Academic journal article The American Midland Naturalist

Understory Light Regime, Shrub Layer and Sand Pine (Pinus Clausa) Regeneration in Four Scrub Stands

Academic journal article The American Midland Naturalist

Understory Light Regime, Shrub Layer and Sand Pine (Pinus Clausa) Regeneration in Four Scrub Stands

Article excerpt

ABSTRACT.-Estimates of direct and indirect light regime were derived from hemispherical canopy photographs for four Florida sand pine (Pinus clausa) scrub stands. Shrub foliageheight density and sand pine regeneration were tallied in 4 me cells around photo points. We found negative correlations between shrub foliage-height density and understory light regime in all four stands. Sand pine regeneration was spatially associated with higher surface light levels and reduced shrub foliage-height density within only one of our four stands, a recently burned (early 1970s) peninsular site. We found more consistent evidence of shade intolerance in our two Ocala sand pine stands. Sand pine regeneration was abundant in high light patches within an open, recently burned site; however, in an adjacent 60-yr-old, closedcanopy stand, regeneration was lacking despite a local seed source. Our two Choctawhatchee sand pine stands were characterized by higher surface light regime values than the two peninsular stands and sand pine seedling growth rates were lower than in the one peninsular stand that had regeneration. The closed-canopy stand had a sparse shrub layer; regeneration was ample in this stand, but did not covary with light regime or shrub foliage-height density. The open-canopy stand was in a coastal dune setting with stunted canopy trees exceeding 100 yr of age. Sand pine regeneration in this stand was not sufficiently abundant to test for spatial association with light regime or shrub layer.

INTRODUCTION

Within its present range, sand pine [Pinus clausa (Chapm. ex. Engelm.) Vasey ex. Sarg) has been separated into two geographically disjunct varieties. Ocala sand pine (Pinus clausa var. clausa D. B. Ward) occurs in the Florida peninsula and Choctawhatchee sand pine (Pinus clausa var. immuginata) occurs primarily in the Florida panhandle. The principal basis for varietal separation was a purported contrast in expression of cone serotiny (Little and Dorman, 1952; Ward, 1963); however, recent studies have reported considerable variability in the degree of cone serotiny, especially within Ocala sand pine. Therefore, a strict view of Ocala sand pine as serotinous and Choctawhatchee sand pine as nonserotinous is oversimplified, and varietal segregation has been questioned (Myers, 1990).

Ocala sand pine possesses many characteristics of a typical shade-intolerant, pioneer species. It is fast-growing, can reach reproductive maturity in <10 yr, and is relatively shortlived; it produces abundant, wind-dispersed seeds; and its regeneration is often cued by disturbance. The prevalence of cone serotiny in many peninsular stands of sand pine underscores the historical role of periodic crown fires in maintaining these populations. Several authors have described mature Ocala sand pine forests that lack understory recruitment by sand pine (Abrahamson et al., 1984; Myers, 1990; Menges et al., 1993). Despite this evidence for shade intolerance, postfire seedlings of Ocala sand pine must cope with aggresive vegetative regeneration of most common shrub-layer associates (all except Florida rosemary (Ceratiola ericoides Michaux), which is a prolific seeder). In general, these shrub associates establish rapidly following disturbance and outgrow sand pine, which regenerates more slowly from seed (Abrahamson, 1984). Sand pine seedlings become established and grow in the shade of this taller shrub layer for several years, until they emerge above it (Price, 1973; Outcalt and Balmer, 1983). This partial shading may, in fact, be required to protect sand pine seedlings from temperatures exceeding 70 C in full sun at the soil surface (Cooper et al., 1959; Brendemeuhl, 1990).

Regeneration dynamics and life-history traits of Choctawhatchee sand pine are less well studied (Brendemeuhl, 1990). Myers (1990) observed that sand pine regeneration is common in the understory of mature, coastal populations of Choctawhatchee sand pine, despite the absence of stand-destroying events. …

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