Positive Seedling-Shrub Relationships in Natural Regeneration of Ponderosa Pine

An understanding of natural regeneration processes, and the stand structural features that influence those processes, is vital to attaining goals associated with natural regeneration. This paper discusses natural regeneration concepts and the interactions that occur between shrubs and natural regeneration of ponderosa pine. The interactions observed recently in a series of related seedling recruitment studies in central Oregon are summarized. Evidence suggests a positive relationship between shrubs and the occurrence of ponderosa pine seedlings during the first two years of the seedling establishment phase. The vast majority of germinant mortality occurs during summer months, and especially the weeks immediately after emergence. Shrubs do not enhance emergence rates, but do enhance overall recruitment rates by reducing rates of first-summer mortality. The mechanism driving these patterns appears to be microenvironmental amelioration by shrubs, abating germinant moisture stress and desiccation. The manifestation of this relationship is a positive spatial relationship between shrubs and small seedlings during this establishment phase. Introduction For most forest management objectives, the recruitment of a cohort of seedlings is the first and most critical process following regeneration harvest or stand-replacing disturbance. The spatial and temporal patterns of seedling recruitment set the stage for all subsequent stand development patterns; hence, the seedling recruitment stage is the first step in determining future stand structures, habitat conditions, susceptibility to disturbances, and silvicultural options. An understanding of the natural regeneration processes in forests of ponderosa pine (Pinus ponderosa P. & C. Lawson), and the stand structural features that have bearing on those processes, is vital to manipulating controllable factors in order to successfully secure natural regeneration. This subject is particularly relevant under the current Forest Service paradigm of ecosystem management, wherein reliance upon natural processes in natural systems management is stressed, and funds for reforestation are scarce. An understanding of regeneration processes is equally important for those management objectives wherein regeneration is undesirable. For example, managers may wish to maintain crown fire resistance by promoting ponderosa pine stands with An abbreviated version of this paper was presented at the symposium on Ponderosa Pine: Issues, Trends and Management, October 18-21, 2004, Klamath Falls, Oregon. 2 Assistant Professor, Department of Forestry and Watershed Management, Humboldt State University, Arcata, California 95521 (e-mail: [email protected]) 3 Associate Professor, Department of Forest Science, Oregon State University, Corvallis, OR 97331 95521 (e-mail: [email protected]) USDA Forest Service Gen. Tech. Rep. PSW-GTR-198. 2005. 95 Positive Seedling-Shrub Relationships—Keyes and Maguire open-understory structures that are free of ladder fuels. Currently, management and research in this region is directed toward dealing with the existing condition of dense ponderosa pine stands. However, it is worthwhile to analyze the processes that have contributed to the development of these dense stands in the era of fire suppression and exclusion. It is reasonable to theorize that the effects of fire exclusion on ponderosa pine regeneration have been both direct and indirect. If the relationship of shrubs to seedling recruitment is positive, for example, it then follows that one effect of fire exclusion may have been the indirect proliferation of pine seedlings by enabling dense shrub understories to establish and persist. An understanding of these and other potential indirect effects of fire exclusion on pine proliferation is essential to prescribing practices that will create and sustain stands with crown fire resistance. Despite an abundance of previous research on the subject, the effects of shrubs on processes of ponderosa pine natural regeneration are still poorly understood. Research efforts have been geographically limited, short in duration, and restricted to a single stage of regeneration, a situation that is typical of regeneration research in other species and regions as well (Clark and others 1999). The result has been sometimes-conflicting and often-confusing advice for forest managers dealing with regeneration objectives. This paper addresses the relationship of shrubs to the establishment of ponderosa pine regeneration with a discussion of shrub effects within the context of recent conceptual models of facilitation and competition that have been applied in similarly xeric ecosystems. We also offer evidence of positive seedling-shrub relationships in a summary of the results of three integrated establishment-phase regeneration studies that were recently conducted in central Oregon. Constraints on Regeneration Several ubiquitous biological and physical factors are capable of limiting seedling recruitment in ponderosa pine forests, and often do so (table 1). Many of these factors may occur simultaneously during any given year, thus severely curtailing opportunities for regenerating ponderosa pine without human intervention. Among those factors, moisture stress represents the most significant of practicably controllable factors. The potential roles of phytotoxins as chemical regeneration inhibitors have also been investigated (Kelsey and Harrington 1979), but were revealed to be absent. In previous reviews of ponderosa pine regeneration processes, Heidmann (1992) and Barrett (1979) both identified aspects of climate having greatest influence on the establishment of seedlings from seedfall. Heidmann (1992) opened his review article, Regeneration Strategies for Ponderosa Pine, with the bluntly succinct statement, “Regenerating ponderosa pine (Pinus ponderosa) is difficult.” He elaborated that, The primary obstacle to regeneration of this species throughout its natural range is drought... Annual precipitation in the western and southwestern United States is generally adequate for tree growth but erratic distribution during the year makes seedling establishment difficult. USDA Forest Service Gen. Tech. Rep. PSW-GTR-198. 2005. 96 Positive Seedling-Shrub Relationships—Keyes and Maguire Table 1—Factors that potentially limit ponderosa pine regeneration, the affected regeneration stages, and relevant literature specific to ponderosa pine (references not specific to ponderosa pine are denoted by parentheses). Factor Regeneration stage Sample reference Seed deficiency flower, seed Pearson 1923 Fowells and Schubert 1956 Daubenmire 1960 Curtis and Foiles 1961 Shearer and Schmidt 1970 Foiles and Curtis 1973 Dahms and Barrett 1975 Barrett 1979 McDonald 1992 Molds seed Fowells and Schubert 1951 Wagg 1958 Roth 1970 Predation seed, germinant, seedling Pearson 1913 Eastman 1960 Hooven 1966 Cochran 1970 Blake and others 1986 Blake and others 1989 Shearer and Schmidt 1971 Damping-off fungi germinant Wagg and Hermann 1962 Roth 1970 Unfavorable forest floor substrate germinant Larsen 1924 Foiles and Curtis 1965 Litterfall burial germinant (Koroleff 1954) (Tappeiner and Helms 1971) Excessive heat germinant Baker 1929 Larson 1967 Moisture stress, drought germinant, seedling Pearson 1923 Hermann 1968 Larson and Schubert 1969 Cleary 1970 Djavanshir and Reid 1974 Frost heaving germinant, seedling Heidmann 1976 Summer frosts germinant, seedling Cochran 1972 Cochran 1984 Excessive shade germinant, seedling Pearson 1936 Atzet and Waring 1970 USDA Forest Service Gen. Tech. Rep. PSW-GTR-198. 2005. 97 Positive Seedling-Shrub Relationships—Keyes and Maguire In his report, Silviculture of Ponderosa Pine the Pacific Northwest, Barrett (1979) summarized the primary constraints on ponderosa pine regeneration with the observations: Prolonged moisture for germination, growth, and life during the critical heat of July and August is critical... Germination usually occurs in early spring, but rapid drying of the germinating medium often causes death of the seedling... Because summers are usually dry, conservation of moisture is critical. In the Pacific Northwest, ponderosa pine forests experience a harsh climate with cold winters and hot, dry summers. Annual precipitation is limited, and a regular period of drought occurs throughout much of the growing season (fig. 1). On the east slope of the Cascade Range, these climatic conditions are exacerbated by the presence of relatively young, poorly developed, mineral deficient, and highly porous volcanic soil types that dominate the region. These climatic and edaphic conditions present, through the moisture stress they induce, significant environmental challenges to tree regeneration, survival, and growth. Pr ec ip ita tio n (m m )