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年在展望山地块（Prospect Hill tract）采伐的64年生红松人工林（red pine plantation）后的林木更新情况开展了第12年的持续监测。2001年的啃食（browsing）发生率仍处于极低水平（不足植株总数的2%）。随着林木平均高度持续增长，啃食发生频次及其对林分（stand）未来结构特征的影响均将维持在低位。总体而言，观测结果表明，在该林分的更新过程中，啃食并未产生显著的长期影响。白蜡树（white ash）是受啃食最严重的树种，目前仍是样地内最常见的树种。在过去五年保持相对稳定后，2001年林木总株密度降至17883株/公顷，对应历史数据依次为：2000年19464株/公顷、1999年19414株/公顷、1998年19958株/公顷、1997年19414株/公顷以及1996年20696株/公顷。过去六年间，主要树种的相对重要性始终保持一致。2001年，白蜡树（36.5%）仍是数量最多的树种，其后依次为红枫（red maple，26.9%）、糖枫（sugar maple，14.4%）以及黑樱桃（black cherry，9.4%），上述占比与2000年相比变化极小。2000年小幅下降后，红栎（red oak）在2001年的植株占比回升至7.5%，其中绝大多数为小型幼苗。总体而言，源自实生苗（seedlings）而非萌条（sprouts）的植株占比降至19.3%，低于2000年的23.1%、1999年的23.4%、1998年的25.4%以及1997年的23.7%。其中大部分实生苗（55.7%）为白蜡树，且多数高度不足0.5米。林木平均株高升至3.46米，2000年至1995年的对应数据依次为：3.20米、3.24米、3.01米、2.92米、2.87米以及2.67米。过去一年间平均树高生长的恢复，大概率反映了高度不足0.5米的幼苗及幼萌条的定植率偏低、死亡率偏高，同时较高大植株的生长仍在持续。高度超过7米的植株包括：20株白蜡树、15株红枫、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）。目前已对所有高度超过7米的植株开展胸径（diameter at breast height, DBH）测量。在5种最常见的树种中，糖枫的平均株高最高（4.75米），其后依次为红枫（3.98米）、黑樱桃（3.69米）以及白蜡树（2.99米）。受大量小型幼苗的影响，红栎的平均株高仅为0.67米。尚有多少幼苗能够存活并在发育中的林分中发挥作用，仍有待观察。我们的下一次采样将在第15年进行。