Subalpine coniferous forests is called as “ecological barriers” in the eastern fringe of the Tibetan Plateau and the upper reaches of Yangtze River, of which in Southwestern China are ecologically important but also a region that is susceptible to global climate change, as well as ideal and key region for related studies. Field work of our study was located in Wanglang National Natural Reserve, Pingwu County. Abies faxoniana, a dominant and widely distributed coniferous tree species along altitudinal gradient was used as experimental material. Considered the sharply altitudinal variation, ecophysiological responses of different altitudinal seedlings to simulated understory warming were studied, of which responses in non-growing season was solely documented. On the other hand, ecophysiological responses of A. faxoniana seedlings to reciprocal transplanting at contrast altitudes was done to detected influence of warming amplitude. The results are as follows:

1. Ecophysiological variations of Abies faxoniana seedlings along altitudinal gradients

Along ascending altitude, specific leaf area (SLA), pigment and nitrogen (N) contents of leaves decreased, whereas both water use efficiency (WUE) and nitrogen use efficiency (NUE) in leaves enhanced, nonstructural carbohydrates (NSC) contents of both leaves and branchlets increased, while the growths of seedlings decreased. Under the influence of seasonal environment, season dependent altitudinal trends were detected on peroxidase (POD), proline (Pro), polyphonnics (PP) and malondialdehyde (MDA) of leaves.

2. Effects of reciprocal transplanting on seedlings of Abies faxoniana

Active responses of photosynthetic potential and adversity tolerance in leaves to non-growing seasonal temperature variations were detected. Elevated non-growing seasonal temperature (ENT) increased pigment contents and the ratio of chlorophyll a to chlorophyll b (Chla/Chlb), enhanced POD activity and Pro content, decreased SLA, MDA and NSC contents. While reverse responses was also detected under lower non-growing seasonal temperature (LNT). In branchlets, temperature related respiration and import of carbohydrates, accompanying with nitrogen allocation between leaves and branchlets, might contribute to symmetrical (N and NSC) and unsymmetrical responses (δ13C) of detected indices to non-growing seasonal temperature variations.

3. Altitude dependent responses of seedlings to simulated non-growing seasonal warming

Compared with seedlings from both bottom and upper altitudinal fringes, mid-altitude plants were more sensitive to ENT, as they showed increases of pigments, protein, total sugar (TS), N, phosphorus (P) and kalium (K) contents as well as SLA and POD activity, and decreases of MDA, Pro and sucrose (Suc) contents as well as Chla/Chlb and carbon to nitrogen (C/N) in one-year old leaves. As for seedlings of the lowest altitude, warming winter increased leaves’ pigment, NSC, P and K contents as well as Suc content of branchlets, but decreased starch (Sta), NSC and NSC/C of branchlets, and protein, MDA and Pro contents as well as Chla/Chlb and POD activity of leaves. Little change was detected in the highest altitudinal seedlings response to ENT, except for MDA and Suc of leaves, and Sta, NSC, NSC/C and K of branchlets. Among all measured indices, MDA, Pro and K contents as well as Sta, NSC and NSC/C of branchlets showed stronger altitude-dependent responses to warming winter in the A. faxoniana seedlings than did the others.

4. Altitude dependent responses of seedlings to elevated growing seasonal temperature

Altitude dependent responses of leaves’ thickness, pigment and N contents to simulated warming were detected in seedlings of A. faxoniana. Seedlings of upper peripheral altitude employed conservative water-use traits (e.g. lower SLA and nitrogen, higher δ13C), formed thicker leaves, preferred distributing limited nitrogen to leaves, and limited both growth and storage (e.g. NSC) of seedlings’ aboveground respond to warming, managed to improve both adversity tolerance and nitrogen supply, tended to profit from such short time warming. Seedlings of middle altitudes also profited from warming with rapid growths of both stem and lateral branch, which might improve the competitive ability in understory. At the lower fringe, the expected endangering of seedlings labeled with significantly decreased of WUE (indicated as δ13C) and disturbed distribution of N, which might result from thinner leaves formed in simulated warming.

5.  The influences of tissues’ age and warming amplitude

Age-dependent responses of both ecophysiological traits and growth to simulated warming were detected. Tissues of current-year old presented more sensitive to warming than those of one-year old. Although in non-growing season, warming amplitude cast significant influence on response of seedlings. Mild elevation of temperature only affected biochemical traits of seedlings, whereas sharp elevation of temperature even changed leaves morphology.

Key words: Abies faxoniana; altitudinal gradient; simulated warming; reciprocal transplanting; non-growing season; growing season; open top chamber