Estimation of Grass Plant Biomass

Transcription

1 1 Module # 12 - Component # 3 Estimation of Grass Plant Biomass Objectives To achieve an understanding of the purpose and methods for estimating Grass plant biomass. Expected Outcomes To be able to understand the purpose of studying the grass plant biomass. To be able to understand the methods for estimating grass plant biomass. To be able to relate the results of a study of the grass plant biomass to fire management decisions.

2 2 Grass Biomass The biomass of grass plants is the total leaf mass of the plants in a given environment or community. It is measured as either live or dry-weight (mass) per unit area. The estimation of herbaceous biomass is important for two reasons: 1. Indicates the total amount of herbage on offer to animals and is in direct correlation with the grazing capacity of the area. 2. Determines the fuel load of the grass layer. This information is invaluable in the determining of fire use for habitat management. The herbaceous / grass layer contributes the most to the fuel load in savannas and consists mainly of grasses. The determination of the fuel load must be time and cost efficient, and easily conducted with accuracy. Several methods exist to determine the phytomass of grass swards. Methods are based on samples that are extrapolated to vegetation units. These methods differ in their approach, practicality and efficiency. The strip sampling and quadrat sampling methods determine the grass phytomass in a given strip or quadrat by harvesting the grass, drying it and determining the mass. However the method of choice by the majority of wildlife managers is the Disc Pasture Method.

3 3 Disc Pasture Method The disc pasture method is widely regarded as the most efficient method for determining the herbaceous layer. The method is as follows: The apparatus used is called a disc pasture meter, as illustrated in Figure #1. The disc pasture meter consists of an aluminium rod of 1,8m (5.9 ft) with an aluminium sleeve of 1,2m (3.9 ft) that slides freely on the central rod. A steel base plate is attached to the lower end of the sleeve rod and an aluminium disc with a diameter of 45,8 cm (18 in.) is attached to the baseplate. The sleeve with the disc attached to it has a total mass of 1,5kg (3.3 lbs.). The central rod is calibrated in 1cm (0.4 in.) intervals starting at the upper end of the sleeve rod when the disc is level with the lower end of the central rod. If calibration data does not exist for the study area it must be calculated. This is done by taking a reading after which a shallow cylinder, corresponding to the size of the disc, is placed over the disc. The cylinder is then firmly pressed to the ground and all the grass material within the cylinder harvested, dried and weighed. A regression of the mass of the grass material harvested on settling height is then drawn. Sample sites are located in the same HVU s (homogenous vegetation units) as for the plant community classification survey. A 200m (655 ft) transect is chosen and 100 readings are taken by recording the disc height at 2m ( 6 ½ ft.)intervals. The disc is released from a standard height of 600mm ( 23 ½ in.) above the ground. The settling height of the disc on the grass sward is taken.

4 4 After the procedure has been carried out in each HVU a mean settling height for each unit is determined. This is then substituted into the regression equation to calculate the estimated fuel load. A = Central rod B = Level of meter reading C = Sleeve D = Disc Figure # 5 - The disc pasture meter.

5 5 Case Study: Msasa Nature Reserve The disc pasture meter was used to obtain estimates for the plant biomass production of the study area. The following regression equation was used: Y = x X (square root of X) Where Y = estimated fuel load in kg/ha X = mean disc height in cm The mean disc heights and estimated fuel loads of the communities are represented in Table # 4. Community Minimum disc height (cm) Maximum disc height (cm) Mean disc height (cm) Fuel load (kg/ha) Table # 4 - Disc height and estimated fuel loads for the communities of Msasa Nature Reserve

6 6 Fuel Loads and Burn Potential The methods and mechanics of veld burning have been covered previously in Module # 2 Component # 4: (Bush encroachment and Fires). However in order to make an informed decision on what to burn and the frequency at which it should be burnt, the results of plant biomass studies are important. The decision to burn or not to burn is recently become a contentious issue as a number of long term studies are completed. It had long been thought that should a block of vegetation support a high enough fuel load, then regular burning was the key to healthy veld management. However in a number of long term studies, where over 20 years adjacent blocks of vegetation closely monitored. One was burnt annually, another burnt biannually, another burnt once every 5 years and others not burnt at all (control). We find that none of the blocks show any significant increase or decrease in species richness, biomass or biodiversity, other than the fact that the burnt blocks are slightly more open having fewer trees; and in comparing trees shows that the unburnt blocks have larger more mature trees. The lack of significant change is largely due to that fact that Savanna ecosystems show a remarkable degree of resilience, and therefore the blocks do not exhibit any long term impact and consequently show little differentiation. The conclusion from these studies is that certain grasslands should be left alone in terms of a burning programme and are only burnt in the event of a natural fire caused by lighting strikes. The Kruger National Park (where most of the above mentioned studies were done) have abandoned their block burning programme in favour of a patch mosaic model, that mimics a natural fire occurrence. In this programme an area is identified based on fuel loads and species composition, and a lit match is dropped and left to burn out naturally.

7 7 However burning, as related previously, may have other management aims not related to habitat health. Burning is frequently used to encourage the movement of game and may be used in certain circumstances to control bush encroachment. Blocks of veld (mixed savanna grassland) that reveal fuel loads < 2000 kg/ha will be unable to sustain a burn, and fires started here quickly burn out. Blocks of veld (grassland) that reveal fuel loads below 4000kg/ha should not be burnt, and doing so invites habitat degradation and the resultant soil erosion. Blocks of veld that have fuel loads between 4000kg/ha and 7000kg/ha may be burnt to remove moribund vegetation (old grasses that suppress new growth) to encourage new growth. Blocks of veld with fuel loads in excess of 7000kg/ha may be burnt to control bush encroachment. However burning these blocks results in an all consuming fire and great care should be taken to prevent its spread. NB. It is important to note that these guidelines will vary according to site-specific conditions and management objectives for an area. For this reason, the burning program for the Msasa reserve differs from these guidelines. To successfully burn to control bush encroachment, the aspect of timing is very important. Fires should be lit in relation to the trees phenology (the study of their natural growth, flowering, leaf abscission and die back cycles). To effectively retard tree growth the burn should take place just after the tree has flowered. At this time the tree has just used up a significant amount of its resources in producing its reproductive structures and is therefore physiologically weak. To burn successfully the environmental conditions should also match the following criteria: 25 o C (77 o F) 30 % relative humidity < 20 kph (12 ½ mph) wind

8 8 Conclusion The considerably high fuel loads recorded in the communities is indicative of the situation on the ground, with the exception of Community 6 where dense patches of Hyparrhenia hirta occurs amongst the otherwise overutilized veld. Community 2 was not considered as the high percentage rock and often steep slopes would have given unreliable results. Grassveld deteriorates if it is not utilized for too long. The frequency of burning depends on the reproduction rate of the grass which in turn is related to rainfall and the degree and proportion to which the veld is utilized. The amount of fuel calculated can now be used to determine a burning program. The management conclusion and recommendations reached with regard to a burning programme are detailed in Module # 14 Component # 1: Game Reserve Management.