Reply to the comment by V.J. Lieffers and K.J. Stadt on "Aging discrepancies of white spruce affect the interpretation of static age structure in boreal mixedwoods"

Abstract

In their recently published discussion paper, Lieffers and Stadt (2003) comment on an article in which we documented aging errors in understory white spruce (Picea glauca (Moench) Voss) in the boreal mixedwood (Peters et al. 2002). We believe that the primary concern raised by Lieffers and Stadt is based upon an extension of our results and conclusions beyond our intended scope. Our objectives in this response are to (i) clarify the scope and intent of our previous paper, (ii) respond to the issue of “accuracy of concept”, and (iii) discuss its relevance to the broader issue of forest regeneration dynamics. Lieffers and Stadt (2003) state: “We are concerned that Peters et al. (2002) will lead readers to believe that recruitment of white spruce in boreal mixedwood forests occurs only for a short period immediately after disturbance.” The intent of our paper was to clarify precisely how aging errors affect the apparent window of opportunity for regeneration in the time period shortly after fire, rather than addressing the separate issue of regeneration over the entire duration of stand development. We attempted to establish this specific scope by (i) focusing in our introduction on reasons why immediate regeneration is observed in studies conducted 1–5 years post-disturbance, while evidence of this short regeneration pulse appears to be absent in most mature stands (Youngblood 1995; Lieffers et al. 1996; Galipeau et al. 1997), (ii) selecting sites for which a seed source was available for immediate regeneration, and (iii) using the phrases “early mixedwood stand development” and “young stands” when discussing regeneration dynamics. We tried to openly acknowledge that the observed regeneration dynamics were confined to the types of sites we sampled, but we stand by our assertion that the aging errors have broader implications: “... an immediate post-fire cohort of white spruce dominates the age structure in young mixedwood stands when intense burns coincide with mast years and when seed sources are nearby. Age structures probably vary under different conditions; however, aging errors will also be prevalent” (Peters et al. 2002, concluding paragraph). We had no intention of implying that other studies that found delayed regeneration were in error; instead, we wanted to point out that studies may need to use cross-dating at the root collar to confirm the validity of their results. Indeed, we agree with the general conclusions of previous authors (Youngblood 1995; Lieffers et al. 1996; Galipeau et al. 1997) that white spruce often establishes immediately after fire, and that in many cases establishment may also be delayed substantially. As pointed out by Lieffers and Stadt (2003), in subsequent research Peters (2003) found two types of regeneration delays: (i) delays of 1–3 years are common when mast years occur 1–3 years after fire, and (ii) regeneration frequently does not occur for up to 30 years after fire, perhaps reflecting the decay of fire-killed logs to a stage appropriate for white spruce regeneration. Data from our studies do not show that there is continuous white spruce regeneration in mixedwood stands during the 30 years after fire. Lieffers and Stadt (2003) raised concerns with our previous statement that the aging errors we observed nearly account for the regeneration lags and extended regeneration that has been reported in older stands that were not crossdated (Peters et al. 2002). In making this comment we intended to refer to the first regeneration pulse that in previous studies often appeared to be delayed for 5–10 years after fire, and peaked 10–20 years after fire. Our results suggest that age structures that depict this pattern would look very different if all trees could be cross-dated both above and below ground. However, we are not suggesting that all regeneration lags are artificial, because they may occur for a variety of reasons. We believe that the accuracy of estimates is important for accuracy of concept. In studies of forest dynamics, age pat-