Knowledge Gaps in the Eco-Physiology of Oak Regeneration

The most important knowledge gaps in the eco-physiology of oak regeneration are:

1. Exact timing of primordia initiation. The timing of primordia initiation in northern red oak is not known. Careful anatomical and morphological studies based on the Quercus morphological index (QMI) (Hanson et al. 1986) could determine the exact timing of initiation. The timing of primordia initiation is probably associated with specific developmental and metabolic stages of the antecedent leaves (Dickson 1994).
2. Antecedent leaf developmental and metabolic state. The antecedent leaf is an older leaf or leaves directly connected by vascular traces to other developing leaves and primordia. The antecedent leaf provides metabolites and perhaps hormones for primordia initiation and subsequent leaf development (Dickson and Shive 1982, Larson 1983, Dickson and Isebrands 1991). These antecedent leaves are the major receptors of environmental stimuli and thus exert considerable control over the development of the apex and primordia initiation. Information on the metabolic state (e.g., QMI stage) of the antecedent leaves during development of the new flush would be helpful for elucidating basic control mechanisms (Dickson 1994).
3. Shoot/root interactions and feedback systems. There is no question that shoot and root growth is controlled by multiple feedback systems. Understanding the complementary functions of shoots and roots is a prerequisite to explaining whole-plant development. This is especially true when trying to understand episodic growth. During the lag phase in northern red oak, more than 90 percent of the photosynthate translocated from maturing leaves is allocated to lower stem and the root system (Dickson 1991). These photosynthate is used in new root growth, storage, and more importantly, in recycling of amino acids, hormones, and other chemical compounds from roots to shoots in the xylem (Dickson 1994).
4. Carbon-nitrogen relations. Related to the above three research areas is a lack of understanding about basic carbon and nitrogen interactions. Nitrogen is absorbed by the roots, reduced in the roots if absorbed as nitrogen, converted to organic nitrogen compounds, and translocated in the xylem to shoots. Distribution within the shoot (leaves, stem, developing leaves, etc.) is largely controlled by the particular amino acid being translocated in the xylem because of differential uptake from the transpiration stream in the stem (Vogelmann et al. 1985). Carbon translocated to the root system is required for energy to run these metabolic processes, and for the production of amino acids and organic acids for transport. Currently produced photosynthate translocated to roots may follow very different pathways compared to stored carbohydrate in roots (Dickson 1989).  Thus, to understand and perhaps control episodic growth, detailed information is required on carbon-nitrogen interactions and differential metabolism in shoots and roots during a flush cycle (Dickson 1994).
5. Reproductive biology gaps. The most complete information for any one species on the reproductive cycle exists for whiteoak (Q. alba L.) (Feret et al. 1982, Merkle et al. 1980, Mogensen 1965, Sharp and Chisman 1961, Sharp and Sprague 1967, Stairs 1964, Turkel et al. 1955). Even here there are many gaps and much disagreement as to what may or may not be the causes of the huge variability in acorn crops among trees, places, and years. And for most of the other species, the reproductive biology is very poorly understood (Beck 1993).
   As was noted earlier, little information is available about the reproductive biology of Q. rubra In fact, no complete detailed life history for any species of oak has been written. Therefore, our knowledge of oak flowering biology is based on a few reports about selected events in only a few species. These references are often combined to tell a developmental "story" about a hypothetical oak-an oak that probably does not exist. Before we try to interpret how various factors influence the production of flowers and acorns, we must develop a solid understanding of the reproductive cycle of individual species. The fragmented information about oak flowering does not give us an accurate picture of the flowering process, but it does indicate where the shortcomings and opportunities exist. We can then ask meaningful questions and do the appropriate research. We must study the biology of Q. rubra if we are truly interested in managing Q. rubra acorn crops (Cecich 1992, Isebrands and Dickson 1994).
6. Unpredictable acorn production. There is no simple or easy explanation, or fix, for the problem of sporadic, unpredictable acorn crops (Beck 1993).
7. Understory development. If a major cause of the regional oak regeneration problem has been the development of dense understories of competing species on mesic and dry-mesic sites, some evidence would be helpful in clarifying whether or not the magnitude of factors inhibiting understory development was sufficient prior to 1930 to account for the existing widespread occurrence of mature oak on mesic sites, and whether these factors have subsequently decreased (Lorimer 1993).
8. Regeneration ecology. We have a significant pool of knowledge for only three of the 31 species of oak reviewed. We have a modest amount of regeneration information for seven species and only limited information on the remaining 21 species (Table: Regeneration-related silvical characteristics). For the 27 native oak species that were not reviewed, there is virtually no information available (D. Smith 1993).