Dataset Information

Walter's two-layer hypothesis revisited: back to the roots!

ABSTRACT: Walter (Jahrb Wiss Bot 87:750-860, 1939) proposed a two-layer hypothesis, an equilibrium explanation for coexistence of savanna trees and grasses. This hypothesis relies on vertical niche partitioning and assumed that grasses are more water-use efficient than trees and use subsurface water while trees also have access to deeper water sources. Thus, in open savannas, grasses were predicted to predominate because of their water use efficiency and access to subsurface water. This hypothesis has been a prominent part of the savanna literature since first proposed. We review the literature on Walter's hypothesis and reconsider his original intentions. Walter intended this hypothesis to be restricted to dry savannas. In his opinion, mesic and humid savannas were controlled by biotic factors and disturbances. We surveyed the global savanna literature for records of vertical niche partitioning by grasses and trees. We find that, within the scope of Walter's original intentions, this hypothesis works remarkably well, and in some cases is appropriate for deserts as well as for dry temperate systems and even some mesic savannas.

SUBMITTER: Ward D

PROVIDER: S-EPMC3679411 | biostudies-other | 2013 Jul

REPOSITORIES: biostudies-other

ACCESS DATA

Publications

Walter's two-layer hypothesis revisited: back to the roots!

Ward David D Wiegand Kerstin K Getzin Stephan S

Oecologia 20121225 3

Walter (Jahrb Wiss Bot 87:750-860, 1939) proposed a two-layer hypothesis, an equilibrium explanation for coexistence of savanna trees and grasses. This hypothesis relies on vertical niche partitioning and assumed that grasses are more water-use efficient than trees and use subsurface water while trees also have access to deeper water sources. Thus, in open savannas, grasses were predicted to predominate because of their water use efficiency and access to subsurface water. This hypothesis has bee ...[more]

PMID: 23266712

Similar Datasets

Project description:The two-layer hypothesis of tree-grass coexistence posits that trees and grasses differ in rooting depth, with grasses exploiting soil moisture in shallow layers while trees have exclusive access to deep water. The lack of clear differences in maximum rooting depth between these two functional groups, however, has caused this model to fall out of favor. The alternative model, the demographic bottleneck hypothesis, suggests that trees and grasses occupy overlapping rooting niches, and that stochastic events such as fires and droughts result in episodic tree mortality at various life stages, thus preventing trees from otherwise displacing grasses, at least in mesic savannas. Two potential problems with this view are: 1) we lack data on functional rooting profiles in trees and grasses, and these profiles are not necessarily reflected by differences in maximum or physical rooting depth, and 2) subtle, difficult-to-detect differences in rooting profiles between the two functional groups may be sufficient to result in coexistence in many situations. To tackle this question, I coupled a plant uptake model with a soil moisture dynamics model to explore the environmental conditions under which functional rooting profiles with equal rooting depth but different depth distributions (i.e., shapes) can coexist when competing for water. I show that, as long as rainfall inputs are stochastic, coexistence based on rooting differences is viable under a wide range of conditions, even when these differences are subtle. The results also indicate that coexistence mechanisms based on rooting niche differentiation are more viable under some climatic and edaphic conditions than others. This suggests that the two-layer model is both viable and stochastic in nature, and that a full understanding of tree-grass coexistence and dynamics may require incorporating fine-scale rooting differences between these functional groups and realistic stochastic climate drivers into future models.

Project description:Polycystic ovary syndrome (PCOS) was hypothesized to result from functional ovarian hyperandrogenism (FOH) due to dysregulation of androgen secretion in 1989-1995. Subsequent studies have supported and amplified this hypothesis. When defined as otherwise unexplained hyperandrogenic oligoanovulation, two-thirds of PCOS cases have functionally typical FOH, characterized by 17-hydroxyprogesterone hyperresponsiveness to gonadotropin stimulation. Two-thirds of the remaining PCOS have FOH detectable by testosterone elevation after suppression of adrenal androgen production. About 3% of PCOS have a related isolated functional adrenal hyperandrogenism. The remaining PCOS cases are mild and lack evidence of steroid secretory abnormalities; most of these are obese, which we postulate to account for their atypical PCOS. Approximately half of normal women with polycystic ovarian morphology (PCOM) have subclinical FOH-related steroidogenic defects. Theca cells from polycystic ovaries of classic PCOS patients in long-term culture have an intrinsic steroidogenic dysregulation that can account for the steroidogenic abnormalities typical of FOH. These cells overexpress most steroidogenic enzymes, particularly cytochrome P450c17. Overexpression of a protein identified by genome-wide association screening, differentially expressed in normal and neoplastic development 1A.V2, in normal theca cells has reproduced this PCOS phenotype in vitro. A metabolic syndrome of obesity-related and/or intrinsic insulin resistance occurs in about half of PCOS patients, and the compensatory hyperinsulinism has tissue-selective effects, which include aggravation of hyperandrogenism. PCOS seems to arise as a complex trait that results from the interaction of diverse genetic and environmental factors. Heritable factors include PCOM, hyperandrogenemia, insulin resistance, and insulin secretory defects. Environmental factors include prenatal androgen exposure and poor fetal growth, whereas acquired obesity is a major postnatal factor. The variety of pathways involved and lack of a common thread attests to the multifactorial nature and heterogeneity of the syndrome. Further research into the fundamental basis of the disorder will be necessary to optimally correct androgen levels, ovulation, and metabolic homeostasis.

Dataset Information

Walter's two-layer hypothesis revisited: back to the roots!

Publications

Walter's two-layer hypothesis revisited: back to the roots!

Similar Datasets

OmicsDI is part of the ELIXIR infrastructure

Tweets