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Forest‐Tree Growth Rates and Probability of Gap Origin: A Reply to Clark
Author(s) -
Lorimer Craig G.,
Frelich Lee E.,
Nordheim Erik V.
Publication year - 1992
Publication title -
ecology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.144
H-Index - 294
eISSN - 1939-9170
pISSN - 0012-9658
DOI - 10.2307/1940185
Subject(s) - understory , diameter at breast height , weighting , canopy , mathematics , tree (set theory) , statistics , growth rate , ecology , biology , forestry , geography , combinatorics , physics , geometry , acoustics
In a previous paper (Lorimer et al. 1988) we described a method for estimating the probability that a randomly selected canopy tree was growing in a gap when it was a sapling, based on its early growth rate. We defined P. as the conditional probability that a tree was suppressed (growing in the understory) as a sapling of 4 cm diameter at breast height (dbh), given an observed sapling radial growth rate greater than or equal to a threshold growth rate x, and 1 P. as the corresponding probability that the tree was growing in a gap. Calculation of these probabilities requires estimates of the following variables: (1) G, the proportion of gap saplings with growth rates -x, (2) Sx, the proportion of suppressed saplings with growth rates -x, (3) Qg, the proportion of all saplings growing in gaps, and (4) Qs, the proportion of all saplings growing in the understory, such that Qg + Q, = 1. Qg and Q, can be viewed as weighting factors for cases where the two kinds of trees are unequally abundant; if Qg and Q, are both equal to 0.5, they drop out of the equation. When gap-origin probabilities are being estimated for currently mature trees, the weighting factors of interest are the overall proportions of surviving mature trees that were growing in gaps or in the understory when they were 4 cm dbh. We designated these weighting factors for mature trees as Qg' and Q,% respectively, to distinguish them from the situation involving contemporary gaps. Estimating the historical analogue Qg, however, is not a straightforward matter. Qg' will vary among stands because of historical differences in the frequency of gap formation, and its value may be quite different from the present value of Qg. Accurate estimates of gap-origin probabilities therefore require stand-specific estimates of Q9', which are most readily obtained from historical growth data.