The material in this section is based largely on information provided by Frelich (2002), who notes that it is possible to infer historical patterns of forest disturbance by using information from a variety of sources, including:
● Fossil evidence. Fossil pollen and other durable plant parts such as seeds, needles, and wood fragments can be preserved in sediments, and provide a means of reconstructing forest composition in the past. Small forest hollows have proved to be particularly useful for assessing changes in forest composition at the local scale (Calcote 1995). The presence of charcoal in forest soil profiles can be used to infer historical fire frequency (see Clark and Royall 1995), and can be subjected to carbon-14 analysis to provide dating of fire events.
● Historical records. Forest managers often routinely collect information on specific disturbance events such as storms or fires, as well as records of timber extraction. Alternative historical sources include newspapers and other media, records made by surveyors or naturalists, and weather records.
● Physical evidence. The most common form of physical evidence is the presence of tip-up mounds, produced as a result of treefalls. The date of formation of the mound can sometimes be determined by excavating it, and ageing the trees growing nearby (Henry and Swan 1974). In northern temperate forests, tip-up mounds can remain visible for 200 years or more. Another form of physical evidence is the presence of cut stumps, which can be used to infer previous logging events.
Historical information can also be gathered from ancient documentary evidence, such as legal records, charters, and historical maps; archaeological artifacts; earth- works and surface features; iconography; and oral tradition (Rackham 2003). Use of such evidence has given rise to the discipline of historical ecology, which aims to identify the historical factors that have influenced the development of vegetation to its current state. The methods arguably lie closer to historical than to ecological research, and are therefore not considered in detail here. However, their potential value should not be underestimated. In the UK, the historical research of Oliver Rackham (Rackham 1986, 2003) has not only transformed our understanding of woodland ecology, but has led to a major shift in conservation policy. So-called
‘ancient woods’, which have a history of management stretching back over many Forest disturbance history | 161
centuries, are now rightly recognized as of exceptional conservation importance (Rackham 2003).
Most commonly, forest disturbance history is inferred from measurements of stand size and age structure. The frequency distribution of tree diameters can usefully be plotted to assess whether or not the stand is even-aged, indicated by a unimodal diameter distribution. However, without measurements of the age of the trees, interpretation of diameter-frequency distributions is subject to a great deal of potential error. To reduce such errors, diameter distributions should ideally be plotted separately for the dominant species in a single homogeneous stand, or for only those trees receiving direct sunlight on the top of the crown. Another struc- tural measure that can be used is the diameter-exposed crown-area distribution, which is obtained by estimating the cross-sectional area of the portion of the crown of each tree exposed to the sun. These areas are then summed for all trees within each size class. The resulting measure represents the proportion of the total exposed crown area of the stand occupied by each size class, and has the benefit of equalizing for the different densities that can occur in various size classes.
Tree ages can be determined most readily by increment corers, at least in temperate forests (see section 3.6.1). Although the age distribution of trees can be very helpful for inferring stand disturbance history, the results may often be open to a variety of different interpretations. For example, the same type or intensity of disturbance can result in stands with different age structures, simply because of variation in the pattern of recolonization following the disturbance. Stands with even-aged struc- tures can be produced by a series of disturbances, rather than just a single stand- levelling disturbance event, such as when seedling recruitment is interrupted by deer browsing or ground fires. Trees within a stand can display a wide variety of ages, reflecting the survival of individual trees following a range of different disturbance events.
Patterns of growth increment can also be obtained from increment cores and can provide a valuable source of information about disturbance history. If a large tree is blown down, then neighbouring trees that were formerly suppressed are likely to display a period of significantly higher radial growth. Analysis of such growth patterns can enable disturbance chronologies to be obtained for entire stands of trees, and enable those trees that were already in the canopy at the time of disturb- ance to be identified. However, care must be taken when inferring disturbance history from increment cores. A tree that has been released may subsequently be overtopped again by a competitor, making the original disturbance more difficult to detect. Trees some distance from a newly created gap can respond to increased light availability, and as a result, estimates of disturbance intensity may be inflated.
Details of the techniques of radial increment pattern analysis are presented by Frelich (2002), who provides the following guidance based on detailed analysis of northern temperate forests in the USA:
● Criteria of 100% and 50% increases in ring width after disturbance are typically used to indicate ‘major release’ and ‘moderate release’ for shade-tolerant
species, the former indicating transition from an understorey to a canopy position.
● For the release to be considered abrupt, indicating a sudden disturbance event, the 50% or 100% release in ring width should occur within a period of 1–5 years.
● Criteria of at least 15 years slow growth before release and at least 15 years of rapid growth after release are used to screen out growth patterns that are not related to the disturbance events of interest.
Use of this method, by applying these criteria, can enable disturbance chronologies to be produced that indicate the proportion of trees that entered the canopy in each decade.
Attention also needs to be paid to sampling approaches when taking cores for increment analysis. Typically, individual trees are selected by using a random or systematic method; for example, the trees closest to randomly located sample points may be selected for coring. Age structure may be represented either as the proportion of individuals in each age class, or as the proportion of area of a plot or landscape occupied by each age class, based on measurements of crown area.
Area-based samples are commonly used in studies of disturbance, where disturb- ance rate is expressed as a percentage of forest area disturbed per unit time.
In some cases it is not possible to obtain complete increment cores, because the trees are hollow or are too large to core. The dbh and crown area of such trees should be measured, so that they can be taken into account in area-based calcula- tions of disturbance. Small trees (5 cm dbh) also tend not to be sampled by using increment borers; instead, a proportion of them may be felled to procure ring measurements.
How many trees should be sampled for increment analysis? The sample size determines the precision of the estimates obtained, and the chance of missing a cohort of trees in the sample. The probability of failure to detect an age class of trees can be calculated from the following equation (Frelich 2002):
Pf (1Py)x
where Pfis the probability of failure to detect age class y,Pyis the proportional area occupied by age class y, and x is the number of independent sample points.
Precision can be estimated by calculating the confidence limits for proportions (Frelich 2002):
where pis the sample estimate of the proportion of points belonging to a given cohort, and Nis the number of sample points. As a rule of thumb, 5–10 cores may be enough to characterize the disturbance history of even-aged stands; 30 or more cores may be needed in complex multi-aged stands (Frelich 2002).
Fitting functions to stand age distributions (see section 3.7.1) can also provide insights into disturbance history. It is also useful to consider the hazard function,
1.96冪(p(1p)/N)
Forest disturbance history | 163
which expresses the chance of disturbance with stand age. For example, an equal hazard function (or equal probability of disturbance across all stand ages) results in the negative exponential age distribution. The formula for the negative exponential is:
A(t) exp((t/b))
where Ais age, tis time in years, and bis the mean stand age. If the cumulative frequency of stand ages is plotted on a semi-log graph, a straight line is obtained if a negative exponential distribution fits (Frelich 2002). Changes in the slope of this graph indicate changes in the rate of disturbance (1/b).
In the case of the Weibull function (see section 3.7.1), when the shape parameter (c) is 1, the distribution is the same as the negative exponential, with a constant hazard function. When c1, disturbance hazard increases with age.
Rubino and McCarthy (2004) provide a recent review of the use of den- drochronological techniques to assess forest disturbance history, and highlight two limitations of radial-growth methods for the assessment of disturbance regimes:
● The length of time used for determining the mean growth rate must be long enough to take account of climatic anomalies (such as extended periods of increased or decreased precipitation), yet permit identification of short-term dynamics.
●Release identification methods may not be able to detect a disturbance event if two release events occur in rapid succession.
Despite such problems, the authors conclude that radial growth analysis is a useful technique for characterizing forest disturbance history, particularly where destruc- tive and invasive sampling is undesirable or prohibited, such as in protected areas (Lorimer 1985).