Impact of Competition on the Growth of Pinus Tabulaeformis in Response to Climate on the Loess Plateau of China

: With climate change, understanding tree responses to climate is important for predicting 18 trees’ growth, and plant competition as a nonnegligible biotic factor plays a key role in such response. 19 However, few studies have investigated how competition affects the response of Pinus tabulaeformis plantations to climate . In our study, we investigated nine 29 - year - old P. tabulaeformis plantation plots 21 (three density gradients). The dendroecological method was used to analyze the impact of competition 22 on tree s response to drought and interannual climate variation. Stand density index was used to indicate 23 the intensity of competition. The results showed that competition modified the climate - growth 24 relationship. Competition increased trees’ sensitivity to drought but the relationship between 25 competition and sensitivity to drought was nonlinear. The competition effect slightly increased under 26 intense competition conditions. Additionally, competition reduced trees’ sensitivity to interannual 27 climate variation. After 1999, the effect of competition was obvious. The sensitivity of small - diameter 28 trees, especially those in middle - and high - density stands, declined. Thus, in the future these trees 29 presumably may exhibit a reduced sensitivity to interannual climate variation and a greater sensitivity 30 to drought. Abstract: With climate change, understanding tree responses to climate is important for predicting 61 trees’ growth, and plant competition as a nonnegligible biotic factor plays a key role in such response. 62 However, few studies have investigated how competition affects the response of Pinus tabulaeformis 63 plantations to climate . In our study, we investigated nine 29 - year - old P. tabulaeformis plantation plots 64 (three density gradients). The dendroecological method was used to analyze the impact of competition 65 on tree s response to drought and interannual climate variation. Stand density index was used to indicate 66 the intensity of competition. The results showed that competition modified the climate - growth 67 relationship. Competition increased trees’ sensitivity to drought but the relationship between 68 competition and sensitivity to drought was nonlinear. The competition effect slightly increased under 69 intense competition conditions. Additionally, competition reduced trees’ sensitivity to interannual 70 climate variation. After 1999, the effect of competition was obvious. The sensitivity of small - diameter 71 trees, especially those in middle - and high - density stands, declined. Thus, in the future these trees 72 presumably may exhibit a reduced sensitivity to interannual climate variation and a greater sensitivity 73 to drought. 74

.The predominant soil type of local area is loess. Genarally, the depth of soil is deep. Due to the 164 differences in microenvironment, there are slight differences in soil properties in local areas. In Apr. 165 2019, we investigated the properties of the soil. The physical properties of the soil are shown in Table  166 S1 and Table S2 in the Supplementary Information.

Data collection 175
To analyze the effect of competition on the response of P. tabulaeformis to climate, 9 impermanent 176 rectangle plots with similar elevations and aspects were set in even-aged pure P. tabulaeformis 177 plantations in May 2018 ( Table 1). All of these monoculture plantations were planted in the 1980s and 178 were not thinned after being planted. These trees were approximately 29 years old when the experiment 179 was conducted. We established three density gradients. In each density gradient, the plots were set in 180 three stands with similar density (total of 9 densities). The average low stand density was 1208 181 stems/hm 2 ; the average middle density was 2275 stems/hm 2 ; and the average high density was 2989 182 stems/hm 2 . In the plots, we measured the diameter at breast height (DBH) of every tree and recorded its 183 coordinate. Since the diameter range was relatively large and, generally, trees of different sizes in a 184 stand responded differently to climate (Chen et al., 2012). Mérian et al.(2011) also showed that in even 185 aged stand (generally even-aged stand with single layer) it is divergent that trees of different size 186 responses to climate. Thus, we sampled them in two size classes. Because the tree sizes of the 187 low-density stands were larger than those of the high-density forest, the diameter was classified 188 according to 65% of the maximum DBH. A DBH greater than 65% of the maximum DBH of the 189 density was defined as the large-diameter class (

Calculation of the competition index 198
To analyze the magnitude of competition, the stand density index (SDI) was used to indicate the 199 competition pressure of a stand (Reineke 1933

Calculation of response indices and the tree ring width index 207
Increment cores were dried in a shaded area and then fixed and polished. All of the cores were 208 measured with LINTAB 6 at 0.01 mm resolution. The COFECHA program was used to assess the 209 cross-dating accuracy (Holmes, 1997). 210 The dated series were used to calculate the basal area increment (BAI) series by using the "dplr" 211 package in R software (Bunn et al., 2020). Then, the BAI series was used to calculate indices for 212 resistance (Rt), recovery (Rc), and resilience (Rs) (Trujillo-Moya et al., 2018). Rt can be characterized 213 as the ability of trees to withstand a period of water deficit without showing a perceptible decrease in 214 tree ring width (Rt<1 indicates a decline in growth). Rc describes the increase in tree ring width after a Rs is the ability to recover the growth level to that before a drought (here, Rs=1 indicates complete 217 recovery to predrought growth; otherwise the tree is still experiencing a legacy effect of the drought; where r is the radius of a tree; t is the year of a tree ring. Dr, preDr and postDr mean BAI in drought, 226 the average BAI of two years before and after a drought, respectively. 227 To analyze differences in response indexes among trees of different densities and diameters, the 228 Mann-Whitney-Wilcoxon test was used (Hollander et al., 1973) . 229 Additionally, the dated series were also used to calculate the tree ring width index chronology 230 (TRWI) of the whole series and of different densities and diameter classes. To develop the standardized 231 tree-ring width chronology (STRWI), the "ModNegExp" method was used to remove tree growth 232 trends. The detrended series was then used to calculate TRWI by using beweight robust mean. In addition, the residual chronology (RTRWI) was also established. This process was conducted by using 234 the R package "dplr" (Bunn et al., 2020). 235

Climate data 236
Monthly climate data (i.e., precipitation and temperature) from 1989 to 2016 for the Lishi area The Pearson correlation analysis was used to calculate the correlation between STRWI and RTRWI 242 and SPEI at time scales of 1-12 months. The STRWI has a higher correlation with SPEI when 243 compared with RTRWI, and the correlation is higher at the time scale of 8 months than that at other 244 time scales (Supplementary Information Table S3,and Table S4). We then selected the 8-month SPEI 245 (SPEI8), to calculate the MSPEI, which was calculated by the weighted mean of SPEI8 246 (Supplementary Information S1). The absolute value of the correlation coefficient between SPEI8 and 247 RTRWI served as the weighting factor. 248

Analysis of trees responding to climate 249
To evaluate the effect of competition on trees' response to drought, we developed a linear 250 mixed-effect model for response indices (values were converted to a normal distribution by the square 251 root, high th-root transformations, and reciprocal. The best method was selected by using 252 "powerTransform" function in the "car" package ) using the "nlme" package in R (Pinheiro et al., where R is the response index (Rt, Rc, Rs); SDI is a fixed effect, β is a random effect derived from the 258 plot; α and ε are the coefficient and error, respectively. All statistical analyses were conducted using R 259

(R Core Team 2018). 260
To study changes in trees' response to climate variation, the response ability (relative basal area 261 increment, RBAI) was calculated using the ratio of the BAI to the MSPEI for each density and 262 diameter. Although the traditional method, moving correlation, has been widely used to test the change 263 in the relationship of climate-growth, this method will reduce the length of the correlation series (at the 264 beginning and the ending, the length of the correlation series equal to the window will loss). In our 265 study, the length of the tree ring width was short. Thus, the moving correlation is not suitable for our 266 study. We used the relative basal area increment (RBAI) to analyze the change in the response in the 267 The RBAI is similar to the meaning of the climate effect on site productivity (Sharma et al., 2018). 269 The response of tree growth (basal area increment, BAI) to climate is related to α, as shown by 270 equation six. The larger the α, the more sensitive the tree is to climate conditions. The ratio (α, also 271 RBAI ) of the basal area increment to the climate index (MSPEI; in order to make RBAI larger than 0, 272 we add 3 to the series of MSPEI) can indicate trees' response to the interannual climate (equations six). 273 where α is the coefficient. 276 However, the volatility of RBAI is very large, which leads to an insignificant trend. Moving average 277 is a method that was widely used to reduce fluctuation of a series (Merens, 2010 conducted to test the significance of the trend before and after 1999 (the MSPEI decreased before 1999 280 and then increased, as shown in Fig. 2). We also analyzed the correlation between the standardized tree 281 ring width chronology (STRWI, which has a closer relationship with climate than residual chronology, 282 Supplementary Information Table S5 and Table S6) and SPEI8. 283 3 Results 284

Tree growth response to drought 285
Competition exerted significant impacts on tree responses to drought. As the density increased, 286 competition increased, while Rt decreased. There were significant differences in Rt among densities 287  Fig. 3a). Between diameters, the decline in the Rt of small-diameter trees was 289 more obvious than that of the large-diameter class, and the difference increased from low density to 290 high density (Fig. 3b). 291 In contrast, the change in the Rc was positively related to the increase in density. Among densities, 292 the differences in the Rc were significant (middle-high density W=6114, P<0.01; low-high density W=1629, P<0.01, Fig. 3c). Between diameters, the Rc of the small-diameter trees was larger than that 294 of the large-diameter trees, and the difference was significant at the high density (W=1156, P<0.01, Fig.  295 3d), showing that the Rc of the small-diameter trees was more sensitive to the density increase than that 296 of large-diameter trees. 297 The Rs slightly declined as the density increased. Among densities, the differences were significant 298 (low-middle density W=4370, P<0.01; low-high density W=4391, P<0.01, Fig. 3e), and the differences 299 between diameters were significant in the middle density stands (W=1422, P<0.01) and high density 300 stands (W=1556, P<0.01, Fig. 3f). large-and small-diameter trees, respectively. Two asterisks indicate significant differences 305 between diameters or densities (P<0.01), *, significant at P<0.05; **, significant at P<0.01. The 306 orange dash line is the value equal to one. 307

Competition effect on trees' response to drought 308
As density increased, the competition increased, which heightened tree sensitivity in response to 309 drought. The results of the linear mixed models showed that the significant effect of increased SDI 310 caused the Rt to decrease, and Rc to increase in the whole series ( Table 2). The form of the model 311 (including Rt^0.5 and Rc^-0.25) also showed that the relationships between SDI and Rt or Rc were not 312 linear ( Fig.S1 and Fig.S3 also showed the nonlinear relationship between competition, and Rt and Rc). 313 It suggested that the rate of Rt decline and the rate of increase in Rc reduced with the increase of SDI. 314 Thus, in dense stand trees resistance to drought and recovery from drought were not sensitive to the 315 change in competition. The Rs also decreased as the competition increased, but this relationship was 316 not significant. 317 When the large-and small-diameter trees were separated, the models showed a similar pattern to that 318 of the whole series. The marginal R 2 of Rt and Rs was larger in small-diameter trees than those in 319 large-diameter trees (Table 2), and compared with large-diameter trees, the response indices showed 320 more obvious changes for small-diameter ones (Fig. 3b, c and f). This result indicated that 321 small-diameter trees were more sensitive to the increase in competition. 322

Growth response to inter-annal climate variation 327
Trees' sensitivity to interannual climate variation was weakened by increased competition. The 328 average correlation value between SPEI8 and STRWI was higher in the low-density stand than in the 329 high-density stand (Fig.4). Before 1999, MSPEI declined (Fig.2), while BAI showed upward trends for 330 all densities and diameters (Fig.5). The result showed that in their early stage their growth was not 331 sensitive to the climate fluctuations. During this stage, the increased competition caused little effect on 332 their sensitivity. Although competition became higher as trees have grown up, the MRBAI did not 333 decline. 334 respectively. The numbers 1 and 2 represent large diameters and small diameters, respectively. 344 After 1999, although the BAI did not show a decline, the upward trends gradually plateaued for 345 small-diameter trees in high-density stands (Fig. 5). From low to high density, the trend of the MRBAI 346 varied from upward to downward. For large diameters, the slope decreased from 5.67 to -0.44, while 347 for the small diameters it decreased from 1.89 to -1.39 (Fig. 6). The result indicated that trees' 348 sensitivity of response to climate change was reduced by the increased competition. In low-density 349 stands, trees' sensitivity of response to long-term climate change increased, while in high-density 350 Additionally, the growth of large-diameter trees showed a higher correlation with interannual climate 352 change. They were more sensitive to interannual climate change than small diameter trees (Fig. 4). 353 During the whole period, when compared with large-diameter trees, small-diameter trees showed a 354 slight increase in early stage (before 1999) and a steeper decline in the second stage (after 1999). 355 Especially in middle-and high-density stands, small diameter tree MRBAI showed a significant 356 decline (Fig.6), which indicates that small-diameter trees were greatly affected by increased 357 competition. In middle-and high-density stands, small-diameter trees' response to climate change was 358 restricted by intensive competition. 359 4 Discussion 360

Competition effect on tree responses to drought 361
Competition exerts impacts on tree responses to drought. Our results also showed that competition 362 significantly increased tree sensitivity to drought. With an increase in stand density, the competition for water and nutrients became more intense (Vernon et al., 2018). Generally, increased competition 364 worsens water resource shortages. We found a negative relationship between the SDI and Rt, which is which made them more vulnerable to hydraulic failure (Ryan et al., 2006). Besides, in their researches, 377 the ages might differ between large-and small-diameter trees. In some cases, larger trees that suffer 378 from weak competition are older than smaller ones that suffer from intense competition. Skubel et al. 379 (2015) revealed that younger trees have more conservative water use strategies, while old trees showed 380 greater variation in water use efficiency. Therefore, the effect might encompass both age and 381 competition. Our experiment was conducted in an even-aged plantation, and the difference in height 382 between the large-and small-diameter trees was not very obvious. Thus, the difference was mostly 383 derived from the different growth rates and competition. Finally, in Martínez-Vilalta's research, the 384 species, Scots pine, is sensitive to high temperature. In contrast, P. tabulaeformis can tolerate drought and high temperature (Zeng et al., 2005). Therefore, differences in species characteristics and forest 386 structure may also lead to differences in responses to drought. Thus, the relationship between competition and response indices was also nonlinear. In addition, 407 previous studies showed that the effect of competition decreased with an increase in water stress 408 (Kunstler et al., 2011;Carnwath and Nelson, 2016). In the long term, trees living in high-density stands 409 suffer from more serious drought stress than those living in low-density stands. To adapt to low soil 410 water content they may maintain a lower ratio of leaf area to relative sapwood (Carnwath and Nelson., 411 2016). This characteristic is conducive to adapting to the increased water stress induced by increased 412 competition 413 However, although small-diameter trees also experienced more serious water deficits than 414 large-diameter trees, their response index (Rt and Rs) was more sensitive to the increase in competition. 415 The result has a great relationship with that the growth of small-diameter tree was more sensitive to the density to high density, the living space of small diameter class decreased more than that of large 420 diameter class ( Supplementary Information Fig. S4). Thus, small-diameter trees were more affected by 421 increased competition and their response was more sensitive to the increase in competition. 422

Response to interannual climate change 423
The results also showed that the correlation between growth and interannual climate change was 424 influenced by competition. The competition effect on their response was not constant throughout the 425 whole period. In the early stage competition may have been at a low level. When they were young they 426 had a smaller LAI than when they were mature so the demand for evapotranspiration was lower. And the water deficit also was not very serious. So the competition effect was not evident. Besides, when 428 they were young trees exhibited high stomatal conductance, photosynthetic rate, and high plasticity 429 In crowded populations, high evapotranspiration leads to the increased moisture providing fewer 435 benefits to individuals compared with low-density stands (Tamai et al., 2015). In high-density stands, a 436 dense canopy and thick litter intercept some of the precipitation so that the effect of the climate 437 was conducted in single-layer stands. The buffer effect might cause little impact on the difference between diameters. Although it is a single-layer stand, its diameter distribution range is large (Table 1)