLUENCE CANOPY TREE DIVERSITY

THOMAS L. POULSON Department of Biological Sciences, University of Illinois at Chicago, Box 4348,

Chicago, Illinois 60680 USA

AND

WILLIAM J. PLATT Department of Botany, Louisiana State University,

Baton Rouge, Louisiana 70803 USA

In old-growth deciduous forests of the eastern United States, where light levels beneath intact canopies are about 1% of full sunlight (Canham 1988b), gaps result in locally elevated light levels. Such increases in light appear necessary for almost all tree species to attain canopy status (e.g., Barden 1980, Runkle 1981, Can- ham 1989). While patterns and mechanisms of replacement among the two species most often codom- inant in these forests, American beech (Fagus grandifolia) and sugar maple (Acer saccharum), have been intensively studied (Cain 1935, Fox 1977, Woods 1979, Donnelly 1986, Canham 1988a), relationships among rates and types of treefall, variability in light levels, and species' responses have not been studied.

We have studied populations of tree species in one old-growth Eastern deciduous forest (Warren Woods, Michigan, USA) for the past 15 yr. We have followed the more abundant canopy species under different spa- tial and temporal patterns of increased light levels as- sociated with different rates of gap formation (Poulson and Platt 1981, 1988, unpublished manuscript). Here we summarize our results to show how size and compass orientation of gaps determine light regimes and how light regimes interact with sapling architecture to influence the diversity of species that reach the canopy. We contrast small, isolated treefalls with small, overlapping treefalls and contrast small, overlapping gaps oriented north-south (N-S) with those oriented east- west (E-W). In addition, we describe patterns to zonation of species in large, multiple treefalls. We con- clude our essay by discussing how changes in latitude influence gap light regimes and, hence, replacement patterns of canopy trees in temperate and tropical forests.

SMALL, ISOLATED GAPS

Low rates of treefall result in isolated gaps, most often single windfalls, in Warren Woods; low light levels in these gaps result in shade-tolerant species dominat-

ing the canopy. Juveniles of beech, the most shade- tolerant species, can survive in shade for >100 yr. Sugar maple, the next most shade-tolerant species, can survive in shade only 20 yr on the average. Thus, subcanopy beech almost always replace canopy maple and frequently replace canopy beech resulting in canopy beech/maple ratios as much as 4 to 1. Other species only rarely attain canopy status.

SMALL, OVERLAPPING GAPS

As rates of treefall increase, maple released by one opening of the canopy may not become fully suppressed before they are re-released by a subsequent gap. In such cases, maple have a net height growth greater than that of beech. Thus, the likelihood that maple will replace beech increases, and maple may even occa- sionally replace itself in the canopy. Hence maple is less dominated by beech in the canopy; ratios are less than 4: 1.

COMPASS ORIENTATION OF SMALL,

OVERLAPPING GAPS

Compass orientation of small, overlapping gaps in- fluences light levels and ingrowth by surrounding trees. Overlapping N-S gaps have higher light levels than overlapping E-W gaps. At solar noon direct sunlight reaches the ground at the northern end of N-S canopy gaps and shines into and beyond expanded gaps (sensu Runkle 1982). In contrast, direct sunlight reaches only edges of canopy trees along northern borders of E-W gaps. Also, canopy ingrowth is slower along edges of N-S gaps because long east-west edges receive low- intensity morning or afternoon sunlight, but little direct midday sunlight. In contrast, the long northern edges of E-W gaps receive high-intensity sunlight most of the day. These differences influence patterns of replacement. Beech, sugar maple, and the next most shade- tolerant species, black cherry (Prunus serotina), reach the canopy in different frequencies in E-W and N-S



554 SPECIAL FEATURE-TREEFALL GAPS AND FOREST DYNAMICS Ecology, Vol. 70, No. 3

gaps. In N-S gaps, sugar maple often and cherry oc- casionally (at the northern end) reach the canopy, whereas in E-W gaps maple less often and cherry almost never reach canopy status.

ZONATION OF SPECIES IN LARGE GAPS

In several large windfalls that occurred in 1975, light levels were initially high because canopy trees knocked down all saplings along the gap axis. In parts of each of these large gaps light levels have remained high where saplings did not resprout and where new treefalls occurred along southern edges. Initial light levels were asymmetrically zoned from least light under the canopy along the south edge to brightest light under canopy along the north edge. Such heterogeneous light levels are resulting in a range of species entering the canopy (in order from least to the most light-demanding: beech, sugar maple, black cherry, yellow poplar [Liriodendron tulipifera] and white ash [Fraxinus americana]).

In one large canopy gap of 800 m2 (expanded gap of 2100 M2) beech and sugar maple were initially present as seedling-sapling stages, yellow poplar germinated from a seed bank after gap formation, and white ash was present both as seeds and as juveniles '0.5 m. Each of these species is likely to attain canopy status, but in different locations dependent on architecture, the relationship of architecture and vertical growth to light regime and crowding by herbs, shrubs, and saplings, and the length of season for extension growth.

White ash should attain canopy status in the brightest area north of the gap axis, where tallest ash are now several metres above tallest sugar maple. Here herbs and shrubs quickly formed a tangle that overtopped and compromised sugar maple. In contrast, great architectural flexibility enabled small ash to adjust to crowding and shading. Ash saplings have few lateral branches that are rarely divided, and any that are shaded for more than a few years do not persist. Ash also responds to local changes in light intensity by varying the position of terminal buds set for the next year's upward growth. Bud size and number vary with light intensity. Large leaf areas can be produced by a single bud (e.g., a large terminal bud of a 6-8 m tall sapling in bright light can produce 38-52 leaflets distributed among 6-8 long compound leaves), but no buds are set where light levels are low. Because ash has the capability to reposition large leaf areas and for rapid extension growth, it can quickly penetrate a growing tangle of herbs and shrubs. In contrast to ash, sugar maple has a month shorter season of extension growth, has many subdivided lateral branches, and does not drop shaded branches readily. The direction of upward growth also is determined by bud set over previous years.

Yellow poplar can grow faster than the other species,

but will not attain canopy status in areas with brightest light. Yellow poplar lacks the architectural flexibility to adjust to severe crowding; it has multiply divided branches that spread widely, and it is committed to straight upward growth. Nonetheless, yellow poplar should reach the canopy in areas with slightly lower light levels just south of the E-W gap axis. Here herbs and shrubs are sparse, and neither ash nor sugar maple is dense. Yellow poplar has numerous thin branches that are quickly shed when heavily shaded. Thus as light decreases around the lower crown, more of the growth is directed in an upward direction. In this region of the gap three yellow poplar are now 13-14 m tall, 3-5 m taller than adjacent ash and once-released sugar maple, and nearly as tall as multiple-released sugar maple. There are two reasons why yellow poplar has done so well. First, yellow poplar has a period of extension growth of 3.5 mo, longer than the 2 mo of ash or 1 mo of sugar maple. Second, seedling yellow poplar did not have to recover from shade suppression as did the initially much taller subcanopy maple; by the second year after germination yellow poplar was growing nearly a metre per year, whereas subcanopy sugar ma- ples took 6 yr before they grew even a half metre per year.

Along all edges of the original canopy gap, multiply released beech and maple, the two least light-demanding species, will attain canopy status. The peripheral beech and maple have shaded smaller juveniles of other faster growing species and thus have competed only with each other for entry into the canopy. At two locations along the edge, beech will outgrow maple and reach the canopy. These are beech that were initially taller than adjacent maple and were not fully shade suppressed. Following gap formation, they grew upward at close to maximum attainable rates and rapidly extended horizontal branches that shaded smaller maple. Elsewhere along the edge, five maple that started at the same height or slightly taller than beech will enter the canopy. Even though the maple take a long time to recover from the shade suppression, their strong apical dominance in bright light provides a sufficient advantage to outgrow beech.

We note that the chance of a species reaching the canopy in large gaps is predicted by the local environ- ment (e.g., light levels, other species present; also see Veblen 1989), growth rate, and architecture, but not by overall frequency or density. For example, the five multiple-released sugar maple reaching the canopy along edges of the large gap represent <0. 1% of the maple in this gap. At the other extreme, the three potential canopy yellow poplar represent nearly 10% of the rel- atively few seedlings that germinated a year or two after the gap formed. Such negative frequency-dependent entry into the canopy (cf. Connell 1989) in less frequent



June 1989 SPECIAL FEATURE-TREEFALL GAPS AND FOREST DYNAMICS 555

large gaps clearly could enhance persistence of rare species more light-demanding than the shade-tolerant dominants, especially if, once in the canopy, the former had a high probability of colonizing any new large gaps (e.g., via seed banks).

NORTH-SOUTH ASYMMETRY OF LIGHT

The initial north-south gradient in light levels in the large gap has become reversed over the past 13 yr. This has affected advance recruitment along edges of the gap. Along the northern edge high initial light intensities promoted rapid growth of saplings, shrubs, and herbs, as well as ingrowth (3-5 m) of adjacent beech canopies. Understory light levels have declined from 25 % to - 2% of full sunlight. Saplings < 10 m are greatly suppressed; recruitment of all species but beech has ceased. In contrast, along the southern edge, where initial light levels were 10/I% full sunlight, rapid growth of saplings, shrubs, and herbs has not occurred, and ingrowth of beech canopies has only been 1-2 m. Light levels have declined to only 5% full sunlight. Re- cruitment of many species has continued to occur, and understory beech and maple have continued to increase in size. Asymmetry of light in gaps thus can have im- portant consequences, since new gaps formed along northern and southern edges of older gaps are likely to differ initially in species composition, size structure of cohorts, and degree of suppression.

LIGHT REGIMES IN TEMPERATE AND TROPICAL GAPS

Within forests not subjected to widespread disturbance, patterns of illumination in gaps should change with latitude. Maximum initial light intensities should be lower in temperate than tropical gaps. Also, initial light intensities should decrease from north to south

in north temperate gaps, while in tropical gaps, initially light should be most intense near the center and decrease concentrically (but only twofold; Denslow 1987) toward the edges. Thus, high light levels should occur under much more restricted conditions (and thus less often) in temperate than tropical gaps. In addition, unlike tropical forests, there should be no systematic association of gap microhabitats (mineral soil; tip-up mounds) conducive for germination and rapid growth (see Putz 1983, 1984, Williamson 1984, Wiemann and Williamson 1988) with high light intensities in temperate forests. Such restrictions, especially if coupled with seasonal constraints on responses to variation in light environments (see Runkle 1989), could greatly limit numbers of high-light-demanding pioneer species (sensu Swaine and Whitmore 1988, Whitmore 1989) in temperate relative to tropical forests.

The relative importance of within-gap light regimes on the population dynamics of shade-tolerant species in forests not subjected to widespread disturbance is less clear. While advance regeneration occurs at all latitudes, the relative importance of responses to differences in light levels (relative to other phenomena, both prior to and after gap formation; see Brokaw and Scheiner 1989, Martinez-Ramos et al. 1989, Schupp et al. 1989) may change with latitude. We suggest that growth rates at different light levels may increase in importance in temperate compared to tropical forests since within-gap light regimes are more heterogeneous and change more extensively with gap size. Hence, there may be greater predictability of forest composition based on growth responses of shade-tolerant species in temperate than tropical forests because of the in- creasing importance of variation in light levels within gaps at higher latitudes.

For reprints of this Special Feature, see footnote 1, page 535.

Ecology, 70(3), 1989, pp. 555-558 ? 1989 by the Ecological Society of America

TREE DEMOGRAPHY AND GAP DYNAMICS IN A TROPICAL RAIN FOREST

MIGUEL MARTINEZ-RAMOS, ELENA ALVAREZ-BUYLLA, AND JOSt SARUKHAN Centro de Ecologia, Universidad Nacional Aut6noma de Mexico,

Ap. Post. 70-275, Coyoacdn 04510, Mexico, DR.

Forests can be considered dynamic mosaics of vege- tation patches of different ages produced by distur- bances and influenced by different abiotic and biotic conditions. Three main phases (gap, building, and ma- ture) have been recognized in the forest regeneration cycle since initial work by Watt (1947). Each phase

poses particular problems and advantages for the regeneration of different species. Whitmore ( 1975, 1982, 1989) and Swaine and Whitmore (1988) have placed tropical trees in either of two categories defined by their light requirements for germination and establishment, suggesting two main routes by which trees may attain



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Gap Light Regimes Influence Canopy Tree Diversity