R-script modified VS-Lite model

Drought is a key factor affecting forest ecosystem processes at different spatio-temporal scales. For accurately modeling tree functioning ? and thus for producing reliable simulations of forest dynamics ? the consideration of the variability in the timing and extent of drought effects on tree growth is essential, particularly in strongly seasonal climates such as in the Mediterranean area. Yet, most dynamic vegetation models (DVMs) do not include this intra-annual variability of drought effects on tree growth. We present a novel approach for linking tree-ring data to drought simulations in DVMs. A modified forward model of tree-ring width (VS-Lite) was used to estimate seasonal- and site-specific growth responses to drought of Scots pine (Pinus sylvestris L.), which were subsequently implemented in the DVM ForClim. Ring-width data from sixteen sites along a moisture gradient from Central Spain to the Swiss Alps, including the dry inner Alpine valleys, were used to calibrate the forward ring-width model, and inventory data from managed Scots pine stands were used to evaluate ForClim performance. The modified VS-Lite accurately estimated the year-to-year variability in ring-width indices and produced realistic intra-annual growth responses to soil drought, showing a stronger relationship between growth and drought in spring than in the other seasons and thus capturing the strategy of Scots pine to cope with drought. The ForClim version including seasonal variability in growth responses to drought showed improved predictions of stand basal area and stem number, indicating the need to consider intra-annual differences in climate-growth relationships in DVMs when simulating forest dynamics. Forward modeling of ring-width growth may be a powerful tool to calibrate growth functions in DVMs that aim to simulate forest properties in across multiple environments at large spatial scales.干旱是影响不同时空尺度下森林生态系统过程的关键因素。为精准模拟树木功能，进而可靠模拟森林动态，考量干旱对树木生长影响的时间进程与影响范围的变异性至关重要，在地中海气候这类强季节性气候区尤为如此。然而，绝大多数动态植被模型（Dynamic Vegetation Models, DVMs）并未纳入干旱对树木生长影响的年内变异性。本研究提出一种将树木年轮数据与动态植被模型内干旱模拟相联结的创新方法。研究使用改进后的树木年轮宽度正向模型（VS-Lite），估算欧洲赤松（Pinus sylvestris L.）在不同季节与立地条件下对干旱的生长响应，并将该响应模式嵌入动态植被模型ForClim中。研究采集了从中西班牙到瑞士阿尔卑斯山沿湿度梯度分布的16个样点的年轮宽度数据（涵盖干旱的阿尔卑斯内陆山谷），用于校准该年轮宽度正向模型；同时使用人工经营欧洲赤松林的清查数据，对ForClim的模型性能进行评估。改进后的VS-Lite模型可精准估算年轮指数的年际变异性，并能模拟出树木对土壤干旱的真实年内生长响应：春季生长与干旱的相关性显著强于其他季节，由此捕捉到欧洲赤松应对干旱的生存策略。纳入干旱响应季节变异性的ForClim版本，对林分断面积与株数的预测精度有所提升，这表明在模拟森林动态时，动态植被模型需要考虑气候-生长关系的年内差异。树木年轮生长的正向建模，或可成为校准动态植被模型生长函数的有效工具，此类模型旨在大空间尺度下模拟多环境中的森林属性。