Natural disturbance in northern Japanese mixed forests

Transcription

1 Natural disturbance in northern Japanese mixed forests Uryu Experimental Forest plan to visit on 16 JUN In natural mixed forests in Hokkaido, the annual mortality of overstory trees has frequently been less than 1% in basal area (Kubota 2000; Yoshida et al. 2006). Nevertheless, extreme mortality occasionally results from strong winds. Such a large disturbance can destroy stand structure; alternatively, it can contribute to sporadic regeneration of several tree species in mixed stands. Together with canopy gap formation, the supply of fallen logs and tip-up mounds is recognized as a significant process through offering seedlings a favorable establishment condition. The M3 Catchment (3.5 ha), located in Uryu Experimental Forest (UREF), Hokkaido University ( N, E) is designated as an experimental catchment to examine the forest stand dynamics and its functional relationships with biogeochemical processes. The east half (left side) is densely dominated by conifer species, besides the west half (right side) is gappy with more broadleaved components.

2 The forest understory is almost exclusively dominated by dwarf bamboo (Sasa senanensis) except for some riparian zones. In September 2004, a severe typhoon struck Hokkaido Island. The maximum wind speeds were recorded as greater than 30-m s -1, causing wind-induced damage of 13.3 % of basal area in this stand.

3 Regeneration on tree-fall-oriented structures Fallen logs and tip-up mounds are recognized as significant establishment microsites for seedling establishments. This is particularly important for forests in this region where dwarf bamboos are exclusively dominate the understory. However, considerable variations are existed in the seedling density among fallen logs and among tip-up mounds. To examine the causes of the variation, we are conducting seedling census in this stand. (Noguchi & Yoshida 2004) Picea glehnii 1.91 (6.6) 5.63 (11.7) Abies sachalinensis 1.92 (6.0) 2.87 (6.3) Relative height from ground surface (m) Seedling density (/m 2 ) Betula platyphylla (4.1) Figure. Seedling densities (except for 3.17 (6.7) current year seedlings) in relation to 1 relative height of quadrats from the ground surface at the end of third 0 growing season. The average and sd (in parentheses) are shown -1 respectively for mound (blue) and for pit (red). -2 (Harada et al. unpublished)

4 Forests in Dorokawa basin Uryu Experimental Forest plan to visit on 16 JUN The Dorokawa basin is located at north of the Lake Shumarinai. The size of the basin is ca.4000ha, consisting of ca. 700 ha of flat area (wetland) and the surrounding mountainous area. The flat area includes marshland, riparian broadleaved forests and Picea glehnii forests. Mountainous area is largely dominated by conifer-broadleaved mixed forests, the main components of which are Abies sacharinensis and Picea glehnii in conifer, and Quercus mongolica, Tilia japonica, Acer mono, Betula ermanii, Betula platypgylla, Kalopanax pictus and Phellodendron amurense in broadleaved species. The current woody volume of the basin is estimated to be 120m 3 ha -1, in which ca. 70% of the volume is broadleaved trees. Non-wooded land covered densely with dwarf bamboos occupied ca.40% in area of the basin, mainly in high altitude or steep slope area. Such area had been considerably expanded by the major typhoon in Picea glehnii preserved forest on wetland Picea glehnii occur in conifer- broadleaved mixed forest, while they often develop pure stands on relatively infertile conditions (e.g. wetland or serpentinous soil area). The preserved forest located along the Dorokawa river shows typical stand structure grown on wetland with acidic peat soil. We established study plot of 0.5 ha in 1992, and are measuring trees with dbh > 5cm. The canopy of the stand consists of Picea glehnii with Betula platyphylla as a scarce associate. The dbh-class distribution shows L-shape, indicating stable size structure. Establishment of trees primarily depends on fallen logs or mounds. Changes in stand structure seemed to be minor with low recruitment and mortality (0.005 and ha -1 year -1, respectively), suggesting slow changes in structure without major disturbances. Number of trees (/ha) DBH-class (cm)

5 Establishment process A phytolith analysis suggested that Picea glehnii colonized the wetland c.a. one thousand years ago. Regarding the relatively long life-span of this species (several hundred years), the current individuals seems to be in earlier generations. Published work: Kawano T et al (2007) Holocene phytolith record at Picea glehnii stands on the Dorokawa mire in northern Hokkaido, Japan. The Quaternary Research 46: Phytolith analysis was applied to clarify a vegetation history at Picea gluhnii stands on the Dorokowa mire, northern Hokkaido. Phytolith assemblages from surface samples and Holocene deposits from the mire are used to clarify the stand-scare vegetation history, with special focus on the dynamics of Gramineae, which includes dwarf bamboo, and the establishment of P. gluhnii stands on the mire. The phytolith records during the early-holocene (ca. 12,000 to 10,000 cal BP) indicate the distribution of Pooideae-dominated vegetation, which reflects the cool climate. Phragmites (reed) dominated vegetation from the early to mid-holocene (ca, 10,000 to 5,000 cal BP) suggests wet conditions like low-moor. The reed was replaced by dwarf bamboo at ca. 5,000 cal BP, although the distribution of dwarf bamboo was scattered. At ca. 1,500 cal BP, dwarf bamboo increased abruptly and has been sustained to the present. Also, Pinaceae type phytoliths began to occur continuously at ca. 1,000 cal BP. Taking into account the relationship between modern vegetation and modern phytolith assemblages on the mire, the occurrence of the Pinaceae type phytoliths from the sediment samples indicates the existence of P. glehnii in the study sites. On the basis of these results, P. glehnii stands with dense undergrowth of dwarf bamboo would have been established in the study sites on the mire by, ca. 1,000 cal BP at least.

6 Selection harvesting in mixed stands After 1950s, most of the stands in the mountainous area have been repeatedly logged with single tree selection manner. We set up long-term study plots to monitor the consequences of the management. Logging may cause further increase in Sasa (dwarf bamboo) cover, as well as decrease in fallen logs on which particular species preferably regenerate. Sustainable logging system may need lower cutting rate with site preparation for seedling establishment. Published work: Noguchi M & Yoshida T (2003) Tree regeneration in partially cut conifer hardwood mixed forests in northern Japan: roles of establishment substrate and dwarf bamboo. Forest Ecology and Management 190: We assessed the relationship of the regeneration (seedling and sapling) densities of seven representative tall-tree species to the past partial cutting and current stand structure. We also determined if differences in tree regeneration were associated to establishment substrates (coarse woody debris (CWD) and root throws) and understory inhibitor vegetation (the dwarf bamboo species: Sasa senanensis). The study was conducted in 17 conifer hardwood mixed stands in a heavy snowfall region in Hokkaido, northern Japan. The results suggest that stand structure, rather than logging intensity, is the primary factor influencing regeneration densities. Total conifer basal area was positively correlated with the regeneration density of individual tree species, including two conifers and three hardwoods. These patterns differ from those observed in old-growth stands in the region. A negative correlation between total conifer basal area and dwarf bamboo coverage suggests that the presence of dense conifer canopies causes an increase in regeneration density of tall-tree species by preventing domination of dwarf bamboo. Picea glehnii, a species that depends for its establishment strongly on CWD, has lower seedling and sapling density in stands with higher logging intensity. This seems to be a result of the decrease in the volume of CWD with increasing logging intensity in these stands. We suggest that both reducing logging intensity and retaining overstory conifers should be considered to develop a sustainable silvicultural system in this region. Providing sufficient CWD and root throws may also be important to ensure natural regeneration of tree species that require these as an establishment substrate.

7 Stand structure and dynamics of an oak (Quercus crispula)-dominated mixed primeval stand Uryu Experimental Forest visit (possibly) In northern Hokkaido, mixed stands are frequently dominated by Quercus crispula. In this type of a stand, we maintain a 1-ha-scale tree census since 1982 to observe its stand dynamics. Quercus crispula share 67 % of basal area with low density of small trees, and Abies sachalinensis, Acer mono and Acer japonicum dominate the sub-canopy and sapling layer. Dwarf bamboo (Sasa senanensis) is densly established in understory. Published work: Takahashi et al. (2003) Stand structure and dynamics during a 16-year period in a sub-boreal conifer hardwood mixed forest, northern Japan. Forest Ecology and Management 174: 39-50

8 The stand structure and regeneration dynamics of trees >2.0 m in trunk height were studied during in a 1 ha plot in a sub-boreal conifer hardwood mixed forest, northern Japan, with a dense dwarf bamboo in the understory. Total density was low in 1982 (651 trees/ha), as compared with other forests in Japan. Quercus crispula was dominant in the upper canopy layer but their saplings were rare, while Acer mono, Acer japonicum and Abies sachalinensis were dominant in the sub-canopy and understory layers with many saplings. Mortality of each species was quite low during the census period (average 0.57% per year), and there was no clear difference among the four species. The age structure of Q. crispula was bell-shaped with a peak at ca. 200 years, while that of the other three species was weakly reverse-j-shaped or a rough plateau. In addition, no recruits growing over a height of 2 m were observed during the census period in Q. crispula, but many recruits of the other species were observed. These suggest that Q. crispula depended on episodic disturbances for the persistence of its population. Recruits of the three species except for Q. crispula did not concentrate in canopy gaps probably because of the dense dwarf bamboo cover there. They showed a negative spatial association with their own canopy trees, but a positive association with canopy trees of Q. crispula. Most of the crowns of the three species (A. sachalinensis and the two Acer spp.) were lower than that of overtopping Q. crispula. These spatial associations between recruits and canopy trees brought about the competitive effect of Q. crispula on the growth rates of other species and that of itself. However, the low mortality of trees taller than 2 m indicates that intra- and interspecific competition was not strong as a structuring force of the tree community. Our long-term study suggests that factors affecting recruitment (disturbances and dwarf bamboo in the forest floor) are more important for species coexistence than intra- and interspecific competition between trees taller than 2 m. Number of trees (/ha) Quercus crispula Abies sachalinensis Acer mono Acer japonicum Tree age class (year)

9 Regeneration management in non-wooded area Uryu Experimental Forest plan to visit on 22 OCT Dwarf bamboos are acknowledged as key to the structure and dynamics of Japanese forests, since they are widely distributed and often form exclusively dense undergrowth in many forests. Several researchers have reported the negative effects of dwarf bamboo undergrowth on the density of tree seedlings (Hiura et al. 1996; Nagaike et al. 1999) and on understory plant species diversity. Sasa kurilensis, a larger species with height 2-3m, is more likely to dominate in higher altitude area with making larger monospecific community than S. senanensis, a smaller species with height 1-2m. Dense growth of both two species inhibits the establishment of other plant species, and harvesting often result in enhancement of their growth rather than tree regeneration (Noguchi and Yoshida 2005). Therefore, elimination of dwarf bamboos is a main issue for enhancing tree regeneration as well as maintaining species diversity in this region. Because dwarf bamboos can produce new shoot from their root stocks, cutting of aboveground stems is generally insufficient. Scarification is conducted widely in northern Japan to get rid of understory dwarf bamboo species before replacement with tree species. Natural regenerating is applied in area with sufficient mother trees, and it generally results in establishment of a pure Betula stand in this region. The major plantation species is Picea glehnii. In the Uryu Experimental Forest, the other plantation species include Abies, Fraxinus, Kalopanax (by root burying), and Quercus (by sowing). Some of treated sites are designated as standard plots for monitoring recovery of woody vegetation for a long period. Published work: Yoshida T et al (2005) Factors influencing early vegetation establishment following soil scarification in a mixed forest in northern Japan. Canadian Journal of Forest Research 35:

10 Scarification is widely conducted in northern Japan to remove understory dwarf bamboo species in degraded forests for replacement with tree species. To explore ways to enhance species diversity and restoration of mixed forest at the treated site, we clarified the mechanisms that lead to compositional heterogeneity of plant species. We evaluated the relative importance of environmental factors (scarification properties, soil properties, light conditions, litter cover, and presence of canopy trees) for the demography of tall tree species (emergence, mortality, and growth) and whole vegetation structure (species diversity and composition) over the two growing seasons immediately following scarification. Of tall tree species, Betula spp. were dominant (60% in total density), followed by Abies sachalinensis (Fr. Schm.) Masters, Acer mono Maxim., and Phellodendron amurense Rupr. Light intensity was an important factor, having mostly negative effects on the demography of these species. Soil factors (e.g., nitrogen content, moisture) affected the demography mainly of shade-intolerant or hygrophilous species. In general, extreme environmental conditions led to the dominance of grasses, forbs, and lianas rather than tall trees. Maintenance of canopy cover, which limits light and supplies seeds as well as litter, proved to be most important in promoting plant species diversification on the scarification site.

11 Published work: Nagai M & Yoshida T (2006) Variation in understory structure and plant species diversity influenced by silvicultural treatments among 21- to 26-year-old Picea glehnii plantations. Journal of Forest Research 11: 1-10 We investigated effects of silvicultural treatments (planting and subsequent treatments) on understory structure and plant species diversity in managed Picea glehnii plantations (21 26 years old) in northern Japan. We evaluated the importance of each treatment (machinery site preparation, planting, weeding, and thinning) in 19 plantations, with considerable variation among treatments overall. The understory had 98 vascular plant species; the most dominant species was a dwarf bamboo Sasa senanensis, followed by tree species Abies sachalinensis and Betula ermanii. Multiple regression analyses showed that thinning negatively influenced plant species diversity. Planting density showed a strong positive correlation with density of seedlings (height <20cm), but few independent variables were correlated with density of saplings (height 20cm and diameter at breast height <1cm). The negative effect of thinning and the positive effect of planting density seem to be related to the existence or reinitiation of dense cover of Sasa senanensis. We present possible mechanisms of response to treatments to generalize the results. We then suggest ways to improve current treatments to meet the goals of wood production and biodiversity conservation in the region.