by Svetlana Savin | 30 January 2019
Published in Forest Ecology and Management 55: 259-278
Forest floor and mineral soil samples were collected from the interdune depressions of 6-, 8-, 17-, and 34-year old Casuarina equisetifolia Forst. & Forst. plantations growing on the coastal sand dunes of Senegal. The accumulation of organic matter as well as all mineral elements in the forest floor except sodium followed a linear pattern. The thickness of the litter layer increased from 4.2 to 8.0 cm. Forest floor biomass increased up to 120.3 Mg ha-1, an average increase of 3.3 Mg ha-1 year-1. Nitrogen in the forest floor accumulated up to 1567 kg ha-1; an average increase of 45 kg ha-1 year-1. Calcium, aluminium, magnesium, iron, potassium and phosphorus in the forest floor accumulated at 6.8, 3.7, 1.8, 1.3, 0.49 and 0.25 kg ha-1 year-1, respectively.
In the mineral soil, total phosphorus and exchangeable calcium decreased with plantation age. Nitrogen in the mineral soil increased by 30 kg ha-1 year-1 while organic matter in the mineral soil increased by 61.5 kg ha-1 year-1. The pH of the mineral soil decreased from 7.3 to 6.5.
As a result of a particular combination of species characteristics and ecological conditions, C. equisetifolia plantations on these coastal sand dunes appear to accumulate forest floor organic matter at rates comparable to those found in boreal and subalpine forests. A nitrogen to phosphorus ratio in the litter layer of 164:1 indicates that decomposition processes are strongly limited by phosphorus. This suggests that introducing strains of nitrogen-fixing bacteria capable of fixing even greater amounts of nitrogen would do little to improve soil fertility. Since there was no sign that accumulation of either organic matter or mineral nutrients had reached steady-state, a minimum of a 34 year rotation would seem justified if soil improvement is the primary management objective. Since the overwhelming majority of the organic matter and nutrient capital is located in the forest floor and upper 2 cm of mineral soil, proper management of this fragile layer following harvest will be extremely important if the benefits realized in the first rotation are to be capitalized on during subsequent rotations.
by Svetlana Savin | 30 January 2019
Published in Forest Ecology and Management 56: 13-28
Casuarina equisetifolia Forst. & Forst. plantations, established to stabilize sand dunes on the north coast of Senegal and thus protect inland agricultural areas, were sampled to determine if growth and yield were affected by microtopography. Microtopographical position, classified as interdune depressions, flanks, or summits, was found to have a highly significant effect on height, diameter, and volume growth. Maximum mean annual height increment was attained at Age 6 years (1.6 m year-1) in interdune depressions, at Age 7.5 (1.3 m year -1) on flanks, and at Age 8.5 (1.1 m year-1) on summits. Diameter distributions showed that the interdune depressions had a greater percentage of large diameter trees than flanks of summits. Maximum mean annual volume increment was 4.2 m3 ha-1 year-1 at Age 18 years for the interdune depressions, 2.0 m3 ha-1 year-1 at Age 29 for flanks, and 1.7 m3 ha-1 year-1 at Age 30 for summits. Thus, different rotation lengths can be recommended for specific microsites depending on microtopographical position and on whether fiber production or soil amelioration is the dominant management objective.
by Svetlana Savin | 30 January 2019
Published in Canadian Journal of Forest Research 33: 2452-2459
Precommercial thinning is often used to control stand density in naturally regenerated balsam fir (Abies balsamea (L.) Mill.) stands. Early stand density control could have beneficial effects on longer term stand stability through a modification of stem shape and root development. To assess the effect of precommercial thinning, two thinned and two unthinned stands were selected. Root sections were collected at 25 cm from the centre of the stem for all major roots (diameter greater than 2 cm). A disk was also cut at breast height level. From these disks, response in radial growth was determined. Detailed root measurements were taken on the sample disks to assess treatment effect on the following variables: asymmetry in root cross-sectional area (CSA) distribution, individual root shape, as well as changes in root shape over time. A quick and pronounced response in root growth occurred. This response was greater than that observed in the trunk. No asymmetry in root CSA distribution was observed at the stand level. Trees allocated more to radial growth above the biological centre of the root both in treated and control stands, but this trend was increased by thinning. Roots also tended to develop T-beam shapes over time, both in control and thinned stands. Most roots initially did not possess an I-beam shape and did not develop one during the course of the study. According to our results, trees respond quickly to the new growing conditions created by thinning by increasing biomass allocation to parts of the roots where mechanical stresses are greater.
by André Boily | 30 January 2019
Published in Tree Seed Working Group – News Bulletin. 46(December) : 10-12.
