Canopy Gap Size Research Articles

Canopy gap dynamics of second-growth red spruce-northern hardwood stands in West Virginia

Forest restoration requires an understanding of the natural disturbance regime of the target community and estimates of the historic range of variability of ecosystem components (composition, structure, and disturbance processes). Management prescriptions that support specific restoration activities should be consistent with these parameters. In this study, we describe gap-phase dynamics of even-aged, second-growth red spruce-northern hardwood stands in West Virginia that have been significantly degraded following early Twentieth Century harvesting and wildfire. In the current stage of stand development, gaps tended to be small, with mean canopy gap and extended canopy gap sizes of 53.4m2 and 199.3m2, respectively, and a canopy turnover rate of 1.4%year−1. The majority of gaps resulted from the death of one or two trees. American beech snags were the most frequent gap maker, partially due to the elevated presence of beech-bark disease in the study area. Gaps ranged in age from 1 to 28 years, had a mean of 13 years, and were unimodal in distribution. We projected red spruce to be the eventual gap filler in approximately 40% of the gaps. However, we estimated that most average-sized gaps will close within 15–20 years before red spruce canopy ascension is projected (30–60 years). Accordingly, many understory red spruce will require more than one overhead release – an observation verified by the tree-ring record and consistent with red spruce life history characteristics. Based on our observations, silvicultural prescriptions that include overhead release treatments such as thinning from above or small gap creation through selection harvesting could be an appropriate activity to foster red spruce restoration in the central Appalachians.

Sugar maple and yellow birch regeneration in response to canopy opening, liming and vegetation control in a temperate deciduous forest of Quebec

We examined how the density, growth and survival of sugar maple ( Acer saccharum Marsh.) and yellow birch ( Betula alleghaniensis Britton) regeneration are influenced by gap size, soil nutrient availability and understory vegetation. We used a factorial combination of (1) three gap sizes (small: <100 m 2; medium: 100–300 m 2; large: ∼1000 m 2); (2) presence/absence of liming (92% CaCO 3 at 500 kg ha −1, 1st year post-harvest); and (3) presence/absence of vegetation control (weeding twice a year; 1st to 3rd year post-harvest). We monitored height increment and survival of 1500 seedlings and saplings of both species from the 3rd to the 6th year post-harvest, and assessed density 6 years post-harvest. Both species exhibited a complex set of density, growth and survival responses across the combination of treatments. Compared to sugar maple, yellow birch had an overall lower density, greater growth, and similar survival rate; the two species attained maximum values in different gap size for density, and similar gap size for growth and survival. Liming had very little or no effect on the species. The growth of yellow birch was slightly but significantly greater when understory vegetation was controlled, particularly in medium and large gaps. These results suggest that a variety of canopy gap sizes can provide the right combination of understory conditions for regenerating these two functionally different tree species.

Estimation of canopy gap size and gap shape using a hemispherical photograph

Gap size and gap shape are two important properties of forest gaps that can influence microsite conditions in a forest stand and determine the recruitment and establishment of trees. There is no universally adopted method for measuring the gap size, although several options are available. In addition, few methods have been proposed for measuring the gap shape. This paper proposes a photographic method of estimating canopy gap size and gap shape. The proposed method is based on a vertical hemispherical photograph of the gap and is thus named the hemispherical photograph method (HPM). We tested the accuracy of the HPM measure of gap size by two ground-based methods and compared the HPM with other methods. Our results indicate that the HPM measurement of the canopy gap size is accurate, but is significantly influenced by the location of the camera. Compared with the ground-based methods, the HPM is more objective and repeatable. Compared with other photographic methods, HPM is more accurate due to the more actual assumptions, but is more labor-intensive because more field measurements are necessary. We conclude that the HPM is a powerful tool for comparative and long-term studies of forest gaps.

Growth, crown architecture and leaf dynamics of saplings of five mangrove tree species in Ranong, Thailand

We investigated the regeneration pattern of mangrove forests, considering the corre- spondence between growth strategies (i.e. sapling growth, crown architecture, leaf dynamics) of 5 canopy-dominant species and the disturbance regime in Ranong, Thailand. Individual canopy gap size and percent gap area were greater in the Sonneratia alba-Avicennia alba (SA) zone, which is located on the most seaward fringe, than in the other inland zones. In canopy gaps, the seaward species S. alba and A. alba showed a higher relative height growth rate than the inland species Rhizophora apiculata, Bruguiera gymnorhiza and Xylocarpus granatum. Under closed canopies, the seaward species showed greater mortality, presumably due to their low shade tolerance, while the inland species demonstrated a net growth in spite of the dark conditions. Leaf longevity of sunlit saplings increased from seaward to inland species. The 2 seaward species had well-branched, slen- der and deeper crowns, while R. apiculata and B. gymnorhiza had wider and flatter crowns, and X. granatum had less-branched, smaller crowns. Phenotypic traits were correlated with each species' growth strategy (potential growth rate and shade tolerance), which corresponded to the disturbance regime in each vegetation zone. Many large gaps may enhance the abundance of S. alba and A. alba in the SA zone, and a few small gaps may prevent establishment and growth of light-demanding species in the inland zones. Accordingly, the correspondence of disturbance regime and growth strategies of canopy-dominant species provides an advantage for successful regeneration, and may contribute to the maintenance of the present species composition in each vegetation zone.

The influence of canopy gap size on soil water conditions in a deciduous broad-leaved secondary forest in Japan

The influence of canopy gap size on soil water conditions in a deciduous broad-leaved secondary forest in Japan

SPATIAL CONTAGIOUSNESS OF CANOPY DISTURBANCE IN TROPICAL RAIN FOREST: AN INDIVIDUAL‐TREE‐BASED TEST

Spatial contagiousness of canopy dynamics-the tendency of canopy disturbances to occur nearby existing canopy openings due to an elevated risk of tree fall around gaps-has been demonstrated in many temperate-zone forests, but only inferentially for tropical forests. Hypothesized mechanisms increasing the risk of tree fall around tropical forest gaps are (1) increased tree exposure to wind around gaps, (2) reduced stability of trees alongside gaps due to crown asymmetry, or (3) reduced tree health around gaps due to damage from prior disturbances. One hypothesized consequence of elevated disturbance levels around gaps would be that gap-edge zones offer relatively favorable prospects for seedling recruitment, growth, and survival. We tested whether disturbance levels are indeed elevated around natural canopy gaps in a neotropical rain forest in French Guiana, and more so as gaps are larger. We followed the fate of 5660 trees >10 cm stem diameter over five years across 12 ha of old-growth forest and analyzed the risk and magnitude of canopy disturbance events in relation to tree diameter and the proximity and size of natural canopy gaps. We found that the cumulative incidence of disturbance over the five-year survey was not significantly elevated around preexisting gaps, and only weakly related to gap size. Also, neither the risk nor the magnitude of canopy disturbances increased significantly with the proximity of gaps. Moreover, canopy disturbance risk around gaps was independent of gap size, while the magnitude of disturbance events around gaps was weakly related to gap size. Tree size was the major driver of disturbance risk as well as magnitude. We did find an elevated incidence of disturbance inside preexisting gaps, but this "repeat disturbance" was due to an elevated disturbance risk inside gaps, not around gaps. Overall, we found no strong evidence for canopy dynamics in this rain forest being spatially contagious. Our findings are consistent with the traditional view of tropical rain forests as mosaics of patches with predictable regeneration cycles.

Germination responses to water potential in neotropical pioneers suggest large-seeded species take more risks.

In neotropical forests, very small-seeded pioneer species (<0.1 mg seed mass) recruit preferentially in small tree fall gaps and at gap edges, but large-seeded pioneers do not. Since water availability is related to gap size, these differences in microsite preference may reflect in part species-specific differences in germination at reduced water potentials. For 14 neotropical pioneer species, the hypothesis is tested that small-seeded species, with shallow initial rooting depths, reduce the risks associated with desiccation by germinating more slowly and at higher water potentials than large-seeded species. Germination occurred both more quickly and at lower water potentials with increasing seed mass. For example, Ochroma pyramidale (seed mass 5.5 mg) had a time to 50 % germination (T50) of 2.8 d and a median base potential for germination (psi(b50)) of -1.8 MPa while Clidemia quinquenervia (seed mass 0.017 mg) had a T50 of 17.6 d and psi(b50) of -1.1 MPa. These data suggest that small-seeded species germinate only in comparatively moist microsites, such as small canopy gaps, which may reduce the risk of drought-induced mortality. Conversely, large-seeded species are able to germinate in the drier environment of large gaps, where they benefit by enhanced seedling growth in a high irradiance environment. The positive association of seed size and canopy gap size for optimal seedling establishment is maintained by differential germination responses to soil water availability coupled with the scaling of radicle growth rate and seed size, which collectively confer greater drought tolerance on large-seeded species.

Mangrove forests in a peri-urban setting: the case of Mombasa (Kenya)

The structure and regeneration patterns of the peri-urban mangrove vegetation of Mombasa at Tudor creek were studied along belt transects at two forest sites of Kombeni and Tsalu. Based on the species importance values, the dominant mangrove species were Rhizophora mucronata Lam. (Rhizophoraceae) and Avicennia marina (Forssk.) Vierh. (Avicenniaceae). Lumnitzera racemosa Willd., reported in an earlier floristic survey, was not encountered. Tree density varied from 1,264 trees ha–1 at Kombeni to 1,301 trees ha–1 at Tsalu and mean tree height was higher at the former site compared to the latter. The size-class structure at both localities showed the numerical dominance of small trees over larger trees. The spatial distribution pattern of adults and juveniles varied greatly between sites and showed a close to uniform pattern (Morisita’s Index Iδ ≪ 1) for adult trees, but a tendency to clustered distribution (Iδ ≫ 1) for juveniles. The present paper shows that unmanaged but exploited peri-urban mangroves are structurally stressed, having enlarged canopy gaps that are characterised by spatial and temporal site heterogeneity that influences regeneration, implying longer periods for canopy closure. Diversifying uses of mangrove products and establishing reserves as no cut zones with regulated harvesting will minimise canopy gap sizes, and promote conservation practices. The proposed management strategy shall boost the ecosystem resilience to both anthropogenic and natural stressors expected in the peri-urban setting in the long run.

Effects of canopy gap size on recruitment and invasion of the non-indigenousRubus alceifoliusin lowland tropical rain forest on Réunion

Abstract:An important challenge of invasion biology is to understand how interactions between species traits and ecosystem properties enable alien species to become invasive at particular locations. We investigated how gap dynamics in a tropical rain forest on the island of Réunion affected the invasiveness of alien plants. In the 12 000-m2study area, alien plants occupied 24.9% of the area of gaps, which represented 5.62% of the forest area, but only 0.8% of the understorey area. The most abundant invasive species wasRubus alceifolius, which formed dense, monospecific stands in the largest gaps (&gt; 25 m2). Although plants could persist in the shade, a germination experiment revealed that canopy openings were essential for seedling establishment. A cyclone that struck the study area in 2002 caused a temporary thinning of the canopy, increasing light levels to above the threshold needed for germination ofR. alceifoliusand also stimulating the growth of established plants. We conclude that the ability of this and other alien species to colonize intact lowland tropical rain forest is strongly influenced by the prevailing gap dynamics. Because gaps are also essential for the regeneration of many native trees in our study area, there is a real danger of the forest being progressively degraded by alien plants. There are no simple solutions to controlling species such asRubus alceifolius, but efforts should be focused mainly upon the larger gaps where the species are most invasive.

Frost-heaving damage to one-year-old Picea abies seedlings increases with soil horizon depth and canopy gap size

We studied first winter frost-heaving damage to one-year-old Picea abies (L.) Karst. seedlings planted in gaps made by group fellings (large circular gaps, ca. 500 m2) and single-tree selection cuttings (small irregularly shaped gaps, ca. 175 m2), as well as in uncut forest. One-month-old seedlings were planted on manually exposed LF, Ae, and B horizons that emulated various intensities and depths of scarification. The three experimental sites were located in multistoried Pinus sylvestris L. or P. abies forests on sandy loam or silt loam in southeastern Norway. Altogether, 5% of seedlings sustained frost heaving damage on the LF horizon, compared with 20% on the Ae horizon and 45% on the B horizon. On average, 31% of the seedlings in large gaps incurred frost-heaving damage compared with 20% in small gaps and 19% in uncut forest. Exposed roots and poorly anchored or uplifted seedlings were recurring classes of damage, especially on the B horizon and in large gaps. The above- versus below-ground biomass ratio of seedlings was higher on the B than on the Ae horizon in uncut forest and large gaps, inferring broken roots. Therefore, to reduce the risk of frost-heaving damage, shallow soil preparation and smaller gap sizes should be used.

Natural regeneration and environmental factors within small-scaled gaps in urban afforestation

In this study, we hypothesized that small-scaled gaps would facilitate the emergence of late successional species and that an abundance of understory vegetation would influence the gap responses of the forest floor vegetation, focusing on the regeneration dynamic within canopy gaps in planted, warm temperate forests. Two different sizes of canopy gaps were formed in two plantations with different understory abundances and the dynamics of the microenvironments and the forest floor vegetation were studied for 4 years pre- and post-gap. Based on our results, the first hypothesis was not supported, and the second hypothesis was supported.

Relating bat species presence to simple habitat measures in a central Appalachian forest

We actively sampled the bat community at 63 sites using detection and non-detection metrics on the Fernow Experimental Forest (FEF) in the central Appalachians of West Virginia using Anabat acoustical equipment May–June 2001–2003 to relate species presence to simple habitat measures such as proximity to riparian areas, forest canopy cover, forest canopy gap width, and forest canopy height. We acoustically detected eight species on the FEF, including the endangered Myotis sodalis. The presence of Lasiurus cinereus, M. lucifugus, M. sodalis, and Pipistrellus subflavus was associated more with riparian areas than upland areas. Both univariate comparisons and multiple logistic regression modeling showed that the probability that clutter-adapted foraging species such as M. septentrionalis and M. sodalis would be detected was greater as forest canopy cover increased or forest canopy gap size decreased, whereas the opposite was true for open-adapted foraging species such as Eptesicus fuscus and L. cinereus. The overall proportion of unidentifiable bat echolocation sequences to those identified to species was related to upland sites with increasing forest canopy cover indicating some sampling bias between cluttered and uncluttered habitats. However, given sufficient sample points, bat community surveys using acoustical detection show the ability to quickly develop generalized habitat associations for rugged areas such as the central Appalachians where traditional mist-net survey efforts often are logistically difficult and are lacking in scope. Moreover, these acoustical surveys also could lend themselves to species-specific predictive mapping of foraging habitat as well as allowing researchers to formulate testable hypotheses about detailed bat habitat relationships to be definitively tested with radio-telemetry techniques.

Response of oriental beech (Fagus orientalis Lipsky) seedlings to canopy gap size

Summary The response of nursery seedlings and wildings of oriental beech ( Fagus orientalis Lipsky) to canopy gap size was studied in a climax beech forest in northern Iran with respect to seedling survival, shoot growth and vitality. These parameters were followed for 2 years after planting in gaps of 50, 200 and 600 m 2 , as well as in the open fi eld. Foliage coloration was used as a criterion for vitality. The survival rate of nursery seedlings and wildings dropped rapidly with increasing gap size from 84 per cent in 50-m 2 canopy openings to �∼� 55 per cent in the 600-m 2 gaps and the open fi eld 2 years after planting. Shoot growth of wildings and nursery seedlings differed in response to gap size variation. Shoot growth of wildings was higher in small gaps than in larger openings, whereas for nursery seedlings the inverse was observed. Most beech seedlings were not discoloured or slightly discoloured under gaps of 50, 200 and 600 m 2 and were slightly to moderately discoloured in the open. There was a general tendency for higher discoloration in larger openings. It is concluded that small non-regenerated canopy gaps (�≤� 200 m 2 ) of beech forests in northern Iran can be restored by planting beech wildings as well as nursery seedlings in small openings, whereas nursery seedlings are preferred in larger gaps (�≥� 600 m 2 ).

Disturbance dynamics of old‐growth Picea rubens forests of northern Maine

AbstractQuestion:How have the spatial and temporal aspects of past disturbance shaped the current structure and composition of old‐growth Picea rubens forests?Location:Northern Maine, USA.Methods:We established three 50 m × 50 m plots in old‐growth Picea rubens forests and mapped the location of trees and saplings. We extracted increment cores from canopy trees, and recorded growth releases indicating past disturbance. By linking spatial data (tree positions) and temporal data (dated growth releases), we reconstructed the location and size of former canopy gaps back to 1920, and determined a more general disturbance chronology extending as far back as 1740.Results:We found no evidence of stand‐replacing disturbances. The disturbance dynamic includes pulses of moderate‐severity disturbances caused by wind storms and host‐specific disturbance agents (spruce budworm, spruce bark beetle) interposed upon a background of scattered smaller canopy gaps. Consequently, rates of disturbance fluctuated considerably over time. Reconstructed canopy gaps were temporally and spatially scattered; during disturbance peaks, they were both larger and more numerous.Conclusions:Despite peaks in disturbance, several of which created relatively large gaps, this system has experienced no significant change in species composition. Instead, the shade‐tolerant Picea rubens has maintained canopy dominance. The patch dynamics described here consist of dramatic structural, not compositional, changes to the forest. The persistence of Picea rubens is attributed to a combination of traits: (1) abundance of advance regeneration; (2) ability to endure suppression and respond favourably to release; and (3) longevity relative to ecologically similar species.

Disturbance dynamics of old-growth Picea rubens forests of northern Maine

Abstract Question: How have the spatial and temporal aspects of past disturbance shaped the current structure and composition of old-growth Picea rubens forests? Location: Northern Maine, USA. Methods: We established three 50 m × 50 m plots in old-growth Picea rubens forests and mapped the location of trees and saplings. We extracted increment cores from canopy trees, and recorded growth releases indicating past disturbance. By linking spatial data (tree positions) and temporal data (dated growth releases), we reconstructed the location and size of former canopy gaps back to 1920, and determined a more general disturbance chronology extending as far back as 1740. Results: We found no evidence of stand-replacing disturbances. The disturbance dynamic includes pulses of moderate-severity disturbances caused by wind storms and host-specific disturbance agents (spruce budworm, spruce bark beetle) interposed upon a background of scattered smaller canopy gaps. Consequently, rates of disturbance fluctuated consid...

Root hydraulic conductivity and whole-plant water balance in tropical saplings following a shade-to-sun transfer

We hypothesized that pioneer and late successional species show different morphological and physiological responses in water use after gap formation. The magnitude of the responses was compared between two pioneer species (Macaranga gigantea and Trema orientalis) and four late successional species (Shorea sp.), in an experiment in which saplings were transferred from shade to sun. Although transpiration demand increased following the transfer, root hydraulic conductivity (Lp(r)) decreased. Lp(r) was sensitive to brief treatments with HgCl(2) (a specific inhibitor of aquaporins). This allows Lp(r) to be divided into two components: cell-to-cell and apoplastic pathways. The Lp(r) of cell-to-cell pathway decreased in all species following the transfer, relating to aquaporin depression in roots. Following the transfer, leaf osmotic potentials at full hydration decreased and both leaf mass per area [leaf mass/leaf area (LMA)] and fine-root surface area/leaf surface area (root SA/leaf SA) increased in almost all species, allowing saplings to compensate for the decrease in Lp(r). Physiologically, pioneer species showed larger decreases in Lp(r) and more effective osmotic adjustment than late successional species, and morphologically, pioneer species showed larger increases in root SA/leaf SA and LMA. Water balance at the whole-plant level should be regulated by coupled responses between the aboveground and the belowground parts. Interspecific differences in responses after gap formation suggest niche differentiation in water use between pioneer and late successional species in accordance with canopy-gap size.

Canopy gaps formed by mangrove trimming: an experimental test of impact on litter fall and standing litter stock in Southwest Florida (USA)

Mangroves in Florida (USA) are subject to horticultural pruning that may increase the size of canopy gaps and alter rates of litter production and accumulation. Mangrove canopy gap formation is a common phenomenon, known to alter abiotic conditions near the forest floor. Using a series of field experiments in Rookery Bay, Florida, the effects of mangrove trimming on canopy density, mangrove litter production, standing litter stocks, and the decomposition rate of Rhizophora mangle leaves on the forest floor were assessed. Litter trap collections over the year following mangrove trimming indicated that pruned mangrove stands (canopy coverage: 42.8±0.9%; mean±S.E.) produced approximately one-half of the litter of mangrove stands with relatively complete canopies (canopy coverage: 72.1±0.5%). However, there was no significant difference between the mass of standing litter on the forest floor beneath reduced canopy and intact canopy mangroves. Also, R. mangle leaves held on the forest floor in fiberglass litter bags at both reduced canopy and control sites did not decompose at different rates over 28 days. These results indicate that while system-wide mangrove litter production should be reduced by the formation of these gaps in mangrove forests, postproduction influences may obscure any site-specific declines in standing litter stocks.

Tree dynamics in canopy gaps in old-growth forests of Nothofagus pumilio in Southern Chile

The gap dynamics of two Nothofagus pumilio lenga stands have been investigated. We evaluated and compared tree diameter distributions, spatial patterns, tree fall and gap characteristics and regeneration responses in gaps in two old-growth forests of Nothofagus pumilio in Southern Chile Shangri-La: 36°60' S, 71°30' W; Reserva Coyhaique: 45°52' S, 72°00' W. In addition, we describe relationships between gap size and regeneration characteristics. We detected some differences in tree and sapling densities between sites. The causes of gap formation and the gap size distribution, mean gap area, and gap fraction were similar, but gap abundance was different at the two sites. The Reserva Coyhaique site had 15 gaps/ha compared to 10 gaps/ha for Shangri-La. Sizes of clumps of trees were within the range of sizes of canopy gaps at both sites. The density of saplings was higher in gaps than under closed forest at R. Coyhaique, but not at Shangri-La. We found that sapling densities were unrelated to gap size in both sites. The lower sapling density in gaps at Shangri-La might be explained by the presence of Chusquea quila, a competitive pioneer bamboo species. The height increment was related to gap size at ShangriLa, but not at R. Coyhaique. Gap size itself does not account for all the variation in recruitment performance in these Southern beech stands.

Landscape distribution and characteristics of large hurricane-related canopy gaps in a southern Appalachian watershed

Hurricane-related winds are a major source of disturbance in coastal ecosystems of the southern United States, but their effects on forests in the southern Appalachian Mountains, >400 km inland, have seldom been documented. In October 1995, remnant winds of Hurricane Opal caused windthrow of individual and patches of trees throughout the mountainous region of western North Carolina, USA. The 2-day storm event was accompanied by over 150 mm of precipitation and gusty, predominantly southeasterly winds with peak velocities at low elevations of up to 26 m/s; peak velocities were over 40% greater at high elevations. In a landscape-scale case study, we spatially located the population of large canopy gaps (groups of ≥10 windthrown trees) within a 2400 ha watershed to determine frequency of occurrence on basin and highland landscape types, dimensions, association with topographic features, and direction of treefall as a measure of wind vectors. The distribution of large gaps was not random within the watershed and occurred at an average density of 1 per 39 ha in the basin (elevation <700 m), which is characterized by hills of low relief and soils with high clay content. In comparison, gap density averaged 1 per 192 ha on the surrounding highlands (elevation >700 m) of high relief and soils with low clay content. Gaps on both landscape types occurred with greater frequency on sites of southeasterly aspect. Sizes of large canopy gaps ranged from 0.1 to 3.9 ha (average 0.7 ha) and were not correlated with landscape type or topography. Gap shape tended to be linear and averaged 2.3 times longer than wide. Direction of mean treefall among gaps was predominantly northwesterly and was strongly associated with aspect on highland, but less so in basin landscape types. Variation in mean treefall direction among gaps suggests that gaps were created by individual gusts of high-velocity, linear winds. A logistic discriminant model based on elevation, azimuth, and slope gradient correctly classified 78% of the study sites. However, gap size and association with topographic features were similar between basin and highland landscapes. Although the southern Appalachian Mountain region is over 400 km from the Gulf of Mexico and the Atlantic Ocean, results from our study suggest that strong winds from hurricane remnants can influence forest structure, with greater impacts on basin than highland landscapes.

TESTING LIFE HISTORY CORRELATES OF INVASIVENESS USING CONGENERIC PLANT SPECIES

We used three congeneric annual thistles, which vary in their ability to invade California (USA) annual grasslands, to test whether invasiveness is related to differences in life history traits. We hypothesized that populations of these summer-flowering Centaurea species must pass through a demographic gauntlet of survival and reproduction in order to persist and that the most invasive species (C. solstitialis) might possess unique life history characteristics. Using the idea of a demographic gauntlet as a conceptual framework, we compared each congener in terms of (1) seed germination and seedling establishment, (2) survival of rosettes subjected to competition from annual grasses, (3) subsequent growth and flowering in adult plants, and (4) variation in breeding system. Grazing and soil disturbance is thought to affect Centaurea establishment, growth, and reproduction, so we also explored differences among congeners in their response to clipping and to different sizes of soil disturbance. We found minimal differences among congeners in either seed germination responses or seedling establishment and survival. In contrast, differential growth responses of congeners to different sizes of canopy gaps led to large differences in adult size and fecundity. Canopy-gap size and clipping affected the fecundity of each species, but the most invasive species (C. solstitialis) was unique in its strong positive response to combinations of clipping and canopy gaps. In addition, the phenology of C. solstitialis allows this species to extend its growing season into the summer—a time when competition from winter annual vegetation for soil water is minimal. Surprisingly, C. solstitialis was highly self-incompatible while the less invasive species were highly self-compatible. Our results suggest that the invasiveness of C. solstitialis arises, in part, from its combined ability to persist in competition with annual grasses and its plastic growth and reproductive responses to open, disturbed habitat patches. Corresponding Editor: D. P. C. Peters.