Transcript document
Energy allocation: Studies on Goldenrods Abrahamson and Gadgil (1973) studied goldenrods along what they described as a 'disturbance gradient'. Species were studied in open, dry, early successional sites, in wet meadow sites (intermediate) and in hardwood climax forest. Because each species was found in two of these site types, comparisons of allocations could be made both across species (overall and within habitat) and across the successional gradient (both across and within species). One problem: Their comparisons involved only above ground plant parts, while root growth differences are also probably important. In these graphs, the x axis is time, so the figure indicates the temporal dynamics of allocation. At any point, the y axis indicates the proportion of biomass allocated to plant organs. These are proportions of an increasing total biomass. Some points to recognize: 1) all goldenrods begin as a basal rosette of leaves, out of which the flowering stem arises. Thus, initially all species allocate only to stems and leaves. 2) allocation to reproduction follows the expected pattern, decreasing as succession advances and diversity increases (dry site > wet site > woods site). 3) we’d expect the opposite pattern in leaf biomass due to differences in access to light (woods site > wet site > dry site) when those patterns are viewed during flowering. Those differences apply both across environments and, where comparisons can be made, within species. Consider allocation to reproduction [N.B. remember that these are proportions of total biomass]: S. speciosa is present in both dry and woods sites. During reproduction it allocates more to reproduction and less to leaves in the dry, open, disturbed site than in the woods site. S. rugosa is present in the woods and meadow site. It allocates more to reproduction and less to leaves in the meadow site (labeled wet) than in the woods site. S. nemoralis is the most 'weedy' of the species studied. It is characteristically a species of open, disturbed sites. Among these species, it has by far the largest proportional allocation to reproduction. Why am I concerned about the absence of measures of root biomass? It might be expected that in open, disturbed sites where you find S. nemoralis soil moisture and soil nutrients are at the lowest levels among these habitat types, and that root allocation would therefore be highest among these species in S. nemoralis. What might this do to our measure of biomass allocated to reproduction? How do the proportions allocated vary among species graphically (without considering roots)? What do these graphs show? 1) The species found mostly in habitats more diverse and successionally advanced, S. rugosa, has the lowest allocation to reproduction at any plant size; the 'weedy‘ species, S. nemoralis, has the highest fraction allocated to reproduction at any plant size. 2) Plants in the woods are, overall, larger than plants in disturbed sites. 3) You need to remember that these figures do not indicate a dynamic reallocation of biomass. Instead, they depict the de novo allocation of biomass as plants grow through the season. 4) We should also consider the impact of root allocation on this picture. We can use data gathered by a series of Field Biology classes studying many of the same species... Species Environment Solidago canadensis woods S. canadensis open S. graminifolia open S. rugosa woods S. nemoralis open % allocated to roots .358 .271 .217 .312 .310 The proportion of biomass allocated to roots was larger in both woods species than in any of the open field species. Let’s take one species, Solidago canadensis, from an open site, which, for purposes of comparison we’ll consider to be ‘like’ the wet meadow of Abrahamson & Gadgil’s original work. We re-calculate biomass allocation, knowing that only 72.9% of total biomass was above ground. Therefore, total biomass (including roots) was allocated as… roots stems leaves flowers with roots .271 .364 .175 .190 without roots ----.50 .24 .26 The average proportions allocated to roots are: in open habitats x = .266 and in woods sites x = .335, i.e. in the woods environment root allocations increase, possibly to compete for below ground resources. Thus, the difference in allocation to flowers and seeds is even larger in a comparison of open to woods plants than it appears in Abrahamson and Gadgil’s numbers. Can we frame these results in terms of the hypotheses about life histories? 1) In habitats characterized by regular or frequent disturbance adult survivorship is uncertain. However, characteristics of early successional environments also produce openings in which biotic interaction (inter and intra-specific competition, at least) is grossly reduced. That enhances the survival prospects for juveniles (seedlings). Thus, the seemingly optimal strategy increases reproductive allocation at the expense of already uncertain adult survivorship. 2) The pressure of density-dependent interactions is believed, according to dogma, to increase as succession advances. Accompanying this change is the necessity to allocate a larger proportion of available energy to growth (individual size), roots, leaves, and anything else which might enhance competitive ability. That decreases reproductive allocation. As well, in the closed environment of the woods (or other successionally advanced, climax communities) there are few chances for seedling establishment, and adult survivorship to be around to exploit them is critical.