Leaf lifespan (longevity) is defined as the time period during which an individual leaf (or leaf analogue) or part of a leaf (see Monocotyledons below in the present Section) is alive and physiologically active. It is expressed in days, months or years. Leaf lifespan relates to the nutrient-use strategy of a plant, thus providing an indirect index of important plant traits such as potential growth rate, nutrient-use efficiency and decomposability of litter. Long leaf lifespan is often considered a mechanism to conserve nutrients and/or reduce respiratory costs in habitats with environmental stress or low resource supply. Species with longer-lived leaves tend to invest significant resources in leaf protection and (partly as a consequence) grow more slowly than do species with short-lived leaves; they also conserve internal nutrients longer. The litter of (previously) long-lived leaves tends to be relatively resistant to decomposition.
Leaf lifespan does not necessarily indicate the phenology of growth or the proportion of the year when a plant is able to perform photosynthesis. This is because leaves may senesce all together (as in deciduous species), or their senescence may be spread out over a much longer period, in which case a plant may maintain several cohorts of leaves of different ages simultaneously, as is the case in many temperate evergreen species. In addition, some tropical evergreen species maintain a continuous green leaf canopy with a very short leaf lifespan, because they have a high and continuous rate of leaf production. Thus, to understand the ability of a plant to exploit its seasonal light environment and the timing of its growth, it is also useful to measure the duration of green leaves, defined by the number of months per year that the leaf canopy (or analogous main photosynthetic unit) is green, This measure constitutes one important component of the timing of plant growth, or phenology. Certain groups of competition avoiders (including some gap colonisers) may have very short periods of foliar display outside the main foliage peak of the more competitive species; some spring geophytes manage important growth at the beginning of the favourable season, before the canopy of the seasonally green species closes; in contrast, many evergreen species have a year-round ability to photosynthesise. Detailed information on monitoring all the aspects of plant phenology can be obtained from several networks set up for this purpose, including the NPN (http://www.usanpn.org/) and Project Budburst (http://www.neoninc.org/budburst/index.php).
Measuring leaf lifespan
Different methods are required for taxa with different kinds of phenological patterns and leaf demographic patterns. In all cases where a general measure is desired, select individuals and leaves by using the same criteria as in Section 3.1.
(A) Dicotyledons. Method 1 (below) is the best, although it is most labour-intensive and takes a longer time period. Methods 2–4 can replace Method 1, but only if the criteria are met.
(Method 1) Periodic census of tagged or mapped leaves. Tracking the birth and death of individual leaves over time at repeated census intervals is the best, although most labour-intensive method. Tag or map individual leaves (not leafy cotyledons) when they appear for the first time at a census event, and record periodically (at intervals of roughly 1/10 of the ‘guesstimated’ lifespan) whether they are alive or dead. Because the census interval length increases relative to the mean lifespan, the accuracy of any individual measure will decrease; however, with a large sample size, the estimate of the mean should remain accurate. The leaf-identification information needs to be recorded on the leaf itself (or nearby on the branch, although in an unambiguously systematic way), and often is done with a brief code (such as colour and/or symbols). Alternatively, a branch and leaf drawing, or map, can be made in which different colours are used at each census event. In this method, the position along the branch that separates leaves produced in consecutive census intervals must be marked. Leaves produced in given census intervals are drawn in such a way that their relative spatial positions are clear. At the first census when a leaf is no longer present (or is visibly mostly or completely dead), it is crossed out on the drawing with the colour of the present census event. For each individual plant, select two or more branches or shoots and sample all leaves on them. Note that a total of at least 40 and ideally 160 leaves per species is necessary (see Appendix 1). To achieve this, we recommend increasing the number of individuals, rather than the number of branches or shoots per individual. Calculate the lifespan for each individual leaf and take the average per individual plant. In addition to providing the most accurate estimate of the mean and median leaf lifespan, this method also provides a frequency distribution and enables estimates of the variance, which most other methods do not provide.
(Method 2) Count leaves produced and died over a time interval. Count (for each shoot or branch) the total number of leaves produced and died over a time interval that represents a period of apparent equilibrium for leaf production and mortality (see below within the present Protocol). We recommend about eight counts over this time interval, but a higher frequency may be better in some cases. Then estimate mean leaf lifespan as the mean distance in time between the accumulated leaf-production number and the accumulated leaf-mortality number (facilitated by plotting leaf production and leaf death against time). This is a good method if the census is long enough to cover seasonal periodicity (so typically it needs to be several months up to a year if seasonal periodicity occurs) and the branch or shoot is in quasi-equilibrium in terms of leaf production and mortality. This period can be much shorter for fast-growing plants such as tropical rain forest pioneers, woody pioneers in temperate zone or many herbs. This technique is useful for plants in their exponential growth phase, and for plants with very long leaf lifespan (because one gets data more quickly). Number of individuals and leaves are the same as in Method 1 above.
(3) Observe a cohort of leaves until one–half have died. This method measures the median leaf lifespan. Count the number of leaves that appear between two census events. Periodically revisit and count the number of leaves remaining. This method is effective when many leaves are produced within a short time period. Care must be taken to make these measures for multiple consecutive cohorts, if seasonal or interannual variation is likely to cause shifts in the median lifespan of different cohorts. Number of individuals and leaves are the same as in Method 1 above.
(4) Counting ‘cohorts’ for many conifers and only some woody angiosperms. For woody angiosperms, it is important to be very familiar with the species. This method is very easy and quick, but can be used only if the species is known to produce foliage at regular known intervals (most frequently once per year) and each successive cohort can be identified either by differences in foliage properties or by scars or other marks on the shoot orbranch. In that case, it is simple to count, branch by branch, the number of cohorts with more than 50% of the original foliage remaining until one gets to the cohort with less than 50% of the original foliage remaining, and use that as the estimate of mean leaf lifespan. This works if there is little leaf mortality for younger cohorts, and most mortality occurs in the year of peak ‘turnover’. Many conifers, especially Pinus and Picea, show this pattern, although some Pinus species may flush more than once per year. This method gives a slight overestimate, because there is some mortality in younger cohorts, and usually no or very few survivors in the cohorts older than this ‘peak turnover’ one. This method can also work (1) if there is some mortality in younger cohorts and a roughly equal proportion of survivors in cohorts older than the first cohort with >50% mortality, or (2) if one estimates %mortality cohort by cohort. This can be tricky. For instance, some conifers may appear to be missing needles (judging from scars) that were never there in the first place because of reproductive structures. Be aware that in Mediterranean-type climates, some species experience two growing seasons each year. Count one shoot (preferably the leader or a dominant shoot in the upper canopy) from at least 10 individual plants.
(5) Duration of green foliage for species that produce most of leaves in a single ‘cohort’ within a small time period and shed them all within a short time period (see Measuring duration of green foliage, below in the present Section). This method is likely to be the least reliable of the five described herein.
(B) Monocotyledons. For some monocots species, the longevity of entire blades can be measured as described above. However, given the growth habits of many monocots (see below within the present Protocol), this may not provide an estimate of green-tissue longevity that is comparable to the measures above for dicots. In some grasses and related taxa, the blade continues to grow new tissue while old tissue becomes senescent over time, making the mean of the whole blade lifespan much longer than the lifespan of a particular section of the blade and, therefore, not particularly meaningful as a measure of tissue longevity. In such cases, the production and mortality of specific zones of the blade can be assessed to estimate the tissue longevity, with an adaptation of Method 2 above.
Measuring duration of green foliage
Observe the foliage of 5–10 individuals of a given species several times throughout the year. We recommend a census for all species in the survey at least once a month during the favourable season (preferably including a census shortly before and shortly after the favourable season) and, if possible, one during the middle of the unfavourable season. The months in which the plants are estimated to have at least 20% of their potential peak-season foliage area are interpreted as ‘green’ months. This census can be combined with assessment of Leaf lifespan. Most species with individual leaf lifespans >1 year will be green throughout the year. Note that in some evergreen species from the aseasonal tropics, individual leaf lifespans can be as short as a few months.
Special cases or extras
(1) Leafless plants. If photosynthetic tissues do not die and fall off as separate units, follow Method 2 (above) for specific zones of the photosynthetic tissues, as specified above for monocotyledons.
References on theory, significance and large datasets: Chabot and Hicks (1982); Coley (1988); Reich et al. (1992, 1997, 2004); Aerts (1995); Westoby et al. (2000); Wright et al. (2002, 2004); Poorter and Bongers (2006).
More on methods: Jow et al. (1980); Diemer (1998); Craine et al. (1999); Wright et al. (2002); Reich et al. (2004).