Dushyantha K. Wijesinghe

TOWARD UNDERSTANDING THE CONSEQUENCES OF SOIL HETEROGENEITY FOR PLANT POPULATIONS AND COMMUNITIES

Several recent studies demonstrate that yield of individual plants, and their allocation of biomass between roots and shoots, can be profoundly affected by the pattern of supply of soil-based resources. Patchy provision of soil-based resources can affect the location of root biomass, as roots often proliferate in nutrient-rich patches. Root system size is important in determining whether plants access nutrient-rich patches, and the proportion of root systems located within such patches. This proportion will alter as growth proceeds. Species with small root systems have a limited ability to place roots in nutrient-rich patches even when they are very close. Of four species with different root system sizes, the growth of the species with the smallest root system was significantly limited by being located in nutrient-poor substrate even when nutrient-rich substrate was only 3.5 cm away, whereas three species with larger root systems were not disadvantaged. Both in the laboratory and in the field, root density is higher in nutrient-rich patches, and such patches can contain roots of many plants. Recent work showing that plants can respond to non-self roots sharing the same nutrient supply suggests that competition will be more severe in nutritionally patchy substrates than in homogeneous environments with the same overall nutrient supply. Taken together, these facts lead to the prediction that inter- and intraspecific plant interactions will be influenced by patterns of nutrient supply. We present evidence supporting this prediction, and indicating that population and community structure are also affected by patterns of nutrient supply. Significant differences in population yield, plant size distribution, and mortality have been recorded between populations growing under patchy and uniform conditions. Plant communities grown from identical seed inocula, with the same overall nutrient supply, provided in different spatial and temporal patterns, differed by up to 44% in total biomass, up to 70% in root biomass, and differed in species composition. These significant effects of heterogeneous resource supply on plants merit further detailed study. We present a framework of predictions of the impacts of different types of spatial heterogeneity in nutrient supply on the performance of single plants, and on plant interactions, plant populations, and plant communities.

Patchy habitats, division of labour and growth dividends in clonal plants.

Natural habitats are patchy in quality. in clonal plants, resource-acquiring structures often occupy sites that differ in quality. Clonal plants can display division of labour in resource-acquisition duties, manifested as local specialization by ramets, which enhances acquisition of each resource from sites of greatest abundance. Physiological integration can re-distribute resources internally from sites of acquisition to clone parts sited where the same resources are scarce. Recent research is showing that such specialization and resource sharing is a highly efficient strategy for acquiring resources and that it can result in considerably greater growth when resources are heterogeneously distributed than when the same quantity of resources is distributed homogeneously.

The effects of spatial scale of environmental heterogeneity on the growth of a clonal plant: an experimental study with Glechoma hederacea

1) Habitat heterogeneity is manifested as patches differing in quality at a variety of spatial scales, durations or contrasts, but little is known about its effects on the capacity of plants to forage for resources and to grow. This paper investigates the effects of the spatial scale of heterogeneity upon growth of the clonal herb Glechoma hederacea. 2) Clones were grown in eight experimental environments, each containing the same total amount of two types of soil distributed in separate patches. Contrast between patch types was the same in all treatments. The number and size of patches differed between treatments, from two 25-cm × 50-cm patches to 64 6.25-cm × 6.25-cm patches. In six of the treatments roots could grow freely between patches. Partitions prevented root growth between patches in the remaining treatments. 3) Although all treatments provided the same quantity of nutrients, clone biomass was dependent on the scale of heterogeneity. Biomass was highest in the 25-cm × 25-cm patch-size treatment and declined significantly at smaller patch sizes. It varied by a factor of four when only the treatments allowing root growth between patches were compared, and by a factor of seven when the treatments preventing root growth between patches were included. 4) Clones displayed a scale-dependent capacity to locate roots selectively in nutrient-rich patches. Although the proportion of biomass allocated by clones to above-ground structures and roots did not differ significantly between treatments, a significantly greater proportion of the root biomass of clones was located in rich than in poor patches in the larger patch-size treatments, promoting more efficient foraging for nutrients in these treatments. As patch size decreased, the proportion of clone root biomass located in the two patch types became more equal. 5) Root: shoot ratio within clones responded to patch scale and quality. In the larger patch-size treatments, in which clones foraged more efficiently, parts of clones located in rich patches had a higher root:shoot ratio than parts of clones located in poor patches, thus enhancing nutrient acquisition from rich patches. However, G. hederacea behaved like plants with a single rooting point in the smaller patch-size treatments, in that root:shoot ratio increased when nutrients were scarce. 6) Thus, the foraging response of G. hederacea was coarse-grained in environments where patches were large. In comparison, G. hederacea apparently responded to environments with small-scale patchiness as if they were homogeneously poor. It could not adjust its morphology rapidly enough to respond to these less predictable environments where changes in patch quality were more frequent.

Advantages of Clonal Growth in Heterogeneous Habitats: An Experiment with Potentilla Simplex

1 The ability to share resources between modules (ramets) is considered to be a benefit of the clonal growth habit. This type of physiological integration may buffer the entire clone against locally adverse conditions resulting from the patchy distribution of resources. The hypothesis that physiological integration is most advantageous in a heterogeneous habitat was tested using the clonal, perennial species Potentilla simplex (Rosaceae). 2 Five artificial habitat types differing in nutrient status were constructed in the glasshouse and the performance of intact and disconnected clones (clones in which all ramets were severed from each other following the establishment of roots) growing in them was compared. The habitat types ranged from homogeneously poor via three levels of spatial heterogeneity in nutrient supply to homogeneously rich. 3 Intact clones growing in the heterogeneous habitats weighed significantly more than their disconnected counterparts, supporting the hypothesis. The benefits of resource sharing were enhanced with increasing patchiness of the habitat. No differences in biomass between intact and disconnected clones appeared in the homogeneous settings. 4 Parent ramets supported their offspring at a large cost in biomass to themselves, but the offspring ramets benefited from clonal integration irrespective of the quality of the patches they occupied. 5 Across all habitats, intact stolons elongated more than the severed ones. This increased mobility of intact clones implies that clonal integration may allow this species to escape from unfavourable patches. The enhanced performance of connected ramets rooted in rich patches shows that clonal integration also enables clones to exploit resource-rich microhabitats, thereby maximising foraging ability and resource acquisition, particularly when resources are distributed in patches. 6 The cumulative effects of parent and offspring biomass and offspring ramet number appear to explain the trend in performance of intact and disconnected clones observed in the homogeneous and heterogeneous habitats.

Performance of a clonal species in patchy environments: effects of environmental context on yield at local and whole-plant scales

Yield of the clonal plant Glechoma hederacea was compared at different spatial scales, in heterogeneous and homogeneous environments providing the same amount of nutrients. For the heterogeneous treatments, environments were created with different patch sizes and different degrees of contrast in nutrient concentration between patches of different quality. Total clone yield differed by almost 2.5-fold across treatments, being highest in environments with large patches and high contrast, lowest in environments with small patches and high contrast, and intermediate under homogeneous conditions. Compared with plants in homogeneous conditions, there were significant increases or decreases in yield at all scales of measurement in many of the heterogeneous treatments. These effects on yield reflected a combination of local responses to growing conditions and modification of these responses due to physiological integration with other parts of the plant growing in contrasting conditions, supporting the proposal of de Kroon et al. (2005 New Phytol 166:73–82). The results show that plant yield at all scales is strongly dependent on environmental context, and that maximum yield can only be realized under a limited range of heterogeneous conditions.

Costs of producing clonal offspring and the effects of plant size on population dynamics of the woodland herb Uvularia perfoliata (Liliaceae)

1 Clonal propagation is an important means of population increase in many temperate woodland herbs. The production of vegetative propagules is often limited by the patchy availability of light in the understorey. A field study and a greenhouse experiment investigated the patterns of clonal growth under different light conditions in the shade-tolerant herb species Uvularia perfoliata. 2 In the field study, vegetative propagation in four populations, two growing under closed canopy and two in gaps, was compared. A significantly higher proportion of plants in gap populations produced clonal offspring than plants from closed-canopy populations, possibly because of higher light availability. The cost to plants of producing clonal offspring was seen as a significantly lower probability of survival and reduced capacity of the survivors for future clonal propagation. 3 In the greenhouse experiment, offspring ramets of three size classes (small, medium and large) from both closed-canopy and gap sites were grown under low and high light conditions. Plants grown from larger ramets from both sites produced more clonal offspring and invested more biomass in reserves and clonal propagation than plants grown from small ramets. Counter to our expectations, plants did not produce more clonal offspring in high light than in low light. However, plants invested more biomass in clonal propagation in high light, especially if they were grown from larger ramets. The only evidence for adaptation to conditions at the site of origin was that increased reserve allocation in high light was more marked in plants from gaps. The effect of ramet size on performance was stronger than the effect of light level or site of origin of plants, although plants grown from larger ramets also benefited more from high light. 4 Unless plants have sufficient biomass and morphological capacity their positive response to high light is delayed. Although clonal propagation is always costly, costs are less severe in gaps where plants have the potential to produce clonal offspring repeatedly in consecutive seasons. Our results also show that plant size has a potential long-term effect on population dynamics of this species, with the advantages of large size expressed over several generations.

Consequences of patchy distribution of light for the growth of the clonal herb Glechoma hederacea

The hypothesis that the clonal plant Glechoma hederacea can forage for light in environments with spatially patchy light supply was tested experimentally using artificial habitats in the glasshouse. There were five light treatments. Two treatments were homogeneous, with uniformly low or high light. Three treatments were patchy, each consisting of one high light patch and one low light patch. In the patchy treatments the position of the light patches differed with respect to the position at which the clone commenced growth. Several morphological responses were predicted if foraging for light through morphological plasticity took place : (I) there should be a greater concentration of ramets and biomass, a greater frequency of branching, greater leaf area, a lower proportional allocation of biomass to stolons and a higher proportional allocation of biomass to leaves in high light than in low light patches ; (2) there should be increased petiole lengths and a higher proportional allocation of biomass to stolons and petioles in low light patches than in high light patches ; (3) different primary stolons should assume different morphologies when exposed, simultaneously, to different light regimes. Structures directly involved in light acquisition in G. hederacea showed the greatest responses to changes in light conditions. In the patchy light treatments the frequency of branching, the leaf area of primary ramets and the proportional allocation to leaf biomass were significantly greater in high light patches and petiole length and proportional allocation to petiole biomass were significantly greater in low light patches. The greater leaf area, coupled with greater branching, allowed clones to harvest light energy more efficiently in high light conditions and the increase in petiole length allowed clones to escape from poor light conditions. However, these foraging responses were shown only when clones grew from low light to high light patches or when different primary stolons were exposed simultaneously to either high or low light. Foraging responses were suppressed when clones grew from high light to low light patches. These results imply that responses to patchy light conditions are not clone-wide, but localised within individual stolons, and that individual stolons can adjust morphology rapidly only when high resource patches are encountered after low resource patches. The converse - adjustment to a low resource patch following a period in a high resource patch - does not occur or occurs slowly, due to physiological integration within stolons and the predominantly acropetal nature of resource translocation.

Temporal and structural components of ramet independence in the clonal perennial herb, Potentilla simplex

Connected ramets within a clone may show different levels of physiological interdependence. Ramets that were formerly physiologically dependent on others may become autonomous over time. Alternatively, clones may be composed, from a very early stage, of completely independent physiologically integrated units, consisting of one or several connected ramets. The perennial herb Potentilla simplex Michx. (Rosaceae) was tested to determine whether newly produced ramets are continuously dependent on other parts of the clone and clones consist of completely interdependent primary stolons. Two stolons of each of a number of P. simplex clones were anchored to the soil (...)