Showing papers by "Peter J. Grubb published in 1998"

A reassessment of the strategies of plants which cope with shortages of resources

Abstract: An attempt is made to review the whole range of plants able to cope with shortages of mineral nutrients, water or light for at least a substantial part of their lives. In the past insufficient attention has been paid to quantitative measurements of the tolerance of various species for lack of resources. Growth rate at a low level of supply and mere survival are the variables to be considered. Problems in defining tolerance arise where a collection of species shows a negative correlation between survival at a low level of supply and growth rate under these conditions; in at least some circumstances a balance between survival and growth rate is important. Plants coping with lack of major resources display one of three strategies: ‘low-flexibility’, ‘switching’ or ‘gearing-down’. The essential features of the low-flexibility strategy are long-lived leaves, low maximum relative growth rates, and inflexibility of form and of gas exchange rates when resource-shortage is relieved, both in seedlings and in adults. Plants which show the switching strategy display the low-flexibility strategy as young seedlings, but are flexibile in form as older plants, and commonly have high relative growth rates then. The gearing-down strategy is based on an ability to reduce strongly the respiration rate when resources are in short supply, both as seedlings and as adult. In some cases this involves shedding of parts which would otherwise consume respiratory substrate. Plants showing this strategy have some characteristics which are the opposite of those shown by plants with the low-flexibility strategy: short-lived leaves, and high flexibility in form and in rates of gas exchange. All three strategies are represented among plants tolerant of the most extreme shortages of nutrients, water and light. It is concluded that while ecologists should attempt to reduce the complexities of nature to an oligo-dimensional framework of generalizing ideas, it is not reasonable to expect that any very simple scheme – such as might be represented by three or four reference points in one plane – will have generality on a world scale.

Responses of juvenile trees to above‐ and belowground competition in nutrient‐starved amazonian rain forest

Abstract: In several tropical lowland rain forests, removal of root competition by trenching around seedlings and saplings has been shown to have little or no impact on growth rate. However, we found that trenching increased the aboveground growth rate of saplings and seedlings of a wide variety of species in an Amazonian caatinga. This low-statured forest develops on waterlogged, humus-rich, white-sand soil that is extremely low in available nitrogen. Analysis of foliar nutrient concentrations and a fertilizer experiment confirmed previous findings that nitrogen is limiting, while measures of soil moisture potential and stomatal resistance gave no indication of water shortage. We suggest that belowground competition has an impact on growth rate because (1) established trees allocate a large proportion of biomass to fine-root production, further reducing the availability of nutrients in an inherently poor soil, and (2) penetration of daylight to the sapling layer is greater than under other tropical lowland rain fo...

The ecology of very small-seeded shade-tolerant trees and shrubs in lowland rain forest in Singapore

Abstract: A study was made in Bukit Timah Nature Reserve of niche differentiation among 11 woody species that have very small seeds (22–460 µg) and establish in deep shade, i.e., where the indirect site factor (isf) under cloudy conditions is ≤2%, and one taxonomically related light-demander (seed mass 33 µg). Comparative observations were made on changing light requirement with age. The species varied from shrubs and treelets (Urophyllum) via small to medium-height trees (Ficus, Pternandra) to tall trees (Gynotroches and Pellacalyx). In general, the shorter species fruited in deep shade while the taller needed direct irradiance. All produced ripe fruit at least once a year.

Nitrogen-richness and protection of seeds in Australian tropical rainforest: a test of plant defence theory

Abstract: Some species invest more in protecting their seeds or embryos than others, yet the reasons behind this disparity have not been clearly identified. In order to test the hypotheses that species with marked protection in the fruit have high nitrogen concentration ([N]) in the seeds, and species with marked protection in the seeds have high [N] in the embryo-cum-endosperm fraction (EE), we determined the [N] in the seeds and EE of 194 species from lowland tropical rainforest in NE Queensland. For species in which the fruit provides marked protection of the seed there is a significant negative linear relationship between [N] and log seed mass. There is a similar relationship for 'unprotected' species, but the intercept on the y-axis is significantly and markedly higher for the group of'protected' species. This relationship holds true when genera rather than species are considered. We are also able to make taxonomically controlled comparisons within six families and within four genera: all six familial comparisons and two of the generic comparisons support the finding for the collection of species as a whole. We then compare the [N] in the EE of species in which the fruit does not offer marked protection, but in which there is a range of investment in the seed wall fraction. Species with > 60% dry mass in the wall have significantly higher [N] than species with less massive walls. Possible advantages of having nitrogen-rich seeds are discussed.

A comparison of 12 tree species of Amazonian caatinga using growth rates in gaps and understorey, and allometric relationships

Abstract: 1. The relative growth rate of saplings of 12 species from an oligotrophic lowland rain forest were measured in treefall gaps and understorey. Mean relative height growth (RH) within treefall gaps was found to be slowest for tall-tree species with branched saplings, intermediate for subcanopy trees and fastest for tall-tree species with unbranched saplings. Most species had similar RH within the understorey. RH values were not related to leaf mass per unit area (LMA) or foliar N concentrations. 2. Allometric relationships between the total leaf area (TLA) and height were dependent upon light conditions; in general saplings of a given height had a greater TLA in treefall gaps than in understorey. The species with the largest estimated TLA values in gaps tended to have the greatest RH values in gaps; no such trend emerged in the understorey. The values of the allometric coefficients were not related to foliar properties. 3. The relationship between stem diameter and height was only weakly dependent on light conditions and the relationship between the growth rates in these dimensions was also weak. The lack of plasticity may reflect the fact that the height–diameter relationship has little bearing on a sapling’s tolerance of shade. 4. One way of accommodating the dependence of allometry upon irradiance is to add RH as a covariate. We derive a relationship between growth rates from this resource-dependent allometric equation and show that it reasonably describes measurements taken in the caatinga forest.

Seed mass and light‐demand: the need to control for soil–type and plant stature

Abstract: Studies on whether or not species which need more light for establishment have smaller seeds should be made between plants of similar mature height living in the same soil and macroclimate. Salisbury's classic comparisons confused differences in light-requirement during establishment with differences in mature height and in the supply of soil resources.