Developmental stages and fine structure of surface callus formed after debarking of living lime trees (Tilia sp.)

Wounding of trees by debarking during the vegetative period sometimes results in the formation of callus tissue which develops over the entire wound surface or on parts of it. This light and transmission electron microscopy study of living lime trees found that the formation of such a surface callus is subdivided into three stages. During the first stage, numerous cell divisions take place in regions where differentiating xylem remains at the wound surface after debarking. This young callus tissue consists of isodiametric parenchymatous cells. Cambium cells, sometimes also remaining at the wound surface, collapse and do not contribute to callus formation. During the second stage, cells in the callus undergo differentiation by forming a wound periderm with phellem, phellogen and phelloderm. In the third stage, a cambial zone develops between the wound periderm and the xylem tissue laid down prior to wounding. This process is initiated by anticlinal and periclinal divisions of a few callus cells only. Later this process extends tangentially to form a continuous belt of wound cambium. Subsequently, this cambium produces both wound xylem and wound phloem and thus contributes to further thickening.     

Appropriate vegetation indices for measuring the impacts of deer on forest ecosystems

Increasing deer density can cause serious degradation of forests in the Americas, Europe, and Asia. To manage deer impacts, evaluating their current impacts on forest ecosystems is necessary, usually via vegetation indices. However, the relationship between vegetation indices and absolute deer density, while taking into account tree size, snow depth, light condition, and the type of understory vegetation, has never been investigated. We examined the relationship between various vegetation indices and absolute deer density in 344 study plots in the deciduous broad-leaved forest of Yamanashi Prefecture, central Japan. In each plot, debarking and browsing, along with the coverage and maximum height of understory vegetation, were surveyed. Estimated deer densities for 82 5 × 5-km mesh units ranged from 0.8 deer/km² to 32.7 deer/km². The percentages of debarked trees within a plot ranged from 0 to 84%. Debarking was promoted by high deer density, small tree size, and thick snow. The effect of tree size on debarking was stronger than that of deer density. Occurrence of browsing on understory vegetation was higher at higher deer densities, and where understory vegetation was dominated by evergreen dwarf bamboo. Coverage and maximum height of understory vegetation were unaffected by deer density but increased with canopy openness and the dominance of dwarf bamboo in the understory. Overall, we predict that debarking of small trees living in heavy snow areas should occur even at low deer densities (<10 deer/km²). Browsing on dwarf bamboo should occur at intermediate deer densities (10–30 deer/km²), while debarking of thick trees living in low snow areas should occur only at high deer densities (≥30 deer/km²). Our study shows that debarking and browsing on understory vegetation are appropriate indices for evaluating deer impacts on forest ecosystems, but that tree size, snow depth, and the type of understory vegetation should also be considered.     

Responses of the mangroves Avicennia marina and Bruguiera gymnorrhiza to oil contamination

The effects of bunker fuel oil on morphological and physiological responses of Avicennia marina and Bruguiera gymnorrhiza were investigated in glasshouse and field experiments. In the glasshouse study, 15-month-old seedlings of A. marina were subjected to oiling or debarking treatments for 6 months. Oiling or debarking of a 5 cm ring of the basal portion of the stem, alone and in combination, reduced leaf CO₂ exchange by over 50% and resulted in the production of adventitious roots immediately above the debarked and/or oiled stem 8–12 weeks after the commencement of treatments. In the field study, sediment oiling at a single dose of 5 l m⁻² of A. marina and B. gymnorrhiza trees reduced electron transport rate (ETR) through Photosystem II (PSII) and PSII quantum yield. Oiling also reduced the photochemical efficiency of PSII (F_v/i_m) in B. gymnorrhiza, but not in A. marina. After 15 weeks of oiling, adventitious roots developed at the base of the stem in A. marina, but not in B. gymnorrhiza. Naturally occurring A. marina seedlings with adventitious roots exhibited lower leaf CO₂ exchange rates, photochemical efficiency of PSII and leaf chlorophyll content than similar seedlings without these roots. These results indicate that bunker fuel oil adversely affects photosynthetic performance of A. marina and B. gymnorrhiza mangroves. A. marina responds to oiling by producing adventitious roots at the base of the stem. Adventitious root production at the base of the stem may be a useful biological indicator of oil or other toxic pollutants in A. marina.     

Transition of a Sambucus nigra L. dominated woody vegetation into grassland by a multi-species herbivore assemblage

We describe and analyse how large herbivores strongly diminished a woody vegetation, dominated by the unpalatable shrub Sambucus nigra L. and changed it into grassland. Density of woody species and cover of vegetation were measured in 1996, 2002 and 2012 in the grazed Oostvaardersplassen. In 2002 and 2012 we also measured density and cover in an ungrazed control site. In 2002 we measured intensity of browsing and bark loss of Sambucus shrubs in the grazed and control sites. In the grazed site the density of Sambucus and Salix spp. declined significantly between 1996 and 2012, and large areas changed into grassland. In the control site the density of Sambucus increased significantly during this period, the density of Salix spp. did not change, and the vegetation consisted of a mixture of woody species and a field layer dominated by tall herbs. In 2002 and 2012 the percentages of dead Sambucus shrubs were significantly higher in the grazed site than in the control site. In 2002 the percentages of twigs browsed and ring barked stems of Sambucus shrubs were significantly higher in the grazed site than in the control site. Our results show that debarking caused mature Sambucus shrubs to die, but that heavy browsing may have helped this process. Our results also point to a significant neighbour effect on the break down of Sambucus, suggesting that Aggregational Resistance and Associational Palatability were both active. Essential conditions for the break down of this woody vegetation were the presence of large herbivores, the low ratio between the areas of summer and winter feeding habitats and the competition amongst herbivores. Browsing may have been responsible for seedling death, as seedlings were found only in the control site and not on the old and newly established grasslands in the grazed site.