4.1. Comparing Beech Regeneration in Gaps and Closed-Canopy Patches
In this beech virgin forest, the density and estimated cumulative biomass of saplings ≥0.5 m height were significantly higher in “understory gaps”, i.e., gaps allowing radiation penetration to at least 3 m above ground, than in closed-canopy patches (p
< 0.001, Wilcoxon rank-sum test). This clearly demonstrates that gap formation has a large influence on the regeneration structure, in support of our first hypothesis. Positive gap effects on regeneration have been observed in many forest types from the boreal zone to the tropics [41
], and they should be especially strong in light-demanding tree species. One would expect that highly shade-tolerant late-successional species such as beech are less dependent on gap formation, and very large gaps might even inhibit the development of shade-tolerant tree saplings, which thrive best in semi-shade [5
]. The evidence from beech-dominated virgin forests on the role of canopy gaps for the regeneration process is indeed contradictory. While some authors found, similar to our study, gaps to have a positive effect on the establishment and growth of beech offspring [33
], others detected no explicit gap influence on the structure of the regeneration [17
]. The latter finding suggests that beech saplings are capable of tolerating the shade cast by the nearly closed canopy in natural beech forests, which is characteristic for large parts of the stand. This may not only be caused by the well-known high physiological shade tolerance of Fagus sylvatica
seedlings and saplings [5
], but may also reflect the fact that canopy gaps increase the light levels on the ground on a much larger area than just in the projected gap area [57
]. The cumulative expanded canopy gap area, i.e. the area of the stand approximately influenced by gap formation, comprises more than a quarter to half (29–55%) of the total area in beech-dominated virgin forests, even though cumulative gap area itself covers only about 13–16% [24
However, there may be other reasons related to methodology, why Nagel et al. [17
] did not detect a significant gap effect on the structure of the regeneration. These authors used a relatively broad gap definition in terms of the regeneration layer, including also gaps that are filled by larger saplings and young trees (up to 15 m in height). If a broader gap definition is used which considers both understory gaps and canopy gaps, the average gap becomes more similar in its light regime to the closed-canopy stand. The presence of larger saplings and understory trees decreases light transmission to the ground and thereby effectively reduces the number of surviving saplings in lower strata. If we had included canopy gaps with such tall saplings and understory trees, it is likely that the statistical difference of tree density in gaps compared to that in closed-canopy plots would have been weakened or even lost. Actually, Nagel et al. [17
] did not compare gaps with the closed-canopy stand, but with average stand conditions, which also may have included plots that were affected by gaps. The difference between gaps and average stand conditions diminishes with a growing proportion of gaps in total stand area.
4.2. The Role of Gaps for Beech Regeneration
Openings in the forest canopy do not only affect the light availability on the projected gap area but cause a range of light conditions in the gap and the surrounding forest area, from deep shade to full sunlight [58
]. By analyzing beech regeneration along belt transects that reached well beyond the projected area of differently sized gaps, and on plots under closed-canopy conditions, our study covers the existing gradient in light availability.
We hypothesized that the position in the gap influences the establishment success of beech seedlings and the height growth of seedlings and saplings, and thus causes heterogeneity in the spatial distribution of sapling density and biomass in the course of time (H2). When an understory gap is created and light levels become more favorable for beech offspring, newly established seedlings usually meet an already existing seedling and sapling population that managed to persist at low densities under closed-canopy conditions. According to the shoot length growth data from Kyjov forest, small seedlings that established prior to gap formation, and many younger saplings from post-disturbance colonization events must have reached or exceeded the height threshold (≥0.5 m) used here to count them as saplings even in new gaps. In newly formed understory gaps, we found an equally low median sapling biomass in most gap positions (Figure 5
) which differed only slightly from that of closed-stand conditions. This finding suggests that there were no differences in pre-disturbance regeneration structure.
Changes over time in sapling density largely depend on the availability of seeds and the conditions for germination and survival. While seed germination seems not to depend on light availability in European beech [6
], radiation intensity has frequently been reported to be a key factor determining the survival and development of seedlings and saplings [9
]. Small saplings of 0.5–1.5 m height showed a median shoot length growth rate of 10–18 cm a−1
in the gaps in Kyjov, which is in the range of growth rates recorded for beech saplings under 9–15% relative light intensity [11
]. At such light intensities, beech is capable of forming stable seedling banks, as observed in a mixed beech-fir-spruce forest in Poland [9
], which implies that mortality rate generally did not exceed the rate of establishment. In accordance, median sapling density was higher in all gap positions than in the closed stand in Kyjov.
The GLMM revealed a significant effect of direct radiation intensity on sapling density, but no effect of diffuse radiation intensity. Generally higher sapling densities were observed in the light regime classes HH
with comparatively high direct radiation as compared to class LH
with high diffuse radiation, suggesting a dominant effect of direct light intensity on sapling density in support of the first part of our second hypothesis (H2). In contrast, high amounts of diffuse radiation seem to promote sapling growth but not seedling establishment. This view is supported by the observation that sapling shoot length growth was higher in gap positions with high diffuse radiation than in positions with low diffuse light, while there was no positive effect of elevated levels of diffuse radiation on sapling density (Table 2
). Thus, the rate of seedling establishment and early survival seems to depend largely on the intensity of direct radiation.
Even though the availability of direct light is comparatively low in the LL
microsites, in old medium gaps they showed the highest median sapling density of all gap positions. Sapling density in these gap positions (LL
) was significantly higher in old medium gaps than in old small gaps. Thus, there must be other factors that positively influenced establishment and survival in the LL
microsites of medium gaps. We suggest that a combination of seed dispersal effects and higher diffuse radiation (due to increasing gap size) likely is responsible for the high sapling density. Beech seeds are dispersed only within small distance to the source tree through barochory and zoochory. Therefore, seed density is typically by far higher below fruiting trees (LL
microsites) than at more distant locations, e.g. in gaps with higher light intensity [7
]. Even if the rate of establishment is comparably low, the higher density of seeds might lead to a higher absolute number of seedlings. The spatial effect of seed availability imprinting on the spatial pattern of tree regeneration in gaps was shown in several studies in previously managed beech forests [40
]. Even though not significant, gap size tended to be an important factor for sapling density in the model calculations (p
= 0.077). Assuming similar seed densities in positions below the canopy, the higher sapling density in LL
microsites of medium gaps indicates that the rate of establishment is higher in medium than in small gaps. As saplings in the gap periphery showed considerably higher shoot length growth rates in medium than in small gaps (Figure 4
), it seems likely that seedling and sapling survival also profit from a larger gap size.
Seedling survival and the associated sapling density could also depend on competition for light in dense sapling populations. However, our sapling density data suggest that plant densities were in most cases <1 m−2 and thus too low to result in significant competition between saplings. Only in gap position HL, median sapling density exceeded 1 m−2 already in new gaps and competition may have resulted in self-thinning processes. Median sapling density and variability in these positions were lower in old gaps compared to new gaps.
Several studies reported negative effects on regeneration establishment and development by root competition from the bordering stand [67
]. Thresholds for categorizing relative intensity of root competition would approximate those for diffuse radiation. The categories of intensity would be inversely arranged, i.e. root competition is relatively high in the gap periphery and low in the gap interior. However, in medium-sized gaps the highest sapling densities and values for biomass were found in the gap periphery (LL
), indicating that establishment and productivity of beech saplings were not fundamentally constrained by root competition.
The significant positive effect of comparatively high direct radiation intensity on sapling density may perhaps relate more to associated thermal effects than to the influence of radiation itself. Notably, air temperature close to the ground and soil surface temperature are typically higher in gap positions which receive high light intensities [70
], and these reach maxima when direct sunlight hits the spot [71
]. A warmer soil surface could positively affect germination and early seedling development [72
], especially at relatively cool sites as in Kyjov forest. Elevated soil surface temperatures could also increase the N mineralization rate in the organic layer [73
], which may facilitate seedling survival on acid, relatively nutrient-poor soils. Finally, more rapid decomposition in the warmer gap positions (HH
) could result in thinner organic layers, which represent an improved seedbed and favor early seedling survival [60
]. A lower thickness of the organic layer along with a higher density of beech regeneration in gap positions receiving relatively high direct sunlight (HH
) was observed in a Slovenian beech-fir forest [40
Gap age (<10 or >10 years) did not significantly affect regeneration density. However, the higher sapling densities in gaps compared to closed-stand conditions suggest that a considerable number of beech seedlings must have established in the first months or years after gap formation, especially in gap positions HH
. As new gaps were not formed immediately before the survey, this early period of colonization was largely missed in our study. Further, the subsequent temporal development of sapling density was not consistent across gap positions (Figure 3
) with either increases, decreases or no changes in density. Therefore, a significant gap age effect did not appear in our data. The observed trends in sapling density between new and old gaps may be interpreted as an expression of interacting effects of seed dispersal (high in LL
), seedling establishment success (high in HH
), and competition (reduced density in HL
in old gaps).
The main determinants of the shoot length growth of seedlings and saplings were the initial size of the plants and the relative amount of diffuse radiation, which is related to gap size [34
]. This finding confirms our hypotheses H3 and H2. Positive effects of increased levels of diffuse radiation on shoot length growth of beech regeneration have frequently been observed [10
], but see [75
]. In other studies that applied the conceptual model of Diaci [38
] for separating light classes, higher growth rates in gap positions receiving relatively high diffuse radiation have been reported as well [44
]. Thus, our findings are in agreement with the results of experiments and field observations on the light response of tree sapling growth. In accordance, beech sapling height was found to increase progressively from the area under the canopy towards the gap interior [44
]. However, the positive effects on sapling height development by increasing diffuse radiation are saturating [12
], i.e., sapling height development likely benefits from increasing gap size only until a certain size is exceeded [78
]. That shoot length growth does not only depend on the abiotic environment but also on plant size supports earlier observations [10
] and is an indicator of the competitive advantage for saplings that established prior to gap formation. The growth data indicate that only in the periphery of small gaps, light intensity fell below a certain critical level, which strongly hampered the height growth especially of medium-sized saplings (1.5–2.99 m tall).
In contrast to the situation with diffuse light, we did not find a significant stimulation of sapling growth rate by elevated levels of direct light. This may result from the well-known sensitivity of young beeches to extended periods of excess radiation, which may cause photoinhibition [79
] and the formation of small, more xerophytic leaves [12
]. Yet, radiation intensity matters: Short episodes of higher direct light intensity, as they occur in sunflecks, can contribute much to the carbon gain of understory plants, as was shown by gas exchange measurements for beech regeneration [80
]. Beech seedlings and saplings have a remarkable potential to adapt to the elevated light levels, which are found in gap positions receiving direct light [52
]. It is not known whether part of the additional carbohydrate gain received through the interception of direct radiation is invested belowground, or in increased diameter growth.
How successful beech regenerates in gaps of different sizes and ages, may best be seen from our data on sapling biomass per ground area. Although we have only biomass estimates and no harvest data, they demonstrate that the regeneration success in small gaps (<100 m2) is clearly highest in gap position HH with comparatively high direct and diffuse radiation. In the other gap positions, either plant establishment (and thus sapling density) or growth rate was limited by low direct light and/or diffuse light.
In medium-sized gaps, sapling biomass reached higher values in most gap positions (except for HH) than in small gaps, probably due to higher levels of diffuse radiation that promoted shoot length growth even in the gap periphery. Here, saplings in the smallest recorded size class (<1.5 m) grew in height at relatively similar rates in all gap positions. Consequently, in these larger gaps, the density of seedlings established prior to, or shortly after, gap formation largely determined sapling biomass. In contrast, subsequently colonizing seedlings (which appeared mainly in HH and LL) contributed only to a minor extent. Thus, early colonizing advanced saplings dominated space filling. The rather low sapling density in gap position LH (median density in new medium-sized gaps: 0.5 m−2) seems to be sufficient to fill the area without any time lag. The biomass data also suggest that competition between saplings plays a decisive role for sapling survival only in later phases of regeneration development, while seedling densities are generally too low for lateral competitive interaction in the early phase of gap filling. In peripheral gap positions (LL and HL), lateral canopy closure and increasing sapling heights and leaf areas in the gap interior (LH and HH) will likely hamper the development of the saplings in the medium term.