Growth and form of Quercus robur and Fraxinus excelsior respond distinctly different to initial growing space: results from 24-year-old Nelder experiments

Initial growing space is of critical importance to growth and quality development of individual trees. We investigated how mortality, growth （diameter at breast height, total height）, natural pruning （height to first dead and first live branch and branchiness） and stem and crown form of 24-year-old pedunculate oak （Quercus robur [L.]） and European ash （Fraxinus excelsior [L.]） were affected by initial spacing. Data were recorded from two replicate single-species Nelder wheels located in southern Germany with eight initial stocking regimes varying from 1,020 to 30,780 seedlings·ha？1. Mortality substantially decreased with increasing initial growing space but significantly differed among the two species, averaging 59% and 15% for oak and ash plots, respectively. In contrast to oak, the low self-thinning rate found in the ash plots over the investigated study period resulted in a high number of smaller intermediate or suppressed trees, eventually retarding individual tree as well as overall stand development. As a result, oak gained greater stem dimensions throughout all initial spacing regimes and the average height of ash significantly increased with initial growing space. The survival of lower crown class ashes also appeared to accelerate self-pruning dynamics. In comparison to oak, we observed less dead and live primary branches as well as a smaller number of epicormic shoots along the first 6 m of the lower stem of dominant and co-dominant ashes in all spacing regimes. Whereas stem form of both species was hardly affected by initial growing space, the percentage of brushy crowns significantly increased with initial spacing in oak and ash. Our findings suggest that initial stockings of ca. 12,000 seedlings per hectare in oak and 2,500 seedlings per hectare in ash will guarantee a sufficient number of at least 300 potential crop trees per hectare in pure oak and ash plantations at the end of the self-thinning phase, respectively. If the problem of epicormic shoots and inadequate self-pruning can be controlled with trainer species, the initial stocking may be reduced significantly in oak.

Impact of anthropogenic activities on vegetation dynamics in a reservoir area: model establishment and a case study of Longkaikou Reservoir in China

Vegetation in hot and arid valleys is a crucial indicator of ecosystem health,but is vulnerable to human activities and environmental change.Using the Longkaikou Reservoir in the Jinsha River in southwestern China as a case study,we developed a spatially explicit model that combined the plant growth,fruiting,seed dispersal,and seed germination stages to reveal the potential impact of multiple human activities(reservoir construction,logging,grazing,and aerial seeding) on the vegetation dynamics of Dodonaea viscosa and Pinus yunnanensis.After reservoir construction,the grassland area of 68 km^(2) in 2003 decreased to 24 km^(2) in 2018,replaced by forest,shrubland,and bodies of water,and the precipitation increased during the dry season,which indicated the improvement of the local plant and soil environment.Our model predicted that when soil moisture decreased by more than 20% compared to current levels,the area of D.viscosa increased greatly at low elevations;however,when at higher soil moisture,P.yunnanensis would occupy more of the study area.Logging and grazing would slightly change the spatial pattern of vegetation and delay P.yunnanensis communities from achieving stability by directly reducing plant biomass.Countermeasures such as aerial seeding would increase the total area by 13.13 km^(2) and 8.09 km^(2) of two plants,respectively,and accelerate the stabilization of plant communities.The effects of multiple human activities on vegetation may counteract each other;for example,logging decreased the P.yunnanensis area whereas aerial seeding increased it,and plant biomass changed in response to this pressure.Given the complex relationships between vegetation and human impacts,our study provides a scientific basis for vegetation restoration and ecological security in this hot and arid valley.