A cross-continental test of the Enemy Release Hypothesis: leaf herbivory on Acer platanoides (L.) is three times lower in North America than in its native Europe

Acer platanoides (Norway maple) is a widespread native tree species in Europe. It has been introduced to North America where it has often established dense stands in both secondary woodlands and relatively undisturbed mature woodlands. In Europe A. platanoides is also extending its original range, but generally seems to exist at much lower densities. One explanation for the ‘aggressiveness’ of invasive plants such as A. platanoides is that they have left behind pests and diseases which limit their population densities in their native lands (the enemy release hypothesis or ERH). To assess the ERH for Norway maple, a large network of collaborators assessed leaf herbivory rates in populations throughout Europe and North America. We found significantly lower total leaf herbivory (1.6% ± 0.19, n = 21 vs. 7.4% ± 1.94, n = 34) and lower fungal damage (1.0% ± 0.35, n = 13 vs. 3.7% ± 0.85, n = 34) in North America than in Europe over a 2 year period, which is consistent with the predictions of the Enemy Release Hypothesis. Across years, the average total leaf herbivory was significantly correlated with average annual temperature of the site (P < 0.05), although this was mostly due to sites in Europe (P < 0.001), and not sites in North America (P > 0.05). Furthermore, only populations in Europe showed very high levels of herbivory (e.g., nine sites had total leaf herbivory ranging from 10.0 to 51.2% in at least 1 year) or leaf fungal damage (only one site in North America showed high levels of fungal damage in 1 year), suggesting the possibility of more frequent episodic outbreaks in the native range. Leaf herbivory and fungal damage are only two aspects of consumer pressure and we do not know whether the differences reported here are enough to actually elicit release from top-down population control, but such large scale biogeographic differences in herbivory contribute towards understanding exotic invasions. Jonathan M. Adams and Wei Fang—equally contributed as first authors. A list of the participating members of the Transatlantic Acer platanoides Invasion Network is given in the Appendix 3.     

Competition for nitrogen sources between European beech (Fagus sylvatica) and sycamore maple (Acer pseudoplatanus) seedlings

To investigate the short‐term consequences of direct competition between beech and sycamore maple on root N uptake and N composition, mycorrhizal seedlings of both tree species were incubated for 4 days (i.e. beech only, sycamore maple only or both together) in an artificial nutrient solution with low N availability. On the fourth day, N uptake experiments were conducted to study the effects of competition on inorganic and organic N uptake. For this purpose, multiple N sources were applied with a single label. Furthermore, fine roots were sampled and analysed for total amino acids, soluble protein, total nitrogen, nitrate and ammonium content. Our results clearly show that both tree species were able to use inorganic and organic N sources. Uptake of inorganic and organic N by beech roots was negatively affected in the presence of the competing tree species. In contrast, the presence of beech stimulated inorganic N uptake by sycamore maple roots. Both the negative effect of sycamore maple on N uptake of beech and the positive effect of beech on N uptake of sycamore maple led to an increase in root soluble protein in beech, despite an overall decrease in total N concentration. Thus, beech compensated for the negative effects of the tree competitor on N uptake by incorporating less N into structural N components, but otherwise exhibited the same strategy as the competitor, namely, enhancing soluble protein levels in roots when grown under competition. It is speculated that enhanced enzyme activities of so far unknown nature are required in beech as a defence response to inter‐specific competition.     

The invasive alien leaf miner Cameraria ohridella and the native tree Acer pseudoplatanus: a fatal attraction?

• 1 The horse‐chestnut leaf miner Cameraria ohridella is an invasive moth in Europe and a serious pest of horse‐chestnut Aesculus hippocastanum. The moth also occasionally attacks sycamore maple Acer pseudoplatanus, when situated beside infested horse‐chestnuts.
• 2 The main objective of the present study was to provide an overview of the relationship between C. ohridella and A. pseudoplatanus and to determine whether C. ohridella has the potential to shift to this native tree.
• 3 In the field, females oviposit on different deciduous tree species. Although less frequently attacked than A. hippocastanum, A. pseudoplatanus was clearly preferred for oviposition over 12 other woody species investigated.
• 4 Surveys in Europe demonstrated that the majority of A. pseudoplatanus trees found beside infested A. hippocastanum had mines of C. ohridella, even though more than 70% of the larvae died within the first two instars. Attack rates and development success greatly varied from site to site. Attack levels on A. pseudoplatanus were not always correlated with those on A. hippocastanum, and mines on A. pseudoplatanus were sometimes observed beside weakly‐infested A. hippocastanum.
• 5 Field observations, experimental exposure of A. pseudoplatanus saplings and rearing trials in a common garden study showed that individual trees may vary in their susceptibility to C. ohridella, whereas there was no evidence that C. ohridella populations vary in their performance on A. pseudoplatanus.
• 6 To date, there is little evidence that C. ohridella represents a major risk for A. pseudoplatanus.

     

元宝枫生长旺季树干液流动态及影响因素

利用热扩散式边材液流探针和多种气象、土壤因子传感器组成的全自动数据采集系统和美国产Licor-6400光合测定系统,于夏秋季节对北京西山地区低山成林元宝枫单株边材液流动态和叶片蒸腾作用进行了系统观测。元宝枫树干边材液流变化受天气的影响,环境胁迫或环境的改善都能改变边材液流的波动特征。在正常情况下,边材液流的日变化呈单峰曲线:日出后树干液流迅速上升,峰值在中午前后出现,然后下降,在次日早晨前达到坡谷。最热月7月液流启动和进入坡谷的时间比其他各月早1~4 h。6月树干上位液流速率大于中位和下位,其他各月树干下位液流速率大于上位和中位,这种差距在7月达2~3倍。多元回归分析表明,在整个生长旺季,元宝枫边材液流变化深受气温、太阳辐射、空气相对湿度、土壤温度和风速等环境因子的影响,但在不同的观测时段和观测部位其影响的主导因子不完全相同,只有空气温度在任何情况下都是影响液流的主导因子;元宝枫边材液流的变异规律较好地说明了其耐旱的生态策略。     

Using management to address vegetation stress related to land‐use and climate change

While disturbances such as fire, cutting, and grazing can be an important part of the conservation of natural lands, some adjustments to management designed to mimic natural disturbance may be necessary with ongoing and projected climate change. Stressed vegetation that is incapable of regeneration will be difficult to maintain if adults are experiencing mortality, and/or if their early life‐history stages depend on disturbance. A variety of active management strategies employing disturbance are suggested, including resisting, accommodating, or directing vegetation change by manipulating management intensity and frequency. Particularly if land‐use change is the main cause of vegetation stress, amelioration of these problems using management may help vegetation resist change (e.g. strategic timing of water release if a water control structure is available). Managers could direct succession by using management to push vegetation toward a new state. Despite the historical effects of management, some vegetation change will not be controllable as climates shift, and managers may have to accept some of these changes. Nevertheless, proactive measures may help managers achieve important conservation goals in the future.     

Evolutionary history of a widespread tree species Acer mono in East Asia

East Asia has the most diverse temperate flora in the world primarily due to the lack of Pleistocene glaciation and the geographic heterogeneity. Although increasing phylogeography studies in this region provided more proofs in this issue, discrepancies and uncertainty still exist, especially in northern temperate deciduous broad‐leaved and coniferous mixed forest region (II). And a widespread plant species could reduce the complexity to infer the relationship between diversity and physiographical pattern. Hence, we studied the evolution history of a widespread temperate tree, Acer mono, populations in region II and the influence of physiographic patterns on intraspecific genetic diversity. Analyses of chloroplast sequences and nuclear microsatellites indicated high levels of genetic diversity. The diversity distribution was spatially heterogeneous and a latitudinal cline existed in both markers. The spatial distribution pattern between genetic diversity within A. mono and the diversity at species level was generally consistent. Western subtropical evergreen broad‐leaved forest subregion (IVb) had a unique ancient chloroplast clade (CP3) and a nuclear gene pool (GP5) with dominance indicating the critical role of this area in species diversification. Genetic data and ecological niche model results both suggested that populations in region II disappeared during the last glacial maximum (LGM) and recovered from south of Changbai Mt. and the Korean Peninsula. Two distribution centers were likely during the LGM, one in the north edge of warm temperate deciduous broad‐leaved forest region (III) and another in the south edge of region III. This was reflected by the genetic pattern with two spatially independent genetic groups. This study highlights the key role of region III in sustaining genetic diversity in the northern range and connecting diversity between southern and northern range. We elucidated the diversity relationship between vegetation regions which could facilitate the understanding of biodiversity origin and maintenance in East Asia.     

元宝枫苗木的水力结构特征

在温室条件下,控制不同干旱梯度,用改良的冲洗法测定了4年生元宝枫苗木的水力结构参数．研究表明,随着小枝水势的降低,水力结构各参数随茎段功能木质部直径的变化可以用不同的方程来模拟;导水率的大小受茎段所在区域的影响,限速区的导水率明显低于非限速区,限速区的存在对苗木个体的生存竞争有利．导水率、比导率和叶比导率都和功能木质部直径和小枝水势呈明显的正相关．较粗茎段的叶比导率远高于多次分枝的未端细小分枝,有利于苗木在干旱时保存那些光合积累较大的器官．在落叶之前,相同直径枝条的胡伯尔值随小枝水势的变化很小,说明苗木水分胁迫主要源于木质部空穴和栓塞．     

Phylogenetic Relationships of Maples (Acer L.; Aceraceae) Implied by Nuclear Ribosomal ITS Sequences

Acer. ITS 1 sequences in twenty-eight species of Acer and a species of Dipteronia in the family Aceraceae ranged from 220 to 242 bp and ITS 2 sequences from 215 to 251 bp. The size of the 5.8S coding region was 164 bp for all species examined in the family. Phylogenetic analysis of ITS sequences placed a very robust clade of section Palmata at the base of the tree. Three species of section Parviflora sensu de Jong (1994), A. spicatum, A. distylum and A. nipponicum, did not form a monophyletic clade. Acer spicatum was separated from the robust clade of A. distylum and A. nipponicum. Molecular tree strongly supports the close relationship among section Platanoidea, Glabra series Arguta, and section Macrantha. The close relationship between sections Pentaphylla and Trifoliata was also strongly suggested in ITS tree. Sections Rubra and Hyptiocarpa appeared to be closely allied with each other. The average rate of nucleotide substitution was estimated as (8.0±1.9)×10⁻¹¹ substitutions per site per year for ITS 1 and (9.0±1.6) ×10⁻¹¹ for ITS 2. Received 17 December 1999/ Accepted in revised form 10 March 2000     

Adenylate control contributes to thermal acclimation of sugar maple fine‐root respiration in experimentally warmed soil

We investigated the occurrence of and mechanisms responsible for acclimation of fine‐root respiration of mature sugar maple (Acer saccharum) after 3+ years of experimental soil warming (+4 to 5 °C) in a factorial combination with soil moisture addition. Potential mechanisms for thermal respiratory acclimation included changes in enzymatic capacity, as indicated by root N concentration; substrate limitation, assessed by examining nonstructural carbohydrates and effects of exogenous sugar additions; and adenylate control, examined as responses of root respiration to a respiratory uncoupling agent. Partial acclimation of fine‐root respiration occurred in response to soil warming, causing specific root respiration to increase to a much lesser degree (14% to 26%) than would be expected for a 4 to 5 °C temperature increase (approximately 55%). Acclimation was greatest when ambient soil temperature was warmer or soil moisture availability was low. We found no evidence that enzyme or substrate limitation caused acclimation but did find evidence supporting adenylate control. The uncoupling agent caused a 1.4 times greater stimulation of respiration in roots from warmed soil. Sugar maple fine‐root respiration in warmed soil was at least partially constrained by adenylate use, helping constrain respiration to that needed to support work being performed by the roots.