Regeneration Following Clearcutting Study at Harvard Forest since 1991

Measurements of regeneration following removal in 1990 of a 64-year old red pine plantation on the Prospect Hill tract were continued for the twelfth year in 2001. Browsing in 2001 remained at very low levels (less than 2% of stems). As mean tree height continues to increase both the amount of browsing and the impact of browsing on future stand characteristics should remain low. Overall, our observations show that browsing has had little long-term impact during the regeneration of this stand. White ash, the most heavily browsed species, remains the most common species in the plots. After remaining quite stable over the past five years, in 2001 the overall stem density of tree species declined to 17,883 stems/ha, compared with 19,464 stems/ha in 2000, 19,414 stems/ha in 1999, 19,958 stems/ha in 1998, 19,414 stems/ha in 1997, and 20,696 stems/ha in 1996. The relative importance of major species has remained the same over the past six years. In 2001, white ash (36.5%) remained the most numerous tree species, followed by red maple (26.9%), sugar maple (14.4%) and black cherry (9.4%). These percentages changed little from 2000. After a slight decrease in 2000, red oak increased slightly to 7.5% of tree stems in 2001, the majority of which were small seedlings. Overall, the percentage of stems that originated as seedlings rather than sprouts decreased to 19.3%, down from 23.1% in 2000, 23.4% in 1999, 25.4% in 1998, and 23.7% in 1997. The majority of these seedlings (55.7%) were white ash, most less than 0.5 m tall. Mean stem height rose to 3.46 m, compared to 3.20 m in 2000, 3.24 m in 1999, 3.01 m in 1998, 2.92 m in 1997, 2.87 m in 1996 and 2.67 m in 1995. The resumption in mean height growth over the past year probably reflects low seedling establishment and mortality of seedlings and young sprouts less than 0.5 m tall along with continued growth of the taller stems. The tallest stems were 20 white ash, 15 red maples, 15 sugar maples, 6 black cherries, 5 pin cherries, 3 paper birches, and 1 trembling aspen greater than 7 m tall. Diameter at breast height (dbh) is now being recorded for all stems taller than seven meters. Of the five most common species, sugar maple had the tallest mean height (4.75 m), followed by red maple (3.98 m), black cherry (3.69m), and white ash (2.99 m). Because of the preponderance of small seedlings, red oak mean height was only 0.67 m. It remains to be seen how many seedlings will survive to play a role in the developing stand. Our next sampling will be done in year 15.

2001年，本研究对1990年采伐展望山地块（Prospect Hill tract）内一处64年生红松人工林后的天然更新情况开展了第12年的持续监测。2001年的动物啃食发生率仍维持在极低水平（不足总茎株数的2%）。随着林木平均高度持续增长，啃食发生量及其对未来林分特征的影响均将维持低位。综合观测结果表明，啃食对该林分的更新过程未产生显著长期影响。白蜡树（white ash）是受啃食最严重的树种，仍是样地内最常见的乔木。

过去五年间乔木总株密度曾保持相对稳定，但2001年整体株密度降至17883株/公顷，2000年、1999年、1998年、1997年、1996年的对应数值分别为19464株/公顷、19414株/公顷、19958株/公顷、19414株/公顷及20696株/公顷。过去六年间，主要树种的相对重要值未发生明显变化。2001年，白蜡树（占比36.5%）仍为数量最多的乔木树种，依次为红花槭（26.9%）、糖枫（14.4%）与黑樱桃（9.4%），该占比与2000年相比变化极小。

红花槭在2000年小幅下降后，2001年的株占比回升至7.5%，其中绝大多数为小型实生苗。整体而言，由实生苗而非萌芽起源的茎株占比降至19.3%，低于2000年的23.1%、1999年的23.4%、1998年的25.4%及1997年的23.7%。其中多数实生苗（55.7%）为白蜡树，且大部分高度不足0.5米。

平均茎株高度升至3.46米，2000年至1995年的对应数值依次为3.20米、3.24米、3.01米、2.92米、2.87米及2.67米。本年度平均高度增长恢复，可能源于高度不足0.5米的实生苗及幼萌芽株的定植率偏低、死亡率较高，同时较高大茎株持续生长所致。

株高超过7米的个体包括：20株白蜡树、15株红花槭、15株糖枫、6株黑樱桃、5株灰樱桃（pin cherry）、3株纸皮桦（paper birch）以及1株颤杨（trembling aspen）。目前已启动对所有株高超过7米的个体胸径（diameter at breast height，dbh）的测量工作。

在五大常见树种中，糖枫的平均高度最高（4.75米），依次为红花槭（3.98米）、黑樱桃（3.69米）与白蜡树（2.99米）。受小型实生苗占比过高影响，红栎的平均高度仅为0.67米。尚有多少实生苗能够存活并在未来林分中发挥作用，仍待进一步观测。我们的下一次采样将在第15年进行。