The Adaptive Role of Heteroblasty in Acacia koa with Implications for Restoration and Management
Plant species that undergo heteroblasty, the sudden change in morphology concurrent with maturation, possess distinct juvenile (or early stage) and mature (or later stage) characteristics that allow for disparate growth strategies during the course of development. Leaf form changes represent one of the most obvious forms of heteroblasty, but the adaptive importance of the distinct phases during development and plasticity in the timing of transition has been inconsistent. In this dissertation, I used a heteroblastic species native to Hawaii with a large climatic range, Acacia koa (koa), to ask questions about the adaptive role of heteroblasty, how the abiotic microclimate affects the rate of development, the role of ecotype in determining the plasticity in the rate of development, and how modification of the heteroblastic program interacts with abiotic stressors not mechanistically connected to the heteroblastic program to affect overall performance and survival. This dissertation presents three experiments designed to identify the abiotic stressors controlling the rate of development, asking how they modify development over time and ontogeny and how these stressors affect the phenotype at the time of phase change (Chapter 2). Using the framework developed in Chapter 2, a field study was used to assess the effect of irradiance on heteroblasty and whether plasticity in the rate of development in response to irradiance is adaptive (Chapter 3). Finally, a second field study tested how irradiance and nutrition interact to affect development rates on a frost prone site, specifically examining mechanisms of frost tolerance in koa and to test whether the ability to retain juvenile foliage adapted to low light conditions confers and adaptive advantage on sites where protection from cold stress concurrently reduces light availability (Chapter 4). I found that increasing irradiance increased the rate of transition (Chapters 2-4), but that the adaptive value of the plastic response to light was dependent on the structure of the canopy (Chapters 3-4). Decreasing water availability likewise increased transition rates (Chapter 2), suggesting the potential of an additional pathway controlling heteroblasty allowing for phyllodinous Acacia species to utilize mature form, drought tolerant phyllodes to establish on arid sites, a finding that was further strengthened by evidence that populations from dry ecotypes transition more quickly than populations from wet ecotypes (Chapter 2). Finally, I found that despite evidence that frost tolerance of koa increases seasonally and in response to temperature gradients, the ability to maintain early-form foliage without a significant reduction in growth rate in low light conditions facilitated improved performance in canopy gaps where the canopy provides protection from cold temperatures (Chapter 4). Collectively in these studies, I found high heteroblastic plasticity over both time and ontogeny that allows koa to maintain performance and survival in partially shaded conditions during establishment, in some cases allowing for increased survival and performance on harsh sites where the absence of a canopy would result in high mortality rates. Moreover, these studies strengthened the evidence for mechanisms controlling heteroblasty in plants and expanded on them by identifying potentially additional mechanisms (e.g., reduced water availability). In the context of koa, I found that alternative management techniques (gap silviculture) could capitalize on hetetoblastic plasticity to increase survival and improve growth form without sacrificing growth rates. This could lead to plantation designs that reduce labor inputs needed to realize a return on investments and restoration plantings that increase the complexity of canopy structure (providing improved habitat for endangered birds) and the rate of survival on harsh and remote sites. Finally, the presence of frost tolerance mechanisms suggests that survival gains could be realized through the use of tree improvement involving selection of superior genotypes.
Jacobs, Purdue University.
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